Wood Hull Inspection Guidance (NVIC 7-95)
Enclosure (1) To NVIC 7-95
Compiled By The Joint Industry/Coast Guard Wooden Boat Inspection Working Group August 1995
Table Of Contents
Acknowledgements
List Of Figures
Glossary
CHAPTER 1. Design Considerations
- Introduction
- Acceptable Classification Society Rules
- Good Marine Practice
CHAPTER 2. Plan Submittal Guide
- Introduction
- Plan Review
- Other Classification Society Rules and Standards
- The Five Year Rule
CHAPTER 3. Materials
- Shipbuilding Wood
- Bending Woods
- Plywood
- Wood Defects
- Mechanical Fastenings; Materials
- Screw Fastenings
- Nail Fastenings
- Boat Spikes and Drift Bolts
- Bolting Groups
- Adhesives
- Wood Preservatives
CHAPTER 4. Guide To Inspection
- General
- What to Look For
- Structural Problems
- Condition of Vessel for Inspection
- Visual Inspection
- Inspection for Decay and Wood Borers
- Corrosion & Cathodic Protection
- Bonding Systems
- Painting Galvanic Cells
- Crevice Corrosion
- Inspection of Fastenings
- Inspection of Caulking
- Inspection of Fittings
- Hull Damage
- Deficiencies
CHAPTER 5. Repairs
- General
- Planking Repair and Notes on Joints in Fore and Aft Planking
- Diagonal Planking
- Plywood Repairs
- Butt Joints in Planking
- Mechanically Fastened Scarfs
- Framing Repairs
- Decayed Frame Heads
- Treating Isolated Decay
- Sheathing of Existing Wood Hulls
ANNEXES
Typical Construction Details C-1
References R-1
Acknowledgments
This Navigation and Vessel Inspection Circular is the result of a joint
effort between the Wooden Boat Industry and the Coast Guard to provide
the latest and most practical methods of wooden boat inspection and repair.
Every effort was made to harness the collective expertise and practical
insight of Coast Guard field inspectors and wooden boat builders, shipyard
repairers, marine surveyors, and operators.
Special thanks goes to the following people who comprised the Joint
Industry/Coast Guard Wooden Boat Inspection Working Group and actively
participated in the revision of NVIC 1-63:
- Mr. Giffy Full, Surveyor, Brooklin, Maine
- Mr. Edward McClave, Consultant, Noank, Connecticut
- Mr. Fred Hecklinger, Surveyor, Annapolis, Maryland
- Mr. K. T. Smith, Builder/Repairer, Yorktown, Virginia
- Mr. Bill Holland, Builder, D'Iberville, Mississippi
- Mr. Meade Gougeon, WEST SYSTEM Developer, Bay City, Michigan
- Mr. Ernie Baird, Repairer, Port Townsend, Washington
- Captain Kenneth Franke, USCG (Retired), Surveyor, San Diego, CA
- Commander Al Moore, USCG (Commandant G-MI/T)
- Commander Bill Uberti, USCG (Commandant G-MVI-1)
- Lieutenant Commander Marc Cruder, USCG (Commandant G-MVI-2)
This NVIC reflects the wooden boat building and repair methods acceptable
at the time of its publishing. It is not meant to be the sole authority
on this subject. Survey and repair methods not discussed in this NVIC which
have proven themselves "seaworthy" should be forwarded to the Coast Guard
(Commandant G-MCO-2) for consideration in any future revision.
List Of Figures And Tables
- Figure A Typical Wood Screw
- Figure B Wood Screw Properly Inserted and Countersunk
- Figure C Stray Current Corrosion
- Figure D Bonding Systems
- Table 4-1 The Galvanic Series of Metals in Seawater
- Notes on the Use of the Galvanic Series Table
- Figure E Common Forms of Scarfs
Glossary of common wooden boat design/construction words (see Annex C for illustrations of typical construction details)
Ashcroft Construction - Double diagonal planking system with
the planks of both skins raking in the same direction.
Backbone - The "spine" of the hull from which the frames radiate.
Back Rabbett - The surface against which the side of a plank lies in a rabbetted member. The end fastenings of the plank penetrate the back rabbett of a stem or sternpost; the lower or inner edge fastenings of a plank penetrate the back rabbett of a keel or horn timber. See diagram below.
- Rabbett line = Outer Rabbett Line
- Apex Line = Middle Rabbett Line, Margin Line
-
- Bearding Line = Back Rabbett Line, Inner Rabbet Line
Ballast - Added weight either within or external to the hull added to improve the stability of a vessel or bring it down to its designed lines.
Balsa Sandwich - End grain balsa wood used as a core between FRP laminates.
Bastard Sawn - Hardwood lumber in which the annual rings make angles of 30 degrees to 60 degrees with the surface of the piece.
Batten - A thin flexible piece of wood.
Beam - A structural member supporting a load applied transversely to it. The transverse members of a deck framing system; the width of a vessel.
Beam Knee - A gusset like member used to connect a beam to a frame.
Bearding Line - The line formed by the intersection of the inside of the planking with the side or face of the keel.
Bending Steam - The process of forming a curved wood member by steaming or boiling the wood and bending it to a form.
Bilge Plank - A strengthening plank laid inside or outside of a vessel at the bilge's turn; also known as "Bilge Stringer".
Binding Strake - An extra thick strake of side or deck planking.
Box Section Mast - A hollow mast of round, square or rectangular section made up of long strips of wood.
Breasthook - Timber knees placed horizontally between two fore ends of stringers to reinforce their connection to the stem.
Butt Block - A short longitudinal piece of wood used to back up the connection of two plank ends.
Buttock - That part of a vessel's stern above her waterline which overhangs or lies abreast of the stern post; the counter.
Buttock Lines - Lines representing fore and aft vertical sections from the centerline outward.
Camber - The curve of a deck athwartships.
Cant Frames - Frames whose plane of support is not perpendicular to the fore and aft line.
Capping - Fore and aft finished piece along the topside of an open boat, often improperly termed gunwale; called a covering board, margin plank or plank sheer in a decked vessel.
Carlin - The fore and aft members of the deck framing system.
Carvel Planked - Smooth skinned planking whose strakes run fore and aft.
Caulking (calking) - Cotton, oakum or other fiber driven into planking seams to make them watertight.
Ceiling - An inner skin of the hull often used to add strength in boats having sawn frames. In some cases the ceiling is not structural but merely serves to line the hull for decorative purposes or for ease in cleaning.
Chain Plate - (Shroud Plate) A flat strip of metal fastened through the hull, either from inside or outside, to which the lower ends of the shrouds are attached.
Check - A lengthwise separation of the wood that usually extends across the rings of annual growth and commonly results from stresses set up in wood during seasoning.
Chine - The line of intersection of the bottom with the side of a vee or flat bottomed vessel.
Clamp - The fore and aft member at the sheer line of the vessel to which the deck beams usually fasten.
Clench Planking - Lapstrake, in which the adjacent planks overlap like clapboards of a house.
Clench Fastening - Securing a nail or rivet by placing a rove (washer) over the inboard side and then bending the fastening over it. In many cases they are simply bent over by driving them against a backing iron, causing them to reenter the frame.
Clinker Built - See clench planking.
Coat, Mast - A protective piece, usually canvas, covering the mast wedges where the mast enters the deck.
Cold Bent (frames) - Frames which are bent on forms and after shaping are fitted to the vessel.
Cold Molded - A method of boat construction using a male mold over which layers of thin wood and/or plywood are diagonally laid and glued together. Can be covered with epoxy or FRP.
Cove Line - A hollowed out decorative line found along the sheer of a boat.
Covering Board - A plank used as a "washboard" or "plank sheer" along the outer edge of the deck. - See Capping.
Cutwater - The forward edge of the stem at the waterline.
Dead Rise - The amount the bottom rises from keel to chine - most properly applied to "Vee" bottom construction but also used in reference to the rising bottom of round bottom boats.
Deadeye - A stout disk of hard wood, strapped with rope or iron, through which holes (usually three) are pierced for the reception of lanyards. They are used as blocks to connect shrouds and chain plates.
Deadwood - The vertical structure built up from the keel to support the cant frames at the stern or stem; longitudinal timbers of a vessel's structural backbone which lie entirely outside the keel, sternpost, and horn timber rabbett lines
Decay - The decomposition of wood substance by fungi.
- (Advanced or typical) - The older stage of decay in which the destruction
is readily recognized because the wood has become punky, soft and spongy,
stringy, ringshaked, pitted or crumbly. Decisive discoloration or bleaching
of the rotted wood is often apparent.
- (Incipient) - The early stage of decay that has not proceeded far
enough to soften or otherwise perceptibly impair the hardness of the wood.
It is usually accompanied by a slight discoloration or bleaching of the
wood.
Deck Head - The underside of the deck.
Diagonal Planking - Planking laid on an angle to the keel.
Displacement - The actual weight of a boat as it "displaces"
its weight when afloat; not to be confused with admeasurement tonnages.
Drift (Pins, Bolts) A long fastening driven (pin) or threaded
(bolt) to receive end nuts, used for joining heavy timbers such as horn
timbers and stern frames; also used to fasten and reinforce wooden panels
on edge, such as rudders and centerboard trunks.
Dry Rot - A term loosely applied to any dry, crumbly rot but
especially to that which, when in an advanced stage, permits the wood to
be crushed easily to a dry powder. The term in actually a misnomer for
any decay, since all fungi require over 20% moisture to grow.
Dutchman - Wooden block or wedge used to fill the void in a badly
made butt or joint; a graving piece or repairing patch in a deck; filler;
shim; short plank.
Edge-Grained Lumber - Lumber that has been sawed so that the
wide surfaces extend approximately at right angles to the annual growth
rings. Lumber is considered edged grained when the rings form an angle
of 45 degrees to 90 degrees with the wide surface of the piece.
Edging - Amount required to be cut away from the edge of a plank
in fitting strakes.
Edge Nailed - A method of fastening a strip plank to adjacent
planks.
Facing - Building one piece of timber on another for strength
or finish purposes.
False Keel - Sacrificial batten added to the keel to protect
the keel from grounding and from marine borers; eg. worm shoe.
Faying - Joining closely together.
Flat-Grained Lumber - Lumber that has been sawed in a plane approximately
perpendicular to a radius of the log. Lumber is considered flat grained
when the annual growth rings make an angle of less than 45 degrees with
the surface of the piece.
Floor or Floor Timber - A transverse structural member lying
across the keel and tying the frames on either side of the keel together.
The central futtock or futtocks of a sawn frame, lying across the
keel. Floor timbers join both sides of a vessel together and make up
the substructure for external keel fastenings, engine beds, and mast steps.
Floorboards - Planking laid on top of the floors to provide a
walkway. Also known as the "sole."
Frame - The transverse structure at each section giving form
to the hull. Frames connect to the keel or keels on and to the clamp or
shelf at the sheer. Also known as "ribs."
Freeing Port - Any direct opening through the vessel's bulwark
or hull to quickly drain overboard water that has been shipped on exposed
decks.
Futtock - Curved parts or sections of transverse frames extending
from the floor timbers to the top timbers.
Garboard - The strake of planking nearest the keel.
Green - Freshly sawed lumber, or lumber that has received no
intentional drying; unseasoned. The term does not apply to lumber that
may have become completely wet through waterlogging.
Grub Beam - A built up beam of short heavy timbers used to shape
a round stern.
Gusset - Any piece that is used to join or strengthen the joint
of two other pieces.
Hanging Knee - A strengthening bracket used between frames and
deck beams.
Heartwood - The wood extending from the pith to the sapwood,
the cells of which no longer participate in the life processes of the tree.
Heartwood may be infiltrated with gums, resins, and other materials that
usually make it darker and more decay resistant than sapwood.
Horn Timber - One or more timbers forming the main support for
an overhanging stern and extending aft from the upper end of the stern
post. Also used for timber connecting the shaft log and body post with
the rudder post.
Horse (n) - The form upon which a small boat is built.
Horse (v) - To drive home, as to horse caulking.
Hot Frame - A frame which, after being softened by heat, is bent
into shape as it is installed.
Joint - The junction of two pieces of wood or veneer.
- Butt Joint - An end joint formed by abutting the squared ends of two
pieces. Because of the inadequacy in strength of butt joints when glued,
they are not generally used.
- Edge Joint - The place where two pieces of wood are joined together
edge to edge, commonly by gluing. The joints may be made by gluing two
squared edges as in a plain edge joint or by using machined joints of various
kinds, such as tongue-and-grooved joints.
- Scarf Joint - An end joint formed by joining with glue and mechanical
fastenings the ends of two pieces that have been tapered or beveled to
form a sloping plane surface, to the same length in both pieces. In some
cases, a step or hook may be machined into the scarf to facilitate alignment
of the two ends, in which case, the plane is discontinuous and the joint
is known as a stepped or hooked scarf joint or scarf joint with nib.
- End Joint - The place where two pieces of wood are joined together end
to end, commonly by scarfing and gluing.
- Lap Joint - A joint made by placing one piece partly over another and
bonding the overlapped portions.
- Starved Joint - A glued joint that is poorly bonded because insufficient
quantity of glue remained in the joint. Starved joints are caused by the
use of excessive pressure or insufficient viscosity of the glue, or a combination
of these, which result in the glue being forced out from between the surfaces
to be joined. This term should only apply to epoxy glues. Joints made with
other waterproof or water resistant glues like resorcinol and urea-formaldehyde
(brown glue) should be starved for maximum strength.
Keelson - An inner keel usually laid over the floors and through
bolted to the keel.
Kerf, Kerfing - To cut or make a channel with a saw blade.
Kiln Dried - As in timber, refers to forced hot air circulation
through a chamber to dry the wood.
King Plank - The centerline plank of a deck.
Knee - See Hanging Knee.
Knot - That portion of a branch or limb which has been surrounded
by subsequent growth of the wood of the trunk or other portion of the tree.
As a knot appears on the sawed surface, it is merely a section of the entire
knot, its shape depending upon the direction of the cut.
Lapstrake - See Clench Planking.
Limber - A hole allowing the free passage of water from one area
to another.
Lignum Vitae - A hardwood used for deadeyes and propeller shaft
bearings.
Making Iron - A large caulking iron used to drive oakum into
plank seams.
Mast Partners - Carlins between deck beams to strengthen the
area where the mast passes through the deck.
Molding - Measurement of a plank or timber from inboard to outboard,
i.e., parallel to the plane in which the member lies; opposed to siding
measured at right angles to such plane. Thus, the molding of a frame is
measured in the thwartship direction while that of a stern piece is its
cross sectional dimension fore and aft.
Nib - The squared off end of a tapered piece such as a scarf.
Noble Metal - A metal most resistant to deterioration due to
galvanic action; the cathodic material.
Oakum - A caulking material of tarred fibers.
Partner - Stiffening or supporting pieces fitted in way of the
passage of a mast through a deck. See Mast Partners.
Paying - The filling of the seam with seam putty, pitch, tar,
or other type of seam sealant after caulking it.
Pitch Pocket - An opening extending parallel to the annual growth
rings containing, or that has contained, pitch, either solid or liquid.
Plank - Strips of wood that form the "skin" of a boat; strakes.
Plank Sheer - See Capping.
Preservative - Any substance that for a reasonable length of
time is effective in preventing the development and action of wood-rotting
fungi; borers of various kinds and harmful insects that deteriorate wood.
Prick Post - An outer post supporting an outboard rudder.
Quarter Knees - Lateral brackets similar to the breast hook used
to join the sheer shelf or clamps to the transom.
Quartersawed Lumber - Another term for Edge-Grained Lumber.
Rabbet - A longitudinal channel or groove in a member which received
another piece to make a joint.
Racking - Two or more structural members working and becoming
loose; structural deformation of the transverse section of a ship's hull.
A vessel is said to be racked if, when viewed end on, it appears to be
leaning or tilting over to one side. Symptoms of racking generally appear
at the junction of the frames with the beams and floors.
Resorcinol - A formaldehyde resin to which a powder hardener
is added to form a strong water resistant wood glue.
Rib - See Frame.
Sampson Post - Any post well attached to the vessels structure
to take excessive loads; used as a bitt.
Scantling - The dimensions of all structural parts used in building
a boat. A full scantling vessel is of maximum required structural dimensions.
Scarf (scarph) (n) - A joint by which the ends of two structural
pieces of timber are united so as to form a continuous piece; a lapped
joint made by beveling off, notching or otherwise cutting away the sides
of two timbers at ends, and bolting, riveting, or strapping them together
so as to form one continuous piece without increase in sectional area at
the joint.
Scarf (v) - To join the ends of two timbers so as to form a continuous
piece in appearance; the joining of wood by sloping off the edges and maintaining
the same cross section throughout the joint.
Scupper - A pipe or tube leading down from a deck and through
the hull to drain water overboard.
Shake - A separation along the grain, the greater part of which
occurs between the rings of annual growth.
Sheer, Sheer Line - The intersection of the deck and the hull;
the longitudinal sweep of the deckline from the stem to the sternpost upward
at the ends in traditional designs, and downwards at the ends in reverse-sheer
designs.
Sheer Strake - The top or uppermost plank in a hull.
Shelf - Line of timbers bridging and thus stiffening frames but
chiefly for supporting the end of the deck beams.
Shipworm - A misnomer for the wood boring mollusk Teredo which
feeds on wood cellulose. Another but different marine borer, the Limnorae,
is also misnamed shipworm.
Siding - Generally the sawn or planed thickness of the planks
or timbers from which wood members are shaped or cut. See Molding.
Sister - As in sister frame or sister keelson. A member attached
to or laid alongside an original member to strengthen it, either as an
original construction technique or as a repair.
Spiling - The edge curve in a strake of planking.
Split - A separation of the wood with the grain due to the tearing
apart of the wood cells.
Spline - As in spline planking. A thin tapered strip of wood
glued and hammered into carvel plank seams which have become enlarged and
spill caulking internally.
Stain - A discoloration in wood that may be caused by such diverse
agencies as micro-organisms, metal, or chemicals. The term also applies
to materials used to impart color in wood.
