2-8 BOAT HULL DESIGN 



The basic objection to wood framing on a fiberglass shell is that the materials have dif- 

 ferent physical properties. Wood swells when wet, rots and requires frequent painting while 

 fiberglass does not. These differences can cause maintenance difficulties which simply would 

 not occur if both the framing and shell were entirely of fiberglass laminate construction. 



From a structural designer's viewpoint, the major objection to composite construction 

 is the form of the wood stiffeners used. These are normally of the "plank on edge" type and 

 the stiffener itself is very rigid locally. This means that the connection of the stiffener to 

 the shell must be carefully made to prevent the formation of a hard spot. See Chapter 3 for 

 a discussion of this problem. Fig. 3-24 indicates the recommended treatment for the attach- 

 ment of wooden framing to a shell laminate. 



It is a temptation to the builder to cover wood framing with a thin layer of fiberglass to 

 avoid painting and improve the appearance of the boat. This practice is very definitely not 

 recommended. It does not solve the problem of wood swelling or rotting in the presence of 

 moisture. It simply hides the problem so that the boat owner's first indication that trouble 

 exists will be cracking or delamination of the covering laminate, or actual failure of the framing. 



The use of metal, usually aluminum, for framing is not common, but it has been used in 

 some designs. The major problems with this type of construction are obtaining a satisfactory 

 bond between the metal and the fiberglass, and eliminating hard spots. In addition, difficul- 

 ties may occur due to the difference in the moduli and in the rates of expansion and contrac- 

 tion with changing temperature. These differences introduce additional shear stresses in the 

 connecting bond or fasteners. 



SELECTION OF LAMINATES 



The choice of type and arrangement of reinforcement for a single skin laminate is based 

 on a number of factors. These factors include strength, rigidity, impact resistance, re- 

 sistance to passage of water, cost of material and labor, ease of handling and appearance. 

 Each of the basic types of reinforcement has its own qualities and these must be utilized in 

 combination to provide the most effective laminate. These are discussed in detail in Chapter 4. 



It is obviously impossible to consider all the possible combinations of reinforcement 

 which might be used. As an indication of the choices which are made, and the reasons for 

 them, three laminates will be discussed. 



The first laminate, Type A: Consists of 1 ply of 10 ounce cloth on the outboard side, 

 1 ply of 2 ounce mat and a varying number of plies of 25-27 ounce woven roving. 



Considering this laminate in detail: The outside ply of cloth is intended primarily for 

 appearance and as gel coat reinforcement. It is used because of its relatively smooth sur- 

 face, which reduces the thickness of the gel coat. This ply also provides good tensile quali- 

 ties for the exterior of the laminate. Some builders replace this ply of cloth with a ply of 

 3/4 ounce mat as gel coat reinforcement. 



The ply of 2 ounce mat serves three purposes. First, it is highly resistant to the pas- 

 sage of water and minimizes the absorption of water by the laminate. Water absorption re- 

 duces the strength of the laminate. Second, mat provides greater thickness per layer than 

 either woven roving or cloth, and thus increases thickness with a minimum of cost. Third, 

 the mat prevents the pronounced weave pattern of the woven roving from showing through 

 the cloth and the gel coat. Note that 2 ounce mat is used here, but that 1-1/2 ounce mat 



