The idea for the Type XXI arose at a conference in November 1942 at Admiral Doenitz’s Paris headquarters. The conference was attended by Professor Walter and the technical experts Schuerer, Broeking and Waas. The Walter boats were discussed as a way of producing submarines with a much greater submerged speed. The V80 a 4 man 76-ton prototype submarine completed in 1940 reached a submerged speed of 28 knots. A problem was the very large amounts of hydrogen peroxide required compared to diesel consumption for example. Helmuth Walter had designed the type XVIII boat with a figure of 8 hull cross section, the lower hull to be used to store hydrogen peroxide. Once the hydrogen peroxide had been consumed within a few hours the submarine would have to revert to an inferior conventional propulsion system. Doenitz learnt to his regret that the Walter U-boat was nowhere near ready for service. He could not accept the responsibility of recommending that the Walter U-boat should be put at once into mass production.
The engineers Schuerer and Broeking suggested taking the basic type XVIII design and using the lower hull to store batteries instead of hydrogen peroxide. The result would be to triple the submarines battery capacity but with a conventional diesel electric system. The possibility existed of a much enhanced underwater performance with a shorter development time than would be required for the Walther submarine. Doenitz, head of the U-boat arm and soon to be head of the Kriegsmarine, agreed to go ahead with the electro-boat idea. Another advantage of using the type XVIII hull design was that hydrodynamic testing had already been carried out. Professor Walter suggested the development of an air intake and expulsion apparatus, later called the snorkel, which would avoid the necessity for the submarine to surface to recharge its batteries.
In June 1943 some time after he had become Commander-in-Chief Doenitz was able to examine the blue prints of the new design. As the Type XXI design developed a number of significant improvements were made compared to the previous Types VII and IX. For the first time in a diesel electric design more power was to be developed under water than on the surface, 5000 hp versus 4000 hp. Low power creeping electric motors were developed which would allow very stealthy under water running at speeds up to 6 knots. In general the Type XXI was much more streamlined than previous U-boats with as few external protuberances as possible. There was no main gun but two anti-aircraft turrets blended in to the fore and aft parts of the conning tower. Each turret had a pair of 20 mm anti-aircraft cannon. The Type XXI had six bow torpedo tubes and no stern tubes, and was able to carry 23 torpedoes altogether. An improved torpedo reloading system allowed 3 salvoes of 6 torpedoes to be fired in 20 minutes. The submarine had a figure of eight cross section for the pressure hull and a streamlined outer hull parts of which were free flooding. The number of free flooding holes had an effect on the water resistance, and hence maximum speed, and the diving time. The best compromise resulted in a maximum submerged speed of 16.8 knots and a diving time of 25 seconds. Because the pressure hull was not circular it was more difficult to calculate the safe diving depth. In deep diving tests depths of more than 200 metres were reached. The boat had two 6 cylinder diesels developing about 2000 hp each when used with superchargers. The two electric motors developed 2500 hp each for the maximum submerged speed and a slightly higher speed on the surface showing that the hull was also well designed for surface running. Unfortunately the superchargers did not work well particularly when snorkelling and were deleted from many of the submarines. As a result the maximum speed on the surface under diesel power was reduced from 15.6 knots down to about 14 knots. The maximum snorkelling speed was only about 6 knots due to oscillation problems with the snorkel which was an extending type and not as strong as other designs. The facilities for the crew were better than in the previous Type VII and IX designs. There was improved air purification and regeneration apparatus, a freezer for better food storage and a washroom with three wash basins and a shower.
In the early stages of the Type XXI design it was expected that two prototypes would be built and thoroughly tested before embarking on mass production but that would probably mean that the submarine would not become fully operational until 1946. With the U-boat war having reached a critical stage Doenitz realised that something would have to be done to speed up the normal production process for a new type of submarine. Doenitz consulted with Albert Speer the armaments minister and he brought in Otto Merker an industrialist with experience of mass production in the automobile industry. A plan was devised to produce the Type XXI straight from the drawing board with a wooden mock up but no actual prototype. In a complete departure from normal submarine construction at that time, the submarine was divided longitudinally into 8 sections which were completed separately and only brought together in the final assembly stage.
