Given your settings in Commander, it's probably "sugar in the tank" as you put it. Should be pushing 18 knots in good weather.
EDIT:
Also, Immelman is right about fuel consumption. I don't recall what the nominal cruising speed should be, but "balls to the wall" in a Uboat might run you out of fuel before you can make it back to base.
Oh yes, and welcome aboard Subsim, DeMeza.