German U-Boat

Submerged U-boats employed several devices to evade allied pursuers equipped with sonar. These consisted of decoys which resembled a submerged submarine and noise-makers to blackout the pursuer’s listening device.

BOLD Canisters

This was a metal canister about 3.9 inches in diameter, filled with calcium hydride which gave off large quantities of gas when mixed with sea water. It was launched from a special tube and on release, sea water seeped into a special valve which reacted with the chemical. The valve would open and shut, causing the canister to stay at a certain depth until the compound was depleted in about 20 to 25 minutes. To underwater locating devices such as sonar, the resulting bubble cloud could resemble a submerged U-boat, and unless the sonar operator was especially skilled, it was often difficult to distinguish from a real target. The allies called this a “Submarine Bubble Target” (SBT). BOLD was widely used from 1942 onwards, with new and improved versions being developed until the end of the war. The last was BOLD 5, and was intended for use at depths of up to 200 meters.

Sieglinde

Similar to BOLD, this device was a slight variation where it was designed to resemble a submerged U-boat moving at 6 knots. These two were normally used together.

Siegmund

This was an anti-sonar device which emitted a series of deafening explosions and were intended to blackout the enemy’s listening gear. The U-boat would make its getaway by altering course or running at high speed during this short period.