2500 



500 + 



Fig. 5. Tests at Stevens Institute towing tank on 3 June 1962: (a) oscilloscope, 

 single sweep, 0.2 ms/cm, (b) Panoramic, single sweep, ^-second tape loop, 

 (c) Panoramic, l6 sweeps, continuous segment and (d) Panoramic, 60 sweeps, 

 continuous segment. 



is no appreciable broadening after 16 consecu- 

 tive sweeps (Fig. 5c) and there is still a sharp 

 frequency spike after 60 consecutive sweeps 

 (Fig. 5d). 



An interesting phenomenon was noted in the 

 offshore Bimini tests where the sea was essen- 

 tially still. The meter output was zero except 

 when long gentle swells produced a Doppler shift 

 output coincident with the below surface orbital 

 motion. There was a poor signal to noise ratio 

 (Fig. 6a). A broad plateau contains the entire 

 forward range of the orbital water motion caused 

 by the swell (Fig. 6b, 6c and 6d) except for 

 the frequencies below 50 cps which were cut off 

 by the combined tape recorder and spectrum 

 analyzer response. 



Frequency meter indications were generally 

 lower than those of the Panoramic analyzer 

 because the instantaneous "peak" frequency is 

 not as well defined or densely distributed in 

 time as appears on a well integrated record. 



Tests at the Lerner Marine Laboratory dock, 

 where near zero wind speed and seemingly low 

 turbulence conditions prevailed, show a more 

 complicated wave structure than produced in the 

 tank (Fig. 7a) . Note the broader structure and 

 some sidebands of Fig. "7b in contrast to Fig. 6. 

 The sidebands were caused by turbulence and/or 

 instantaneous variation in flow velocity (Fig. 7c). 

 Additional integration time produced a somewhat 

 denser version (Fig. 7d) and the low frequency 

 peak is due to reflections from a barge riding at 



131 



