Re: Outboard motor size
Dories traditionally used either natural ballast (300# of codfish) or a dory stone to balance the boat, so I'll guess you'll want to put some ballast forward -- depending on many other factors.
I'd sure guess that you'd get 20mph with a 15hp 4-stroke, depending on prop size, boat weight when wet and loaded, water conditions, etc. But that's a guess -- I'm sure someone here can be more specific. Any chance of you borrowing an outboard for testing before purchase?
"The enemies of reason have a certain blind look."
Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.