Kitabı oku: «Blood, Tears and Folly: An Objective Look at World War II»

Cracking the naval codes: Enigma

In 1920 the organization known in the First World War as I.D. 25, and usually referred to as Room 40, changed its name to the Government Code and Cypher School (GC&CS). This name served to hide its true purpose, which was to protect British official communications and intercept foreign ones. It was a part of the Secret Intelligence Service, and in 1925 it was moved from a building behind Charing Cross railway station to the SIS headquarters at 54 Broadway, London, near St James’s Park underground station, a site conveniently close to the Foreign Office. A few days before the outbreak of war its name was again changed – to Government Communications Headquarters – and it moved to an endearingly ugly but conveniently secluded Tudor-Gothic Victorian house at Bletchley Park, about fifty miles north of London. Its principal task was to read German radio messages encoded on Enigma machines.

The Enigma started its career as a commercial enciphering machine, a sort of typewriter that scrambled text using notched wheels or rotors. The message could be unscrambled by a recipient using an identical Enigma with its rotors adjusted to the same settings, known only to the sender and the receiver. When the Germans bought some Enigma machines they adapted them to make them more difficult to counter. The improved machine had plugs, varying the circuits, which the operators changed every twenty-four hours according to a dated instruction book of ‘keys’. This gave an astronomical number of alternatives for each letter.

The story of the breaking of the Enigma can be said to start in October 1931, at the Grand Hotel, Verviers, a town in Belgium not far from the German border. Hans-Thilo Schmidt, a highly placed official in the German Defence Ministry, made contact with Rodolphe Lemoine, an agent of the Deuxième Bureau, the French military intelligence service. Lemoine, a widely travelled linguist, had been born in Berlin, the son of a jeweller. A naturalized French citizen, he had taken his French wife’s name and, despite being a successful businessman, he went to work as a secret agent. Lemoine got along well with his fellow Berliner. Schmidt’s father was a professor and his mother a baroness, but this highly intelligent and well educated veteran of the First World War found it difficult to manage on the salary he earned as a clerk distributing cipher material. He offered to sell the operator’s instruction manual for the Enigma machine and some other notes and manuals. He also offered to continue to supply information about the updating of the machine and its codes as well as details of the workings of the German High Command (where Schmidt’s elder brother was now a lieutenant-colonel and chief of staff of the signal corps). At one time Rudolf had been the head of the cipher section – Chiffrierstelle – where Hans-Thilo now worked, and it had been Rudolf’s decision to purchase the Enigma machines that his brother was offering to compromise.

FIGURE 5

German Enigma coding machine

It was Lemoine’s stated belief that every man had his price, and at their first meeting he offered Hans-Thilo Schmidt payments equal to three times what he was getting in salary. The Service de Renseignement in Paris approved the deal and gave Schmidt the codename HE, which spoken in French eventually gave way to the German word Asche, ash. Although the material from Hans-Thilo enabled the French to read a few messages, the complex wiring inside the machine made it a daunting challenge. Paris, deciding that cracking such a machine was beyond their resources, offered to share the task with the British, but the Secret Intelligence Service in London was not interested. The French decided to give everything they had collected to the Poles.

The successes of French and British linguistic cryptanalysts, working on methods perfected during the First World War, persuaded their masters to ignore the problems of mathematical cryptanalysis. This was why the Enigma machine defeated them. The Poles had superior mathematicians, more men familiar with the German language, and the will to succeed at a task no one else believed possible.

By 1933 the Poles had rigged up a reproduction of the Enigma. They kept the French informed about their progress and the French faithfully passed to Warsaw the new codes and whatever mechanical changes to the machine Hans-Thilo could discover. However over a period of five years few of these messages got to the codebreakers. The Polish high command wanted its men to crack the German codes without outside help, and it doled out the material from Hans-Thilo only in small amounts when the codecrackers were stuck.

