2-18 BOAT HULL DESIGN 



time. This pressure will occur only under the most severe sea conditions, which must be 

 judged in relation to the size of the boat. The intention here is that severe conditions are 

 those in which the operator of the boat is subjected to acute discomfort. It is recognized 

 that this does not represent a normal operating condition, but rather one due to some emer- 

 gency, such as an approaching storm, when the boat must be driven as hard as the operator 

 is physically capable of driving it. 



The pressure given by the nomograph will not occur, at any one time, over a large area 

 of the bottom, and there are areas of the bottom, both forward and aft where it will never 

 occur. The pressure to be used for design purposes will vary with the type of member be- 

 ing designed and its location in the boat. These pressure variations with location and ex- 

 tent of area loaded were investigated during the PT boat tests (13), and the results, with 

 some modifications, are summarized in Fig. 2-15. 



For the design of the bottom shell the pressure to be used is the full pressure varied 

 only for longitudinal location. The formula is: 



P D = P H + P I X F l (2 - 3) 



p = design pressure 

 D 



P H 



pressure due to the normal static head of 

 water taken in the at-rest position 



p = impact pressure from the nomograph, Fig. 2-lU 



F = a factor for the shell impact pressure, depending 

 1 on location along the length of the hull shown in 

 the top graph of Fig. 2-15 



It can be seen from the figure that one-half of the maximum impact pressure may be 

 expected at the bow, the maximum may be reached anywhere between the forward one- 

 quarter point and midships, and that the impact pressure drops off to zero at the stern. 

 For practical construction, the whole bottom laminate forward of station 6 or 7 may be 

 designed for the maximum impact pressure, and the laminate aft for the impact pressure 

 at station 6 or 7. In this manual reference to station numbers is based on the water 

 line length being divided into ten stations from, forward. 



For the design of transverse members the variation in pressure depends on the 

 longitudinal location of the member in the boat, and on the span of the member in the 

 transverse direction. The formula is: 



P D = P H + P I X F 1 X F T < 2 - 4 > 



p = design pressure considered acting as a 



T) 



uniform pressure on the shell area supported 

 by the transverse 



p = static water pressure as previously defined 



H 



P = pressure from nomograph, Fig. 2-lU 



F, = factor taken from the top graph on Fig. 2-15 



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



