History
Nazi Germany
Name	U-1231
Ordered	14 October 1941
Builder	Deutsche Werft AG, Hamburg
Yard number	394
Laid down	31 March 1943
Launched	18 November 1943
Commissioned	9 February 1944
Fate	Surrendered on 13 May 1945 Transferred as war booty to the Soviet Union
Soviet Union
Name	N-26
Acquired	November 1945
Commissioned	13 February 1946
Decommissioned	29 December 1955
Renamed	B-26 on 9 June 1949 KPB-33 on 15 September 1952 UTS-23 on 27 December 1956
Stricken	13 January 1968
Fate	Broken up for scrap in 1968
General characteristics
Class and type	Type IXC/40 submarine
Displacement	1,144 t (1,126 long tons) surfaced 1,257 t (1,237 long tons) submerged
Length	76.76 m (251 ft 10 in) o/a 58.75 m (192 ft 9 in) pressure hull
Beam	6.86 m (22 ft 6 in) o/a 4.44 m (14 ft 7 in) pressure hull
Height	9.60 m (31 ft 6 in)
Draught	4.67 m (15 ft 4 in)
Installed power	4,400 PS (3,200 kW; 4,300 bhp) (diesels) 1,000 PS (740 kW; 990 shp) (electric)
Propulsion	2 shafts 2 × diesel engines 2 × electric motors
Speed	18.3 knots (33.9 km/h; 21.1 mph) surfaced 7.3 knots (13.5 km/h; 8.4 mph) submerged
Range	13,850 nmi (25,650 km; 15,940 mi) at 10 knots (19 km/h; 12 mph) surfaced 63 nmi (117 km; 72 mi) at 4 knots (7.4 km/h; 4.6 mph) submerged
Test depth	230 m (750 ft)
Complement	4 officers, 44 enlisted
Armament	6 × torpedo tubes (4 bow, 2 stern) 22 × 53.3 cm (21 in) torpedoes 1 × 10.5 cm (4.1 in) SK C/32 deck gun (180 rounds) 1 × 3.7 cm (1.5 in) Flak M42 AA gun 2 x twin 2 cm (0.79 in) C/30 AA guns
Service record (Kriegsmarine)
Part of:	31st U-boat Flotilla 9 February – 31 August 1944 11th U-boat Flotilla 1 – 30 September 1944 33rd U-boat Flotilla 1 October 1944 – 8 May 1945
Identification codes:	M 43 319
Commanders:	Kapt.z.S. Hermann Lessing 9 February 1944 – March 1945 Oblt.z.S. Helmut Wicke March – 13 May 1945
Operations:	2 patrols: 1st patrol: a. 18 October 1944 – 31 January 1945 b. 1 – 5 February 1945 c. 13 – 20 April 1945 d. 22 – 24 April 1945 2nd patrol: 27 April – 14 May 1945
Victories:	None

German submarine U-1231 was a Type IXC/40 U-boat built for Nazi Germany's Kriegsmarine during World War II.

German Type IXC/40 submarines were slightly larger than the original Type IXCs. U-1231 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.^[1] 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).^[1]

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).^[1] 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-1231 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.^[1]

U-1231 was ordered on 14 October 1941 from Deutsche Werft AG Weser in Hamburg-Finkenwerder under the yard number 394. Her keel was laid down on 31 March 1943 and was launched on 18 November 1943. About three months later she was commissioned into service under the command of Kapitän zur See Hermann Lessing (Crew 21) in the 31st U-boat Flotilla on 9 February 1944.

After completing training and work-up for deployment U-1231 was transferred to the 33rd U-boat Flotilla for front-line service on 1 October 1944. The U-boat left Bergen on 18 October 1944 for the first war patrol operating unsuccessfully against Allied shipping in the North Atlantic and off the coast of Canada. After returning to Flensburg on 5 February 1945, Lessing was relieved as commander by Oberleutnant zur See Helmut Winke (Crew X/39). In April 1945 'U-1231 left Kiel for the North Atlantic, again operating without success. After the German surrender, Winke took U-1231 to Dundee, from where the U-boat was transferred to Lisahally.

In November 1945, U-1231 was allocated to the Soviet Union as war booty and was transferred to Libau via Copenhagen between 24 November and 5 December 1945. Renamed N-26 the U-boat was commissioned into the Soviet Navy and served with the Red Banner Baltic Fleet. On 29 December 1955, having been re-designated B-26, the U-boat was decommissioned and placed into reserve and used for training purposes. Struck from the list on 13 January 1968 and sold for scrap the U-boat was later broken up in Riga.