by Kahn, David
Indeed, so valuable was this technique that when kisses were needed, the British would sometimes sow mines anew in the channels that the Germans had swept and so provoke a flurry of messages that such and such a route was closed between two specified points—messages enciphered in both readable Dockyard and unreadable Enigma. The British called this technique “gardening.”
Once, for example, the Admiralty messaged the Air Ministry that “special planting of willow is urgently required.” Two days later, the British harvested from the airways a German “most immediate” message. Perhaps with the help of a Dockyard kiss, G.C.&C.S. solved it within three days. The message was, “Squares BF2927 and 2928 [off the north Brittany coast] closed owing to danger of mines.”
Despite the help of kisses, solutions remained irregular. On April 30, 1941, for example, Bletchley teleprinted to the Admiralty’s Operational Intelligence Centre eighteen messages: eleven general naval and seven U-boat messages. They were the first solutions to be sent in a week, and all of them were at least twelve days old.
The paucity of solutions meant that the O.I.C.’s Submarine Tracking Room had to obtain much of its information on U-boat locations from direction-finding, or DF. The DF section was headed by retired Lieutenant Commander Peter Kemp, who had lost a leg in a submarine in the late 1920s. When war threatened, he was called up for intelligence duty. Feeding him his data were eight direction-finding posts, from the Shetlands through Wick and Cupar in Scotland; Scarborough, Winchester, Lydd, and Land’s End in England; and Gibraltar; later stations were added at Chelmsford, in Iceland, and eventually on the Azores. All were connected to Scarborough, the oldest intercept station, halfway up England’s east coast. When Scarborough heard a U-boat radio message, it notified the other stations through direct landlines to take bearings on it. The stations reported these bearings to Kemp’s section in the Admiralty, which took up a quarter of the Submarine Tracking Room.
Here Kemp had rigged an ingenious device that avoided the constant penciling in and erasing of successive bearings on charts. On a sloping board he pasted a chart of Britain and the North Atlantic. On its frame were scales indicating the bearings, in degrees, from each station. He drilled holes through the position of each direction-finding station and threaded a black string through each. On one end of the string was a pin; on the other, a weight. When a bearing from a station came in, Kemp or his watch leader pulled up the string and stuck the pin into the station’s scale at the proper bearing. He did the same for the bearings from other stations. The area where the strings converged marked the location of the transmitter. This was a fix.
Every bearing, and consequently every fix, had a margin of error. How great it was depended upon cold fronts at sea, electrical storms, the different groundings of the DF posts, their weather, the height of the ionosphere, the experience of the DF operator. At the better stations the operators erred in their bearings usually by no more than plus or minus 3 degrees. In general, with half a dozen bearings, DF could locate a submarine within 25 miles. If this fix was close to a convoy, Kemp would have a signal sent to the convoy escort to warn him of the proximate danger.
At the time that Kemp’s unit was supplying a substantial portion of the intelligence the O.I.C. was receiving, that agency was undergoing a significant change in personnel in its Submarine Tracking Room. The room’s first head was a paymaster commander who had tracked U-boats for Room 40 in World War I: Ernest W. C. Thring. He set the tone of caution and skepticism so essential to accurate intelligence. When it was claimed, often hotly, that a U-boat had been sunk, he remained cool. “He lay skeptically at the center of his web,” wrote one who observed him, “unimpressed by oil, unpersuaded by a corpse floating, according a reluctant ‘probable’ to what others might regard as overwhelming circumstantial evidence.”
But as no signs of a breakthrough appeared in the Battle of the Atlantic, dissatisfaction grew with Thring’s conservatism. He believed that guessing a submarine’s future movements was too dangerous. Moreover, he was over sixty and disinclined to accept that both Britons and Germans might be using methods different from those of the previous war; he was difficult with coworkers, and his health was declining. In January 1941 Thring was replaced by his assistant.
The move was revolutionary: the assistant was not a career officer but a thirty-seven-year-old barrister appointed a temporary commander in the Royal Naval Volunteer Reserve; for the hidebound navy to appoint a civilian to such a critical department was unprecedented. But those who sought the change knew their man.
