HMS Graph

HMS Graph (pennant number P715) was a German Type VIIC U-boat used by the British Royal Navy during World War II. Commissioned as the U-570 in the German 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 She was withdrawn from service in 1944 due to problems maintaining her.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 an island 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 769 t when surfaced, and 871 t when submerged. The boat was 67.1 m long, with a beam of 6.2 m, and a draught of 4.74 m. The diesel-electric propulsion system provided a maximum speed of 18.8 kn surfaced or 7.6 kn submerged. The U-570 had a fuel capacity of 109 LT which gave a range of 7500 nmi at 10 kn. The test depth of the submarine was 230 m.

The main armament consisted of five 21 in torpedo tubes; four in the bow, and the fifth in the stern. A total of 14 torpedoes could be carried – 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 a 8.8cm C/35 deck gun (with around 150 rounds of ammunition) and a 2cm Flak 30 anti-aircraft gun. A 28 cm Stereoscopic rangefinder was carried for use with the anti-aircraft gun, and a number of machine guns were also carried.

U-570 was laid down by Blohm & Voss at Hamburg on 21 May 1940. The submarine was launched on 20 March 1941.

Kriegsmarine service
The 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 13 km 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 sixteen U-boats to the area. The 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.

The submarine was commanded by Kapitänleutnant Hans-Joachim Rahmlow. He was an experienced naval officer, but had only recently transferred to U-boats, having previously been a gunnery and coastal defences specialist. He had commanded the training submarine GS U-58 (1939), 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 on-board) 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.

The U-570's inexperienced crew was not unique for the time. British interrogation of rescued crew-members of the GS U-501—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, 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, she had been attacked by a Lockheed Hudson bomber of 269 Squadron, flown by Sergeant Mitchell and operating from Kaldaðarnes, Iceland. However, the Hudson's bomb-racks failed to release its depth charges during the attack.

The boat surfaced at position 62.25°N, -18.58333°W at around 10:50 am, immediately below a second 269 Squadron Hudson. Flown by Squadron Leader James Thompson, it was patrolling the area after being summoned by radio by Mitchell. Rahmlow, who had climbed out onto the bridge, heard the approaching Hudson's engines and ordered a crash-dive. Thompson's aircraft reached the submarine before she was fully submerged and dropped its four 250 lb depth charges—one detonated just 10 yd from the boat.

The U-boat quickly resurfaced and around ten of the crew emerged. The Hudson fired on them with machine guns, but ceased when the U-boat crew displayed a white sheet. An account of what happened was subsequently given to British naval intelligence interrogators by the captured crew members—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 Reykjavik; it reached the scene some 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-570's assistance after receiving this report and the GS U-82 (1941) responded, but was prevented from reaching the U-570 by Allied air patrols.

The U-570's transmission was in plain language and it was intercepted by the British. Admiral Percy Noble, commander of Western Approaches Command, immediately ordered a small armada of 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 the U-570's crew to take to the water, then sink her. The first vessel to reach the U-boat—the anti-submarine trawler HMT Northern Chief in fact arrived at around 10pm, and was guided to the scene by flares dropped by the Catalina. The aircraft returned to Iceland after circling the U-570 for thirteen 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 Kingson Agate, two anti-submarine whalers, the Royal Navy destroyer HMS Burwell (H94) and the Canadian destroyer HMCS Niagara (I57).

At daybreak, there was a series of signal lamp messages between the Allies and Germans, with the Germans repeatedly requesting to be taken off as they were unable to stay afloat, and the British refusing to evacuate them until they 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 the U-570 with small bombs and fired on the Northern Chief (which returned fire). No damage was done and Burwell ordered the aircraft away by radio.

The weather worsened; several attempts to attach a tow-line to the U-boat were unsuccessful. Believing the Germans were being obstructive, Burwell's captain, S.R.J. Woods ordered warning shots to be fired with a machine gun, but five of the German crew were accidentally hit and slightly wounded. With much difficulty, an officer and three sailors from the trawler HMS 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 the Kingson Agate. The remaining crew were taken on board HMCS Niagara, which by this time had come alongside the U-boat.

The ships began slowly sailing to Iceland with the 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, the submarine was beached as she had been taking on water and was thought to be in danger of sinking.

Salvage and repair
Two days after the submarine's arrival, a British submarine commander—Lieutenant George Colvin—together with a team of engineering warrant officers and civilian technical experts arrived at Þorlákshöfn from Britain to carry out the initial examination and salvage of U-570.

