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American Twin 20-mm Gun. Many wartime submarines in the Pacific used this twin hand-operated version of the 20-mm Oerlikon light AA gun. It was mounted on a platform on the conning tower or on deck, and was also used against small surface targets

The Polaris system for firing ballistic missiles has already been described, but the Russians are now the only submarine power to employ cruise missiles. The Americans dropped their Regulus I and Regulus II systems because they both involved operation on the surface, which has become too risky for submarines. One wonders, therefore, about the value of the Russian Shaddock and Sark missiles in the early missile submarines.

The ultimate in submarine firepower is the American Trident project, formerly known as the Underwater Long-Range Missile System (ULMS). The project calls for ten submarines of about 15,000 tons displacement (submerged), each one armed with 24 Trident missiles capable of travelling 6,000 miles. The size of missile would permit even more multiple warheads than the Poseidon, as well as decoys and jamming devices to enable its warheads to beat the most advanced defenses. The cost is estimated to be $1,000 million per submarine, but the detente between Russia and the West will hopefully allow such a terrifying idea to be cancelled.

Hedgehog Attack Pattern. The Hedgehog mortar could be fired while the U-Boat was still held in the Asdic beam. The elliptical spread of the bombs was intended to give the best chance of hitting a submarine, and a detonation meant a certain "kill" except in very shallow water

The depth-charge remained the principal weapon against the submarine throughout the Second World War, although it was supplemented by the aircraft bomb, and by specialised weapons such as Hedgehog. After 1945 a variety of launchers replaced the old-style depth-charge throwers and racks, to eliminate the need for an escort to pass over the submarine before dropping her charges.

Quick-reaction defense

British Depth-charges. Left: The Mk VII (Heavy) was simply a charge with an added weight to make it sink rapidly. Right: The Mk VII (Aircraft Pattern) was the first airborne depth-charge. Centre: the Mk VII was the standard British depth-charge and differed little from the "D" Type of the First World War

The British favoured a three-barrelled mortar firing full-sized depth-charges. The first pattern was the Squid, but it was replaced by the Limbo, which had more range and could be trained over a wider arc. Limbo also had the advantage of being able to pre-set depth-charges with data supplied direct from the Sonar plot, and it remains in service today as a useful quick-reaction defense against a submarine which gets within a mile.

The US Navy and others favoured a rocket projectile to carry the explosive charge to the submarine. There are several versions of this type of weapon such as the now obsolete American Weapon Alfa, the Bofors quadruple launcher and the Norwegian Terne, but they all suffer from the basic weakness of carrying a small explosive charge, which reduces their chances of damaging a submarine.

The homing torpedo has been turned against the submarine, and it makes a potent weapon, especially when dropped by a helicopter. One way of destroying submarines by homing torpedoes is to programme the torpedo in a descending spiral; another is to use wire guidance. Significantly the three most advanced anti-submarine missile systems in existence today are merely delivery systems for homing torpedoes.

American Aircraft Depth-charge. During the war considerable time was spent on re-designing the old "ash-can" shape of the depth-charge to improve its flight-path and its rate of sinking. This was the US Navy's airborne depth-charge

The American Asroc system works like Subroc, but in reverse. It fires a missile on a ballistic trajectory to the target area, and then parachutes a torpedo into the sea. The French Lamafon is a 21-in torpedo with wings and tail, which is launched by rocket and then glides to its target area. The Anglo-Australian Ikara system differs from the earlier two in that the missile is actually a small delta-winged aircraft carrying a torpedo; its virtue is that it can be flown on a revised course to counteract evasive manoeuvres by the submarine, and when it reaches the area the carrier breaks up and releases the homing torpedo.

The original Asdic and hydrophones which were the only detection devices of the Second World War have been much improved and developed. These devices are now known as active and passive Sonar respectively, and the way in which they can be used has changed. Some U-Boats found that they could hide under "thermal layers" of seawater of a different temperature, which made the Asdic beams bounce off. This has been countered by the Variable Depth Sonar (VDS), which is simply a Sonar transducer towed behind an escort and lowered to search beneath a thermal layer. The US Navy has developed very powerful Sonar sets, and they are frequently mounted under the bow, where the hull turbulence is at a minimum. Helicopters can use their dipping Sonars for the same purpose, and formidable machines like the Sikorsky Sea King have space for homing torpedoes as well.

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