402 



SUBMARINE TARGET STRENGTHS 



measurements, and is apparent in Figures 3 to 6. 

 Much of it is due to experimental error, and may not 

 be real. Repeatable asymmetry in target strength- 

 aspect curves has occasionally been found at San 

 Diego and is illustrated in Figure 15 for three runs on 

 an S-boat, where a sharp dip is evident just off the 

 port bow at an aspect angle of about 340 degrees. 

 This dip may be attributable to particular features 

 of the construction of the S-boat, such as the lack of 

 any surfaces normal to the sound beam so that specu- 

 lar reflection cannot occur at that aspect. It is also 

 possible that this decrease in target strength is a 

 characteristic of bow echoes and that the aspect 

 angles were in error by as much as 20 degrees; low 

 bow target strengths are conspicuous in the re- 

 sults of the optical measurements. On the other hand, 

 the variability of echo intensities at other aspects is 

 so large that this dip may not be real, even though it 

 appeared during all three runs. 



Horizontal asymmetry in the indirect measure- 

 ments is apparent in Figures 5 and 6. This asym- 

 metry may be attributed largely to the asymmetrical 

 models used, in other words, the port and starboard 

 sides were not the same. Not only were the models 

 asymmetrical, but, as shown in Section 23.2.2, models 

 of the same submarine appeared to differ rather 

 markedly. Because of these model differences, the 

 observed data cannot be used to confirm the exist- 

 ence of asjTnmetry in the target strength in the hori- 

 zontal plane. 



23.3 



DEPENDENCE ON SPEED 



Almost no data are available on the variation in 

 target strength with the speed of the submarine. 

 If echoes come only from the hull and conning tower 

 of the'submarine, it can be argued theoretically that 

 the target strength of the submarine itself should not 

 change as the speed is changed. But if a layer of air 

 bubbles immediately surrounding the submarine con- 

 tributes appreciably to the echoes received, then the 

 target strength would be expected to depend on the 

 speed and depth of the submarine; the scattering of 

 sound from bubbles is discussed in Chapters 26 to 

 35. In addition, turbulence in the water adjacent to 

 the submarine, which would depend on the speed of 

 the submarine, may be responsible for part of the 

 reflection of sound. 



So far, little evidence has been uncovered to iden- 

 tify a layer of air or turbulent water surrounding the 

 submarine as an effective reflector of sound (see Sec- 



tion 33.3), although bubbles have been observed on a 

 submerged submarine traveling through the water'^ 

 (see Section 27.1.1). Most direct measurements have 

 been made on submarines at a creeping speed be- 

 tween 1 and 3 knots; none have been made on a 

 stationary, balanced submarine. Some data describe 

 tests at 6 knots, but no significant difference has been 

 observed between these results and results at lower 

 speeds. At 6 knots, however, reasonably strong 

 echoes were received from a wake behind a fleet- 

 type submarine at periscope depth. As the sound 

 beam crossed the submarine from bow to stern, 

 echoes grew stronger, faded and died out completely 

 for a short time. Then strong echoes were received 

 for several hundred yards behind the submarine, 

 which were attributed to reflection from the wake. 



23.4 



DEPENDENCE ON RANGE 



At long ranges, target strength is practically inde- 

 pendent of range. Close to the submarine, however, 

 the target strength will decrease with range. This 

 phenomenon has two causes. First, at very short dis- 

 tances from the submarine, the submarine reflects 

 more like a plane or a cylinder than a sphere, and the 

 inverse fourth power law does not apply. In other 

 words, the target is not equivalent to a point source, 

 and the target strength decreases. This effect de- 

 pends on the aspect of the submarine and diminishes 

 as the range exceeds the maximum radius of curva- 

 ture of the submarine. 



Secondly, at short ranges the effective portion of 

 the sound beam may cover only part of the area ex- 

 posed by the submarine. Such an effect would be 

 expected primarily at beam aspect, since geometric 

 foreshortening reduces the area exposed by the sub- 

 marine at other aspects. For example, a sound beam 

 12 degrees wide will not cover the entire length of a 

 300-ft submarine, at beam aspect, at ranges less than 

 475 yd. However, only those areas on the submarine 

 giving rise to nonspecular reflection would be af- 

 fected, so that if most of the reflection were specular, 

 arising in a small area amidships, the target strength 

 as measured very close to the submarine from per- 

 fectly aimed pulses would not depend on the beam 

 width. From Figures 1 to 5 in Chapter 22, it appears 

 that most of the reflection is concentrated amid- 

 ships. Thus the effect of beam width, though present, 

 is probably not important except possibly at very 

 short ranges, as long as nonspecular reflection is 

 neglected. 



