The Type XXI U-boat

The Type XXI U-boat

The Type XXI U-boat was one of the few types of weapon that, despite being unable to take part in combat, completely changed the character of naval actions at sea.

It was the first true submarine in history. The other types of vessels designed so far were basically “diving ships” that might have been underwater for some time, but most of the time they had to be on the surface because their underwater range was severely limited. Under water they were slow and not manoeuvrable, and often had to emerge onto the surface to charge their electric batteries. The Type XXI was designed from the outset as a true submarine whose natural environment was to be the deep seas.
Unfortunately, due to various reasons, it did not have the expected impact on the course of  maritime activities during the Second World War, despite the expenditure by the Third Reich of enormous material and energy resources on this ingenious project. Eventually, only three submarines of the XXI Type managed to perform combat patrols by the end of the war.

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The concept is born
The origins of the Type XXI began at the end of 1942, when it became clear that the Allies were getting better and better at fighting German submarines, and in that situation the existing “workhorses” of the battles for the Atlantic – the Type VII and Type IX - were becoming outdated. Allied warships and aircraft sank more and more U-boats month by month, and the new German-made improvements in the construction of the old types and electronic warfare equipment to fight the Allies were not working to their desired extent. An excellent example of such a change in construction was the removal of 88 mm or 105 mm onboard guns to allow more anti-aircraft weapons to be fitted. Unfortunately, apart from a few cases, it did not happen that the U-boat was suddenly able to protect itself against air attack. In addition, the greater number of AA guns on the conning tower increased the boat’s underwater resistance, thereby reducing its range and speed. Even technological innovations such as Schnorchel (“snorkel” - a special device used to draw air for diesel engines during submerged travel) and radar warning electronic devices such as Metox and Naxos had failed to restore the balance.
At that time, Professor Helmut Walter worked intensively on the design of new types of submarines with improved hydrodynamic silhouette and a new type of atmospherically-independent propulsion, which achieved an underwater speed of 26 knots (V 80 – the author). The result of this effort was the Walter turbine project of the Type XVIII submarine with a displacement of 1,652 tons and an underwater speed of 24 knots. However, in May 1943, the concept of use of the Type XVIII hull design was introduced and it was fitted with a conventional diesel-electric drive. This idea was enthusiastically welcomed, and not only because the bigger hull gave new possibilities for designing completely new technological solutions. Due to the very serious situation of the U-bootwaffe, the project was approved on 19 June by Dönitz, and the whole program of construction of the Type XXI was accepted the following day. The pace of the work was really fast, as the detailed design and production work was carried out in parallel. This resulted in the fact that on 18 December 1943, the “Section I” was ready. The first U-boat of Type XXI (U 3501 – the author) was launched at the Schichau Shipyard on 19 April 1944, and by the end of the war German shipyards managed to launch as many as 131 such submarines. Still, only two of them achieved the status of operational readiness and performed combat patrols. Looking at it from this point of view, the whole programme ended in a complete failure for the Third Reich.

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The project

Thanks to the fact that the hull of Professor Walter’s Type XVIII project was used, a lot of time was saved, because the model tank tests and hydrodynamic calculations were already done for this model. But besides these, almost all the rest had to be designed from scratch. The Type XXI had a very streamlined outer hull and was designed to minimize hydrodynamic resistance. There were no guns on the outer deck, and two 20 mm twin-barrel anti-aircraft guns were mounted in streamlined turrets perfectly suited for both ends of the conning tower. In turn, all the protruding elements on the conning tower such as Schnorchel, the radar antenna and the radio direction finder antenna were lowered and hidden inside the hull when they were not in use. The idea of ​​an open combat bridge was also abandoned. The Type XXI had three individual posts - for the watch officer and two observers (as in modern submarines – the author).
The rigid hull was made of 26 mm thick (37 mm around hatches – the author) in the ship’s central sections and 18 mm thick in the bow and stern sections made of special steel alloy (St 52 HP) and aluminium. On the cross-section, the rigid hull resembled an “8” with its upper part larger than the bottom, which greatly increased the space of interior.


