I was playing with some numbers, and after about a year tracking engine hours and fuel-ups the "worst" calculation is 1.22 gph (actual) for my boat's engine (Volvo Penta MD-22P rated 57hp). It averages about 7.5-8 knots cruising speed in a 10k headwind. So using those two numbers I get 6+ nmpg. (Best case was 1.1 GPH).

It's pretty easy to find published numbers on power boats, but not sailboats. What kind of mileage does your boat get under power?