anti-submarine weapons

anti-submarine weapons. The object of an anti-submarine (A/S) weapon was (and is) to pierce or rupture a submarine's pressure hull and let the submariner's worst enemy—the sea itself—make the kill. Although only tactical A/S weapons are reviewed here, ground and moored mines (see mine warfare, 2) were responsible for numerous submarine losses during the Second World War—probably one-third of those suffered by the Royal Navy.

A submarine on the surface, or to a lesser extent at periscope/Schnorchel depth, was vulnerable to torpedoes fired by other submarines as well as to guns, air-dropped weapons, or ramming; but, except when torpedoed by a submerged submarine, a boat could often dive deep quickly enough to avoid being hit provided that a sharp visual or electronic lookout was being kept. It was therefore imperative for A/S units, aircraft in particular, to attack without delay, which led to more than a few ‘own goals’ due to wrong identification. The most effective aerial means of detecting a submarine on the surface was centimetric radar; at night this could be used in conjunction with a powerful searchlight. The Americans also developed the Magnetic Anomaly Device (MAD) to detect submarines from the air, even when submerged, and a Catalina flying boat had the first success with this when it sank a U-boat in the Straits of Gibraltar in 1944.

If a submarine was fully submerged the hunters had to explode a weapon in the right position at the right depth which might be anything down to 300 m. (985 ft.). The exact depth was always hard to estimate. Moreover, an attack could often be predicted by a submarine commander who was able to alter course, speed, and depth drastically, and/or release a bubble-making decoy (the Germans called the device a Pillenwerfer) at the critical time.

A single A/S weapon stood little chance of success unless it had a homing capability: the US Navy therefore produced the Fido air-dropped, acoustic-homing torpedo in 1943 (see also guided weapons). The Japanese developed a non-homing torpedo that spiralled down to 200 m. (656 ft.) in the (mostly vain) hope of hitting its target at some point.

The most common weapon was the depth charge (D/C). Similar in all navies, it was simply a strong canister of high explosive—90–136 kg. (198–300 lb.) in weight but less for small patrol craft—actuated by a hydrostatic device at a pre-set depth. The British ‘Mark VII Heavy’ had the deepest maximum setting at 259 m. (850 ft.): lethal range was 9 m. (29.5 ft.). D/Cs dropped from aircraft had fins like the A/S bombs aircraft also carried. Typically, surface warships dropped D/Cs in patterns, like the Five of Clubs, with a spacing of 37–55 m. (120–180 ft.), three being rolled over rails at the stern and one being discharged on each quarter by throwers. A depth bracket could be achieved by alternating heavy charges (quicker sinking) with lighter ones. Of all German U-boats destroyed 43% were sunk with D/Cs.

The attacker's ASDIC lost contact in the final 180 m. (590 ft.) of a D/C run over the target: that ‘deaf-time’ as it was called, together with the D/Cs' sinking time, afforded the submarine precious moments to take vigorous evasive action. However, even if the explosions were not close they could cause extensive damage, shake a crew's morale, and, most important, keep the submar ine deep and impotent. The answer to ‘deaf-time’ was ahead-throwing weapons which only the Allies developed satisfactorily. A widely fitted type was the British Hedgehog (adopted by the US Navy in parallel with, and preferred to, the smaller Mousetrap) comprising 24 light projectiles mounted on six rows of spigots. Each row tilted to compensate the ship's rolling; and each spigot was slightly angled to spread the weapons over a circular area, some 40 m. (130 ft.) in diameter, well ahead of the firing ship so that it could maintain contact with the target. The disadvantage of Hedgehog was that it only exploded on contact: it did not have a depth charge's deterrent value.

Squid, its uniquely British successor in late 1943, was a large three-barrelled mortar, eventually mounted in pairs, which fired projectiles carrying 91 kg. (200 lb.) of powerful Minol II (see explosives). These sank rapidly at 13.4 m. (44 ft.) per second (nearly three times as fast as a D/C) and exploded at an automatically set depth in triangular patterns with 37 m. (120 ft.) sides. A double mounting produced two depth layers 18 m. (59 ft.) apart. Double Squid, besides reintroducing the deterrent effect, theoretically raised the kill probability to 50% from 6% with depth charges and 20% with Hedgehog.

Richard Compton-Hall

Bibliography

Compton-Hall, R. , Chapter 9, ‘The Enemy Above’, in The Underwater War 1939–1945 (Poole, Dorset, 1982).
Hackmann, W. , Asdics in World War II: measures and countermeasures (London, 1984), ch. 12.