Van Oortmerssen 



DISCUSSION 



C. M. Lee 



Naval Ship Research and Development Center 

 Bethesda, Maryland, U.S.A. 



I would like to refer to the results given in Figures 2 and 4. 

 Figure 2 presents the amplitude of the oscillatory force in the hori- 

 zontal direction for the circular cylinder and Figure 4 presents the 

 drift force in the horizontal direction. At Ka equals. 6, which cor- 

 responds to the wavelength being approximately 10 times greater than 

 the radius of the circular cylinder, the ratio of the drift force to the 

 oscillatory force is about. 3, if we assume that the wave amplitude is 

 of equal magnitude to the cylinder radius. That means that drift force 

 would comprise about 30 per cent of the oscillatory force. Thus, as 

 the author has already indicated, the drift force cannot be ignored in 

 the calculation of wave forces on a vertical circular cylinder. I would 

 like to point out that when there are multiple wave frequencies as in 

 the case of irregular seas there could exist very slowly -varying for 

 ces which in practical cases could be regarded almost like a steady 

 force. The existence of such a force can easily be recognized if we 

 assume two regular waves of slightly different frequencies pinging 

 upon the body. The hydrodynamic pressure on the body contributed by 

 the velocity-square term in the Bernoulli equation would contain a 

 term associated with the difference -frequency of the two frequencies. 



REPLY TO DISCUSSION 



G. van Oortmerssen 



Netherlands Ship Model Basin 

 Wageningen, Netherlands 



I agree with Mr. Lee that the drift force is very important, 

 and especially for moored objects, in irregular seas. The drift has 

 an important influence on the resulting behaviour of the structure. The 

 results presented in this paper dealt only with the drift force in regul- 



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