Four of the six victims of 16–17 October 1943 were sunk by aircraft, which were increasingly proving even more deadly to U-boats than well-trained and -equipped escorts. The aircrew of the specialist anti-submarine squadrons were themselves now highly trained and had gained great experience in four years of warfare. At first their role had been little more than that of forcing down U-boats in daylight, though even the early anti-submarine aircraft were equipped with depth-charges. By 1943, however, better aircraft, with better equipment, were successfully attacking and killing U-boats at a distance from convoys. The weak spot in Dönitz’s oceanic dispositions was the Bay of Biscay, for across it almost all his U-boats had to pass, from their bomb-proof bunkers in the French west coast ports, to the North Atlantic. They began by making the passage submerged during daylight hours but on the surface at night, so that they might recharge their batteries and achieve the higher speed possible on their diesels. In the first three years of the war, they made the passage safely. In the first twenty-six months only one was sunk by aircraft in the Bay, and then because Bletchley had supplied accurate intelligence and the Coastal Command aircraft engaged, a Whitley of JO2 squadron was equipped with an early model of search radar.
Coastal Command recognised that its so-called Biscay offensive was ineffectual. The aircraft were types pensioned off from Bomber Command, their search equipment was feeble, even their offensive weapons were of limited effect. During 1942 there were improvements. The most important was the development of a visual sighting device, the Leigh Light, so called after its inventor, an RAF officer. Mounted typically in a Wellington medium bomber, and steerable, it was illuminated after on-board search radar had detected the target. The pilot then flew down the beam of light, switched on only after the U-boat had lost the chance to dive deep, when depth charges were dropped either around the diving boat or into its submerging swirl.
From the spring of 1943, air attack on U-boats attempting to enter the North Atlantic either across the Bay of Biscay or, having circumnavigated the British Isles from German ports, through the Faroes–Iceland gap, was increasingly successful. On 5 May, for example, U-663 was attacked in daylight by a Sunderland flying-boat and sunk with depth charges. On 8 May a Halifax heavy bomber sank the U-boat tanker 490 with depth charges in the Bay of Biscay. On 31 May U-563 was attacked in the Bay of Biscay by a Halifax, then by another, finally by two Sunderlands which destroyed her. On the same day, U-440 was depth-charged by a Sunderland and sunk, almost on the edge of the Atlantic.30 The destruction was not all one-way. In late 1942 Dönitz had begun to equip U-boats with extra anti-aircraft guns and issued orders to “fight it out on the surface.” It was an unequal struggle, for the U-boats, particularly if caught at night in a Leigh Light beam, were far more vulnerable than their attackers. Nevertheless, considerable numbers of aircraft were hit by heavy fire when pressing home their attacks, as the pilots did with great bravery; understandably, for the sighting of a U-boat on the surface came very rarely in hundreds of hours of patrolling an empty sea. In the excitement of the moment, aircraft were flown to within a few hundred feet of the surfaced U-boat, in the effort to assure that a straddle of shallow-set depth charges would send it to the bottom. Sometimes the U-boat’s firepower destroyed the attacker, killing the crew immediately or leaving the survivors to die slowly in life rafts. The same, of course, happened to the U-boat crews whose boats were sunk by air attack. Unlike those forced to abandon ship near convoys, who stood the chance of being rescued by an escort—as the famous ace Otto Kretschmer was on 9 March 1942, by HMS Walker, with most of his crew of U-99—sinking by an aircraft in the open sea led either to immediate death or to a longer agony in a lifejacket. The same went for the crews of unsuccessful aircraft forced to ditch, either because of offensive flak from the U-boat “fighting it out on the surface” or as a result of interception by German fighter aircraft based on the west coast of France. A particularly grisly fate awaited damaged models of the earlier twin-engined Wellington, which could not sustain altitude if one of its engines was knocked out. Clay Blair calculates that twenty-nine aircraft were shot down by their intended victims, though three were sunk while defending themselves.
On balance, however, the advantage lay with the aircraft. In the concerted Bay of Biscay offensive of April–September 1943, thirty U-boats were sunk and nineteen damaged and forced back to port. “Fighting it out on the surface” proved a suicidal tactic, even when U-boats sailed in groups, and it had to be abandoned. During 1943, moreover, even more lethal air-to-surface (and sub-surface) weapons appeared, including air-launched rocket projectiles, typically fired by the ancient Swordfish biplanes flown off British escort carriers, and 6-pounder anti-tank guns mounted in land-based Mosquitoes. Both could penetrate the U-boat pressure hull, with disastrous effect. The dominant air-launched weapon, however, proved eventually to be the so-called Mark 24 Mine, cover name for an American acoustic torpedo which homed on the propeller noises—cavitation—of a submerged U-boat. U-boats began to receive the equivalent Zaunkönig acoustic torpedo during 1943, which homed on the engine and propeller noises of escorts; several were sunk or seriously damaged until a decoy noise maker, towed astern, was developed. U-boats could not use decoys, since their safety lay in keeping silent. Because the Mark 24 Mine was successfully kept secret, U-boats attempting to make off submerged after being surprised by aircraft fell victim. The Mark 24s “were curious weapons to use . . . there was always an anxious time-lag before any sign of a result, sometimes as long as 13 minutes. When it came it might consist of nothing more than a brief disturbance of the sea’s surface.”31
What occurred beneath the surface does not bear contemplation. Unlike a depth charge, which could damage a U-boat badly enough to force it to surface without wrecking the boat, the Mark 24 was an impact weapon, which ruptured the pressure hull, causing it to flood too rapidly for any hope of escape by the crew. Airmen who achieved a kill were bound to experience a sense of satisfaction at the culmination of a search process which occupied hundreds of hours of fruitless flying time; but even the most hardened must have shuddered at the thought of the reality of their victims’ fate. There were one or two accounts by survivors of their experience in leaving a U-boat making its way to the bottom, and they tell of a screaming mass of humanity, fighting for egress against the inrush of water, all comradeship lost, every man for himself, imbued by blind and selfish panic.
