German submarine U-644 was a Type VIIC U-boat of Nazi Germany's Kriegsmarine during World War II. The submarine was laid down on 1 December 1941 at the Blohm & Voss yard at Hamburg, launched on 20 August 1942, and commissioned on 15 October 1942 under the command of Oberleutnant zur See Kurt Jensen.
Attached to 5th U-boat Flotilla based at Kiel, U-644 completed her training period on 31 March 1943 and was assigned to front-line service.
German Type VIIC submarines were preceded by the shorter Type VIIB submarines. U-644 had a displacement of 769t when at the surface and 871t while submerged. She had a total length of 67.1m (220.1feet), a pressure hull length of 50.5m (165.7feet), a beam of 6.2m (20.3feet), a height of 9.6m (31.5feet), and a draught of 4.74m (15.55feet). The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of NaNPS for use while surfaced, two Brown, Boveri & Cie GG UB 720/8 double-acting electric motors producing a total of 750PS for use while submerged. She had two shafts and two 1.23abbr=onNaNabbr=on propellers. The boat was capable of operating at depths of up to .
The submarine had a maximum surface speed of and a maximum submerged speed of . When submerged, the boat could operate for at ; when surfaced, she could travel at . U-644 was fitted with five 53.30NaN0 torpedo tubes (four fitted at the bow and one at the stern), fourteen torpedoes, one 8.82NaN2 SK C/35 naval gun, 220 rounds, and one twin 2sp=usNaNsp=us C/30 anti-aircraft gun. The boat had a complement of between forty-four and sixty.
In April 1943, U-644 was on patrol between Shetland and Jan Mayen. British signals intelligence had pinpointed the U-boat's position and HMS Tuna was dispatched to hunt down U-644. On 7 April, the British submarine detected the U-boat only 500m (1,600feet) away and fired a spread of five torpedoes. Two torpedoes hit U-644, which sank with all 45 crew members on board, there were no survivors.