500 



WAKE GEOMETRY 



wake should change to a mirror image of its former 

 geometry, but in the majority of cases this expecta- 

 tion is not entirely confirmed. Perhaps some as yet 

 undiscovered parameter is responsible for this puz- 

 zling behavior. 



results from actual variations of the wake structure. 

 The wake thicknesses were plotted as a function of 

 the distance astern and examined for a possible 

 systematic variation. The slope of the bottom of the 

 wake up to 800 yd astern was found to be 5 minutes 



These ratios are smaller than those previously found ^ for incidental destroyer wakes, and 

 are believed to be more accurate. 



Aside from the miscellaneous results just described, 

 an attempt was made to investigate systematically 

 the variation of the thickness h of the wakes of two 

 yachts, the USS Jasper (PYcl3) and the E. W. 

 Scripps, with distance astern up to 3,000 ft and with 

 speed from 3.5 to 11 knots. For either vessel, no 

 systematic changes of h could be noted. A fair average 

 of all measurements of h was 1.70 times the draft, or 

 2.9 times the screw depth for the Jasper, and 1.11 

 times the draft, or 3.0 times the screw depth for the 

 Scripps (overall length 104 ft, draft 12 ft, single 

 screw 8 ft above the keel). 



Scattered measurements made on the wakes of 

 numerous large surface vessels of all types gave an 

 average ratio of thickness to draft of 2.02. The wakes 

 of small craft appear to be relatively thicker, with a 

 thickness to draft ratio of the order of four. The only 

 wake depth shallower than the draft was from a 

 carrier wake 4,000 yd from the ship. For a speed- 

 boat, h appears to increase considerably with speed. 



All these observations were made with a fathometer 

 ranging downward from a measuring boat in the wake 

 being investigated. Later measurements * were made 

 with a fathometer mounted on the deck of a sub- 

 merged submarine, ranging upward at the surface of 

 the ocean. This method, for several reasons men- 

 tioned in Section 30.1.2, provided more accurate data 

 than was possible with the former. The accuracy of 

 the individual thickness determination is such that 

 the range of wake thicknesses summarized in Table 1 



of arc (or 4 ft per 1,000 yd) upward for the USS 

 Rathburne (ex-DD113) and 16 minutes of arc (or 14 ft 

 per 1,000 yd) downward for the Ewing. In other 

 words, the differential quotient of the thickness of the 

 wake with respect to the time, which will be required 

 in the later discussion of the decay rate of wake 

 strength, has the following values as upper limits: 



1 dh 



— — = —0.08 mm"' for the Rathburne at 10 knots, 

 h at 



I dh 

 h dt 



= 0.04 min~' for the Ewing at 13 knots. 



Additional information on the rate of widening of 

 destroyer wakes is found in a report by UCDWR.° 

 Wakes were laid by three different modern destroyers, 

 running past the E. W. Scripps at 15 knots. The 

 Scripps was hove to and recorded the sound level of a 

 transducer carried repeatedly across the wake by a 

 50-ft motor launch. The sound level records showed 

 definite breaks whenever the source crossed what 

 may be called the acoustic boundaries of the wake; 

 the time between these breaks was multiplied by the 

 speed of the launch to give the width of the wake, 

 suitable allowance being made for the occasional 

 crossing occurring as much as 30 degrees away from 

 the perpendicular transit. The plot of the entire data 

 collected in this manner (Figure 22 in reference 5) 

 suggested to the experimenters that new wakes widen 

 more rapidly than old ones, with a total included 



