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The standard Mark VII depth-charge had already been modified for use in aircraft, but it was also made heavier to make it sink faster. This was an attempt to reduce that gap between the time the Asdic beam lost contact with the U-Boat, and the explosion of the first charge; obviously the shorter the time interval the more precise was the attack. Another more deadly weapon was the Mark X depth-charge, a 15-ft long canister packed with Minol, one of the new explosives developed during the war. This 1-ton monster could only be fired from a torpedo-tube, and when it exploded at great depth its concussive effect produced damage over a greater radius. It had its drawbacks however: being the equivalent of a full pattern of ten ordinary depth-charges it could blow the ship's stern off if set too shallow or if the ship was moving too slowly.

In 1941 the first "Hedgehog" appeared. This was a spigot mortar for firing 24 small bombs well ahead of the ship in an elliptical pattern. Each bomb had a 32-lb charge of Torpex (another new explosive) and a hit from one could sink a U-Boat. It allowed the firing ship to hold the target in the Asdic beam, but unlike the ordinary depth-charge a near miss did no harm at all to the U-Boat. In 1943 the British produced a fearsome weapon called "Squid", which fired three full-sized depth-charges ahead of the ship, and thus combined the advantages of both the earlier weapons.

To be on the receiving end of these weapons was a harrowing experience. For a start there was the audible "pinging" of the Asdic, and then if a searching escort gained contact would come the repeated concussion of patterns of depth-charges, each one containing 300 lb of high explosive. Light bulbs were shattered and small leaks could be started in the pressure hull. These were particularly dangerous as the mixture of seawater with the sulphuric acid in the batteries generated chlorine gas, which attacks the mucous membranes of the breathing passages.

A prolonged attack could keep the U-Boat down until her air-supply was exhausted even if she had not suffered structural damage. Evasive action was difficult because the noise of the electric motors could be detected by the enemy. If a submarine was trapped by a number of escorts and kept down until her air-supply ran out the choice of options was not inviting: to stay down and die of asphyxiation or blow tanks and try to fight it out on the surface.

Other ideas were germinating. At the end of 1941 the British converted a small merchant ship, the ex-German banana boat Empire Audacity into the first "escort carrier", HMS Audacity. She carried only six Martlet fighter aircraft, which had to be parked aft on her small wooden flight deck as there had been no time to provide a lift or hangar. The purpose of this conversion was to provide a defense against the Focke-Wulf Condors which were causing a great deal of trouble to the convoys running from the British Isles-to Gibraltar, but it proved that small utility aircraft carriers were a possibility. Audacity served for only a month before she was torpedoed off Portugal during a fierce convoy battle, but her aircraft had made so much difference that more conversions were ordered.

The entry of the United States into the war in the same month changed the whole situation, for she alone could provide the numbers of aircraft and the building resources to convert more escort carriers. As a result of strenuous British entreaties six mercantile hulls were converted in April and May 1942, and five of them were immediately transferred to the Royal Navy. However, the Americans were badly equipped for anti-submarine warfare, despite their decisive intervention in 1917, and the first result of their entry into the war was merely an inflation of the losses of merchant shipping. The U-Boats moved over to the Caribbean and the East Coast of the United States, where they found so many juicy targets that they christened the early months of 1942 the "Happy Times". It took only 21 U-Boats to sink 500 ships in six months.

The Americans' unpreparedness cannot be blamed on surprise, for the US Navy had been escorting ships bound for the British Isles to a "Mid-Ocean Meeting Point", called MOMP for short, since September 1941. One destroyer had been sunk and another severely damaged by U-Boat torpedoes during this quasi-war, and the British had freely handed over information about their anti-submarine measures. Furthermore, the Admiralty had ordered 50 American-designed escorts early in 1941.

How was it, then, that the US Navy appeared to have no way of protecting shipping in its own coastal waters? The answer is that senior officers still doubted the wisdom of convoys, even after all experience had shown how vulnerable unescorted ships were to submarine attack. The Americans, having a large number of elderly destroyers, and no specialised escorts like the British sloops and corvettes, pinned their hopes on "hunting groups", or high-speed patrols by destroyers to seek out the U-Boats before they could attack. It was seriously held by some officers that convoy was too defensive a measure to appeal to the aggressive American spirit - it might suit the more dogged, patient British but it was too old-fashioned an idea to be used by dynamic Americans.

Of course this argument had been the one used by the British from 1914 to 1917 to justify their own "aggressive" tactics, and it ignored the inescapable fact that it was impossible to cover the ocean with patrol vessels. The submarine by virtue of its invisibility need only hide until the patrolling group had passed, and this is just what happened. During the "Happy Times" U-Boat commanders reported that they could almost set their watches by the American patrols, which signalled their approach by plumes of smoke and impressive bow waves as a squadron of destroyers tore past at 30 knots. Once past, the length of their patrol line ensured that they would not be back for some time, and the U-Boat could rely on a free hand in running down solitary merchantmen once more. Furthermore security was bad, the American coastline was ablaze with lights, and the merchantmen chattered to one another in plain language, giving their positions regularly. Under these circumstances it is hardly surprising that 505 ships (nearly a third of the total for the whole year) were sunk in American waters before June 1942, when the US Navy finally organised all shipping into convoys.

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