DEPENDENCE ON SHIP TYPE 



431 



very shallow, from 60 to 110 ft deep. The sound beam 

 was not highly directional; the total vertical beam 

 width between points where the response was 10 db 

 down was 20 degrees. Thus if the tran.sdiicer were 

 level, the sound beam would strike the bottom at a 

 range of only about 126 yd, for water 60 ft deep. 

 Consequently the bottom undoubtedly reflected part 

 of the incident sound in much the same way as the 

 surface, and contributed to the intensity of the 

 echoes received at the transducer. 



Assume, for example, that both the surface and 

 bottom reflected sound perfectly, so that at the 

 particular ranges used the sound beam could spread 

 in only one direction — horizontally. In this extreme 

 case, the intensity of the echo would be inversely pro- 

 portional, not to the fourth power of the range, but 

 to the square of the range. This assumption, of course, 

 is not realistic, but the result suggests that for the 

 New York measurements the actual drop is some- 

 where between inverse fourth and inverse square; 

 perhaps the echo intensity actually varies more 

 nearly inversely as the cube of the range over a shal- 

 low reflecting bottom. This relatively slow increase 

 of transmission loss with increasing range may ac- 

 count for much of the range dependence for moving 

 vessels in the New York data. Even an inverse 

 square dependence of echo level on range fails to 

 account, however, for the observed variation on still 

 vessels shown in Figures 5 and 6, where the echo 

 level actually increases rapidly with increasing 

 range. 



Another possibility might account for the depend- 

 ence of target strength on range for stationary ves- 

 sels. Sound incident on the hull of the ship will be 

 reflected downward where the hull is curved slightly 

 downward, then reflected upward from the bottom. 

 It is possible that the curvature of the hulls of the 

 surface vessels measured is such that the rays re- 

 flected to the bottom will strike the bottom and be 

 reflected back to the transducer only at longer 

 ranges, so that target strengths measured at long 

 r&nges will be greater than target strengths measured 

 at short ranges. This explanation may account for 

 the stronger range dependence for stationary ves- 

 sels than for moving vessels, although it must be re- 

 garded as highly tentative in the absence of further 

 substantiating evidence. 



In all, it has been well established that the target 

 strength of surface vessels on which measurements 

 have been made apparently increases with range. 

 This increase is much greater at beam aspects than 



at off-beam aspects. The exact rate of increase is un- 

 certain becau.se many causes are respon.sible; meas- 

 ured rates vary from 4.5 to 105 db per kyd at ranges 

 between 200 and 500 yd. The dependence of target 

 strength on range arises from (1) smaller coverage of 

 the target by directive transducers at close ranges; 

 (2) incorrect evaluation of the transmission loss 

 neglecting surface and bottom reflections; and (3) 

 the dimensions and curvature of the target, in so far 

 as they reduce specular reflection at close ranges. 

 Probably none of these effects, however, can explain 

 the enormous observed range dependence for an- 

 chored vessels. Further measurements would be re- 

 quired to show the extent to which this observed 

 effect is generally found. 



24.5 



DEPENDENCE ON SPEED 



Very little information is available on the varia- 

 tion of target strength with the speed of the ship, for 

 moving vessels. At San Diego, speeds of 10, 15, and 

 20 knots were employed; Table 1 lists target strengths 

 without separating the speeds at which they were 



a 30 



z 

 z 



UJ 



cc 

 t) 10 



300 400 



RANGE IN YARDS 



500 



Figure 11. Range dependence at different speeds for 

 beam aspect (San Diego). 



measured. Figure 11 shows beam target strengths 

 plotted as a function of range for three different 

 speeds, 10, 15, and 20 knots, from the San Diego 

 measurements. The dependence on range is evident, 

 even in only twenty observations, but no significant 

 dependence on ship speed is apparent. Ship speeds 

 were not estimated or measured in the New York 

 tests. As already mentioned, this same lack of de- 

 pendence on ship speed is characteristic of wake 

 echoes. 



24.6 



DEPENDENCE ON SHIP TYPE 



No clear dependence of target strength on ship 

 type is indicated by the evidence now available. 

 While a large number of different vessels have been 



