The Type XXI is a German diesel-electric attack submarine dating to the late Second Weltkrieg. It is the first of its kind to be designed for combat operations mostly underwater, recharging air for its engines. The Type XXI was designed in modular sections, allowing for mass-production at different shipbuilding facilities, for final assembly at a designated location. Its passive sonar, electric torpedo reloading systems, quiet engine and hydrodynamic hull were revolutionary at the time. A lot of modern submarine designs take inspiration from the venerable Type XXI; its modernity relative to its contemporaries has resulted in its continued use by naval forces around the world even in the 1960s. Thus, it is no surprise that the design has become popularised in Oceanyka, mainly used to raid cargo vessels at sea and engage larger warships.
As a submarine, the Type XXI is specialised at hunting down capital ships and cargo vessels, taking them out with a distance with its acoustic torpedoes. However, it is fairly weak to dedicated anti-submarine tactics and equipment.