Understanding and Prediction of Ship Motions 



which were used. When the 5.82 ft ship model is scaled up to a 392 ft prototype 

 length, wave conditions A, B, and C correspond respectively to sea states 5, low 

 7, and high 7. The hull form used was that of a DD692 destroyer. Wave condi- 

 tion C was the most severe that could be produced in the Davidson Laboratory 

 tank; from Fig. 1 it may be noted that the significant wave height for wave C was 

 9.3% of the model length. 



Figures 2-4 show typical results from Dalzell (1962b). They present the 

 pitch f.r. function, both amplitude and phase, for three speeds which correspond 

 to F = 0, 0.18, 0.37. The abscissa of these curves is a non-dimensional fre- 

 quency, obtained by dividing the actual frequency of encounter by the frequency 

 of a wave with wavelength equal to model length. The ordinate for f.r. function 

 amplitude is |t (co)\l/7t and for phase the ordinate gives the lag of maximum 

 bow-up pitch after the wave crest at the longitudinal center of gravity (LCG), in 

 degrees. These quantities were calculated from wave and motion records by 

 standard spectral techniques, although some manipulating of the wave height 

 record was necessary, since the probe was located ahead of the model bow, and 

 surface elevation was required at the LCG.* 



1.2 



1.0 



0.8 



3 0.6 



0.4 



0.2 



WAVE A 



- 300 < 



- 270 I 



WAVEC 

 WAVE B 



WAVE A 



240 t 



0.2 



0.4 



0.6 0.8 1.0 



FREQUENCY, a 



1.2 



1.4 



Fig. Z - Dalzell' s pitch frequency response function, 

 Model DD 692 Froude number = (from Dalzell (1962b)) 



*The reference elevation at the LCG was the wave height which would have oc- 

 curred there in the absence of the model. 



13 