Stealer - In the shell planking toward the ends of a vessel a
strake introduced as a single continuation of two tapering strakes. One
of (usually the shorter or narrower of) the two planks which are butted
into a single plank as double continuation or as the short piece notched
into a larger plank to add width not available on one board.
Stern Frame - The frame work around the inside of the transom.
Stopwater - A softwood dowel driven across a lap, scarf, or butt
joint in the backbone structure or elsewhere, to prevent seepage of water
into the hull; any contrivance to accomplish this purpose.
Strake - One of the rows or strips of planking constituting the
surface of the hull.
Strip Planking - Carvel construction where each plank is edge
nailed to the adjacent planks.
Taffrail - A timber rail around the aft deck of a vessel.
Treenail - (Trunnel) A wood dowel used as a fastening; often
fitted with a wedge in the dowel end to hold it in place. Dense wood such
as locust is used for the dowel.
Wane - A defective edge or corner of a board caused by remaining
bark or a beveled end.
Warp - Any variation from a true or plane surface. Warp includes
bow, crook, cup and twist or any combination thereof.
Weathering - The mechanical or chemical disintegration and discoloration
of the surface of wood caused by exposure to light, action of dust and
sand carried by winds and alternate shrinking and swelling of the surface
fibers with the variation in moisture content brought by changes in the
weather. Weathering does not include decay.
Welt - A strip of wood fastened over a flush joint or seam for
strengthening purposes; a seam batten.
Wicking - A caulking material such as oakum or cotton , used
to wrap a fastening in order to protect it from moisture.
Worm Shoe - A non-structural piece of wood placed at the bottom
of the keel to protect the keel from marine borers.
CHAPTER 1: Design Considerations
- Introduction
Watercraft have evolved over centuries of trial and error to the more
"modern" state of the art vessels we are now familiar with. Wood as a boat
building material is still used in many parts of the world as the most
readily available, easy to work, repairable material for marine applications.
Even with the advent of composites, fiber reinforced plastic (FRP) and
lightweight metals, wood will for many years to come, continue to be a
major factor in the design of boats.
- Acceptable Classification Society Rules
Lloyds Register of Shipping Rules and Regulations for the Classification
of Yachts and Small Craft is the standard adopted by reference in Coast
Guard regulations for the design and construction of wooden small passenger
vessels. Lloyds Rules apply to vessels of up to 50 meters (164 feet scantling
length).
Other classification society standards may be accepted on a case-by-case
basis.
- Good Marine Practice
No single publication contains all the innovations found in the design
of wooden vessels. This circular and the readings referenced in Annex R
form a basis of good marine practices from which owners, designers, builders,
inspectors and surveyors can, along with experience, maintain the highest
level of small passenger vessel safety.
- Typical Construction Details
Annex C contains several illustrations of typical construction details.
An index of these illustrations can be found on page C-1.
CHAPTER 2: Plan Submittal Guide
- Introduction
This chapter is intended as a general reference and guide for submitting
the plans for a proposed vessel to the Coast Guard. It is not a complete
text on naval architecture or a commentary on classification society rules.
Plans should be submitted in accordance with the appropriate regulations
to the Marine Safety Center (MSC) in triplicate.
- Plan Review
- Plans for small passenger vessels of wooden construction are generally
reviewed by the local Officer in Charge, Marine Inspection (OCMI). For
vessels over 65 feet in length and/or vessels incorporating novel designs
or specifications not entirely addressed by acceptable Classification Society
Rules, plan review will be conducted by the (MSC).
- Lloyd's Rules and Regulations for the Classification of Yachts
and Small Craft should be used as a reference for designs as well as
application to existing vessels.
- Other Classification Society Rules And Standards.
- Direct reference to Lloyd's Rules is based on the familiarity that
Coast Guard inspectors and technical personnel have with reviewing a vessel
designed to those standards. This does not prevent a design from being
based on the rules of another classification society or on some other standard.
The burden of proof rests with the designer to show, with thorough engineering
documentation and logic, that a proposed vessel meets a level of safety
at least equivalent to that prescribed by Lloyd's Rules.
- Another useful "historical" reference that may be used as a plan
review guide is "Merchant Marine Safety Instruction 14-60" dated 14 April
1960. This instruction contains scantling tables for 500 wooden T-Boats
up to 60 feet in length, which have been approved for routes ranging from
rivers to oceans. The scantlings in this reference are from a sampling
of vessels certificated based on years of satisfactory service similar
to the present "Five Year Rule" noted below.
- THE FIVE YEAR RULE
- Definition. The "Five Year Rule" is defined as:
"When scantlings differ from such standards and it can be demonstrated
that craft approximating the same size, power and displacement, have been
built to such scantlings and have been in satisfactory service insofar
as structural adequacy is concerned for a period of at least 5 years, such
scantlings may be approved. A detailed structural analysis may be required
for specialized types or integral parts thereof." Determinations for meeting
this rule are made for each case on individual basis by the OCMI.
- Burden Of Proof . The burden is upon the designer or owner
to show the similarities between the proposed vessel and an existing vessel.
The Coast Guard approving authority may need documentation showing the
similarities in size, power, displacement and scantlings, and may conduct
a survey and/or underway check of the similar vessel's performance in the
anticipated operating area. Scantlings can vary greatly for similar sized
wooden vessels depending on materials used.
- Satisfactory Service. The service life of small passenger
vessels vary greatly depending on location, maintenance, and use. An inner
harbor tour boat experiences a vastly different service environment than
does a deep sea party fishing vessel, and is normally designed quite differently.
An existing vessel used as a basis for a proposed new vessel should have
experienced at least the same operating environment planned for the new
vessel for five years, showing satisfactory service. A similar relationship
of experienced service to expected service should be presented to the OCMI
for an existing vessel changing service into Coast Guard certification.
CHAPTER 3: Material
- Shipbuilding Wood
Wood is an engineering material. Douglas Fir, Southern Yellow Pine (long
leaf), and White Oak furnish most of the wood used for boat and shipbuilding
in the United States. Of these, Douglas Fir is the predominant choice due
to availability and relatively rapid growth.
- When requirements call for strength, moderate to good decay resistance
and ability to hold fastenings well (frames, keels, stems, etc.), the following
woods are most commonly used:
- Douglas Fir
- Southern Yellow Pine (long leaf)
- Teak
- Western Larch
- White Oak
- Where light wood, which is easy to work and is warp and decay resistant,
is required (planking, etc.) the following woods are most commonly used:
- Cypress
- Mahogany
- Cedar (Port Orford, Northern White, Western Red and Alaska)
- Tangile (Philippine hardwood)
- Where light, easily worked and strong woods of moderate to low decay
resistance are required, the following woods have found favor:
- Sitka Spruce
- Western Hemlock
- White Pine
- Yellow Poplar
There are many other varieties suitable for boat use. These are listed
together with their properties in The Encyclopedia of Wood and Wood
- A Manual for its use as a Shipbuilding Material (References 1 and
10).
- Bending Woods
Unseasoned White Oak is the choice bending wood. It bends readily and
is high in decay resistance. Red Oak, Hickory, Rock Elm, White Ash, Beech,
Birch, and hard Maple, also bend readily but do not have the decay resistance
of White Oak. White Oak and its best substitute, Rock Elm, are expensive
and hard to obtain, but do the best job. For a further discussion of the
effects of bending and bending ratios of various types of woods see "Bent
Frames", Wooden Boat, No. 86, page 87.
It is important to remember that bending woods are unseasoned and therefore
should show a moisture content over 15% (18% is desirable). Attempting
bends with dry wood results in cracks across the grain particularly in
hulls with sharp bends at the turn of the bilge.
- Plywood
Plywood is a built up board of laminated veneers in which the grain
of each "ply" is perpendicular to the ones adjacent to it. Its chief advantages
lie in more nearly equal strength properties along the length and width
of the panel, resistance to change in dimensions with moisture content
and resistance to splitting. Major disadvantages are low decay resistance
and the difficulty of painting it properly.
Plywood is excellent where strength is needed in more than one direction
and where the relatively large size of the panels available can be utilized.
It is no stronger than the wood from which it is made and is not a cure-all
for wood structural problems.
Plywood is made from several types of wood and in many different types
and grades. In general, "Marine-Exterior" type of fir plywood or its equivalent,
technical or Type 1 hardwoods are the only plywoods acceptable for use
as hull planking. These plywoods are identical with ordinary "Exterior"
type in that they are bonded with waterproof glue by a process using heat
and pressure. Their advantage lies in the fact that the interior plies
contain few gaps and thus its strength, ability to hold fastenings and
resistance to decay are much higher than "Exterior". "Marine" plywood is
more expensive than "Exterior" but provides additional safety and durability.
Fir plywood is graded according to the appearance of the exterior veneers.
These grades run from grade "N" intended for natural finish and grade "A",
suitable for painting, down through grade "D", the poorest quality. Each
side is graded. For example, a panel may be graded "Marine Exterior A-B"
where "Marine Exterior" refers to the type of bonding used and the allowable
defects in the inner plies, while "A-B" refers to the appearance of the
two sides of the panel.
Marine plywood is usually available only in appearance grades B-C and
better. The strength of the wood is indirectly reflected in the grading
since the poorer grades have openings, splits, pitch pockets and other
defects which adversely affect strength and decay resistance.
All plywood is marked with its classification. This classification may
appear on the panel back, on its edge or both. Marine plywood is clearly
marked "Marine".
- Wood Defects
Wood, being a natural material, is not uniform in quality and is subject
to defects. Some of these affect only the appearance of the wood. Others
affect the strength of the wood and are of importance.
Boat building and repair craftsmen carefully select each piece for the
intended use. Often a load of timber, even milled from the same tree, will
display a variety of defects. Wood with knots, checks, excessive warp,
splits and pitch pockets should be rejected for use particularly in hull
structure applications.
- Mechanical Fastenings; Materials
Mechanical fastenings should be of material suitable for the service
intended. Ferrous fastenings should be hot-dipped galvanized. Among the
usual non-ferrous types brass is not acceptable in salt water applications
as it will corrode from de-zincification and is inherently soft and weak.
Caution should be used in selecting fastening material because of the
problem of galvanic action which can arise if dissimilar metals are used
close to one another. A bronze washer used with a steel bolt will result
in the eating away of the steel. Proper selection of fastening materials
will significantly prevent corrosion and thereby extend their service life.
Marine applications of stainless steel alloys (chromium-nickel) are
subject to a phenomenon known as contact corrosion or more commonly, crevice
corrosion. Stainless steels which are in contact with each other or
placed in tight joints (nuts and bolts), swage connections (standing rigging),
or used to fasten wood planking below the waterline, corrode at an alarming
rate. The vehicle of crevice corrosion is electrolytic cell formation.
If the stainless steel is unable to naturally form a thin film of chromium
oxide to shield the material from attack, corrosive liquids such as salt
water are able to establish electrolytic cells with chloride ions and corrosion
takes place. In short, stainless steel depends on oxygen to provide protection
against crevice corrosion.
Grade 316 L (passive) stainless steel is the most accepted material
for marine applications due to the introduction of molybdenum to the alloy.
For example: grade 304 stainless steel has 18% chromium and 8% nickel in
the alloy while grade 316 L has 18% chromium and 10% nickel and 3% molybdenum.
Grade 304 is quite susceptible to crevice corrosion when employed in tight
spaces and unable to generate chromium oxide. The 316 L material will last
longer in the same application.
Chandlers usually stock only brass and stainless steel, both being very
unsuitable for underwater fastenings. The grade of stainless is rarely
mentioned and is often only Type 304.
Generally, stainless steel fasteners should not be used underwater.
However, they are used quite frequently, but only if all of the following
conditions are met will they be satisfactory:
- Austenitic grade at least Type 304, preferably Type 316.
- Not passing through wet wood.
- Ample sealant under the head and in between mating surfaces.
- The item to be fastened is less noble than stainless; i.e. all
the copper alloys and, with some risk of hole enlargement, steel and iron.
Note: Condition (b) indicates that stainless wood screws
should never be used underwater.
The choice of stainless steel fasteners below the waterline should be
carefully considered based on the water salinity, grade of stainless steel
fastener available, and material of other fasteners and fittings in the
hull. Stainless steel may be subject to varying degrees of accelerated
crevice corrosion. For more information, see Metal Corrosion in Boats,
(Reference 13).
The number, size, type and spacing of fastenings for various applications
are given in Lloyd's Rules and Regulations for the Classification of
Yachts and Small Craft, Part 2, Chapter 4.
A general guide for use of the various types of fastenings follows:
- Screw Fastenings
- Lead Holes. Lead holes for wood screws should be about 90%
of the root diameter of the screw for hardwoods and about 70% of the root
diameter for softwoods. For large screws and for hardwoods, a shank hole
of a diameter equal to the shank of the screw and of a depth equal to the
shank may be used to facilitate driving. Lag screws should always have
a shank hole.
The lead hole for the threaded portion of a lag screw should have a
diameter of 65-85% of the shank diameter in oak and 60-75% in Douglas Fir
and Southern Pine with a length equal to the length of the threaded portion.
Denser woods require larger lead holes and the less dense require smaller
holes. For long screws or for screws of large diameter, lead holes slightly
larger than those recommended here should be used. The threaded portion
of the screw should be inserted by turning and not by driving with a hammer.
Where possible, screws should be selected so that the unthreaded shank
penetrates the joint for greatest strength and corrosion resistance, and
to facilitate the drawing together of the members. In this case, the shank
hole shall extend the full length of the shank. If conditions prevent the
shank from extending through the joint, the shank hole shall extend
completely through the member containing the head, to prevent threads
from engaging in that member, which might prevent the joint from drawing
up.
Figure A: Typical Wood Screw
Figure B: Wood Screw Properly Inserted And Countersunk
- Lubricants. Suitable lubricants such as wax, grease, or heavy
paint, but never soap should be used on screws, especially in dense wood,
to make insertion easier and prevent damage to the screw.
- Depth. Penetration of the threaded portion for at least a
distance of 7 screw diameters for hardwoods and 10-12 in softwoods is required
for maximum holding power.
- Loading. If possible, screws should be placed so that they
are loaded across the screw and not in the direction of withdrawal.
The spacing, end distance and edge distances for wood screws should
be such as to prevent splitting the wood. Lag screws should follow the
rules for bolts. For further information concerning wood screws, see Wooden
Boat, Issue 54 & 55 (Reference 17).
- Nail Fastenings
Hot dipped galvanized cut boat nails have traditionally and are still
being used in boat building. Barbed or annular ring nails have been successful
and are suitable depending upon their application (usually smaller scantling
vessels). Smooth, thinly coated or plated nails, with small irregular heads
and long tapered shanks such as horseshoe nails and ordinary "cut nails"
(i.e. hardwood flooring nails) will not provide sufficient holding power
and should not be used. In addition, wire nails are not acceptable for
hull construction.
- Lead Holes. Lead holes for nailed joints may be 3/4 of the
diameter of the nail without causing loss of strength.
- Types Of Load. If possible, nails should be loaded across
the nail and not in the direction of withdrawal. This is especially important
in end grain.
- Spacing Of Nails. The end and edge distances and spacings
of the nails should be such as to prevent splitting of the wood.
- Boat Spikes And Drift Bolts
- Lead Holes. Lead holes for boat spikes should be the size
of the short dimension of the spike and should extend approximately 75%
of the spike depth. The lead holes for drift bolts should be slightly less
than the bolt diameter and of a depth equal to the bolt length.
- Type Of Load. Where possible, spikes and drift bolts should
not be loaded in withdrawal. This is especially important in end grain.
- Insertion. A clinch ring or washer may be used under the head
to prevent crushing of the wood. Spikes should be driven with the edge
of the chisel point across the grain to avoid splitting the wood.
- Spacing of Spikes and Drift Bolts. The end distance, edge
distance and spacing of the spikes should be such as to avoid splitting
the wood.
- Bolts. Bolt holes should be of such diameter as to provide
an easy fit without excessive clearance. A tight fit requiring forcible
driving of the bolt is not recommended.
- Placement Of Bolts In Joint. The center to center distance
between bolts in a row should be not less than four times the bolt diameter.
The spacing between rows of bolts should be 5 times the bolt diameter
for a bolt whose length from the bottom of the head to the inner side of
the nut when tightened is 6 times the bolt diameter or longer. For short
bolts, this distance may be decreased but in no case should be less than
3 times the bolt diameter.
The "end distance" from the end of a bolted timber to the center of
the bolt hole nearest the end should be at least 7 times the bolt diameter
for softwoods and at least 5 times the bolt diameter for hardwoods. These
requirements should be relaxed where necessary in the case of bolted planking
butts to allow the "front row" of fastenings on each side of the butt to
be bolts.
The "edge distance" from the edge of the member to the center of the
nearest bolt hole should be at least 1 1/2 times the bolt diameter. For
bolts whose length is over six times their diameter, use one half the distance
between bolt rows and in no case below 1 1/2 times the bolt diameter.
For perpendicular to the grain loadings (joints at right angles), the
edge distance toward which the load act, should be at least 4 times the
bolt diameter.
- Bolting Groups
In general, all groups of bolts should be symmetrical in the members.
The individual fasten-ings should be offset slightly as necessary to avoid
placing more than one on the same grain.
- Washers. The importance of washers, especially under the heads
of fastenings which may be loaded in tension either because of external
stresses or because of swelling stresses, cannot be overstated. The weak
link in most metal-fastened wood structures is not the tensile strength
of the wood or of the fastenings, nor the withdrawal resistance of threaded
fastenings. The weak link is almost always the cross-grain crushing strength
of the wood under the heads of the fastenings. Care should be exercised in drawing
nuts down on the bolts too tight and crushing the wood.
- Wickings. A suitable wicking should be fitted in way of the
faying surface of the joint at each through bolt subject to moisture.
- Adhesives
Household glues having low moisture resistance have tendencies towards
early joint failure and should be avoided in marine applications.
Resorcinol and Phenol-Resorcinol resin type marine glues have been used
for many years and are satisfactory for most new construction and repair
applications. Resorcinol age hardens and becomes brittle and inelastic
over time and should be limited to rigid surfaces where shear, vibration
and impact forces are unlikely.
