Motion and Resistance of a Low-Wat erp lane Catamaran 



of this part of the paper. 



II . 2 - Design Procedure. 



Designing a catamaran hull form with good resistance quali- 

 ties is a much easier problem than predicting the resistance of a 

 given hull form. For instance, for the purpose of reducing the resis- 

 tance, the crossflow around each demihull is eliminated. Hence, in 

 a design problem, we have to deal only with the hull form without 

 crossflow, and we need not be concerned with the complications 

 crossflow would create. Since the basic flow around a demihull is 

 not straight but is curved due to the presence of another demihull, 

 the geometry of a demihull cannot have symmetry with respect to 

 both sides if the crossflow is to be avoided. The amount of asymme- 

 try depends upon the geometry of, as well as the distance between, 

 the demihulls. Since hull geometry and hull distance are the objecti- 

 ves of the design problem for a catamaran hull form, it is very diffi- 

 cult to solve the problem in one step. To simplify the situation, a 

 concept of an effective hull form is introduced as follows. 



Concept of an Effective Hull Form. 



We define a monohull in a straight uniform flow to be an 

 effective hull for a demihull in a curved flow, if the flow relative to 

 the hull is the same in both cases. If a demihull were towed alone 

 and then towed with another demihull, a different effective hull form 

 would result, even though its physical geometry had been kept the 

 same. To maintain the same effective hull form, changes must be 

 introduced to the hull geometry. Whenever the distance between 

 demihulls is varied, a corresponding variation in hull geometry is 

 also needed. 



Since the geometrical difference between a demihull and its 

 effective hull can be determined after the effective hull geometry 

 and the hull distance are given, it is rather logical to divide the de- 

 sign problem into two steps. In the first step an effective hull form 

 is developed, and in the second step the geometry of the demihull is 

 determined. 



Designing an Effective Hull Form for the Demihull of a Catamaran. 



The viscous drag and the free wave system of a demihull 

 are considered to be the same as those of its effective hull form. 

 Hence, the resistance of a catamaran can be optimized by developing 

 an optimum effective hull form. Since effective and geometric hull 



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