The capture of U-570

On the morning of 27 August 1941, a few dozen nautical miles south of Iceland, the German submarine U-570 surfaced. Moments later depth charges from a British patrol aircraft came down on her. A dozen or so hours later a white sheet was flying from her conning tower, an American flying boat with a British crew was circling overhead, and a Royal Navy armed trawler was rolling on the swell alongside. It was the only case in the war of a U-boat surrendering to an aircraft – and the boat went on to sail under the enemy’s flag as HMS Graph.

A popular chronology of the war states that the boat was “captured by the British Royal Navy”. This is a simplification. U-570 was forced to surface and surrender by aircraft of RAF Coastal Command; British warships and a Canadian destroyer reached the scene only during the night, and the following day took over the prisoners and the boat.

The boat and her crew

U-570 was a Type VIIC boat, the most numerous type of German submarine. She was built by Blohm & Voss in Hamburg, launched on 20 March and commissioned into the Kriegsmarine on 15 May 1941. After training in the Baltic she moved to Norway, to the base near Trondheim. She was commanded by Kapitänleutnant Hans-Joachim Rahmlow, an experienced artillery and coastal defence officer who had, however, only recently transferred to the submarine arm and had not yet made a single war patrol. The same was true of most of the crew: the first watch officer had previously served in destroyers, the second had just been commissioned, and of the 44 men only four, including the engineer, had a war patrol behind them.

This was no exception. In the summer of 1941 Admiral Karl Dönitz’s U-boat fleet was expanding rapidly – German yards were turning out a dozen or more new boats a month – and crew training had been cut to the minimum. More and more boats were putting out into the Atlantic with men who had never been through a depth-charge attack.

South of Iceland

At the end of August the German radio intelligence service (the B-Dienst) established that Allied ships were gathering in the waters south of Iceland. Dönitz sent a dozen or so boats into the area. One of them was U-570, which on 23 or 24 August (sources give both dates) left Trondheim on her first war patrol. After the patrol she was to put in at her new base at La Pallice in France.

Iceland, however, was no longer defenceless. Since May 1940 it had been occupied by the British, who had built airfields there for aircraft patrolling the convoy routes, and since July 1941 the Americans had been stationed there too. Lockheed Hudson bombers of No. 269 Squadron RAF flew from the airfield at Kaldaðarnes in the south of the island, and Consolidated PBY Catalina flying boats of No. 209 Squadron from Reykjavík. The mid-Atlantic remained beyond their reach, but the waters south of Iceland were already being patrolled by aircraft every day.

The Hudson’s attack

On 27 August, after four days at sea, Rahmlow kept the boat submerged for most of the morning because many of the crew were seasick. Earlier that morning the boat had been spotted by Sergeant Mitchell’s Hudson, but its depth charges failed to release. Mitchell called up a second aircraft of the squadron by radio, piloted by Squadron Leader James Thompson. At around 10.50 a.m. U-570 surfaced at 62°15′N, 18°35′W – almost directly beneath Thompson’s approaching Hudson, which could not be seen through the periscope. Rahmlow, who had come up onto the bridge, heard the engines and ordered an immediate dive. The boat did not get under in time. Thompson dropped four depth charges, one of which exploded a few metres from the hull.

The explosions almost rolled the boat onto her side. The lights went out, the glass on the instruments cracked and water began seeping in. The crew concluded that seawater had got into the batteries and that they were giving off poisonous chlorine gas. The men in the engine room fled in panic to the bow, and no one restored the power, although – as the British later established – it would not have been difficult. Rahmlow, convinced that everyone would be poisoned if they stayed submerged, ordered the boat to surface. A dozen or so men came out on deck, and the Hudson opened fire on them with its machine guns. The sea was too rough to man the anti-aircraft gun, and the Germans did not know that the aircraft had no bombs left. So they hung out a white sheet. Thompson ceased fire and reported that the boat was surrendering.

The British had no ship anywhere near. Thompson’s Hudson circled over U-570, and then a second one, which broke off its flight from Scotland to Iceland. After three hours Flying Officer Edward Jewiss’s Catalina arrived from Reykjavík and kept watch over the boat for hours more, with orders that if no ship arrived in time, the crew was to be made to jump into the water before dark and the U-boat sunk. The Germans meanwhile reported their position to headquarters in plain language, destroyed the radio, smashed the Enigma cipher machine and threw its parts overboard together with the secret documents. Dönitz ordered boats in the vicinity to go to her aid, but U-82, which tried to reach the scene, was driven off by aircraft.

Night and the tow

U-570’s intercepted signal reached Western Approaches Command, which sent ships to the scene. The first to arrive, at around 10 p.m., guided by the Catalina’s flares, was the armed trawler Northern Chief. Her captain signalled to the Germans that if they tried to scuttle the boat he would open fire and would not pick up survivors. During the night the trawler Kingston Agate, two smaller escort vessels, the British destroyer Burwell and the Canadian destroyer Niagara came up.

