Hello everyone. Myself and a buddy have just purchased a 31 Fountain CC with twin Merc 250 EFI’s. We left Port Royal Sound and went out to the Betsy Ross and drifted a few hours. The boat was totally full of gas when we left (220 gallons). When we got back to the marina and topped off, it took a lot more fuel than we expected. We figured we were getting a little better than half a mile to the gallon. We have changed props since then, amd went to a higher pitch, with about a 6mph gain in cruise speed. We went from 39mph at 4200rpm, to 45mph at 4200rpm. This prop change was done by a knowledgeable, reputable marine mechanic, who is loaning us these props to try for a while.
We have not been out again since, just inshore running.
My question is, does anyone have a similar boat/motor combo that can tell me what fuel consumption should be close to? I don’t mind admitting, we are fresh water boat owners who have fished inshore a lot over the years, and are stepping up to offshore fishing! We have been told by several people that we should expect around or slightly over 1mpg with this boat.
Sounds to me like you were over propped from the start. I’m not sure the RPM range of those motors, but I would think they should be turning upwards of 5500 rpms. Those speeds you posted were at WOT? If so you’re really lugging those motors while cruising.
Just over 1 mpg sounds about right. I do not know that exact rig but comparing to a similar rig with same size yamahs gotta be somewhat similar. You really need to get a flow scan for that fuel system. It would allow you to really dial in your best cruise RPM/speed/fuel burn. A very small cost with a big ROI.
I had a 25’ Downeaster pilot house with a 10’ beam. When I bought it it had twin merc. 150s and after a year I replace with a pr. of 225 EFIs Boy did they love gas! Less than 1 mpg at 3200, nothing but trouble with the mers. sold it and bought a Bert with twin 350 hp EFI GMs that get a little over 1 mpg at 3200 and a lot better ride and AC.
What props were on it?
What distance was run?
How much fuel was burned?
Cruising or running?
1/2 a mile a gallon is poor…at least a mile to the gallon.
Those engines(what year) are old school and thirsty.
Props make a lot of difference. What did your mechanic put on now?
I’ve run one before and they fly.
Get it propped right and you’ll be a lot happier…and less poorer.
Find out max rpm and find the correct pith/diameter/blades.
those 1st generation fuel injected motors were not much better on fuel than the old carbed motors.
increasing your cruise speed isn’t necessarily a good thing, if you cause your top end rpms to be reduced to much.
your WOT rpms should be 5400-5500, and your cruise speed will be what ever it is.
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