Type XXI submarines is a class of diesel–electric U-boats built for Nazi Germany's Kriegsmarine from 1943 to 1945 during the Second World War. The concept of Type XXI submarines emerged in 1943 when it became clear the Type VII and Type IX still being produced, had become obsolete and the designed successor Type XVIII still faced blocking problems with its new propulsion system. 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 of escaping detection. As they incorporated many batteries to increase the time they could spend submerged, they were sometimes referred to as Elektroboote (German: "electric boats") in the German Navy. They were equipped with a snorkel, which allowed them to cruise at periscope depth on their diesel engines, and recharge their batteries without having to surface. The design included many general improvements as well: much greater underwater speed by an improved hull design, power-assisted torpedo reloading and better crew accommodations.
In order to speed up the delivery of the new U-boats, a new mass production method of assembling prefabricated sections was introduced, and production began before design work was completed. The inexperienced factories which constructed the boats were unable to meet quality standards, and most of the produced U-boats were mechanically unreliable. The massive building program suffered also from the Allied Strategic bombing campaign. One hundred and eighteen Type XXIs were commissioned during the war, but only one executed a combat patrol in the last week of the war. After the war, captured Type XXIs were examined by several navies, and many post-war submarine designs were influenced by them. These include the Soviet , Chinese Ming, American , British Porpoise, and Swedish classes.
Background A World War II-era submarine relied on diesel engines for surface cruising, and on battery-powered electric engines for underwater travel. Submerged speed and range were limited by the battery capacity. For example, the standard ocean-going U-boat Type VII had a surface range of at cruising speed of , but only a submerged range of at a cruising speed of . A top speed of could only be maintained for one hour. Because of these limitations, a submarine was in fact rather a surface vessel with the capability to dive for tactical reasons. Once a submarine was detected and submerged, its chances to execute a successful attack were quite low.
In 1933 Hellmuth Walter, a German engineer working at Germaniawerft sought to overcome these limitations, and started to experiment with hydrogen peroxide powered engines for high-speed underwater propulsion. In 1939 Walter received a contract to build the experimental submarine V-80 which in the autumn of 1940 could achieve an underwater speed of with an air-independent propulsion system consisting of a hydrogen peroxide powered steam turbine. A diesel-electric powered submarine spent most of its time on the surface cruising on diesel engines and its outer hull was that of a classic surface ship, but as the Walter turbine provided high underwater speeds, a more streamlined outer hull was needed for submerged cruising. The V-80 had a fish-like hull with a smooth exterior.
At this time, the German Navy was reluctant to deviate much resources into developing this experimental concept into a practical design. The existing U-boat types were still performant enough and it would take at least three years before this experimental design could be put into use. Only in February 1942, Karl Dönitz, the head of the German Navy, realized that this alternate submerged propulsion could be the answer to the growing threat of Allied long-range aircraft escorting convoys and patrolling the bay of Biscay. He ordered four small, experimental Type XVII Walter U-boats, and one slightly larger ocean-going vessel, the ocean-going . By September 1942 these U-boats were still under design and there were still many issues to be solved before the propulsion system could be used on combat U-boats. But as it was realized that the existing U-boat types would soon become obsolete, a further twenty-four Type XVII were ordered and, well before the lessons learnt from the small Walter U-boats could be incorporated into the design, two prototypes for a large 1500-ton ocean-going Type XVIII Walter U-boat were ordered.
In April 1943, Allied advances in radar, HF/DF and long-range air escort finally rendered the existing U-boat types fighting the battle of the Atlantic completely obsolete. A more performant U-boat type was urgently needed, but the Walter U-boat project was far from ready: Germany lacked the resources to produce the hydrogen peroxide fuel needed for these turbines, which consumed far more fuel than a U-boat could store in her tanks. There remained also many mechanical and practical teething problems with the Walter-turbine so alternate solutions were investigated. German engineers took up the idea of using the large turbine room and hydrogen peroxide tanks of these Type XVII and Type XVIII U-boats to install larger electric engines and extra battery cells, which would vastly increase underwater speed and range. On 19 June 1943, the German Navy ordered these ideas to be worked out as a new Type XXI submarine, which was to replace the Type IX submarine, and on 29 June Dönitz approved the Type XXI as the successor for the Type VII for operations against the North-Atlantic convoys.
Design
Using the Type XVIII design for the hull, armament, and control systems, and the new supercharged diesel engine intended for the Type VIIC/42 submarine, the Type XXI was underpowered and slower on the surface than its predecessors the Type VII and Type IX, but low surface speed was not considered a problem. Being designed primarily for submerged use, the Type XXI's maximum surface speed was lower than that of the Type IX (18.2 knots) but its submerged speed was more than twice that of the Type IX (17.2 knots versus 7.7 knots), because they were equipped with much more powerful electric drive motors and had less drag. With its superior submerged speed and endurance, a Type XXI could position itself undetected for a convoy attack, and with a new quick-loading device, it could fire eighteen torpedoes in twenty minutes.
