Testing Ship Models in Transient "Waves 



Fig. 13 - Heave response for 

 Model 4606 at zero speed in 

 head waves; transient test com- 

 pared with regular wave test 

 results 



1.4 



1.2 



LEGEND 



REGULAR WAVES 

 TRANSIENT RUN 4 

 TRANSIENT RUN 6 

 TRANSIENT RUN 3 

 TRANSIENT RUN 5 



'0 .2 .4 .6 .8 1.0 



Frequency in cycles per second 



quite good over most of the frequency range. The solid curves, which are also 

 shown on these plots, were computed on the basis of slender-body hydrodynamic 

 theory by Newman [6] for a roughly similar hull, Series 60, Block 0.70. 



The Newman computation neglects everything but buoyancy. For many 

 models tested at the David Taylor Model Basin, the resonant frequencies of 

 heave and pitch are sufficiently high so that in a zero speed test, the wave length 

 components that produce significant pitch and heave motions act at frequencies 

 which are considerably below resonance. The resulting motions are essentially 

 a wave force measurement or the response of the ship without ship dynamics. 

 The comparison between computed and measured zero speed response is very 

 impressive. 



A detailed frequency analysis was conducted for one test shown in Figs. 13 

 and 14, and wave height transform amplitudes were computed for the same wave 

 program measured without the model in the water. This was done in order to 

 remove the hypothesized effect of model generated waves. A comparison of 

 analyzed wave heights measured under these various conditions is presented in 

 Fig. 15 which shows a large percentage variation in the high frequency range. 



The heave/pitch ratio for these tests is shown in Fig. 16. The ratio demon- 

 strates a considerable consistency except for some of the regular wave values. 

 These deviations, together with Fig. 13, lead to a strong suspicion that the heave 

 measurements during the regular wave tests at 0.4 and 0.55 cps were low. 



The results of this second series of transient tests added further experi- 

 mental support for the utility of transient waves in ship model testing. In most 

 cases, frequency response functions could be estimated with about 10 percent 



523 



