HMS Graph (pennant number P715) was a German Type VIIC U-boat captured and recommissioned by the British Royal Navy during World War II.

Refitted for use by the Allies, she 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.

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.

Earlier that morning, a Lockheed Hudson bomber of 269 Squadron, RAF, flown by Sergeant Mitchell and operating from Kaldaðarnes, Iceland, had attacked her.

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.

[19] 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;[20] U-82 responded, but Allied air patrols prevented U-82 from reaching U-570.

[22] 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.

[18] 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.

After a quick search failed to find the U-boat's Enigma machine, they attached a tow line and transferred the five wounded men and the submarine's officers to Kingston Agate.

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.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.

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.

[31] 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.

The dock was evacuated while a volunteer shipyard worker cut the armed torpedoes free with an oxyacetylene cutter under the officers' supervision.

[41][42] 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.

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.

[46] 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.

[28] 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 22 miles (35 km)–and somehow destroying her.

A system of coded messages, hidden in the text of apparently ordinary, personal letters, was used to order Otto Kretschmer to report on this.

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.

[56] 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.

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.

[58] On the afternoon of 21 October 1942, about 50 nautical miles (90 km; 60 mi) north-north-east of Cape Ortegal (44°31′N 7°25′W / 44.517°N 7.417°W / 44.517; -7.417), 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.

[59] In early 1943, Marriott was awarded the Distinguished Service Order for "great courage, skill and determination in a successful submarine patrol".

[60] After the war, examination of German records showed the submarine attacked was the U-333, badly damaged after being rammed by the Flower-class corvette HMS Crocus off the coast of West Africa.

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.

At 1 am on 1 January 1943, at the position 70°51′N 21°56′E / 70.850°N 21.933°E / 70.850; 21.933, 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.