The Germans improved their coding machines while the Poles improved their codecracking ones, constructing what they called a bombe, a computer consisting of six linked Enigma machines. In September 1938 the fears of the Polish intelligence chiefs came true. Hans-Thilo was transferred to Göring’s Forschungsamt and the supply of codes ended. But by now the codecrackers had learned to manage without his help.¹⁴

On 24 July 1939, with war not far away, the Poles invited the French and the British to Warsaw to show them in great detail the work they were doing breaking the German codes. They showed them the bombe, a method of using overlaid perforated sheets, and calculations about wiring. Mathematical talent was at the heart of the Polish work. Some of the most notable breakthroughs had been made by Marian Rejewski, a young mathematician who has been described as one of the greatest cryptanalysts of all time.¹⁵

As a going-away present both French and British representatives were given a ‘replica’ Enigma machine. Although as the Germans got nearer to war they changed the codes, and added an extra rotor to their Enigmas to make the machine codes more complex, this gift was beyond price. Gustave Bertrand, a senior French intelligence officer, described how he brought the machine to the chief of Britain’s Secret Intelligence Service.

On 16 August 1939 I was on my way to London accompanied by Uncle Tom – the diplomatic courier of the British Embassy in Paris – who was carrying a diplomatic bag with the Enigma machine. At Victoria Station Colonel Menzies, head of the [S]IS wearing the rosette of the Legion of Honour in the buttonhole of his dinner-jacket (he was going to a soirée) was waiting for us: triumphal welcome! Which occasioned him to say one day [that the French intelligence service] had done him a ‘considerable service on the eve of the war’.¹⁶

Considering how little accurate information his SIS was able to supply about even peacetime Nazi Germany, the ill-judged rejection of the French offer of the Enigma secrets and a total lack of any preparations for war, Colonel Menzies did not exaggerate.

Poland was invaded, but keeping ahead of the Germans the Polish codecrackers moved to France. Then France fell too. The French team escaped the German invaders, set up shop near Uzès in the unoccupied sector of France, and continued their work, sending their solutions to London (enciphered in Enigma!). Many Polish and French cryptologists ended up as captives of the Germans, but all managed to convince their interrogators that Enigma was beyond their abilities. The Germans believed them; such is the power of self-deception.

In 1943 some Polish members of the original team reached England after harrowing experiences. According to one notable historian of the Enigma story: ‘The Poles reaped the customary reward of the innovator whose efforts have benefited others: exclusion. The British kept Rejewski and the others from any work on Enigma, assigning them instead to a signals company of the Polish forces in exile, where they solved low-level ciphers. It was not one of Britain’s finest hours.’¹⁷

Bletchley Park was sited halfway between Oxford and Cambridge universities, and with more and more big wooden huts built in the grounds, GC&CS were able to recruit and accommodate academics including the junior dean and mathematical tutor of Sidney Sussex College who arrived on the first day of war. Hurrying back from an International Chess Olympiad in Buenos Aires came Britain’s chess team. More such men and women followed.

No one in the world had ever attempted to break machine-enciphered messages on a regular basis with all the urgency that war brings to such a task. The resulting intelligence – eventually to be called Ultra – came solely from radio transmissions in Morse code. No teleprinter message or telephone conversation was included, and since most of the vital messages were sent by those means, the greater proportion of secret enemy communications were never intercepted. Sometimes a radio message in Enigma was answered by telephone or teleprinter, or vice versa, so that only one side of a dialogue was available. Radio reception was often subject to interference, and errors were commonly included, making the job even more perplexing.

The British took up where the Poles left off. The first break-throughs came from analysis of the uncoded message prefixes that told the recipient the key settings for the Enigma machine. Analysing the electrical wiring in rotors and plug boards, mathematicians and ‘probability specialists’ soon ‘reduced the odds against us by a factor of 200 trillion’.¹⁸ However there was still about a million to one against the men trying to conquer the Enigma messages. Wheel order and ring settings, the two most vital secrets, were sometimes guessed at by ‘sheet stacking’, a technique the Poles had pioneered. Holes – one per letter – punched in large sheets of paper allowed light through to reveal the pattern of the day’s key.

The repetitive phrases used in much of the traffic also helped: especially the formal way in which people and organizations were addressed in full. Often the codebreakers were waiting for the Germans to describe something that had already happened, such as a bombing attack or a weather report. Sometimes the same message was sent in a low-grade code – already cracked – and in Enigma too. In that case the two messages could be compared.

