There seemed to be some mystical curse/blessing that worked to slow down the development of high speed submarines in Axis navies during that period. The Japanese had boats that could do 19 knots submerged before the war started, but never did much with them. The later I-201 design looked so much like a Nautilus class boat that you almost have to wonder if the stories about copying and adapting the Walter hulls in the early nuclear boats were true, or just a case of the USN not wanting to admit they were ripping off a Japanese design instead. (A bit like the old story of the Zero being a copy of an American plane, when it was entirely a Japanese design and, at least in the area of the flight controls, several years ahead of anybody else.)

Americans didn't put as much effort into high underwater speed, which may have been influenced by the dearth of radar on Japanese escorts, and its presence on the submarines, giving the fleet boats the ability to stay surfaced most of the time. They were "advanced enough" and that was sufficient. If nothing else, they did have better heads than their enemies' boats.

J.T. McDaniel