Motion c id Resistance of a Low-Waterplane Catamaran 



and heave in head seas. It would seem to me that the non-head-sea 

 case may be more critical for certain combinations of ship speed and 

 wave heading angles. If you have calculated these modes of motions, 

 did you consider any viscous effects, particularly roll-damping, as 

 Salvesen, et. al. had in their paper when they included an empiri- 

 cal viscous damping term. 



REPLY TO DISCUSSION 



Choung M. Lee 



Naval Ship Research and Development Center 

 Bethesda s Maryland, U.S.A. 



I will answer Professor Newman's discussion first. As I 

 discussed in the paper, the under-estimation of the damping is expect- 

 ed to come from the deficiency of the 'strip theory which cannot take 

 care of a three-dimensional flow when a catamaran has forward 

 speed. I agree with Professor Newman's viewpoint that the forward 

 speed effect on the motion of LWP catamarans could well be repre- 

 sented by a thin- ship approach. The difficult part is to incorporate the 

 three-dimensional wave interference between the two hulls. 



As to Professor Beck's question, I have pointed out that I 

 formulated for five degrees of freedom of motion. The only numerical 

 results we have obtained so far is up to coupled heave and pitch mo- 

 tion in head seas. As to roll damping, I suspect that we may have to 

 introduce a similar kind of supplemental damping to the one introduc- 

 ed in the heave and pitch motion for LWP catamarans. However, at 

 the present stage we are not sure if the supplemental damping should 

 necessarily be governed by viscous effect alone as treated by Salve - 

 sen, Tuck and Faltinsen in their SNAME paper of 19 70. More signifi- 

 cant factor governing the damping could be the forward speed effect. 

 I hope that our future investigation on this subject would clarify the 

 question. 



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