Acoustic torpedo

The first passive acoustic torpedoes were developed nearly simultaneously by the United States Navy and the Germans during World War II.

On the Allied side, the US Navy developed the Mark 24 mine, which was an aircraft-launched, anti-submarine passive acoustic homing torpedo.

The German U-boats now had an effective "fire and forget" weapon capable of homing-in on attacking escorts and merchant ships and doing so in close quarters of only three or four hundred yards.

[1] By summer of 1943, the German U-boat campaign was experiencing severe setbacks in the face of massive anti-submarine efforts integrating Coastal Command attacks in the Bay of Biscay, the deployment of merchant aircraft carriers in convoys, new anti-submarine technologies such as hedgehog and improved radar, and the use of dedicated hunter-killer escort groups.

[2] Two actions in May 1943 have a claim for the first U-boat sunk by a US Navy's Mark 24 "mine", actually an acoustic homing torpedo designed in 1942 for antisubmarine warfare.

The escort carrier USS Bogue had been tracking the Japanese submarine I-52 which was on its way to France carrying raw materials and minerals.

After a rendezvous with the German submarine U-530, which transferred a Naxos radar detector to I-52, the Japanese unit was spotted on the surface by two Grumman Avenger aircraft that dropped depth charges.

A US version codenamed FXR was deployed at the end of September 1943 on all transatlantic escort vessels[7] but was soon replaced by the more effective Fanfare noisemaker.

The downside of the Foxer was that it also rendered the ship's own ASDIC ineffective and concealed any other U-boat nearby that could home in on the convoy.

The AN/SLQ-25 Nixie (and AN/SLQ-25A and variants) is a towed decoy deployed on USN and allied surface ships for defending against passive acoustic homing torpedoes.

However, after a protracted journey to Kronstadt the two Royal Navy officers were not allowed access to the submarine and returned home empty handed.