HMS Graph
HMS "Graph (pennant number P715) was a German Type VIIC U-boat that the British Royal Navy captured during World War II. Commissioned as U-570" in Nazi Germany's "Kriegsmarine" in mid-1941, she was attacked and captured on her first patrol. She provided both the Royal Navy and United States Navy with significant information on German submarines, and carried out three combat patrols with a Royal Navy crew, becoming the only U-boat to see active service with both sides during the war. She was withdrawn from service in 1944 due to problems maintaining her. While being towed  to the breakers for scrapping, she ran aground on the Isle of Islay, off the west coast of Scotland. Some of the wreck was removed as scrap but some wreckage remains there to the present day. Design and construction. The submarine was built to the German Type VIIC design. She had a displacement of when surfaced, and when submerged. The boat was long, with a beam of , and a draught of . The diesel-electric propulsion system provided a maximum speed of surfaced or submerged. The "U-570" had a fuel capacity of which gave a range of at . The test depth  of the submarine was . Her main armament consisted of five 53.3-cm (21-inch) torpedo tubes; four in the bow, and a fifth in the stern. She could carry a total of 14 torpedoes – five in the torpedo tubes, seven reloads inside the pressure hull, and a further two in water-tight canisters outside the pressure hull. The boat was fitted with an 8.8-cm C/35 deck gun (with around 150 rounds of ammunition), and a 2-cm Flak 30 anti-aircraft gun with a 28-cm stereoscopic rangefinder. She also carried several machine guns. Blohm & Voss laid down "U-570  at Hamburg on 21 May 1940. The submarine was launched on 20 March 1941. Kriegsmarine service. U-570" was commissioned into the "Kriegsmarine" on 15 May 1941. After a series of short testing and commissioning trips in the Baltic, she moved to Norway where she carried out short training voyages and fired practice torpedoes. By 25 July, she had moved to the German U-boat base at Lofjord, part of Trondheimsfjord, around north of Trondheim. In late August 1941, B-Dienst (the German naval codebreaking organisation) became aware of a large concentration of Allied merchant ships in the region of the  North Atlantic south of Iceland. Admiral Karl Dönitz ordered 16 U-boats to the area. "U-570" was to be one of these and, on the morning of 24 August, she put to sea on her first war patrol. Her planned mission was to patrol the area south of Iceland before proceeding to the U-boat base at La Pallice, France. She carried provisions for four weeks at sea. "Kapitänleutnant" Hans-Joachim Rahmlow commanded "U-570". He was an experienced naval officer, but had only recently transferred to U-boats, having previously been a gunnery and coastal defence specialist. He had commanded  the training submarine , but had carried out no war patrols. Likewise, the First Watch Officer (second-in-command) had only served a few months with the U-boat branch, after serving on destroyers, and the Second Watch Officer had little experience, having only recently been commissioned. The engineer was the only officer (and one of only four crewmen) who had served on a U-boat war patrol. While the boat's petty officers had several years of navy service, many of the enlisted crew were still new to the German navy and had only a few months of U-boat  training. "U-570"s inexperienced crew was not unique for the time. British interrogation of rescued crew-members of —sunk on her first patrol in September 1941—revealed that 41 out of 48 crew were on their first war patrol. Capture. On 27 August 1941, "U-570" spent much of the morning submerged. She had been four days at sea and this was to give respite to a crew that was suffering acutely from seasickness (several had been incapacitated). Earlier that morning, a Lockheed Hudson bomber of 269 Squadron, RAF, flown by Sergeant Mitchell and operating from Kaldaðarnes, Iceland, had attacked her. The  attack failed when the Hudson's bomb-racks failed to release its depth charges. "U-570" surfaced at position at around 10:50 am, immediately below a second 269 Squadron Hudson, flown by Squadron Leader James Thompson. Thompson was patrolling the area after Mitchell summoned him by radio. Rahmlow, who had climbed out onto the bridge, heard the approaching Hudson's engines and ordered a crash-dive. Thompson's aircraft reached "U-570" before she was fully submerged and