ABSTRACT 



Unified slender-body theory has been applied to pre- 

 dicting the motion of small-waterplane-area twin-hull high- 

 speed (SWATH) ships in head seas. Using this theory, 

 numerical results indicate an improvement in accurately 

 predicting the effects of added-mass and damping coeffi- 

 cients when compared with strip theory. However, the same 

 improvement is not achieved in predicting the effects of 

 exciting forces and motions. Corrections in the numerical 

 computation of previous work have been implemented; previous 

 calculations for following seas have also been repeated. 

 The pitch motions of SWATH 6D in following seas are now in 

 good agreement with the results of scale model experiments. 

 However, the motion results of SWATH 6A in following seas 

 show large unexplained peak values when the encounter 

 frequency becomes very small. 



ADMINISTRATIVE INFORMATION 

 This work was performed under the General Hydromechanics Research Program 

 administered by the DTNSRDC Ship Performance Department and was authorized by the 

 Naval Sea Systems Command, Hull Research and Technology Office. Funding was pro- 

 vided under Program Element 61153N, Task Area SR 0230101, and Work Unit 1572-031. 



INTRODUCTION 

 An analytical method for predicting the motions of small-waterplane-area twin- 

 hull (SWATH) ships has been developed using the strip theory developed earlier by 



1* 

 Lee. The method has been further improved for the computation of heave and pitch 



motions in head seas by adding the surge effect on the pitch exciting moment and by 



2 

 correcting the viscous-damping terms developed by the present author. The numerical 



results correlate well with experimental results for a moderate speed range. 



The original strip theory has several limitations in application. First, when 



the encounter frequency is very small, strip theory cannot be properly applied. The 



small encounter frequency occurs when a ship proceeds with a fairly high speed in 



following seas. Second, the strip theory does not predict the motion correctly when 



a ship moves at high speed in head seas. The fundamental assumption of the strip 



theory is that the encounter frequency should be far larger than the longitudinal 



3 

 gradient of the body surface multiplied by the forward speed. 



*A complete listing of references is given on page 41, 



1 



