LISTENING AND ECHO RANGING 



487 



Figure 4. Fathometer record of wake echoes from Coast Guard cutter Ewing. 



is a result of the sound traveling to the ocean surface 

 twice and returning again to the submarine. The 

 dark streaks at the top of this figure result from the 

 acoustically reflecting region formed behind the 

 submarine conning tower, presumably as a result of 

 cavitation originating around the conning tower. 

 The record at the far left is that of the ocean surface 

 after the submarine arrived at a depth corresponding 

 to the scale limit of the recorder and the scale was 

 shifted to bring the record nearer the center of the 

 paper. The small indentations and undulations of the 

 record are produced by the surface swells. 



Reflection from a surface ship wake under which 

 the submarine is passing produces in these records a 

 shaded area protruding below the ocean surface, as 

 shown in the next three illustrations. Figure 4 repre- 

 sents the record of a wake laid by the USCGC Ewing, 



proceeding at 13 knots. The submarine in this case 

 passed under the wake at a point 350 yd behind the 

 Ewing. This record was suitable for transmission loss 

 calculations, according to the principles which will be 

 described in Section 32.3.3. The result was a trans- 

 mission loss of 42 db with 21-kc sound traversing the 

 wake twic" Note that as a result of the large trans- 

 mission loss, the record of the ocean surface is almost 

 blotted out in the center of the wake, which had a 

 thickness of 15 ft. 



The same effect is apparent in Figure 5, showing a 

 wake record originating from the destroyer, USS 

 Hopewell (DD681), proceeding at 10 knots. The 

 distance astern is not accurately known, but it is 

 roughly several hundred yards. The transmission 

 loss at 21 kc through the center of this wake was 

 32 db for the double path. The cause of the extrane- 



