Added Resistance of Ships in Waves 



assumption leads to a first order term in the potential which does not contain 

 forward speed effects. We may expect, therefore, that the expression for the 

 added resistance is also independent of the speed. 



Therefore we shall apply the expression for the drift force at zero forward 

 speed to the problem of the moving body as well: ar= f,^. For the determina- 

 tion of the frequency parameter k, we shall use the frequency of encounter: 



27Tg 



1/2 



277 



V COS /3 



Although not being consistent with the mathematical theory we shall apply this 

 procedure, since we know that it gives very reliable results in determining the 

 ship motions. 



THEORETICAL AND EXPERIMENTAL RESULTS 



To determine the drift force and added resistance for an actual ship, a 

 computer program has been written based on Eq. (3). The values of the potential 

 4>^, the motion amplitudes, and the phase difference are calculated by a method 

 of Tasai (7) for a two-parameter family of cross sections. 



Numerical results are obtained for a "Series 60" model (Cg = 0.65) in head 

 waves at zero forward speed. To compare the results with experimental data, 

 the model was tested in the sea-keeping laboratory of the Netherlands Ship 

 Model Basin. The results are plotted against the wavelength ratio \/L in Fig. 1. 

 The agreement between theory and experiment is satisfactory for wavelengths 



Fig. 1 - Longitudinal force component F^ 

 vs wavelength ratio \/L for "Series 60" 

 model (Cg = 0.65) (F^ = 0) in head waves 

 at zero forward speed 



643 