Urea-type adhesives such as Weldwood Plastic Resin glue are available
in water mix one-part and two-part resin/hardener mixes. Use of ureas requires
special care particularly with the two part system as, unlike epoxy resins,
the urea is applied with resin on one surface and the hardener on the other.
Clamp pressure is then applied and the cure begins.
Epoxy resins are available for a wide variety of marine applications
and have been found to provide excellent adhesion in all areas of boat
building. In the early 1960's epoxy adhesives were introduced to western
boat builders by the Gougeon Brothers of Bay City, Michigan, through their
registered trademark WEST SYSTEM. Epoxy resins are two part adhesives and depend on accurate mixing ratios to yield high strength joints. Epoxy is also an excellent filler material when thickened to high or low density with micro fibers, micro balloons or colloidal silica
Not all woods are easily joined. Wet wood (above 18% moisture content)
is difficult to glue. Normal seasoned wood of most species can be glued.
Strong joints can be made bonding either face or side grain of the wood.
These joints can be very nearly as strong as the wood itself. It is impossible
to join end grain with glue and get joints which are even 20% as strong
as the wood. A scarf or some other form of joint which gives a surface
approaching side grain condition must be used where end connection is desired.
As with any chemicals the manufacturer's instructions must be carefully
followed. Curing temperature and surface condition are important. The temperature
must be about 70 degrees Fahrenheit or higher for a full cure of resorcinol
resin glue. Faying surfaces should be well fitted. Smooth surfaces make
the strongest joints with resorcinol, however a roughened surface for epoxy
joints is generally helpful in improving bond strength, especially with
hardwoods, such as oak.
- Wood Preservatives
The use of wood preservatives is not required. However, their use in
wood under severe service conditions may pay for itself many times in decreased
decay and borer attack and thus decreased repair and replacement costs.
Their proper use should be encouraged since it increases the chance of
the vessel remaining sound until her next inspection and thus contributes
to maintaining a reasonable standard of safety.
Wood preservatives used for protection against decay fungi and marine
borers either kill the organism or prevent it from growing. For marine
use the preservative must offer no toxic hazard to the crew, must be free
from objectionable odors and must be able to remain in the wood and do
its work in the presence of moisture. No known wood preservative is ideal
for marine use but certain ones have proved effective for specific applications.
There are two general classes of wood preservatives, oil soluble and
water soluble. Both have been used in the marine industry.
- Oil Soluble Preservatives.
- Coal Tar Creosote. One of the most effective of the oil soluble
preservatives is coal tar creosote. This preservative is highly toxic to
wood attacking organisms, is relatively insoluble in water and is easy
to apply. It has a distinctive unpleasant odor, is somewhat of a fire hazard
when freshly applied and causes skin irritation in some individuals. Its
main disadvantage is that it is a hazardous material to the environment
and thus has become unavailable for boat building applications. However,
some older vessels with deadwood, keel, stems and heavy timbers which were
originally treated with creosote, are still in service.
- Copper Naphthanate Solutions. Copper naphthanate solutions
form one of the most used groups of marine wood preservatives. A three
percent solution, equivalent to one half of one percent copper by weight,
provides good protection against decay when properly applied. The protection
afforded against marine borers is slight. Wood treated with copper naphthanate
is a distinctive green color. Much of the "treated wood" which can be purchased
is preserved with copper naphthanate. The paintability, glue bonding ability,
and structural stability of the wood is only slightly affected by the copper
salts. These properties will vary, however, depending upon the oil used
as a solvent. It is important to note that this substance poses a serious
health hazard to humans. Full body protection should be worn during application.
- Pentachlorophenol Solutions. "Penta" solutions have proven
satisfactory for marine use. Field tests have shown that a 5% solution
offers adequate protection against decay when proper application techniques
are used. Little if any protection against marine borers is provided.
Pentachlorophenol does not give wood any distinctive color. In itself,
it affects the characteristics of wood very little. The final effect of
the preservation treatment on physical characteristics depends upon the
petroleum solvent used. Pentachlorophenol solution remains effective for
approximately 2-3 years before it begins to break down.
- Water Soluble Preservatives.
- Water Soluble Preservatives. Copper naphthanate and "penta"
are often combined with water repellents. These repellents aid in stabilizing
the moisture content of the treated wood. This is a material aid in reducing
the chance that decay growth conditions will occur. In order to be effective
these solutions should contain no less than 5% pentachlorophenol or 2%
copper in the form of copper naphthanate.
- Solvents. Almost any petroleum product from mineral spirit
to used engine oil can be used as a vehicle for the preservative depending
upon local conditions. In general, the heavier high viscosity residuum
types offer the best retention. The choice of solvent is usually a compromise
of effectiveness, paintability and initial cost.
- Water Preservatives. Waterborne preservatives include zinc
chloride, tanalith, copper arsenite, chromated zinc arsenate and many others.
Their major applications are those in which the leeching out of the preservative
by moisture is not a problem. In general, these preservatives have not
proven satisfactory for severe marine service. Some preserved wood obtained
for repair use may have been pressure treated with one of these preservatives.
It can give satisfactory service if care is taken to use it in a location
where it is protected from the action of rain and sea water
- Methods Of Treatment.
- Pressure Treatment. In the commercial treating of wood a method
utilizing high pressure is often used. This method requires expensive equipment
and is seldom seen in a boat yard. Nonpressure treatments available to
the boat yard are brushing, cold soaking, and various types of "hot and
cold" bath processes.
- Brush Treatment. The simplest way of applying a preservative
solution is to brush it on. Every crack and check must be flooded with
preservative if the treatment is to be effective. Small pieces such as
butt blocks can be dipped into the preservative. Solutions of pentachlorophenol
or copper naphthanate available commercially, have proved effective when
used in this way.
"Penta" stock solutions are available in what is know as 1:5 and 1:10
strengths, (i.e. the solution must be diluted one part of solution to five
or ten parts of solvent to achieve a "normal" wood preserving solution).
These stock solutions are used without dilution for applications such as
preserving cracks, holes resulting from old fastenings, and coating joints
and hard to get spots. Care must be exercised since wood preservatives
are toxic. When using the brush-on method the entire surface must be thoroughly
coated.
- Soaking. Cold soaking in copper naphthanate or "penta" solutions
for periods of up to 48 hours provides much better retention of the preservative
than does a brushing. An even better method consists of heating the wood
in a hot preservative bath and then transferring it to a cold bath of preservative.
The heating causes the air entrapped in the wood to expand. The sudden
cooling sets up a vacuum which aids preservative penetration.
Preservative solutions or other chemicals which release copper ions
into wood or into the bilgewater should be avoided in vessels containing
ferrous fastenings. Copper ions are more stable than iron, and will spontaneously
plate out on steel or on zinc coatings, replacing equal numbers of iron
or zinc ions, which go into solution (replacement corrosion). While the
amount of direct wastage of iron or zinc from this mechanism is likely
to be minimal, the presence of copper-plated regions on the surface of
the steel fittings cause them to become small, isolated galvanic cells.
The further corrosion of the steel or galvanizing may be significantly
increased by the presence of copper surface inclusions.
Copper naphthanate (Cuprinol), Chromated Copper Arsenate (CCA) and Ammoniacal
Copper Arsenate (ACA) wood preservatives are one common source of copper
ions in the wood or bilgewater. Another source is the addition of chemical
treatments to bilgewater. A traditional solution to the problem of sour
bilges due to generation of hydrogen sulfide gas by bacteria breaking down
spilled diesel fuel is to dissolve copper chloride crystals in the bilgewater.
|
CHAPTER 4: Guide To Inspection
- General
Intelligent inspection of wooden vessel construction requires knowledge
and judgment. Inspection is made to determine that the vessel is safe and
has a reasonable chance of remaining so until the next scheduled inspection.
A good basic knowledge of wood construction and the deficiencies to which
it is susceptible is essential.
- What To Look For
Problems in wooden vessels group themselves into three categories:
- Time
- Decay
- Wood Borers
- Corrosion
- Stress
- Cracks
- Broken members
- Failure of fastenings
- Failure of caulking
- Damage
- Hull damage due to collision, grounding or to normal wear and tear
- Structural Problems
In wooden vessels structural problems develop in nearly new vessels
as well as in older ones. Deterioration, especially that caused by decay
and wood borers, can occur with surprising rapidity. Boats which have been
free of such infestations can become infected with slight changes in service
area or operation. Fastening problems in new wooden vessels can also develop
as a result of several types of corrosion.
Poor selection of wood structural materials or lack of ventilation will
often make themselves known in the first year of a vessel's service life.
That the vessel was sound at its last inspection has less bearing on the
present condition of a wooden vessel than on one of steel.
- Condition Of Vessel For Inspection.
If practicable, inspect the vessel out of the water with the interior
of the hull opened up as much as possible. The bilges and forepeak should
be dry and reasonably clean. Excess tackle, tools and gear which might
interfere with proper inspection should be cleared away. This is not always
possible; however, hard to inspect (and thus hard to maintain) areas should
not be missed
Where the interior of the hull has closely fitted ceiling or paneling,
sufficient access should be provided to allow examination of the interior
at selected locations. This can be accomplished on lighter scantling vessels
by cutting inspection openings in the ceiling which will also aid in providing
ventilation to combat dry rot. On heavy timbered vessels, borings or core
samples may be used to show the condition of hidden structures. Apparent
soundness of the ceiling should not be taken as indicative of soundness
beneath.
In some cases access for frame inspection may be made by removal of
sheer/waterline and/or garboard planks for inspection from the outside.
In any case, visual inspection must be accomplished to ascertain conditions
under ceilings. Full ceiling vessels often lack ventilation between frames
therefore making them a likely place where decay can be found.
Some vessels will be found with poured concrete, ballast ingots or other
interferences which make internal bilge inspection and condition of floor
frames/fastenings and keel bolts difficult to evaluate. Where it is possible
to remove some of the material without damaging the hull or internal structural
members, sufficient access should be made for examination. Careful documentation
of conditions found must be accomplished to avoid unnecessary removal of
internals.
The vessel's underwater body should not be filled, faired or
painted before it is examined. Coatings cover a multitude of defects such
as cracks, bleeding or loose fastenings, discolored wood due to rot, and
borer attack.
- Visual Inspection
An overall examination of the hull of a wooden vessel which has been
in service can give the inspector an idea of the portions where deficiencies
can be expected. Distorted planking, pulled butts, local damage, and unexplained
wetness or weeping are tell tale indications.
Particular attention should be paid to the garboard area, stem, stern,
transom, region under the covering boards, the wind and water area, and
around hull fittings. It is impossible to list each area of trouble in
each type of boat. In general, areas which are hard to maintain, have poor
ventilation or are subject to heavy stresses display the most deficiencies.
- Inspection For Decay And Wood Borers
Serious deterioration of a wooden hull goes on within the wood itself
with little or no outward sign until it is well advanced. In order to spot
decayed wood, which has not progressed to the point where the wood appears
eroded and spongy, sounding with hammer can be of use.
Unsound wood will give a dead or dull sound. Heavy timbers whose interiors
are rotted may give a distinctive drum-like tone where the sound is not
that of good solid wood, the member is suspect. Often, the first indication
of "wet rot" is a distinctive musty odor which permeates the interior spaces
of a closed up vessel. Deteriorated wood will be spongy when probed and
repairs generally require complete renewal of the affected wood.
- Decay. Decay in wood is caused by various fungi which are
living organisms whose growth depends upon suitable temperature (50 degrees
to 90 degrees F), suitable food (wood), moisture, and oxygen. Wood that
is dry will not rot nor will waterlogged wood. In order to provide a condition
suitable for fungus growth, wood must be moist (from 20 to 80% moisture
content). This condition is promoted by poor ventilation. A well designed
vessel should have adequate ventilation of its enclosed spaces. Bilges,
cabins, etc., of vessels in service should be opened periodically to allow
a change of air. Good ventilation of interior structure in wooden hulls
is one of the most effective measures in the prevention of decay.
It should be realized that decay progresses rapidly and that it is more
economical to eliminate small decayed areas early than become involved
in costly major replacements caused by neglected decay.
Moisture meters can be of use particularly in areas where FRP overlays
or paint may hide deteriorated wood. Use of the moisture meter and/or hammer
should be followed up with probing or boring to develop the extent of the
defect. Core sampling can be used to determine depth of deterioration.
It is imperative that indiscriminate probing and boring be avoided.
Holes made by a probe or drill on the exterior are potential entry ways
for wood borers. In the hull interior they allow moisture penetration and
thus aid in starting decay. Probing and boring should be done carefully
and only where there is an indication from non-destructive testing that
the material is unsound, not as a matter of routine.
Holes made by boring should be plugged with dowels or plugs which are
glued in place, not merely driven into the wood. Plugs and dowels should
preferably be treated with wood preservative to prevent future trouble.
Areas which have been probed should be filled with a suitable compound.
When covering boards or other obscuring construction is involved, it is
often difficult to locate deteriorated members by probing. In such cases,
when bolted or screwed fastenings are used, check for tightness of randomly
selected fastenings. If the member is solid, the fastenings thus set up
will take hold at the beginning of the turn. If serious decay is present
the fastening will turn freely and fail to take a bite, indicating soft
and spongy wood.
Decay is most often found in the following locations:
- Internally.
- All areas that are poorly ventilated, i.e. at the stem, transom, and along the sheer.
- In the bilge especially at the turn and along the keel.
- The lower courses of bulkhead planking.
- Areas under refrigerators, freshwater tanks and valves and other areas where fresh water can accumulate.
- In the area of butt blocks and longitudinal members where dirt and debris may have retained fresh water.
- At the heads of frames caused by fresh water leakage through defective covering boards and from condensation.
- Where the futtocks of sawn frames join and at the faying surfaces where the frames abutt the hull planking.
- At the terminal ends of frames, floors, engine foundations, etc. where end grain is present.
- Externally
- In joints where fresh water has penetrated.
- Around deck metallic fastenings and penetrations.
- At covering board joints.
- In mast fastening locations and within natural checks or compression
cracks.
- Under spar hoops, gaff jaws, mast partner deck penetrations, and
any other areas where wood is covered with metal or leather chafing gear.
Under freezing temperature conditions wood structural members with a
high moisture content, particularly in the bilge areas, may appear quite
sound when, in fact, they may be in advanced stages of decay. Periodic
examination of these areas should be conducted before freezing sets in
or after, allowing sufficient time for thawing.
The other principal form of deterioration which goes on within the wood
is wood borer attack.
- Marine Borers. Marine borers are present to a varying degree
in almost all the salt and brackish waters of the world. They attack practically
every species of wood used in boat construction. There is no sure method
of protection from their attack. The two principal methods are to physically
keep the worm away from the wood (sheathing) and to make the wood unattractive
to the worm (toxic substances and coatings). The main types of marine borers
are listed in the following paragraphs.
- Mollusks. (Often called shipworms) There are several species
of Teredo and Bankia in this group. Though they vary in detail, their attack
upon wood follows the same pattern.
They start their lives as tiny free swimmers. Upon finding a suitable
home, even a tiny crack in a sheathed bottom, they attach themselves and
quickly change form. As a pair of cutting shells develop on their heads
they bury themselves in the wood and feed upon it. Their tails or "syphons"
always remain at the entrance to their burrow but, as the worms grow, their
heads eat channels in the wood. The entrance holes always remain small
and hardly noticeable but the interior of the wood becomes honeycombed.
When they are not crowded, some species of shipworm can grow to lengths
exceeding four feet. One species, (Teredo Navalis) can burrow up to 3/4"
per day.
- Martesia. These are wood boring mollusks which resemble small
clams, they enter the wood when they are small and do their damage within.
They do not grow to the length of shipworms but, nevertheless, they can
do considerable damage. Their main area is in the Gulf of Mexico.
When borer attack is just starting it is possible to burn the holes
clean with a torch and then fill them with a suitable compound. If the
attack is extensive, however, the only method acceptable is to replace
the affected wood.
The first principle in reducing the chance of borer attack is to keep
the worm away from the wood. This is accomplished by sheathing or by toxic
paints. If the protective coating is broken borers can enter. To prevent
this, sheathing where fitted, should be unbroken and in good condition
and the bottom paint should be free from scratches, nicks and scrapes before
the vessel is launched.
Wormshoes, rubbing strakes and similar members whose protective coatings
have been broken should be inspected carefully. If they have heavy borer
infestation they should be replaced. Care should be taken to see that the
infestation has not progressed from them to the main part of the hull structure.
Though wormshoes are usually separated from the hull by felt or copper
sheathing, this separation is never 100% effective.
Marine borers die when removed from salt water for any period of time.
A vessel which has been out of the water for a few days and is essentially
dry will probably have no live borers.
- Termites. Classified as a wood boring worm found principally
in tropical areas, the winged variety often infest masts and wood appendages
of large sailing craft, particularly those with solid (grown) spars which
have developed surface checks or compression cracks.
Termites burrow deep into the wood leaving tunnels which fill with water
and promote decay. Hammer testing and use of the moisture meter can often
detect subsurface termite colonies. If borer infestation is suspected under
canvas deck coverings or in areas where wood is covered or sheathed with
metal, leather or composite overlayment, the covering should be removed
to facilitate further examination.
- Corrosion And Cathodic Protection
- General. Most wooden boats relay on metal fastenings for structural
integrity, and those fastenings are subject to corrosion. Because of the
great structural importance of the relatively small mass of metal in the
fastenings, a small amount of corrosion can cause major problems, therefore,
the inspection of fastenings is crucial. Many casualties to wooden
vessels involving structural failures are caused by corrosion of the fastenings.
Underwater metal fittings of wooden vessels (but usually not individual
fastenings) are often protected electrically from corrosion by a process
called cathodic protection. Wood in contact with cathodically protected
fittings is often deteriorated by the chemicals produced by the protection
process.
In inspecting fastenings, several fundamental facts must be kept in
mind. First, most corrosion of metal fastenings in wood proceeds from the
surface to the interior at a fairly constant rate which can be predicted
quite accurately by experience if the metal, the temperature, and the nature
of the surrounding wood are known.
