Type XXI and XXIII U-boats were a next generation of German diesel-electric Elektroboot (“electric boat”) submarines designed and deployed by Nazi Germany during the Second World War. Type XXI were large ocean-going submarines and Type XXIII were small coastal submarines designed to operate in the shallow waters of the North Sea, Black Sea and Mediterranean Sea, where larger Type XXI Elektro boats were at risk i
Although only four of the Type XXI submarines were completed during the war, and only two were sent for combat patrol (and these were not used in combat), Type XXI and XXIII revolutionized the submarine design.
After the war, several navies obtained XXIs and operated them for decades in various roles, and large navies introduced new submarine designs based on them. These include the Soviet Whiskey, US Tang, UK Porpoise, and Swedish Hajen classes, all based on the Type XXI design to a significant extent.
Type XXI and XXIII U-boats 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 to escape detection or to attack.
These subs incorporated a large number of batteries to increase the time they could spend underwater, to as much as several days, and they only needed to surface to periscope depth for recharging via a snorkel.
The new U-boat design included many general improvements as well; much greater underwater speed by an improved hull design, greatly improved diving times, power-assisted torpedo reloading, and greatly improved crew accommodations.
All six bow torpedo tubes could be reloaded faster than a Type VIIC could reload one tube. Consequently, Type XXI submarine could fire 18 torpedoes in less than 20 minutes. It also featured a very sensitive passive sonar for the time, housed in the “chin” of the hull.