Below a certain depth there is a rather dramatic increase in propulsion efficiency. This is at the point when the water pressure becomes high enough to suppress cavitation. (Those bubbles you always see in movies.) On the surface, a lot of the effort is wasted forming the bow wave and wake, and the screws cavitate, so that a 6' pitch screw maybe moves the boat only 2-1/2" instead. Deep enough, screw efficiency can increase to very close to 100%, and water flows tightly along the hull, without forming energy-sapping waves. One way a boat can tell when it's reached this depth is that revolutions will suddenly increase, and the speed will jump.

At least is theory, this should have worked for u-boats as well, but this effect happens only at considerable depth, and most u-boats didn't get deep enough to notice it. (At least, not on purpose.)

Schnorchel speed is limited by, among other things, the strength of the mast. In the old German boats anything much above 6 knots could start to bend the mast. The fact that the schnorchel itself produces a wake, which can be seen from the air, also limits speed.

J.T. McDaniel