Seizing the Enigma: The Race to Break the German U-Boat Codes, 1933-1945

by Kahn, David

Lemp was one of the German navy’s most successful U-boat commanders. A jaunty young man with pudgy cheeks, he had joined the navy in 1931 at age eighteen. At the outbreak of the war, he commanded the U-30, a three-year-old Type VIIA, the Kriegsmarine’s principal operating submarine, known for its excellent performance.

  On the very first day of the war with Britain, Lemp became the first U-boat captain to sink an enemy ship. But this opening of the Battle of the Atlantic harmed Germany more than it helped her. For Lemp had torpedoed the unarmed passenger liner Athenia, which he had apparently mistaken for an armed auxiliary cruiser. The loss of 118 lives and the apparent flouting of a Hague convention convinced Britain that Germany had resumed the unrestricted submarine warfare of World War I; consequently she at once initiated all measures against this aggression.

  Lemp made eight cruises in the U-30. In August 1940, after an especially successful cruise, he was acclaimed for having put a torpedo into the battleship Barham and for having sunk, it was claimed, nine ships. (The actual number was six; U-boat captains, like fighter pilots, continually overestimated successes.) Dönitz, who seems to have had a soft spot for Lemp, awarded him the Knight’s Cross to the Iron Cross.

  In November, Lemp commissioned the U-110, one of the new large Atlantic Type IXB boats. The submarine, 252½ feet long and displacing 1,050 tons, could cruise for 12,400 miles. Top speed was 18.2 knots on the surface, 7.3 submerged. It carried twenty-two torpedoes, could submerge in about half a minute, and could dive to 330 feet.

  Most of the enlisted men were new, having just come from their various training schools. Lemp brought two of his three officers with him from the U-30. The exception was his second in command, Lemp’s cousin, who was not popular with the crew members. They regarded him as a Jonah because two of his ships had been sunk under him. With 4 officers, 15 noncoms, and 27 seamen, the crew totaled 46. Lemp made a good impression on the men. He was open, avoided spit and polish, and knew how to weld them together He didn’t throw his weight around, but it was clear that he was the boss. The Knight’s Cross gained him automatic respect. The men believed they were serving under one of the best commanders in one of their country’s most important forces.

  Because of trouble in the U-110’s diesels, she did not sail on her maiden war patrol until March 9, 1941. As she searched the vast wastes of the North Atlantic for prey, the seamen stood four-hour watches, the machinists six-hour. Though it was a scramble to eat and sleep in the four hours off, the watches were often boring. Excitement came only during the rare spurts of action. During her first cruise, the U-110 sank two ships. Lemp used the classic tactic of slipping between the first and second columns of a convoy at night, firing torpedoes from the bow at one column and from the stern at another, and then diving deep to avoid the expected attack.

  One of Lemp’s log entries reveals his thoughtless enthusiasm:

  16.3 [1941] 0022. Tanker passes within 100 meters.… Stern shot.… Hit. Tanker flies in the air with a great flash and is atomized.… Night lit up like day by the light of the fires of exploding tankers. U-boat stays between tanker and destroyer 3,000 meters [2 miles] away in the middle of the convoy as if in a spotlight. Since destroyer turns toward U-boat with big bow wave, emergency dive.

  During the depth-bomb attack that followed, Lemp showed his leadership. Often he would give an order just to let the crew hear the lack of anxiety in his voice. His calmness soothed the younger men. And the boat survived.

  Another log entry a week later indicates his honesty and self-confidence. After missing three shots, the last at only 500 yards, “(1) Out of mistrust of torpedoes, (2) out of fury, I order an artillery attack.” But one of the seamen had forgotten to remove the muzzle tampion from the 4.1-incher, and when the gun was fired, it exploded and damaged the U-boat. No longer able to dive fast, the U-110 was ordered back to Lorient, on the Atlantic coast of France, for repairs; after its return Dönitz visited the boat and awarded the Iron Cross to several crew members.

  Following two weeks of repairs and furlough, the U-110 sailed again on April 15. The next day, Dönitz radioed the location of her operating area. He was hoping to concentrate the nine U-boats then in the North Atlantic against the convoys. Experience had shown that having one of the subs command the others on the spot did not work; the lead submarine sometimes had to submerge and so lost contact with the rest. Control had to be exercised from Dönitz’s headquarters in France. This required constant radio communications, all enciphered in the Enigma machine. The responsibility for both ciphering and communicating fell on each U-boat’s radiomen.

