FATHOMETER STUDIES 



497 



Figure 5. Wake of submarine during crash dive. 



Figure 6. Swirl behind submarine after crash dive. 



is only about 0.1 degree, or one-tenth of the diver- 

 gence of the acoustic wake. In addition, the thermal 

 wake appears to be much narrower than the acoustic 

 one. 



Since the thermal and acoustic measurements were 

 not made on the same day, it is by no means certain 

 that the thermal and acoustic wakes behave as 

 differently as these observations would seem to sug- 

 gest. The 1-degree divergence found acoustically ap- 

 plied to wakes less than 10 minutes old, while the 

 available thermal data were apparently all for wakes 

 more than 10 minutes old. Furthermore, the rate of 

 widening may possibly depend on oceanographic 

 factors, such as the temperature gradients in the 

 surface layers of the sea. 



31.3 



FATHOMETER STUDIES 



At the U. S. Navy Radio and Sound Laboratory 

 [USNRSL], numerous measurements with a record- 

 ing fathometer have been carried out on the wakes of 

 a number of different surface vessels and submarines.' 

 Some of these wakes were investigated systemati- 

 cally, and the width and depth of the wake was de- 



termined as a function of the distance from the wake- 

 laying vessel and of its speed. 



31.3.1 



Surface Vessel Wakes 



With surface ships, two methods were used for 

 measuring the wake width. When ranging on the 

 wakes of ships which happened to be passing, the 

 survey boat, carrying the fathometer, crossed the 

 wake as nearly perpendicularly as could be judged 

 while speed and distance measurements were made. 

 Some inaccuracy arose in the judgment of the angle 

 of crossing when very far behind the wake vessel. 

 However, the error introduced into the measured 

 mdth by assuming perpendicular crossing was usu- 

 ally negligible. When the survey boat was still farther 

 astern, a greater error was present in determining the 

 onset and disappearance of the wake record. The 

 duration of recording was measured on chart paper 

 (see Figures 3 to 6 in Chapter 30) by a caliper and 

 rule. Fi'om the speed of the chart paper and of the 

 survey boat, the width may be calculated. 



In the other method, which was suitable at close 

 range when working with an assigned vessel, the 