On the morning of 28 August the Germans signalled by lamp, asking to be taken off the boat, which they said was sinking. The British refused until the crew had made the boat secure – they feared the Germans would leave the valves open behind them. The confusion was compounded by a Norwegian Northrop N-3PB seaplane of No. 330 Squadron, which, unaware of the surrender, dropped small bombs on U-570 and fired on Northern Chief before it was called off by radio. The weather was worsening, and repeated attempts to pass a tow failed. The captain of Burwell, thinking that the Germans were deliberately obstructing, ordered warning shots to be fired; the bullets slightly wounded five of U-570’s crew. In the end a lieutenant and three seamen from Kingston Agate paddled across to the boat on a life raft, made a cursory search – the Enigma was already gone – and secured a tow. The wounded and the officers were taken aboard the trawler, and Niagara took off the rest of the crew. None of the 44 Germans was killed.

The group of ships, with U-570 in tow and covered by relays of Hudsons and Catalinas, limped to the coast of Iceland at dawn on 29 August. The boat was taking in water, so she was beached at Þorlákshöfn on the south coast of the island.

What the British gained

Two days later the submarine officer George Colvin flew in from Britain with a team of mechanics and engineers. They found the boat heeled over on her side and, inside, a knee-deep slurry of oil, seawater, flour, peas, bread and clothing. The damage, however, proved slight: leaks in several tanks, roughly a third of the battery cells cracked, a dented bow. No chlorine was found. Colvin wrote in his report that an experienced crew would have repaired the damage without difficulty, stayed submerged and probably escaped the aircraft. After surrendering, the Germans had tried to destroy equipment, but apart from the radio and the torpedo data computer they had done so half-heartedly.

Unlike with U-110 in May, the British did not capture an Enigma or current cipher keys aboard U-570. They did, however, find copies of enciphered signals together with their plain text, and a U-boat commander’s handbook that helped the cryptanalysts at Bletchley Park understand the abbreviations and jargon of the German navy in the signals they were already reading. The German command, which learned of the boat’s fate from the British press, investigated the matter and concluded that even if documents had fallen into enemy hands, communications would remain secure. It did not know that since June the British had been reading the U-boat cipher thanks to the capture from U-110.

The boat was raised, towed to the base at Hvalfjörður and given temporary repairs alongside the depot ship Hecla. On 23–26 September she was inspected by two naval officers of the still formally neutral United States, who took away a torpedo for examination in America. On 29 September U-570, with a British crew, set out on the surface under escort for Barrow-in-Furness, where her arrival on 3 October was filmed by the newsreels. Unlike with U-110 there was nothing to hide: the Germans knew of the surrender, and too many people had taken part in the operation.

Trials in Britain yielded valuable lessons. It turned out that a Type VIIC boat could dive much deeper than had been assumed – to more than 200 metres, according to various studies – whereas British depth charges could at that time be set to a maximum of about 150–170 metres. The fuzes were quickly changed. The Zeiss periscope, the sensitive hydrophones and the shock-absorbing machinery mountings that kept the boat quiet were rated highly; the cramped crew quarters were criticised. The boat also served as a model for mock-ups of a U-boat interior on which boarding parties were trained, intended in future to seize surfaced boats before their crews could scuttle them.

Berndt’s escape

U-570’s officers were sent to the prisoner-of-war camp at Grizedale Hall in Cumbria, which held many German naval officers, among them the U-boat ace Otto Kretschmer. The prisoners, who knew of the surrender from British newspapers, secretly convened a “court of honour”. It found Rahmlow – in absentia, since he was held elsewhere – and his deputy, the first watch officer Bernhard Berndt, guilty of cowardice. During the night of 18–19 October Berndt escaped from the camp. According to some accounts, he intended to reach Barrow, a few dozen kilometres away, and destroy the boat in order to redeem himself. He was stopped by men of the Home Guard; when he tried to run, he was shot dead. The British afterwards moved Rahmlow to a camp for army and air force prisoners.

HMS Graph

On 5 October 1941 the boat was commissioned into the Royal Navy as HMS Graph (pennant number P715). The name began with “G” for German, alluded to the many graphs drawn up during the trials and to the German title Graf. Churchill at first wanted to hand the prize over to the Americans or to a Yugoslav crew in the Mediterranean, but the Admiralty objected. From October 1942 Graph made three war patrols. On 21 October 1942 in the Bay of Biscay she fired four torpedoes at the German U-333 under Peter-Erich Cremer; the British believed they had sunk her, but the Germans spotted the torpedo tracks and managed to evade them. At the turn of 1942 and 1943 the boat patrolled off the coast of Norway. She was then used for training and, for lack of spare parts, was withdrawn from service in 1943. On 20 March 1944, while being towed to the breakers, she broke her tow in a storm and was wrecked on the rocks of the west coast of the island of Islay. She was the only U-boat to fight on both sides during the war.

Place in the war

The capture of U-570 did not change the course of the Battle of the Atlantic, but it illustrated its changing conditions well. The patrol aircraft, a rarity over these waters a year earlier, had proved a dangerous opponent – and from a base in Iceland, which the British had occupied precisely in order to close the North Atlantic to the U-boats. The German command drew the lesson that the new crews were poorly trained: training was lengthened, and experienced seamen began to be distributed among the new boats. The British gained knowledge of the diving depth, quietening and equipment of Type VII boats, which served to improve depth charges and anti-submarine tactics in 1942–1943, when the Battle of the Atlantic reached its peak. In the autumn of 1941 American destroyers began escorting convoys in the western Atlantic, which soon led to the first clashes with U-boats, and the number of aircraft over the ocean grew month by month. The air gap in the middle of the Atlantic, where U-boats could operate without fear of aircraft, was not closed, however, until the spring of 1943.