Testing Ship Models in Transient Waves 



First of all, it was recognized that there is no particular virtue in having a 

 model at the point of contraction of the wave transient, since the amplitude of an 

 ideal wave transform is invariant with position. In fact, there is a strong pos- 

 sibility that nonlinear water or model behavior would be accentuated near the 

 point of highest wave amplitude and that surge modulation effects would be sig- 

 nificant. In addition, passing through a longer program before it coalesced 

 would mean that more controlled energy could be imparted to the wave excita- 

 tion, which, other things being equal, would lessen the effects of extraneous 

 noise. And finally, the practical virtue of not having to conduct a precisely 

 timed meeting of model and wave is still another motivation for altering and ex- 

 tending the duration of the wave transient. 



A second major source of error was believed to be in the wave height meas- 

 urement. Besides the previously mentioned wave reflections from the beach, 

 there was a considerable possibility that waves generated by the model were 

 being picked up by the side wave probe. Although there are some advantages in 

 measuring a wave signal at its geometric reference point, they seem to be out- 

 weighed by the readily demonstrated fact that waves are generated much more 

 efficiently by the model in the abeam direction rather than in the ahead direction. 



Test Series 2 



A second series of transient tests was conducted during January 1963; in 

 these tests an attempt was made to profit from the lessons learned in the initial 

 tests. The model selected was a Series 60, Block 0.60 hull form. Model 4606. 

 Heave and pitch were measured in ahead waves. Attention was focused on the 

 zero- speed case in order to minimize the number of factors affecting the ex- 

 periment. 



During these tests, wave height was measured with two sonic probes. One 

 was placed in the same location as in the previous tests, approximately 12 ft 

 abeam of the model center of gravity, and the other was located 19 ft 2 in. ahead 

 of the center of gravity. 



The wave program employed was considerably longer than that used in the 

 previous test series. The excitation signals from both Series 1 and 2 are dis- 

 played for comparison in Fig. 11, which shows that the duration of the control 

 voltage for the second test was doubled, that is, raised from 40 to 80 seconds. 



A typical transient test recording of this series is shown in Fig. 12. The 

 wave height and motions are seen to be of a form considerably different from 

 that observed during the first tests; they resemble the varying frequency and 

 amplitude characteristics that would be predicted by theory. One immediately 

 obvious result is that the side wave measurement contains a peculiar null in its 

 envelope which is not present in the ahead wave measurement, an anomaly which 

 is most likely due to model-created waves generated to the side. 



In Figs. 13 and 14, respectively, frequency response operators obtained 

 from four transient tests in heave and pitch are compared with regular wave 

 measurements made during the same series of tests. Agreement seems to be 



521 



