The G7es (T5) "Zaunkönig" ("wren") was a passive acoustic torpedo employed by German U-boats during World War II. It was called the GNAT (German Navy Acoustic Torpedo) by the British.

The forerunner of the Zaunkönig was the G7e/T4 Falke, codename "Falcon," which was introduced in March 1943, but saw limited use for trial and was rapidly phased out of service in favor of the later model that received a number of significant enhancements. It was faster, had more range, possessed a magnetic or contact detonator and could be equipped with a percussion pistol.

The T5 torpedo was capable of travelling at 24 knots (44 km/h) and had an effective range of about 5000 metres against convoy escorts vessels proceeding at speeds between 10 (18 km/h) and 18 knots (33 km/h). The homing system consisted of two hydrophone receivers and altered the direction of the rudder via an electropneumatic device. The acoustic homing torpedo was specifically designed as to be attracted by the pitch of an escort's propellers and would — even if aimed inaccurately — explode under the ship's stern.^[1]

There were three variants:

• Two flat-nosed versions which contained four sets of magnetostriction hydrophones.
• A round-nosed version which contained two magnetostriction hydrophones inside a funnel-shaped baffle.

The acoustic homing torpedo required a minimum distance of 400 metres (1,300 ft) to lock onto the target after launch. The detection range of the hydrophones varied significantly according to circumstance, but 450 m (500 yards) was considered reasonable for a ship moving at 15 knots.^[2] After at least one unconfirmed instance of a U-boat (U-972) sinking after being hit by its own torpedo, and another circle-back that forced the U-862 to crash-dive, the BdU ordered submarines to dive to 60 metres (200 ft) and go completely silent after launching acoustic torpedoes, to minimize the risk.

The first 80 T5s were delivered on 1 August 1943, and the weapon was first used in a large-scale maneuver against the North Atlantic convoys ONS 18/ON 202 in late September 1943. The commanders reported a number of torpedo strikes and recorded the sinking of nine commercial steamers and 12 escort ships after the battle. In fact, only six merchant ships and three escort vessels, a destroyer, a frigate and a corvette, were sunk. However, despite some initial success, in particular sinking destroyers and corvettes, the Zaunkönigs effectiveness was quickly nullified by the introduction of a noise making decoy known as Foxer. The NDRC worked on countermeasures to an acoustic torpedo long before its introduction by the Germans. At the end of September 1943, it was promptly installed on all transatlantic escort vessels.^[3]

Grossadmiral Dönitz, not satisfied with the development of countermeasures, realized that his brief success against the North Atlantic trade convoys was only a flare up and recorded:

[it is] finally clear that surface warfare for U-boats had come to an end. It was now a matter of filling in time till the new type should be ready for action.^[3]

In spite of highly effective Allied countermeasures, a total of over 700 T5s were fired in combat, sinking 77 ships.^[4]

• G7e torpedo
• List of World War II torpedoes of Germany

• John Campbell (2002) [1985]. Naval Weapons of World War Two. Conway Maritime Press. ISBN 0-87021-459-4.

• Helgason, Guðmundur. "A brief synopsis of German torpedo evolution during World War II". German U-boats of WWII - uboat.net.
• More indepth details of G7es (TV) Acoustic Homing Torpedo at uboataces.com