During the First World War, 178 of the 365 German U-boats built by Germany had been lost at sea, despite the then Allied lack of any effective means of detecting submerged boats. Acoustic methods were employed, and aircraft and airships attempted to spot U-boats from the air in shallow waters, almost always unsuccessfully. Most U-boats sunk, at least forty-eight, fell prey to mines in mine barriers. Ramming, by warships or merchantmen, accounted for another nineteen, attack by British submarines seventeen. Destruction by depth-charge, the specific anti-submarine weapon, was responsible for only thirty losses.7
The depth-charge was a bomb containing usually 40 pounds of high explosive, dropped from a rack over the stern or projected abeam, and activated by a pressure fuse, which could be set to explode at a chosen depth. It created high-pressure waves and, if detonated close enough to a U-boat hull, cracked its plates. Accurate depth-charging was fatal but accuracy was difficult to achieve; throughout the Battle of the Atlantic, but particularly in the early days, damage rather than destruction was a common outcome of depth-charging. After 1942, depth-charging was supplemented by the firing of large numbers of contact bombs, from the Hedgehog and subsequent Squid systems, which, given accurate location, could be deadly. In mid-1943 another weapon appeared, the Mark 24 Mine so-called; in fact, an acoustic torpedo, dropped from an aircraft to home on U-boat propeller sounds. Lethal in most circumstances, it suffered from the disadvantage of being deemed so secret that it could be launched only under special conditions. Aircraft also dropped depth-charges and fired high-explosive rockets against surfaced or submerging U-boats, those motoring to Atlantic patrol lines across the Bay of Biscay providing most of the targets.8
Most of these developments lay far in the future at the outset of the U-boat war, when the advantage lay heavily with the Germans. The advantage would have been decisive, had Dönitz been able to deploy the numbers he desired and would eventually achieve. Even so, the advantage was enhanced by the enemy’s lack of anti-submarine warships. Britain in 1939 appeared to have a sufficiency of escorts. The Royal Navy deployed 128 destroyers and 35 sloops.9 Most of the destroyers, however, including the superb Tribal and Javelin classes, were high-speed ships, designed to accompany the battle fleet and lacking the endurance to linger as convoy escorts. Many of the older destroyers had been built during or soon after the Great War and were coming to the end of their lives. The Hunt-class destroyers, designed specifically as escorts, were entering service but were too few in number as yet—only twenty—to tip the balance. The sloops were generally too old to be effective. A whole new generation of escorts—slow but sturdy corvettes, modelled on South Atlantic whalers, and speedier frigates—were in gestation but had not yet reached the fleet. Trawlers and drifters from the fishing fleet had been pressed into service; but they were too small and too slow to make capable escorts. The result was that convoys had too few escorts to be able, when attacked, to defend themselves.
Convoy was adopted by the Admiralty at the outset of the war, in sharp contrast to its policy during the First World War. Then the admirals had resisted it, for wholly mistaken reasons. Convoy was a hallowed British maritime practice, which had protected British trade from attack by the French fleet and roving corsairs throughout the wars of the French Revolution and empire. At the outbreak of the Great War, however, and until late in 1917, the Admiralty had mistakenly calculated that, given the submarine’s ability to mount attacks submerged, the practice of massing merchantmen in convoy merely multiplied the targets available to a predator, which the navy lacked the means to locate. It was therefore better, it was concluded, to let ships sail independently, thereby, apparently, forcing submarines to choose their targets singly and, so it was supposed, with greater difficulty. The Anti-Submarine Division of the Admiralty also shrank from the task of providing escorts for what it believed were 5,000 sailings into and out of British ports each week.
