Smith and Cummins 



speed of the drive system from to 3.5 cycles per second. The amplitude of 

 the excitation eccentric was fixed at one inch. The frequency spectrum of the 

 excitation signal is shown in Fig. 3. 



,vw 



Fig. 3 - Excitation force spectrum 



The measurement system was as shown in Fig. 4. DTMB block gauges 

 were used to measure forces and moments. High resolution film t3rpe potenti- 

 ometers were used to measure the motion. All data was recorded simultane- 

 ously on Sanborn strip chart recorders and FM analog magnetic tape. The in- 

 strumentation system, exclusive of recorders, has an accuracy of 0.2 percent, 

 a dynamic range of 60 db, and a frequency response which is essentially flat 

 from to 100 cycles. Phase shift between any two channels was held to less 

 than one part in 10,000. The tape recorder, however, the system's weakest link, 

 is accurate to only 1-1/2 to 2 percent, and its dynamic range is limited to 38 to 

 42 db. 



Test Procedures 



As previously mentioned, tests were conducted over a range of Froude 

 numbers, from to 0.4. For each test condition, the model and carriage were 

 accelerated to the appropriate speed with the oscillator turned off. Care was 

 taken to ensure that the model had reached a steady state condition and that all 

 energy in the model-free surface memory had dissipated. When steady state 

 conditions were established, the recorders were turned on and allowed to run 3 

 to 5 seconds before the excitation. Excitation was started with an initial fre- 

 quency setting of 3.5 cycles per second, and was swept from 3.5 cps to in 



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