400 



SUBMARINE TARGET STRENGTHS 



30 



20 



5 10 





 BOW 



30 



60 90 



BEAM 

 ASPECT ANGLE IN DEGREES 



ISO 



ISO 

 STERN 



Figure 14. Optical comparison of two models of U570 (HMS/M Graph). 



The theoretical results at off-beam aspects are 

 not very realistic, since they assume a perfectly 

 smooth and curved reflecting surface. In Figure 13, 

 port and starboard sides were averaged in the optical 

 and acoustical measurements to simplify the illus- 

 tration. A considerable difference is evident between 

 these three curves, which may be attributed in part 

 to the errors in the models as well as to possible dif- 

 ferences arising from the different methods used. 



Since different models of the U570 were used at 

 MIT and at Moimtain Lakes, the target strength 

 of each model was measured optically, after the 

 model used at Mountain Lakes had been finished in 

 the same way as the MIT model. The results are 

 reproduced in Figure 14 for the model originally used 

 at Mountain Lakes and for the model first tested at 

 MIT. Unfortunately, the Mountain Lakes model 

 was not measured at aspects within 30 degrees of the 

 beam, so that the comparison is not complete. Since 

 the beam target strengths measured both optically 

 and acoustically agree quite well, however, the tar- 

 get strengths of both models are probably the same 

 at beam aspect. 



The differences between the two curves in Figure 



14 cannot be ascribed to differences in the methods, 

 since both models were tested in the same way — 

 optically. Instead, these differences, which near the 

 bow are as great as 5 db, must be due to differences 

 in the models themselves. Generally higher values 

 were obtained for the Mountain Lakes model. In 

 particular, diving planes at the bow, and horizontal 

 and vertical rudders at the stern increased bow and 

 stern reflections from the Mountain Lakes model. 

 The MIT model had a knife-edge finish at bow and 

 stern. Furthermore, the difference in the profile of 

 the conning towers and the supporting structures for 

 the two models would be expected to reduce somewhat 

 the reflections of the MIT model at stern aspect. 



In view of these discrepancies between different 

 models of the same submarine, and between the 

 different indirect methods of determining target 

 strength, too much reliance cannot be placed on the 

 general differences in submarine classes shown in 

 Tables 2 and 3. 



23.2.3 



Asymmetry 



To a first approximation, the shape of a submarine 

 may be represented by an ellipsoid which is sjth- 



