Why depth-charge attacks needed several ships

A depth-charge attack requires the escort to pass over the submarine, creating a period in which the attacker loses contact before releasing its weapons. Another ship can continue tracking the target during that run. Two ships making crossing attacks improve the prospect of a successful pattern, while a third holds contact. Japanese escorts lacked later offboard aids such as towed sonar and helicopters, which provide separate detection and attack positions. Helicopter-launched torpedoes avoid the need for the same depth-charge run, although some navies retain helicopter depth charges. Escort numbers therefore mattered alongside the equipment carried by each ship.