Second, when a fastening is loaded in shear, like many bolts are, its
strength is related to its cross-sectional area. Because the area varies
as a function of the diameter, a fastening which is corroded to one-half
its original diameter retains only one-quarter of its original shear strength.
Third, fastenings which are loaded in withdrawal tensile rather than in
shear and which rely on threads or friction for their holding power (such
as screws, lags, nails, and drifts) may lose their effectiveness completely
when only a small fraction of their original diameter is lost to corrosion.
The metals used for hull fastenings in wood boats are steel (often coated
with zinc, or galvanized, to increase corrosion resistance), bronzes (alloys
of copper with metals other than zinc), copper, nickel-copper (Monel),
stainless steels (alloys of iron with chromium and nickel), and occasionally
aluminum.
Fastenings can suffer from four principal classes of corrosion - simple
electrochemical corrosion, galvanic corrosion, replacement corrosion, and
stray current corrosion. Stainless steel fastenings are also susceptible
to a form of corrosion called crevice corrosion.
- Simple Electrochemical Corrosion. Simple electrochemical corrosion
is the normal way in which metals combine with oxygen to reach their more
stable form as metallic oxides. In sea water, dissolved oxygen and chloride
ions (from salt) are the principal instigators. Simple electrochemical
corrosion rates are quite predictable for most metals. The process involves
two different types of reactions which take place at distinct locations
on the metal-water interface. An interface of metal and wet wood is the
same as an interface of metal and water. At the anodes, the free electrons
are absorbed in a reaction that consumes the oxygen which is dissolved
in the surrounding water or in the water absorbed by the surrounding wood.
In open water, the sites of the anodes and the cathodes may be microscopically
small and intermixed - the metal may appear to corrode more or less uniformly.
For a fastening buried in wood however, the area exposed to oxygen is often
limited. The heads of fastenings tend to support oxygen consuming cathode
reactions and are thus protected from wastage, while the deeper-buried
shanks are where the anode reaction, and the physical wastage takes place.
For this reason, exposed or shallow buried heads are often the least-corroded
parts of hull fastenings. This is why hull fasten-ings in wooden boats
cannot usually be adequately assessed without withdrawing them.
- Galvanic Corrosion. Different metals have different levels
of chemical stability in water, causing them to have different tendencies.
These differences in stability are measurable as different electrical potentials,
or voltages. These potentials are tabulated in the "Galvanic Series". (See
Table 4-1 on page 4-17 at the end of this chapter.)
When two metals which have different potentials and which are immersed
in the same body of water or wet wood are brought into direct physical
contact or connected together with a metallic conductor, electric current
flows between them, altering their corrosion rates from those which existed
in the isolated state. The corrosion rate of the less stable metal (which
had the more negative potential) increases, while that of the more stable
metal (which had the more positive potential before the connection was
made) decreases by an equal amount. The less stable metal is now said to
be undergoing galvanic corrosion, an accelerated form of electrochemical
corrosion, while the more stable metal is now receiving cathodic protection,
with the other metal serving as a sacrificial anode. In order for
galvanic corrosion to occur, the two different metals (dissimilar metals)
must be connected electrically (by contact or by a direct metallic link,
and they must be immersed in the same body of liquid or wet wood
(either of which is called an electrolyte.) Two or more metals,
electrically connected in a common body of electrolyte are called a galvanic
cell.
Galvanized steel (steel coated with zinc) is an example of an intentional
galvanic cell - the zinc acts as a sacrificial anode for the steel in the
case of a small penetration of the coating. In addition, despite being
less stable than steel galvanically, the zinc is considerably more corrosion
resistant than the steel when it's not acting as a sacrificial anode for
a large area of steel. There's a lesson here - the Galvanic Series should
be used only to predict the nature of galvanic interactions between metals
- not to predict their relative corrosion rates. For example, aluminum,
which is also less stable galvanically than steel, also has a lower corrosion
rate than steel if it is galvanically isolated.
The ratio of the exposed areas of the two metals which make up a galvanic
cell is an important factor in what happens to the metals. In the case
of a cell made up of a small piece of copper (a stable metal) and a large
piece of steel (an unstable metal) the corrosion rate of the steel would
be only slightly increased by the connection, while the copper might be
completely protected from corrosion. If the area ratio were reversed (a
large area of copper to a small area of steel), the corrosion rate of the
steel (already high) would be greatly increased, while the corrosion rate
of the copper (already low) would be decreased only slightly. In the first
case, if the copper is in contact with wood, the cathodic protection it
receives comes at a price. The increased conversion of oxygen to hydroxyl
ions which accompanies the protection will cause deterioration of surrounding
wood. Regardless of the area ratio, painting the copper will decrease not
only the adverse affect on the wood but the detrimental galvanic effect
on the steel as well. Painting the steel may decrease the total galvanic
effect, but will concentrate what there is at small imperfections in the
paint film, causing severe localized pitting which could be disastrous
to thin material found in fuel or water tanks.
In general, galvanic connections should be avoided in wooden vessels,
unless they are made for a very good reason (like cathodic protection)
and the consequences (like wood damage around protected metals) have been
fully considered and mitigated (such as by painting the protected metals).
- Replacement Corrosion. If a metal fitting or fastening is
placed in an electrolyte which contains ions of a more stable metal, typically
a galvanized steel or stainless steel fitting in pressure treated wood
containing copper salts, the copper ions coming into contact with the fastening
will "plate out" as a solid copper film on the surface of the fastening,
with equal numbers of zinc or iron atoms ionizing, or going into solution.
The replacement reaction itself is a one-for one process, and if the stable
copper ions are depleted from the electrolyte, the replacement stops. However,
the thin surface coating of copper on the steel fastening results in a
galvanic cell, which accelerates the fastening corrosion rate.
The three principal causes of replacement corrosion to wooden boat fastenings
are, in descending order of frequency and the likelihood of significant
damage:
- Copper wood preservative salts. These include copper napthenate
from green Cuprinol, which is usually brushed on, and chromated copper
arsenate (CCA) and ammoniacal copper arsenate (ACA), which are used in
pressure treating softwood lumber.
- Copper salts dissolved in bilgewater. Copper chloride is occasionally
used as a cure for the sour bilges (hydrogen sulfide) caused by bacterial
decomposition of spilled diesel and lube oils
- Nearby copper-alloy fittings or fastenings. After a long period
of time, wood around corroding copper alloy fittings or fastenings becomes
saturated with copper ions. Any steel, galvanized steel, or stainless steel
fastening driven into that area can suffer some replacement and consequent
accelerated corrosion from galvanic effects. The effect only extends for
a few inches at most around the copper alloy fitting, however, it's prudent
not to use galvanized or stainless steel fastenings for refastening boats
previously fastened with copper alloy fastenings, whether or not the original
fastenings are removed.
- Stray-Current Corrosion. Stray-current corrosion is a magnified
version of the galvanic corrosion suffered by the more negative metal in
a galvanic cell. In the galvanic cell, the metal is connected to another,
more positive, metal, which draws electrons from it and causes the anode
reaction rate of the negative metal to increase to supply those extra electrons.
In stray current corrosion, a metal comes into contact with the positive
side of a DC electrical system, the negative side of which is grounded
to the seawater. The effect is the same, but since the driving voltage
is now 12 volts or more, instead of the few tenths of a volt found in galvanic
cells, the resulting corrosion rate can be catastrophic.
Typical sources of stray current are submersible bilge pumps, bilge
pump float switches, and electrical wiring connections in the bilge area
which might become submerged in the bilgewater. Fittings can be subject
to stray current corrosion by coming into direct contact with a chafed
positive (hot) DC wire or, more commonly, indirectly by a DC fault current
to the bilgewater. Fittings which pass through the hull and are in contact
with the outside seawater are most susceptible. In the case of an indirect
stray current path through the bilgewater, fittings which are in direct
contact with both the bilgewater and the outside seawater are most susceptible.
Stray current corrosion generally causes deep pitting of the objects
it affects, and is almost always highly localized to within a few feet
of the source of the stray current. In addition, the effected metal parts
will appear to be unusually bright or shiny. A DC stray current may cause
complete disintegration of a substantial fitting within a few days or even
less. The magnitude of the DC stray current may be a few amps in severe
cases, but usually not high enough to cause overcurrent protective devices
to trip. Stray current can discharge batteries quickly, but in boats with
shore-powered battery chargers, a substantial DC stray current may continue
to flow indefinitely.
- Bonding Systems
In order to protect against the potentially disastrous effects of DC
stray currents, many non-metallic hulled boats have a network of wires
which connect hull fittings which are at risk of stray current corrosion
with the negative, or ground side of the battery, usually via the engine
block. This network is called a bonding system. In the case
of a direct fault to a bonded fitting, sufficient current will probably
flow to trip the overcurrent protective device, stopping the stray current.
In the case of an indirect stray current (the wire in the bilgewater),
it is unlikely that a sufficient current will flow to trip the circuit,
even with a bonding system. In this case the bonding system and the stray
current will share the fault current. An indirect fault, however, is often
limited by the corrosion of the exposed metal at the source of the fault,
which eventually stifles the current flow.
FIGURE D-1: Typical Bonding System
The bonding system ties the thru-hulls electrically to the negative terminal of the battery. When a hot wire touches the thru-hull, the electrical path presented by the bonding wire has so much less resistance than the electrolytic path of the stray-current cell that a high current flows in the bonding system. This should cause a fuse to blow or a circuit breaker to trip, interrupting the stray current flow. Even if this does not happen, however, the amount of current that flows in the stray-current circuit before the battery becomes discharged, and the resulting corrosion of the affected fitting, are greatly diminished.
FIGURE D-2: Cathodic Protection Distributed By The Bonding System
When there are no stray currents, the shaft zinc may protect not
only the shaft and prop, but also any fitting connected to the bonding
system. This often results in alkali damage to the wood around those fittings.
On wooden boats, bonding systems can cause unexpected problems. First,
by connecting together a number of underwater fittings and fastenings,
the bonding system may provide the metallic links which turn otherwise
isolated dissimilar metals into a galvanic cell. Second, the bonding system
often inadvertently supplies unneeded or unwanted cathodic protection to
objects connected to the bonding system by connecting those objects to
the propeller shaft's sacrificial zinc anode. This cathodic protection
of underwater metal hull fittings often causes damaging alkali delignification
of the surrounding wood.
The fittings on a wood boat which are most susceptible to stray-current
corrosion are those in the bilgewater or those which are in close physical
proximity to wires, while those most susceptible to alkali delignification
are those above the bilgewater level, but below the waterline. In this
area the wood is wet enough to be a fairly good electrolyte, but there
is little flushing action to remove accumulations of cathode reaction products.
The hydroxyl ions produced by the cathode reaction on cathodically protected
metals can concentrate in these locations, damaging the wood and often
producing visible deposits of sodium hydroxide (lye) crystals which appear
as a white mound of salt around fastenings.
Bonded vessels should be checked with a electrical potentiometer by
a qualified electrical specialist for electrical leakage to ensure that
the boat is not over zinced. This is especially true after a vessel has
been found to have extensive wood repair due to alkali deterioration. Repairing
the wood, without determining the cause (via a corrosion survey) is a poor
practice as it would only be treating the symptom.
- Painting Galvanic Cells
Care must be taken in painting metals which are connected galvanically
to other metals. In the case of steel and copper-alloy fittings, it would
seem to make sense to worry more about the coating of the steel, since
it is more prone to corrosion than the copper alloy. If however, those
fittings are connected together, forming a galvanic cell, painting the
steel but not the copper may result in a tremendously unfavorable area
ratio for a few spots on the steel that are inadvertently not coated well.
When painting galvanic cells, one should always try to make the area ratio
more favorable to the susceptible metal. The answer is to paint both metals,
and to pay particular attention to reducing the exposed area of the cathode
of the cell (the copper).
- Crevice Corrosion
Stainless steels are subject to a particular type of corrosion called
crevice corrosion, which is a severe form of pitting. Crevice corrosion
can destroy a fastening in a few years while only damaging a small fraction
of the total mass of the fastening. The austenitic stainless steels (including
the most commonly encountered types, 304 and 316) derive their corrosion
resistance from a surface oxide film which is self-repairing in air or
in the presence of oxygen dissolved in an electrolyte. In stagnant areas
like wet wood or underneath marine growth or paint, however, oxygen can
be depleted by cathodic activity, allowing the ever-present chloride ions
to destroy the film in small areas, which then undergo unpredictable and
exceedingly rapid corrosion. Unfortunately, wet wood is a nearly perfect
environment for crevice corrosion. Stainless steel must be used with great
caution as a fastening material for wooden boats, and inspectors should
be suspicious of all stainless steel fastenings, especially wood screws,
used on boats in saltwater service. Type 316 contains more nickel and chromium
than type 304, and it also contains molybdenum, which inhibits crevice
corrosion to a certain extent, but it is not completely immune. Barbed
or "ring" nails of type 316 are available, but wood screws of type 316
are generally not available.
- Inspection of Fastenings
A boat is no better than its fastenings. The most common type of fastenings
found on wooden boats are screws, however, certain types of construction
utilize nails, bolts or rivets. Most hull fastenings are concealed from
view, being countersunk and covered; therefore their inspection is difficult.
Regardless of the type of fastenings involved, inspection to ascertain
condition is necessary in most plank on frame boats.
For purposes of uniformity careful fastening inspection must be carried
out on all vessels. Removal of fastenings should be conducted as follows:
- For Cause - Saltwater And Freshwater Service. Remove fastenings
whenever inspection reveals the probability of defects such as when a plank
or planks are "proud" and have moved away from the frames or indications
of loose bungs, rust bleeding from fastening holes etc., are noted.
Particular attention should be given to exposed hull fittings and through
bolts accessible inside the hull, such as keel bolts, chine bolts, and
double frame, clamp, and floor timber bolts. These are as important to
the total hull structure as plank fastenings. They should be sounded with
a hammer or wrench tightened and, if suspect, some should be pulled for
inspection. Often a bolt will be completely wasted away in the middle,
at the faying surface of the joint, and will break and come out when pried
up. This is caused by moisture accumulation which, besides wasting the
fastenings, forms an excellent place for wood decay to start.
- Periodic. Inspection of fastenings can prevent planking/frame
failure. Random sampling of fasteners should be part of a regular maintenance
program for continuously monitoring the structural condition of the vessel.
Therefore for vessels designed and built to Subchapter "T" Inspection Standards,
random sampling of fastenings should begin at the 10th year of age and
every 5th year thereafter in salt water service and 20th year of
age and every 10th year thereafter in fresh water service.
For existing vessels not originally built to Subchapter "T" Inspection
Standards but certificated later in life, random sampling should begin
at the 5th year of age and every 5th year thereafter in salt water service,
and 10th year of age and every 10th year thereafter in fresh water service.
Scope of Periodic Random Sampling Of Fastenings.
- Remove a minimum of eight fastenings per side below the waterline.
- Concentrate sampling in the following areas:
- Garboard seams
- Stem joints
- Plank ends in areas of bent planks
- Shaft log(s)
- Under engine beds where vibration is maximum
- In vessels of cross plank (CHESAPEAKE BAY DEADRISE) construction, specificallyinspect
fastenings at the keel and chine joints, at transom attachments, and over
the propeller(s).
It is extremely important that the type, material, and location of the
fastenings removed, along with a description of their condition be accurately
documented. This includes areas of the vessel which have undergone refastening
as well. Use of a camera is invaluable in recording areas of interest during
inspections.
Composite, cold molded and laminar built-up wooden hulls often depend
on adhesives and resins for fastening purposes. Inspection of these type
vessels requires common sense and good judgement to identify the method
of construction used and thereby determine the extent of inspection required.
Generally, these vessels do not require periodic random sampling of fastenings
by removal except for cause.
- Inspection Of Caulking
The art of caulking is an ancient one which requires experience and
a certain "touch". A good caulker makes his work look easy but it is a
skill which takes much experience to develop.
Caulking materials are subject to deterioration. It is advisable to
search the seams in any doubtful areas and re-caulk. Caulking should be
uniform and well "horsed" home. This can be checked with a probe or knife.
Care should be taken that the caulking has not been driven clear through
the seam. Over caulking is as bad as under caulking.
Extensive trouble with caulking may be indicative of structural problems,
which often includes broken or deteriorated fastenings and/or frames. If
a hull "works" excessively, caulking may be squeezed out. In such cases,
the hull structure will have to be made sound before caulking will hold.
In old hulls, where the seams have become enlarged from repeated re-caulking,
copper or lead strips may have been nailed over the seams to act as caulking
retainers. These are a temporary remedy and are an indication of poor general
condition of the vessel. It is advisable that such strips be removed and
the seams inspected for excessive width, poor caulking and decay. In some
cases, wide seams can be repaired by the use of thin wedge shaped splines
driven into the wide seams and bedded in marine adhesive. This procedure
requires excellent workmanship and should be pursued with caution. In most
cases where garboard seams have widened beyond caulking limits, refastening
of the keel, frames and renewal of the garboard planks may be the only
acceptable methods of repair.
- Inspection of Fittings
Rudder and propeller struts and fastenings should be examined carefully.
If suspect, random removal of fastenings for inspection should be accomplished.
The steering arrangement should be inspected from the steering wheel to
the heel pintle. Wear in the carrier bearing and excessive
clearances elsewhere should be corrected. Tiller lines should be in
good condition with shackles moused and bolts cottered.
The shaft log glands should be in good condition and the deadwood should
be sound. This is often neglected and is a potential cause of leakage.
Propeller shaft cracks are sometimes found at the keyway. A careful
examination here is essential. Magnetic particle testing is usually not
available in a small boatyard so the inspector must depend on visually
locating surface cracks. Dye penetrant testing is relatively inexpensive
and can be useful when deemed necessary.
Some older boats are still fitted with AM radio hull grounding plates.
These are usually copper sheet metal of several square feet in area attached
to the underwater hull. Use of AM radio equipment is no longer found on
small passenger vessels. To minimize the mixing of metals below the waterline,
the old ground plates should be removed and the hull inspected, repaired
as found necessary, and recoated.
Inspection of hardware fastenings should also be accomplished including
cleats, bitts, chain plates, etc., where threaded fasteners hold load bearing
as well as structural parts.