Rodger Winn had worked under Thring since August 1939. He was of medium height, broad, with powerful shoulders but a twisted back and a limp, the consequences of childhood poliomyelitis, which had kept him from realizing his ambition to join the navy. In the Submarine Tracking Room he often rested his hands on the edge of a table to take some of the weight off his legs. He was hardworking, intellectually honest, and pleasant to deal with, in part because of his good sense of humor and the many stories he told about his legal experiences. He was also hard-driving, intolerant of laziness or stupidity and, like Thring, inflexible in his standards of evidence. But above all he was willing to forecast U-boat movements. He felt that if he beat the law of averages by only 1 percent, he was ahead of the game in terms of Allied lives and ships saved and U-boats sunk.
Soon after Winn took over, the whole Operational Intelligence Centre moved from the subbasement of the Admiralty to the so-called Citadel, a modern, bombproof concrete bunker at the rear of the Admiralty. The Submarine Tracking Room, deep within the bowels of the Citadel, was dominated by a huge table, about seven feet square, brilliantly illuminated by overhead lights. On the table lay a map of the North Atlantic, dotted with pins and markers indicating Allied convoys and German U-boats. Stuck to the walls of the room were graphs of sinkings and of the construction of ships by the Allies. On a side table was spread the German naval grid, with its map of the vast ocean spaces marked off into lettered rectangles, each subdivided into numbered squares. Here Winn and his half-dozen assistants assembled information from air and sea sightings of submarines, from sinkings, agents’ reports, direction-finding, and aerial photographs, and from intercepts teleprinted from Hut 8, their most important source. They collated these varied data to determine the number and location of U-boats at sea and, if possible, their identity, all to generate the U-boat plot.
The work involved an incredible mass of detail. Bits of seemingly contradictory information had to be reduced to their common germ of truth. Other bits had to be fitted into an overall pattern. Recollections of earlier references to a submarine or to an episode similar to the one at hand had to be summoned. Winn and his staff had to have intimate knowledge of convoys and U-boats, sources, and situations to make sense of it all and to recommend actions.
A little while after the flamboyant, abrasive Admiral Sir Max Horton took over as commander in chief Western Approaches, the man who largely ran the Battle of the Atlantic, he attacked Winn at a meeting of the antisubmarine warfare committee for an inadequate evaluation. Winn replied that if Horton would give him half an hour, he would lay out all the intelligence available to him at the time. When Horton arrived, he was confronted with a pile of varied intelligence reports. “It’s all yours, sir,” said Winn, “and”—slipping in a needle—“your chief of staff in Liverpool is in a devil of a hurry for the answer.” Horton settled down to intense study. After a while, however, he turned to Winn and confessed that most of it was outside his province. With a smile, he extended his hand and said, “Goodbye, Rodger. I leave it to you.” And thereafter he did.
Winn provided his information to the Operations and Anti-Submarine Warfare divisions, but he dealt most intimately with the Trade Division’s Movements Section, which controlled convoys and ordered their reroutings. This section, just across a corridor, was headed by Commander Richard A. Hall, son of a famous World War I naval intelligence chief. He, like Winn, could hold strong views about what was happenin
g at sea. One officer said of Hall and Winn, “If those two ever stop bickering, we shall lose the war.”
By the spring of 1941, with the volume of solutions low and delays lengthy, an occasional sighting by a British ship or airplane or an actual attack on a convoy comprised virtually the only information the O.I.C. had about the location and movements of German submarines. Though its Submarine Tracking Room could tally the number of U-boats on patrol fairly accurately, it did not know—and direction-finding could not tell it—where Dönitz was sending them. So it could do little more than guess where the convoys should be routed to steer them around the wolfpacks. And the number of U-boats increased by a third from January to April 1941.
