Gerritsma and Beukelman 



The differences, therefore, are seen to be completely a result of a different 

 assumption of the importance of forward speed on strip theory, independent of 

 what axis system is used. The assumption of Gerritsma and Beukelman seems 

 to be justified by experiment. The derivation of the equations of motion by 

 Vassilopoulos and Mandel, due to Abkowitz, seems the most rigorous and satis- 

 fying. However, the evaluation of the motion derivatives by Gerritsma and 

 Beukelman, due in part to Korvin-Kroukovsky, seems to yield better results. 



This discusser therefore feels it most practical to use the former work to 

 study the mathematics of motion and the latter to evaluate the motion derivatives. 



REPLY TO THE DISCUSSION BY E. V. LEWIS 



J. Gerritsma and W. Beukelman 



Technological University 



Delft, Netherlands 



The authors are grateful to have Professor Lewis' comments on their paper. 

 The definition of m, which is omitted in the paper, is given by 



m'dx = m = a . 



It should be noted that 



and not 



/ 



xdm' = - I m'd> 

 I X dm' = I m'dx , 



as suggested by Professor Lewis. 



The work reported in this paper was recently extended for the zero forward 

 speed case. 



These tests were carried out in a wide basin to avoid wall influence, due to 

 reflected waves. The results support the conclusions of the present paper. 



Within the very near future the restrained tests in waves with the segmented 

 model will be carried out in our Laboratory. The results will be compared with 

 calculated values. 



* * * 



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