These two objections to convoy were separate and different but were dissolved by analysis of the second. In April 1917, Commander R. G. H. Henderson, RN, dissected the figures for maritime trade and established that only 120–140 arrivals and departures each week were by ocean-going ships, those on which Britain’s survival depended; the rest were by coastal and short-sea-crossing vessels which it was not vital to protect. Because of the enormous number of destroyers and other small warships that been built during the war, provision of escorts to convoy the essential merchantmen was not seen to be a difficulty at all. The only problem was to learn the technique of convoy. Once that was mastered, sinkings began to fall. In October 1918, tonnage lost was 178,000 tons against an average of 550,000 tons a month during 1917. Most ships sunk were sailing independently; losses from convoy were under 2 per cent.10
The Admiralty’s immediate adoption of convoy in September 1939 averted any large-scale toll of sinkings in the first year of the war. There were several ancillary reasons for that, the paucity of U-boat numbers being one and the confinement of the U-boats to German bases far from the shipping routes another. The most spectacular U-boat successes, indeed, were achieved against naval targets, particularly the torpedoing of the British battleship Royal Oak inside the protected anchorage of Scapa Flow in October 1939. It owed much to intelligence success. A German captain, who had visited the Orkneys just before the declaration of war, reported that he had heard the defences of the eastern approaches to the anchorage were neglected; aerial photographic reconnaissance confirmed the existence of a gap. Dönitz then briefed the thrusting young U-boat commander, Gunther Prien, about the possibility of making an entrance at slack water under cover of darkness. On 13 October, U-47 found its way through the defences, fired torpedoes which detonated Royal Oak’s magazine and sent it to the bottom with most of its crew. Militarily the attack was not significant, for Royal Oak was obsolete; its sister R-class battleships would have to be hidden from the Japanese in east African ports after Pearl Harbor, so low was their ability to defend themselves. Nevertheless, the attack was a humiliation for the Royal Navy, besides being an awful warning of the vulnerability of capital ships to unorthodox attack, particularly when at anchor, as Pearl Harbor, Taranto and the Italian attack on Alexandria were subsequently to demonstrate.11
The comparative ineffectuality of Dönitz’s U-boat campaign of September 1939–July 1940 was to be sharply reversed after the fall of France. In the immediate aftermath, the German navy hurried supplies of torpedoes and other submarine warfare material to the French Bay of Biscay ports—Lorient, Brest, La Pallice, Saint-Nazaire, Bordeaux—which were henceforth to be the bases for its U-boats during the Battle of the Atlantic; the first arrived in the Bay of Biscay, at Lorient, on 7 July. The Biscay ports provided Dönitz’s submarine fleet with direct access to Britain’s Atlantic trade routes, shortening by hundreds of miles those from Germany’s bases and sparing it attack on passage in the constricted waters of the North Sea.
As soon as the Biscay bases were acquired, Dönitz embarked on the realisation of his plan to defeat Britain, and its surviving allies, by destruction of its Atlantic convoys. Advantage seemed on his side. The number of U-boats, which had to survive only one outward passage from German shipyards to the French ports in order to become effective, was increasing. The number of British escorts, and of replacements of British merchantmen lost to attack, was increasing much more slowly. Dönitz’s belief in his ability to win the naval—and thereby the European—war, by destruction of the Atlantic shipping trade, seemed ready to be realised.
By a strange reversal, the First World War fears of the Admiralty, that it lacked the escorts necessary to protect convoys, seemed about to be confirmed in a subsequent war twenty years later. In the second half of 1940, the Royal Navy, wholly committed to the concept of convoy as it was, was attempting to protect much larger convoys than it had organised in 1917–18 with far fewer warships. In 1918 a typical oceanic convoy of 16–22 merchantmen was protected by seven destroyers, first-class warships of a speed (over 30 knots) double that of a U-boat on the surface, where U-boats usually attacked. In the winter of 1940, convoys of as many as thirty ships or more might be protected by only one inadequate escort.
An example was Convoy SC7 (convoys were identified by acronyms, usually denoting point of departure, and numbered consecutively; those most used were HX, originating in Halifax, Nova Scotia; later New York, OB, outbound from Britain; CU, Caribbean–United Kingdom; MK, Mediterranean–United Kingdom; SL, Sierra Leone; PQ, Britain–North Russia). SC7 originated in Sydney, Novia Scotia, and consisted of thirty-five ships, all slow, four of them inland freighters from the American Great Lakes. The only escort was the sloop Scarborough, built in 1930 with a top speed of 14 knots and so slower than a surfaced U-boat. On the fourth day out, 8 October 1940, the convoy ran into a gale and that night into U-boats. Over the course of the next ten days, SC7, though reinforced by two more sloops and two corvettes, and attended by a Sunderland flying-boat, lost seventeen ships. The horror of the experience scarcely bears thought. For the torpedoed seamen, even if they were able to launch lifeboats or floats, there was no hope of rescue. The convoy could not stop; the escorts’ duty was to stay with the merchantmen. The survivors of sunken ships drowned or died of exposure.12
SC7 was a ghastly example of the pre-1917 fears of the anti-convoy Admiralty, which thought there were not enough escorts to protect merchantmen, encountering the plans of a commander, Dönitz, who had contrived means to maximise the offensive power of what should have been an inferior weapon, the U-boat. The U-boat was the realisation of an ancient conception, the idea of the invisible weapon. Most of its early forms had been devised to undermine, literally, the power of the British surface fleet, as had the first practical submarine, invented by the Irish-American, J. P. Holland, in 1900. The Holland boat, however, had been intended, like all its ineffectual predecessors, to attack submerged. The genius of Dönitz—he was a sort of evil genius—was to perceive that the submersibility of the U-boat should be used merely to protect it from counterattack, once its presence was detected, and that in offence it should be used on the surface, where it could achieve speeds superior to most of its targets, the merchantmen, and not greatly inferior to those of all but first-class escorts.
