You’ve probably seen the movie. Submarine crewmen look up anxiously. Suddenly there’s a terrific explosion. Lights flicker off and on. Streams of water spurt from leaks. The attackers work the submarine over again and again, straining the crew to—or nearly to—the breaking point. Sometimes the submarine crawls away; sometimes the next scene show her survivors on board the attacking ships. In either case, you’re reminded that antisubmarine warfare, particularly during World War II, usually meant depth charging. However, you’ve probably never seen a live depth charge because this weapon died off with the advent of fast submarines in the postwar years.
During World War I, when the depth charge was invented, a submerged submarine was an elusive target, usually only indicated by the swirling water where a periscope had been or by the track of a torpedo. The only prewar antisubmarine weapon was a sweep, a towed submerged explosive developed by the Royal Navy. If a destroyer towing a sweep happened to pass over a submarine and the device snagged the boat, the destroyer could detonate the explosive electrically. But that rarely happened.
In the fall of 1914, British Grand Fleet commander Admiral Sir John Jellicoe was very concerned by the threat of U-boats that might lurk off his base at Scapa Flow. He had a force of destroyers equipped with prewar sweeps but knew that they offered only the most limited protection to his fleet. In December Jellicoe requested that HMS Vernon—the British underwater-weapons establishment based on board the Vernon—develop a mine that would explode reliably underwater. Ships spotting U-boats would drop the mines. At the very least, they might drive off submarines threatening the fleet (Jellicoe was probably under no illusion that they would have much of a chance of killing U-boats).
HMS Vernon’s solution was to develop a hydrostatic pistol that would reliably set off an attached explosive charge at a set depth, deep enough, for example, not to damage the ship dropping it. Other navies tried a simpler approach: attaching the sinking charge to a float. When the cord connecting the two became taut, the charge was at more or less the desired depth, and it was set off. This solution was substantially less reliable than HMS Vernon’s, and it made for a far bulkier weapon that could not be carried in quantity.
HMS Vernon’s depth charge became available in 1916 (the design for the standard Type D was approved in June 1915). By late in the war, British destroyers on convoy or fleet escort duty typically carried six, two of them in throwers (installed beginning in August 1917). These ships could not afford any more top weight, and they also had to accommodate antisubmarine circling torpedoes, antiship torpedoes, and guns. Ships on special submarine-hunting duty often carried many more depth charges, perhaps up to 50 of them.
When the United States entered World War I, the British gave their depth-charge design to their new ally. The Bureau of Ordnance developed an improved hydrostatic pistol and in 1918 produced a depth charge double the weight of the British type. U.S. destroyers were considerably larger than nearly all their British counterparts, so they could easily accommodate more depth charges, typically about 30. Along with the heavier depth-charge battery came a different approach; U.S. Navy instructions emphasized convoy escorts hunting down detected U-boats rather than merely frightening them off.
Not many U-boats were sunk by depth charging during World War I because there was no way for antisubmarine ships or craft to remain in contact with their targets long enough to pour on enough charges to do fatal damage. No single charge had much chance of killing a submarine, although there were sometimes grossly overoptimistic accounts of what even a near-miss might do. Postwar experience with surrendered U-boats cured that. More important, between the wars the Royal Navy and the U.S. Navy developed active sonar (Asdic in British parlance). It offered two enormous advances, the first being that there was now a way of detecting a submerged submarine, although the chances of simply picking one up were poor. Moreover, once a submarine was detected, the hunter had a good chance of maintaining contact.
While no single depth-charge attack was likely to kill a U-boat, even though World War II salvoes dwarfed those of the previous war, sonar made it possible to keep attacking. Now it made sense for an escort to leave a convoy to chase down a U-boat; killing that submarine would save other convoys from attack. (If you want the flavor of such hunts, watch the The Cruel Sea, a 1953 film based on a best-selling novel by Nicholas Monsarrat.)
Attack after attack required very heavy depth-charge batteries. By 1943 the Royal Navy was firing 8 and even 14 charges in each attack, and some escorts carried as many as 144 depth charges. You can get a sense of what was involved by looking at the after-half of a contemporary U.S. antisubmarine ship—a destroyer escort—and counting the depth charge throwers (K-guns) ranged down the ship’s side, each with its stock of reloads. This was far beyond anything imagined during World War I.
The depth charge was a relatively crude weapon, and it didn’t help that a captain had to maneuver the stern of his ship into place to use it. As she approached to within a few hundred yards of a submerged submarine, a sonar-equipped surface ship lost contact, because the sonar beam was tilted down at only a shallow angle. When contact was lost, the attacker would accelerate, so that the submarine could not move far before the depth charges sank toward her. Submarine commanders learned to listen for the acceleration (indicated by the sound of the attacker’s propellers) and then maneuver radically.
Depth charges took some time to sink. The deeper the submarine, the greater the lost-contact range and the greater, too, the dead time between when the charge was dropped and when it sank to the submarine. Attacking a submarine that could make probably all of 5 knots underwater was difficult enough, but the advent of fast submarines ruined depth-charge tactics altogether.
A solution, conceived as early as 1941, was for an escort to fire charges over her bow before contact was lost. Veterans of the 1950s and 1960s will remember the Hedgehog, a mortar firing 24 contact-fused bombs. An alternative favored by the Royal Navy (but adopted in only a few U.S. ships) was Squid, a mortar firing fast-sinking depth charges ahead of the escort. (The postwar Royal Navy successor was the longer-range Limbo.)Range could be longer, and even if the charge did not hit the submarine it would explode with the sort of physical and psychological effects advocates of depth charging always claimed.
All of these depth-charge weapons have died out in favor of homing torpedoes such as the ubiquitous Mk 46. But perhaps they will return. The U.S. Navy now operates in littoral waters shallow enough for diesel (not nuclear) submarines to lie on the bottom. Homing torpedoes rely on the Doppler generated by target motion to distinguish a live submarine from a bump on the bottom. An imaging sonar would probably show that the bump on the bottom really is a submarine. If a salvo of depth charges did not kill the sub, their explosions would probably convince her commander that he was in trouble. Once he ran for it, the homing torpedoes would have their chance.
Norman Friedman is a consultant on global naval strategy, naval trends, and naval warfare who has authored more than 30 books, including Fighting the Great War at Sea: Strategy, Tactics and Technology (Naval Institute Press, 2014).