The Second World War (17 page)

The principal enemy of convoy was the submarine, or U-boat (
Unterseeboot
). As in 1914 the Germans also deployed a number of surface commerce raiders, including both orthodox warships and converted merchantmen, but their number was small; between September 1939 and October 1942 less than a dozen auxiliary raiders gained great waters, of which the most successful,
Atlantis
, sank twenty-two vessels before interception and destruction by HMS
Devonshire
in November 1941. Germany’s battleships, battlecruisers, pocket battleships and cruisers occasionally raided the sea lanes, but they too were few in number, and judged too valuable, to be risked often, particularly after the humiliating defeat of the pocket battleship
Graf Spee
off Montevideo by three British cruisers in December 1939. German aircraft achieved some success as ship destroyers – in May 1941, a peak month, they sank 150,000 tons (the average displacement of a Second World War merchant ship was 5000 tons) – and mines, whether laid by aircraft, surface ship or submarine, were a constant menace. German fast coastal craft, known to the British as E-boats, were prolific minelayers in British coastal waters in 1941-4, and constituted a relentless threat to coastal convoys; in April 1944 a raid on an American troop convoy practising disembarkation for D-Day at Slapton Sands in Devon drowned more GIs than were lost off Normandy on 6 June itself. However, the attacks of aircraft and surface ships, large and small, on merchant shipping were extraneous to the real battle at sea in European waters in the Second World War. That was one, as Winston Churchill rightly denoted, between the convoy escort and the U-boats.

In September 1939 Karl Dönitz, the German U-boat admiral, had fifty-seven U-boats under command, of which thirty were short-range coastal types and twenty-seven ocean-going. The German navy’s pre-war expansion programme, the ‘Z-plan’, called for the construction of a fleet of 300, with which Dönitz claimed he could certainly strangle Britain to death. He was to achieve that total in July 1942, allowing him to maintain 140 boats on operations and sink shipping at an annual rate of 7 million tons, a figure which exceeded British building of replacement shipping more than five times. By then, however, thanks to the inescapable dynamic of warfare, almost every term in the equation by which he had calculated the inevitability of Britain’s strangulation by U-boat tactics had changed to his disadvantage. Requisition and chartering of foreign ships had added 7 million tons to the British merchant fleet, the equivalent of a year’s torpedoing. American shipyard capacity, enormously expanded by an emergency mobilisation, had been added to the British, promising an output of 1500 new ships in 1943 (including many of 10,000-15,000 tons), more than three times as many as the U-boats were sinking. Naval construction in the United States would add 200 escorts a year to the fleet between 1941 and 1945. Over 500 of these would go to join the Royal Navy’s escort fleet in the North Atlantic which, having reached a strength of 374 in March 1941, had almost doubled since the outbreak. Long-range aircraft based in North America, Iceland and Britain were progressively reducing the ‘air gap’ in which U-boats could safely operate on the surface, their preferred mode because of their low submerged speed; and integral aircraft protection for convoys, provided by ‘escort carriers’, was soon to level a direct threat against attacking U-boats. Only in facilities for basing his boats had Dönitz’s position improved; in electronic and cryptographic warfare the conflict hung in the balance; the promise of secret underwater weapons, which favoured Germany, could not be realised for some years. Nevertheless the U-boats had already inflicted severe material and psychological damage on the Allied, particularly the British, war effort; and in mid-1942 the eventual outcome of the Battle of the Atlantic was evident to no one. The ‘statistics, diagrams and curves’ were pregnant with menace.

