"As long as the motor isn'
"As long as the motor isn't trying to push too much or too little boat, you can use some rule of thumb numbers.
A motor will burn 10% of it's max horsepower in gas at wide open throttle.
So your 115 would normally guzzle 11.5 gallons per hour if you open her up.
Every motor has a sweet spot which is advertised in tests as a rig's "cruise speed".
It is normally somewhere around the 2/3's throttle mark. It is actually a point where the motor has reached maximum timing advance but you still have throttle left. If you pull the "hood" with the motor not running and move your throttle, you will notice there is a point where the timing linkage stops advancing.
That is (normally) around the 4000 rpm mark give or take a bit.
At the "cruise speed" the motor only uses about 40-50% as much gas as at WOT. On your motor, I would expect 4-5 gallons per hour.
When you translate that into mpg, you will find that the numbers are even better than idle speeds where you burn less gas per hour but the distance travelled is obviously much less as well.
Those numbers are generally pretty close in real life to (thumb) numbers. Of course a lighter load will save a bit, and obviously heavier will burn a bit more.
My 140 horse is rigged to a pair of 40 gallon tanks. I don't go out and zip up and down the lake to show how fast I can get to the other side very much.
For cruising around, doing some fishing and travelling canals I find my average gas consumption is about 6 gallons per hour (in combined running) - so I get about 7 hours out of a tank - that tends to last all weekend because most of us have the motor shut off more than it's on
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