Upon reflection, I’m probably wrong on that 50-100 MPG number. I guess I measured something wrong (starting gas level) or over estimated my distance or something. I need to get a better number for that. My number was based on a test 20 mile run I did on a slow river, 10 miles up and then 10 miles back down, and I thought I only used about 1/4 of a gallon, which would be 80 MPG. I knew it was a small sample so I fudged it to 50-100, thinking that had to cover it. But now that I look it up online, it looks way out of line.

In my defense, the number sounded kind of reasonable if you think of the big differences between auto MPG, motorcycle MPG and dirt bike MPG. Also, just subjectively, when I go out fishing in my zodiac I never use much fuel - I think I use more gas cutting the grass than running the outboard in an average summer. It seems like you can run around forever on a fraction of a gallon of gas. But then, I rarely travel 80 miles when I'm out fishing a lake.

Searching on the internet, I’m seeing numbers like 4 MPG and 6 MPG – the highest is 10 MPG. I'm sure I'm doing better than that - that’s all with bigger boats and bigger engines and bigger propellers and running flat out with boat on a plane. I’m running a new 6HP Tohatsu on an inflatable Zodiac with 400 lbs of boat and contents and usually about half throttle, which gives me a speed of 5-7 MPH. I haven’t yet found any numbers for a setup like that. I also have “lifters” (fin-looking devices) installed at the bottom of the transom and an inflatable keel, which improves the performance. All those things should work for me, although probably not enough to reach anywhere near 80 MPG. Maybe 20 MPG? 30 MPG?

I did find this chart for a 25HP motor on a bigger, but still lightweight zodiac:
RPM MPH GPH MPG
1000 2.4 0.1 47.00
1500 3.5 0.1 34.50
2000 4.5 0.3 14.83
2500 5.3 0.5 10.60
3000 6.0 0.8 7.50
3500 6.9 1.2 5.96
4000 14.2 1.4 10.52
4500 18.2 1.6 11.38
5000 21.3 1.8 11.83


I wonder if those low RPM numbers are a quirk of the testing instruments or if they are real – 47 MPG at 2.4 MPH? I had picked out the half-throttle point as what seemed most efficient, in terms of speed versus how much effort the engine seemed to be making judging by the noise it made. Maybe the secret is to throttle down a little from there.

Bottom line, yes, range matters a great deal, and I purposely favored a small outboard and lightweight boat on the theory that fuel economy is much greater. If that's not true, then it might change my approach somewhat. If it is true then my choices are okay, but I still need to get a better handle on the exact numbers so I can plan better.