Kapitänleutnant Hans-Joachim Rahmlow had joined the Kriegsmarine in 1928, and had served nearly all of his career in coastal artillery. However around November 1940 he first set foot on a submarine. His first boat had no patrols under his leadership and he moved to U-570 in May 1941. U-570 was a brand new build Type VIIC U-boat, on her commission date KptLt Rahmlow took command. In August U-570 was ordered to take part in an operation to sink a concentration of allied shipping in the Atlantic.
The weather, as it often is in the Atlantic, was horrific. Four days into her patrol (August 27th) the seas had battered the submarine and most of her crew were seasick. In addition she'd been attacked by a RAF Hudson bomber from 269 Squadron in Iceland, however during the attack run the bombers depth bombs had failed to release, allowing U-570 to escape. So KptLt Rahmlow ordered the morning to be spent submerged. At about 1050 she re-surfaced to face the battering of the sea.
On the U-570 KptLt Rahmlow suddenly heard over the stormy weather the sound of aircraft engines. He immediately ordered a crash dive. Bursting out of the grey mist the Hudson hurtled towards the rapidly submerging submarine. Just as U-570 became fully waterlogged, Sqn Ldr Thompson released his four 250lb depth bombs in a line. They were all fused to detonate at fifty feet. One of the charges detonated just ten metres from the bow, the blast smashed into the bow of the submarine and crumpled it and shorted out the electrics. In the dark and confusion inside the metal coffin it's possible the blast may have knocked KptLt Rahmlow unconscious. The engine room crew suddenly scrambled forward yelling warnings about chlorine gas escaping. The next sound the crew heard was the clang of the watertight doors to the engine room being slammed shut. With no way to get the power back, in darkness and with the risk of choking on chlorine the crew blew ballast and surfaced. They tumbled out of the hatches to man the anti-aircraft guns and fight off the Hudson.
Sdn Ldr Thompson saw the U-570 sitting in a pool of bubbles and froth from his depth bomb and the subsequent surfacing. He turned towards her and the nose gunner fired a burst from his machine gun at the submarine. On board U-570 the crew quickly found that with the pitching of the sea and the submarine it was impossible to lay their AA weapons on the circling bomber. Expecting another stick of bombs at any second they quickly hoisted a flag of surrender.
The next morning a seaplane from 330 Squadron reached the area, spotting the submarine and trawler the Norwegian pilots launched an attack on the U-570 and strafed the HMT Northern Chief, which promptly returned fire. Luckily one of the destroyers saw what was happening and ordered the plane away.
After these tests she was renamed HMS Graph (G for "German" and from the German word Graf) and entered service with the Royal Navy conducting two war patrols. Although she made several attacks, including one on an enemy submarine she failed to score any hits. Eventually lack of replacement supplies meant she was decommissioned and used for target trials of depth charges. After resisting the battering she was sold for scrap, but broke free of the tow rope used to send her to the breaker's yard. Her hulk came to rest on Coul Point, on the west coast of Islay. She remains there to this day.
Of the people involved, the crew of the 269 Squadron Hudson, Sqn Ldr Thompson and Flying Officer Coleman (the Hudson's Bomber/Navigator), each received the Distinguished Flying Cross. KptLt Rahmlow remained a POW and died on 13th of June 1967.