The capture of U-110

Around noon on 9 May 1941, in the North Atlantic east of Cape Farewell at the southern tip of Greenland, the German submarine U-110 attacked convoy OB 318 and sank two ships. A few minutes later the British escort smothered her with depth charges and forced her to the surface. The crew abandoned the boat in haste, convinced that she was about to go down. She did not sink. A boarding party from the destroyer HMS Bulldog went aboard and carried off everything that could be lifted: an intact Enigma cipher machine, and the code books and tables used by the German radio operators. U-110’s commander, Fritz-Julius Lemp – the same man who on the first day of the war had sunk the liner Athenia – died in the water. The British gave the affair the code name Primrose and kept it secret until the end of the war; the Germans never learned that their boat had fallen into enemy hands.

Some chronologies place the capture of U-110 under 8 May, linking it with heavy convoy losses and with the “salvage of another Enigma machine”. Convoy OB 318 was under attack from the evening of 7 May to 10 May, but U-110 herself was captured on 9 May. The phrase “another Enigma machine” also needs explaining: what proved most valuable was not the machine itself but the documents found beside it.

The Atlantic in the spring of 1941

May 1941 was one of the hardest months of the early years of the Battle of the Atlantic. According to figures compiled by uboat.net, German submarines hit 66 ships that month, more than in any month since the autumn of 1940. After the losses of March, when Prien and Schepke were killed and Kretschmer was captured, Vice-Admiral Karl Dönitz moved his patrol lines further west, into the mid-Atlantic, where convoys outward bound from Britain were less well protected or already dispersing. The British responded by extending the escort. From the spring, escort groups based in Iceland took over convoys from the ships based in the British Isles and led them further west, and in London a plan was being prepared for continuous escort across the whole ocean, with the participation of Canadian warships.

At the same time another war was being fought, of which almost no one knew. From the start of the war the codebreakers at Bletchley Park had been reading more and more Luftwaffe and Army messages enciphered on Enigma, but the naval version of the machine resisted them longest. The Kriegsmarine used eight rotors instead of five, changed its daily keys according to secret tables, and additionally enciphered the settings of each message using bigram tables, lists of pairs of letters. Without captured documents the codebreakers of Hut 8, where Alan Turing worked, could break only isolated days of traffic, and then only weeks late. That is why the Royal Navy was hunting for German cipher material. In February 1940, as the Germans were extending the submarine war, rotors were recovered from survivors of U-33, sunk in the Firth of Clyde. On 4 March 1941, during the Lofoten raid, sailors from the destroyer Somali took spare rotors and the February keys for the home waters cipher from the burning trawler Krebs. On 5 April, during the attack on convoy SC 26, the corvette Arbutus tried to seize the surfaced U-76, but the boat sank before anyone could take anything from her.

The first of the spring successes came at the beginning of May. On 7 May, in waters near Iceland, British cruisers and destroyers intercepted the German weather ship München. The idea came from a young Bletchley Park analyst, Harry Hinsley, who reasoned that weather ships, which spent several weeks at a time at sea, must carry Enigma keys for the following month locked in a safe. The München’s crew threw the machine overboard, but a boarding party from Somali found the home waters keys for June. Two days later fortune brought the British far more.

Convoy OB 318

OB 318 sailed from Liverpool on 2 May 1941. It consisted of 38 ships, most of them sailing in ballast or with general cargo for North American ports. At first it was escorted by the 7th Escort Group, and on 7 and 8 May, south of Iceland, it was taken over by the Iceland-based 3rd Escort Group, commanded by Commander Addison Joe Baker-Cresswell in the destroyer Bulldog. The group included, among others, the destroyers Broadway and Amazon and the Flower-class corvette Aubrietia. The escort was to accompany the ships to a dispersal point at roughly 35 degrees west, from where each would sail on to its destination independently.

On the evening of 7 May the convoy was sighted by Kapitänleutnant Herbert Kuppisch’s U-94. He reported it to Dönitz’s staff and after dark, submerged, let the leading escorts pass and sank two ships from the middle of the formation at close range. For several hours afterwards the escorts hunted him with depth charges; damaged, he had to withdraw. On the evening of 8 May two more boats made contact with the convoy: Lemp’s U-110 and the U-201 of Adalbert Schnee. On the morning of 9 May the two commanders, contrary to the custom of the German U-boat arm, met on the surface and agreed a joint attack: Lemp was to strike from ahead of the convoy, Schnee from astern. They were counting on the escort having already turned back to base, as it usually did at that longitude.

