Motion and Resistance of a Low-Waterplane Catamaran 



Several examples of comparisons between the theoretical predictions 

 and the experimentally obtained motions are given. 



In dealing with the resistance problem, it was found that, 

 by using the concept of an effective hull form, the design problem 

 of a catamaran and a conventional hull became the same. Hence, no 

 additional theoretical development was necessary. Because of this, 

 it was possible to devote this section exclusively to the catamaran 

 design procedure and design examples. 



Based on the motion work, the principal dimensions and 

 hull coefficients which control ship motions in a given seaway can be 

 specified. These specifications constitute part of the hull-form design 

 conditions. Within all the design contraints, an optimum catamaran 

 hull form based on powering considerations can be developed by fol- 

 lowing the design procedure. When this is done, a table of hull offsets 

 is available which can be used to make a final check on ship motions. 

 Since the hull characteristics required for ship motion considera- 

 tions may conflict with those required for the optimum power requi- 

 rement, a compromise between ship motion and ship powering is 

 necessary. Based on the work given in this paper, a well-balanced 

 design can be developed. 



I - MOTION OF CATAMARAN. 



I . 1 - Back g round . 



One of the obvious advantages of a catamaran is the large 

 available deck area. If this large deck area is to be efficiently utili- 

 zed, it must behave as a stable platform. From a seaworthiness 

 viewpoint there are some special features associated with twin-hull 

 configurations. 



First, an increase of overall beam results in a decrease 

 in natural period in roll. A smaller natural period in roll makes 

 catamarans very jerky ships. Most conventional monohull ships have 

 a greater natural period in roll than in pitch. In case of catamarans, 

 the pitch period may be slightly larger than that of monohull ships of 

 equivalent length and displacement. This fact together with the de- 

 crease in the roll period for catamarans tends to bring the natural 

 period for roll and pitch closer to each other. This could cause 

 simultaneous excitation of large roll and pitch motions, which make 

 very uncomfortable riding for the crews. 



Second, the existence of a cross-deck structure between 



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