- Hull Damage
Most hull damage can be seen readily. Cracked and broken members are
obvious faults.
Likely locations for cracks or breaks are in areas of high stress or
where the structure undergoes a sudden change in shape. The turn of the
bilge is the prime location for breaks of this type. The harder the turn,
the more chance that damage has been done. Bent frames are particularly
susceptible to breakage under bilge stringers, especially when the stringers
are substantially thicker than the planking or when there are large diameter
fastenings in the stringers.
Wood hulls are more prone to secondary damage remote from the site of
collision or grounding than are steel hulls. Damage may consist of sprung
butts, pulled fastenings, sprung or cracked frames and misalignment of
the structure. In inspecting any damaged wooden hull, the entire vessel
should be checked.
- Deficiencies
When deficiencies are encountered an evaluation must be made of their
extent and their effect on seaworthiness. The following factors must be
weighed in making this determination:
- Is the defect progressive and, if so, how can its progress be arrested?
- How long will it be before the area in question is next inspected?
- Is the work contemplated necessary to restore seaworthiness or to
prevent the vessel from becoming unseaworthy, or is it a maintenance measure
to prolong the life of the vessel?
Specific requirements detailing the nature and extent of required repairs
should be written. However, with wooden vessels the general rule "renew
as original" while applicable, is not always practical nor necessarily
the best way to effect repairs. Most accepted methods of marine repair
may be used as long as the vessel's strength is not adversely affected.
Wood is a natural material, its quality cannot be controlled as closely
as with a manmade product such as steel. Consequently the inspector should
check the material to be used in repair work. Special attention must be
given to the type of wood proposed for each purpose and for any inherent
defects.
Many deficiencies, particularly surface defects or scars caused by chafing,
freezing and other forms of exterior deterioration are not as serious as
they may first appear. Do not be hasty in requiring the correction of minor
defects of this nature in otherwise sound seasoned planking.
Requirements For Adequate Repairs Are:
- Use of good material comparable in properties to that replaced.
- Repairs extensive enough to ensure that the hull is at as strong
as the original.
- Construction details and fastenings at least equivalent in strength
and in quality to those replaced.
- Good workmanship.
TABLE 4-1: The Galvanic Series Of Metals In Seawater
Voltages are those measured against a silver/silver chloride (Ag/AgC1) reference electrode.
Noble or Cathodic Metals |
Designation |
Voltage
Potential |
Graphite
|
C |
+ 0.27 V |
Platinum |
Pt |
+ 0.24 V |
Titanium |
Ti |
+ 0.02 V |
Incoloy |
825 |
+ 0.02 V |
Ag/AgCl Reference Electrode |
|
0.00 V |
316 Stainless Steel (passive) |
|
- 0.03 V |
Monel 70 %, 30 % cu/ |
400,K-500 |
- 0.06 V |
304 Stainless Steel (passive) |
|
- 0.06 V |
Silver |
Ag |
- 0.10 V |
Nickel Ni |
|
- 0.13 V |
Silver Brazing Alloys |
|
- 0.13 V |
Inconel 600(passive) |
|
- 0.13 V |
Ni-Al Bronze |
C63x,C954-8 |
- 0.16 V |
Cu-Ni (70-30) |
C715-9, C964 |
- 0.18 V |
Lead |
Pb |
- 0.20 V |
Cu-Ni (80-20 and 90-10) |
C710, C706 |
- 0.22 V |
"Nickel Silver" |
C745-70, C97x |
- 0.25 V |
Phosphor (Tin) Bronze |
C524, C903-5, C92x |
- 0.26 V |
Silicon Bronze |
C655, C872 |
- 0.25 V |
Manganese Bronze |
C675, C86x |
- 0.29 V |
Admiralty Brass |
C443-5 |
- 0.30 V |
Aluminum Brass |
C687-90 |
- 0.30 V |
Lead-Tin solder |
|
- 0.30 V |
Copper |
C10x, Cllx, C12x |
- 0.31 V |
Tin |
Sn |
- 0.31 V |
Naval Brass/"Bronze"(Tobin Bronze) |
C464 |
- 0.33 V |
Yellow and Red Brass |
C23x-27x, C83x-85x |
- 0.33 V |
Aluminum Bronze |
C606-24, C952-3 |
- 0.34 V |
Stainless Steel 316 (active) |
|
- 0.39 V |
Stainless Steel 304 (active) |
|
- 0.49 V |
|
Low Alloy Steels |
- 0.58 V |
Steel, Cast Iron |
|
- 0.63 V |
Aluminum |
Alloys |
- 0.87+/-.10 V |
Zinc |
Zn |
- 1.00 V |
Magnesium |
Mg |
- 1.60 V |
Notes on the Use of the Galvanic Series Table
All values are for sea water at room temperature.
Average variability is +/-.04 Volts for alloys containing nickel or
iron, +/-.02 V for copper alloys without nickel.
Sign of corrosion potential assumes that the "COMMON" or negative (Black)
terminal of the voltmeter is connected to the reference electrode and the
"VOLTS-OHMS", or positive (Red) terminal is connected to the metal to be
measured. The reference electrode must be immersed in the same body of
electrolyte as the metal being measured, preferably in close proximity.
To use Zinc as a reference electrode instead of Ag/AgCl add +1.00 volts
to the potentials listed in this table. For example, low alloy steel should
measure -.58V +1.00 V, or +0.42V against zinc, and magnesium should
measure - 1.60V + 1.00V,or -0.60V against zinc. Extremely accurate measurements
should not be attempted with zinc as a reference, since it isn't as stable
as the Ag/AgCl electrode.
Metals are receiving cathodic protection when their measured potentials
are more negative than their natural corrosion potentials listed here,
and are generally completely protected from corrosion when their potentials
measure .20V to .25V more negative than the values listed in this chart.
Metals are receiving stray current or are the anode of a galvanic system
(these are equivalent situations) when their potentials measure more positive
than the values listed in this chart. Metals in this situation are generally
suffering accelerated corrosion.
Copper alloy designations: Alloys numbered C100 to C799 are wrought
alloys, those numbered C800 to C999 are casting alloys. "x" indicates a
range of alloys sharing the preceding digits.
|
CHAPTER 4: Guide To Inspection
- General
Intelligent inspection of wooden vessel construction requires knowledge
and judgment. Inspection is made to determine that the vessel is safe and
has a reasonable chance of remaining so until the next scheduled inspection.
A good basic knowledge of wood construction and the deficiencies to which
it is susceptible is essential.
- What To Look For
Problems in wooden vessels group themselves into three categories:
- Time
- Decay
- Wood Borers
- Corrosion
- Stress
- Cracks
- Broken members
- Failure of fastenings
- Failure of caulking
- Damage
- Hull damage due to collision, grounding or to normal wear and tear
- Structural Problems
In wooden vessels structural problems develop in nearly new vessels
as well as in older ones. Deterioration, especially that caused by decay
and wood borers, can occur with surprising rapidity. Boats which have been
free of such infestations can become infected with slight changes in service
area or operation. Fastening problems in new wooden vessels can also develop
as a result of several types of corrosion.
Poor selection of wood structural materials or lack of ventilation will
often make themselves known in the first year of a vessel's service life.
That the vessel was sound at its last inspection has less bearing on the
present condition of a wooden vessel than on one of steel.
- Condition Of Vessel For Inspection.
If practicable, inspect the vessel out of the water with the interior
of the hull opened up as much as possible. The bilges and forepeak should
be dry and reasonably clean. Excess tackle, tools and gear which might
interfere with proper inspection should be cleared away. This is not always
possible; however, hard to inspect (and thus hard to maintain) areas should
not be missed
Where the interior of the hull has closely fitted ceiling or paneling,
sufficient access should be provided to allow examination of the interior
at selected locations. This can be accomplished on lighter scantling vessels
by cutting inspection openings in the ceiling which will also aid in providing
ventilation to combat dry rot. On heavy timbered vessels, borings or core
samples may be used to show the condition of hidden structures. Apparent
soundness of the ceiling should not be taken as indicative of soundness
beneath.
In some cases access for frame inspection may be made by removal of
sheer/waterline and/or garboard planks for inspection from the outside.
In any case, visual inspection must be accomplished to ascertain conditions
under ceilings. Full ceiling vessels often lack ventilation between frames
therefore making them a likely place where decay can be found.
Some vessels will be found with poured concrete, ballast ingots or other
interferences which make internal bilge inspection and condition of floor
frames/fastenings and keel bolts difficult to evaluate. Where it is possible
to remove some of the material without damaging the hull or internal structural
members, sufficient access should be made for examination. Careful documentation
of conditions found must be accomplished to avoid unnecessary removal of
internals.
The vessel's underwater body should not be filled, faired or
painted before it is examined. Coatings cover a multitude of defects such
as cracks, bleeding or loose fastenings, discolored wood due to rot, and
borer attack.
- Visual Inspection
An overall examination of the hull of a wooden vessel which has been
in service can give the inspector an idea of the portions where deficiencies
can be expected. Distorted planking, pulled butts, local damage, and unexplained
wetness or weeping are tell tale indications.
Particular attention should be paid to the garboard area, stem, stern,
transom, region under the covering boards, the wind and water area, and
around hull fittings. It is impossible to list each area of trouble in
each type of boat. In general, areas which are hard to maintain, have poor
ventilation or are subject to heavy stresses display the most deficiencies.
- Inspection For Decay And Wood Borers
Serious deterioration of a wooden hull goes on within the wood itself
with little or no outward sign until it is well advanced. In order to spot
decayed wood, which has not progressed to the point where the wood appears
eroded and spongy, sounding with hammer can be of use.
Unsound wood will give a dead or dull sound. Heavy timbers whose interiors
are rotted may give a distinctive drum-like tone where the sound is not
that of good solid wood, the member is suspect. Often, the first indication
of "wet rot" is a distinctive musty odor which permeates the interior spaces
of a closed up vessel. Deteriorated wood will be spongy when probed and
repairs generally require complete renewal of the affected wood.
- Decay. Decay in wood is caused by various fungi which are
living organisms whose growth depends upon suitable temperature (50 degrees
to 90 degrees F), suitable food (wood), moisture, and oxygen. Wood that
is dry will not rot nor will waterlogged wood. In order to provide a condition
suitable for fungus growth, wood must be moist (from 20 to 80% moisture
content). This condition is promoted by poor ventilation. A well designed
vessel should have adequate ventilation of its enclosed spaces. Bilges,
cabins, etc., of vessels in service should be opened periodically to allow
a change of air. Good ventilation of interior structure in wooden hulls
is one of the most effective measures in the prevention of decay.
It should be realized that decay progresses rapidly and that it is more
economical to eliminate small decayed areas early than become involved
in costly major replacements caused by neglected decay.
Moisture meters can be of use particularly in areas where FRP overlays
or paint may hide deteriorated wood. Use of the moisture meter and/or hammer
should be followed up with probing or boring to develop the extent of the
defect. Core sampling can be used to determine depth of deterioration.
It is imperative that indiscriminate probing and boring be avoided.
Holes made by a probe or drill on the exterior are potential entry ways
for wood borers. In the hull interior they allow moisture penetration and
thus aid in starting decay. Probing and boring should be done carefully
and only where there is an indication from non-destructive testing that
the material is unsound, not as a matter of routine.
Holes made by boring should be plugged with dowels or plugs which are
glued in place, not merely driven into the wood. Plugs and dowels should
preferably be treated with wood preservative to prevent future trouble.
Areas which have been probed should be filled with a suitable compound.
When covering boards or other obscuring construction is involved, it is
often difficult to locate deteriorated members by probing. In such cases,
when bolted or screwed fastenings are used, check for tightness of randomly
selected fastenings. If the member is solid, the fastenings thus set up
will take hold at the beginning of the turn. If serious decay is present
the fastening will turn freely and fail to take a bite, indicating soft
and spongy wood.
Decay is most often found in the following locations:
- Internally.
- All areas that are poorly ventilated, i.e. at the stem, transom, and along the sheer.
- In the bilge especially at the turn and along the keel.
- The lower courses of bulkhead planking.
- Areas under refrigerators, freshwater tanks and valves and other areas where fresh water can accumulate.
- In the area of butt blocks and longitudinal members where dirt and debris may have retained fresh water.
- At the heads of frames caused by fresh water leakage through defective covering boards and from condensation.
- Where the futtocks of sawn frames join and at the faying surfaces where the frames abutt the hull planking.
- At the terminal ends of frames, floors, engine foundations, etc. where end grain is present.
- Externally
- In joints where fresh water has penetrated.
- Around deck metallic fastenings and penetrations.
- At covering board joints.
- In mast fastening locations and within natural checks or compression
cracks.
- Under spar hoops, gaff jaws, mast partner deck penetrations, and
any other areas where wood is covered with metal or leather chafing gear.
Under freezing temperature conditions wood structural members with a
high moisture content, particularly in the bilge areas, may appear quite
sound when, in fact, they may be in advanced stages of decay. Periodic
examination of these areas should be conducted before freezing sets in
or after, allowing sufficient time for thawing.
The other principal form of deterioration which goes on within the wood
is wood borer attack.
- Marine Borers. Marine borers are present to a varying degree
in almost all the salt and brackish waters of the world. They attack practically
every species of wood used in boat construction. There is no sure method
of protection from their attack. The two principal methods are to physically
keep the worm away from the wood (sheathing) and to make the wood unattractive
to the worm (toxic substances and coatings). The main types of marine borers
are listed in the following paragraphs.
- Mollusks. (Often called shipworms) There are several species
of Teredo and Bankia in this group. Though they vary in detail, their attack
upon wood follows the same pattern.
They start their lives as tiny free swimmers. Upon finding a suitable
home, even a tiny crack in a sheathed bottom, they attach themselves and
quickly change form. As a pair of cutting shells develop on their heads
they bury themselves in the wood and feed upon it. Their tails or "syphons"
always remain at the entrance to their burrow but, as the worms grow, their
heads eat channels in the wood. The entrance holes always remain small
and hardly noticeable but the interior of the wood becomes honeycombed.
When they are not crowded, some species of shipworm can grow to lengths
exceeding four feet. One species, (Teredo Navalis) can burrow up to 3/4"
per day.
- Martesia. These are wood boring mollusks which resemble small
clams, they enter the wood when they are small and do their damage within.
They do not grow to the length of shipworms but, nevertheless, they can
do considerable damage. Their main area is in the Gulf of Mexico.
When borer attack is just starting it is possible to burn the holes
clean with a torch and then fill them with a suitable compound. If the
attack is extensive, however, the only method acceptable is to replace
the affected wood.
The first principle in reducing the chance of borer attack is to keep
the worm away from the wood. This is accomplished by sheathing or by toxic
paints. If the protective coating is broken borers can enter. To prevent
this, sheathing where fitted, should be unbroken and in good condition
and the bottom paint should be free from scratches, nicks and scrapes before
the vessel is launched.
Wormshoes, rubbing strakes and similar members whose protective coatings
have been broken should be inspected carefully. If they have heavy borer
infestation they should be replaced. Care should be taken to see that the
infestation has not progressed from them to the main part of the hull structure.
Though wormshoes are usually separated from the hull by felt or copper
sheathing, this separation is never 100% effective.
Marine borers die when removed from salt water for any period of time.
A vessel which has been out of the water for a few days and is essentially
dry will probably have no live borers.
- Termites. Classified as a wood boring worm found principally
in tropical areas, the winged variety often infest masts and wood appendages
of large sailing craft, particularly those with solid (grown) spars which
have developed surface checks or compression cracks.
Termites burrow deep into the wood leaving tunnels which fill with water
and promote decay. Hammer testing and use of the moisture meter can often
detect subsurface termite colonies. If borer infestation is suspected under
canvas deck coverings or in areas where wood is covered or sheathed with
metal, leather or composite overlayment, the covering should be removed
to facilitate further examination.
- Corrosion And Cathodic Protection
- General. Most wooden boats relay on metal fastenings for structural
integrity, and those fastenings are subject to corrosion. Because of the
great structural importance of the relatively small mass of metal in the
fastenings, a small amount of corrosion can cause major problems, therefore,
the inspection of fastenings is crucial. Many casualties to wooden
vessels involving structural failures are caused by corrosion of the fastenings.
Underwater metal fittings of wooden vessels (but usually not individual
fastenings) are often protected electrically from corrosion by a process
called cathodic protection. Wood in contact with cathodically protected
fittings is often deteriorated by the chemicals produced by the protection
process.
In inspecting fastenings, several fundamental facts must be kept in
mind. First, most corrosion of metal fastenings in wood proceeds from the
surface to the interior at a fairly constant rate which can be predicted
quite accurately by experience if the metal, the temperature, and the nature
of the surrounding wood are known.
Second, when a fastening is loaded in shear, like many bolts are, its
strength is related to its cross-sectional area. Because the area varies
as a function of the diameter, a fastening which is corroded to one-half
its original diameter retains only one-quarter of its original shear strength.
Third, fastenings which are loaded in withdrawal tensile rather than in
shear and which rely on threads or friction for their holding power (such
as screws, lags, nails, and drifts) may lose their effectiveness completely
when only a small fraction of their original diameter is lost to corrosion.
The metals used for hull fastenings in wood boats are steel (often coated
with zinc, or galvanized, to increase corrosion resistance), bronzes (alloys
of copper with metals other than zinc), copper, nickel-copper (Monel),
stainless steels (alloys of iron with chromium and nickel), and occasionally
aluminum.
Fastenings can suffer from four principal classes of corrosion - simple
electrochemical corrosion, galvanic corrosion, replacement corrosion, and
stray current corrosion. Stainless steel fastenings are also susceptible
to a form of corrosion called crevice corrosion.
- Simple Electrochemical Corrosion. Simple electrochemical corrosion
is the normal way in which metals combine with oxygen to reach their more
stable form as metallic oxides. In sea water, dissolved oxygen and chloride
ions (from salt) are the principal instigators. Simple electrochemical
corrosion rates are quite predictable for most metals. The process involves
two different types of reactions which take place at distinct locations
on the metal-water interface. An interface of metal and wet wood is the
same as an interface of metal and water. At the anodes, the free electrons
are absorbed in a reaction that consumes the oxygen which is dissolved
in the surrounding water or in the water absorbed by the surrounding wood.