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 Type VII. This gave these boats great underwater range and dramatically reduced the time spent on or near the surface. They could travel submerged at about for about 75 hours before recharging batteries, which took less than five hours using the snorkel due to the new supercharged diesel engines.
The Type XXI was also much quieter than the VIIC, making it more difficult to detect when submerged, and the design eliminated protruding components that had created drag in earlier models. The ability to outrun many surface ships while submerged, combined with improved dive times (also a product of the new hull form), made the Type XXI much more difficult to pursue and destroy. It could sprint when positioning for an attack, while 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 Type XXI was also equipped with a pair of electric "creep motors" for silent running. In addition, the Type XXIs had better facilities than previous U-boat classes, with much roomier crew berths, and a freezer to prevent food spoilage.
Characteristics Type XXIs had an overall length of The boats' beam was , the draught was , with a height of . The pressure hull had a length of and had a width of . The boats displaced when surfaced and when submerged. The complement consisted of five officers and fifty-two enlisted men. Constructional, test and crush diving depth were , and respectively.
Propulsion As the emphasis was laid on enhanced submerged performance, Siemens-Schuckert designed new GU265/30 double-acting electric motors which were more powerful than the diesel engines. Two of these large electrical motors were installed in the original walter-turbine compartment, together with two small GW323/28 electrical motors for slow silent running. The Type XXI had two MAN six-cylinder four-stroke M6V40/46KBB diesel engines with a total of for use on the surface and two double-acting electric motors with a total of for underwater use. These engines powered two shafts, which gave the boats a top surface speed of and when submerged. Cruising range was at on the surface and at submerged.
Armament The Type XXI was the first U-boat to be equipped with six bow torpedo tubes, but had no stern torpedo tube. Six torpedoes were carried within the tubes and another seventeen could be stored in racks immediately behind the tubes. Reloading was done by hoisting a torpedo on a rail hanging from the upper pressure hull. An electrically powered trolley could push the torpedo into the torpedo tube much faster than had been possible in manual reloading. A full second salvo of six torpedoes was possible five to six minutes after the first salvo, and a third salvo could be fired within twenty minutes. It was intended to make it possible to substitute twelve mines for four torpedoes. Four torpedo tubes could contain three mines each and eject these mines with compressed air. But no Type XXI was ever equipped with the needed minelaying gear. The anti-aircraft armament consisted of two twin 2 cm C/38 guns, mounted in two turrets on the enclosed bridge. The design of this enclosed bridge and anti-aircraft turrets came from the Type XVII. As designed, these turrets were equipped with twin 3 cm M44 guns, but the development of these new guns was delayed and instead the well-proven but obsolete 2 cm C/38 was installed.
Sensors Just like all other U-boat types, the Type XXI was equipped with a passive sound listening device, the Gruppenhorchgerät, often abbreviated to GHG. Until 1943 the sensors of the GHG in a U-boat were mounted as far as possible below the waterline towards the bow, with the receiver membranes fitted into the hull plating. In order to give the GHG a better performance, and to allow the GHG to be used at higher submerged speeds, the sensors for the Type XXI were mounted in a 'balcony', a round construction protruding from the keel at the bows. The receivers were arranged in two horizontal 'horseshoe' lines, which allowed even accurate sound location in all directions except the sector right aft. This arrangement allowed the GHG to be used effectively both surfaced and submerged at all depths. The increased submerged speed of the Type XXI necessitated the installation of the receivers behind steel plates. The range of the GHG depended much on the size and speed of the target, but a ship moving at 10 knots could be detected at a range of minimum with an accuracy of 1 degree. The GHG could not give bearings and distance to target accurate enough for torpedo firing control. As it was intended that the Type XXI attack submerged without the use of periscopes, an active sonar called 'S-gerät' was installed for this purpose. The S-gerät has a radius of . Because the sound pulses of the S-Gerät could give away the presence of a U-boat, target acquisition was done with the GHG and the S-gerät was only activated when the attack started.
The Type XXI boats were fitted with the FuMO 65 Hohentwiel U1 with the Type F432 D2 radar transmitter. It was intended to install a FuMO 84 Berlin II and FuMO 391 Lessing as from the summer of 1945. The FuMo 84 was a retractable, tactical radar with a wavelength of 9 cm, which could be operated at periscope depth, whilst the FuMO 391 was an air search radar operating at a wavelength of 2.4m.
Construction Section building