From time to time a rotor or two was retrieved from the pocket of a rescued U-boat crewman. By summer 1941 much-improved bombes came into use at Bletchley Park. These were 10-feet-high electro-mechanical machines like calculators. Using rotors like those inside the Enigma machines, the bombe searched rapidly through, not every possibility but the limited number chosen by the operator. The clickety-clickety sound came to a sudden stop when the bombe found a letter substitute for which the operator was looking. This would provide a setting that could be tried on a British encoding machine that had been adapted to perform like an Enigma. The printer started and produced a long strip of text. If it was ‘a good stop’ it might mean a batch of messages could be broken.¹⁹

If it was a navy message it would then go to the Royal Navy section in one of the huts, where a dozen or more men juggled with the German military jargon to make an intelligible message in English. This done, English and German versions might be sent by teleprinter to the Submarine Tracking Room in the OIC (Operational Intelligence Centre) in the Admiralty’s Citadel near Trafalgar Square in London.

The ‘key’, the settings of the rotors and the plugs, changed daily, or sometimes every two days. It was the most difficult problem, but even after the settings were discovered, the deciphered messages still had to be translated and made intelligible. Many messages were long, and included codenames and complicated service references that would be baffling to any civilian. There were technical aspects of stores, requisitions, meteorology, aviation or maritime matters. There were newly minted technical words and acronyms. A great deal of the intercepted material was banal and of little or no possible use as intelligence. And of course everything was in German. For some messages a result in two or three days was too late, and this was usually the case with the constantly moving war at sea.

The military wisely gave way to the boffins in the matter of getting useful results quickly. The first stage was a job for mathematicians; the second stage for men who spoke German. One of the Bletchley Park team remarked:

The rise to prominence of the translators – their pivotal position was already an accomplished fact by the winter of 1940–41 – inaugurated a revolution which gave primacy to the end over the beginning.²⁰

A navy, army or air expert assessed and annotated the translation to underline its significance. He would add, for instance, references to a unit, a place or person mentioned, bringing into use the big card-reference system that each service maintained. The next stage was the drafting of a signal to the field commander who could make use of the intelligence. The syntax of the message was changed, or ‘sanitized’, so that if the Germans encountered it they would not be able to identify the original message and guess how it had been obtained.

The British army monitors at Chatham, on the Thames Estuary, and later other stations too, listened to all German short-wave radio traffic. According to the weather, reflections from the upper atmosphere would sometimes enable transmissions from U-boat to U-boat in the farthest reaches of the Atlantic to be heard.

The Enigma coding machine was a remarkable invention. The most surprising aspect of the story is that the experts agree that a few simple changes to the original design could have made it fool-proof in use, and its output totally invulnerable. Messages were usually broken because of German carelessness and lax procedures. The Luftwaffe provided most opportunities; the navy was far more careful, so the German navy’s Enigma codes were not broken on anything like a regular basis. A founder member of the Bletchley Park team commented: ‘At any time during the war, enforcement of a few minor security measures could have defeated us completely.’²¹

In any case it wasn’t necessary to understand what a message said to benefit from knowing where your enemy was. High Frequency Direction Finding (HF/DF or what the Royal Navy called ‘Huff Duff’) could estimate the position of U-boats (or enemy surface ships) by getting a compass bearing on the transmissions. BP staff also made valuable assessments of German preparations and operations by studying the volume, character and point of origin of signals traffic.

Oscilloscope patterns (which depict voltage or current fluctuations on a cathode ray tube) were also photographed and filed to provide a ‘fingerprint’ that could positively identify a radio transmitter, and this in effect was enough to identify a ship or U-boat. Such fingerprinting was another of Lt Merlin Minshall’s ideas. It was in this way that the Bismarck was positively identified.²²

Britain’s Secret Intelligence Service retained control of their GC&CS, and so deciphered material went to their Broadway HQ for distribution to whoever ‘C’ – Colonel Stewart Menzies, the organization’s chief – thought deserving. The Royal Navy did not trust SIS with this task, and right from the beginning the navy kept its facilities at Bletchley Park entirely separate from those of the army and the RAF. The admirals had not been satisfied with SIS since November 1939 when Menzies, an army man, was appointed to be ‘C’. The navy said that previous SIS chiefs – Captain Smith-Cumming and Admiral Sinclair – had established a tradition that ‘C’ would always be a job for a sailor.

So all naval material was handled independently and went to the OIC (Operational Intelligence Centre) in Whitehall. In early 1941 the OIC moved to the Citadel (a lumpy brown granite building between Admiralty Arch and Horse Guards; it is still there, mercifully hidden under ivy).