In open water, the sites of the anodes and the cathodes may be microscopically
small and intermixed - the metal may appear to corrode more or less uniformly.
For a fastening buried in wood however, the area exposed to oxygen is often
limited. The heads of fastenings tend to support oxygen consuming cathode
reactions and are thus protected from wastage, while the deeper-buried
shanks are where the anode reaction, and the physical wastage takes place.
For this reason, exposed or shallow buried heads are often the least-corroded
parts of hull fastenings. This is why hull fasten-ings in wooden boats
cannot usually be adequately assessed without withdrawing them.
- Galvanic Corrosion. Different metals have different levels
of chemical stability in water, causing them to have different tendencies.
These differences in stability are measurable as different electrical potentials,
or voltages. These potentials are tabulated in the "Galvanic Series". (See
Table 4-1 on page 4-17 at the end of this chapter.)
When two metals which have different potentials and which are immersed
in the same body of water or wet wood are brought into direct physical
contact or connected together with a metallic conductor, electric current
flows between them, altering their corrosion rates from those which existed
in the isolated state. The corrosion rate of the less stable metal (which
had the more negative potential) increases, while that of the more stable
metal (which had the more positive potential before the connection was
made) decreases by an equal amount. The less stable metal is now said to
be undergoing galvanic corrosion, an accelerated form of electrochemical
corrosion, while the more stable metal is now receiving cathodic protection,
with the other metal serving as a sacrificial anode. In order for
galvanic corrosion to occur, the two different metals (dissimilar metals)
must be connected electrically (by contact or by a direct metallic link,
and they must be immersed in the same body of liquid or wet wood
(either of which is called an electrolyte.) Two or more metals,
electrically connected in a common body of electrolyte are called a galvanic
cell.
Galvanized steel (steel coated with zinc) is an example of an intentional
galvanic cell - the zinc acts as a sacrificial anode for the steel in the
case of a small penetration of the coating. In addition, despite being
less stable than steel galvanically, the zinc is considerably more corrosion
resistant than the steel when it's not acting as a sacrificial anode for
a large area of steel. There's a lesson here - the Galvanic Series should
be used only to predict the nature of galvanic interactions between metals
- not to predict their relative corrosion rates. For example, aluminum,
which is also less stable galvanically than steel, also has a lower corrosion
rate than steel if it is galvanically isolated.
The ratio of the exposed areas of the two metals which make up a galvanic
cell is an important factor in what happens to the metals. In the case
of a cell made up of a small piece of copper (a stable metal) and a large
piece of steel (an unstable metal) the corrosion rate of the steel would
be only slightly increased by the connection, while the copper might be
completely protected from corrosion. If the area ratio were reversed (a
large area of copper to a small area of steel), the corrosion rate of the
steel (already high) would be greatly increased, while the corrosion rate
of the copper (already low) would be decreased only slightly. In the first
case, if the copper is in contact with wood, the cathodic protection it
receives comes at a price. The increased conversion of oxygen to hydroxyl
ions which accompanies the protection will cause deterioration of surrounding
wood. Regardless of the area ratio, painting the copper will decrease not
only the adverse affect on the wood but the detrimental galvanic effect
on the steel as well. Painting the steel may decrease the total galvanic
effect, but will concentrate what there is at small imperfections in the
paint film, causing severe localized pitting which could be disastrous
to thin material found in fuel or water tanks.
In general, galvanic connections should be avoided in wooden vessels,
unless they are made for a very good reason (like cathodic protection)
and the consequences (like wood damage around protected metals) have been
fully considered and mitigated (such as by painting the protected metals).
- Replacement Corrosion. If a metal fitting or fastening is
placed in an electrolyte which contains ions of a more stable metal, typically
a galvanized steel or stainless steel fitting in pressure treated wood
containing copper salts, the copper ions coming into contact with the fastening
will "plate out" as a solid copper film on the surface of the fastening,
with equal numbers of zinc or iron atoms ionizing, or going into solution.
The replacement reaction itself is a one-for one process, and if the stable
copper ions are depleted from the electrolyte, the replacement stops. However,
the thin surface coating of copper on the steel fastening results in a
galvanic cell, which accelerates the fastening corrosion rate.
The three principal causes of replacement corrosion to wooden boat fastenings
are, in descending order of frequency and the likelihood of significant
damage:
- Copper wood preservative salts. These include copper napthenate
from green Cuprinol, which is usually brushed on, and chromated copper
arsenate (CCA) and ammoniacal copper arsenate (ACA), which are used in
pressure treating softwood lumber.
- Copper salts dissolved in bilgewater. Copper chloride is occasionally
used as a cure for the sour bilges (hydrogen sulfide) caused by bacterial
decomposition of spilled diesel and lube oils
- Nearby copper-alloy fittings or fastenings. After a long period
of time, wood around corroding copper alloy fittings or fastenings becomes
saturated with copper ions. Any steel, galvanized steel, or stainless steel
fastening driven into that area can suffer some replacement and consequent
accelerated corrosion from galvanic effects. The effect only extends for
a few inches at most around the copper alloy fitting, however, it's prudent
not to use galvanized or stainless steel fastenings for refastening boats
previously fastened with copper alloy fastenings, whether or not the original
fastenings are removed.
- Stray-Current Corrosion. Stray-current corrosion is a magnified
version of the galvanic corrosion suffered by the more negative metal in
a galvanic cell. In the galvanic cell, the metal is connected to another,
more positive, metal, which draws electrons from it and causes the anode
reaction rate of the negative metal to increase to supply those extra electrons.
In stray current corrosion, a metal comes into contact with the positive
side of a DC electrical system, the negative side of which is grounded
to the seawater. The effect is the same, but since the driving voltage
is now 12 volts or more, instead of the few tenths of a volt found in galvanic
cells, the resulting corrosion rate can be catastrophic.
Typical sources of stray current are submersible bilge pumps, bilge
pump float switches, and electrical wiring connections in the bilge area
which might become submerged in the bilgewater. Fittings can be subject
to stray current corrosion by coming into direct contact with a chafed
positive (hot) DC wire or, more commonly, indirectly by a DC fault current
to the bilgewater. Fittings which pass through the hull and are in contact
with the outside seawater are most susceptible. In the case of an indirect
stray current path through the bilgewater, fittings which are in direct
contact with both the bilgewater and the outside seawater are most susceptible.
Stray current corrosion generally causes deep pitting of the objects
it affects, and is almost always highly localized to within a few feet
of the source of the stray current. In addition, the effected metal parts
will appear to be unusually bright or shiny. A DC stray current may cause
complete disintegration of a substantial fitting within a few days or even
less. The magnitude of the DC stray current may be a few amps in severe
cases, but usually not high enough to cause overcurrent protective devices
to trip. Stray current can discharge batteries quickly, but in boats with
shore-powered battery chargers, a substantial DC stray current may continue
to flow indefinitely.
- Bonding Systems
In order to protect against the potentially disastrous effects of DC
stray currents, many non-metallic hulled boats have a network of wires
which connect hull fittings which are at risk of stray current corrosion
with the negative, or ground side of the battery, usually via the engine
block. This network is called a bonding system. In the case
of a direct fault to a bonded fitting, sufficient current will probably
flow to trip the overcurrent protective device, stopping the stray current.
In the case of an indirect stray current (the wire in the bilgewater),
it is unlikely that a sufficient current will flow to trip the circuit,
even with a bonding system. In this case the bonding system and the stray
current will share the fault current. An indirect fault, however, is often
limited by the corrosion of the exposed metal at the source of the fault,
which eventually stifles the current flow.
FIGURE D-1: Typical Bonding System
The bonding system ties the thru-hulls electrically to the negative terminal of the battery. When a hot wire touches the thru-hull, the electrical path presented by the bonding wire has so much less resistance than the electrolytic path of the stray-current cell that a high current flows in the bonding system. This should cause a fuse to blow or a circuit breaker to trip, interrupting the stray current flow. Even if this does not happen, however, the amount of current that flows in the stray-current circuit before the battery becomes discharged, and the resulting corrosion of the affected fitting, are greatly diminished.
FIGURE D-2: Cathodic Protection Distributed By The Bonding System
When there are no stray currents, the shaft zinc may protect not
only the shaft and prop, but also any fitting connected to the bonding
system. This often results in alkali damage to the wood around those fittings.
On wooden boats, bonding systems can cause unexpected problems. First,
by connecting together a number of underwater fittings and fastenings,
the bonding system may provide the metallic links which turn otherwise
isolated dissimilar metals into a galvanic cell. Second, the bonding system
often inadvertently supplies unneeded or unwanted cathodic protection to
objects connected to the bonding system by connecting those objects to
the propeller shaft's sacrificial zinc anode. This cathodic protection
of underwater metal hull fittings often causes damaging alkali delignification
of the surrounding wood.
The fittings on a wood boat which are most susceptible to stray-current
corrosion are those in the bilgewater or those which are in close physical
proximity to wires, while those most susceptible to alkali delignification
are those above the bilgewater level, but below the waterline. In this
area the wood is wet enough to be a fairly good electrolyte, but there
is little flushing action to remove accumulations of cathode reaction products.
The hydroxyl ions produced by the cathode reaction on cathodically protected
metals can concentrate in these locations, damaging the wood and often
producing visible deposits of sodium hydroxide (lye) crystals which appear
as a white mound of salt around fastenings.
Bonded vessels should be checked with a electrical potentiometer by
a qualified electrical specialist for electrical leakage to ensure that
the boat is not over zinced. This is especially true after a vessel has
been found to have extensive wood repair due to alkali deterioration. Repairing
the wood, without determining the cause (via a corrosion survey) is a poor
practice as it would only be treating the symptom.
- Painting Galvanic Cells
Care must be taken in painting metals which are connected galvanically
to other metals. In the case of steel and copper-alloy fittings, it would
seem to make sense to worry more about the coating of the steel, since
it is more prone to corrosion than the copper alloy. If however, those
fittings are connected together, forming a galvanic cell, painting the
steel but not the copper may result in a tremendously unfavorable area
ratio for a few spots on the steel that are inadvertently not coated well.
When painting galvanic cells, one should always try to make the area ratio
more favorable to the susceptible metal. The answer is to paint both metals,
and to pay particular attention to reducing the exposed area of the cathode
of the cell (the copper).
- Crevice Corrosion
Stainless steels are subject to a particular type of corrosion called
crevice corrosion, which is a severe form of pitting. Crevice corrosion
can destroy a fastening in a few years while only damaging a small fraction
of the total mass of the fastening. The austenitic stainless steels (including
the most commonly encountered types, 304 and 316) derive their corrosion
resistance from a surface oxide film which is self-repairing in air or
in the presence of oxygen dissolved in an electrolyte. In stagnant areas
like wet wood or underneath marine growth or paint, however, oxygen can
be depleted by cathodic activity, allowing the ever-present chloride ions
to destroy the film in small areas, which then undergo unpredictable and
exceedingly rapid corrosion. Unfortunately, wet wood is a nearly perfect
environment for crevice corrosion. Stainless steel must be used with great
caution as a fastening material for wooden boats, and inspectors should
be suspicious of all stainless steel fastenings, especially wood screws,
used on boats in saltwater service. Type 316 contains more nickel and chromium
than type 304, and it also contains molybdenum, which inhibits crevice
corrosion to a certain extent, but it is not completely immune. Barbed
or "ring" nails of type 316 are available, but wood screws of type 316
are generally not available.
- Inspection of Fastenings
A boat is no better than its fastenings. The most common type of fastenings
found on wooden boats are screws, however, certain types of construction
utilize nails, bolts or rivets. Most hull fastenings are concealed from
view, being countersunk and covered; therefore their inspection is difficult.
Regardless of the type of fastenings involved, inspection to ascertain
condition is necessary in most plank on frame boats.
For purposes of uniformity careful fastening inspection must be carried
out on all vessels. Removal of fastenings should be conducted as follows:
- For Cause - Saltwater And Freshwater Service. Remove fastenings
whenever inspection reveals the probability of defects such as when a plank
or planks are "proud" and have moved away from the frames or indications
of loose bungs, rust bleeding from fastening holes etc., are noted.
Particular attention should be given to exposed hull fittings and through
bolts accessible inside the hull, such as keel bolts, chine bolts, and
double frame, clamp, and floor timber bolts. These are as important to
the total hull structure as plank fastenings. They should be sounded with
a hammer or wrench tightened and, if suspect, some should be pulled for
inspection. Often a bolt will be completely wasted away in the middle,
at the faying surface of the joint, and will break and come out when pried
up. This is caused by moisture accumulation which, besides wasting the
fastenings, forms an excellent place for wood decay to start.
- Periodic. Inspection of fastenings can prevent planking/frame
failure. Random sampling of fasteners should be part of a regular maintenance
program for continuously monitoring the structural condition of the vessel.
Therefore for vessels designed and built to Subchapter "T" Inspection Standards,
random sampling of fastenings should begin at the 10th year of age and
every 5th year thereafter in salt water service and 20th year of
age and every 10th year thereafter in fresh water service.
For existing vessels not originally built to Subchapter "T" Inspection
Standards but certificated later in life, random sampling should begin
at the 5th year of age and every 5th year thereafter in salt water service,
and 10th year of age and every 10th year thereafter in fresh water service.
Scope of Periodic Random Sampling Of Fastenings.
- Remove a minimum of eight fastenings per side below the waterline.
- Concentrate sampling in the following areas:
- Garboard seams
- Stem joints
- Plank ends in areas of bent planks
- Shaft log(s)
- Under engine beds where vibration is maximum
- In vessels of cross plank (CHESAPEAKE BAY DEADRISE) construction, specificallyinspect
fastenings at the keel and chine joints, at transom attachments, and over
the propeller(s).
It is extremely important that the type, material, and location of the
fastenings removed, along with a description of their condition be accurately
documented. This includes areas of the vessel which have undergone refastening
as well. Use of a camera is invaluable in recording areas of interest during
inspections.
Composite, cold molded and laminar built-up wooden hulls often depend
on adhesives and resins for fastening purposes. Inspection of these type
vessels requires common sense and good judgement to identify the method
of construction used and thereby determine the extent of inspection required.
Generally, these vessels do not require periodic random sampling of fastenings
by removal except for cause.
- Inspection Of Caulking
The art of caulking is an ancient one which requires experience and
a certain "touch". A good caulker makes his work look easy but it is a
skill which takes much experience to develop.
Caulking materials are subject to deterioration. It is advisable to
search the seams in any doubtful areas and re-caulk. Caulking should be
uniform and well "horsed" home. This can be checked with a probe or knife.
Care should be taken that the caulking has not been driven clear through
the seam. Over caulking is as bad as under caulking.
Extensive trouble with caulking may be indicative of structural problems,
which often includes broken or deteriorated fastenings and/or frames. If
a hull "works" excessively, caulking may be squeezed out. In such cases,
the hull structure will have to be made sound before caulking will hold.
In old hulls, where the seams have become enlarged from repeated re-caulking,
copper or lead strips may have been nailed over the seams to act as caulking
retainers. These are a temporary remedy and are an indication of poor general
condition of the vessel. It is advisable that such strips be removed and
the seams inspected for excessive width, poor caulking and decay. In some
cases, wide seams can be repaired by the use of thin wedge shaped splines
driven into the wide seams and bedded in marine adhesive. This procedure
requires excellent workmanship and should be pursued with caution. In most
cases where garboard seams have widened beyond caulking limits, refastening
of the keel, frames and renewal of the garboard planks may be the only
acceptable methods of repair.
- Inspection of Fittings
Rudder and propeller struts and fastenings should be examined carefully.
If suspect, random removal of fastenings for inspection should be accomplished.
The steering arrangement should be inspected from the steering wheel to
the heel pintle. Wear in the carrier bearing and excessive
clearances elsewhere should be corrected. Tiller lines should be in
good condition with shackles moused and bolts cottered.
The shaft log glands should be in good condition and the deadwood should
be sound. This is often neglected and is a potential cause of leakage.
Propeller shaft cracks are sometimes found at the keyway. A careful
examination here is essential. Magnetic particle testing is usually not
available in a small boatyard so the inspector must depend on visually
locating surface cracks. Dye penetrant testing is relatively inexpensive
and can be useful when deemed necessary.
Some older boats are still fitted with AM radio hull grounding plates.
These are usually copper sheet metal of several square feet in area attached
to the underwater hull. Use of AM radio equipment is no longer found on
small passenger vessels. To minimize the mixing of metals below the waterline,
the old ground plates should be removed and the hull inspected, repaired
as found necessary, and recoated.
Inspection of hardware fastenings should also be accomplished including
cleats, bitts, chain plates, etc., where threaded fasteners hold load bearing
as well as structural parts.
- Hull Damage
Most hull damage can be seen readily. Cracked and broken members are
obvious faults.
Likely locations for cracks or breaks are in areas of high stress or
where the structure undergoes a sudden change in shape. The turn of the
bilge is the prime location for breaks of this type. The harder the turn,
the more chance that damage has been done. Bent frames are particularly
susceptible to breakage under bilge stringers, especially when the stringers
are substantially thicker than the planking or when there are large diameter
fastenings in the stringers.
Wood hulls are more prone to secondary damage remote from the site of
collision or grounding than are steel hulls. Damage may consist of sprung
butts, pulled fastenings, sprung or cracked frames and misalignment of
the structure. In inspecting any damaged wooden hull, the entire vessel
should be checked.
- Deficiencies
When deficiencies are encountered an evaluation must be made of their
extent and their effect on seaworthiness. The following factors must be
weighed in making this determination:
- Is the defect progressive and, if so, how can its progress be arrested?
- How long will it be before the area in question is next inspected?
- Is the work contemplated necessary to restore seaworthiness or to
prevent the vessel from becoming unseaworthy, or is it a maintenance measure
to prolong the life of the vessel?
Specific requirements detailing the nature and extent of required repairs
should be written. However, with wooden vessels the general rule "renew
as original" while applicable, is not always practical nor necessarily
the best way to effect repairs. Most accepted methods of marine repair
may be used as long as the vessel's strength is not adversely affected.
