Vevhagen 



TABLE I 



The low frequency drifting force 



This part of the drifting force has been estimated as follows; 

 The measured wave height record t,a(t) is squared £,^(t). This 

 squared wave height function can again be analysed by a spectral 

 analysis. The spectral density function of jl, (t) is determined. 

 For an example see Fig. 15. As can be seen from this figure it 

 contains the sum and difference frequencies of the original wave 

 height spectrum. The difference frequencies are now of special 

 Interest. They are related to the envelope of the original wave 

 height record. 



The r.m.s. value of the low-frequency energy variation Is; 



E di 



.= [ I 



1/2 



Sj^ 2((o) dw 



diff. freq. 

 (TcAff Is proportional to the mean square value of the fluctuating 

 parr of the wave height envelope. The energy In the fluctuating part 

 of the envelope curve depends largely on the occurrence of "high" 

 waves. As discussed earlier the occurrence of "high" waves Is pro- 

 portional to 1/T per unit time. So the relation between the r.m.s. 



974 



