Force Pulse Testing of Ship Models 



Fig. 4 - Analog measurement system 



about 20 seconds, care being taken to see that the exciter was stopped at zero 

 amplitude. Recording continued until any remaining motions had ceased. Typi- 

 cal recording time ranged from 28 to 48 seconds, depending on carriage speed. 

 Test results were as shown in Fig. 5. It should be recognized that the towing 

 structure, and thus the towing carriage, was used as a reference for all meas- 

 urements. 



When the model was excited by a force restricted in bandwidth, with negli- 

 gible energy above 5 cycles per second, the measured signals were excellent, 

 with a dynamic range and accuracy limited only by the 40 db range of the analog 

 tape recorder. However, when a relatively broadband signal was used, such as 

 an impulse or step function, the model, strut and carriage structure were ex- 

 cited at their own natural frequencies and produced oscillations which almost 

 completely masked the motion responses of the model. 



DATA ANALYSIS 



One of the inherent disadvantages of a digital record is that it provides in- 

 formation about the corresponding time function at the sampled instants only. 

 Discrete samples completely define a continuous function, f(t), only if the func- 

 tion is absolutely band- limited, and then only if the sampling frequency is at 



445 



