Motion and Resistance of a Low-Waterplane Catamaran 



assess the contribution of the main hull to the total wavemaking re- 

 sistance of a catamaran, the C w curve of a single main hull without 

 a strut was also computed and is plotted in Figure 21. This curve 

 was computed on the assumption that the main hull alone had the same 

 displacement volume as Model 527 6-E. In other words, the diameter 

 was increased but its centerline depth was not changed. To facilitate 

 a direct comparison of wavemaking resistance, this C w curve was 

 computed by using the wetted surface of Model 527 6-E rather than 

 that of the main hull form. This C w curve gives the limit as the size 

 of a strut is reduced to zero while the total displacement volume is 

 kept constant. Figure 22 gives the same information at a 28 -ft draft. 



It is important to note that, at high Froude number, the 

 wavemaking resistance of a submerged main hull cannot be ignored. 

 The wavemaking resistance of a SWATHS can be greatly reduced by 

 diminishing the strut size while the total displacement volume is kept 

 the same. 



II. 4. - Conc luding Remarks about the Catamaran Hull Form Design 

 Procedure. 



Based on the concept of an effective hull form, the problem 

 of designing a catamaran essentially becomes the same as that of 

 designing a conventional single hull. The direct link between these 

 two design problems is a computer program developed to obtain off- 

 sets of a demihull form offsets of its effective hull at a specified hull 

 distance. By using this computer program, a pair of monohulls can 

 be converted into a pair of demihulls of a catamaran without altering 

 the original hydrodynamic properties. 



In general, the SWATHS is a special case of catamaran 

 configuration. Due to the simplicity of its hull geometry, designing a 

 SWATHS theoretically is much simpler than designing a conventional 

 catamaran. Two design examples for SWATHS were given. Except 

 in the case of a strut with blunt endings and quick curvature reversals, 

 comparisons of theoretical C w curves with experimental C r curves 

 showed good agreement. 



How low the resistance level of a catamaran hull form can 

 be designed depends both upon our ability to design a conventional 

 single hull and on the design constraints imposed. To enhance the 

 good resistance performance of a catamaran, restrictions imposed 

 on the hull geometry should be kept to a minimum. For developing a 

 specific type of catamaran, the major effort should be directed toward 

 developing an effective hull form first, rather than toward developing 

 a catamaran hull form directly. In so doing, not only would the expense 



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