BOAT HULL DESIGN 2-7 



widely with different physical properties of the core. Some cores, notably wood, have suf- 

 ficiently high flexural moduli to cause an appreciable effect on the flexural rigidity of the 

 sandwich. Others have such low shear moduli that the effect of shear deflection must be 

 taken into account. In designing a sandwich the effect of the core material used must be 

 investigated to determine whether the core should be considered effective. 



Because of the great increase in stiffness and load carrying capacity, sandwich con- 

 struction is frequently used for relatively large flat panels, particularly where the presence 

 of stiffeners would be objectionable for arrangements. When the shape is complex, the pre- 

 formed cores are generally difficult and expensive to shape. This difficulty may be overcome 

 by using foamed in place resins with appropriate molds or troweled in place filled resins. 

 Common applications of sandwich construction are for bulkheads, decks and cabin tops on 

 larger boats. 



Many different materials have been tried as cores but the most common being used are 

 balsa wood, foamed resins, and honeycombs. A detailed discussion of these core materials 

 is presented in Chapter 4 and physical properties are given in Chapter 5. 



Core material for use in shell and weather deck sandwiches must have the ability to 

 prevent water migration within itself and between the core and the fiberglass facings. 



Another highly desirable quality is lightweight. In any boat the application of lightweight 

 materials is an advantage since they will permit a reduction in the portion of the displace- 

 ment which must be allotted for the hull structure. This will allow a correspondingly larger 

 portion of the displacement for useful load in the form of stores, fuel, etc. , or increase 

 speed with the same load. For small boats made of sandwich construction to be competitive 

 with single skin and frame construction the lightweight core should provide a lower over-all 

 weight and should also provide positive buoyancy. If the sandwich cannot be made buoyant 

 its lower unit weight compared to an equivalent thickness of solid fiberglass laminate will at 

 least reduce the requirement for positive buoyancy. 



Balsa wood cores and unicellular foamed plastic cores can provide positive buoyancy. 

 Honeycombs made of heavy cotton duck and resin impregnated paper are satisfactory but 

 must be handled with extreme care to insure complete resin impregnation for watertightness 

 and to obtain a good bond between the core and faces. Improperly constructed honeycomb 

 laminates in boat hulls have, in the past, caused water migration trouble and their use is not 

 recommended for primary hull construction unless the fabricator has had considerable suc- 

 cessful experience with them. Their use should be limited to decks, cabins, flats and bulk- 

 heads where they will be used to maximum advantage. 



Composite Construction 



The term "composite", as used in the marine industry, refers to a boat or ship whose 

 framing is made of one material while the shell and decks are of another. The use of this 

 term began during the transition period when ships were built with iron framing and wood 

 plank shells and decks. For fiberglass boats, it refers to hulls with shell and decks of 

 fiberglass laminate and framing of wood or metal. The wood framing is by far the most 

 commonly used. 



Composite construction has been used successfully on a number of boats. There are 

 basic technical objections to composite construction, which will be discussed, but properly 

 applied it can produce a successful boat. 



