Hull General 

 Surface Boundary or Comparisons 



Method Description Evaluation Specific (Accuracy) 



Pinkster & Strip Near Field Beam Limited 



Van Oortmerssen waves 



(1977) only 



Kim & Dalzell Strip Near Field All wave Limited 



(1979) Directions to 



Slendership 



Molin (1979) 3-D Source Far Field - Similar to 

 Sink Faltinsen & 



(Diffraction) Michelsen 



It appears that for determination of the mean drift force, 

 the method of Faltinsen and Michelsen (1974) has the most 

 general and immediate applicability, whereas, the method of 

 Kim and Dalzell (1979) offers significant advantages for the 

 future . 



For the problem of predicting the slowly varying forces, the 

 method of Kim and Dalzell (1979) is a very rigourous and 

 sophisticated approach from which the mean drift force may 

 also be obtained. The unique feature of this approach is the 

 "near field" evaluation of the boundary condition where the 

 pressures are integrated over the hull surface. This approach 

 discloses the effects of the response motions and other body 

 properties, which are obscured and/or averaged out with the 

 "far field" approach. 



The mean drift force was shown to be composed of five terms 

 which are a function of: 



2 



a. (Relative wave height) 



b. Integral (around the hull surface) of the square 

 of the velocity. In other words, the Bernoulli 

 quadratic. 



C.I Two terms related to the radiation potential: 



> One term is due to gyroscopic coupling, and 

 d.J the other is a force rate x displacement 



e. Second order potential. 



Kim and Dalzell (1979) have shown that (in at least one case) 

 the first two terms are dominant and that the second order 

 potential term may be neglected. It should be remarked that 

 the procedures reported by Kim and Dalzell and by Pinkster 

 and Van Oortmerssen both make use of the "weak scattering" 

 assumption introduced by Salvesen. This is not quite the 

 same as the slender body assumption although for certain wave 

 directions, the assumptions have the same effect. 



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