The Type XXI U-boat

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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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