The other ingredient of the Dönitz idea was that of the wolfpack (Rudel). His time as commander of a U-boat in the Great War had persuaded him that the deployment of single U-boats was wasteful. Better, he convinced himself in the war’s aftermath, to mass them in groups which could, first, detect convoys by forming a patrol line—similar to that organised by Nelson with his frigates—and then close for the kill. SC7 had had the misfortune to fall under attack by one of Dönitz’s earliest wolfpacks. It overwhelmed the escort. At one stage seven U-boats were operating against four escorts, since Scarborough had detached itself to hunt for one of the first predators, U-48, which it did not find; nor did it find its convoy again.
The other ingredient of the wolfpack method was central control from headquarters; after June 1940 from La Pallice. The medium of control was radio, just as it had been during von Spee’s cruiser campaign against British shipping in the Pacific and the Indian Ocean in 1914. Radio, as before, overcame the limitations of visual sighting, which so restricted Nelson’s ability to command the Mediterranean, because a single visual sighting, transmitted by radio, allowed La Pallice to concentrate a wolfpack against a convoy even if the U-boats constituting it had been scattered across several hundred miles of sea. Pack strategy, plus radio, was a deadly weapon against the convoy system.
All strategies, however, have weaknesses. Radio was the weakness of the pack system. Bletchley, supplied by intercepts from the listening stations, was provided with the material by which Dönitz controlled his U-boats. The difficulty was to break it. By late 1940, Bletchley had had no success against the German naval keys. Unlike those of the recently founded German air force (Luftwaffe), the Kriegsmarine’s operators came from a long-established signal service, which had strict procedures and severe schooling. Not only were German naval signallers trained not to make mistakes—for Bletchley the most fruitful source of breaks into the Luftwaffe traffic; the whole German naval signalling system operated on the belief that the enemy was listening. The Kriegsmarinetherefore strove not only to keep enciphering secure but also to limit the amount of material transmitted, on the sound principle that the smaller the quantity of intercepts, the harder it is for an enemy to find a way into them.
Assurance of secure encipherment was attempted by two principal means: enlarging the number of rotors used in naval Enigma machines and designing certain keys to be used only by officers. Even before the war, naval Enigma operators were issued eight rotors from which to select three; from 1 February 1942 onwards Atlantic and Mediterranean U-boats used four rotors in an adapted machine.13 The “Officer” keys introduced were versions of the Heimisch key, the Süd key, and Triton, known as Shark at Bletchley, by far the most important since it was the key used in Atlantic U-boat operations from February 1942. Officer keys were regularly broken but usually with some delay.14
Limitation of material transmitted was achieved by the devising of “short” signals, a form of code which was enciphered within longer messages or used simply as answers to enquiries from U-boat headquarters at La Pallice (later Berlin). Most short signals, transmitted as “digraphs” (two-letter groups), referred to a chart of the Atlantic and adjoining waters, which was divided into an irregular grid. Bletchley, beginning with some captured material, managed to reconstruct some of the grid by April 1940. In May 1941, as a result of the celebrated capture of U-110, it reconstructed the grid of the whole North Atlantic and most of the Mediterranean. The Germans, who constantly reviewed the security of their signal system, became concerned in mid-1941 that the U-boat position transmissions might have been compromised and introduced a more complex short signal by relating positions at sea to fixed points of reference—Franz, Oscar, Herbert, etc.—arbitrarily chosen and changed at short intervals. When deciphered, a typical Enigma order to a U-boat now read: “If boat is in a fit condition for night attacks occupy as attacking area the northern waters of the 162-mile-squares [of the naval grid] whose central points lie 306 degrees 220 miles and 290 degrees 380 miles respectively from Point Franz. If boat not in a fit position, report by short-signal ‘No.’ “15
Bletchley managed to overcome the difficulty thus presented quite quickly, a vital matter since the position reports provided the data by which the Admiralty rerouted convoys, on passage, away from wolfpack patrol lines. Other short signals used by U-boats at sea were sighting and battle reports and announcements of expected dates of return to port. Most useful of all were the short weather reports, essential to Dönitz’s headquarters in positioning U-boats. Bad weather, paradoxically, was welcomed by convoy commodores and escort commanders, since it usually prevented U-boats from attacking. The short weather reports became a fruitful source of decrypts because, early in the Battle of the Atlantic, Bletchley found that they were rebroadcast by a meteorological shore station in a code that it could read; later, because the reports were made in three-letter groups, Bletchley discovered that U-boat operators were not using the fourth rotor on their Enigma machines, thus greatly simplifying the mathematics of decryption.16