Type XXI U-Boat

Type XXI submarines were a class of German diesel–electric Elektroboot (German: "electric boat") submarines designed during the Second World War. One hundred and eighteen were commissioned, but only one executed a combat patrol in the last week of the war.

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. They incorporated many batteries to increase the time they could spend submerged, to as much as several days, and they only needed to surface to periscope depth for recharging via a snorkel. The 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. However, the design was also flawed in many ways, with the submarines being mechanically unreliable and, according to post-war analysis, vulnerable to combat damage. The Type XXI submarines were also rushed into production before design work was complete and the inexperienced factories which constructed the boats were unable to meet quality standards.

After the war, captured Type XXIs were examined by several navies, and many post-war submarine design were influenced by them. These include the Soviet Whiskey, Chinese Ming, American Tang, British Porpoise, and Swedish Hajen III classes.