BOAT HULL DESIGN 



2-29 



Thwarts: Consider as a simply supported beam loaded by two 2 00 pound men, sitting 

 at one -third of the span from each end. 



Factor of Safety 



Bending Moment, M = F :•: — 



200 x 51} 



3600 in. -lbs, 



(2. 10) 



, I PLY CLOTH 



12 .. 



"V 



2 PLI ES MAT 



3-1/2" 



x 3 PLIES MAT 



Fig. 2-2 0. Thwart cross -section 



Consider a channel shaped thwart integrally molded with the gunwale as shown in Fig. 

 2-19. The cross-section and laminate construction shown in Fig. 2-20 gives the following 

 results: Ultimate tensile stresses from Table 5-6. 



Stress in Cloth - tension at bottom edge of flange = 5350 psi 

 Factor of Safety = "~^'^ = 1|.5> Satisfactory 



21,100 

 5350 



Stress in Hat - tension at bottom edge of flange 



li.9 Satisfactory 



= 2250 psi 



Factor of Safety = 



2250 



Tensile and compressive stresses are used instead of the flexural stresses given in 

 Chapter 5 because the flexural stresses are obtained from tests of bending in a plane 

 perpendicular to the plane of the laminate. No experimental information is available 

 regarding flexural stresses when the bending is in the plane of the laminate. 



DESIGN EXAMPLE 2-2. OPEN SAILBOAT 



This sailboat is intended for use as a day sailer and for class racing. It is a sloop 

 rigged, centerboard boat and similar in size, rig, and general layout to many classes in 

 widespread use. The transom is to be suitable for a 5 horsepower outboard motor. 



P rincipal Dimensions - See Fig. 2-21 for details. 



Length Over -all: 14 ft. - in. 



Beam: 5 ft. - 2 in. maximum 



Sail Area: 131 sq. ft. 



