Here is some info for the type C (see page 14 for speed)
http://www.uboatarchive.net/Manual.htm
For patrol speeds - most think that 7knots surfaced is the best speed.
If you then dive to schorkel depth - your speed will change to 4 to 5 knots - I guess this is the same engine revs so may be the best speed.
I would use the same test for decks awash - set speed on surface (7knots) set decks awash and check speed.