dropped its four depth charges—one detonated just from the boat. "U-570" quickly resurfaced and around 10 of the crew emerged. The Hudson fired  on them with machine guns, but ceased when the U-boat crew displayed a white sheet. The captured crew members later recounted to British naval intelligence interrogators what had happened—the depth charge explosions had almost rolled the boat over, knocked out all electrical power, smashed instruments, caused water leaks, and contaminated the air on the boat. The inexperienced crew believed the contamination to be chlorine, caused by acid from leaking battery cells mixing with sea-water, and the engine-compartment crew panicked and fled forward to escape the gas. Restoring electrical power—for the underwater electric motors and for  lighting—would have been straightforward, yet there was nobody remaining in the engine compartment to do this. The submarine was dead in the water and in darkness. Rahmlow believed the chlorine would make it fatal to stay submerged so he resurfaced. The sea was too rough for the crew to man their anti-aircraft gun so they displayed a white flag to forestall another, probably fatal, depth charge attack from the Hudson—they were unaware the aircraft had dropped all its depth charges. Most of the crew remained on the deck of the submarine as Thompson circled above them, his  aircraft now joined by a second Hudson that had been en route from Scotland to Iceland and had broken off its journey to lend assistance. A radio request for help saw a Consolidated Catalina flying boat of 209 Squadron being scrambled at Reykjavík; it reached the scene three hours later. The German crew radioed their situation to the German naval high-command, destroyed their radio, smashed their Enigma machine, and dumped its parts overboard along with the boat's secret papers. Admiral Dönitz later noted in his war diary that he ordered U-boats in the area to go to  U-570s assistance after receiving this report; responded, but Allied air patrols prevented "U-82" from reaching "U-570". "U-570"s transmission was in plain language and the British intercepted it. Admiral Percy Noble, commander of Western Approaches Command, immediately ordered several ships to race to the scene. By early afternoon, fuel levels had forced the Hudsons to return to Iceland. The Catalina, a very long-range aircraft, was ordered to watch the submarine until Allied ships arrived. If none came before sunset, the aircraft was to warn "U-570"s crew to take to the water, then sink her. The first vessel to reach "U-570 , the anti-submarine trawler HMT Northern Chief", arrived around 10pm, guided to the scene by flares the Catalina dropped. The Catalina returned to Iceland after having circled "U-570" for 13 hours. The German crew remained on board "U-570" overnight; they made no attempt to scuttle their boat as "Northern Chief" had signalled she would open fire and not rescue survivors from the water if they did this ("Northern Chief"s captain, N.L. Knight, had been ordered to prevent the submarine from being scuttled by any means.) During the night, five more Allied vessels reached the scene: the armed trawler "Kingston  Agate", two anti-submarine whalers, the Royal Navy destroyer , and the Canadian destroyer . At daybreak, the Allies and Germans exchanged signal lamp messages, with the Germans repeatedly requesting to be taken off as they were unable to stay afloat, and the British refusing to evacuate them until the Germans secured the submarine and stopped it from sinking—the British were concerned that the Germans would deliberately leave behind them a sinking U-boat if they were evacuated. The situation became more confused when a small float-plane, (a Northrop N-3PB of 330 (Norwegian) Squadron), appeared. Unaware of the surrender, it  attacked "U-570" with small bombs and fired on "Northern Chief", which fired back. No damage was done and "Burwell" ordered the aircraft away by radio. The weather worsened; several attempts to attach a tow-line to "U-570" were unsuccessful. Believing the Germans were being obstructive, "Burwell's" captain, S.R.J. Woods ordered a machine gunner to fire warning shots, shots that accidentally hit and slightly wounded five of the German crew. With much difficulty, an officer and three sailors from "Kingston Agate" reached the submarine using a Carley float (a liferaft). After a quick search failed to find the U-boat 's Enigma machine, they attached a tow line and carried out the