Thus far the Battle of the Atlantic (Churchill had coined the term) had passed through four distinct phases. From the outbreak of the war until the fall of France, the U-boat fleet had been confined by geographical constraints and Hitler’s concern for the sensitivities of neutrals to operating within the immediate vicinity of the British Isles. After June 1940, when Germany gained possession of the French Atlantic ports (where in January 1941, with remarkable prescience, Hitler ordered the construction of bomb-proof U-boat ‘pens’ to begin), the fleet began to operate in the eastern Atlantic, concentrating in particular on the ‘Cape route’ to West and South Africa and occasionally penetrating into the Mediterranean, since the Italians were proving themselves inept submariners. From April to December 1941, thanks to their increasing expertise in anti-convoy tactics, and despite the delineation of an American ‘Neutrality Zone’ in which the United States Navy gave notice of its intention to attack marauding submarines, the U-boat captains began to extend their operations into the central and western Atlantic; after June 1941, when Britain began to run convoys to North Russian ports with war supplies, the U-boats, frequently supported by German warships and shore-based aircraft, also began to operate in Arctic latitudes. Finally, after December 1941, Dönitz’s men carried the submarine war to the Atlantic coast of the United States and into the Gulf of Mexico, where, during a gruesomely named ‘Happy Time’ of several months caused by the US Navy’s temporary inability to organise convoy on coastal routes, they sank coastwise shipping by hundreds of thousands of tons.

Until June 1940 the U-boats had been confined by the same facts of geography as had kept the German High Seas Fleet close to their home bases during the First World War. Using the Baltic as their training ground (as they were to do throughout the war), they attacked British shipping in the North Sea but were denied egress via the Channel by the mine barrier in the Dover Straits and could reach the Atlantic only by making the long passage round the north of Scotland – if, that is, they had the range to do so. Few had that range. Only eight of the Type IX were truly oceanic, with a range of 12,000 miles; eighteen could cruise as far as Gibraltar; the remaining thirty could not leave the North Sea. Despite these limitations the U-boats had some notable successes, including the sinking of the battleship
Royal Oak
in the Royal Navy’s main base at Scapa Flow in October 1939 and the aircraft carrier
Courageous
which was sunk while flagrantly neglecting anti-submarine precautions in home waters. From the outbreak of war to the fall of France, total merchant sinkings in the North Atlantic did not exceed 750,000 tons and 141 ships.

The capture of the French Atlantic ports in June 1940, however, transformed the basis of U-boat operations. Possession of Brest, Saint-Nazaire, La Rochelle and Lorient put Dönitz’s boats on the doorstep of Britain’s trade routes, thus ensuring that the pattern of sinkings, thitherto arbitrary and sporadic, should become regular and consistent. As soon as his crews cleared the Bay of Biscay they found themselves astride the route from Britain to the Cape along which travelled Nigerian oil and South African non-ferrous ores; and by reaching out only a little further into the Atlantic they could attack convoys carrying meat from the Argentine and grain from the United States.

Ships sailing individually were desperately vulnerable to interception. As the Royal Navy’s experience in the First World War had proved, independent sailing presented U-boats with a succession of targets: a captain who missed one lone ship on a well-used trade route could still count upon the appearance of another and thus achieve a respectable success rate by the operation of the probability factor alone. Convoy upset probability. Because the submerged speed of a submarine was at best equal to and often lower than that of a merchant ship, a U-boat captain who was wrongly positioned for an attack when a convoy hove into view would miss all the ships in it and might have to wait days before another appeared, with no greater certainty than before of finding himself correctly positioned to attack it.

Dönitz, a First World War submarine captain, had recognised the mathematical disadvantage at which his naval arm operated and conceived a method to overcome it. By experimentation with surface torpedo-boats, during the period when Germany was denied U-boats by the Versailles Treaty, he demonstrated that ‘packs’ of submarines, if disposed in a chain on the surface where their speed exceeded that of merchant ships, could identify the approach of convoys across a wide band of ocean, be concentrated against one by radio command from shore and inflict mass sinkings by a concerted raid in numbers that would overwhelm the escorts. Once Germany acquired the French Atlantic ports, it was these ‘wolf pack’ tactics that were to make the Battle of the Atlantic the knife-edge struggle for advantage which overshadowed Churchill’s conduct of the British war effort from mid-1940 to mid-1943.

Convoy, which was the Royal Navy’s defence against the wolf packs, offered only partial protection to the Atlantic lifeline. The naval escorts themselves – in the early days perhaps only two or three destroyers and a corvette were available to shepherd forty freighters and tankers across 3000 miles of ocean – were little direct threat to a determined U-boat formation. Asdic, the echo-sounder used to detect submerged U-boats, was ineffective beyond 1000 yards and reflected only range and bearing, not (until 1944) depth. The depth charges used to attack U-boats, triggered by water-pressure fuses, had to be set by guess and fractured the U-boat hull only if detonated close by. Most U-boat attacks, moreover, were delivered from the surface at night, when radar was more useful than Asdic, but until 1943 radars were too primitive to give early warning or accurate ranging.