Attack and surfacing

But the escort had not left. When Lemp, around noon, penetrated the convoy at periscope depth, he saw British warships around the merchantmen. He fired his torpedoes all the same and sank two British steamers: the Esmond, sailing in ballast to Sydney, Nova Scotia, and the Bengore Head, carrying coal from Belfast to Montreal. The whole crew of the first was rescued; one man died from the second. Lemp was already aiming at another ship when lookouts on the corvette Aubrietia spotted his periscope. The corvette located the boat by ASDIC, raced in at full speed and dropped a pattern of depth charges. Bulldog and Broadway joined the hunt.

U-110 did not manage to reach a safe depth. According to the survivors, the explosions damaged the hydroplanes and rudder and ruptured the compressed-air lines, while chlorine began to escape from cracked battery cells. The boat went out of control and broke surface in the midst of three British warships. They immediately opened fire, thinking the Germans meant to man the deck gun. Broadway turned to ram, and Lemp, seeing this, gave the order to abandon ship.

At that moment Baker-Cresswell remembered, as he later recounted, a lecture on a staff course about searching enemy vessels for code books. He ordered the firing to stop and the ramming to be called off. Broadway altered course at the last moment, scraped along the U-boat’s hull, damaging her own bow and propeller in the process, and dropped two shallow-set depth charges to hurry the crew’s escape. The Germans jumped into the icy water. The boat’s stern and the base of the conning tower were already awash, and Lemp believed that U-110 was about to sink with her secret documents. That is why, according to his officers’ accounts, he gave neither the order to destroy the cipher material nor the order to scuttle; he was the last to leave the boat, together with the first watch officer, Dietrich Loewe, and the engineer officer.

Lemp’s death

But the boat did not sink. According to Loewe, Lemp, swimming beside him, noticed that U-110’s bow and conning tower were still above water and that the destroyer was not ramming. He shouted that they must go back aboard and sink the boat, or at least throw the Enigma and the documents overboard, and began to swim towards her. Wind and waves, however, were pushing the drifting boat away from the men in the water. Loewe lost sight of his commander just as a boat carrying the boarding party was being lowered from Bulldog. Lemp was never seen again.

The circumstances of his death remain disputed. Some of the German sailors claimed after the war that he had been shot in the water by the British in the boarding boat. A war correspondent who had sailed in U-110 testified that he too had been fired at. The leader of the boarding party, Sub-Lieutenant David Balme, firmly denied this: according to him his men did not fire a single shot, and Lemp, aware of the consequences of his mistake, chose death in the sea. That is also the official British version. All that is certain is that fifteen members of the crew died, including the commander. Some fell to the warships’ fire as the boat surfaced; others died of exposure and exhaustion in the water, because Aubrietia first picked up the survivors of the sunken Esmond. When she came back for the Germans she pulled 34 men from the sea, two of whom died on board. Thirty-two were taken prisoner.

The boarding party

Balme and a few sailors rowed across to U-110 and, weapons in hand, climbed down through the conning tower hatch, expecting resistance or demolition charges. According to his account the boat was empty, the lights were on, and the compartments were in disorder after the crew’s flight. For several hours the sailors, forming a human chain, passed up and ferried across to Bulldog everything that looked important: charts with the German naval grid, the logbook, signal books, orders and, above all, a machine which in the radio room looked like an odd typewriter and which was unscrewed from the table because it struck one of the sailors, a former telegraphist, as out of place. Next to it lay code books and tables. No one in the boarding party knew at the time what they had actually captured. For a while Balme and his men were left alone aboard U-110, because Bulldog had moved off to investigate a fresh submarine contact.

Baker-Cresswell then took U-110 in tow and headed for Iceland, hoping to bring the whole boat into port. The weather was worsening and the damaged hull was taking in water. The following day, 10 May, U-110 sank under tow; so say most sources, though some speak of two days. For the secrecy of the operation this was a gain rather than a loss: the German prisoners, kept below decks in Aubrietia from the start, had seen only that a boat full of British sailors was heading for the U-boat, but did not know what happened next. They could therefore assume that U-110 had sunk soon after they abandoned her.

Convoy OB 318, meanwhile, fought on. On 9 May Schnee’s U-201 sank one ship and damaged another before being driven off, and on 10 May, as the convoy was already dispersing, U-556 sank two more and damaged one. According to the figures, the convoy lost five ships, and three more were sunk after dispersal, near their ports of destination.

Operation Primrose

The Admiralty quickly grasped what Bulldog had brought back. Had the Germans learned of the boat’s capture, they would have had to assume the worst and change their keys, perhaps even their whole cipher system. Everyone involved was sworn to silence, and the affair was given the code name Primrose. The prisoners were kept apart from the survivors of the merchant ships and taken via Iceland to Britain, and later on to camps in Canada. Loewe was never entirely convinced that the boat had sunk and tried to pass on a warning in a simple code in a letter to his family, but British censors intercepted it. When Bulldog put into Scapa Flow, envoys from Bletchley Park were waiting for her; they photographed the documents and took them away.