  Of the four radiomen aboard the U-110, the most junior was Radioman Third Class Heinz Wilde. Open-faced and friendly, he had been a radio amateur as a teenager in Breslau before the war. A naval officer had come to one of his club’s meetings in 1937 and persuaded many of the members to join the navy. Wilde entered on November 1, 1939. After basic training, he went to the naval communications school at Flensburg, close to the Danish border. Here he deepened his knowledge of radio, and here he was introduced to the Enigma cipher machine. The instructors boasted that the machine was the best and couldn’t be solved. This emphasis on the machine’s unparalleled cryptographic security, Wilde thought, led many students to think its physical security not so terribly important. Wilde, the best in his class, was assigned to U-boats; after specialized submarine training, including instruction in acoustic detection, he was assigned to the U-110.

  There, in the closetlike radio room, he stood four-hour watches, chatting or reading a book and playing phonograph records over the loudspeaker system for the crew while listening for a signal through the whistlings, peepings, and static that filled his earphones. As soon as he heard the tone of the transmitter, he would turn down the phonograph, stop talking or reading, and begin taking down the Morse message. All messages were taken down, even those addressed to other U-boats; all were deciphered and given to the captain. The locations of other submarines and of convoys were entered on charts. Wilde never received more than four messages on a watch; sometimes none came in. To minimize the number of transmissions and to eliminate clues to its location, a U-boat did not signal receipt of a message. Garbled transmissions, which are difficult or impossible to decipher, were relatively rare; to ensure that important messages were not missed, U-boat headquarters repeated each one half an hour to an hour after the first transmission. During an action, the radioman worked the listening apparatus. This array of hydrophones gave the range and bearing of sound sources, such as torpedo explosions and attacking enemy warships; Wilde thought the data it yielded were imprecise.

  On April 21, the day after the U-110 reached the waters west of Ireland where she would begin her patrol, Dönitz radioed her an attack location, AL58 on the Kriegsmarine grid, 400 miles west of Ireland’s west coast. The next day, Dönitz ordered “Attack!” Three days later, Lemp sank a 2,500-ton ship steaming alone. He missed on April 28 with a single shot on a fishing trawler. Dönitz moved the U-110 south, then north, then west over the eastern Atlantic, perhaps on the basis of German codebreaking that told him where convoys might be.

  On Thursday, May 8, Lemp saw smoke and the mast of a warship. Coming up on her starboard, he discovered that she was escorting a slow-moving convoy heading west at 7 or 8 knots. He transmitted a sighting report, which was intercepted by the British Admiralty. It determined the location of the transmitter by a direction-finding fix, then compared the location with its plot of convoys at sea. At 7:07 P.M., Greenwich mean time, the Admiralty warned OB 318 that it was being shadowed. The convoy altered course 30° to port, away from the U-110.

  At about the same time, Dönitz ordered other U-boats to report their positions and directed Lemp to maintain contact with OB 318 and to attack if possible. But the convoy’s turn away broke Lemp’s contact until the listening apparatus again detected the convoy. However, Lemp was unable to take action because the moon was too bright. He decided to attack the next day, when two other U-boats were expected to be
nearby. At 2:16 A.M. local time on Friday, May 9, he again reported the convoy’s position; Dönitz ordered several U-boats to concentrate. A few hours later, the U-201, commanded by Lieutenant Adalbert Schnee, hove into sight. Using light blinkers to communicate, the two captains agreed to attack that day, Lemp first.

  Early that morning, Baker-Cresswell came onto the bridge of the Bulldog to watch his group take up its day screening positions. It was to be his last day with the convoy. His ship would leave late in the afternoon with sufficient fuel to return to Iceland. By that time the convoy could disperse; it would then be at about 34° west longitude, and no Axis submarine had ever sunk a ship that far west. His good feelings were reinforced by the arrival of his breakfast, wrapped in a napkin and served by his faithful steward. As the morning wore on and nothing happened, those feelings seemed to be justified.

  But Lemp had moved into position to attack. From in front of the convoy and to starboard, at periscope depth, he fired three torpedoes at a diagonal distance of 800 yards, or half a mile, at three steamers. Just as Baker-Cresswell was preparing to exchange noon positions with the commodore of the convoy’s merchantmen, he saw a spout of water on the Esmond’s starboard side. A few moments later, the Bengore was hit. Her stern rose almost to the vertical and the crated cargo on her deck cascaded into the sea. It looked, one witness said, like “a child pouring toys out of a box.”

