Bold (also calledBolde, a term derived fromkobold) was aGermansonar decoy, used byU-boats during theSecond World War from 1942 onwards. It consisted of a metal canister about 10 cm (3.9 in) in diameter filled withcalcium hydride. It was launched by an ejector systemcolloquially referred to asPillenwerfer (English: "pill thrower"). When mixed withseawater, 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 30 m (98 ft). The device lasted 20 to 25 minutes. It replicated the echo of anAsdic-sonar submarine contact.[1]
TheRoyal Navy called it SBT (Submarine Bubble Target).
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