528 



OBSERVATIONS OF WAKE ECHOES 



sive report on these investigations, a summary of pre- 

 liminary results has been made available for the 

 purposes of this volume. 



The experiments with craft other than the USS 

 Jasper (PYcl3) itself followed a single pattern. The 

 recording vessel, the Jasper, was lying to in the open 

 sea, and the wake vessel approached and passed her 

 while maintaining a straight course. As the wake 

 vessel approached, the Jasper echo-ranged on her, 

 training the sonar projector with the aid of a pelorus 

 manned on the flying bridge. When the wake vessel 

 came abreast of the Jasper at the time of closest ap- 

 proach, the training of the sonar projector was halted, 

 and its true bearing was held fixed and approximately 



Table 6. Decay rate of wake of 40-ft launch. 



15 

 24 

 30 



6.8 + 0.6 

 7.0 + 0.4 

 7.0 + 0.5 



4.0 

 2.5 

 3.0 



20 

 20 

 20 



perpendicular to the wake until the end of the run. 

 When possible, recording was continued, either con- 

 tinuously or intermittently, until no more echoes 

 from the wake were detectable above the background 

 of reverberation. 



When the Jasper was studying her own wake, a 

 different technique was necessarily adopted. In this 

 case, the Jasper ran on a straight course at 12 knots 

 for 10 or 15 minutes; then she turned around and ran 

 ba,ck parallel to her original course with her sonar 

 projector trained abeam so that the sound beam was 

 directed normal to the original course. Recording was 

 continued until a wake echo was no longer discernible 

 above the reverberation. 



A sample graph of the peak echo level received 

 from a wake against time is shown in Figure 7, which 

 also contains the echo levels received from a sphere 

 3 ft in diameter buoyed behind the wake at a depth 

 of 6 ft. On this run three different signal lengths, re- 

 peated in cyclical succession, were used to give wake 

 echoes. This system of interchanging signal lengths 

 was used on a number of wakes. On some occasions 

 the gear used for cycling the pulses was not in order, 

 and five or six echoes were recorded at one pulse 

 length before the pulse length was changed manually. 

 On a few occasions only one pulse length was used 

 throughout the run. 



The initial wake strength is determined by the 

 initial echo level — the level of the wake echo at the 

 time (zero time) when the stern of the wake vessel 

 has just passed out of the sound beam. This time can 

 only be estimated, and sometimes echoes were not 

 recorded until some time after the wake was laid. 

 In such cases the values to be used for initial echo 

 level are obtained by extrapolating the observations 

 available back to the zero time. The decay of the 

 wake is measured as the slope in decibels per minute 

 of the echo level-time curve. Some thought was given 

 tp the possibility of two decay rates in the wake, the 

 dividing line between them being rather sharp in time, 

 but the data were not sufficiently well defined to 

 allow such a distinction to be made. Therefore, only 

 one decay rate was obtained for each wake. This is 

 the rate at which the echoes seemed to decay steadily 

 for several minutes before they became indistin- 

 guishable. 



2 4 6 8 10 



TIME IN MIN AFTER CROSSING SOUND BEAM 



Figure 7. Decay of wake from E. W. Scripps. Run 1 

 of Table 8, 24 kc. 



It seems fairly certain that there is a systematic 

 increase in W with the size of the wake vessel, but 

 Table 7 shows that the magnitude of the effect is not 

 very large. All that can be said about the speed of 

 the wake vessels during these experiments is that 

 they seemed to be running within their normal range 

 of operating speeds. 



These averages include echoes obtained both with 

 10-msec and 30-msec pulses, as the number of avail- 

 able data was small and the increase of W30 maeo over 

 Ww msec is moderate (see Table 8). Presumably, the 

 standard deviation would not be reduced much by 

 separating the results according to signal length. 



The wake strength appears to increase with fre- 



