Russ,
In general, the lower then number of turns or windings in a motor, the faster is runs. It also means that current is used much faster so that the run time from a particular sized battery is shorter than for a motor with more windings ('there ain't no such thing as a free lunch', kind'a deal). Is there a formula for figuring all this stuff? Probably, but I have no idea what it is, except that it is NOT simple. It would have to take the shape/type of hull and everything in the drive system into account. I'm just too lazy! The 'easy' way is to look around and find out what 'everyone' else is using and how it performs. (Not exactly the answer you were looking for, right?)
Have fun...
- 'Doc