Bold (decoy)
Bold (a term derived from kobold) was a German sonar decoy, used by U-boats during the Second World War from 1942 onwards. It consisted of a metal canister about  in diameter filled with calcium hydride. It was launched by an ejector system colloquially referred to as Pillenwerfer (English: "pill thrower"). When mixed with seawater, the calcium hydride produced large quantities of hydrogen which bubbled out of the container, creating a false sonar target. A valve opened and closed, holding the device at a depth of about . The device lasted 20 to 25 minutes. The Royal Navy called it SBT (Submarine Bubble Target). See also
 Sieglinde (decoy)

External links
 Sonar decoys at uboataces.com

Sonar decoys
Weapons countermeasures
Sonar
U-boats