Chapter 35 

 SUMMARY 



THE WAKE of a moving ship scatters and attenu- 

 ates sound. The following sections summarize 

 existing data in the form of rules for predicting the 

 geometry of acoustic wakes — their depths and 

 widths, the attenuation of sound crossing wakes, and 

 the scattering of sound from wakes. 



In some cases, these rules are based on few ob- 

 servations. Moreover, the degree of reliability of most 

 of the rules is difficult to assess, and an adequate ap- 

 praisal of it in most cases can be reached only by 

 study of the detailed expositions given in the pre- 

 ceding chapters. 



35.1 



WAKE GEOMETRY 



For surface ships, the depth h of an acoustic wake is 

 approximately twice the draft of the wake-laying 

 vessel, and is practically constant up to distances of 

 at least 1,000 yd behind the ship (see Section 31.3.1). 

 The depth of the wake laid by a surfaced submarine 

 decreases from about 30 ft at a distance 100 yd be- 

 hind the screws to about 20 ft at a distance astern of 

 1,000 yd. The wake of a submerged submarine, run- 

 ning at a periscope depth with a speed of 6 knots, 

 reaches the ocean surface at distances astern greater 

 than 100 yd, corresponding to a half-angle of diver- 

 gence at the screws of about 5 degrees in the vertical 

 direction (see Section 31.3.2). 



The width w of a wake increases with the range r 

 behind the wake-laying vessel. For destroyer and 

 destroyer escort wakes at distances astern greater 

 than 100 yd, the wake fans out laterally in a regular 

 manner, with the wake edges including a total angle 

 of 1 degree (see Section 31.2). 



At distances less than 100 yd astern, the wake 

 geometry is less regular and depends upon the speed 

 of the destroyer in a complicated manner. This de- 

 pendence may be represented by the following equa- 

 tion: 



which is valid at distances astern r less than r*. For r* 

 the values in Table 1 , which were deduced from aerial 



Table 1. Dependenee of r* and w* on ship speed. 



photographs (see Section 31.1) of destroyer wakes, 

 should be used. At distances astern greater than r*, 

 one can compute the wake width by the equation 



w = w* + 0.017(r - r*) 



(2) 



using the same values of r* and w* as before. 



Acoustic determinations of the width of destroyer 

 wakes (see Section 31.3.1) are much less accurate 

 than the photographic measurements, and seem to be 

 in moderate agreement \vith the predictions made on 

 the basis of equation (2). 



The acoustic properties of the wake apparently 

 vary with position inside the wake, although no defi- 

 nite predictions can yet be made for a particular 

 wake. Outside the boundaries estabhshed by the 

 above relationships, the acoustic effects produced by 

 the water are no greater than those typical of the 

 ocean with no wakes present. 



35.2 



ABSORPTION BY WAKES 



w = —r = 0.85r, 



(1) 



When sound from a shallow projector is received on 

 a shallow hydrophone, the transmission loss is in- 

 creased by an amount Hw if a wake is present between 

 the projector and the hydrophone. This attenuation 

 by the wake Hw may be expressed as 



Hy, = Kex (3) 



where K^ is the attenuation coefficient in decibels per 

 yard, and x is the length in yards of the sound path 

 within the wake [see equation (2) of Chapter 32]. 



541 



