G7es torpedo

The G7es or Zaunkönig T-5 (wren in German) was a torpedo employed by German U-boats during World War II. It was known as the GNAT (German Navy Acoustic Torpedo) to the British.

Description
The forerunner of the T-5 was the G7e/T4 Falke, codename "Falcon," which was introduced in March 1943, but only fired by three U-boats by September of that year when the G7es (which was faster, had more range, possessed a magnetic exploder and could also be equipped with a percussion pistol) became available.

The torpedo was electric and had an effective range of 5700 meters at a speed of 24 knots (44 km/h). This torpedo employed passive acoustic homing to find its target, becoming active after a straight run of 400 meters. The homing mechanism consisted of two hydrophone receivers which sensed the sound waves of ship propellers and altered the direction of the rudder via a electropneumatic device.

There were two main variants:
 * A flat-nosed version which contained two sets of magnetostriction hydrophones.
 * A round-nosed version which contained two magnetostriction hydrophones inside a funnel-shaped baffle.

Use
The 400 meter limit was employed for safety reasons, even though there were at least two instances of U-boats (U-972 in December 1943 and U-377 in January 1944) sinking after being hit by their own torpedo. This risk was later mitigated by requiring submarines to dive to 60 meters and go completely silent after launching acoustic torpedoes.

The first 80 T-5s were delivered on 1 August 1943, and the weapon was first used in September. Despite some success, in particular sinking destroyers and corvettes, the T-5 often detonated behind the enemy ship because the acoustic steering was very imprecise. This was particularly evident at its first large-scale use from 20 to 24 September 1943 in the attacks on convoy ON-202. The commanders reported a number of torpedo strikes and recorded the sinking of nine commercial steamers and twelve escort ships after the battle. In fact only six merchant ships and 3 escort vessels, a destroyer, a frigate and a corvette were sunk. A total of 640 T-5s were fired in combat, sinking 45 ships. The T-5 was countered by the introduction by the Allies of the Foxer noise maker.

The T-5 was nicknamed Zerstörerknacker (destroyer cracker) by the German submariners, as it was especially used against convoy escorts.