Background

Years ago boat building epoxy resins were generally formulated for wooden boats. Prior to the advent of epoxy resins wooden boats were built using "traditional" methods such as carvel planking, caulked plank on frame, lapstrake, etc. In the mid 1970s Meade Gougeon and his brother, Jan, showed that wood veneers could be glued with epoxy resin and, more importantly, that wood could be protected from moisture egress by coating all exposed sides with epoxy resin. This use of epoxy resin made monocoque (single piece) hull construction possible. These hulls can be constructed using veneer, wood strips and the like and the effect of water on these materials could largely be ignored so long as they were epoxy coated (using fiberglass where appropriate) and maintained so as to keep the epoxy coating intact. Wood/epoxy hulls generally require no more maintenance than all fiberglass hulls. Since traditionally built hulls partially rely for their watertight integrity on wood getting wet and swelling; they can have significant maintenance problems.

One of the chief requirements for epoxy resins used in wooden boat building is that the resin cure sufficiently at room temperature and that the resin system retain enough flexibility and resiliency so that the epoxy coating does not crack from impact during normal use. Epoxies formulated with these properties paramount generally become quite flexible and rubber-like at temperatures in excess of 145F. It can be said that their modulus (a measure of stiffness) has dropped sufficiently so that their behavior has changed from glass-like to rubber-like. While tensile, compressive, flexural, and shear strengths have dropped significantly at this temperature, they are still sufficient to bond wood together. The coating qualities of the epoxy are largely unaffected. Obviously, one should carefully choose a painting scheme when completing a wood/epoxy boat to minimize the effects of heat - any color you like so long as it is light (save the dark for accents on hull and cabin sides).

The strength and stiffness of a wood/epoxy boat primarily comes from the wood. The role of the epoxy is to hold the wood together and to protect it from moisture. Except for strip planked and the "corners" of stitch and glue boats epoxy/fiberglass laminates add little to the overall strength of wooden boats. Strip planked boats generally have fiberglass sheathing on both the inner and outer hull sides. Here the epoxy/glass laminate holds the planks in a fixed position relative to each other so that the hull is strong. For a boat built this way to break it is necessary to shear the fiberglass laminate from the wood. Stiffness results from two layers of fiberglass cloth (parallel planes) held a fixed distance apart (I-Beam). Still, the wood strip core adds significantly to the overall stiffness and strength of the boat. A boat built this way could be thought of as a "wood-cored fiberglass boat". Standard room temperature cured boat building epoxies are perfectly adequate for this construction because the wood takes the load.

But, what happens when the core has little strength or stiffness - a balsa or foam cored boat for example? Here, unlike the "wood-cored fiberglass boat" described above, the skins (epoxy and fiberglass laminates) take the entire load. This significantly changes the requirements of the epoxy resin used in the laminates. In Part 2 we'll explore why. Look for it next week.