Churchill expressed his concern in a broadcast to the nation. After paying homage to the shipwrights, the longshoremen, the minesweepers, and the merchant seamen “who go out in all weathers and in the teeth of all dangers to fight for the life of their native land,” he declared. “Still, when you think how easy it is to sink ships at sea and how hard it is to build them and protect them, and when you remember that we have never less than two thousand ships afloat and three or four hundred in the danger zone; when you think of the great armies we are maintaining and reinforcing in the [Near] East, and of the worldwide traffic we have to carry on—when you remember all this, can you wonder that it is the Battle of the Atlantic which holds the first place in the thoughts of those upon whom rests the responsibility for procuring the victory?”
Goaded by this and by mounting ship losses, with direction-finding inadequate, with the need for codebreaking intensifying, Harry Hinsley, the long-haired undergraduate who had become one of the chief analysts of Hut 8’s output, had an idea.
12
A TRAWLER SURPRISED
GERMANY THIRSTED FOR WEATHER INFORMATION. SHE NEEDED it for the air operations that supported her blitzkriegs, for the massive bombardments that were to bring Britain to her knees, for the planned invasion of Britain. But she was at a disadvantage. Weather moves from west to east, and for Germany to observe the phenomena that would determine the weather over Britain and Europe a few days later, she had to penetrate the North Atlantic, an area long dominated by British sea power.
Germany began this penetration with three airplanes in April 1940, after the occupation of Denmark and Norway, and soon expanded to a full weather reconnaissance squadron. But airplanes could not furnish repeated observations from one place, as ground stations or ships could. Spot reports of air pressure, temperature, humidity, and wind, at one point at sea level and at another, say, 300 miles away at 5,000 feet, could not take the place of twice-daily reports of several factors from various heights plus measurements of water temperature and sea conditions day after day, in bad weather and good, and farther west than German planes could reach. Moreover, the data obtained in flight were often imprecise: the crews sometimes estimated the direction of a light surface wind by firing a burst from the forward machine gun and observing which way the spray drifted.
By the summer of 1940, with the Battle of Britain in full force, the Germans were using ships for weather observation. On July 23, the Adolf Vinnen was positioned north of Iceland. Three weeks later, the Hinrich Freese sailed to relieve her. On September 14, the Sachsen sailed as a replacement for two stations on Greenland that the British had closed. She was commanded by Captain Otto Kraul, an experienced whaler.
The demand for weather information continued to grow. The Naval War Staff had declared that an “augmented weather reporting service is necessary” for Operation SEALION—the invasion of Britain—and that it was to be provided by weather buoys, U-boats, and fishing vessels. U-boat Command objected, saying that its submarines should not be endangered by such secondary missions and should be used exclusively in their primary function: to sink enemy ships. But the war staff overruled it, and U-boats were required to transmit regular weather reports.
These, however, could not be fully satisfactory. In the first place, they were transmitted from each U-boat’s operating area, not from the weather factories in the northern and western Atlantic. In the second place, they lacked the regularity of reports from a ship in a single location. Even the subsequent postponement of SEALION did not end the need for weather information; it would be needed for the raids on British shipping that German surface forces were to undertake.
In the middle of November 1940, however, the navy found itself facing a crisis in weather ships. Three vessels that it had obtained for that purpose proved inadequate. And two of the four northern weather-reporting ships had been sunk. At 1:10 A.M. on October 24, the signal station at the Stadlandet headland on the coast of occupied Norway reported cannon fire and searchlight beams out at sea; the Adolf Vinnen had been sunk by a British submarine, survivors said. On November 16, the Hinrich Freese, visiting the German meteorological station on the Arctic island of Jan Mayen, radioed frantically: “Am being chased by enemy vessels.” Nothing further was heard, even when she was ordered to report, and the navy concluded that she had been lost. Then an accident took the Sachsen out of service for a while, leaving only one weather ship operating in the north.
