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ANTONY PRESTON, JOHN BATCHELOR
The aces soon found that a night attack on the surface made a U-Boat almost invulnerable. The British escorts' Asdic could not detect a surfaced submarine, and it took an exceptionally sharp lookout to spot the conning tower of a U-Boat. Kretschmer went a step further, and took his U-Boat inside the columns of the convoy, the last place an escort commander would think of looking, and from this "sanctuary" he could sink ships with impunity while his torpedoes lasted. The answer to this tactic was surface- warning Radar, but in 1940 no escorts were fitted with it, so merchant ships were fitted with illuminant rockets known as Snow- flakes; when these were launched at the orders of the escort commander any U-Boat near the convoy would find itself suddenly exposed to view.
It was at this time that Admiral Dönitz began to intervene more directly in the conduct of U-Boat operations. Realising that the exploits of the aces could not be emulated by the newer submarine commanders and crews, he was anxious to use the large numbers of new boats in "wolf packs". The essence of the wolf pack tactics was the swamping of a convoy and its escorts by a co-ordinated attack from a group of U-Boats. To achieve this a U-Boat which made contact with a convoy was given strict orders not to attack but to signal its course and position to U-Boat Headquarters, which would then make contact with other U-Boats in the area and direct them to the original U-Boat's position. When the pack was assembled it was launched against the luckless convoy in a series of night attacks, night after night if necessary.
The chief danger from wolf pack tactics was that a U-Boat was both fast and hard to detect on the surface. Running at 17 knots on her diesels, a U-Boat could outpace the trawlers, corvettes and sloops which made up the bulk of convoy escorts in 1940 and 1941. Destroyers were faster, but too few. and in any case the lack of a radar set in all but a few ships made it difficult for any escort to sight a U-Boat. The new tactics were introduced between October 1940 and March 1941, and they proved deadly.
Another factor contributing to their success was the aerial reconnaissance provided by Focke-Wulf Condor aircraft operating from French airfields. These four-engined aircraft were able to locate convoys and shadow them for the benefit of the U-Boats, and until air-cover could be provided for convoys there was virtually no defense against them.
As stated before, the small size of the Type VII U-Boats prevented them from ranging too far in pursuit of targets, and to offset this a "U-tanker" was introduced. The Type XIV was known to the Germans and the British as the "Milch Cow", and each one could carry 432 tons of spare fuel and four torpedoes for transfer on the surface. Only ten were completed in 1941/42, and as Allied anti-submarine forces were told to give them top priority all were sunk. A further ten were cancelled because the growing threat from aircraft made refuelling on the surface too dangerous. For the same reason a series of much larger supply boats, the Types XV and XVI were also cancelled.
Italian Brin. The 1,000-ton Brin Class of four boats were completed in 1938, but a year later another two were added to replace an earlier pair transferred to General Franco during the Spanish Civil War. Armament: Eight 21-in torpedo-tubes (forward); one 3·9-in gun; four 13·2-mm machine-guns. Speed: 17·4 knots (surfaced), 8·7 knots (submerged)
The British adopted several counter-measures to meet the new threat. In May 1941 the first Type 271 surface search radar set went to sea in a corvette, and as it could detect a conning tower at 2,5 miles or more it put paid to surface attacks at night. Another device was High-Frequency Direction-Finding, known for short as Huff-Duff or H/F-D/F. Its principle was well known, but the British had been able to produce a set of great sensitivity which was small enough to be installed in an escort, and this meant that the vital signals sent by a shadowing U-Boat could be traced to within a quarter of a mile. The result was that the U-Boat could at least be forced to dive, and thus silenced; an immediatealteration of the convoy's course then gave the U-Boats the lengthy task of relocating the convov and re-assembling the wolf pack.
March 1941 was a bad month for the U-Boats. Five U-Boats were sunk in the North Atlantic, including the three aces Prien (U47), Kretschmer (U99) and Schepke (U100). A month later U110 fell into British hands for long enough to allow boarders to recover her code-books. The results of this exploit was good enough for the Admiralty to keep its secret until well after the end of the Second World War. Even today it is not known exactly how much knowledge of German cyphers was gained, but it can be assumed that some of the successes against U-Boats in 1941 were attributable to the capture of U110.
There was no quick answer to the Focke-Wulf Condor, but an interim remedy was to fit some merchant ships with a catapult for launching a single Hurricane fighter aircraft. Although there was no way of recovering the fighter, it was a fair exchange for the degree of immunity conferred on the convoy if the shadower was shot down. Ideally each convoy needed its own aircraft carrier, but in 1941 this was quite beyond the Royal Navy, even if there had been sufficient aircraft to equip the carriers.
The other area in which improvements were made was weaponry. The difficulty in holding a submarine contact with Asdic as the contact came closer to the searching ship has already been mentioned: the answer was to provide a weapon which could project bombs or depth-charges ahead of the ship, while she still held the U-Boat in the Asdic beam. Development of such weapons took time, and in the meantime it was only possible to increase the number of depth-charges which could be dropped. Before the war two depth-charge throwers and a short rack of charges was considered enough, but by 1941 most escort destroyers had surrendered a gun on the quarter-deck for a heavy outfit of four throwers and two extended racks of depth-charges. Better reloading gear was provided so that an escort could keep up a continuous attack, and the pattern of dropping charges was revised to give the maximum chance of destruction.