Intelligence in War: The Value--And Limitations--Of What the Military Can Learn About the Enemy (36 page)

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.
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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.
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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
Kriegsmarine
therefore 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

BLETCHLEY AND THE BATTLE OF THE ATLANTIC

Winston Churchill confessed that he would much rather have faced the danger of a German invasion of Britain than had to sustain resistance to the U-boat war. The point is understandable. An invasion would have taken place within the dramatic unities of time, place and action. The U-boat war went on and on, always destructive and formless and apparently endless. As long as Dönitz could find boats and crews to send westward into the waters of the Atlantic, across which Britain’s necessities were convoyed, ships would be sunk, sailors drowned, cargoes lost, and the issue of the war suspended in precarious balance.

Yet, despite that perception, the Battle of the Atlantic, like any great battle, can be seen in retrospect to have had chronology and shape. It divides into five broad periods. From September 1939 until July 1940, the combat between Dönitz’s U-boat arm and the Royal Navy was not strictly a Battle of the Atlantic, since Germany’s lack of forward bases made passage into great waters difficult and largely confined the U-boats to the seas around Britain. There were rarely more than ten U-boats on station, often as few as four, and, though few were lost, only nineteen in the first ten months of the war, little shipping was sunk either. Dönitz’s dream of deploying 300 U-boats, to sink 100,000 tons of shipping a month (about twenty ships, given the current average size of ocean-going merchantmen), seemed a fantasy.

Then, following the fall of France and the Franco-German armistice of July 1940, Germany acquired occupation rights over French territory which included the ports of the Atlantic coast. Dönitz at once set up his U-boat command headquarters there, in the chateau of Kerneval near Lorient, and began to bring his flotillas out of the narrow waters of the Baltic and North seas to the Bay of Biscay. Sinkings at first rose but then fell again as the Royal Navy deployed more escorts and Dönitz was obliged to commit a huge proportion of newly built U-boats to training. Throughout the war, the
Kriegsmarine
never skimped on training, subjecting new boats and crews to as much as a year’s practice in the Baltic before allowing them to the “front.”