The atom bomb was not available when the first research on the walter drive was going on (1939). The type XXI would have been quite devistating, but one with the sprint ability the walter had, would be much more of an advesary.
Remember, the PBYs could not hear under water, they were in the air. They could not see down more than 100 meters, if that. The Walter boat would have dissapeared for the airborne scout.
It takes a long time for a deph charge to drop to 400 ft, which the type XXI was found to exceed with not so much a creak while under tests by the allies following the war. By the time the bomb got to the deph, the walter would have been long away from the area.
Why do you all think the type XXI, became the basis for early american cold war subs, and most if not all soviet subs until the second generation nuclear subs? Because it was way ahead of its time, and would have been a very scary advesary for the allies to deal with.
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