I- 

 I 



The Influence of Freeboard on Wetness 



40 



30 - 



20 



10 



10 



20 



30 



40 



W WIND SPEED (Jinots"^ 



50 



Fig. Z - Significant wave height vs 

 wind speed 



REPRESENTATION OF THE SEA 



Sea spectra are needed in the analysis in order to obtain motion response 

 spectra and a modified form of the Darbyshire formulation has been used. The 

 curve of significant wave height against wind speed shown in Fig. 2 was used and 

 the three Darbyshire spectra are shown in Fig. 3. 



The equation of the Darbyshire spectrum is 



hJ "^^ " ^^-^ ^''P \ 0.00847 [(f-f°) + 0.042: 



df 



where 



H^ = IE 



f > 



h/ = spectral ordinate, 

 f = frequency, 

 f^ = frequency of the peak value of the spectrum. 



H,/3 = 1.65H. 



This latter value of Hj^j is that derived by Darbyshire from his analysis. 

 The spectrum in this form cannot be combined directly with response operators 

 which are expressed in terms of wave length to ship length ratios, nor in fre- 

 quencies of encounter. If the response curves for one ship speed and for varying 



599 



