498 



WAKE GEOMETRY 



Figure 7. Wake of PT boat at 25 knots. 



survey boat was towed by the vessel laying the wake; 

 the latter is called the wake vessel. Measurements can 

 then be made on a wake whose lifetime is effectively 

 constant, in other words, for a constant boat speed, 

 the survey boat is in a wake of the same age at all 

 times. While the wake vessel maintained a steady 

 course, the survey boat under tow was moved in and 

 out of the wake on either side by using the helm. At 

 the moment the survey boat passed the wake edge, 

 as indicated by the fathometer record, the record was 

 marked and a signal was sent to two observers on the 

 wake vessel. One of these observers was on the stern 

 and followed the transverse movement of the survey 

 boat with a pelorus. At the instant of signaling, the 

 angle of the fathometer mounting on the survey boat 

 relative to the axis of the wake ship was noted. The 

 other observer was on the bridge, and at the signal he 

 instantaneously observed the ship's compass course, 

 for use in correcting for the angle of yaw of the wake 

 vessel. The record was marked at the time of sig- 

 naling the observers on the wake vessel, so that the 

 data could be discarded if it were found that the 

 survey boat was not exactly at the wake edge. 



The wake of the Jasper (overall length 127 ft, 

 draft 12 ft, beam 23 ft) was studied by the second 



method over the range from 50 to 500 ft astern. All 

 the measured widths, expressed in feet, agree with the 

 formula 



w = Wo + 0.0625y<, (1) 



where Wa is the extrapolated width at the stern of the 

 wake vessel, v the speed of the ship in feet per second, 

 and t the time in seconds since its passage. By dif- 

 ferentiating this formula with respect to the time, 



1 dw 

 = 0.0625 = sm a , (2) 



V at 



where a, the total angle of divergence of the wake 

 edges, is 3.5 degrees. For similarly small distances be- 

 hind destroyers, the wake edges were found to include 

 an angle of about 50 degrees (see Section 31.1). The 

 conspicuous discrepancy between this value and the 

 corresponding one for the Jasper is doubtless due to 

 the different type of construction of these ships. The 

 extrapolated value Wa = 10 ft, in formula (1), is very 

 nearly one-half the ship's beam. At distances greater 

 than roughly 5 ship lengths, the divergence of the 

 Jasper's wake ceased at a width of perhaps two and 

 a half times the ship beam; only random measure- 

 ments by the first method were available for this 

 region, however, and thus a small divergence angle of 

 about 1 degree caimot be ruled out as far as large 

 distances behind the Jasper are concerned. 



The measurement of wake thickness was carried 

 out by proceeding into a wake and either remaining 

 in its center while measuring distances and speeds, 

 or by crisscrossing in order to investigate the thick- 

 ness at points across the wake. Crisscrossing was 

 necessary in order to locate the wake when operating 

 at distances when the wake was not visible. A given 

 wake will frequently have different acoustic trans- 

 parencies at different points along its width. In some 

 cases the thickness is the same along the width, and 

 the greater transparency at the edge is caused by its 

 less effective scattering properties. In other cases the 

 wake is thinner at the edges. Some wakes are quite 

 flat at the bottom, others are rounded at top and 

 bottom, or may have one side which sinks below 

 the other at both top and bottom. 



A wake cross section asymmetrical in the vertical 

 plane parallel to the beam of the vessel was frequently 

 observed and is apparently correlated with wind di- 

 rection. Such records were first noted when operating 

 with one engine of a twin-screw vessel and were 

 thought to be the result of this asymmetrical source. 

 The wake of a sailing vessel was investigated next, 

 and was found to have an even more pronounced 