The navy’s system worked reasonably well. By the end of July 1941 – due largely to the capture of some Enigma machine rotors and dated settings in February, March and May – there was regular interception of Enigma signals. These were being added to all the other signals intelligence, collectively known as SIGINT, which included data from diplomats, foreign newspapers and secret agents, as well as from interrogating, bugging and planting stool-pigeons among captured U-boat crewmen. It was all analysed and put together, and with these sources the Admiralty’s Submarine Tracking Room tried to predict the intention of the U-boat captains.

Radio transmissions from U-boats were few and far between. Getting one solitary fix on a U-boat simply put a coloured pin into a big plotting table. From that there was no telling which way the submarine was heading. Was it outward-bound, with a full complement of torpedoes, or was it limping home with engine troubles? Or was it part of a ‘rake’ of other U-boats across the expected route of a convoy? The Submarine Tracking Room – using all its resources – tried to answer such questions, often guessing right and telling transatlantic convoys to change route and avoid the places where the U-boat packs were waiting. This rerouting of convoys was to become the most effective counter to the U-boats, and – since signals avoided all references to German movements – it was unlikely to reveal British successes with Enigma.

By 1941 the German armed forces had thousands of Enigma machines in use. One of the disadvantages to such machines was that they could be lost or stolen, and so could the keys, the settings for rotors and plug boards. Ever since the remarkable luck of finding some Enigma settings in the patrol boat VP2623, captured off Norway long before in April 1940, the codebreakers had been crying out for more.

On 23 February 1941 during a commando raid on the Lofoten Islands, northern Norway, the destroyer HMS Somali fought off a suicidal attack by a tiny German trawler, Krebs, which was left holed and beached. The Somali’s signals officer volunteered to search the little ship. From it he got some spare rotors and the Enigma ‘keys’ for the month of February. It was a splendid haul, more important to the war than the destruction of ships and factories and oil that had been the purpose of the commando raid.

It wasn’t the rotors found on Krebs that pleased the men at BP (Bletchley Park) – they already had them. The keys however were very valuable; with them current messages could be read without any delay. Even outdated keys were useful, enabling older messages to be examined. Such keys were always printed in water-soluble ink, and a dousing in seawater was enough to render them illegible. Ian Fleming, later to gain fame as the creator of James Bond, was at this time in naval intelligence and he came up with a hare-brained scheme to get some more of these keys. A captured airworthy German plane would be crash-landed in the sea and (it was hoped) found by a German ship. The rescued airmen would thereupon seize control of the ship and grab its codes. It says a lot about the desperate need for settings that this idea was taken seriously and a captured Heinkel made ready. Fortunately one of the boffins came up with a more practical adventure. The bounty of the February intercepts revealed that German weather-ships out in the Atlantic used Enigma. It also disclosed their movements. Why not capture one of those? The weather-ships were on station for months, so they might have longer lists of keys.

The destroyer Somali was used also in this engagement, which depended upon closing on the little weather-ships as quickly as possible in the hope that not everything would be tossed overboard. In the case of the weather-ship München the seizure and boarding went according to plan, and among the bundles of paperwork seized there were German Enigma keys for June. This would not enable the Bletchley Park men to read the whole of German naval traffic, for the navy had many different keys. But now the important ‘home waters’ radio traffic would be available. A second weather-ship, the Lauenberg, was intercepted on 28 June, just as the next month’s settings were due to come into use. Now traffic up to the end of July could be read.

By this time there had been another dramatic success in the story of Enigma. It was early morning on Friday 9 May 1941 and the outward bound convoy OB 318 was seven days out of Liverpool. There was reason for the British sailors to feel that the most hazardous part of the voyage was done; no U-boat had made a kill this far west. But trailing the convoy came U-110, one of the big long-range Type IX submarines. It was commanded by Kapitänleutnant Lemp, the man who sank the passenger liner Athenia in the first hours of the war. He now wore the Knight’s Cross – or ‘tin tie’ – at his collar. With him to the U-110 he had brought his cousin, who was regarded as a Jonah by the crew, after having two previous boats sunk under him.