  Baker-Cresswell, recovering from his astonishment, swung the Bulldog to starboard and raced to where he thought the U-boat might be, determined to destroy her. At the same time one of Escort Group 3’s corvettes, the Aubretia, which had detected the incoming torpedoes on her asdic, increased speed and turned to starboard. Two minutes later she obtained an asdic contact, then lost it, so the captain stopped her engines to improve the reception. A minute later the Aubretia spotted a periscope dead ahead, about 800 yards away, traveling from port to starboard. She sped toward it and dropped a full pattern of depth charges, set to explode at 100 and 225 feet.

  The crew of the U-110 heard these explosions and were shaken by the speed of the attack; they had thought they would have fifteen minutes to dive and get away. The explosions, however, were distant. The submarine continued in attack mode. A few minutes later, Lemp, at the periscope, turning his submarine for a stern shot, spotted a destroyer coming at him with great speed. “Down deep!” he ordered. The crew ran forward to speed the dive.

  But no sooner had the vessel begun to tilt than at least a score of depth charges, dropped by the Aubretia and set to discharge at 150 and 385 feet, exploded very close to the submarine. They blew out the main electric motor switch, stopped the electric motors, shattered all depth meters, and started leaks in the oil bunkers; the submarine started to take water and sank even deeper. The plates that formed the deck of the control center, which normally butted one another, overlapped from the pressure. Wilde and others thought it was the end. Although they were frightened and felt helpless, nobody screamed or wept. Lemp, meanwhile, was trying to blow the tanks to get the boat to rise. Suddenly, the men felt her moving upward, perhaps pushed up by depth charges.

  Her rise caused a patch of water on the surface to become disturbed, drawing the eyes of the men on the Aubretia, the Bulldog, and another destroyer, the Broadway. Then, at 12:35, the U-boat burst up from the seemingly vacant sea. Water streamed from her uppers, and she rolled in the slight swell. Inside, the crew members felt the motion and knew, to their great relief, that they were on the surface. Lemp, instead of releasing pressurized air through a valve, opened the hatch. A cloud of dust blew out The crew was ordered to put on life vests and to get out. Ventilation ports and sea strainers were opened to let in water.

  Baker-Cresswell saw red when the U-boat surfaced. She had just sunk two ships, and now this embodiment of all the evil he was fighting had appeared before him. Firing his heavy guns, he ordered 12 knots—suitable ramming speed. But as he saw the German crew boiling out of the conning tower, he realized that they were abandoning ship.

  At that moment, there flashed into his mind a story he had heard at the Naval Staff College in Greenwich in the mid-1920s. It may have made an impression because it involved the father of a fellow student, Lieutenant Louis Mountbatten. During World War I, the Russians had salvaged a German codebook from a German cruiser that had grounded in the Baltic, the Magdeburg. They had delivered the code to Mountbatten’s father, who was first sea lord, the head of the Royal Navy. The codebook had enabled the Admiralty to solve many German coded messages during the war, to great advantage.

  As Baker-Cresswell saw the U-boat rocking on the surface of the ocean, he asked himself, “Is there a chance we can do another Magdeburg?” And he ordered full astern to stop the Bulldog from ramming the sub.

  2

  THE WRECK OF THE MAGDEBURG

  ON THE AFTERNOON OF AUGUST 24, 1914, A GRAY GERMAN warship steamed out of the East Prussian harbor of Memel toward the most fateful accident in the history of cryptology. She was the Magdeburg, a four-stacker, what the Germans called a small cruiser to differentiate the type from the larger light cruisers. She was new (three years old), well armed (twelve fast-firing 4-inch guns), fast (27.6 knots)—and unlucky. Her acceptance test had not gone well: her commissioning was delayed several months. She never participated, as was intended, in the fall 1912 naval maneuvers. Some equipment was still not in order when she was declared “ready for war” and when the ancient city of Magdeburg, southwest of Berlin, for which she was named, sponsored her in two days of festivities. One of her turbines gave trouble. And, unlike her sister ships, which got assignments suitable for cruisers, the Magdeburg became a torpedo test ship.

