Pien and Lee 



uniform flow, there is no circulation on this symmetrical section. 

 However, if we place this section in a curved flow, we have to curve 

 the section in such a way that its original plane of symmetry coinci- 

 des with the flow in order to maintain the stagnation points and thus 

 avoid the creation of circulation on this section. Similarly, we have 

 to modify the single-hull geometry in the presence of another demi- 

 hull so that there is no shift in the stagnation points. In this case, 

 however the beam of each demihull is relatively large in comparison 

 with the distance between the demihulls. The flow curvatures on two 

 sides of a demihull are quite different, and the usual practice of ad- 

 ding thickness distribution to a mean cambered surface may not al- 

 ways be applied here. Instead, we have chosen the following proce- 

 dure. 



The Douglas program, used to compute the source distribu- 

 tion, has been extended to trace offbody streamlines. Our objective 

 is to obtain the distortion in the effective hull form placed at the 

 location of one demihull in the presence of another for the purpose 

 of obtaining its final geometry. We shall measure the distortion of the 

 effective hull form due to the flow curvature with respect to the mid- 

 ship section. In other words, the midship section of the demihull is 

 identical to that of its effective hull form. Let us consider an after- 

 body plan of the effective hull form placed at the midship section. If 

 a number of points along a given station of this body plan are chosen 

 as starting points of streamline tracing, all streamlines are parallel 

 in a uniform flow in the absence of the other demihull, and the sta- 

 tion section defined by these points will not be distorted. Due to the 

 presence of the other demihull, however, the streamlines so traced 

 will no longer be parallel but will be distorted according to the flow 

 curvatures due to the other demihull. The points obtained by inter- 

 secting these distorbed streamlines by a cross plane at the corres- 

 ponding longitudinal location will then define the required hull cross 

 section of the demihull. In this manner the afterbody of the demihull 

 is obtained. Similarly, by placing a forebody plan at the midship 

 section and by reversing the direction of the uniform flow at infinity, 

 we can obtain the forebody of the demihull. 



A computing program has been developed, using the exis- 

 ting Douglas program as a starting point for computing the body plan 

 of a demihull. Using the offsets of the effective hull form as input 

 data, the required offset table of the demihull is generated by this 

 computing program. By using this computer program, an existing 

 single hull can easily be converted into a demihull of a catamaran 

 without changing its hydrodynamic properties. 



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