transfer of the five wounded men and the submarine's officers to "Kingston Agate". The remaining crew were taken on board HMCS "Niagara", which by this time had come alongside "U-570". The ships began slowly sailing to Iceland with "U-570" under tow, and with a relay of Hudsons and Catalinas constantly patrolling overhead. They arrived at dawn on 29 August at Þorlákshöfn. There, they beached "U-570" as she had been taking on water and was thought to be in danger of sinking. Salvage and repair. Two days after  U-570s arrival, a British submarine commander—Lieutenant George Robson Colvin—together with a team of engineering warrant officers and civilian technical experts, arrived at Þorlákshöfn from Britain. They then carried out the initial examination and salvage of "U-570". Colvin's team was able to restore lighting and buoyancy; a refloated "U-570" was towed around the coast to the British naval base at Hvalfjörður. There "U-570" was docked to the depot ship HMS "Hecla" for repair, with the aim that "U-570" could make the trip to Britain under her own power. The British discovered that the depth charge damage to the  U-boat was not critical—there were leaks in some of the ballast tanks and a small leak in a fuel tank. Around one third of the battery cells were cracked and the bow had been buckled. Water had leaked in through a valve that had been unseated by the explosions and through glass gauges that had broken; other damage was minor and no evidence of chlorine gas found. In his report, Colvin stated his opinion that there was no evidence of any damage control being carried out and that an experienced submarine crew would have been easily able to  improvise repairs, stay submerged and likely evade the air-attack. After their surrender, the German crew had attempted to destroy instruments and fittings, but, with the exception of the wrecked radio and the damaged torpedo firing computer, the attempt appeared half-hearted and the damage was not significant. Also, useful papers had missed destruction. Copies of encrypted signals and their corresponding, plain-language, German texts were found—material of use to the British Enigma code breaking effort. A significant discovery was the U-boat commander's handbook, which provided context and background information for decrypted messages. The British, unfamiliar with  German naval procedures, abbreviations and jargon, had often found German naval traffic hard to understand even when decrypted. "U-570" spent three weeks at Hvalfjörður, being repaired and taking short sea trials to test the engines and steering. Between 23 and 26 September, she was carefully inspected by two US Navy officers who had been sent from the United Kingdom to Iceland for that purpose; one of the submarine's G7a torpedoes was off-loaded, handed over to the Americans, and later sent to the United States. An eyewitness recalled that at one point, a Hudson bomber flew low over "U-570  and HMS Hecla", signalling with a Morse lamp, "This ****** is mine." On 29 September, the submarine set out for the UK, manned by a Royal Navy prize crew under the command of Lieutenant Colvin. Escorted by the S class destroyer HMS "Saladin" and HMS "Kingston Agate", she sailed on the surface as her diving planes had been damaged by the beaching at Þorlákshöfn. Her arrival at Barrow-in-Furness on 3 October was filmed by Pathé News newsreel cameras, and reported in the press. The capture would later be featured in British propaganda. The capture of several other U-boats , such as the , which had sunk whilst under tow, was kept secret to conceal the seizure of their code-books and Enigma machines. "U-570"s situation had been reported to the German high command. Also, so many ships, aircraft, and personnel had been involved in her capture that any attempt at secrecy would have been futile. "U-570" was placed in a dry-dock in the Vickers shipyard in Barrow. Her repair was complicated by depth-charge damage to her bow—her plating had been buckled, trapping four electrically powered torpedoes in their torpedo tubes. Two officers from the Royal Navy's  Department of Torpedoes and Mines Investigations had the task of retrieving them for examination. The dock was evacuated while a volunteer shipyard worker cut the armed torpedoes free with an oxyacetylene cutter under the officers' supervision. One of the officers—Lt Martin Johnson—then removed the magnetic pistols (detonators) from the torpedoes and made them safe—a dangerous task as the