  During one of her cruises in 1913, when she sailed to the Canary Islands off the northwestern shoulder of Africa to test the range of the naval radio station at Neumünster, her radio officer, a young lieutenant named Walther Bender, bought a puppy. Schuhmchen, the puppy, became a favorite of the crew. Later, in Kiel, whenever Bender spent the night ashore, Schuhmchen went down to the gangway in the morning as the launch shoved off from the dock to return to the Magdeburg. How did he know that Bender was aboard?

  The Magdeburg was part of the Baltic Fleet. When war with Russia, France, and England broke out in August 1914, she dropped her test assignment and undertook more typical cruiser tasks. These were directed against the Russians, whose empire included the countries bordering the eastern Baltic: Finland, Estonia, Latvia, and Lithuania. In her first operation, the Magdeburg and another small cruiser, the Augsburg, arrived off Liepaja, Latvia’s naval port, to lay mines. They gained an unexpected success: the Russians, thinking the appearance of the two ships portended a major fleet operation, blew up their own ammunition and coal dumps and scuttled ships in the harbor entrances. In the two ships’ second and third operations, they shot up some lighthouses and a signal station and laid a minefield not far from the mouth of the eastern arm of the Baltic Sea, the Gulf of Finland, at whose farther end lay the Russian capital, St. Petersburg (now Leningrad).

  A few days later, on August 23, the commander of a new flotilla ordered his vessels, which included the two cruisers, to assemble for operations. The Magdeburg, in Danzig, then a German port, went first to Memel, at the extreme east of Prussia, for some gunnery exercises meant to reassure the population, nervous because the border with Russia was not far from the city limits. The next afternoon the warship set out for the rendezvous, and early on the twenty-fifth it joined the Augsburg, three torpedo boats, a submarine, and three other warships off Hoburgen lighthouse on the southern tip of the Swedish island of Gotland. There the officers were told the plan. The ships were to slip by night behind a Russian minefield believed to protect the entrance to the Gulf of Finland and attack whatever Russian ships they found. At 8:30 A.M. that same day, the flotilla set out, moving northeast at the fairly high speed of 20 knots. The sailors aboard the Magdeburg, who suspected the presence of enemy armored cruisers, thought the assignment was a suicide mission.

  By 5 P.M.,
in a calm sea, the air misty, the navigational plots of the Magdeburg and the Augsburg differed by a mile. But this raised no concern, since the Magdeburg was to follow the flagship by half a mile: if the Augsburg struck a mine, the Magdeburg could avoid hitting any herself.

  Soon, however, fog—common in those waters in summer—rolled in. By 9 P.M., it was so thick that even with binoculars an officer on the bridge of the Magdeburg could not see the lookout on the stern. At 11 P.M., the Augsburg, intending to run along the supposed Russian minefield before swinging east to enter the Gulf of Finland, turned onto a course south-southeast ½ east—and ordered the Magdeburg to do the same. She did so, maintaining the same speed, about 15 knots, that had kept her at the proper distance from the Augsburg during the afternoon. Her captain, Lieutenant Commander Richard Habenicht, had soundings taken. These showed the depth decreasing: 190 feet, 141 feet, and, at 12:30 A.M., now of August 26, 112 feet.

  At the same time the radio shack reported that a message was coming in; four minutes later it was decoded and on the bridge. It ordered that course be altered to east-northeast ½ east. The helmsman spun the wheel and, at 12:37, just as he reported that the new course was being steered, still at 15 knots, the luckless vessel hit something. She bumped five or six times and, shuddering, stopped. The cruiser had run aground. As a consequence of her navigation error, which put her a mile south of the Augsburg, she had struck shallows 400 yards off the northwestern tip of Odensholm, a low, narrow, sandy island 2½ miles long at the entrance to the Gulf of Finland.

  At once, Habenicht sought to free his ship. He reversed engines; he rocked her with various engine speeds; he assembled the entire 337-man crew on the quarterdeck to push the ship’s stern down and her bow up and then went full speed astern; he had the crew carry munitions aft. The ship didn’t budge. Soundings showed that at the bow, where the Magdeburg normally drew 16½ feet, the water was only 16 feet deep to port and 9 feet to starboard; at the stern, with normal draft just under 20 feet, the depths were 13 and 17. The vessel needed to rise between 3 and 7 feet. The tides of the Baltic, measured only in inches, would not suffice for this.