A depth charge is an explosive device which is designed for use in anti-submarine warfare. Rather than being designed to hit the submarine itself, a depth charge is meant to cause damage with its concussive shock wave, ideally forcing the submarine to surface, so that it can be attacked by ships and aircraft overhead. These devices often play a prominent role in submarine movies, as fans of this particular genre may have noted.
The concept of the depth charge was initially developed in 1910 by the British, with early versions being rolled out by 1914 for the purpose of attacking German submarines. Early depth charges were very crude, but their designers learned quickly, developing increasingly effective weapons which became especially useful in the Second World War, when German U-Boats patrolled the Atlantic looking for victims.
Depth charges include a large quantity of explosives, to ensure that a large shock wave will develop, and a detonator, which is usually set to go off at a particular depth, or after a certain amount of time has elapsed. The first depth charges were just barrels filled with explosives on a timer which were rolled from the back of a ship. Modern versions are often finned, so that they will penetrate the water quickly and evenly, rather than straying off target, and in addition to conventional explosives, they can also be equipped to carry nuclear warheads.
In some cases, a depth charge can score a direct hit on a submarine, causing damage with the explosives. More commonly, however, depth charges must be laid on in a field, exploding at different heights in the hopes that one will eventually catch the submarine in its shock wave. When a submarine is attacked with depth charges, it can attempt to submerge to avoid them, and it can also try to outrun them, but the sound of the submarine's engines will alert listeners on the surface, allowing them to target the submarine more precisely. If the submarine is damaged by the shock wave, it will need to surface, making it a sitting duck for the attacker.
Originally, depth charges had to be deployed from ships. Modern versions can also be dropped from aircraft, which may work in concert with a ship, or on their own. Ships and aircraft which deploy depth charges must also watch out for return shots from the submarine, because the sounds of the deployment betray the location of the ship or aircraft dropping the charges.
Another weapon used in anti-submarine warfare is the torpedo. Torpedoes are designed to penetrate a submarine and detonate, ripping a hole in the submarine which will cause it to sink. A well-placed torpedo can also cause secondary detonations of weapons stored in the submarine, causing the ship to blow itself up. Submarines can also fire torpedoes at targets.