Pien and Lee 



the two hulls above the water can result in slamming of the bottom 

 of the cross-deck either by a train of sharp-crested waves or by a 

 large vertical motion of the ship. The slamming of the cross-deck 

 can cause structural damages due to water impact and the hull vibra- 

 tion initiated by the impact. 



Thus, designing catamaran hull forms which could avoid the 

 afore- said disadvantages requires different experience and knowledge 

 of seaworthiness characteristics from what is needed for monohull 

 ships. 



Motion of a ship is mainly excited_by waves, and unless 

 ships are deeply submerged, like submarines, the influence of waves 

 cannot be avoided. Wave influence on the hull could be minimized on 

 a ship whose main hull is submerged and connected to a deck by a 

 vertical strut. This is the main idea behind the semisubmersible or 

 low-waterplane-area catamarans as they are referred to in this 

 paper. 



From a motion standpoint, the concept of the small-water- 

 plane-area-twin-hull-ship (SWATHS) configuration may be traced to 

 the so-called wave-excitationless forms which are extensively studied 

 by Motora and Koyama 1 (1966). This configuration, which has small 

 waterplane area but carries large volume beneath the waterplane, in- 

 creases the natural period of heave, since the natural period is pro- 

 portional to the square root of the ratio of virtual mass to waterplane 

 area. This fact means that only long waves may excite a large motion. 

 Moreover, as investigated by Motora and Koyama, depending on the 

 scantling of the strut width and height and maximum breadth of the 

 submerged hull, the wave damping 2 can be reduced to a small value 

 in a certain frequency range. Smaller wave damping means a smaller 

 wave excitation force and moment ; see Newman (1962). 



Caution is necessary, however, in reducing the damping 

 factor of a dynamic system for the purpose of reducing the forcing 

 function. If we let c be the damping coefficient of a harmonically 

 excited mass-dash pot-spring system and F the forcing function, 



References are listed on page 539. 



2 Here the term "wave damping" means the damping associated 



with generation of progressive water waves which carry away ener- 

 gy supplied by an oscillating body. 



466 