Now greatly pressured, the navy obtained, on November 23, four fishing vessels for use as weather ships. They had the capacity to be rebuilt for cruises longer than the three weeks or so that sufficed for fishing. One of them was the fourteen-year-old, steam-powered, 139-foot-long München, named for the lively capital of Bavaria. She had to be fitted out with additional water tanks, reserve fuel bunkers, more space for provisions, room for three or four more men, and radios. The Sachsen was mentioned as a norm: she carried a 150-watt transmitter, a 40-watt portable transmitter, two portable receivers, one panoramic receiver, and one broadcast receiver. The München was worked on at the Seebeck dockyard in Bremerhaven, which estimated six weeks for the job. Her sand ballast was replaced by poured concrete. Marine engineers thought at first that the vessel should be strengthened for the ice. But Kraul advised that this was not necessary if the München did not plan to seek shelter in estuaries where she might be frozen in. This advice reduced the time needed for rebuilding.
The high command of the navy declared specifically that weather ships were not warships but auxiliary vessels belonging to the train of the Kriegsmarine. As a consequence, the original civilian crews were retained. But some new personnel, such as the weather observers and some of the radiomen, were members of the navy. They wore mufti but were given armbands saying “Deutsche Wehrmacht” that they were to wear in case of enemy contact. Despite the noncombatant nature of the weather ships, the navy acquiesced in the proposal made by the Naval Group Command North after it lost the Adolf Vinnen and the Hinrich Freese: that the weather ships be armed for self-protection. But because of the haste to get the ships to sea, the München had time only to have a machine gun installed. The vessels were equipped with cipher systems for communicating the results of their observations to Germany.
One was the Wetterkurzschlüssel, or Short Weather Cipher. This booklet, issued in October 1940, was printed on absorbent pink paper in water-soluble ink. Its 21 pages consisted mostly of tables. Each table condensed weather observations and locations into a single letter. These substitutions reduced the time it took to transmit a message from minutes to seconds, decreasing congestion on German radio circuits as well as the chance of enemy interception and direction-finding. The observer aboard ship converted his measurements into these letters in the order prescribed (see the Appendix for an example). He then added his ship’s two-letter signature. The time of the observation was presumed to be within an hour of the radioing of the message.
The Germans knew that the meaning of these messages would quickly be ascertained by British meteorological cryptanalysts, donating valuable information to Britain. To prevent this, and to maintain security in general, the Germans enciphered their abbreviated weather messages in the Enigma. Each weather ship thus carried an Enigma and its associated manuals and key lists. The latter specified the Enigma settings t
hat were to be used each day for a month throughout the German navy; sometimes two or even three months’ worth of keys had to be carried, depending on the length and dates of a cruise. The keys specified the three rotors to be placed in the machine in left-to-right order, the position of the alphabet rings on each rotor, the setting of the three rotors, and the plugboard connections. Also included were the indicators book, which listed the three-letter message-grade indicators that, when tapped out on a set-up Enigma, yielded the message key—the position to which the rotors were turned at the start of the encipherment of a nonweather message—and the bigram tables.
Weather messages abbreviated this procedure. The Short Weather Cipher listed twenty-six three-letter groups that served as ready-made message keys. Each was represented by a letter: EDM, for example, by A. The encipherer chose one of these positions, say, that represented by Q. Then, with the rotor order and alphabet ring position specified in the current Enigma keying document, he turned the rotors until the three letters that Q represented appeared in the windows in the machine’s lid. Then he enciphered his Short Weather Cipher text; MZNFPED plus, say, MR as signature, might become TVUOIGMVX. The encipherer prefixed the key indicator Q to this and, in front of that, a WW, meaning Wetterkurzspruch or “short weather message.” The radioman, who was also the encipherer, would tap this out on a specified frequency in 15 to 20 seconds. He sent it “blind”—not addressed to any station. But any station that received the message was to acknowledge receipt. If the ship received no confirmation, she repeated her transmission fifteen minutes later on a different frequency. The main weather stations became Norddeich, for weather ships operating south of Iceland, and Kootwijk, in the Netherlands, for those north of Iceland; more often than not, however, other stations replied. The receiving station deciphered the message using Enigma, added a time group, and teletyped the Short Weather Cipher letters to the navy’s weather central, which reconverted them into the original data.
