Sheathing Of Existing Hulls, Wood
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.
Excerpted from Wood Hull Inspection Guidance (NVIC 7-95) |
