2-2 BOAT HULL DESIGN 



dockings are required, such as a club launch, or a passenger ferry, the side laminate 

 should be increased to the same thickness as the bottom laminate. 



Side framing for runabouts may be designed to withstand a pressure equivalent to a head 

 of water six inches above the deck edge. If several side frames are used, they are usually 

 made the same size throughout. A factor of safety of 4 is suggested for this loading. Side 

 framing designed to the above criterion will usually support a lower maximum load than the 

 shell laminate can support. The reason for this difference is that the side laminate thick- 

 ness, selected as explained above, is chosen partially to resist effectively local impact 

 loads due to striking docks, partially to reduce torsional deflection of the hull, and 

 partially to prevent side panel vibration. The side framing in the forward end of a 

 boat should be designed as bottom framing especially when a Vee form is used since 

 it is subject to similar impact loadings. 



The next shell design to be considered is for displacement type boats. These boats do 

 not travel fast enough in a seaway to have the large impact pressures experienced by a high 

 speed planing boat. The shell is therefore designed for a static water pressure. 



In the case of the cruising sailboat, the shell and framing should be designed to with- 

 stand a head of water corresponding to that experienced when the boat is heeled to the point 

 where the cockpit starts to flood. Depending on the height of the cockpit coaming this height 

 will vary from 6 inches to a foot or more above the deck edge. Since this is a normal load- 

 ing condition a factor of safety of four is suggested. Deflection is not normally a problem 

 with boats of this type. 



The displacement type power boat occupies a position midway between the planing power 

 boat and the cruising sailboat, as far as the shell load is concerned. The shell design load 

 for this type of boat cannot be taken from the empirical formulation presented for planing 

 boats, but should be greater than the static pressure existing on the bottom with the boat at 

 rest because of the additional loading experienced in a seaway. For these boats a bottom 

 design pressure equivalent to a water head of 2 feet above the main deck forward of Station 7 

 and 6 inches above the main deck aft of Station 7 is recommended for the shell and framing. 



In designing the framing for displacement boats no reduction factors for location and 

 span, as are used for planing boats, should be applied. The pressures used here are not 

 local transient impact pressures, such as those that occur on the planing hull, but are con- 

 sidered as a uniform load over the entire bottom of the boat. A factor of safety of four is 

 recommended for use with this design loading. As in cruising sailboats, deflection is not 

 normally a problem with this type of boat. 



As with planing hulls, the side shell is proportioned on the basis of the bottom shell. 

 For normal service this thickness may be taken as approximately 80 per cent of the bottom 

 thickness. The side framing is designed to withstand a water head to a height six inches 

 above the deck edge, as in planing boats. 



To assist the designer in making preliminary estimates for the bottom laminate of a 

 cruising sailboat, Fig. 2-16 has been compiled from existing designs. The Figure indicates 

 required section modulus and stiffener spacing as a function of the cube root of the displaced 

 volume. The Figure is for guidance only and in the detail design stage the laminate and the 

 stiffener spacing must be determined for the particular boat by the detailed design methods. 

 Fig. 2-16, in conjunction with the unit weights given in Fig. 2-12, will aid the designer in 

 making preliminary weight calculations. 