The first stage in the construction was the basic steel building stage carried out at many steelworks throughout Germany. At this stage the pressure hull, outer hull, bulkheads, decks and tanks were constructed. Many of the steelworks involved had no previous shipbuilding experience. The basic steel modules were then transported by inland waterways to the section outfitting stage. At this stage yards with experience of submarine construction would complete the sections to a high state of readiness. Finally the sections or modules were transported to the final assembly yards with access to a river or the sea for the launching. The sections would be arranged on the slipways and then carefully lined up for the welding stage. Four welders would weld two sections together over a period of about 8 hours with no stoppages allowed either for weather or air raids. There were three final assembly yards Blohm & Voss, Hamburg; Deschimag Weser AG, Bremen and Schichau, Danzig.
The modular construction system allowed the U-boats to be constructed more quickly but many of them were completed with a number of faults and had to keep returning to the shipyards as a result. The de-centralised construction of sections gave some protection against air raids but the assembly yards were still vulnerable. Late in the war the assembly yards were heavily bombed which slowed construction. Ideally the final assembly stage would have taken place in reinforced concrete bunkers but they were not ready in time. A lot of pressure was put on the shipyards to produce a large number of U-boats but it worsened some of the problems. For example some sections were sent forward with parts missing in order to keep to the scheduled date. Some of the early boats were constructed without a snorkel and had to return to the yards to have them fitted causing more delays. The hydraulic system originally intended for the type XVIII boats turned out to be over complicated and had to be redesigned. Another problem when the U-boats were tested was that the turning circle was too large but it was too late to change the single rudder design. Additionally the diesel and electric motors were not separated by a clutch, because it was assumed that the large battery installation would always need charging. As a result some electric motors running in reverse caused water to be sucked into the diesel engines.
However despite all the faults the Type XXI was probably the right concept as a U-boat to take over from the Type VII and Type IX designs. Although considerably larger than the Type VII the Type XXI could still dive fairly rapidly. The Type XXI could carry 23 torpedoes compared to 14 for the Type VII and with 6 tubes at the bow and a rapid reloading system could fire many more torpedoes in a short time. Also the Type XXI could carry more fuel and provisions, enough for a long voyage in the Atlantic without needing to be resupplied.
The most revolutionary aspect of the Type XXI was its underwater speed and endurance. The speed of over 16 knots would give it a much better chance of penetrating an escort screen and getting into a good firing position to sink a number of merchant ships. The high speed would also give it a much better chance of escape from the escorts following an attack. Also using the creeping electric motors a stealthy disengagement was possible at 6 knots for up to 48 hours.
The poor underwater performance of the Type VII meant that it was very difficult to get into a firing position even with a convoy travelling at only 10 knots. In the early part of the war type VII boats attacked on the surface at night but that was becoming increasingly difficult by the time the Type XXI was being considered. Also hunter-killer groups when able to hunt a U-boat away from a convoy could take advantage of the poor underwater performance to stay with a U-boat, such as the Type VII or Type IX, until it was forced to surface.
Normally accepting a new U-boat into the Kriegsmarine involved the crew in ‘standing-by’ the U-boat as it neared completion and then taking it over on the commissioning day. The crew was then involved in testing the U-boat along with the construction yard. When all was satisfactory the boat was taken for a shake-down cruise in the Baltic with a training flotilla and the crew was trained in the latest operational techniques. Because so much of the equipment was new the whole process took much longer with the Type XXI compared to the Type VIIs and Type IXs. Type XXI crews made desperate efforts to prepare their boats for operations before the war ended. However many completed boats needed modifications or repairs which could not be done in time.
A number of boats left German ports heading for the U-boat bases in Norway but only two commenced war patrols before the war ended. On April 30 1945 U-2511 put to sea from Bergen, Norway, with orders to go to the Caribbean. The boat was detected by an anti-submarine patrol group off the Scottish coast but easily escaped. On May 3, after receiving the surrender order by radio, the commanding officer, Schnee, had the British cruiser HMS Suffolk in his sights at a range of 600 metres and went through the motions of an attack, braking off at the last minute. He dived under the target and returned to Norway without being detected. The other war patrol was commenced by U-3008 which sailed from Wilhelmshaven just before the surrender. U-3008 carried out an undetected dummy attack on a British convoy before returning to port.
Although the Type XXI was a revolutionary design the programme as a whole was a failure because the type did not become operational in sufficient numbers before the war ended. Despite using the sectional construction system not enough boats reached a stage of operational reliability and there was no possibility of making an impact on the Battle of the Atlantic. Statistically 120 boats were delivered but most were constantly having to return to the yards for repair or modification or were still training and not ready for combat. Unfortunately all the cutting of corners and the pressure to produce the U-boat in large numbers lead to mistakes in detail design which added to the problems.