pistols were sensitive mechanisms and large enough to produce a lethal explosion on their own. For this act he was awarded the George Medal on 8 December 1942. Squadron Leader Thompson, his navigator/bomb-aimer—Flying Officer John Coleman , and Flying Officer Edward Jewiss (the pilot of the 209 Squadron Catalina), all received the Distinguished Flying Cross on 23 September 1941. The captain of "Kingston Agate", Henry L’Estrange, was awarded the Distinguished Service Cross for his part in the capture. German response. Initially, all the German naval high command knew of "U-570"s situation was her radio message, saying she was under air-attack and unable to submerge; they only learned of her capture from later British press reports. They were concerned for the security of their communications and "Vizeadmiral" Erhard Maertens, the Director of the Naval Intelligence Command, was  ordered to report on this. He concluded that in the worst-case scenario—that is, the British had secured "U-570"s codebooks and Rahmlow had revealed to them his memorised, secret keyword—communications would be compromised until a new list of Enigma machine settings came into force in November. However, he believed this worst case to be unlikely and that "U-570"s crew would have almost certainly destroyed their secret material. Even if they had not, the additional security of the commander's secret keyword would defeat British cryptanalysis. In fact, the British code-breakers at Bletchley Park found the extra security  of the keyword procedure to be simply of "nuisance value". "U-570"s crew had indeed destroyed their Enigma machine and code-books but the Germans were unaware of the Royal Navy's earlier capture of 's secret material, thanks to which the British had been breaking German naval cyphers since June 1941. British code-breaking was not seriously impeded until February 1942, when a new naval Enigma cypher remained unbroken for 10 months—the so-called "Shark Blackout". Apart from Rahmlow, "U-570"s officers were taken to an officers' prisoner-of-war camp at Grizedale Hall in Cumbria. This was nicknamed "U -boat Hotel" by the British as, during the early part of the war, the majority of prisoners were naval officers rescued from sunken U-boats. There, a "Court of Honour" convened by other German prisoners, including captured U-boat ace Otto Kretschmer, tried Rahmlow, "in absentia", and "U-570"s other officers. Rahmlow and his second-in-command, Bernhard Berndt, were found "guilty of cowardice"; the other two officers were "acquitted". On the night of 18/19 October, Berndt escaped from the camp. A detachment of the Home Guard apprehended him, shooting him when he tried to escape. According to some sources, he  had escaped from the camp with the stated intention of redeeming himself by making his way to "U-570"s dockyard at Barrow–a distance of only –and somehow destroying her. Another source states he was forced to make an escape attempt by a group of senior German prisoners who enforced a brutal regime of punishing those who held anti-Nazi views or who co-operated with the British, and that Berndt only broke away and ran from the Home Guard when he realised they were returning him to Grizedale Hall; they shot him dead after he ignored warning shots. The British  placed Rahmlow in a camp with German Army and Air Force prisoners to avoid further incidents of this kind. The German high command recognised "U-570"s loss could be partially blamed on the crew's lack of training and experience (during the early part of the war, U-boat training had been cut down to two months). Both this and mounting U-boat losses, including many boats sunk on their first patrol, prompted the Germans to put more resources into training. Also, the Germans broke up existing veteran crews and dispersed the men amongst the U-boat fleet, so the crews  of all newly commissioned boats would include a core of experienced, long-serving crewmen. Months later, the German command was still trying to discover the fate of "U-570"s codebooks. A system of coded messages, hidden in the text of apparently ordinary, personal letters, was used to order Otto Kretschmer to report on this. They were unaware that the Allies had discovered this channel of communication with German prisoners. Royal Navy service. The disposition of "U-570" was initially uncertain. Winston Churchill was in favour of handing her over to the Americans for repair, both for propaganda and as a means of  deepening