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Inside the rigid hull there were sets of batteries, the total number of which was three times that in comparison to the previous model (hence the colloquial name of the Type XXI - electroboot – the author). Also new were the SSW 2 Gu 365/30 “Herta” electric motors. They had much more output (2,500 horsepower) than the previously built models. Moreover, two additional SSW GV 323/28 electric motors with 113 horsepower each were added, each of which was to be used only to move the ship in a “silent” mode while dodging tracking enemy ships (at a speed of 6 knots underwater it was practically undetectable – the author). The battery packs were in two separate sections - right and left side at the bottom of the rigid hull. In each of these sections, each battery was connected to three batteries in a separate compartment (Akkuräume). The four bow battery sections were located under the fore crew compartment (Section VI), two aft compartments below the living quarters behind the ship’s command post (Section IV). Individual batteries were built from AFA 44 MAL 740E cells. The total weight of all the batteries on board the U-boat Type XXI was 238.8 tons. Their full working time was 20 hours, and charging time was very short at only four hours.
In addition, for the needs of Type XXI, a number of innovations were used from the Type VIIC/42 U-boat, whose production was stopped. These were electric dashboards and diesel engines. These were very good and tested under the harshest operational conditions the six-cylinder MAN M6V 40/46 KBB engines, but equipped with a special exhaust gas turbocompressor of BBC Mannheim Vta 450 type, which increased their output from 1,050 to 2,000 horsepower.

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The Type XXI armament consisted of six torpedo tubes at the bow (two more than in the Type VII), but the new torpedo reloading system was a novelty. Due to its use, the XXI submarine was capable of firing three, six-torpedo salvos in just 20 minutes, which dramatically increased the number of targets in the convoy that could be attacked. Due to the greater amount of space inside the ship, the U-boat Type XXI was able to carry even more torpedoes - up to 23!
 Compared to its predecessor Type VIIC, the Type XXI ship was much longer - 76.7 m compared to 67.1 m, had a much greater displacement – 1,819 tons compared to 865 tons - and was much more spacious inside (beam 7.6 m - author’s note). But perhaps the most important feature distinguishing it from its predecessor was its underwater speed - a maximum of 17 knots compared to 7 knots and battery capacity that allowed it to move under water at 5 knots continuously for 72 hours compared to 45 hours for the VIIC type. In addition, the slim and streamlined XXI hull made it much more difficult to detect by the active sonar of the enemy’s surface ships, and the significant muting of the mechanisms working inside the rigid hull and the limitation of cavitation and turbulent flow made it almost undetectable by passive sonar under the water. Post-war tests in the United States showed that Type XXI emitted the same noise level underwater at 15 knots, like an American Balao type at 8 knots. A trial conducted in 1946 proved that an escort ship was unable to detect it from a distance of 200 metres.
With the use of air regeneration system and snorkels, this type of vessel could have operated continuously underwater for more than 10 days.
The sensor system that it was equipped with (GHG and SU-Gerät “Nibelung”) was able to monitor and locate up to 50 potential targets within a 7 kilometer radius in real time.
With 5 officers and 52 seamen, the crew had exceptionally luxurious living conditions for those days. There were three showers and three bathrooms and 51 berths on board, which should not be surprising, as it was designed for cruises of up to five months, mostly immersed.

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The construction
From the very beginning, the Type XXI was planned in a modular system. It was a revolutionary way of building and assembling individual sections (also called modules) at production facilities spread throughout the Third Reich. This resulted in the most economical use of resources needed for production, especially shipyards, equipment and the limited number of workers.
In this way, it was also possible to reduce the time spent by individual sections on shipyard ramps, and to increase the number of workers involved in the construction of the sections, which in turn enabled them to gain a lot of experience by specializing in specific construction tasks. Decentralization also forced Allied bombers to attack many more small targets scattered over the Third Reich. The standard methods used to build submarines required the use of heavy equipment to install large components such as diesel, electric and batteries inside the rigid hull through specially designed construction holes, which was possible only in shipyard conditions. The new modular concept allowed for easy and quick installation of these devices through open spaces at both ends of almost every section.

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