TESTING SHIP MODELS IN 

 TRANSIENT WAVES 



Lt. Cdr. M. C. Davis, USN and Ernest E. Zarnick 



David Taylor Model Basin 



Washington, D.C. 



ABSTRACT 



The seaworthiness characteristics of a ship design are often deter- 

 mined by a series of model tests in regular waves. This report de- 

 scribes a new nnodel test procedure w^hich makes use of a transient 

 •wave disturbance having energy distributed over all ■wave lengths of in- 

 terest. With the use of this transient wave technique, the testing time 

 required to characterize a model is reduced by an order of magnitude. 

 In this report, the basic behavior of a unidirectional transient wave is 

 discussed, and a simple Fourier transformation is developed in order 

 to link wave height records measured at any two separated points along 

 the path of such a wave. A particular form of transient wave, which is 

 approxinnately sinusoidal with linearly varying frequency, has been 

 used to test successfully a number of shipmodels. The results of these 

 tests are presented and the practical problems in generating, measur- 

 ing, and analyzing transient wave tests are discussed. 



INTRODUCTION 



The linear theory of ship motion in a random seaway has become generally 

 accepted as a useful approximation to the actual nonlinear phenomena involved 

 in ship-wave interaction. As outlined in the pioneering work of St. Denis and 

 Pier son [l] the random sea surface can be visualized as a superposition of two- 

 dimensional sinusoidal waves continuously distributed in amplitude, wave length, 

 and direction. The total ship response in any degree of freedom is found from a 

 summation of the responses to each individual wave component. 



The primary role of a ship model testing facility, in providing information 

 for quantitative full-scale motion prediction using this theory, is to measure the 

 response of a model to sinusoidal waves of unit amplitude over the entire range 

 of ship speed, wave length, and direction of encounter. The Harold E. Saunders 

 Maneuvering and Seakeeping Facility, located at the David Taylor Model Basin, 

 is admirably suited to conduct such an investigation in head and oblique waves. 



The scope of a complete model measurement program is without parallel in 

 other engineering fields which perform frequency response testing of dynamic 

 systems. A numerical example will illustrate the large number of tests which 



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