2-14 BOAT HULL DESIGN 



Boats intended for rougher service than would be considered normal, such as frequent 

 beachings in rocky areas should be strengthened with an additional ply of woven roving. For 

 large open sailboats of the "Thistle" type, 2 plies of 1-1/2 ounce mat may be substituted for 

 the 2 ounce mat, due to the large torsional loading from the sails and the lack of a deck. 



The above scantlings are recommended for "round bottom" boats whose shells have 

 curvature in the transverse direction and have some form of centerline stiffening in the 

 bottom. This centerline stiffener or keel is very important as it provides support for the 

 shell when in the water and also when in storage. This member will usually be in the form 

 of an external keel and skeg. Another common method of adding stiffness to the boat in the 

 vicinity of the keel is to lap the middle and /or inner plies of the laminate to form a double 

 thickness in this area. In the case of a flat bottomed boat, such as a conventional skiff, it 

 is recommended that a stiffener of parallel strands of roving be added approximately midway 

 between the centerline and the side. These stiff eners should be 1/2 inch thick and of varying 

 width from a minimum of 1 inch for a 12 foot boat to 3 inches foran 18 foot boat. If desired, 

 the single stiffener may be replaced by a number of smaller longitudinal stiffeners, spaced 

 at equal intervals across the hull, whose total width is equal to that of the single stiffener. 



Sometimes it is desired to produce a fiberglass version of an existing class of wood 

 racing sailboats. In this case the determining factor in choosing the scantlings may prove 

 to be the weight limitations of the class rules, and the laminates recommended here may 

 prove to be too light. The designer is referred to the tables of physical properties in 

 Chapter 5 for unit weights of laminates with various types of reinforcement. 



The most difficult shell loading to evaluate is that experienced by a relatively high speed 

 planing power boat travelling in waves. Anyone who has experienced a ride on this type of 

 boat knows that severe impact loads occur on the bottom due to high accelerations. 



Although some investigations have been made, (10, 11, 12), there is no simple theory 

 available which predicts the maximum impact pressure on the bottom of a boat at high speed. 

 This is particularly true in the case of small light boats. Also, very little experimental data 

 are available on this subject. After consideration of several possible approaches to the 

 problem of predicting bottom pressures for use in design, an empirical relationship has been 

 developed which is considered practical and conservative, particularly for small boats. 



The development of this equation consists basically of considering the impact pressure 

 developed when the boat strikes the water. This pressure is given by the formula: 



p = 1/2 p v 2 (2.1) 



where p = pressure in pounds per square foot 



v = speed in feet per second 



p = mass density of water in slugs per cubic foot, 1.992/4. 

 for salt water at 5>0 degrees F 



