Chapter 26 



INTRODUCTION 



26.1 



WHAT 4RE WAKES? 



THE APPEAHANCE of a Streak of foamy, churned 

 water behind a ship under way, known as the 

 ship's wake, is famiUar to every mariner. Because the 

 wake extends along the path of the ship over a 

 length many times the ship's length, it is hard to get 

 a good view of the wake as a whole, even if a some- 

 what elevated vantage point, such as the bridge or 

 masthead, is chosen. Figure 1 shows the wake of an 

 antisubmarine patrol vessel (PC488) in a quiet sea, 

 as viewed aft from the crow's nest. 



The observer in an airplane enjoys ideal condi- 

 tions for the visual study of wakes. Figures 2 to 6 

 describe better than verbal descriptions what a wake 

 looks like from a great height. The first four were 

 taken from altitudes of 2,500 to 3,000 ft, the plane in 

 level fhght overtaking a destroyer, the USS Moale 

 (DD693), which was proceeding on a straight course 

 at constant speeds of 16, 20, 25, and 33 knots. By 

 way of a scale, the ship had an overall length of 

 376 ft, a beam of 41 ft, and a draft of 13 ft. Figure 6 

 illustrates what happens to the wake as the ship 

 turns; the foam on the curved section of the path is 

 seen to be visually more dense, especially along the 

 outer edge of the wake. As in turning, acceleration of 

 the ship on a straight course increases the visual den- 

 sity of a wake. Incidentally, the irregular white 

 streaks appearing in Figures 4 to 6 are foam patterns 

 on the water. All the photographs show the delicate 

 bow-wave pattern, fanning out astern with a much 

 greater angle of divergence than the actual wake. 

 Figure 7 is a close-up taken from an altitude of 300 

 ft, of the bow wave and the wake of another de- 

 stroyer, the USS Ringgold (DD500). The visible 

 structure' of the wakes laid by large ships does not 

 dififer markedly from that of the wakes of vessels of 

 destroyer size, as may be seen in Figure 8, which 

 gives a view from an altitude of 4,000 ft of a large 

 aircraft carrier, the USS Saratoga (CV3). 



Beyond the obviously foamy and turbulent nature 



of wakes, visual inspection does not reveal any of 

 their physical properties. The discovery that 

 "wakes," using this term in a loose sense, are capable 

 of affecting the propagation of sound energy through 

 the water has suggested a new distinction: an acoustic 

 wake is defined as a volume of the ocean which has 

 acquired, because of the passage of a ship through it, 

 a greater though transitory capacity for absorbing 

 and scattering sound. The expression "volume of the 

 ocean" is used advisedly, because acoustic wakes 

 have a definite vertical extension, often rather 

 sharply bounded. Acoustic wakes under the surface, 

 originating from submerged submarines, are of par- 

 ticular interest. During a level run at periscope 

 depth, the upper boundary of the wake, spreading out 

 bodily from the screws, does not reach the ocean sur- 

 face imtil several hundred yards behind the sub- 

 marine. Several aerial views of submarine wakes, 

 both during surface runs and after a crash dive, are 

 reproduced in Figures 1 to 6 of Chapter 31. 



The temperature distribution in the top layer of 

 the ocean may be disturbed by the passage of a ship 

 in such a manner as to leave a thermal wake, detect- 

 able by sensitive thermocouples. Evidently, experi- 

 ments must decide to what extent thermal and 

 acoustic wakes coincide with the body of water called 

 a wake by a visual observer. This problem is dis- 

 cussed in Chapter 31, dealing with the geometry of 

 wakes. However, one interesting feature will be men- 

 tioned here : the acoustic and thermal manifestations 

 of a wake may persist over periods of half an hour or 

 more, often long after visible traces of the ship's 

 passage have disappeared. 



26.2 NAVAL IMPORTANCE OF WAKES 



Wakes can be important in naval warfare in two 

 general ways. In the first place, they may interfere 

 with the successful operation of acoustic devices, by 

 scattering or absorbing sound. In the second place, 

 they may provide a method for detecting, tracking. 



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