Sec. 73.27 



FIXED-APPENDAGE DESIGN 



705 



are described by H. F. Harvey Jr., and 0. J. 

 Streever [SNAME, 1953, pp. 431-403]; also by 

 D. P. Graham, F. E. Cook, and H. S. Preiser in 

 their paper "Cathodic Protection in the U. S. 

 Navy; Research-Development-Design" [SNAME, 

 Nov 1956]. 



73.27 Design Notes for Locating Echo-Rang- 

 ing and Sound Gear on Merchant Vessels. A 

 good underwater sound installation for trans- 

 mitting and receiving, whether of the fixed or 

 retractable type, involves a neat combination of 

 acoustic and mechanical engineering, hydro- 

 dynamics, and naval architecture. Aside from the 

 purely acoustic features involved, the most 

 important single factor in an efficient design is 

 to shape the sound head or sound dome so that it 

 is free from cavitation and separation. It must 

 also be placed under the ship in a position where 

 it is clear of air entrained in the streams which 

 flow past it. 



A sound head may be in the form of a stream- 

 lined body of revolution, resembling the hull 

 form of a stubby true submarine vessel or an 

 airship. When carried by a post or strut, it forms 

 an assembly which is usually retractable. Such 

 a form is reasonably free from flow noises but it 

 does not lend itself to swinging bodily in azimuth 

 for horizontal search. Furthermore, when yawed 

 slightly it is no longer a streamlined body. A 



vertical, streamlined 2-diml enclosure, with the 

 sound head rotating inside it, can rarely be made 

 long enough to be entirely free of separation, air 

 bubbles, and possible cavitation along its after 

 or trailing portion. 



A fore-and-aft location of the sound gear at a 

 distance abaft the stem of about 0.15F^, where V 

 is in kt and the .r-distance is in ft, appears to be 

 as satisfactory as any. More important, however, 

 than both shape and fore-and-aft position may 

 be the sound-head distance below the hull. A 

 deep projection keeps the sound head always 

 under water when the vessel is pitching. Further, 

 it is below the boundary layer and beneath the 

 streams of air bubbles entrained at the bow, near 

 the free surface, and flowing aft under the hull. 

 This usually requires that the head be extensible 

 and retractable. 



For the ABC ship at say 20 kt, the distance 

 abaft the stem would be 0.15(20)^ or 60 ft, cor- 

 responding to about Sta. 2.35. At a distance of 

 4 ft below the keel the head should be well below 

 the most disturbed portion of the boundary 

 layer. For a fishing trawler traveling at say 10 kt, 

 the head could be at a distance of 0.15(10)'^ or 

 15 ft from the bow. If also placed 4 ft below the 

 keel it should be in a good listening position and 

 should remain submerged unless the vessel is 

 pitching deeply. 



