Description
The key improvement in the Type XXI was greatly increased battery capacity, roughly triple the Type VIIC. This gave these boats great underwater range, and dramatically reduced the time spent on or near the surface. They could travel submerged at about 5 kn (5.8 mph; 9.3 km/h) for two or three days before recharging batteries, which took less than five hours using the snorkel. The Type XXI was also far quieter than the VIIC, making it harder to detect when submerged.
The Type XXI's streamlined and hydrodynamically clean hull design allowed high submerged speed. The ability to outrun many surface ships while submerged, combined with improved dive times (also a product of the new hull form), made it far harder to chase and destroy. It also gave the boat a 'sprint ability' when positioning itself for an attack. Older boats had to surface to sprint into position. This often revealed a boat's location, especially after aircraft became available for convoy escort. The new hull design also reduced visibility via marine or airborne radar when surfaced; whether this was a goal of the design or coincidence is still debated.
They also featured a hydraulic torpedo reloading system that allowed all six bow torpedo tubes to be reloaded faster than a Type VIIC could reload one tube. The Type XXI could fire 18 torpedoes in under 20 minutes. The class also featured a very sensitive passive sonar for the time.
The Type XXIs also had better facilities than previous U-boat classes, including a freezer for food.
Read more about this topic: German Type XXI Submarine
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