In daylight, submerged, Lemp fired three torpedoes. Two merchant ships were hit. One ship’s stern tipped up so steeply that crates on her deck rolled into the ocean like ‘a child pouring toys out of a box’.²³ One of the convoy’s escorts spotted the racing white wake of his periscope, sped towards it and dropped a pattern of depth charges. But Lemp escaped and without delay came back up to periscope depth just in time to see the destroyer coming at him. The second scatter of depth charges was close. The underwater explosions stopped the electric motors, started leaks in the oil bunkers and sent the U-boat plunging downwards. Lemp ordered the tanks blown. This not only stopped the descent but brought U-110 back up to the surface with a boiling of water that got the immediate attention of the deck crews of all three destroyers. Flustered, Lemp failed to have the pressure valve released, so that when the hatch opened a great cloud of dust came pouring out of the submarine. So did the crew, who jumped into the sea with their captain.

The captain of HMS Bulldog was shouting for a boarding party almost as soon as the mortally damaged U-110 surfaced. Ignoring the probability that the Germans had set explosives before abandoning ship, a 20-year-old sub-lieutenant, rowed out by five sailors, clambered aboard the slippery hull of the wallowing boat and went down into the dark interior followed by his men. They formed a human chain to pass back the codebooks, charts and, having unscrewed it from its mounting, the Enigma machine itself. With great thoroughness and an iron nerve the sub-lieutenant scoured through everything on the U-boat from charts to ‘art studies’. He searched through discarded clothes to find anything of value to intelligence from the contents of wallets to recreational reading matter. It took three or four hours to get everything valuable aboard Bulldog.

From the water Lemp watched the British go aboard his command. Realizing that his detonators had failed and knowing that he was responsible for the destruction of the Enigma machine and all the secret material, he seems to have deliberately allowed himself to drown. (Ex-U-boat men – and at least one account²⁴ – say that Lemp swam back to the U-boat to sink her and was shot by the British boarding party as he climbed on to the deck, but I find no evidence to support this allegation.)

HMS Bulldog’s captain tried to tow the U-boat back to port but failed. He showed masterly restraint and impeccable good sense in keeping his extraordinary success secret, so that no news of it could leak back to German intelligence, but he little knew what a tremendous coup he had brought off. The men from BP had never had such a wonderful collection of data: a new machine, spare rotors and a list of the prescribed rotor settings for the length of the U-boat’s cruise – three months – and a mountain of helpful material that enabled them to read most Enigma machine messages in the code Hydra for the rest of the war. It also helped with the big warship code Neptun and the Mediterranean codes Sud and Medusa.

The ups and downs of the Enigma struggle meant that messages were sometimes read almost immediately and at other times there were long delays. Most messages were never read. The naval Enigma was the most difficult to crack and many successes came from captured current German keys. Without such helpful clues the German naval secrets could seldom be tapped. And yet it was only because Bletchley Park had been set up with its Enigma machines and bombes that the captured keys could be used.

But Enigma was only one part of the Atlantic battle. Nothing was more decisive than the rate at which merchantmen and escorts could be constructed in British and North American shipyards. The construction of U-boats was equally telling, and so were the global demands that took warships and U-boats to other parts of the world. The weather played a part, and so did the successes of the German B-Dienst, the service which intercepted British signals. The availability of Allied air cover and of long-range aircraft, and, the extent of Luftwaffe reconnaissance and anti-shipping operations, influenced the monthly figures, as did the operations of German surface raiders. The rationing of food and petrol was vital to the struggle, as was the improving technology of anti-submarine weapons – such as the hedgehog depth-charge thrower – and the use of better explosive charges. Just as deadly in effect were airborne and shipborne radar and, perhaps most important of all, ‘Huff Duff’, which provided ever better ‘fixes’.

The Germans never suspected that the British might be reading their Enigma traffic on a regular basis. The chief of the German navy’s Signals, and the head of the Naval Intelligence Service, assured Dönitz that it was not possible to crack such machine codes. After the war Dönitz still believed them. To some extent of course this was true.

By 23 June 1941 British penetration of the Hydra traffic had given the OIC (Operational Intelligence Centre, of which the Submarine Tracking Room was a part) a great deal of supplementary data, including details of all German inshore traffic and thus minelaying operations, as well as the routine messages that marked the beginning and end of a U-boat’s cruise.

