|Ordered:||25 August 1941|
|Laid down:||18 November 1942|
|Launched:||8 January 1944|
|Commissioned:||18 March 1944|
|Fate:||Sunk, 15 December 1944|
|Class and type:||Type VIIC submarine|
|Height:||9.60 m (31 ft 6 in)|
|Draught:||4.74 m (15 ft 7 in)|
|Complement:||4 officers, 40–56 enlisted|
The submarine was laid down on 18 November 1942 at the Howaldtswerke yard in Kiel as yard number 32, launched on 8 January 1944 and commissioned on 18 March under the command of Kapitänleutnant Horst Creutz.
German Type VIIC submarines were preceded by the shorter Type VIIB submarines. U-400 had a displacement of 769 tonnes (757 long tons) when at the surface and 871 tonnes (857 long tons) while submerged. She had a total length of 67.10 m (220 ft 2 in), a pressure hull length of 50.50 m (165 ft 8 in), a beam of 6.20 m (20 ft 4 in), a height of 9.60 m (31 ft 6 in), and a draught of 4.74 m (15 ft 7 in). The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of 2,800 to 3,200 metric horsepower (2,060 to 2,350 kW; 2,760 to 3,160 shp) for use while surfaced, two Garbe, Lahmeyer & Co. RP 137/c double-acting electric motors producing a total of 750 metric horsepower (550 kW; 740 shp) for use while submerged. She had two shafts and two 1.23 m (4 ft) propellers. The boat was capable of operating at depths of up to 230 metres (750 ft).
The submarine had a maximum surface speed of 17.7 knots (32.8 km/h; 20.4 mph) and a maximum submerged speed of 7.6 knots (14.1 km/h; 8.7 mph). When submerged, the boat could operate for 80 nautical miles (150 km; 92 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 8,500 nautical miles (15,700 km; 9,800 mi) at 10 knots (19 km/h; 12 mph). U-400 was fitted with five 53.3 cm (21 in) torpedo tubes (four fitted at the bow and one at the stern), fourteen torpedoes, one 8.8 cm (3.46 in) SK C/35 naval gun, (220 rounds), one 3.7 cm (1.5 in) Flak M42 and two twin 2 cm (0.79 in) C/30 anti-aircraft guns. The boat had a complement of between forty-four and sixty.
The U-boat sailed from Horten Naval Base in Norway for her first war patrol on 15 November 1944, and headed for the waters off Land's End. Despite repeated requests for reports by the German U-boat Command, none were received. The U-boat was eventually listed as "missing" at the end of January 1945. After the war, the Allies attributed the loss of U-400 to a depth charge attack by the frigate HMS Nyasaland on 17 December 1944, about 30 nautical miles (56 km) SE of Kinsale, Ireland.
The wreck of U-400 was finally identified by nautical archaeologist Innes McCartney and historian Axel Niestle in 2006, about 10 miles (16 km) north-west of Padstow, Cornwall, at position Coordinates:  close to the wrecks of two other U-boats, U-325 and U-1021. All three submarines were sunk in the Bristol Channel by a deep-trap minefield.
Previously recorded fate
U-400 was noted as sunk in mid-December 1944 in the British minefield 'HX A1' off the Cornish coast.
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- Gröner 1991, pp. 43-46.
- Helgason, Guðmundur. "The loss of U-325, U-400 and U-1021". German U-boats of WWII - uboat.net. Retrieved 13 January 2010.
- "War Mystery Solved". www.cix.co.uk. Retrieved 13 January 2010.
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- Helgason, Guðmundur. "The Type VIIC boat U-400". German U-boats of WWII - uboat.net. Retrieved 26 December 2014.
- Hofmann, Markus. "U 400". Deutsche U-Boote 1935-1945 - u-boot-archiv.de (in German). Retrieved 26 December 2014.