HMS Aubrietia (K96): The Corvette That Really Captured the Enigma Machine

An Unremarkable Corvette, at First

HMS Aubrietia was a Flower-class corvette built by George Brown and Company at Greenock, laid down on 27 October 1939, launched on 5 September 1940, and completed on 23 December that year. She displaced 940 tons, carried a single 4-inch gun and depth charge throwers, and was, in almost every respect, an entirely typical example of a class built cheaply and quickly to plug an urgent wartime gap in convoy escort numbers.

Nothing about her specification suggested she would go on to play a part in one of the most significant intelligence coups of the entire Second World War.

The Real Story Behind U-571

On 9 May 1941, while escorting Convoy OB 318, Aubrietia spotted the periscope of the German submarine U-110 and depth charged it, forcing the submarine to surface in a badly damaged state.

The man commanding U-110 is worth knowing about, because he had already made himself notorious. On 3 September 1939, the first day of the war, Fritz-Julius Lemp was in command of U-30 when he torpedoed the liner Athenia off the Irish coast, killing more than a hundred civilians including women, children and Americans. He had taken her for an armed merchant cruiser. Germany denied the sinking outright for years and claimed the British had done it themselves, and the boat's log was altered to remove the entry. So the officer who fired the most infamous shot of the war at sea, on its very first day, was also the officer whose boat handed the Allies the means of reading German naval signals twenty months later.

Boarding parties from HMS Bulldog then captured the submarine's Enigma cipher machine and its codebooks intact, a properly pivotal moment for Allied codebreaking efforts at Bletchley Park and one of the most significant intelligence victories of the entire Battle of the Atlantic.

U-110's captain, Kapitänleutnant Fritz-Julius Lemp, tried to swim back to his own boat as the boarding party, led by Sub-Lieutenant David Balme, worked through it; he was never seen again, and it was Balme's men, not Lemp's own crew, who ended up carrying the Enigma machine and its cipher material across to Bulldog.

It is worth being clear about what boarding meant that morning. David Balme was twenty years old. He went down the conning tower ladder of a submarine that had just been depth charged to the surface, first and alone, not knowing whether she was flooding, whether she had been booby-trapped, whether the scuttling charges had been set, or whether there was anybody still alive down there. His orders had been about as broad as orders get: bring back whatever you can, documents, books, charts, the wireless settings, anything. His party then formed a human chain and passed the contents of the boat up the ladder and across to Bulldog by whaler, hour after hour, while the submarine settled under them.

And the machine itself was, in truth, the least valuable thing they took. The Allies already understood how an Enigma worked. What made naval Enigma so much harder than the army and air force versions was the way the Germans disguised the message setting: the indicator that told a receiving operator how to arrange his machine was itself encoded, using printed lookup tables of letter pairs known as bigram tables, which were not derivable by any amount of cleverness. You either had them or you did not. Balme's men brought back the bigram tables, the short signal codebook, the weather codebook, the officer settings and the current keys.

The effect at Bletchley Park was immediate and then lasting. Hut 8 could read Kriegsmarine traffic currently, which meant convoys could be routed round waiting U-boats rather than into them, and within about a month the sinkings had dropped by roughly a third. The longer-term gain was greater still: reading real messages taught the codebreakers the shape of them, the standard phrases, the formats, the habits of individual operators, and that knowledge kept working long after the captured keys had expired. It is the difference between being given the answer and being taught the method.

To stop the Germans ever suspecting what had been taken, U-110 herself was quietly allowed to sink under tow the following day, and the whole affair, code-named Operation Primrose, was kept secret for the best part of seven months, so effectively that even the U-110 survivors pulled from the water had no idea their Enigma machine had been captured rather than lost with the boat.

Which is why the secrecy mattered more than the capture. Everything the material bought would have evaporated the instant Berlin suspected that a U-boat had been boarded rather than sunk. So U-110 was allowed to fill and go down under tow the next day, her survivors were kept isolated from anyone who might work out what had happened, and the men who had done it were decorated without any citation that gave away why. Balme received the Distinguished Service Cross for what was, on paper, an unspecified act of gallantry. The full story did not become public for another thirty years.

David Balme lived a long time with it. He was twenty when he went down that ladder and ninety-five when he died, at Lymington in Hampshire, on 3 January 2016. In between he came home, went into the family wool business, married in 1947, brought up three children, hunted with the New Forest Hounds and said nothing whatever about the most important thing he had ever done, because he was not permitted to. Nobody could be told, which meant that for thirty years the Distinguished Service Cross on his chest had no story attached to it. When the Enigma secret was finally relaxed in the 1970s he was able to talk about it at last, and in his later years he was given a Bletchley Park badge and a certificate signed by the Prime Minister. It is a modest sort of recognition for what he went down that ladder and did, and it took the best part of seventy years to arrive.

It is worth noting, with a certain wry satisfaction, that this was a wholly British operation, involving Aubrietia and Bulldog, roughly six months before the United States even entered the war, whatever a certain well-known Hollywood submarine film might have later suggested to the contrary.

Eighty-Five Convoys and a Second Success

Aubrietia went on to a remarkably relentless convoy escort career, protecting shipping across the Atlantic, the Mediterranean and off North Africa, including during Operation Torch, and is credited with escorting no fewer than 85 convoys between January 1941 and April 1945, an extraordinary tally that speaks to just how central these small, unglamorous corvettes were to keeping Britain supplied throughout the war.

She had a second brush with U-boat hunting success on 15 May 1944, when she was one of several Allied units ordered into the hunt for the German submarine U-731 as it tried to force the Strait of Gibraltar.

The boat was sunk with the loss of all 54 of her crew, though it was HMS Kilmarnock, HMS Blackfly and two American Catalina flying boats, rather than Aubrietia herself, that delivered the fatal depth charge attacks.

A Norwegian Retirement

Aubrietia was decommissioned on 29 July 1946 and sold in 1948 to a Norwegian company, renamed Arnfinn Bergan, going on to a further merchant career before finally being scrapped in Norway in December 1966, a quiet, ordinary end for a ship whose single most famous action had, in fact, been anything but ordinary.

Her own retirement has a certain symmetry to it that is easy to miss. The Norwegian company that bought her was Kosmos, one of the great whaling firms of Sandefjord, and after a spell as a buoy boat she was taken to Kiel in the summer of 1951 and converted into a whale catcher. She then worked every whaling season until the winter of 1964 and 1965. The Flower class had been adapted from a whale catcher in the first place, because in 1939 the Admiralty needed an escort that ordinary commercial yards could build and a whale catcher was the right size, shape and price; so the ship that took the Enigma material out of U-110 spent the last fourteen years of her working life doing the job her own design had been borrowed from. She was laid up at Sandefjord in May 1965 and broken up the following year.