Sonar decoy

A decoy acts as false targets for human operators and/or sonar-homing weapons such as acoustic torpedoes.

The first submarine decoys were the German Bold fitted to U-boats of World War II.

On contact with sea water, the calcium hydride decomposed to produce a trail of hydrogen gas bubbles that acted as a bubble curtain and reflected ASDIC impulses to produce a false target.

The container trapped hydrogen and floated, with a crude spring valve to maintain buoyancy to keep it at a constant depth.

Later decoys, such as Sieglinde, were motorised and could deploy their false target away from the host submarine, increasing safety.