3 

 Hydro-dynamic Design of an S Semi submerged Ship 



plane area. Therefore, in head seas, the maximum heave and pitch 

 motions are usually at wave lengths approximately five times the 

 length of the hull. This means that a long LWP catamaran (say longer 

 than 300 feet) will only experience large pitch and heave motions in 

 extremely severe sea conditions. On the other hand, a 100-foot cata- 

 maran will experience in ocean operation maximum pitch and heave 

 motions a large percentage of the time. 



This aspect of the motion responses of LWP catamarans is 

 well-known ; however, it is less recognized that the added mass and 

 damping coefficients as well as the exciting forces are all much 

 smaller than for conventional hulls and that the maximum pitch and 

 heave motions are extremely sensitive to small changes in these 

 quantities. In particular, it is important to recognize that the damping 

 coefficient for certain catamaran configurations can be so small that 

 it results in pitch and heave motions several times larger than for 

 conventional hulls as shown in Reference 1. 



Another seakeeping aspect which deserves attention is the 

 pitch and heave motions in following seas. Some of the LWP catama- 

 rans have about twice as much pitch motions in following seas than 

 conventional monohulls and the maximum pitch motions occur at wave 

 lengths of the order of magnitude of ship length (see Reference 1). 

 Dr. Lang has demonstrated that these vertical motions in following 

 seas can be considerably reduced by use of automatic control surfaces. 

 I would like to ask Dr. Lang if he is of the opinion that LWP catama- 

 rans in general will need automatic control surfaces in order to have 

 acceptable motions in following seas. 



SALVESEN, N. , "Seakeeping Characteristics of Small-Water- 

 Area-Twin-Hull Ships", presented at AISS, SNAME, USN Ad- 

 vanced Marine Vehicles Meeting, Annapolis, Maryland, 17-19 

 July 1972, and published in the Journal of Hydronautics, Vol. 7 

 No. 1, Jan 1973. 



575 



