BOAT HULL DESIGN 2-19 



The factor F T varies with the length of span expressed as a function of the half-breadth 

 of the boat at the transverse. This means that for a boat with a center keel and transverse 

 framing spanning the entire half-breadth, the design load is a uniform pressure equal to 

 0. 68 times the maximum pressure at the longitudinal position of the transverse in question. 

 This reduction is due to the fact that only a portion of the shell area supported by the trans- 

 verse will be subjected to the maximum pressure at any one time. 



For the design of longitudinals the variation in pressure is dependent on the longitudinal 

 location of the member. The formula is: 



P 



D„ + p„ x F, x F (2. 5) 



D - H L I 1 L 



Pp = design pressure considered acting as a 



uniform pressure on the portion of the shell 

 area supported by the longitudinal 



Pr, = static water pressure as previously defined 



Pj = pressure from the nomograph, Fig. 2-lU 



F-, = factor taken froir, the top graph on Fig. 2-l£ 



F T = factor taken from the bottom graph on Fig. 2-15 



To obtain the design pressure for a longitudinal with a particular span, or distance 

 between transverse supports, calculate the design pressure p^ at the ends of the span and 

 use the average of these two values as a uniform load on the portion of the shell supported 

 by the longitudinal. 



In designing the components of the bottom structure using the pressures obtained by the 

 methods just described, a factor of safety of 1.5 on the ultimate wet strength of the material is 

 suggested. This low factor of safety is permissible because the pressure used is for an 

 extreme loading condition. 



An alternative criterion which should be checked on all runabout designs is the deflection 

 of the bottom. The assigning of an allowable deflection is a difficult problem since it depends 

 on tne reaction of the passengers in a boat. People who are accustomed to wood hulls some- 

 times regard the flexing of a fiberglass boat as a sign of weakness. The reasons that fiber- 

 glass hulls may flex more than wood hulls are that the hull thickness is reduced because of 

 the higher strength of the fiberglass laminate, and that the number of transverses is re- 

 duced compared to that required for a wood hull to prevent working and leaking at the seams. 



From a structural point of view, limited deflection of the shell in a boat hull is not, in 

 itself, a major problem. However, the passenger reaction previously mentioned can be a 

 serious one from the standpoint of customer acceptance. For guidance, it is recommended 

 that the deflection of bottom structure under the maximum bottom pressure be limited to 

 1/100 of the span. 



The side thickness of the planing boat is usually sized on the basis of the required 

 bottom thickness to provide a balanced hull laminate. From a study of a number of existing 

 designs which are believed to represent good current practice, the side laminate thickness 

 may be taken as 75 to 80 per cent of the thickness of the bottom hull laminate with a mini- 

 mum thickness of 1/8 inch. If a boat is intended to be used in service where frequent 



