Palve, et al:

Allies name for GERMAN NAVAL ACOUSTIC TORPEDO (GNAT)

Actually, the G7es/T5 "Zaunkonig" passive acoustic torpedo........

Used by U-boats against escorts. The sensors of the acoustic torpedo were designed to detect propellor revolutions at a frequency of 24.5 kilohertz. This was the
frequency that was generated by propellors that drove a vessel at approximately 15 knots. The hydrophones could not detect an escort vessel that proceeded considerably faster or slower than 15 knots. The homing device was inoperative for the first 400 meters, which allowed the u-boat to keep clear. After the
acoustical device was activated, it locked onto the loudest noise. The torpedo
would then home in on the propellor cavitation of the target, and not the cavita-
tion that was generated by the slow revolutions of the u-boat's propellor.

Hope this helps,

ROBERT M.