Enigma intercepts also revealed the positions of five tankers, two supply ships and a scouting ship positioned for the commerce-raiding cruise by Bismarck and Prinz Eugen. When plans were made to attack these auxiliary vessels, the navy decided to leave two of them – the tanker Gedania and the scout Gonzenheim – unmolested. To attack them all might prompt the Germans to guess their Enigma had been penetrated. By chance the Royal Navy happened upon those ships too, and so the whole lot was sunk. As feared, this massacre – five of the sinkings occurring over a three-day period – made the Germans investigate the possibility that Enigma was insecure. They decided that the spate of sinkings was probably a coincidence, but additional security measures were introduced just to be on the safe side.

One of the planned measures was a fourth rotor for the navy’s Enigma machines. A shiny new Enigma machine had been recovered from U-570, which in August 1941 surfaced near Iceland and fell victim to an RAF plane. The Enigma machine had been built with an extra window, all ready for the extra rotor when it was issued. The sight of it sent a shudder through the personnel at BP, for the mathematicians calculated that it would multiply their already Herculean task by a factor of 26! They were right: 1942 brought the four-rotor machine and a year of darkness for the men peering into the German navy Enigma. Sinkings went from 600,000 tons in the second half of 1941 to 2,600,000 tons in the second half of 1942.²⁵

A last word about Bletchley Park is a cautionary one. Considerable opportunities for intelligence gathering were neglected because the pre-war SIS took no interest in foreign radio transmissions other than messages. It wasn’t until 1940 that there was any attempt to intercept or analyse radar or radio navigation signals. GC&CS was responsible not only for cryptography and communications intelligence but also for safeguarding British communications. Whatever its glittering, and much trumpeted, successes at the former, its role as guardian was a chronic and dismal failure.

The Submarine Tracking Room

The Submarine Tracking Room used a wide range of incoming data. Anything that could possibly help was funnelled here. For the admirals perhaps the most disconcerting thing about this place, where Britain’s most vital battle was being fought and from which came the operational decisions that sent orders to the warships, was that by 1941 no regular naval officer was anywhere to be seen.

The room was run by Rodger Winn, a 30-year-old lawyer with degrees from Cambridge and Harvard. He had drifted into this job as a civilian, after volunteering to interrogate enemy prisoners. Winn would never have passed a Royal Navy physical examination: childhood polio had left him a hump-backed cripple who walked with a limp. Neither would he have found favour in the peacetime navy, for he had limited respect for authority. Like many barristers he was a talented story-teller with a sharp tongue. The security of having a well paid profession waiting for him encouraged him to stand firm against authority. When a decision of Winn’s was challenged by an admiral, Winn put a vast heap of reports, sightings, charts and intercepts on to the admiral’s desk and politely asked him for his solution.

When Winn joined the Tracking Room staff, the emphasis was on plotting the present state of the Atlantic battle, rather than predicting the future. But such was Winn’s talent for reading the minds of the U-boat men, that in January 1941 his boss was moved out of the Tracking Room and Winn was given sole command of it and made a commander in the Royal Navy Volunteer Reserve. And when in January 1942 he chose an assistant, it was another ‘civilian’, a bespectacled insurance broker from Lloyd’s.

Most of the Tracking Room’s floor space was occupied by a seven-foot-square plotting table depicting the North Atlantic. It was covered with sheets of white paper and brightly lit from above, like a billiards table. Pins showed the progress of the convoys on their two-week voyages across the ocean, while others showed the U-boats on their month-long sorties. All the evidence of U-boat activity – radio fixes, sightings, signals and sinkings – was written in pencil on to the paper table cover. The coloured pins revealed the source: red-topped pins for a fix, white for a sighting, blue for Enigma intercepts. Red lines showed the extreme limits of air cover. A cluster of pins showed where a convoy was at that moment under attack by a wolf pack.

Another large table was covered by a captured map to show the German navy grid. There was also a map showing the Huff Duff stations that took bearings on U-boat radio messages. Strings could be stretched across the map to intersect. A ‘good fix’ was within 40 or 50 miles, a ‘very good fix’ meant within 10 to 15 miles. On one notable occasion a U-boat was found within three miles of its fix in the Baltic, and sunk by a Coastal Command plane, all within 30 minutes of the U-boat’s tell-tale radio transmission.²⁶

The walls of the Tracking Room were covered in charts and graphs showing such things as U-boat sinkings and estimated production. There were pictures too, including a photograph of Dönitz. Winn tried to make this room as he imagined Admiral Dönitz had his operations room.

