Force Pulse Testing of Ship Models 



generating wave pulses. An oscillatory excitation is imposed on the model by 

 means of a variable speed drive which sweeps from the highest frequency de- 

 sired down to the lowest frequencies which can be treated in our basin. Because 

 of the method of generation, the very high frequency content of the step or im- 

 pulse response is avoided. Because of the shape of the pulse, the separation 

 into the various frequencies is achieved with good accuracy. And because of the 

 length of the pulse, the intense concentration of the information is eased. 



The third achievement is a system of significantly improved absolute accu- 

 racy, about two percent. The present limiting factor is the use of magnetic tape 

 in the data handling path. It is possible that the use of tape can be avoided, with 

 a further significant improvement. 



The last major advance has been the use of a new system for converting 

 data from analog to digital form. This system has the capability of converting 

 as many as 6,000 data spots per second, distributed among the various channels 

 of data. It has been possible to sample the data at a rate of 30 spots per cycle 

 of the highest frequency investigated. 



The net result of all these improvements has been a very high signal to 

 noise ratio. In the range of greatest interest, the noise is 45 db below the sig- 

 nal level. As a result, we have been able to characterize the model from fre- 

 quencies so low that shallow water and wall effects become significant (in a 

 basin 240 ft x 360 ft x 20 ft deep!) up to higher frequencies than any previously 

 investigated. And this entire range was covered in a pair of runs lasting per- 

 haps 50 seconds. 



The system has been in use only a short time, and we have much more to 

 learn about it. The earlier, unreported tests produced a vast amount of infor- 

 mation about how not to run the experiment. This time we have been more suc- 

 cessful, but we have discovered a number of additional refinements which will 

 be necessary before we can do all that we wish with the system. These proposed 

 changes will be discussed in a later section. 



THE EXPERIMENT 



This initial experiment was primarily designed to provide an evaluation of 

 pulse techniques as a method of obtaining the frequency response relationship 

 between exciting forces and the motions of a ship. A Series 60 Block 0.60 ship 

 form was oscillated in pitch and heave. All forces and responses were meas- 

 ured and the damping and added mass terms in pitch and heave were computed. 

 This permits a direct comparison with the results obtained by Gerritsma [5] for 

 a similar form. 



Experimental Details 



As the effect of surge upon heave and pitch is generally considered to be 

 small, it was decided to restrict the analysis to these latter two modes only. 



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