ABSTRACT
By mid-1942, the ubiquity and increasing effectiveness of Allied anti-submarine counter-measures had impelled Dönitz to press for the technical means to employ submerged U-boat operations for the remainder of the war. Conversion of existing types with the schnorkel was only a short-term palliative because of their low underwater mobility. What was needed was a U-boat with high underwater speed and endurance. This, it seemed, could be achieved by accelerating the development of novel submarine types powered by the Walter turbine, which could be run submerged, powered by the decomposition of oxygen-bearing hydrogen-peroxide fuel. High submerged speed was achieved by sacrificing surface performance and optimizing the streamlined hull for underwater travel. However, as the technical difficulties with the system became more apparent, it was soon realized that they would prevent this type becoming operational in the near-term. Development was therefore started on a hybrid U-boat, combining the Walter-boat’s streamlined hull, with a powerful, but conventional, diesel-electric propulsion train and a very large battery, capable of being recharged while submerged by using the schnorkel. This was the Type XXI U-boat.1
