Depth charge

Introduced as the first underwater antisubmarine weapons of the First World War, depth charges were never terribly satisfactory weapons, but had the advantage of being cheap and requiring only low technology. The first versions were no more than metal barrels, containing a large explosive charge and a fuze that was set for a given depth; free-falling and formed a barrage with no guidance; later models had streamlined cases for faster sinking.

"Barrage" is a key word; depth charges had no serious chance of harming a submarine maneuvering at depth unless enough were dropped such that they fell close, especially before accurate sonar was available. Aircraft had somewhat better luck in dropping depth charges on a submarine that they could see submerging.

Due to the incompressibility of water, the area of effect of a depth charge was surprisingly small. To produce more than stress-inducing noise, the depth charge had to detonate within a few meters of the submarine. Nevertheless, a long, resource-intensive "pin down" of a submarine could keep it down until it was forced to surface because it was out of air. Surfaced submarines were very vulnerable to the guns of even small warships, much less aircraft.

Originally, depth charges simply were rolled from a rack at the stern of the attacking vessel. This only allowed linear patterns. Effectiveness improved when various explosive or compressed gas powered launchers were added, which could throw depth charges away from the axis of the ship's course.

It was rarely practical, however, to throw a depth charge directly ahead of the ship. If it detonated at a shallow depth, it could damage the ship. All depth charges, however, suffered from the problem that they were intended always to explode, causing immense disturbance in the water and often causing acoustic contact with the submarine to be lost. One of the first substantially improved replacements was the hedgehog (weapon), which threw a pattern of small, contact-fuzed explosive projectiles ahead of the ship. These exploded only if they made contact with the submarine, so their noise was useful in locating the target. The initial hedgehogs had a small explosive charge and rarely could sink a submarine, as could the larger explosive charge of a precisely aimed depth charge.

Depth charges enjoyed some resurgence with the use of antisubmarine helicopters, which could localize the submarine with rows of sonobuoys and use of dipping sonar, so a depth charge could be placed with much greater precision. In general, however, depth charges are now obsolete, having been replaced by guided antisubmarine torpedoes.