Chapter 24 



SURFACE VESSEL TARGET STRENGTHS 



MUCH LESS IS KNOWN about surface vessel tar- 

 get strengths than about submarine target 

 strengths. Few measurements have been made of the 

 sound-reflecting characteristics of ships, and much 

 of the available information has been extracted from 

 experiments where the investigation of the target 

 strength was only incidental to other studies. No 

 mathematical analyses or measurements on scale 

 models have been attempted, so that all target 

 strengths reported here are the results of direct 

 measurements. 



Experimental conditions have been far from con- 

 trolled during these measurements. Ship speed, 

 course, range, and especially aspect angle have been 

 difficult either to estimate accurately or to maintain 

 closely. Many of the tests were made completely at 

 random on vessels happening to pass in the vicinity. 

 Various types of ships served as targets — destroy- 

 ers, freighters, tankers, coal colliers, transports, and 

 Liberty ships — with the result that although many 

 measurements were made, the data on each ship are 

 too scanty to afford a comparison between different 

 ships. Many variables might have significantly af- 

 fected the measured echo levels — ship speed, length, 

 width, draft, hull curvature, course, range, aspect 

 angle, sea state, wind force, temperature gradients — 

 so many that a clear-cut separation of variables is 

 out of the question. Furthermore, the results are so 

 few in number, compared with other underwater 

 sound measurements, and the scatter of values is so 

 wide, that only the most tentative and general con- 

 clusions may be suggested at the present time. 



One of the most important generalizations that 

 may be suggested is the difference between the re- 

 flecting properties of moving vessels and still vessels. 

 Ships under way are known to entrain air along their 

 sides as they move through the water (see Section 

 27.3) With still vessels, on the other hand, en- 

 tramed air seems less likely. Since small air bubbles 

 are extremely efficient scatterers of sound, it is rea- 

 sonable to expect that sound striking a moving vessel 



might be scattered diffusely by the air bubbles along 

 the sides, just as sound is scattered by the wake laid 

 by the ship. A still vessel, however, might be ex- 

 pected to reflect sound specularly. Such an hypothe- 

 sis seems to bring some coherence into the observed 

 data. Therefore, it is largely from this point of view 

 that surface vessel target strengths are examined, in 

 this chapter, as a function of aspect angle, range, 

 ship speed, ship type, pulse length, and frequency. 



24.1 TECHNIQUES OF SAN DIEGO 

 MEASUREMENTS 



Surface vessel target strengths have been measured 

 by only two groups, the University of California 

 Division of War Research at the U. S. Navy Radio 

 and Sound Laboratory, San Diego, California 

 [UCDWRJ, and Bell Telephone Laboratories, New 

 York, New York [BTL].' The measurements off San 

 Diego were made from November 12 to 17, 1943, 

 during a program mvestigating the acoustical prop- 

 erties of wakes laid by ships at various speeds.^ 



During these tests the USS Jasper (PYcl3) echo 

 ranged on two flush-decked World War I destroyers, 

 the USS Crane (DD109) and the USS Lamberton 

 (DMS2, ex-DD119), which followed straight courses 

 at speeds of 10, 15, and 20 knots in deep water. A 

 standard Navy JK transducer was used, sending out 

 pulses 10 msec long at a frequency of 24 kc. 



The Jasper ranged on the destroyer as it ap- 

 proached; then, just as the beam of the destroyer 

 passed, the Jasper began to range on its wake. Con- 

 sequently no target strengths were measured at as- 

 pect angles beyond about 110 degrees from the bow. 

 Errors in the estimated aspect angles were quite 

 large because of unknown deviations of the destroyer 

 from its normal course but could not be evaluated. 

 Ranges varied from 112 to 660 yd. 



Echoes from the destroyer and its wake were re- 

 ceived and recorded oscillographically on moving 

 picture film, with the equipment employed in the 



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