How to get real MPG?

[serpa4]

Yes, I know how to do it. Easy.
But what is the best way in a boat?
I assume fill up at a dock so that the boat is "Level". Run at my desired cruise for say 50 miles. Far enough to use a measurable amount of fuel. Doesn't do much to go use 1/2 gallon and then the variation in filling up could be 1/2 gallon.
When done, fill up at the same marina.
So, what is the best way to measure distance? Phone w/gps? Boats chart plotter?
Then filling up at the marina, I would like to totally top it off. That may mean some back flows and burps a lot to get a perfectly full tank. I'm sure they won't like that. I don't think that filling it up till it stops then say 2 more clicks etc is very accurate.
From what I have read, my MPG is too high. 2018 Venture/200hp suzuki, 100% full of fuel and water, packed for a trip.
RPM / MPH / MPG off suzuki dedicated MFD.
1500 / 8.18 / 4.8
2000 / 9.56 / 3.87
2500 / 14.3 / 4.36
3000 / 17.6 / 4.28
3500 / 22.9 / 3.58
4000 / 26 / 3.25
4500 / 30 / 2.54
Did not do WOT or go over 30mph.

[baref00tin]

I would say the best way to measure your engine performance is to make sure you have equipment that supports NMEA2000 and install a gateway to connect the two. This will give you real time engine stats and performance.

[C-Wolfe]

You could use portable tank where you can much more accurately measure your fuel consumption. I have a 12G that I sometime carry to give me extra range that I can connect directly to my fuel filter/water separator. So far I did it once, I started a track on my chart plotter outside the harbor on a run I knew I will burn more then 12 G in one go. I switch to the portable tank and ran the boat until I run out of gas, stop the track and did the math. Calm day, light load I do a bit over 4 MPG at 20 MPH, I should do it again since I never go out on such a light load.

[ssobol]

[digger]

I have a floscan in-line with fuel line. Shows gallons per hour, and also records total gallons used (resettable). Keep track with gps miles covered and divide by gallons used.

[jkidd]

You should be able to read it right off of the motors computer. All you need is a NMEA 2000 network and the Suzuki Digital gauge. It will read it right off of the fuel injection. Mine will be within a tenth of a gallon when I refill. It will give you gallons per hour flow, fuel remaining, mile per gallon, range to empty. You do have to have a way to read gps speed into the NMEA 2000 network. That can be done with a NMEA 2000 GPS antenna or reading it from an MDF display that has built in gps and NMEA 2000 builtin.

[serpa4]

[jkidd]

Show us a picture of your helm.

[jkidd]

This is one of my gauges that I can change any of the fields on it to read anything on the NMEA 2000 network. This one can have 10 pages of info.
You can also configure your MFD to display the same info.

[Foggy]

Micro-calculations for researching exact mpg on a perfect flat sea, known load,
etc is an interesting exercise leading to a possible disappointment. Day to day
differences in wind, sea state, current, load, even altitude at different locations
will be different from your carefully researched data, which, if followed precisely
for a specific trip, could make you come up short on fuel.

More practical is to know precisely where your fuel gauge(s) show an empty fuel tank.
Know this by running the tank(s) dry until your engine(s) stop and mark the
gauge(s) [or take/record a photo]. Refuel using mindfully anticipated jerry jug fuel.

Then used this time proven fuel macro-calculation for your daily nautical adventures:
1/3 of a tank to get there
1/3 of a tank to get back
1/3 of a tank for an emergency

Aye.

[thataway]

I have put in flow meters in almost all of my boats. Unfortunately all of their motors were too old to get the information off of the engine's computer.

Foggy gives you excellent advice. You need to know what your fuel flow is at that specific time and on the total of that run. There are so many variables, such has current, wind, tide and seas, which will make a large difference in your MPG or range.



Above is the fuel consumption of "Thataway" the C Dory 25 I last owned. it was made by a previous owner, but I confirmed that it was accurate. At 3000 RPM I was getting about 2.5 nmpg at 8 nautical miles per hour. At 17.5 statute miles per hour & 4500 RPM, I was getting about 3 statute miles per hour.

The numbers posted by serpa4 . seem a little off. I am not sure why, but something is wrong in the way they are being measured? Yes, different 4 stroke engines, different hulls, and conditions; but getting 4.28 mpg at 3000 RPM/& 17.6 miles per hour (I have to assume statute miles, not nautical). Is the boat super light-one person minimal fuel and water etc? The C Dory line are sensitive to weight, as well as windage. Is it possible that current or tail wind etc--or accidental nautical vs statute? How was the mileage measured? Generally with the 200 hp outboards the C Dory 25 gets slightly better mileage than the Venture 26--but it is pretty close.

The other factor is that you are getting 30 mph 4500 RPM, and we don't know your WOT either in RPM or speed. What is the loading of the boat? If these numbers are true--then very light boat, and perfect environmental conditions.

In these boats I have compared the fill up to fill up and they will agree with an average fuel consumption or total fuel used as measured by the flow meter. But it is not easy to get the exact level of fill up with each attempt. The other factor, is the way I boat, it would be a very difficult day to run 50 miles at a constant RPM and constant conditions. I can't think of many places where you can do this.