Dönitz’s staff assumed that U-110 had been lost attacking the convoy, like many other boats. Lemp and his boat vanished from the German reports without trace, and until the end of the war the German navy believed its ciphers were secure; later changes of procedure, introduced as a precaution, did not stem from any knowledge of the loss of U-110. The secret was kept even from allies. Winston Churchill told President Franklin Delano Roosevelt about it only in January 1942, when the United States was already at war. Baker-Cresswell received the Distinguished Service Order and promotion to captain, and Balme the Distinguished Service Cross. According to Baker-Cresswell himself, King George VI, when presenting the decorations, called the capture of U-110 the most important single event of the whole war at sea.

What was really captured

The machine itself, contrary to the common phrase about “another Enigma machine”, was not the greatest novelty for the British. They had known how Enigma was built since the summer of 1939, when, at a meeting near Warsaw, the Polish codebreakers of the Cipher Bureau, who had been the first to break it, handed over to the British and French replicas of the machine that they had reconstructed and the results of their work. The wiring of the additional naval rotors had become known from the captures from U-33 and the Krebs. From U-110 the British did obtain the first complete, working naval machine, but the documents proved far more valuable.

Among them were the bigram tables used to conceal the settings of individual messages, which Bletchley Park had previously reconstructed with great labour; current key settings, including those of the officer key, reserved for the most secret messages; the short signal book (Kurzsignalheft), with which U-boats encoded brief convoy-sighting reports so as not to be located by direction-finders; and the short weather cipher. The U-110 documents also made it possible in June 1941 to break, for the first time, the German navy’s reserve hand cipher (Reservehandverfahren), whose decrypts went on for years to provide clues for breaking Enigma.

The U-110 haul did not give the British the key to German traffic for ever. The daily keys lasted only a few weeks, after which they had to be broken again. Together with the June keys from the München, however, the U-110 material allowed Hut 8 to read the home waters cipher in June – Heimische Gewässer, known at Bletchley Park as “Dolphin” (the Germans later named it “Hydra”) – almost in real time. At that time the U-boats in the Atlantic also used this cipher. The July keys came with the capture of another weather ship, the Lauenburg, on 28 June. Most important, though, was what the codebreakers learned along the way: familiarity with the form and content of typical messages, procedures and abbreviations gave them a stock of probable fragments of plain text, so-called “cribs”, with which from August 1941 they began to break the keys without the help of captures, using the bombes and Turing’s statistical method, which developed an idea of the Polish cryptologist Jerzy Różycki. The delay in reading was usually anything from a dozen or so hours to a few days.

Place in the war

The effects could be seen on the map of losses. From the summer of 1941 the Admiralty, knowing from decrypted messages where the patrol lines lay, increasingly routed convoys so as to avoid the wolf packs. According to figures compiled by uboat.net, U-boats hit 65 ships in June 1941 but only 21 in July. Historians attribute a large part of this fall to evasive routing made possible by signals intelligence, although other factors also mattered: the growing escort, patrol aircraft, and Dönitz’s transfer of some of his boats to other hunting grounds. The decrypts also helped to hunt down German supply ships on the high seas. Intelligence derived from broken Enigma ciphers, later known as Ultra, became one of the Allies’ most important weapons in the Battle of the Atlantic.

The advantage was not lasting. On 1 February 1942 the Atlantic U-boats switched to a new, four-rotor version of Enigma and a key of their own, called “Shark” at Bletchley Park. For ten months the British could not read it. The breakthrough came only with the documents taken on 30 October 1942 from the sinking U-559 by sailors from the destroyer Petard, two of whom were drowned; among them was a new edition of the short weather cipher that the British already knew from U-110. In August 1941 a British aircraft forced U-570 to surrender, but her crew managed to destroy their cipher material; the boat was commissioned into the Royal Navy as HMS Graph. The Americans did not capture U-505 until June 1944.

The secret of U-110 outlived the war. The British naval historian Stephen Roskill described the capture of the boat in his book The Secret Capture in 1959, but he could not yet say why it had been kept secret; the role of the U-110 material in breaking Enigma was revealed only in the 1970s, when works on Ultra began to be published. In 2000 the American film U-571, in which the German cipher machine is captured by Americans, caused such outrage in Britain that the matter was raised in the House of Commons, and the closing credits mentioned the real captors from Bulldog, Broadway and Aubrietia.

The capture of U-110 did not decide the Battle of the Atlantic, which went on for another four years and had its hardest period still ahead. But it was one of several links – alongside the Lofoten raid, the capture of the weather ships and, earlier, the work of the Polish cryptologists – thanks to which the British, in the critical year of 1941, began to read the signals of the German U-boat fleet. It was a bitter irony that this came about through Lemp, a commander whose first attack of the war, the torpedoing of the Athenia, brought Germany a propaganda disaster, and whose last cost it a loss of which it never learned.