It was the measures taken to route convoys away from known or suspected U-boat patrol lines which best assured their safety, together with ancillary measures – particularly aerial patrol – to force U-boats to submerge while convoys passed by. Until May 1943 a shortage of aircraft and their shortness of range left an ‘air gap’ between North America, Iceland (available as a base to Britain after the German invasion of Denmark in April 1940) and Britain itself in which U-boats operated without fear of surveillance; the gap was closed when the very-long-range Liberator (B-24) with an endurance of eighteen hours came into service. Rerouteing, on the other hand, was a stratagem employed from the very beginning of the Atlantic war, and there was always a strong sense of direct conflict between the two sides. On the German side, the officers of the B-Dienst (Observer Service) used wireless intercepts and decrypts of cipher transmissions to establish convoy positions and read their orders; on the British (later Anglo-American) side, the cryptographers of the Government Code and Cipher School at Bletchley and the staff of the Admiralty Operational Intelligence Centre monitored the signals sent between U-boats and from Dönitz’s headquarters at Kernevel, Lorient (after March 1942 Berlin), to detect the formation of patrol lines and the vectoring of wolf packs against their targets. Rerouteing was by far the most successful of convoy protection measures. Between July 1942 and May 1943, for example, the British Admiralty and US Navy Department intelligence centres managed to reroute 105 out of 174 threatened North Atlantic convoys clear out of danger, and minimised attacks on another 53 by rerouteing; only 16 ran directly into wolf-pack traps and suffered heavy loss.

The success achieved by Captain Rodger Winn, RNVR, and later by his American counterpart, Commander Kenneth Knowles, depended ultimately upon the skill of the Bletchley Park cryptographers in decrypting the Kernevel U-boat traffic fast enough for its significance to be applied to convoy operations. That traffic was, of course, enciphered on the Enigma machine, and the ‘Shark’ key used by the U-boat service proved particularly resistant to Bletchley’s efforts; it was not broken until December 1942 and then not regularly until 1943. Much of the vital radio intelligence used by the Operational Intelligence Centre up to that time was of lower-grade, position-fixing quality. High Frequency Direction Finding (HF/DF or ‘Huff Duff’) enabled ships to detect and locate shadowing U-boats from the transmissions they sent back to U-boat headquarters, and so for convoys to be rerouted or protecting aircraft summoned. Meanwhile, because of the Admiralty’s ill-advised persistence in the use of a book code instead of a machine cipher, the B-Dienst was able to read convoy traffic and direct wolf packs on to chosen routes with sometimes disastrous effect.

The crux of this radio intelligence war began with the move of the U-boats from the eastern to the central Atlantic after April 1941. Substitution and rationing in Britain had allowed the import requirement to be reduced from 55 to 43 million tons, but the minimum level of subsistence was approaching and had to be measured against a rate of sinking which threatened to outstrip replacement building. In February 1941 the United States had enacted a Lend-Lease law which, in effect, allowed Britain to borrow war supplies against the promise to repay after victory; and from April 1941 the United States was operating a Neutrality Patrol which effectively excluded U-boats from the Atlantic west of Bermuda, under the terms of the Pan-American Neutrality Act of 1939. However, the U-boat fleet now had over 2000 miles of ocean in which to intercept convoys and was adding to its numbers at a considerably higher rate than it was losing U-boats on operations; during 1941 the building rate exceeded 200, while the total lost since September 1939 was less than fifty.

The eight months of extended U-boat warfare in the Atlantic in 1941 therefore proved extremely successful to the German navy. In May it suffered the loss of the great battleship
Bismarck
, unwisely unleashed as a commerce raider, at the end of a great chase by most of the British Home Fleet; but that defeat was offset by the sinking of 328 merchant ships of 1,500,000 tons at a time when the British yards were launching less than a million tons of new construction annually. The casualties took down with them almost every category of material of which the home islands stood in dire need – wheat, beef, butter, copper, rubber, explosives and oil, as well as military equipment.