The Type XXI series of U-boats developed by the German Navy during World War 2 were revolutionary on many levels and were looked upon by the service as its war-winning design - intended to take back control of the vital Atlantic Theater.
The Type XXI was designed specifically for lengthy submerged operation and this centered on high-powered electrical machinery with density cells built directly into the hull.
This allowed the boats to produce more power submerged than the vessel could create when on the surface - a decided advantage when compared to the diesel-electric-powered counterparts of the day with their closed-cycle turbine engines.
Due to its heavily-dependent electric design, the Type XXI series was also known as the "Elektroboot" ("Electronic Boat").