Type XXI U-boats, also known as "Elektroboote" (German: "electric boat"), were a class of German diesel-electric submarines designed and operated during the Second World War. They were the first submarines designed to operate primarily submerged, rather than as surface ships that could submerge as a means to escape detection or launch an attack.
The key 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 Type VIIC. 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 Schnorchel. The Type XXI was also far quieter than the VIIC, making it harder to detect when submerged.
The Type XXI's streamlined and hydro-dynamically clean hull design allowed high submerged speed. The ability to outrun many surface ships while submerged, combined with improved dive times (also a product of the new hull form), made it far harder to chase and destroy. It also gave the boat a 'sprint ability' when positioning itself for an attack. Older boats had to surface to sprint into position. This often revealed a boat's location, especially after aircraft became available for convoy escort. The new hull design also reduced visibility by marine or airborne radar when surfaced; whether this was a goal of the design or coincidence is still debated.
They also featured an electric torpedo-reloading system that allowed all six bow torpedo tubes to be reloaded faster than a Type VIIC could reload one tube. The Type XXI could fire 18 torpedoes in under 20 minutes. The class also featured a very sensitive passive sonar for the time, housed in the "chin" of the hull.
The Type XXIs also had better facilities than previous U-boat classes, including a freezer for food.
Between 1943 and 1945, 118 boats were assembled by Blohm & Voss of Hamburg, AG Weser of Bremen, and Schichau-Werke of Danzig. Each hull was constructed from eight prefabricated sections with final assembly at the shipyards. This new method could have pushed construction time below six months per vessel, but in practice all the assembled U-boats were plagued with severe quality problems that required extensive post-production work to rectify. One of the reasons for these shortcomings was that sections were made by companies having little experience in shipbuilding, following a decision by Albert Speer. As a result, of 118 Type XXIs completed, only four were fit for combat before the Second World War ended in Europe.
It was planned that final assembly of Type XXI boats would eventually be carried out in the Valentin submarine pens, a massive, bomb–hardened concrete bunker built at the small port of Farge, near Bremen. Construction of the pens was between 1943 and 1945, using around 10,000 concentration camp prisoners and prisoners of war as forced labour. The facility was 90% completed when, in March 1945, it was badly damaged by Allied bombing with Grand Slam "earthquake" bombs and abandoned. A few weeks later, the area was captured by the British Army.
U-2511 and U-3008 were the only Type XXIs to go on war patrols, and neither sank any ships. U-2511 had a British cruiser in her sights on 4 May when news of the German cease-fire was received. She made a practice attack before leaving the scene undetected.
In 1957, U-2540, which had been scuttled at the end of the war, was raised and refitted as research vessel Wilhelm Bauer of the Bundesmarine. She was operated by both military and civilian crews in a research role until 1982. In 1984, she was opened to the public by the Deutsches Schiffahrtsmuseum (German Maritime Museum) in Bremerhaven, Germany.
U-2518 became French submarine Roland Morillot. She saw active service during the Suez Crisis in 1956, and remained in commission until 1967. She was scrapped in 1969.
Four Type XXI boats were assigned to the Soviet Union by the Potsdam Agreement; these were U-3515, U-2529, U-3035, and U-3041, which were commissioned into the Soviet Navy as B-27, B-28, B-29, and B-30 (later B-100) respectively. However, Western intelligence believed the Soviets had acquired several more Type XXI boats; a review by the U.S. Joint Intelligence Committee for the Joint Chiefs of Staff in January 1948 estimated the Soviet Navy then had 15 Type XXIs operational, could complete construction of 6 more within 2 months, and could build another 39 within a year and a half from prefabricated sections, since several factories producing Type XXI components and the assembly yard at Danzig had been captured by the Soviets at the end of World War II. U 3538 — U 3557 (respectively TS-5 – TS-19 and TS-32 – TS-38) remained incomplete at Danzig and were scrapped or sunk in 1947. The four boats assigned by Potsdam were used in trials and tests until 1955, then scuttled or used for weapon testing between 1958 and 1973. The Type XXI design formed the basis for several Soviet design projects, Projects 611, 613, 614, 633, and 644. These became the submarine classes known by their NATO codes as Zulu, Whiskey and Romeo submarine classes.
The U-3017 was commissioned into the Royal Navy as HMS N41. She was used for tests until being scrapped in November 1949.The United States Navy took over the U-2513 and U-3008, operating them both in the Atlantic. In November 1946 President Harry S. Truman visited U-2513; the submarine dived to 440 feet (130 m) with the President on board. The U-2513 was sunk as a target in 1951; U-3008 was scrapped in 1956.
The only boat to survive intact is Wilhelm Bauer (ex-U-2540). The wrecks of other Type XXI boats are known to exist. In 1985, it was discovered that the partially scrapped remains of U-2505, U-3004, and U-3506 were still in the partially demolished "Elbe II" U-boat bunker in Hamburg. The bunker has since been filled in with gravel, although even that did not initially deter many souvenir hunters who measured the position of open hatches and dug down to them to allow the removal of artifacts. The wrecks now lie beneath a car park, making them inaccessible.
U-2513 lies in 213 feet (65 m) of water 70 nautical miles (130 km) west of Key West, Florida. The boat has been visited by divers, but the depth makes this very difficult and the site is only considered suitable for advanced divers. Four other boats lie off the coast of Northern Ireland, where they were sunk in 1946 as part of Operation Deadlight. Both U-2511 and U-2506 were found by nautical archaeologist Innes McCartney during his Operation Deadlight expeditions between 2001 and 2003. Both were found to be in remarkably good condition.
The Type XXI design directly influenced advanced post-war submarines, the GUPPY improvements to the American Gato-, Balao-, and Tench-class submarines and the Soviet submarine projects designated by NATO as the Whiskey, Zulu and Romeo classes. The Chinese built Romeo-class submarines were based on Soviet-supplied designs. The subsequent Ming class, some of which are still in operation in 2013, is in turn based on the Romeo.