We put on a lot of miles this summer: sail, power only and combined. I don't have the exact numbers here at home, but I calculated a season average of 0.47 gal/hour on our Yanmar 3GM30F, pushing a fat 34' cruising hull, heavily loaded, at 5.5 knots, with 1.6:1 reduction gear at an average of 2800 rpm, and a 13x13 fixed 3-blade prop. I figure that yields 11.7 nm/gallon.

For interest, we put on a new prop this season in an effort to increase potential speed. At our past cruising rpm (2500), the new prop made no appreciable difference in speed, but at 3000, we could add a knot (to 6), yet never exceeded 0.5 gal/hour. With the old prop, we would have burned over 0.6 gal/hr at 5 knots if we were lucky. I conclude the new prop adds efficiency, and my mechanic advisor applauds the higher rpm. But there are so many variables in this analysis!