Normal fuel economy?

[tjello327]

Hello forum,
I am in the final testing stages for my 1986 125hp force. It is mounted to a 17 ft. Larson Bowrider (pretty deep sides & made from fiberglass.) The engine runs smooth at all rpm ranges. I just welded up the prop but it is still about one inch too small on diameter but all the blades are equal now. Before repairing the prop, the boat would only go about 30 mph and sucks the gas down. I would get around a 2 mile lake about 4 times on 6 gallons of gas running about 3/4% to 100% throttle. After repairing the prop and adding a 200lb. person in the boat, it now runs at about 38mph and still has about the same fuel economy. I just installed a new tach and its max rpm is about 4600-4800. Is this normal fuel economy? I havent checked timing at wide open throttle under load yet, but did check it at idle which it was at the zero mark.

Thanks,
Tom

 

[MWBROWN]

Re: Normal fuel economy?

Rule of thumb is about 10% of horsepower per hour.
Or for you 12.5 gallons per hour at WOT.
Translated to MPG, about 4 MPG is average from what I have read.
You should really look for a used good prop as size and balance are very important. A vibrating prop can ruin your lower unit.

MIke

 

[tjello327]

Re: Normal fuel economy?

In that case, seems like I have below normal economy. What things should I be looking at to improve this. Could my prop be causing this much by being a little too small?

Thanks.

 

[MWBROWN]

Re: Normal fuel economy?

Yes, too small won't drive per revolution as it is supposed to.
Also, the pitch is important.
My 85 hp came stock with a 13" x 19" pitch.
I am guessing yours should be about the same.
You could look at a prop seller to see what pitch you should be running,
or try the prop forum on this site.
Also, mercruiseparts.com will show you what your engine came with stock,
if there is only one choice.

Hope this helps.