Wood is a natural material, its quality cannot be controlled as closely
as with a manmade product such as steel. Consequently the inspector should
check the material to be used in repair work. Special attention must be
given to the type of wood proposed for each purpose and for any inherent
defects.
Many deficiencies, particularly surface defects or scars caused by chafing,
freezing and other forms of exterior deterioration are not as serious as
they may first appear. Do not be hasty in requiring the correction of minor
defects of this nature in otherwise sound seasoned planking.
Requirements For Adequate Repairs Are:
- Use of good material comparable in properties to that replaced.
- Repairs extensive enough to ensure that the hull is at as strong
as the original.
- Construction details and fastenings at least equivalent in strength
and in quality to those replaced.
- Good workmanship.
TABLE 4-1: The Galvanic Series Of Metals In Seawater
Voltages are those measured against a silver/silver chloride (Ag/AgC1) reference electrode.
Noble or Cathodic Metals |
Designation |
Voltage
Potential |
Graphite
|
C |
+ 0.27 V |
Platinum |
Pt |
+ 0.24 V |
Titanium |
Ti |
+ 0.02 V |
Incoloy |
825 |
+ 0.02 V |
Ag/AgCl Reference Electrode |
|
0.00 V |
316 Stainless Steel (passive) |
|
- 0.03 V |
Monel 70 %, 30 % cu/ |
400,K-500 |
- 0.06 V |
304 Stainless Steel (passive) |
|
- 0.06 V |
Silver |
Ag |
- 0.10 V |
Nickel Ni |
|
- 0.13 V |
Silver Brazing Alloys |
|
- 0.13 V |
Inconel 600(passive) |
|
- 0.13 V |
Ni-Al Bronze |
C63x,C954-8 |
- 0.16 V |
Cu-Ni (70-30) |
C715-9, C964 |
- 0.18 V |
Lead |
Pb |
- 0.20 V |
Cu-Ni (80-20 and 90-10) |
C710, C706 |
- 0.22 V |
"Nickel Silver" |
C745-70, C97x |
- 0.25 V |
Phosphor (Tin) Bronze |
C524, C903-5, C92x |
- 0.26 V |
Silicon Bronze |
C655, C872 |
- 0.25 V |
Manganese Bronze |
C675, C86x |
- 0.29 V |
Admiralty Brass |
C443-5 |
- 0.30 V |
Aluminum Brass |
C687-90 |
- 0.30 V |
Lead-Tin solder |
|
- 0.30 V |
Copper |
C10x, Cllx, C12x |
- 0.31 V |
Tin |
Sn |
- 0.31 V |
Naval Brass/"Bronze"(Tobin Bronze) |
C464 |
- 0.33 V |
Yellow and Red Brass |
C23x-27x, C83x-85x |
- 0.33 V |
Aluminum Bronze |
C606-24, C952-3 |
- 0.34 V |
Stainless Steel 316 (active) |
|
- 0.39 V |
Stainless Steel 304 (active) |
|
- 0.49 V |
|
Low Alloy Steels |
- 0.58 V |
Steel, Cast Iron |
|
- 0.63 V |
Aluminum |
Alloys |
- 0.87+/-.10 V |
Zinc |
Zn |
- 1.00 V |
Magnesium |
Mg |
- 1.60 V |
Notes on the Use of the Galvanic Series Table
All values are for sea water at room temperature.
Average variability is +/-.04 Volts for alloys containing nickel or
iron, +/-.02 V for copper alloys without nickel.
Sign of corrosion potential assumes that the "COMMON" or negative (Black)
terminal of the voltmeter is connected to the reference electrode and the
"VOLTS-OHMS", or positive (Red) terminal is connected to the metal to be
measured. The reference electrode must be immersed in the same body of
electrolyte as the metal being measured, preferably in close proximity.
To use Zinc as a reference electrode instead of Ag/AgCl add +1.00 volts
to the potentials listed in this table. For example, low alloy steel should
measure -.58V +1.00 V, or +0.42V against zinc, and magnesium should
measure - 1.60V + 1.00V,or -0.60V against zinc. Extremely accurate measurements
should not be attempted with zinc as a reference, since it isn't as stable
as the Ag/AgCl electrode.
Metals are receiving cathodic protection when their measured potentials
are more negative than their natural corrosion potentials listed here,
and are generally completely protected from corrosion when their potentials
measure .20V to .25V more negative than the values listed in this chart.
Metals are receiving stray current or are the anode of a galvanic system
(these are equivalent situations) when their potentials measure more positive
than the values listed in this chart. Metals in this situation are generally
suffering accelerated corrosion.
Copper alloy designations: Alloys numbered C100 to C799 are wrought
alloys, those numbered C800 to C999 are casting alloys. "x" indicates a
range of alloys sharing the preceding digits.
Chapter 5: Repairs
General Wood boat construction varies widely from locality
to locality and boat to boat. All types of repairs which an inspector may
encounter cannot be listed. Representative types and standards which are
given here are intended as a general guide to good practice and as an aid
in evaluating required repairs. Repair standards for wooden hulls should
be developed for each locality on the basis of prevailing conditions and
practice.
Planking Repair And Notes On Joints In Fore And Aft Planking
When planking is replaced, the frames and other structures should be thoroughly
inspected and placed in good condition. Holes made by old screw fastenings
should be properly reamed clean and may have the cavities filled with an
epoxy mixture thickened so as to provide a filler which will hold fastenings
like wood. Since nail fastenings depend upon the swelling of the wood around
them after they are driven for their holding power, this technique should
not be used for holes made by old nail fastenings. Holes made by old nail
fastenings should be properly reamed clean and filled with dowels set in
a suitable adhesive. When fastenings are loose it does little permanent
good to harden up those which exist. Additional fastenings, properly placed,
are the preferred repair where there is sufficient room to obtain good
holding power without seriously weakening the planking or frames. If there
is not room, holes in the sub-structure from the old fastenings may be
repaired as noted above and new slightly oversized fastenings may be driven.
Loose planking can also result from deteriorated frames and other sub-structure
in which case refastening is useless unless the structure is first made
sound. Replacement fastenings should be at least equal in size, number,
and of the same material as those of the rest of the planking. Mixing fastening
materials invites galvanic corrosion and should be avoided. Use of stainless
steel fastenings in underwater body salt water plank fastenings can result
in early fastening failure due to crevice corrosion and should also be
avoided. (See Page 4-12 for details on crevice corrosion).As a rule of
thumb, the replacement plank should extend at least six frame spaces and
no portion of a plank shorter than six frame spaces should be allowed to
remain. Where special conditions govern, this rule may be modified but,
as a lower limit, the replacement plank should be at least 5 feet long
and its butts should be spaced in accordance with the rule for butts in
this chapter.5-1
When hull planking is placed on a boat, it should have the concave side
of the annual rings facing toward the frame. This prevents "cupping" as
the moisture content of the wood changes. Deck planking which generally
sees drier service should be placed with the grain on edge or vertical.
If slash grained planks are used, especially when the planking stock is
not fully dried and the boat is painted a dark color, it is entirely possible
that the planks will dry out in service, and the edges of planks whose
ring curvature is inward will lift. Some builders, based on the moisture
content of the planking and the expected service conditions, will intentionally
place the concave ring curvature outward in the topsides. This is good
boat building practice, and it should not be prohibited.It is sometimes
necessary to shape the inboard side of a replacement plank to fit tightly
against the frames. The use of shims or packing pieces for this purpose
should not ordinarily be allowed.Flats, "dutchmen" or short lengths of
planking are normally not acceptable since they will not hold fastenings
and are structurally unsound.
Diagonal Planking The same principles apply to diagonal planking
but due to the relatively short lengths of the individual planks, a portion
of a plank is seldom replaced. Because the proper repair of double and
triple diagonal planking is expensive and time consuming, short cuts involving
the use of dutchmen and backing blocks are sometimes attempted. These should
not be permitted. Most other planking systems follow the same basic principles
of repair as outlined here. Good workmanship and care are the major requirements
for proper repair. See Wooden Boat Restoration and Repair (Reference
6).
Plywood Repairs Small surface defects may be repaired using
commercial fillers (epoxy putty, etc.). In allowing this type of repair
the wood must be decay free and all damaged wood removed. Minor repairs
of this type are satisfactory where basic strength has not been affected.
The danger lies in covering up progressive defects such as decay which
grow worse under the repair material. Damaged areas up to a foot square
can be successfully repaired by cutting the damaged area away in a rectangular
or oval shape, installing a backing block of equal thickness as the damaged
plywood, and shaping an insert piece to suit the cut-out. The repairs should
be set in place with marine adhesives, i.e. Resorcinol glue or epoxy, and
fastened with wood screws. Filling, fairing and coating complete the repair.
Large panel damages should be evaluated to determine if a beveled insert
section can be used for the repair or if the entire panel must be replaced.
Each plywood repair must be evaluated as to cause, location, materials
and strength achieved through the method selected. For detailed repair
methods refer to Wooden Boat Restoration and Repair (Reference 6).
Butt Joints In Planking
Planking butts should not terminate on frames in normal construction.
They should be located between frames on proper butt blocks, though in
light construction with narrow strakes, they may sometimes be found as
glued scarf joints at the frames and in some construction with massive
framing they may be found butted on the frames. As a rule of thumb, butts
in adjacent planks should be at least three frame spaces apart for transversely
framed, longitudinally planked vessels.
Those butts which fall in the same frame bay should be separated by
at least three solid strakes. This is not always possible, especially at
the end of the vessel, but serves to illustrate the principle of keeping
butts separated as much as possible. Where frame spacing is unusual the
following rule may be used as a guide.
Butts in adjacent strakes should be no closer together than 5 feet.
If there is a solid strake between, they should be no closer than 4 feet.
Butts should be shifted so that three or more do not fall on a diagonal
line.
To be effective a butt block must have adequate size (See page C-12).
If the frame spacing allows, its length should be at least 12 times the
planking thickness. Its thickness should be one to one and a half times
the planking thickness and its width at least 1" greater than the strake
width. Prior to installation it is recommended that the faying surface
of the butt block and strakes be coated with a wood preservative. The top
of the butt block should be curved or chamfered to allow for water run
off. Avoid butting the block hard against the frames to minimize decay.
The fastenings of the strake to the butt block should be of equal strength
to that of original butts. The fastening size should be equal or larger
and no fewer number of fastenings should be allowed. Through bolts or machine
screws are preferred fastenings in butt blocks because the joint will achieve
maximum strength. Care should be exercised to avoid over tightening so
as not to crush the planking or split the butt block.
Plywood butt blocks should be avoided because plywood has somewhat less
strength than the "along the grain" strength of the basic wood from which
it is made. Plywood is also prone to delamination and rot precipitation.
For new construction or for repairs "not in kind" the following table lists the suggested number of fastenings for planking:
- Suggested Minimum number of fastenings for planking to butts and frames.
|
Number of |
Number of Fastenings in Frame |
|
Width of
Plank
(inches) |
Fastenings
in Butt
Each Plank |
1/2-l Inch
of Plank
Thickness |
l-l 1/2 Inch
Plank
Thickness |
1 1/2-2 Inch
Plank
Thickness |
3-4 |
3 |
2 |
2 |
2 |
4-6 |
4 |
2 |
2 |
2 |
6-7 |
5 |
3 |
2 |
2 |
7-8 |
5 |
3 |
3 |
2 |
8-10 |
6 |
3 |
3 |
3 |
Glued Scarf Joints
For a glued scarf joint, the plain scarf without nibs (see Figure E) is the simplest and
strongest. Water resistant glue or epoxy resin should be used and the slope of the joint should
be 1/12 or flatter for maximum joint efficiency.
Scarf Slope |
Typical Joint Efficiency for a well |
(depth/length) |
made glued joint without nibs |
1/12 |
90% |
1/10 |
85% |
1/8 |
80% |
1/5 |
65% |
These efficiencies can be attained only with optimum adhesive conditions and excellent workmanship.
Mechanically Fastened Scarfs
Mechanically fastened scarf joints are most often nibbed, hooked, or keyed to provide extra
axial restraint and to aid water tightness.
The surface of scarf joints should be smooth and flat to ensure good fit and adhesion.
Fastenings should be adequate in size and number and arranged so as to prevent splitting the
wood.
There is considerable advantage in the use of split-ring timber connectors
in mechanically fastened joints including backbone scarf joints. Timber
connectors should be considered between the futtocks of full double-sawn
or alternating double-sawn frames which are in line with heavy concentrations
of inside or outside ballast.
Most mechanically fastened scarf joints are nibbed at the ends for a
depth of approximately 15% of the depth of the member, giving a joint length
of at least 6 times the depth.
A scarf joint which is fastened by mechanical means alone cannot, even
under the best of conditions, produce a joint approaching a solid member
in strength.
Glued Butt Joints
Glued butt joints never give joint efficiencies of over 20% and should
not be permitted. Refer to THE ENCYCLOPEDIA OF WOOD (Reference 1).
Framing Repairs
Sister Frames
Damage to frames can be repaired by the use of sister frames though
it is preferred that the frame be replaced if practicable.
The preferred type of sister frame is one of equal size to the damaged
one and as long as possible. They should extend at least 18" or approximately
four plank widths beyond the damaged area. This frame should be fastened
to the planking and other structure with fastenings at least equal in size
and number to those of the damaged member.
Care should be taken when recommending that sister frames be of greater
size than the damaged frame they reinforce. The weakening effect of bending
is inversely proportional to the square of the bend ratio (see "Bent Frames",
Wooden Boat No. 86, page 87.). This means that using a sister frame
which is deeper (larger in molded dimension) than the original frame will
produce a more severe bend ratio in the sister frame, and may actually
result in the sister frames being weaker than the original frames, despite
being larger. Often the original frames broke because their bend ratio
was too severe in the first place. Successful sister frames may be kerfed
if necessary to ease the severity of the bend when that was the problem
with the original frames. This greatly increases the effective tensile
strength of the sisters without any necessity for greater cross-section.
It is important to note that bending sister frames into hard spots in
the hull caused by broken frames may cause locally severe bends in the
sisters, which will very likely cause them to break in service. If the
hard spot cannot be corrected (this usually requires removal of the original
frames), it is actually better to let the sister frame bend fair, spanning
the hard spot and then to shim it to the planking rather than bending it
into the hard spot.
Long sister frames, well tied in to the main structure of the vessel
should not normally butt against damaged frames, though this is acceptable
where it forms the best method of tying in the new frame. If the frames
abutt, a good bedding compound or adhesive is required to exclude moisture
from between the pieces.
Where structural or machinery interference or other reasons prevent
fitting a long sister frame well tied into the other structure, a shorter
"partial sister" may be fitted as a temporary repair. This should extend
as far as practical on both sides of the damage and should be securely
fastened to the damaged frame by bolting or equivalent means as well as
to the planking and other structure. Provisions should be made to exclude
moisture from between the pieces. Temporary repairs of this nature should
be monitored closely, followed by evaluation for consideration of further
repairs or acceptance as permanent repair. Unusual or nonstandard repairs
accepted as permanent should be properly documented in the vessel's permanent
file.
A good wood preservative is recommended for use on all faying surfaces.
Ensure that precautions are taken that water cannot accumulate at the top
of the partial frame and initiate decay. A sister frame should not be used
as a repair for decayed frames. The decayed wood will eventually "seed"
the sound wood with decay spores in spite of any attempts to prevent it
by the use of wood preservatives or to isolate the new wood with sealing
compounds. When extensive decay is present in a frame the only permanent
repair is to replace it and any adjacent wood affected. If the decay is
localized, or such that frame replacement is not practical, the decayed
section of frame may be cropped out, and replaced with a new section, using
a maximum scarf angle, suitable adhesive, and by mechanically fastening
the new scarf joint. A sister frame of the appropriate dimensions may then
be placed next to, and centered around the new scarf joint in the original
frame. This repair may be considered permanent after proper monitoring
and evaluation as previously described.
Where frame damage is evident but sister framing is not practical, consideration
can be given to installing interframes between the affected frames or to
strengthening damaged or weakened frame areas with fitted metal frames.
Such repairs require excellent design considerations and workmanship and
should be undertaken with caution.
Decayed Frame Heads
Heads of frames under covering boards often become decayed due to lack
of ventilation and accumulation of fresh water leakage. With sawn frames,
this can be corrected by replacing the upper futtock. If the futtock is
long or the frame is in one piece, it can often be cropped off well below
the rot (at least 2 feet is a good rule) and a piece spliced in using a
glued and screwed scarf joint of proper dimensions. As an alternate measure
a lap joint of sufficient length may replace the scarf. Repairs to more
than two adjacent damaged frame heads should not be made by short cropping
but should be made by renewing the frames or replacing the damaged sections
by scarfing and then sistering the frame.
One of the principal causes of frame head decay is entry of water from
deck leakage or condensation into the exposed end-grain at the head of
the frame. This problem can be reduced greatly by angle cutting the frame
tops slightly short of the underside of the deck, leaving a 1/8" to 1/4"
space for ventilation, and, most importantly, by painting the end grain
of the frame heads to prevent entry of moisture. The slight gap between
the frame heads and the deck also ensures that if the sheer strakes should
shrink slightly, the covering boards (margin planks) will not be lifted
off the shear strakes by the frame heads.
Treating Isolated Decay
A method which can arrest the progress of incipient decay, at least
temporarily, is as follows:
The affected area is scraped clear of all decayed material and for some
distance into apparently clear sound wood. A strong preservative solution,
for example l:10 pentachlorophenol stock solution, is applied freely. This
is allowed to soak in and dry. Repeated applications are made until the
wood refuses to take any more preservative. Often a small "cofferdam" can
be made to retain a pool of preservative over the area. To be effective
the preservative must sink in and sterilize the wood for a considerable
distance since decay sends out spores ahead of the damaged area
After the treatment is completed the cavity made by the scraping may
be left unfilled but should be painted. Filling it will simply hide any
additional rot still working.
This method is a temporary repair only. It will usually slow decay growth,
but will seldom eliminate all traces of decay.
