What is "sieglinde"

Sieglinde was a sonar decoy used during the Second World War by German U-boats. The Sieglinde was installed in chambers on the sides of the U-boat. It could be ejected to a considerable distance from the boat when attempting to hide from a seeker's sonar equipment. The Sieglinde was powered by electric motors, allowing it to move at , and to periodically ascend or dive, thus imitating the sonar return of an actual submarine. This allowed the real U-boat to slip away quietly from pursuing ships. It was typically used in combination with Pillenwerfer (or BOLD) decoys. Also, the Nazi naval command experimented with Alberich, an experimental sound-absorbing tile that was designed to reduce the U-boat's sonar return. The Alberich's ability to absorb or reduce sound reflections was rapidly reduced by contact with water, however, and in addition the adhesive used to install the sheets of treated rubber was not strong enough to securely hold them in place during operation, so it was seldom used in actual operations.