496 



WAKE GEOMETRY 



Figure 3. Wake of surfaced submarine at 15 knots. 



Figure 4. Wake of surfaced submarine at 20 knots. 



almost without ripples, during the entire day, and the 

 Scripps maintained the same speed of 9.5 knots dur- 

 ing all four runs. Evidently, the geometric and 

 physical properties of wakes are difficult to reproduce 

 in repeated experiments, even under ideal weather 

 conditions. The wake laid in Run 2 gave distinct 

 60-kc echoes at an age of 30 to 40 minutes; during 

 this period the wake width measured on the sound 

 range recorder trace increased from 82 to 100 yd, 

 corresponding to a rate of widening of about 2 yd 

 per minute. No similar tests for the persistence of 

 wakes were made during the other runs. 



Comparison of Figures 8 and 9 reveals that there 

 is no systematic difference between the widths found 

 for different sound frequencies. In particular, the 

 alternating use of 45 and 60 kc during Run 4 gave 

 results which are mutually consistent and agree 

 quite well with the 24-kc graphs. 



At the stated speed of the Scripps, approximately 

 300 yd per minute, a rate of widening of 5 to 6 yd per 

 minute means that the total angle of divergence of 

 the wake is about 1 degree. This figure is in excellent 

 agreement with the angle of divergence found for 

 destroyer wakes from aerial photographs. If the 



Scripps wake had the same great initial angle of 

 divergence (about 50 degrees) as the destroyer wakes, 

 it could not have been discovered by acoustic width 

 measurement, because this method lacks the neces- 

 sary "resolving power" along the time axis. At wake 

 ages greater than 10 minutes, the rate of widening 

 appears to decUne steadily, and the angle of diver- 

 gence must decrease correspondingly. However, it 

 should be remembered that these observations were 

 made on a calm sea. 



The rate of widening of thermal wakes can be 

 studied by carrying a sensitive thermocouple across 

 the wake at increasing distances astern. These in- 

 vestigations are still in an exploratory stage, but 

 they are mentioned here because preliminary results 

 have been reported for two wakes laid by the E. W. 

 Scripps.^ Thus a comparison of the thermal and 

 acoustic dimensions of the wakes laid by the same 

 vessel became possible. Between the ages of 10 and 

 60 minutes, the thermal wakes of the Scripps showed 

 a linear increase in width from 30 to 50 yd. The rate 

 of widening is about 0.4 yd per minute and the speed 

 of the Scripps was 6 knots, or 200 yd per minute. 

 Hence, the angle of divergence of the thermal wake 



