Chapter 31 



WAKE GEOMETRY 



IN THIS CHAPTER information of rather heterogene- 

 ous origin, concerning the dimensions of wakes, is 

 brought together. Some types of acoustic observa- 

 tions are in themselves eminently valuable for deter- 

 mining the geometric characteristics of wakes. How- 

 ever, a good deal has to be known about the geometry 

 of wakes in order to plan and execute their investiga- 

 tion by acoustic methods. Such knowledge has been 

 provided by visual and photographic observations. 

 Brief reference also will be made to thermal wakes, 

 although very little is known about them so far. It is 

 undecided whether or not the visual, acoustic, and 

 thermal manifestations of the same wake agree as to 

 the volume of the sea from which they originate; this 

 problem deserves further study. 



31.1 WAKE GEOMETRY FROM AERIAL 

 PHOTOGRAPHS 



The serial views of destroyer wakes shown in 

 Figures 2 to 6 of Chapter 26 were selected from a 

 large series of photographs, made available by the 

 Photographic Interpretation Center, U. S. Naval Air 

 Station, Anacostia. They show wakes of the destroyer 

 USS Moah (DD693) proceeding on a straight course 

 at constant speeds, ranging from 16 to 34.5 knots. 

 For each speed, photographs of three or more differ- 

 ent runs were measured, so that the results represent 

 a fair average. The following conclusions are drawn 

 from measurements made on the original prints. 



Immediately behind the screws the wake diverges 

 with an included angle of about 50 degrees. Indi- 

 vidual angles measured on different photographs vary 

 between 40 and 60 degrees, but no clear-cut depend- 

 ence on speed is indicated; these variations may well 

 be spurious. It may be mentioned in passing that the 

 wake of a stationary propeller ' showed an angle of 

 divergence of about 20 degrees. At a certain distance 

 astern, the wide divergence of the destroyer wake 

 ceases rather abruptly, and thereafter the wake 

 spreads with a total included angle of about 1 degree. 



This angle too appears to be independent of the speed 

 with which the wake is laid. However, the distance 

 astern at which the transition from the 50-degree 

 divergence to the l-degree divergence occurs in- 

 creases very markedly with the speed of the de- 

 stroyer. At 16 knots, it is about 65 ft, and at full 

 speed about 280 ft; this variation of distance with 

 speed is not linear, as far as present experience indi- 

 cates. The numerical values are given in Section 35.1. 

 Observations of several different types, which will be 

 reported in the rest of this chapter, all seem to indi- 

 cate that the wake spreads out with a large included 

 angle immediately behind the wake- laying vessel, 

 and that at distances astern greater than about 100 

 yd the wake spreads out with a very small included 

 angle, of the order of 1 degree. However, none of 

 these other observations have the same high intrinsic 

 accuracy as the measurements on aerial photographs. 

 Therefore, the results of these measurements, as in- 

 complete as they are, have been selected for inclusion 

 in Section 35.1. The large initial divergence of a wake 

 is quite conspicuously demonstrated in Figures 1 and 

 7 of Chapter 26, showing the wakes of a submarine 

 chaser and destroyer, respectively. 



Aerial photographs also furnish interesting infor- 

 mation on the cross-sectional structures of wakes. For 

 instance. Figures 2 to 5 of Chapter 26 reveal that at 

 short distances astern and at speeds less than 25 

 knots the destroyer wake has a dense core and edges 

 that stand out conspicuously; with increasing dis- 

 tance, this internal structure gradually fades out. At 

 speeds above 30 knots, the destroyer wake appears 

 to be so strongly turbulent that the core is largely 

 obliterated. 



Tranverse structure of a different kind is illustrated 

 by the submarine wakes seen in Figures 1 to 6. The 

 wake of a surfaced submarine shows bifurcation, or 

 twin structure, both at 15 and 20 knots in Figures 3 

 and 4. The same illustrations clearly differentiate a 

 short wake section immediately behind the sub- 

 marine, which has a large angle of divergence, from 



494 