By 1944, the U-boats were fighting a lost battle. Dönitz’s efforts to deploy boats capable of cruising undetected and at long range—the snorkel models, which could breathe submerged, and the Walther electro-boats, running on hydrogen peroxide, which did not need to breathe externally at all—were unsuccessful. The snorkel, hated by the crews for its tendency to burst their eardrums, so reduced the boat’s speed submerged as to negate its theoretical advantages, while the Walther boats were too shoddily built to achieve the results of which they should have been capable. After four years of war, and despite episodes of offensive success which had shaken governments on both sides of the Atlantic, the U-boats were a beaten force.
The pre-war elite, which had opened the campaign against Allied shipping, were dead or captured. Their successors, who had hit British shipping so hard in 1940–41, and had rampaged along the American coast in 1942, had suffered the same fate. There were few survivors of the great convoy battles of early 1943. The new commanders and new crews of 1944 paid a terrible price for their inexperience. Dönitz never relaxed his standards of training. Even so, U-boats remorselessly fell victim to the increasingly efficient British, Canadian and American escort groups and their associated air squadrons. Of the 591 U-boats which sailed between May 1943 and May 1945, 138 were sunk either in their first voyage or under a new captain, often within a few days of putting to sea. Of the seventeen U-boats which attempted to escape from Germany to Norway in May 1945, all but one newly commissioned, all were sunk.32
What had begun as an unequal struggle between an inadequate fleet of British escort ships, with primitive detection devices and crude underwater weapons, had swelled during the course of the war into a major anti-submarine campaign, prosecuted on the Allied side by an ever-larger armada of British, Canadian and American destroyers, sloops, frigates, corvettes and, critically, escort aircraft carriers, supported by an eventually very large complement of land-based aircraft. In the course of six years of bitter conflict, the Allies had introduced a succession of increasingly efficient detection devices and underwater weapons, including sonar able to register depth as well as bearing, centimetric radar and high-frequency direction-finding, a wide variety of depth charges, filled eventually with the very powerful Torpex explosive, and several models of forward-firing contact weapons, particularly Hedgehog and Squid. The Americans had also developed the deadly Mark 24 Mine (“Wandering Annie”), to be dropped from aircraft.
In response, the U-boat arm, though greatly expanded in number, from 57 in 1939 to a total of 1,153 built by May 1945, scarcely developed at all. The Types VII and IX, the former undersized, the latter unhandy, supplied most of its ocean-going strength throughout the war; their experimental successors were all unsatisfactory in one way or another. Their weapons, after early faults had been corrected, scarcely improved either; only the Zaunkönig acoustic torpedo was a real improvement, and by the time of its appearance the wolfpacks had been driven from the convoy routes. The U-boats’ passive detection devices—hydrophones, able to detect ship noises at long distance—were excellent, but no active detection device, such as centimetric radar, useful as it would have been despite its traceable emissions, was ever developed. The U-boats’ own radar-detection devices, such as Metox, were crude and came to be distrusted by their users. As a result, U-boat captains, like those of Nelson’s frigates, depended throughout the war on line of sight, enhanced only by forming patrol lines in exactly the same way as Nelson had done. Radio extended their range a little by providing a captain who got a sighting with means to summon others; but because of the danger of interception, and the relatively low superiority of speed of a surfaced U-boat over that of a convoy, patrol lines had to be kept quite short. As Dönitz was frequently to comment, even the best-placed patrol line, operating against convoys controlled by a headquarters that could steer them away from identified U-boat traps, was often intercepted by boats only at the extremity, leaving the rest to make up hundreds of miles of sea room before they could achieve an attacking position. Long-range aircraft, which might have found convoys at a greater distance than a U-boat captain could see from his conning tower, were in even shorter supply on the German than the Allied side; and the German Condors not only lacked the range of the B-24 Liberator, the dominant aircraft of the Atlantic battle, but were based exclusively in France, and thus limited in coverage, while their Allied equivalents flew from Northern Ireland, Cornwall, Iceland and North America and so were eventually able, with the introduction of Very Long Range models, to oversee the whole Atlantic.