In response to the urgent need for ways to attack dived submarines, British shipbuilders John I. Thornycroft designed the depth-charge thrower in 1917.
Soon after the outbreak of World War I a number of destroyers were fitted with the ‘modified sweep’, a 60.96m (200ft) loop of wire fitted with explosive charges. The upper leg of the loop was kept buoyant by wooden floats, while a ‘kite’ depressed the lower leg of the loop. It was intended to be streamed when a submarine had submerged after being sighted, and could not be towed at more than 10 knots.
An electric indicator showed if any obstruction fouled the sweep, allowing the operator to detonate the charges. As it took some 20 minutes to deploy and because it restricted the ship’s maneuverability, the sweep was heartily disliked by its operators, but it has now gone down in history as the first dedicated anti-submarine warfare weapon, and it is credited with sinking U.8 in March 1915 and UC.19 in December 1916.
Also issued early in the war was the lance bomb, a 9.07-13.6kg (20-301b) charge on the end of an ash pole. They were intended to be used against submarines alongside, and in April 1916 the skipper of a drifter disabled UB.13 by rushing aft and hurling a lance bomb down onto the submarine’s casing.
The paravane was a torpedo-shaped device with lateral fins, and was towed from the bow of a ship to cut the mooring wires of mines. In that role it was very effective, but in 1915 it was adapted as the ‘high speed submarine sweep’, with two explosive paravanes towed from each quarter. In theory one of them would foul a submerged U-boat, which would detonate the charge, or, if the destroyer wished to get rid of them in a hurry, they could be detonated electrically. It was as cordially detested as the modified sweep, and also sank only two U-boats, UB.18 in December 1916 and UC.16 in October 1917.
Scientists were already at work trying to find precise ways of locating submerged submarines from the noise of their electric motors. The first non-directional passive hydrophones were issued to small craft in 1915, but they were inaccurate and required the vessel to be virtually motionless while the operator strained to distinguish any meaningful noise. Collaboration between French and British scientists promised much for the future, but the two unrestricted U-boat campaigns had to be fought without the benefit of their work.
By far the most effective ASW weapon yet developed was the depth charge, which appeared towards the end of 1916 in pitifully small numbers. In essence it was a massive charge of 3001b (136kg) of TNT or amatol, detonated by a hydrostatic device (ie, preset to explode at selected depths). From 1917, production was good enough to supply all destroyers and patrol craft with depth charges, rising from 140 per month in July 1917 to 500 a month in October and 800 in December. The wasserbom was disliked by the U-boat crews, who had to endure the concussion of near-misses during hunts which might last hours, during which light bulbs were shattered and blown rivets caused leaks in the pressure hull. In 1917 up to 300 depth charges were used each month, and in the last six months the monthly expenditure was 2000.
A variety of howitzers and bomb throwers was developed, ranging from 7.5in calibre up to 11in calibre, but these fired contact-fused charges, and were only effective against surfaced U-boats. By 1918, the famous destroyer-builder John I. Thornycroft had produced the first depth-charge thrower, a stubby mortar capable of hurling the charge well clear of the ship. The first submarine to be sunk by a depth-charge attack was UC.7, attacked by the motor boat Salmon in July 1916.
