We traveled over 800 miles this past summer on a 25' sailboat and over 95%
of that was under power with a 6 hp 4 stroke Yamaha. The mileage per gallon
varied depending on wind and current but it averaged out at over 15 MPG or
about .43 gal per hour. I mostly ran the boat at or just below 6 MPH which
was about .7 MPH below WOT. Our friends on a 26' with an 8 HP diesel did
even better by about 20% I have nothing to back this up, but I think I'd
have done even better with my 7.5 HP Honda motor which may have allowed me
to back the throttle off even more and to have driven into a few really
strong head winds better. I opted for the 6 because it was almost new and
the Honda is 20 years old.