RATE OF WIDENING 



495 



FiGUBE 1. Wake of surfaced submarine at 6 knots. 



FiGTjRE 2. Wake of surfaced submarine at 10 knots. 



the long wake proper, whose edges show little diver- 

 gence. Thus, the general wake contour is quite similar 

 for destroyers and surfaced submarines. 



Figure 7 gives a remarkable aerial view of a PT 

 boat and its wake. The wake proper is narrow and 

 compact, without visible structure, but the bow 

 wave, for a distance astern of several ship lengths, is 

 visually much longer than the wake. 



31.2 



RATE OF WIDENING 



The rate of widening of an acoustic wake can be 

 determined by measuring the gradual increase of the 

 duration of the echoes obtained with a horizontal 

 sound beam, as long as the signal length is much 

 shorter than the wake width. In practice, subtracting 

 the signal length from the measured length of the 

 echo will correct for the prolongation of the echo 

 length due to the finite signal length and will make 

 possible a direct determination of the wake width. 



An analysis along these lines was made for four 

 wakes laid by the E. W. Scripps on November 28, 

 1944. The Scripps passed between the echo-ranging 

 vessel, the USS Jasper (PYcl3), which was hove to, 

 and a target sphere 3 ft in radius buoyed at a center 



depth of 6.5 ft. The wakes were laid at right angles to 

 the line connecting the sphere with the Jasper, each 

 run being made in a new location of undisturbed 

 water. All echoes were recorded oscillographically 

 and sound range recorder traces were obtained simul- 

 taneously. The signals consisted of pulses of 0.5, 1, 

 and 3 msec long, transmitted in cyclic succession. 

 The duration of the 3-msec echoes was measured 

 both on the oscillograms and on the recorder traces; 

 the results, expressed in yards, are plotted in Figures 

 8 and 9 as functions of the time elapsed since the 

 Scripps passed. The slope of these curves is the rate 

 of widening. In order to find the width at any time, 

 the plotted values of the echo length should be 

 diminished by 2.4 yd. 



The average rate of widening of the Scripps wake, 

 up to the age of 10 minutes, is 5 yd per minute for the 

 chemical recorder traces, and 6 yd per minute for the 

 oscillograms. This difference of 1 yd per minute can 

 hardly be regarded as significant, in view of the dif- 

 ferences between the several runs. Note that in both 

 illustrations the graph of Run 2 is located between 

 5 and 10 yd above the graph of Run 1, though both 

 runs were made with 24-kc sound. The origin of this 

 shift remains obscure, as the sea was unusually calm. 



