BOAT HULL DESIGN 



2-5 



tions, built-in bunk tops and sides, shelves and so forth can be used as the longitudinal 

 frames to reduce the number of stiffening members. For these reasons, longitudinal fram- 

 ing is frequently preferred in fiberglass hulls. 



The essential requirements for a frame are that it withstand all the applied loads and 

 provide sufficient stiffness to maintain the form of the entire structure with a minimum 

 expenditure of material. The construction of the fiberglass frame may vary with the 

 size requirement. 



For small boats, where the required frame sizes are small, frames are sometimes 

 made of unidirectional roving built up to form a solid rectangular cross-section; Fig. 2-7. 



ELL LAMINATE 



ONAL FIBERS 



Fig. 2-7. Stiffener for small boats - solid laminate 



This type of stiffener has the advantage of simplicity and good bending characteristics. 



For larger boats and higher loadings, where greater strength and rigidity is required, 

 the use of the solid stiffener becomes uneconomical because of the amount of material re- 

 quired. In most hulls a closed shaped stiffener is used which consists of a solid or hollow 

 core or form covered with a fiberglass laminate. This, in effect, makes a closed box or 

 semicircle section when combined with the shell. Typical configurations are shown in 

 Figs. 2-8 and 2-9. Usually the cores or forms, over which the stiffeners are molded, are 

 used to give the desired shape and are not considered to contribute to the strength. The 

 effect of the core material on the rigidity of the stiffeners may be considered if warranted. 

 The material for the form is chosen for lightweight, workability, and ability to withstand 

 the handling required in the molding process. Typical form choices are cardboard mailing 

 tubes for the half-round stiffeners, Fig. 2-8 and balsa wood or unicellular foams for the 

 hat stiffeners, Fig. 2-9. A detailed discussion of low-density core materials is given in 

 Chapter 4. non- structural 



form 

 non-structural 



FO 



LAMINATE 



Fig. 2-8. Half-round stiffener, with 

 hollow core 



Fig. 2-9. Hat stiffener with full 

 lightweight core 



