Anti-Submarine Warfare - WWI

While there were a few submarines used before the First World War, most notably Hunley, the real tale of the war against the submarine didn't begin until the outbreak of that war.

A paravane being hoisted out¹

In 1914, nobody was really sure how to attack a submerged submarine, or even how to reliably detect one. Submarines were detected by lookouts and attacked on the surface with guns, or if they had just submerged, by ramming. The initial British plan involved the use of explosive paravanes, closely related to those used for minesweeping. These were not particularly effective, and navies began to look for alternatives. One that was used extensively was the naval mine, from small fields off enemy bases to the massive North Sea Mine Barrage. Various types of nets were tried to detect submarines passing checkpoints, none of which worked particularly well. Even weirder and more useless were the various hand-thrown weapons that the British issued to their sailors.

Q-ship HMS Underwing²

At the beginning of the war, all of the various powers expected the traditional Prize Rules to apply. This meant that the submarine had to surface, board and secure the safety of the passengers. Only escorted ships could be fired upon without warning. To take advantage of this, the British introduced the Q-ship, an armed vessel disguised as a merchant ship. When the U-boat came alongside to take the ship under prize rules, the Q-ship would open fire, sinking it. In practice, it didn't work that well. Q-ships sunk 14 U-boats, but twice that number were lost.

Lusitania sinking

In reaction to the Q-ships, among other reasons, the Germans began unrestricted submarine warfare, a policy of sinking unescorted ships without warning. This policy was implemented in early 1915, then abandoned mid-year due to American diplomatic pressure after the sinking of the ocean liner Lusitania. That said, the limited number of torpedoes a U-boat could carry meant that small ships were often attacked using guns on the surface, as they were usually lightly armed and easy to sink.

An early depth charge, simply thrown over the side

During 1915, the Allies finally came up with an effective weapon against submerged submarines. The depth charge was a powerful bomb triggered at a specific depth. The first models were fired by a lanyard attached to a float, while later ones used a pressure fuse. Water is excellent at transmitting shocks, and depth charges did their damage through blast effect. The British Type D, with 300 lbs of explosives, was estimated to be able to destroy a submarine within 70', and damage any within 140', although the actual numbers were significantly smaller.

A depth charge exploding; note depth charge in the foreground

The biggest limitation on depth charges was that they had to be dropped directly off the ship, which made it impossible to cover a large enough area to reliably kill submarines. The British developed several different weapons capable of throwing bombs up to a mile to use against submarines on or near the surface. These were not particularly effective, due to their low rate of fire and resulting inability to cover large areas. A more effective weapon was the depth-charge thrower, which would fire depth charges to the sides of the ship, expanding the area covered by an attack significantly.

Depth charge tracks on the stern of a destroyer

In February of 1917, with the British blockade of Germany growing more painful, the Germans reintroduced unrestricted submarine warfare. Allied shipping losses rose quickly, reaching 860,000 tons in April. The British attempted to step up their hunting of submarines, aided by the use of radio intelligence (both codebreaking and direction-finding). However, this did not prove effective, and eventually convoying was introduced.

Escort HMS Geranium

Convoying is the practice of gathering a group of merchant ships and escorting them from one point to another. Besides allowing the escorts to be concentrated, it has other benefits that were not discovered until WWII. Specifically, submarines of the time took a long time to reload their torpedoes and were slow while submerged. This meant that they could only sink a few ships per convoy, regardless of the size of the convoy.³ Convoys had their drawbacks, though. A convoy was slower than ships sailing individually, and the arrival of a mass of ships tended to overwhelm port facilities, so using them considerably reduced carrying capacity.

Submarine HMS R3

Besides convoys, other methods were taken to deal with the U-boats. Submarines were one of the leading killers of other submarines during the war, accounting for almost 10% of U-boats lost. This was universally the result of a submerged submarine seeing a surfaced one, either in transit or while on patrol. The British made extensive use of this, and even built a specialized class of submarine, the R class, specifically for this task. The R class had excellent underwater performance, foreshadowing the famous German Type XXI by a quarter-century.

Curtiss Large America flying boat

Aircraft were also used extensively. Initially, lighter-than-air vehicles were the method of choice, due to their long endurance. Later, flying boats were used extensively. Aircraft were not particularly effective, sinking only 5 submarines in total, but their presence was a powerful deterrent to submarines. Late in the war, the British began "scarecrow" patrols, flying obsolete aircraft along the coast to force U-boats to dive.⁴

SC Tube passive hydrophone

The biggest problem during the war was the lack of a means of detecting submerged submarines. One early British attempt was to train seagulls to land on periscopes, but it ended about as well as you'd expect. More successful was the use of magnetic induction loops to detect submarines attempting to gain access to harbors and other areas of shallow water, but these were limited to small areas. The majority of research and success came from underwater listening devices known as hydrophones. A number of these were developed, but almost all suffered from a major flaw: to avoid self-noise blanketing the sounds of the submarine, a ship had to come to a dead stop and shut down most auxiliaries. The early hydrophones were also omnidirectional, which made it hard to locate a submarine accurately enough to attack. Later versions were directional, and by the end of the war, a few towed systems had been developed which could be used while the ship was under way. These systems were not particularly effective, as only 3 sinkings were made by patrol groups using acoustic detection.

Destroyer HMS Tetrarch with an oil slick produced by a submarine

The story of ASW in WWI is both interesting in how many later developments were foreshadowed and astonishing in how primitive the efforts made were. But work during the war and in the immediate postwar years laid the foundation for the great battles between submarines and their opponents during WWII.

For more on WWI ASW, see Hunters and Killers, Volume 1.