Specifications
The engineers Schuerer and Broeking suggested taking the basic type XVIII design and using the lower hull to store batteries instead of hydrogen peroxide. The result would be to triple the submarines battery capacity but with a conventional diesel electric system. The possibility existed of a much enhanced underwater performance with a shorter development time than would be required for the Walther submarine. Doenitz, head of the U-boat arm and soon to be head of the Kriegsmarine, agreed to go ahead with the electro-boat idea. Another advantage of using the type XVIII hull design was that hydrodynamic testing had already been carried out. Professor Walter suggested the development of an air intake and expulsion apparatus, later called the snorkel, which would avoid the necessity for the submarine to surface to recharge its batteries.
In June 1943 some time after he had become Commander-in-Chief Doenitz was able to examine the blue prints of the new design. As the Type XXI design developed a number of significant improvements were made compared to the previous Types VII and IX. For the first time in a diesel electric design more power was to be developed under water than on the surface, 5000 hp versus 4000 hp. Low power creeping electric motors were developed which would allow very stealthy under water running at speeds up to 6 knots. In general the Type XXI was much more streamlined than previous U-boats with as few external protuberances as possible. There was no main gun but two anti-aircraft turrets blended in to the fore and aft parts of the conning tower. Each turret had a pair of 20 mm anti-aircraft cannon. The Type XXI had six bow torpedo tubes and no stern tubes, and was able to carry 23 torpedoes altogether. An improved torpedo reloading system allowed 3 salvoes of 6 torpedoes to be fired in 20 minutes. The submarine had a figure of eight cross section for the pressure hull and a streamlined outer hull parts of which were free flooding. The number of free flooding holes had an effect on the water resistance, and hence maximum speed, and the diving time. The best compromise resulted in a maximum submerged speed of 16.8 knots and a diving time of 25 seconds. Because the pressure hull was not circular it was more difficult to calculate the safe diving depth. In deep diving tests depths of more than 200 metres were reached. The boat had two 6 cylinder diesels developing about 2000 hp each when used with superchargers. The two electric motors developed 2500 hp each for the maximum submerged speed and a slightly higher speed on the surface showing that the hull was also well designed for surface running. Unfortunately the superchargers did not work well particularly when snorkelling and were deleted from many of the submarines. As a result the maximum speed on the surface under diesel power was reduced from 15.6 knots down to about 14 knots. The maximum snorkelling speed was only about 6 knots due to oscillation problems with the snorkel which was an extending type and not as strong as other designs. The facilities for the crew were better than in the previous Type VII and IX designs. There was improved air purification and regeneration apparatus, a freezer for better food storage and a washroom with three wash basins and a shower.
In the early stages of the Type XXI design it was expected that two prototypes would be built and thoroughly tested before embarking on mass production but that would probably mean that the submarine would not become fully operational until 1946. With the U-boat war having reached a critical stage Doenitz realised that something would have to be done to speed up the normal production process for a new type of submarine. Doenitz consulted with Albert Speer the armaments minister and he brought in Otto Merker an industrialist with experience of mass production in the automobile industry. A plan was devised to produce the Type XXI straight from the drawing board with a wooden mock up but no actual prototype. In a complete departure from normal submarine construction at that time, the submarine was divided longitudinally into 8 sections which were completed separately and only brought together in the final assembly stage.
The first stage in the construction was the basic steel building stage carried out at many steelworks throughout Germany. At this stage the pressure hull, outer hull, bulkheads, decks and tanks were constructed. Many of the steelworks involved had no previous shipbuilding experience. The basic steel modules were then transported by inland waterways to the section outfitting stage. At this stage yards with experience of submarine construction would complete the sections to a high state of readiness. Finally the sections or modules were transported to the final assembly yards with access to a river or the sea for the launching. The sections would be arranged on the slipways and then carefully lined up for the welding stage. Four welders would weld two sections together over a period of about 8 hours with no stoppages allowed either for weather or air raids. There were three final assembly yards Blohm & Voss, Hamburg; Deschimag Weser AG, Bremen and Schichau, Danzig.
