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12 Fundamentals of Wood/Epoxy Composite Boatbuilding Wood has many advantages as a boatbuilding material. It’s probably easier, less expensive, and more satisfying to build a boat from wood than from any other material. Wood is relatively easy to cut and shape and almost everyone has some experience with it. It’s satisfying to work with because it’s beautiful. Wood is readily avail- able and it costs less than steel, aluminum, or fiberglass. Although lumber prices have increased, wood is still comparatively inexpensive in terms of both its own cost and the cost of tools to work it. Most importantly, however, wood has physical characteristics that make it ideal for boatbuilding. Its strength, stiffness, light weight, and resistance to fatigue give wood advantages over other materials. While wood has many advantages as a structural ma- terial, it also has some well-known disadvantages, most of which are caused by the passage of water in and out of its cells. Wood can rot. It shrinks and swells with changes in moisture and temperature, and it loses some of its strength and stiffness when its moisture content is high. In the past, difficulties arose in constructing boats with wood because of changes in the condition of the wood caused by variations in moisture content. As its moisture level increases, the wood changes in dimension and loses some of its strength and stiffness. The design of boats built of wood had to make allow- ances for this instability. To a very great extent, the use of WEST SYSTEM Brand epoxy overcomes the problems previously associated with wood construction. All joints in boats built with the methods described in this book are bonded with, and all surfaces encapsulated in, epoxy. In this way, every piece of wood, inside and out, is covered with a barrier coating of WEST SYSTEM epoxy through which no significant amount of water or air can pass. As a result, the moisture content of the wood is stabilized. This stabilization means that the wood will shrink and swell very little. The moisture level at which the stabi- lization occurs and at which the wood remains ensures a continuation of design strength and stiffness. Encap- sulation in WEST SYSTEM epoxy also prevents dry rot, not only by stabilizing moisture content, but also by restricting oxygen supply to the wood surface. Since World War II, there has been tremendous research and development in the field of epoxy and other thermosetting plastics. Wood research had focused on technologies and products suited to the construction industry. We are among the few to work specifically to develop a wood/epoxy composite material and the only group to test it exhaustively, especially in high-cycle fatigue. Our efforts clearly show that a wood/WEST SYSTEM epoxy composite is one of the best structural materials currently available for building boats when strength and stiffness-to-weight are primary considera- tions. Because of the superiority of the composite in fatigue, wood/epoxy hulls are less liable to failure over time and after hard use than hulls built of other ma- terials. We will explain why this is so in this chapter and discuss WEST SYSTEM epoxy in greater detail in Chapter 4. See Appendix B for additional data on wood’s mechanical properties. Engineering for Wooden Boats Boats present unique engineering problems. They require an outer skin, which may or may not be load- bearing, but which must withstand and deflect tons of water. Boats must also survive the kind of high point loads that occur during launching and hauling out, or if objects are struck at sea. Even small vessels have tremendous amounts of vulnerable hull and deck surface which must be properly supported to retain their streamlined shapes. Large sailing rigs may cause torsion and bending in hulls that have inadequate shear bracing. Finally, boats must survive these loads continu- ously over many years. The overall task in addressing these problems is the successful integration of proper design, construction methods, and material choices. Boats demand a material that is strong and lightweight. It must also be stiff, as demonstrated by the ability to resist deformation under load. Within certain limits, the lighter and stiffer a boat is, the better its potential for performance and durability. The less a hull weighs, the faster it will move under a given amount of power. The stiffer a boat, the better it holds its true shape and resists “softening” or weakening through flex and fatigue. Boat- building materials must retain their strength and stiffness over time and after use, and the boat’s structural compo- nents and construction method must be designed to fully exploit the materials’ capabilities. These principles may be best understood by thinking of a boat hull as a box beam enclosed by hull planking,PDF Image | Gougeon Brothers on Boat Construction
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