Painting of wood structures not only prevents decay, but also prevents
rapid short-term changes of moisture content which result in structurally
damaging dimensional changes. The proper coating of wood structures can
be as important as coating of steel structures in maintaining structural
integrity.
Sheathing Of Existing Wood Hulls
Although rejected by wood boat purists, various reinforced resin systems
have been tried with some success, both as new construction methods for
cold molded wood construction, and as a method to restore strength and
water tightness to existing plank-on-frame constructed boats. Over the
past 20 years, several systems have proven themselves successful in service,
and have been recognized by local OCMIs on a case-by-case basis for certified
small passenger vessels. The following guidance is provided to assist local
offices in evaluating potential sheathing systems.
|
Chapter 5: Repairs
General Wood boat construction varies widely from locality
to locality and boat to boat. All types of repairs which an inspector may
encounter cannot be listed. Representative types and standards which are
given here are intended as a general guide to good practice and as an aid
in evaluating required repairs. Repair standards for wooden hulls should
be developed for each locality on the basis of prevailing conditions and
practice.
Planking Repair And Notes On Joints In Fore And Aft Planking
When planking is replaced, the frames and other structures should be thoroughly
inspected and placed in good condition. Holes made by old screw fastenings
should be properly reamed clean and may have the cavities filled with an
epoxy mixture thickened so as to provide a filler which will hold fastenings
like wood. Since nail fastenings depend upon the swelling of the wood around
them after they are driven for their holding power, this technique should
not be used for holes made by old nail fastenings. Holes made by old nail
fastenings should be properly reamed clean and filled with dowels set in
a suitable adhesive. When fastenings are loose it does little permanent
good to harden up those which exist. Additional fastenings, properly placed,
are the preferred repair where there is sufficient room to obtain good
holding power without seriously weakening the planking or frames. If there
is not room, holes in the sub-structure from the old fastenings may be
repaired as noted above and new slightly oversized fastenings may be driven.
Loose planking can also result from deteriorated frames and other sub-structure
in which case refastening is useless unless the structure is first made
sound. Replacement fastenings should be at least equal in size, number,
and of the same material as those of the rest of the planking. Mixing fastening
materials invites galvanic corrosion and should be avoided. Use of stainless
steel fastenings in underwater body salt water plank fastenings can result
in early fastening failure due to crevice corrosion and should also be
avoided. (See Page 4-12 for details on crevice corrosion).As a rule of
thumb, the replacement plank should extend at least six frame spaces and
no portion of a plank shorter than six frame spaces should be allowed to
remain. Where special conditions govern, this rule may be modified but,
as a lower limit, the replacement plank should be at least 5 feet long
and its butts should be spaced in accordance with the rule for butts in
this chapter.5-1
When hull planking is placed on a boat, it should have the concave side
of the annual rings facing toward the frame. This prevents "cupping" as
the moisture content of the wood changes. Deck planking which generally
sees drier service should be placed with the grain on edge or vertical.
If slash grained planks are used, especially when the planking stock is
not fully dried and the boat is painted a dark color, it is entirely possible
that the planks will dry out in service, and the edges of planks whose
ring curvature is inward will lift. Some builders, based on the moisture
content of the planking and the expected service conditions, will intentionally
place the concave ring curvature outward in the topsides. This is good
boat building practice, and it should not be prohibited.It is sometimes
necessary to shape the inboard side of a replacement plank to fit tightly
against the frames. The use of shims or packing pieces for this purpose
should not ordinarily be allowed.Flats, "dutchmen" or short lengths of
planking are normally not acceptable since they will not hold fastenings
and are structurally unsound.
Diagonal Planking The same principles apply to diagonal planking
but due to the relatively short lengths of the individual planks, a portion
of a plank is seldom replaced. Because the proper repair of double and
triple diagonal planking is expensive and time consuming, short cuts involving
the use of dutchmen and backing blocks are sometimes attempted. These should
not be permitted. Most other planking systems follow the same basic principles
of repair as outlined here. Good workmanship and care are the major requirements
for proper repair. See Wooden Boat Restoration and Repair (Reference
6).
Plywood Repairs Small surface defects may be repaired using
commercial fillers (epoxy putty, etc.). In allowing this type of repair
the wood must be decay free and all damaged wood removed. Minor repairs
of this type are satisfactory where basic strength has not been affected.
The danger lies in covering up progressive defects such as decay which
grow worse under the repair material. Damaged areas up to a foot square
can be successfully repaired by cutting the damaged area away in a rectangular
or oval shape, installing a backing block of equal thickness as the damaged
plywood, and shaping an insert piece to suit the cut-out. The repairs should
be set in place with marine adhesives, i.e. Resorcinol glue or epoxy, and
fastened with wood screws. Filling, fairing and coating complete the repair.
Large panel damages should be evaluated to determine if a beveled insert
section can be used for the repair or if the entire panel must be replaced.
Each plywood repair must be evaluated as to cause, location, materials
and strength achieved through the method selected. For detailed repair
methods refer to Wooden Boat Restoration and Repair (Reference 6).
Butt Joints In Planking
Planking butts should not terminate on frames in normal construction.
They should be located between frames on proper butt blocks, though in
light construction with narrow strakes, they may sometimes be found as
glued scarf joints at the frames and in some construction with massive
framing they may be found butted on the frames. As a rule of thumb, butts
in adjacent planks should be at least three frame spaces apart for transversely
framed, longitudinally planked vessels.
Those butts which fall in the same frame bay should be separated by
at least three solid strakes. This is not always possible, especially at
the end of the vessel, but serves to illustrate the principle of keeping
butts separated as much as possible. Where frame spacing is unusual the
following rule may be used as a guide.
Butts in adjacent strakes should be no closer together than 5 feet.
If there is a solid strake between, they should be no closer than 4 feet.
Butts should be shifted so that three or more do not fall on a diagonal
line.
To be effective a butt block must have adequate size (See page C-12).
If the frame spacing allows, its length should be at least 12 times the
planking thickness. Its thickness should be one to one and a half times
the planking thickness and its width at least 1" greater than the strake
width. Prior to installation it is recommended that the faying surface
of the butt block and strakes be coated with a wood preservative. The top
of the butt block should be curved or chamfered to allow for water run
off. Avoid butting the block hard against the frames to minimize decay.
The fastenings of the strake to the butt block should be of equal strength
to that of original butts. The fastening size should be equal or larger
and no fewer number of fastenings should be allowed. Through bolts or machine
screws are preferred fastenings in butt blocks because the joint will achieve
maximum strength. Care should be exercised to avoid over tightening so
as not to crush the planking or split the butt block.
Plywood butt blocks should be avoided because plywood has somewhat less
strength than the "along the grain" strength of the basic wood from which
it is made. Plywood is also prone to delamination and rot precipitation.
For new construction or for repairs "not in kind" the following table lists the suggested number of fastenings for planking:
- Suggested Minimum number of fastenings for planking to butts and frames.
|
Number of |
Number of Fastenings in Frame |
|
Width of
Plank
(inches) |
Fastenings
in Butt
Each Plank |
1/2-l Inch
of Plank
Thickness |
l-l 1/2 Inch
Plank
Thickness |
1 1/2-2 Inch
Plank
Thickness |
3-4 |
3 |
2 |
2 |
2 |
4-6 |
4 |
2 |
2 |
2 |
6-7 |
5 |
3 |
2 |
2 |
7-8 |
5 |
3 |
3 |
2 |
8-10 |
6 |
3 |
3 |
3 |
Glued Scarf Joints
For a glued scarf joint, the plain scarf without nibs (see Figure E) is the simplest and
strongest. Water resistant glue or epoxy resin should be used and the slope of the joint should
be 1/12 or flatter for maximum joint efficiency.
Scarf Slope |
Typical Joint Efficiency for a well |
(depth/length) |
made glued joint without nibs |
1/12 |
90% |
1/10 |
85% |
1/8 |
80% |
1/5 |
65% |
These efficiencies can be attained only with optimum adhesive conditions and excellent workmanship.
Mechanically Fastened Scarfs
Mechanically fastened scarf joints are most often nibbed, hooked, or keyed to provide extra
axial restraint and to aid water tightness.
The surface of scarf joints should be smooth and flat to ensure good fit and adhesion.
Fastenings should be adequate in size and number and arranged so as to prevent splitting the
wood.
There is considerable advantage in the use of split-ring timber connectors
in mechanically fastened joints including backbone scarf joints. Timber
connectors should be considered between the futtocks of full double-sawn
or alternating double-sawn frames which are in line with heavy concentrations
of inside or outside ballast.
Most mechanically fastened scarf joints are nibbed at the ends for a
depth of approximately 15% of the depth of the member, giving a joint length
of at least 6 times the depth.
A scarf joint which is fastened by mechanical means alone cannot, even
under the best of conditions, produce a joint approaching a solid member
in strength.
Glued Butt Joints
Glued butt joints never give joint efficiencies of over 20% and should
not be permitted. Refer to THE ENCYCLOPEDIA OF WOOD (Reference 1).
Framing Repairs
Sister Frames
Damage to frames can be repaired by the use of sister frames though
it is preferred that the frame be replaced if practicable.
The preferred type of sister frame is one of equal size to the damaged
one and as long as possible. They should extend at least 18" or approximately
four plank widths beyond the damaged area. This frame should be fastened
to the planking and other structure with fastenings at least equal in size
and number to those of the damaged member.
Care should be taken when recommending that sister frames be of greater
size than the damaged frame they reinforce. The weakening effect of bending
is inversely proportional to the square of the bend ratio (see "Bent Frames",
Wooden Boat No. 86, page 87.). This means that using a sister frame
which is deeper (larger in molded dimension) than the original frame will
produce a more severe bend ratio in the sister frame, and may actually
result in the sister frames being weaker than the original frames, despite
being larger. Often the original frames broke because their bend ratio
was too severe in the first place. Successful sister frames may be kerfed
if necessary to ease the severity of the bend when that was the problem
with the original frames. This greatly increases the effective tensile
strength of the sisters without any necessity for greater cross-section.
It is important to note that bending sister frames into hard spots in
the hull caused by broken frames may cause locally severe bends in the
sisters, which will very likely cause them to break in service. If the
hard spot cannot be corrected (this usually requires removal of the original
frames), it is actually better to let the sister frame bend fair, spanning
the hard spot and then to shim it to the planking rather than bending it
into the hard spot.
Long sister frames, well tied in to the main structure of the vessel
should not normally butt against damaged frames, though this is acceptable
where it forms the best method of tying in the new frame. If the frames
abutt, a good bedding compound or adhesive is required to exclude moisture
from between the pieces.
Where structural or machinery interference or other reasons prevent
fitting a long sister frame well tied into the other structure, a shorter
"partial sister" may be fitted as a temporary repair. This should extend
as far as practical on both sides of the damage and should be securely
fastened to the damaged frame by bolting or equivalent means as well as
to the planking and other structure. Provisions should be made to exclude
moisture from between the pieces. Temporary repairs of this nature should
be monitored closely, followed by evaluation for consideration of further
repairs or acceptance as permanent repair. Unusual or nonstandard repairs
accepted as permanent should be properly documented in the vessel's permanent
file.
A good wood preservative is recommended for use on all faying surfaces.
Ensure that precautions are taken that water cannot accumulate at the top
of the partial frame and initiate decay. A sister frame should not be used
as a repair for decayed frames. The decayed wood will eventually "seed"
the sound wood with decay spores in spite of any attempts to prevent it
by the use of wood preservatives or to isolate the new wood with sealing
compounds. When extensive decay is present in a frame the only permanent
repair is to replace it and any adjacent wood affected. If the decay is
localized, or such that frame replacement is not practical, the decayed
section of frame may be cropped out, and replaced with a new section, using
a maximum scarf angle, suitable adhesive, and by mechanically fastening
the new scarf joint. A sister frame of the appropriate dimensions may then
be placed next to, and centered around the new scarf joint in the original
frame. This repair may be considered permanent after proper monitoring
and evaluation as previously described.
Where frame damage is evident but sister framing is not practical, consideration
can be given to installing interframes between the affected frames or to
strengthening damaged or weakened frame areas with fitted metal frames.
Such repairs require excellent design considerations and workmanship and
should be undertaken with caution.
Decayed Frame Heads
Heads of frames under covering boards often become decayed due to lack
of ventilation and accumulation of fresh water leakage. With sawn frames,
this can be corrected by replacing the upper futtock. If the futtock is
long or the frame is in one piece, it can often be cropped off well below
the rot (at least 2 feet is a good rule) and a piece spliced in using a
glued and screwed scarf joint of proper dimensions. As an alternate measure
a lap joint of sufficient length may replace the scarf. Repairs to more
than two adjacent damaged frame heads should not be made by short cropping
but should be made by renewing the frames or replacing the damaged sections
by scarfing and then sistering the frame.
One of the principal causes of frame head decay is entry of water from
deck leakage or condensation into the exposed end-grain at the head of
the frame. This problem can be reduced greatly by angle cutting the frame
tops slightly short of the underside of the deck, leaving a 1/8" to 1/4"
space for ventilation, and, most importantly, by painting the end grain
of the frame heads to prevent entry of moisture. The slight gap between
the frame heads and the deck also ensures that if the sheer strakes should
shrink slightly, the covering boards (margin planks) will not be lifted
off the shear strakes by the frame heads.
Treating Isolated Decay
A method which can arrest the progress of incipient decay, at least
temporarily, is as follows:
The affected area is scraped clear of all decayed material and for some
distance into apparently clear sound wood. A strong preservative solution,
for example l:10 pentachlorophenol stock solution, is applied freely. This
is allowed to soak in and dry. Repeated applications are made until the
wood refuses to take any more preservative. Often a small "cofferdam" can
be made to retain a pool of preservative over the area. To be effective
the preservative must sink in and sterilize the wood for a considerable
distance since decay sends out spores ahead of the damaged area
After the treatment is completed the cavity made by the scraping may
be left unfilled but should be painted. Filling it will simply hide any
additional rot still working.
This method is a temporary repair only. It will usually slow decay growth,
but will seldom eliminate all traces of decay.
Painting of wood structures not only prevents decay, but also prevents
rapid short-term changes of moisture content which result in structurally
damaging dimensional changes. The proper coating of wood structures can
be as important as coating of steel structures in maintaining structural
integrity.
Sheathing Of Existing Wood Hulls
Although rejected by wood boat purists, various reinforced resin systems
have been tried with some success, both as new construction methods for
cold molded wood construction, and as a method to restore strength and
water tightness to existing plank-on-frame constructed boats. Over the
past 20 years, several systems have proven themselves successful in service,
and have been recognized by local OCMIs on a case-by-case basis for certified
small passenger vessels. The following guidance is provided to assist local
offices in evaluating potential sheathing systems.
Improper methods of reinforced resin overlay, or overlay of an unsound
structure will generally not be long lasting. This is especially true of
sheathing plank-on-frame vessels whose hulls tend to flex or work. The
new laminate generally has little flexibility along its length and breadth,
tending to age harden and develop "tension cracks" which destroy water
tightness and strength.
An evaluation should be made considering, but not limited to, the following
items:
- In the hull, even a hairline crack can allow undetected entry of
marine borers.
- With old structure which has been painted or preserved, a good bond
is very difficult to attain and will require mechanical fastening in addition
to the adhesive strength of the resin.
- Any rot present will continue to grow worse under the sheathing if
the proper conditions of moisture and heat develop.
- It is difficult to acquire enough strength from a reinforced resin
coating to make up that lost from an unsound substructure.
- It is difficult to check the soundness of the substructure once the
sheathing system has been applied.
- Boats which have been sheathed may be susceptible to interior deterioration
from inadequate ventilation. Evidence of visible hog or sag along the keel
or sheer lines, erratic moisture meter readings or areas soft to probing
should be thoroughly investigated.
There are three sheathing systems with which the Coast Guard is familiar,
and that have been used on certificated small passenger vessels currently
in service. They each have different methods of application which require
varying degrees of hull preparation. These are:
- Vaitses Overlay. This is a hull sheathing system developed
by Alan Vaitses of Mattapoisett, Massachusetts, which uses conventional
polyester resin reinforced with a lay-up of fiberglass matt and woven roving
mechanically fastened with nails, wood screws or preferably heavy staples
during the application. After fastening is complete, several layers of
matt are applied to complete the job. This system was specifically designed
for overlay of existing vessels, and has been successfully used for vessels
from yachts to heavy timbered commercial fishing vessels from 20 to 50
feet long. A key feature of the Vaitses Overlay system is that it requires
minimum hull prepara-tion. Details and specific guidance on hull preparation
and proper application of this method are provided in Reference #15: Covering
Wooden Boats with Fiberglass.
- W.E.S.T System Overlay. This hull sheathing system, developed by the Gougeon Brothers of Bay City, Michigan, consists of overlays of plywood or cedar strips applied diagonally to the hull, and held in place with non-corrosive staples, while fully saturated in epoxy resin. Proper wet out and temperature/humidity control are essential to achieve a good bond. Sheathing should be conducted under cover, protected from direct sunlight and wind/weather. Details and specific guidance on hull preparation and the various methods of application of this method are provided in reference #5: The Gougeon Brothers on Boat Construction and #6 Wooden Boat Restoration and Repair.
- Fiberglass Planking System (C-FLEX). The main component of this system utilizes fiberglass rod reinforced high strength material and continuous fiberglass roving formed into 12" wide planks. This material is applied over wooden hulls perpendicular to the plank line to withstand the expansion/contraction of the wood planks, and is securely fastened to the planking with bronze staples. A moisture-cured elastomeric polyurethane adhesive designed for marine applications, which will adhere to wet wood, treated wood, and virtually all the various types of marine planking woods, is used to bond the material to the planking. Being an elastomeric, it will withstand extreme stretch and compression forces without breaking its bond, a quality essential in preventing delamination caused by the "working" of the hull. This method requires careful hull preparation and application. Further information can be obtained by contacting Seeman Fiberglass Inc., 6117 River Road, Harahan Louisiana 70123
Approval and use of hull sheathing systems should not be limited strictly to the above, however, the systems outlined here have demonstrated a successful operational history. Other methods must be carefully considered by the local OCMI on a case-by-case basis.
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