Hull mounted, protruding transducer for improving echo integration in bad weather

Abstract

A 38 kHz split beam survey transducer has been mounted on the tip of a 4 m vertical protrusile stabilizing keel of the United States NOAA fishery research vessel R/V Miller Freeman. Air blocking problems, generally observable on all hull mounted transducers, were reduced substantially, and excellent acoustic conditions were achieved with the new mounting for wind speeds up to 35 knots (19 m/s). The improvements are demonstrated through comparative, sequential echo integration of the air close to the transducer with the keel in retracted and extended positions, and through echo recordings on fish. Empirical functions for residual air bubble corrections of echo integration data will presumably work well up to wind speeds where target identification by trawl is problematic because of safety.