Need 5.7LX fuel flow data for cruise planning

[Have Fun Today]

I'm planning some longer cruises for next summer on big lakes with fewer fuel stops available. This summer I'm keeping a log of fuel used vs time on the engine, the idea is to form a crude set of fuel planning charts so I have an idea of how far I can go. I understand factors such as wind, current, water temp, boat loading, etc will affect the data so I'll keep it conservative.

My question is this: Is there some fuel flow data already compiled so the rest of us don't have to do this on our own? I have a '97 Merc 5.7LX Alpha One driving a 23.5 foot Chaparral with cuddy.

Thanks in advance!

 

[Zackman]

Re: Need 5.7LX fuel flow data for cruise planning

You may want to try boattest.com. 97 is a little old for their database, but you may find something similar. They usually include fuel consumption numbers in their tests. You may also have some luck if you seach archives from the numerous boating magazines.

 

[QC]

Re: Need 5.7LX fuel flow data for cruise planning

Yeah, I'm with Zack. Find data there from other 5.7 Carbed engines. Get a couple of comparable boats. My guess is that you will find that all of them get around 3.5 MPG at cruise. I am a specs nut and they usually come in very close at cruise RPM. The lightest boats with MPI are usually best, heaviest with carbs the worst, but still between 3.4 and 3.8 MPG at cruise for 23 footers . . . I know you want GPH too, and you can always get that from the data as well. In fact almost all of them get their best economy between 25 and 30 MPH, so there again is your data . . .

 

[tommays]

Re: Need 5.7LX fuel flow data for cruise planning

I was out with My fuel flow meter yesterday ( refilled the tank today and it was spot on )

My 3.0 got as high as 5.5 MPG and as low as 1.2 MPG (apparently you use more fuel than you think idling in and out of a channel at the ramp )

I found very small things like a little more chop in the water or small increase in wind speed and direction could change it buy 2 MPG

And a more comfortable ride seemed to result in worse MPG

All in all i think without a fuel flow meter it is a crap shoot as to how far your going on a tank as i found minor changes made a pretty dramatic change in the tank range

Tommays

 

[QC]

Re: Need 5.7LX fuel flow data for cruise planning

My 3.0 got as high as 5.5 MPG and as low as 1.2 MPG

What MPG did you get around 25 MPH?

 

[tommays]

Re: Need 5.7LX fuel flow data for cruise planning

You could get up towards 5+ MPG at 25MPH but wind and sea conditions and the trim necessary to make the best ride at that speed could knock it down to 3.5 MPG

triming the bow up a bit for a smother ride always resulted in worse MPG

In general we don't have much smooth water here so that 5.5 MPG is a very brief window as soon as you push through some swells you were down to 3.5

And in the end the the real world burn that day (52 miles ) was 3.4 MPG

Last week 85 miles and much calmer water and it was at 4.25 MPG


Tommays

 

[Have Fun Today]

Re: Need 5.7LX fuel flow data for cruise planning

Great info, thanks. Sounds like a fuel flow gage might be a good investment, than I can do the math and compute MPG real-time.

 

[QC]

Re: Need 5.7LX fuel flow data for cruise planning

triming the bow up a bit for a smother ride always resulted in worse MPG

That's counter-intuitive to me. I know I lose 5 MPH when I jam the bow down with both down trim and down trim tabs, but I absolutely gain speed the other way i.e. bow up. I guess the proper way to gauge trim affect on efficiency would be to cruise at say 25 MPH with the bow down, trim in. And then cruise at that exact same 25 MPH with the bow a little bit up, and then the bow as far up as that speed allowed, again all points EXACTLY at the same speed. I would be VERY surprised if the best economy was not with the bow up some, if not as far as the prop etc. would allow . . . Now if you start digging a hole, then no way, but at an "efficient" bow up trim position it would seem to have to be more fuel efficient than with the bow jammed down . . .

 

[Have Fun Today]

Re: Need 5.7LX fuel flow data for cruise planning

Assuming throttle position (and thus fuel flow) remain constant, then anything that increases speed, like adjusting trim, should increase the MPG:

miles per gallon = (miles per hour) / (gallons per hour)

 

[tommays]

Re: Need 5.7LX fuel flow data for cruise planning

I said getting high it enough to be more comfortable when pounding through chop hurt the MPG

My boat is small and does not have a lot of water line length,it has a very fine line between trimmed up and trimmed up to far

I did not say any bow rise would hurt MPG BUT was surprised by how small and amount of extra trim increased fuel use

You would never notice it without a flow meter




Tommays

 

[riverjet502]

Re: Need 5.7LX fuel flow data for cruise planning

Just a thought here...

Why not buy a cheap hand held GPS. Fill your boat up with gas and go for a ride? Say for an example you like to cruise at 3600 rpm's. Turn your GPS on and go for a ride at your favorite cruiseing speed. The GPS will tell you more acurately how fast you are going then your boat's dream on speedo, plus you will know exactlly how many miles you went, when done with your ride, fill up your boat with gas and figure your milage like you do on your car. This will give you a very good idea how long your tank will last in your real world conditions. PLus a GPS is fun to play with. Plus an excellent tool to use when doing performance upgrades. I never head out on the water without mine.

 

[Have Fun Today]

Re: Need 5.7LX fuel flow data for cruise planning

Got one already and I'll be doing just that. I'm starting to see that fuel planning isn't as exact as it is in airplanes. I'll compile some data specific to my boat for planning purposes and always pad it to be extra conservative for all of the variations in wind, current, wave height, etc. Then I'll be installing a fuel flow gage so I can adjust for peak mileage during the trip. The goal is for fuel used to be less than fuel planned for.

Thanks for the inputs!