Listed below are the known cases where Allied forces boarded German Kriegsmarine vessels before the end of the war.

1939

3 September
The freighter Hannah Böge is captured south of Iceland by the British destroyer HMS Somali (Captain Randolph Stewart Gresham Nicholson), the first ship to be boarded.

1940

12 February
U-33 (Kapitänleutnant Hans-Wilhelm von Dresky) was sunk in the Firth of Clyde off Scotland by the British minesweeper HMS Gleaner (Lieutenant Hugh Percival Price). Several rotors from an Enigma machine were captured from the submarine.

16 February
The supply ship Altmark (Heinrich Dau) was boarded in Jossingfiord, a Norwegian fjord, by men from the British destroyer HMS Cossack (Captain Philip Louis Vian). 299 prisoners were discovered in the hold of the ship and set free.

26 April
The patrol boat (Vorpostenboot) V2623 was captured by the Royal Navy.

1941

3-4 March
The patrol boat (Vorpostenboot) Krebs was captured and put out of action during a British raid on Loften Islands in Norway. Documents recovered enabled the British to read part of the German radio codes in March and April.

3 May
The weather ship München was captured by the British destroyer HMS Somali (Captain Clifford Caslon). Documents recovered enabled the British to read part of the German radio codes in May and June.

9 May
U-110 (Kapitänleutnant Fritz-Julius Lemp) was captured by the British destroyers HMS Bulldog (Commander Addison Joe Baker-Cresswell), HMS Broadway (Lieutenant Commander Thomas Taylor) and the corvette HMS Aubretia (Lieutenant Commander Vivian Funge Smith).
HMS Bulldog was on a ramming course with the submarine but in the last moment changed course so she was damaged but did not sink. After the crew abandoned ship believing that she was sinking the British managed to retrieve the Enigma code machine ciphers and code books, a capture that can be regarded as one of the main events of the naval war. To cover up the capture the submarine was allowed to sink.

4 June
The supply tanker Gedania (H. Paradies) was captured by the British cruiers HMS Marsdale.

15 June
The supply tanker Lothringen was captured by the British cruiers HMS Dunedin.

28 June
The weather ship Lauenburg was located by the British destroyer HMS Bedouin (Commander James Abernethy McCoy) and boarded by men from the destroyer HMS Tartar (Commander Lionel Peyton Skipwith). Documents recovered enabled the British to read part of the German radio codes in June and July.

27 August
U-570 (Kapitänleutnant Hans-Joachim Rahmlow) surrendered to a Lockheed Hudson from RAF Squadron 269 flown by J.H. Thompson and was towed to Iceland. She later became the British submarine HMS Graph.

10 September
U-501 (Korvettenkapitän Hugo Förster) was boarded by men from the Canadian corvette HMCS Moosejaw (Lieutenant Frederick Ernest Grubb) after she had been rammed and had started to sink.

1942

30 October
U-559 (Kapitänleutnant Hans Heidtmann) was hit by depth charges and as she was sinking she was boarded by three men who managed to salvage material that enabled the British to crack the four wheel Enigma codes.

1943

17 February
U-205 (Kapitänleutnant Friedrich Bürgel) was sunk by depth charges from the British destroyer HMS Paladin (Lieutenant Commander Lawrence St George Rich), some sources state that important material was recovered from the submarine before she sank.

17 April
U-175 (Kapitänleutnant Heinrich Bruns) was attacked by the US Coast Guard cutter USS Spencer and she was boarded before she sank.

1944

6 March
U-744 (Oberleutnant zur See Heinz Blischke) was attacked by the escort forces of Convoy HX 280 and before she sank boarding parties from the Canadian covertte HMCS Chilliwack and the Canadian frigate HMCS St. Catharines recovered some material, most of which was lost again before it reached the destroyer when the small boats capsized.

4 June
U-505 (Oberleutnant zur See Harald Lange) was captured was of Africa by the US Navy task force 22.3 and valuable material recovered.

1945

12 April
U-1024 (Kapitänleutnant Hans-Joachim Gutteck) was captured by the British frigates HMS Loch Glendhu (Lieutenant Commander Edric Guy Philip Bromfield Knapton) and HMS Loch More (Lieutenant Commander Robert Arthur Dillon Cambridge) near the Isle of Man but she sank while in tow.

Sources used

W. A. B. Douglas & Brereton Greenhous - Out of the Shadows: Canada in the Second World War
George Franklin - Britain's Anti-submarine Capability 1919-1939
Mac Johnston - Corvettes Canada: Convoy Veterans of WWII Tell Their True Stories
Jak P Mallmann Showell - Enigma U-Boats: Breaking the code
David Syrett - The Defeat of the German U-Boats: The Battle of the Atlantic
John Ward - Submarines of World War II
Bruce Allen Watson - Atlantic Convoys and Nazi Raiders: The Deadly Voyage of HMS Jervis Bay
Gordon Williamson - Wolf Pack: The Story of the U-Boat in World War II

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