The key improvement in the Type XXI was greatly increased battery capacity, roughly three times that of the Type VIIC. This gave these boats enormous underwater range, and dramatically reduced the time spent near the surface. They could travel submerged at about five knots (9 km/h) for two or three days before recharging the batteries, which took less than five hours using the snorkel. The Type XXI was also much quieter than the VIIC, making it more difficult to detect when submerged.

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