"The submarine was then lying broadside on to the surf and listing heavily to starboard... The interior of the submarine was unlit and was in a chaotic state; leaks of oil and water from the broken gauge glasses of internal tanks had combined with vast quantities of provisions, flour, dried peas and beans, soft fruit, clothes, bedding, and the remains of scores of loaves of black bread to form a revolting morass that in places was knee-deep. It was subsequently discovered that in this ship the crew's W.C. had been converted into a food locker and overturned buckets of excrement added to the general noisome conditions."

- Lieutenant GR Colvin, RN

Colvin's team was able to restore lighting and buoyancy; U-570 was refloated and towed around the coast to the British naval base at Hvalfjörður, where she was docked to the depot ship HMS Hecla for repair so she 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, sometimes 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 eye-witness 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 the trawler 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 GS U-110 (1940) (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.

She 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 G7e 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 quite large enough to produce a lethal explosion. 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 the pilot of the 209 Squadron Catalina- Flying Officer Edward Jewiss were all awarded the Distinguished Flying Cross on 23 September 1941. The captain of the 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 head of navy communications, 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 the 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". The 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 the GS U-110 (1940)'s secret material, thanks to which, the British had been breaking German naval cyphers since June 1941. British code-breaking would not be seriously impeded until February 1942, when a new naval Enigma cypher would remain unbroken for ten months—the so-called "Shark Blackout".

Apart from Rahmlow, the U-570's officers were taken to an officers' prisoner–of–war camp at Grizedale Hall in Cumbria. This was nicknamed the 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, Rahmlow (in absentia) and the U-570's other officers were tried by a "Court of Honour" convened by other German prisoners, including captured U-boat ace Otto Kretschmer. 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. However, he was soon apprehended by a detachment of Home Guard and was shot 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 the U-570's dockyard at Barrow–a distance of only 22 mi–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 realized 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 recognized the U-570's loss could be partially blamed on the crews 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, existing veteran crews were broken up and dispersed 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 the 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. However, they were unaware that this channel of communication with German prisoners had been discovered by the Allies.

Royal Navy service
The disposition of the boat 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 19 September 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 230 m—much deeper than the British thought for this kind of boat. At the time, Royal Navy depth charges had a maximum depth setting of 170 m 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 the boat more stealthy by reducing sound transmission into the hull. In particular, her Zeiss periscope was singled out for praise by both the British and the Americans who examined the boat. 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 submarines, the existing class of small US Submarine, but the project was dropped.

Full scale models of her pressure hull were constructed, and used during the summer of 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 were taught where to quickly search 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 50 nmi north-north-east of Cape Ortegal (44.51667°N, -7.41667°W), 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, twelve minutes later, he observed its conning-tower against the setting sun. After a 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".

It was only after the war that examination of German records showed the submarine attacked was the GS U-333, badly damaged after being rammed by the Flower-class corvette HMS Crocus off the coast of West Africa. The torpedoes' tracks were seen by German lookouts, the U-boat successfully evaded them and the torpedoes then self-detonated for unknown reasons. The commander of the U-333 was Peter-Erich Cremer. In his post-war account of the attack, he suggested the rattling and banging noises heard by Graph's crew were due to the severe damage previously inflicted on the U-333. His route back to France closely hugged the Spanish coastline, a pattern followed by other U-boats, and he 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, and was without incident. She 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 70.85°N, 21.93333°W, she sighted the German heavy cruiser Admiral Hipper, returning from the unsuccessful attack on Convoy JB51B, better known as the Battle of the Barents Sea. However, the Hipper was too far off and travelling too fast to be attacked. At 4:23am, she sighted two German destroyers; the German vessels were moving erratically and at low speed. Graph closed to a range of 7000 yd 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. She 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 HMS Stoic, where he was awarded a DSC.

Decommissioning
Defects, exacerbated by a shortage of spare parts, led to her 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 in February 1944. 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 19 March 1944 her tow–rope broke in gale-force winds and, driven by wind and waves, she ran aground at a position 55.80167°N, -6.475°W, 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 1947. In 1966, the salvage diver Keith Jessop and others removed some more of the wreck. However, they did so without the permission of the individual who owned 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 clearly 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 5 m of water; the site has been visited and photographed by recreational divers.

One of the Kriegsmarine battle flags of the 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.