526 



OBSERVATIONS OF WAKE ECHOES 



outstanding feature of the table is the greater strength 

 of the wake laid by surfaced submarines, compared 

 with those from submerged runs. However, during a 

 dive the wake strength does not decline steadily. In- 

 stead, repeated peaks occur. Some of the peak values 

 even equal the strength of the surface wakes, as il- 

 lustrated in Figure 6. These peaks are undoubtedly 

 connected with the diving operations, movement of 

 diving planes, blowing of tanks, and other operations. 

 While the surface values of W are surprisingly con- 

 sistent — about — 15 db — only the order of magni- 

 tude of the subsurface strength can be regarded as 

 established, perhaps —25 db to —30 db. The relative 

 acoustic weakness of wakes behind submerged sub- 

 marines probably results from several causes, such as 

 lack of entrained air and the reduction of cavitation 

 and bubble production at the higher pressure. A small 

 but definite increase of IT with pulse length as the pulse 

 length changes from 10 to 100 msec is found for both 

 submerged runs of the USS S-18 (SS123). The in- 

 crease is small and results largely from the extension 

 of the wake along the axis of the sound beam. Even 

 for a wake whose thickness is less than the signal 

 length, the echo will vary with signal length when the 

 transducer is pointed obliquely at the wake. Only 

 for normal incidence of the sound beam is the change 

 of target strength with pulse length a simple, readily 

 predictable effect. 



33.4 ECHOES FROM SURFACE 



VESSEL WAKES 



The San Diego group has studied echoes from 

 wakes laid by numerous surface craft. Early experi- 

 ments were carried out in San Diego harbor. During 

 1944, the group carried out a large program of record- 

 ing echoes from the wakes of a number of surface 

 vessels, including aircraft carriers, destroyers, and 

 some small craft. Wakes for this program were laid on 

 the open sea off San Diego. 



33.4.1 Echo Ranging at Wakes in 

 San Diego Harbor, 1943 



For these experiments an echo-ranging transducer 

 was mounted on a barge moored to one side of the 

 harbor channel.' Most of the measurements were 

 made on wakes laid by a motor launch (length 40 ft, 

 beam 11 ft, draft 23-^ ft) traveling at 4 to 6 knots. 

 Incidental results were also obtained by echo ranging 

 at wakes produced by other vessels which happened 



to pass; these vessels probably did not travel at full 

 speed in the harbor. 



The chief interest of these experiments, which have 

 been reported in detail in reference 1, lies in the fact 

 that short signals — only 9 msec long — were trans- 

 mitted alternately at 15, 24, and 30 kc. Thus it is 

 possible to analyze the results for a possible depend- 

 ence on frequency both of the wake strength and its 

 decay rate. The absolute values of the wake strength 

 appear to be less reliable, for two reasons. First, 

 difficulties with the calibration of transducers seem 

 to have been experienced during the early phases of 

 the San Diego wake studies; such would affect the 

 absolute values of W, without impairing the results 

 concerning the dependence of W on frequency. Sec- 

 ond, the measurements in the shallow waters of San 

 Diego harbor are likely to have been disturbed by 

 bottom-reflected sound; indeed, an apparent de- 

 pendence of W on the range was explainable only 

 as caused by some peculiarity in the bottom contour. 



The results of these early measurements are sum- 

 marized in Tables 5 and 6. Values of the wake strength, 

 obtained with 9-msec signals at three different fre- 

 quencies, are collected in Table 5, which also contains 

 the attenuation coefficients a and the wake indices ^o 

 used in these computations. 



There is no information available on the wake age 

 at which the observations on the larger vessels were 

 made; probably the age did not exceed a few minutes, 

 and the initial wake strength had decayed only 

 slightly. The decay of the wakes laid by the launch 

 was studied systematically over a period of 12 min- 

 utes, after which time the echo intensity had dropped 

 to the reverberation level. While the decay of the 

 15-kc echoes appeared to start immediately after pas- 

 sage of the launch, the echoes at 24 and 30 kc main- 

 tained their initial strength for about 2 minutes before 

 they began to decay. The decay of the echo intensity 

 follows a simple exponential law, to a good approxi- 

 mation; thus the strength of the echo expressed on a 

 decibel scale decreases linearly with time. The decay 

 rates found are listed in Table 6. Within the errors of 

 observation, there is no dependence of the decay rate 

 on frequency. But the wake strength W seems to in- 

 crease with frequency. From the average W for each 

 frequency of the vessels contained in Table 5, exclud- 

 ing the launch and the three fishing boats, the follow- 

 ing differences are found : 



TF24- T^i5 = +1.0db 



TF30 - Wn = -1-4.6 db. 



