I took a 300 mile on the 103 Lost Pearl this past week and I felt this would be a good time to check my fuel mileage. After running my speed 9.5- 10.5 and 300 miles and one night of anchoring out running a generator the entire trip I came up with 1.19 MPG. I have two generators." My main is a 30kw and my smaller one is 7.6kw. I only run the 30kw when anchoring, cooking or running all the airs. The 30kw drink 1.75 at full and the 7.6 drinks .53 at full so witch would you run. I would love to know do boats vary much in fuel mileage. Most seem to get around a mile per gallon whether larger or small. I live inland and this holds true up here but then again I'm kinda the one of few over 65 feet around here.
displacement hulled motoryachts don't vary fuel milage much on size. Planing hulls vary greatly depending on size.....A 55' MY usually burns 2.5 GPM, a 75' Motoryacht as much as 7GPM..... etc etc....
The key at slow speed is to realize that even a single knots will make a huge difference When you say 9.5 to 10.5mph, that can be a 60 to 75% difference l. As soon as you hit hull speed and go just beyond it your fuel burn goes up drastically. It is extremely obvious if you have flow meter On the 70 footer I run, from 10kts to 11kts flow goes from 18gph to over 30 . Push to 11.5 to 12kts and you re better off on plane doing 22kts on 85gph So whatever you do, try to keep it just below hull speed (1/4 to 1/2 kts below)