German submarine U-123 was a Type IXB U-boat of Nazi Germany's Kriegsmarine that operated during World War II. After that conflict, she became the French submarine Blaison (Q165) until she was decommissioned on 18 August 1959.

Construction
U-123 was laid down on 15 April 1939 at the AG Weser yard in Bremen as yard number 955. She was launched on 2 March 1940 and commissioned on 30 May, with Kapit\xe4nleutnant Karl-Heinz Moehle (Crew 30) in command. He was relieved on 19 May 1941 by Kptlt. Reinhard Hardegen (Crew 33), who was relieved in turn on 1 August 1942 by his watch officer, Oberleutnant zur See Horst von Schroeter (Crew 37b). He remained in command until the boat was decommissioned on 17 June 1944.

Design
German Type IXB submarines were slightly larger than the original German Type IX submarines, later designated IXA. U-123 had a displacement of 1,051 tonnes (1,034 long tons) when at the surface and 1,178 tonnes (1,159 long tons) while submerged. The U-boat had a total length of 76.50 m (251 ft), a pressure hull length of 58.75 m (192 ft 9 in), a beam of 6.76 m (22 ft 2 in), a height of 9.60 m (31 ft 6 in), and a draught of 4.70 m (15 ft 5 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 metric horsepower (740 kW; 990 shp) 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 18.2 knots (33.7 km/h; 20.9 mph) and a maximum submerged speed of 7.3 knots (13.5 km/h; 8.4 mph). When submerged, the boat could operate for 64 nautical miles (119 km; 74 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 12,000 nautical miles (22,000 km; 14,000 mi) at 10 knots (19 km/h; 12 mph). U-123 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) SK C/30 as well as a 2 cm (0.79 in) C/30 anti-aircraft gun. The boat had a complement of forty-eight.