Each day at noon the information from the table was translated into a situation report. The most vital parts of the plot were used each day to update Churchill’s map in the War Room. Once a week, during the night, staff renewed all the white paper sheets on the plotting table, carefully transferring all the current data to them. The Submarine Tracking Room became a favourite place to take VIPs. For extra security the plotting table used coded references so that visitors would not get an accurate or complete view of what was happening.

Around the Tracking Room there were offices for the watchkeepers. Winn’s office had a glass front so that he could see the civilian watchkeepers plotting the convoys while RNVR officers (wearing mufti) plotted the enemy movements. Civilian day-workers kept the records and card indexes. A nearby office held telex machines connected by direct line to Bletchley Park. Arriving Enigma messages were brought into the Tracking Room by a Wren (Woman’s Royal Naval Service) who was called ‘the secret lady’.²⁷ One of the tasks of the Tracking Room staff was to compile an account of each new U-boat. Such a record might start with the low-grade radio traffic sent while the new boat was working-up in the Baltic. From that time onwards every possible detail of boat and crew would be filed for reference. Names, sinkings, medals and commendations, damage and refits were all noted.

Because the Royal Navy gave most of its officers sea duties between spells at a desk, the plotting table often showed the hazards of friends who had recently worked here. One officer, Commander Boyle, was well known to the men in the Tracking Room and was married to a secretary who worked there. He was escort commander for a convoy of eleven tankers and his friends watched the plot day after day as its ships were picked off one at a time. Finally only one ship survived but Boyle was saved.²⁸

By using his background material, and watching all the movements on the plotting table, Winn made decisions about re-routing convoys or even detaching precious warships if the situation required it. Each morning he phoned to the Western Approaches Command in Liverpool and spoke with RAM Coastal Command too. When Enigma material was available, it made Winn’s job incomparably easier. Often he phoned directly to those other civilian boffins at Bletchley Park and asked them to keep a lookout for messages with some known component.

B-Dienst

The German navy’s Observation and Cryptanalytic Service, Beobachtungs und Entzifferungs Dienst, was housed at 72 Tirpitzufer, Berlin. In the first two years of war there is no doubt that the German navy gained more advantage from the intelligence provided to them by their Beobachter-Dienst than did the Royal Navy from the Enigma work at Bletchley Park. The men of the German navy’s B-Dienst (modelled after the Royal Navy’s Room 40) had been listening to British fleet signals long before war began, and they broke the British convoy code (BAMS: British and Allied Merchant Ship Code) without difficulty. The Germans could read a great deal of the Royal Navy traffic too, and Dönitz planned his Atlantic operations upon the wealth of material he got from B-Dienst.

The British Admiralty had resisted the idea of having coding machines. The Typex, resembling the Enigma, had been offered to them but was turned down. One would have thought that a sample of what a potential enemy was using might have been recognized as a worthwhile investment. Lord Louis Mountbatten, when still a Lieutenant-Commander, had drawn attention to the weakness of the whole system of Royal Navy codes and was ordered ‘to mind his own business’.²⁹

The Royal Navy’s refusal to use ciphering machines made it easier for the Germans. Even the convoy designations gave vital information: ONS was a slow outward-bound convoy to North America (Nova Scotia); HX homeward bound from Halifax. From such tags it was possible to guess the routes, and merchantmen were listed by name with a short description of their cargo: war material, eight aircraft on deck, locomotives on deck, chemicals, machine parts. Thus Britain’s most critical supply convoys were exposed to attack and continued to be so until the codes were changed in the summer of 1943.

All the same the U-boat commander’s task was not an easy one. The convoys maintained radio silence most of the voyage. On a clear day, an alert lookout on a submarine conning-tower might spot a convoy’s smoke (against clear sky or cloud) at 50 miles. In calm seas, and at the right depth, a submarine’s hydrophones might pick up the sound of a convoy at the same distance. Yet endless chivvying by the convoy commodores discouraged smoke, and the North Atlantic’s clear days, and calm days, are not numerous. Lookouts, even German lookouts, are not always alert. We can therefore think in terms of a 25-mile visibility or less.

Always we must remember the speeds at which the opposing units could travel. It has been nicely depicted by a historian who suggested that we think of the Atlantic in terms of European distances: a U-boat in Vienna is told to attack a convoy in London. On the surface he can move at the speed of a pedal cycle, submerged he will go at approximately walking pace. Then we understand why convoys sometimes escaped intact, despite the men of B-Dienst.