Type XXI submarines is a class of diesel–electric U-boats built for Nazi Germany's Kriegsmarine from 1943 to 1945 during the Second World War. The concept of Type XXI submarines emerged in 1943 when it became clear the Type VII and Type IX still being produced, had become obsolete and the designed successor Type XVIII still faced blocking problems with its new propulsion system. 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. As they incorporated many batteries to increase the time they could spend submerged, they were sometimes referred to as Elektroboote in the German Navy. They were equipped with a snorkel, which allowed them to cruise at periscope depth on their diesel engines, and recharge their batteries without having to surface. The design included many general improvements as well: much greater underwater speed by an improved hull design, power-assisted torpedo reloading and better crew accommodations.