Davis and Zarnick 



of the exciting wave and (2) transfer function computation would not involve the 

 ratio of two rapidly varying transform amplitudes. 



Another desirable feature of an adequate comparison between transient and 

 regular wave testing was highlighted by the variations in heave transfer function 

 measurement using regular waves (as observed in Fig. 13). To minimize ques- 

 tions as to the accuracy of tests in regular waves, a large number of tests should 

 be conducted throughout the frequency range — many more than are usually 

 called for in routine testing. 



Test Series 3 



The third and final series of transient tests to be presented in this report 

 was conducted in April 1963, using Model 4889. Heave and pitch were again 

 measured in ahead waves, and wave height was provided by two sonic probes 

 mounted 12 and 20 ft ahead of the model center of gravity. 



The program used in this series is shown in Fig. 17. Based on the observed 

 frequency behavior of previous transient tests, both amplitude and frequency 

 sweep rate were varied so as to yield the proper cancellation of basin frequency 

 response. 



10 Sec 



Fig. 17 - Excitation voltage program 

 for third series of transient tests 



Regular and transient tests were conducted at zero speed and at a model 

 speed corresponding to a Froude number of 0.14. The transfer function plots — 

 heave, pitch, and heave/pitch — are presented in Figs. 18, 19, and 20, respec- 

 tively, for zero speed. The corresponding phase data are presented in Fig. 21. 

 The agreement between the regular wave tests and the transient tests is impres- 

 sive. Of special interest is the heave/pitch ratio, which, of course, is independ- 

 ent of wave-height measurement error. The agreement between regular and 

 transient wave tests shown in these figures presents the strongest indication of 

 the potential accuracy of the transient technique, and incidentally, of the linear- 

 ity of motion response of a ship in waves. Note also that the pitch transfer 

 function in Fig. 19 demonstrates, convincingly through a close- spaced series of 

 regular wave tests, the lack of smoothness of pitch response when examined 

 in detail. 



526 



