Type 21 U-boat

U-3008 in U.S. Navy service during 1948.
Class overview
Name:	Type XXI U-boat
Operators:	Kriegsmarine Postwar:  French Navy  German Navy  Royal Navy  Soviet Navy  United States Navy
Cost:	5,750,000 Reichsmark per boat
Built:	1943–45
Building:	267
Planned:	1170
Completed:	118
Cancelled:	785
Preserved:	1
General characteristics
Class and type:	Submarine
Displacement:	1,621 t (1,595 long tons) surfaced 1,819 t (1,790 long tons) submerged
Length:	76.70 m (251 ft 8 in)
Beam:	8 m (26 ft 3 in)
Draught:	6.32 m (20 ft 9 in)
Propulsion:	Diesel/Electric 2× MAN M6V40/46KBB supercharged 6-cylinder diesel engines, 4,000 PS (3,900 shp; 2,900 kW) 2× SSW GU365/30 double-acting electric motors, 5,000 PS (4,900 shp; 3,700 kW) 2 × SSW GV232/28 silent running electric motors, 226 PS (223 shp; 166 kW)
Range:	15,500 nmi (28,700 km; 17,800 mi) at 10 knots (19 km/h; 12 mph) surfaced 340 nmi (630 km; 390 mi) at 5 knots (9.3 km/h; 5.8 mph) submerged
Test depth:	240 m (787 ft)
Complement:	5 officers, 52 enlisted men
Armament:	6 × 53.3 cm (21 in) torpedo tubes (bow) 23 torpedoes (or 17 torpedoes and 12 mines) 4 x 2 cm (0.79 in) C/30 anti-aircraft guns

Type XXI U-boats, also known as "Elektroboote" (German: "electric boat"), were a class of German diesel-electric submarines designed during the Second World War. The submarines were produced prematurely, and all of those built had significant defects. As a result, only four of the submarines were completed during the war, and only two were sent for combat patrol and these were not used for actual combat.

They were the first submarines designed to operate primarily submerged, rather than spending most of their time as surface ships that could submerge for brief periods as a means to escape detection or to attack. They incorporated a very large number of batteries to improve the time they could spend underwater, as much as several days, and only needed to surface to periscope depth for recharging via a snorkel. The design included many general improvements as well; much greater underwater speed by an improved hull design, greatly improved diving times, power assisted torpedo reloading, and greatly improved crew accommodations.

After the war, several navies obtained XXIs and operated them for decades in various roles, and almost every navy introduced new submarine designs based on them. These include the Soviet Whiskey-class submarines, US Tang-class submarines, and the UK Porpoise-class submarines, all of which were based on the XXI design to some extent. The design remains the basis for diesel-electric submarines.

The main features of the Type XXI were the hydrodynamically streamlined hull and conning tower, and the large number of battery cells, roughly triple that of the German Type VII submarine. This gave these boats great underwater range, and dramatically reduced the time spent on or near the surface. They could travel submerged at about 5 knots (9.3 km/h; 5.8 mph) for two or three days before recharging batteries, which took less than five hours using the snorkel. The Type XXI was also much quieter than the VIIC, making it more difficult to detect when submerged.