then-neutral America's engagement in the Battle of the Atlantic. The Americans were eager to have her, but the Royal Navy objected both to this and to Churchill's other idea – to have her serve in the Mediterranean with a Yugoslav crew. Instead, she was commissioned into the Royal Navy as HMS "Graph" on 5 October 1941, and assigned the Royal Navy pennant number P715. She was given a name beginning with a 'G' to signify "German", i.e., denoting that "Graph" was a captured vessel. The name Graph was also chosen owing to the extensive testing carried out  on her (and therefore the many "Graphs" drawn up), but was also a play on the German word "Graf" meaning "Count". Trials. Once seaworthy, meticulous trials were conducted to measure every aspect of "Graph"s sailing and diving characteristics. Even the Zeiss binoculars found on board were carefully tested. "Graph"s safe diving depth was discovered to be —much deeper than the British thought for this kind of boat. At the time, Royal Navy depth charges had a maximum depth setting of so the Germans could dive out of their reach. Depth charges were soon modified to take account of this. The  boat's acoustic and magnetic characteristics were examined by different Admiralty research establishments. The Allied technical experts found much to praise about her design and construction. "Graph"s auxiliary machinery was on rubber mountings, making her stealthier by reducing sound transmission into the hull. The British and the Americans who examined "Graph" singled out her Zeiss periscope for particular praise. The American officers who carried out her initial inspection in Iceland recommended it be copied as quickly as possible for possible US Navy use. Her underwater acoustic equipment was found to be a sophisticated array of hydrophones that was significantly better  than the British equivalents. The main criticism of the boat was poor and cramped crew accommodation, which would degrade crew performance on long patrols. In mid-1942, "Graph" was carefully studied by the US Navy, which then had an interest in a new, smaller submarine that would be roughly her size – around two-thirds the length and half the displacement of the "Gato"-class boats that formed the bulk of the US submarine fleet. She was considered superior in many ways to the two experimental "Mackerel"-class, the existing class of small US submarine, but the project was dropped. Full-scale models of  her pressure hull were constructed, and used during mid-1942 for underwater tests of experimental shaped charge anti-submarine bombs. In a highly secret British project, "Graph" was also used as a model for the construction of three, full-sized, mock-ups of the control compartment, wardroom and radio room of a Type VII U-boat. These were used to train specialist groups of sailors, who would form boarding parties whenever a damaged U-boat was blown to the surface. They were trained to operate a U-boat's ballast-tank valves, to reverse any scuttling attempts by the crew, and  to search quickly for cryptographic equipment and documents. Active service. After completing trials under the command of Lieutenant Commander E.D. Norman, "Graph" was placed under the command of Lieutenant Peter Barnsley Marriott, who had assisted in the trials. She departed from Holy Loch for her first Royal Navy war patrol on 8 October 1942, with the intention of patrolling the Bay of Biscay. On the afternoon of 21 October 1942, about north-north-east of Cape Ortegal (), "Graph" dived to evade a German Focke-Wulf Fw 200 long-range patrol aircraft. A loud hydrophone contact made Marriott believe a nearby  submarine had likewise dived and, 12 minutes later, he observed its conning-tower against the setting sun. After pursuing the German boat, "Graph" fired four torpedoes. Explosions were heard, and also banging noises, leading the British to believe they had hit the other submarine and the banging noises were caused by her breaking up as she sank. In early 1943, Marriott was awarded the Distinguished Service Order for "great courage, skill and determination in a successful submarine patrol". After the war, examination of German records showed the submarine attacked was the , badly damaged after being rammed by the Flower-class  corvette HMS "Crocus" off the coast of West Africa. German lookouts had seen the torpedoes' tracks, enabling "U-333" to evade the torpedoes; the torpedoes then self-detonated for unknown reasons. The commander of "U-333" was Peter-Erich