Bold (decoy)
Bold 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.
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.
