|Ordered:||7 December 1940|
|Builder:||DeSchiMAG Seebeckwerft, Bremerhaven|
|Laid down:||30 June 1942|
|Launched:||1 April 1943|
|Commissioned:||7 September 1943|
|Fate:||Sunk by a mine, 27 April 1944|
|Class and type:||Type IXC/40 submarine|
|Height:||9.60 m (31 ft 6 in)|
|Draught:||4.67 m (15 ft 4 in)|
|Test depth:||230 m (750 ft)|
|Complement:||4 officers, 44 enlisted|
|Part of:||4th U-boat Flotilla|
German Type IXC/40 submarines were slightly larger than the original Type IXCs. U-803 had a displacement of 1,144 tonnes (1,126 long tons) when at the surface and 1,257 tonnes (1,237 long tons) while submerged. The U-boat had a total length of 76.76 m (251 ft 10 in), a pressure hull length of 58.75 m (192 ft 9 in), a beam of 6.86 m (22 ft 6 in), a height of 9.60 m (31 ft 6 in), and a draught of 4.67 m (15 ft 4 in). The submarine was powered by two MAN M 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of 4,400 metric horsepower (3,240 kW; 4,340 shp) for use while surfaced, two Siemens-Schuckert 2 GU 345/34 double-acting electric motors producing a total of 1,000 shaft horsepower (1,010 PS; 750 kW) for use while submerged. She had two shafts and two 1.92 m (6 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 18.3 knots (33.9 km/h; 21.1 mph) and a maximum submerged speed of 7.3 knots (13.5 km/h; 8.4 mph). When submerged, the boat could operate for 63 nautical miles (117 km; 72 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 13,850 nautical miles (25,650 km; 15,940 mi) at 10 knots (19 km/h; 12 mph). U-803 was fitted with six 53.3 cm (21 in) torpedo tubes (four fitted at the bow and two at the stern), 22 torpedoes, one 10.5 cm (4.13 in) SK C/32 naval gun, 180 rounds, and a 3.7 cm (1.5 in) Flak M42 as well as two twin 2 cm (0.79 in) C/30 anti-aircraft guns. The boat had a complement of forty-eight.
She was completed in September 1943 and spent the next seven months on working up cruises in the Baltic Sea near Swinemünde in order to get the crew and boat ready for operational cruising in the Battle of the Atlantic, which was at a critical stage.
As the boat worked up on 27 April 1944, just days off its intended first cruise, she struck a sea mine in the bay and sank, nine of her crew going down with the boat, and 35 being rescued by nearby ships. The boat was salvaged in August 1944, but was too badly damaged to repair, and was broken up for components for use in other U-boats. Her surviving crew were largely transferred to other units, principally U-3009, on which they surrendered in May 1945. The mine had been air-dropped over the bay by the Royal Air Force, who had realised the use that the area was being put to through photo-reconnaissance flights. The air-dropping of mines was a frequent tactic of the RAF, and achieved dividends off many German harbours.
- Gröner 1991, p. 68.
- Busch, Rainer; Röll, Hans-Joachim (1999). German U-boat commanders of World War II : a biographical dictionary. Translated by Brooks, Geoffrey. London, Annapolis, Md: Greenhill Books, Naval Institute Press. ISBN 1-55750-186-6.
- Busch, Rainer; Röll, Hans-Joachim (1999). Deutsche U-Boot-Verluste von September 1939 bis Mai 1945 [German U-boat losses from September 1939 to May 1945]. Der U-Boot-Krieg (in German). IV. Hamburg, Berlin, Bonn: Mittler. ISBN 3-8132-0514-2.
- Gröner, Erich; Jung, Dieter; Maass, Martin (1991). U-boats and Mine Warfare Vessels. German Warships 1815–1945. 2. Translated by Thomas, Keith; Magowan, Rachel. London: Conway Maritime Press. ISBN 0-85177-593-4.
- Sharpe, Peter (1998). U-Boat Fact File. Great Britain: Midland Publishing. ISBN 1-85780-072-9.
- Helgason, Guðmundur. "The Type IXC/40 boat U-803". German U-boats of WWII - uboat.net. Retrieved 7 December 2014.
- Hofmann, Markus. "U 803". Deutsche U-Boote 1935-1945 - u-boot-archiv.de (in German). Retrieved 2 February 2015.