Cremer. In his post-war account of the attack, he suggested the rattling and banging noises "Graph"s crew had heard were due to the severe damage previously inflicted on "U-333". His route back to France closely hugged the Spanish coastline, a pattern followed by other U-boats, and he had also believed that Marriott was aware of this and had been lying in  wait. "Graph's" second war patrol was from 19 November 1942 to 8 December 1942, also in the Bay of Biscay. During the patrol, she was ordered to intercept the Italian cargo ship "Cortellazzo", which was passing through the area "en route" from Bordeaux to Japan with a cargo of 2000 tons of machinery. However, "Graph" failed to find the ship and the patrol was without incident. "Cortellazzo" was sunk within days by HMS "Redoubt", one of the destroyer escorts of an Allied troop convoy, after "Redoubt" had ordered "Cortellazzo" crew to abandon ship and had picked them up. "Graph  departed from Lerwick on her third war patrol on 24 December 1942 with three other Royal Navy submarines; their task was to patrol the waters off the coast of Norway in the area of Altafjord. At 1am on 1 January 1943, at the position , she sighted the German heavy cruiser Admiral Hipper", returning from the unsuccessful attack on convoy JW 51B, better known as the Battle of the Barents Sea. "Hipper" was too far off and travelling too fast to be attacked. At 4:23am, "Graph" sighted two German destroyers moving erratically and at low speed. "Graph" closed to a  range of and fired four torpedoes. Again, explosions convinced the British that hits had been scored and a destroyer probably sunk, but again all the shots had missed. "Graph" returned to Lerwick on 13 January 1943. "Graph" undertook no further war patrols. In 1943, she was assigned for training duties and command of her passed to Lt. D. Swanston. Peter Marriott went on to command , where he was awarded a DSC. Decommissioning. Defects, exacerbated by a shortage of spare parts, led to "Graph" being placed in reserve. Also, her batteries and her MAN diesel engines had a comparatively short service  life; German submarine batteries required annual replacement, unlike British batteries, which were intended to last for the lifetime of the boat. She was decommissioned from active service on 21 June 1943. She saw some use as a target, to determine the damage caused by depth charges in full-scale trials. After surviving these experiments, she was to be towed by the Royal Navy rescue tug HMRT "Growler" from Chatham to the Clyde for scrapping. But on 20 March 1944 her tow–rope broke in gale-force winds and, driven by wind and waves, she ran aground at a position , near  Coul Point, on the west coast of Islay, Scotland. "Growler" was unable to re-float the submarine and she was abandoned. "Graph" was partially salvaged and scrapped in 1961. In 1966, the salvage diver Keith Jessop and others removed some more of the wreck, without the permission of the owner of the wreck's salvage rights; they paid £400 in compensation to the owner, but were still prosecuted and fined £50. Some remains of HMS "Graph" still remained visible at low tide on the rocks near Saligo beach in 1970, with the pressure casing of the conning tower  and periscope tube visible (the cladding, railings and other parts had all broken off in the Atlantic storms many years before). Today, the remains of the wreck lie in about of water; the site has been visited and photographed by recreational divers. One of the "Kriegsmarine" battle flags of "U-570" was presented to Squadron Leader Thompson and is now part of the collection of the RAF Museum. Other surviving relics from the boat include her typewriter, held by the museum at Bletchley Park, a small celestial globe used for navigation, that is owned by a private collector, and a German  sailor's cap, that was taken as a souvenir by one of the officers of HMCS "Niagara" and is in the Canadian War Museum. Another battle flag is claimed to have come into the possession of a young apprentice fitter at the Vickers Barrow shipyard and still survives. The Control Room clock is in the possession of the family of a former submarine officer from the Royal Navy. The boat's sextant manufactured by C.Plath of Hamburg was presented to Midshipman A.C.Lowe RNVR in April 1943 by Capt M. Richmond DSO OBE RN to recognise that, at that point, A.C.  Lowe was the youngest officer to hold a Watchkeeping Certificate. The sextant remains in his family's possession.