Editing U-boat (section)

===U-boat developments===
[[File:Type XXI section.jpg|thumb|A prefabricated segment of a Type XXI U-boat. The cross-section shows clearly the '8'-shaped hull, where the lower part was used to store large batteries hence the name of 'ElektroBoot']]
In 1940 the Germans made successful tests with the [[German submarine V-80|V-80]] experimental submarine featuring a new type of propulsion: on the surface it used the classic Diesel engines but submerged it used a revolutionary [[hydrogen peroxide]] [[air-independent propulsion|air-independent]] [[hydrogen peroxide#Propellant|propellant]] system designed by [[Hellmuth Walter]]. With this Walter-turbine, a U-boat could achieve underwater speeds of more than 20 knots, much more than the 4-knot cruising and 6-knot maximum speed of electrical engines powered by batteries. Four more experimental [[German Type XVIIB submarine|Type XVIIA]] U-boats with Walter turbines were built and tested, but the Germans could not put this design in use for a big frontline U-boat.{{Sfn|Blair Vol2||pp=10-11}}{{Sfn|Costello & Hughes||pp=284–285}} Unlike a classic U-boat that could recharge its batteries with the diesel engines, once a Walter U-boat had consumed its hydrogen peroxide propellant it could not submerge anymore. The Germans did not possess the resources and plants to produce sufficient hydrogen peroxide to operate a fleet of Walter submarines. Despite these limitations, 24 frontline Type XVIIB coastal submarines were ordered, but only three were built and none were operational before the end of the war.{{Sfn|Blair Vol2||p=312}}

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.{{Sfn|Breyer||pp=7–13}} Based on the design of an Atlantic Walter U-boat, the [[German Type XXI submarine|Type XXI]] "{{lang|de|Elektroboot}}" was designed to boost submerged performance. Smaller [[German Type XXIII submarine|Type XXIII]] coastal {{lang|de|Elektroboote}} were also taken into production. These {{lang|de|Elektroboote}} were mass-produced, with prefabricated segments constructed at different sites and then assembled at the bigger shipyards.{{Sfn|Blair Vol2||pp=312–313}}{{Sfn|Costello & Hughes||pp=285-287}}
[[File:Submarine snorkel.png|thumb|The {{lang|de|Schnorkel}} mast and air flows]]
After the [[German invasion of the Netherlands]] in 1940, the Germans captured some Dutch submarines equipped with a [[Submarine snorkel|''Schnorchel'']] (snorkel) but saw no need for them until 1943. The {{lang|de|Schnorchel}} 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.{{Sfn|Breyer||p=8}} 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 {{convert|8|kn|km/h}}, lest the device snap from stress. Whilst running submerged with the {{lang|de|Schnorchel}}, the [[Gruppenhorchgerät]] was useless because of interference with the noisy diesel engines. But the {{lang|de|Schnorchel}} allowed the old Type VII and IX U-boats to operate in waters which were previously denied to them.{{Sfn|Costello & Hughes||p=284}}