432 



SURFACE VESSEL TARGET STRENGTHS 



3 40 



UJ 

 OS 



o 

 S 30 



; 20 



S> 10 



g 





 BOW 



Curve 



20 40 



Ship 



60 80 100 120 140 160 ISO 

 BEAM STERN 



ASPECT ANGLE IN DEGREES 



Length Draft Water Range 

 Tonnage in in depth in 



feet feet in feet yards 



Navy Transport 12,000 



Liberty Ship 10,000 



325 

 300 



25 

 20 



100 300-510 

 60 300-450 



Figure 12. Variation in target strength between simi- 

 lar ships (New York). 



their estimated tonnages, lengths, and drafts; fur- 

 thermore, both were measured at roughly the same 

 ranges. It is possible that the fluctuation and varia- 

 tion normally encountered in underwater sound 

 transmission may be responsible for the 10 db differ- 

 ence between the two curves, or that bottom reflec- 

 tion may be the cause, since the transport was under 

 way in water 100 ft deep while the Liberty ship was 

 under way in water almost half as deep. Even per- 

 fect bottom reflection, however, cannot account for 

 the observed difference between the two curves, 

 which suggests faulty calibration, widely variable 

 transmission, or large unsuspected systematic differ- 

 ences between the two ships. 



0.01 



0.1 



Curve 



Figure 13. Effect of pulse length on measured echo levels (New York). 



made,' the scatter is so great that any correlation be- 

 tween target strength and ship draft and tonnage is 

 obscured. 



As an example of the variation in target strength 

 between one ship and another, as measured at New 

 York, Figure 12 illustrates target strength plotted 

 against aspect angle for two large ships of nearly 

 equal dimensions. The difference in their target 

 strengths cannot be attributed to the difference in 



24.7 DEPENDENCE ON PULSE LENGTH 

 AND FREQUENCY 



Although surface vessel target strengths have not 

 been systematically investigated as a fimction of 

 pulse length, early studies at New York reported a 

 dependence of echo amplitude on pulse length for 

 pulse lengths of 0.05 to 110 msec' The results of 

 these measurements are reproduced in Figure 13, 