The modular construction system allowed the U-boats to be constructed more quickly but many of them were completed with a number of faults and had to keep returning to the shipyards as a result. The de-centralised construction of sections gave some protection against air raids but the assembly yards were still vulnerable. Late in the war the assembly yards were heavily bombed which slowed construction. Ideally the final assembly stage would have taken place in reinforced concrete bunkers but they were not ready in time. A lot of pressure was put on the shipyards to produce a large number of U-boats but it worsened some of the problems. For example some sections were sent forward with parts missing in order to keep to the scheduled date. Some of the early boats were constructed without a snorkel and had to return to the yards to have them fitted causing more delays. The hydraulic system originally intended for the type XVIII boats turned out to be over complicated and had to be redesigned. Another problem when the U-boats were tested was that the turning circle was too large but it was too late to change the single rudder design. Additionally the diesel and electric motors were not separated by a clutch, because it was assumed that the large battery installation would always need charging. As a result some electric motors running in reverse caused water to be sucked into the diesel engines.
However despite all the faults the Type XXI was probably the right concept as a U-boat to take over from the Type VII and Type IX designs. Although considerably larger than the Type VII the Type XXI could still dive fairly rapidly. The Type XXI could carry 23 torpedoes compared to 14 for the Type VII and with 6 tubes at the bow and a rapid reloading system could fire many more torpedoes in a short time. Also the Type XXI could carry more fuel and provisions, enough for a long voyage in the Atlantic without needing to be resupplied.
The most revolutionary aspect of the Type XXI was its underwater speed and endurance. The speed of over 16 knots would give it a much better chance of penetrating an escort screen and getting into a good firing position to sink a number of merchant ships. The high speed would also give it a much better chance of escape from the escorts following an attack. Also using the creeping electric motors a stealthy disengagement was possible at 6 knots for up to 48 hours.
The poor underwater performance of the Type VII meant that it was very difficult to get into a firing position even with a convoy travelling at only 10 knots. In the early part of the war type VII boats attacked on the surface at night but that was becoming increasingly difficult by the time the Type XXI was being considered. Also hunter-killer groups when able to hunt a U-boat away from a convoy could take advantage of the poor underwater performance to stay with a U-boat, such as the Type VII or Type IX, until it was forced to surface.
Normally accepting a new U-boat into the Kriegsmarine involved the crew in ‘standing-by’ the U-boat as it neared completion and then taking it over on the commissioning day. The crew was then involved in testing the U-boat along with the construction yard. When all was satisfactory the boat was taken for a shake-down cruise in the Baltic with a training flotilla and the crew was trained in the latest operational techniques. Because so much of the equipment was new the whole process took much longer with the Type XXI compared to the Type VIIs and Type IXs. Type XXI crews made desperate efforts to prepare their boats for operations before the war ended. However many completed boats needed modifications or repairs which could not be done in time.
A number of boats left German ports heading for the U-boat bases in Norway but only two commenced war patrols before the war ended. On April 30 1945 U-2511 put to sea from Bergen, Norway, with orders to go to the Caribbean. The boat was detected by an anti-submarine patrol group off the Scottish coast but easily escaped. On May 3, after receiving the surrender order by radio, the commanding officer, Schnee, had the British cruiser HMS Suffolk in his sights at a range of 600 metres and went through the motions of an attack, braking off at the last minute. He dived under the target and returned to Norway without being detected. The other war patrol was commenced by U-3008 which sailed from Wilhelmshaven just before the surrender. U-3008 carried out an undetected dummy attack on a British convoy before returning to port.
Although the Type XXI was a revolutionary design the programme as a whole was a failure because the type did not become operational in sufficient numbers before the war ended. Despite using the sectional construction system not enough boats reached a stage of operational reliability and there was no possibility of making an impact on the Battle of the Atlantic. Statistically 120 boats were delivered but most were constantly having to return to the yards for repair or modification or were still training and not ready for combat. Unfortunately all the cutting of corners and the pressure to produce the U-boat in large numbers lead to mistakes in detail design which added to the problems.
Specifications
Displacement
Surfaced: 1621 tons
Submerged: 1819 tons
Dimensions
Length: 76.7 metres
Beam: 6.6 metres
Draught: 6.3 metres
Propulsion
Diesel engines: Two MAN M6V 40/46; 2000 hp each
Electric motors: Two SSW 2 GU 365/30; 2500 hp each
Creep motors: Two SSW GW323/28; 113 hp each
Battery: Three 124-cell batteries
Fuel capacity: 250 tons
Performance
Maximum speed
Surfaced: 15.65 knots
Submerged: 16.8 knots
Range
Surfaced: 11150 nm at 12 knots
Submerged: 285 nm at 6 knots
Weapons
Bow tubes: six 21 in(533 mm)
Torpedoes carried: 23
Alternative: 14 torpedoes plus 12 TMC mines
Guns: Two twin 20mm anti-aircraft canon
Officers and Crew: 57
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