From Mary Bellis,
Your Guide to Inventors.
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The depth charge or bomb is a waterproof weapon used by ships or aircraft to attack submerged submarines.

First Depth Charges

The first depth charges were developed by the British in World War I for use against German submarines or U-boats, beginning in late 1915. They were steel canisters, the size of an oil drum, filled with TNT explosives. They were dropped off the side or stern of a ship, on top of where the crew estimated the enemy submarines were. The canister sank and exploded at a depth that was preset by the use of a hydrostatic valve. The charges often did not hit the submarines but the shock of the explosions still damaged the submarines by loosening the submarine enough to create leaks and forcing the submarine to surface. Then the naval ship could use its guns, or ram the submarine.

The first depth charges were not effective weapons. Between 1915 and the end of 1917, depth charges destroyed only nine U-boats. They were improved in 1918 and that year were responsible for destroying twenty-two U-boats, when depth charges were propelled through the air over distances of 100 or more yards with special cannons, increasing the damage range of the naval ships.