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|Wood Fasteners, Mechanical|
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:
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:
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).
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.
Boat Spikes And Drift Bolts
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.
In general, all groups of bolts should be symmetrical in the members. The individual fastenings should be offset slightly as necessary to avoid placing more than one on the same grain.
Excerpted from Wood Hull Inspection Guidance (NVIC 7-95)