We all know that the Type XXI had a LOT more battery endurance than the Type VII’s.

But how far (how long) could it go as a function of speed?

Assuming a new Type XXI with fully charged batteries submerged and held constant speed, how many hours could it go:

@3-knots = 164-hours

@5-knots = 68-hours

@10-knots = 11-hours

@15-knots = 2-hours

Realistic maximum underwater speed – not quite 16-knots for about 45minutes. According to Roessler, [1] U3507 in experimental configuration (With many flooding slots closed flush) reached 16.5 knots for about an hour. It was calculated, that the resistance of the flooding slots amounted to approximately 28% of total form resistance. The above speeds/time with a theoretical full battery and a clean hull – not exactly combat condition after two months on patrol.

[1] Eberhard Roessler’s impressive publications “The U-Boat” and “Anatomy of the Ship: Type XXI U-Boat”