The Walter U-boats had very large hulls in order to store the fuel for submerged propulsion. Once it became clear these Walter U-boats would not be operational in time, the Walter U-boat hull design was reused with a different approach: the space for the hydrogen peroxide tanks was used to store much larger batteries. With the much increased battery power U-boats were also able to reach much higher speeds and endurance when submerged. Based on the design of an Atlantic Walter U-boat, the Type XXI "" was designed to boost submerged performance. Smaller Type XXIII coastal were also taken into production. These were mass-produced, with prefabricated segments constructed at different sites and then assembled at the bigger shipyards. thumb|The mast and air flows After the German invasion of the Netherlands in 1940 the Germans captured some Dutch submarines equipped with a Schnorchel (snorkel), but saw no need for them until 1943. The was a retractable pipe that supplied air to the diesel engines while submerged at periscope depth, allowing the boats to cruise submerged on diesel engines and recharge their batteries. It was far from a perfect solution: problems occurred with the device's valve sticking shut or closing as it dunked in rough weather; since the system used the entire pressure hull as a buffer, the diesels would instantaneously suck huge volumes of air from the boat's compartments, and the crew often suffered painful ear injuries. Speed was limited to , lest the device snap from stress. Whilst running submerged with the , the Gruppenhorchgerät was useless because of interference with the noisy diesel engines. But the allowed the old Type VII and IX U-boats to operate in waters which were previously denied to them. Finally, Allied radar eventually became sufficiently advanced that the mast could be detected.
The Walter U-boats had very large hulls in order to store the fuel for submerged propulsion. Once it became clear these Walter U-boats would not be operational in time, the Walter U-boat hull design was reused with a different approach: the space for the hydrogen peroxide tanks was used to store much larger batteries. With the much increased battery power U-boats were also able to reach much higher speeds and endurance when submerged. Based on the design of an Atlantic Walter U-boat, the Type XXI "" was designed to boost submerged performance. Smaller Type XXIII coastal were also taken into production. These were mass-produced, with prefabricated segments constructed at different sites and then assembled at the bigger shipyards. thumb|The mast and air flows After the German invasion of the Netherlands in 1940 the Germans captured some Dutch submarines equipped with a Schnorchel (snorkel), but saw no need for them until 1943 The was a retractable pipe that supplied air to the diesel engines while submerged at periscope depth, allowing the boats to cruise submerged on diesel engines and recharge their batteries. It was far from a perfect solution: problems occurred with the device's valve sticking shut or closing as it dunked in rough weather; since the system used the entire pressure hull as a. buffer, the diesels would instantaneously suck huge volumes of air from the boat's compartments, and the crew often suffered painful ear injuries. Speed was limited to , lest the device snap from stress. Whilst running submerged with the , the Gruppenhorchgerät was useless because of interference with the noisy diesel engines. But the allowed the old Type VII and IX U-boats to operate in waters which were previously denied to them. Finally, Allied radar eventually became sufficiently advanced that the mast could be detected.