ECHOES FROM SUBMARINE WAKES 



525 



AGE OF WAKE IN MINUTES 



2 3 





400 600 



DISTANCE ASTERN IN YARDS 



Figure 5. Dependence of wake strength on distance astern. Plot for USS S-18, submerged to a depth of 45 ft, for run 2 

 of Table 1. Echo-ranging vessel and submarine were proceeding on parallel courses at constant speeds of 8 and 6 knots 

 respectively. 



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plot illustrates some significant features of the obser- 

 vations. The individual points of the diagram are 

 computed from the average of five successive echo 

 levels, and the scattering of these averages gives a 

 good idea of the magnitude of echo fluctuations en- 

 countered in practice. Signals 10, 30, and 100 msec 

 long were sent out in cyclic succession, so that the 

 three curves for the different pulse lengths refer to the 

 same wake. Despite the large echo fluctuations, there 

 is good evidence for an increase of W with the signal 

 length To. The steep rise of the curves at zero distance 

 astern probably is due to the stern of the submarine. 



Up to 500 yd astern — corresponding to a wake age 

 of 2 minutes — the wake strength changes very little, 

 if any. But when the observations were resumed at 

 670 yd astern, the decay of the wake had definitely 

 set in. The values given in Table 4 suggest that for 

 this wake the decay rate increased with increasing 

 pulse length. 



All reliable numerical values of W have been col- 

 lected in Table 4, together with the values of the wake 

 index used in the individual computations. The latter 

 will permit the computation from W of the corre- 

 sponding target strength of the wake, if desired. The 



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