Better gas mileage

[Btrent1999@gmail.com]

Have a 13-19 prop not getting very good mileage was reading on this site or some where that a prop change might get the mpg up ? Thanks

 

[foodfisher]

Re: Better gas mileage

Try the prop forum with more details.

 

[jerryjerry05]

Re: Better gas mileage

First you need???? The motor,size of the boat. The rpm's at wide open throttle.
Do a compression/spark test.Gotta make sure the motors running right?
Usually any Force/Chrysler should be getting about 5200-5500 rpm at wot.
Also what's the load on your boat? People,coolers,full tank(how many gallons)dogs, ski's,anchor(s) and misc.junk??
My tank is 100 gal and I don't use or need that much so I don't fill it up. Full that adds 800#
Condition of the prop? New old? Nicks or excesive wear? My dad had a 65hp johnson.His performance was starting to go. He measured the prop. It's aluminum and had worn down almost a 1/4in.after almost 20 years in salt water.
Post your results,by then someone will move this too the prop forum.

 

[steelespike]

Re: Better gas mileage

Marine fuel use is measured in gallons per hour.Rule of thumb gph is 10% 0f the hp at wide open throttle.
ex. 100 hp will use "about" 10 gph at wot. The faster you can get from here to there at a given throttle setting gets you your best mileage. called" best cruising speed".
Usually the best cruising speed is in the area just on plane to about 1000 rpm higher.
The only way to get a really accurate fuel use is with a flow meter.Some of them will calculate mpg.
If you get us your present prop size and light load max rpm and speed by gps we can give you an idea if your setup seems to be working.

 

[blackhawk180]

Re: Better gas mileage

What jerryjerry said. There are some very qualified folks on this forum that can help but not without the info jerry is asking for.

 

[MH Hawker]

Re: Better gas mileage

Rule one,, boats like a lot of fuel.

 

[QC]

Re: Better gas mileage

Rule number two. Unless there is something dramatically wrong you're not going to see swings of more than 5%. Very hard to justify buying props for that money. If you don't have a spare, then it is the perfect reason to try something else.

Ultimately, as others have noted, the best thing you can do is get rid of weight, trim her properly, and make sure she's running right. Proper trim can make a significant difference in fuel use as well.

 

[jerryjerry05]

Re: Better gas mileage

My boat has twin 85's It's a 21ft center console.
I never carry too much junk unless the women folk go along to do some sunning.
Coolers 1 for fish and the other for food.
Not a lot of other stuff.
Standard prop was a 17. I thought I needed more speed and better performance????
I changed to a pair of 19p My fuel economy went DOWN!!! 15-20%
I only got 3 mph more and couldn't really see any difference in performance.

 

[smokeonthewater]

Re: Better gas mileage

sounds like you need to put the 17's back on

 

[Btrent1999@gmail.com]

Re: Better gas mileage

sounds like you need to put the 17's back on

Sorry guys it's 120 on a quantum 17 1/2 bass boat runs about 44mph gps @5200-5400 rpms runs strong starts easy seems to be ok just sucks gas I,ll try to check gph thing just trying to get more gph lol thanks

 

[jerryjerry05]

Re: Better gas mileage

I changed the props back after a week of wasting gas. 1989
You might loose a few MPH but your economy will go up with a 17p

 

[steelespike]

Re: Better gas mileage

Maybe I missed something. Is it a 120 Force? and what year? Do you know the gear ratio?

 

[emilsr]

Re: Better gas mileage

QC is right; you may see a 5% variance, then again it may be 0%.

I've done a lot of prop testing over the years and its hard to measure the mpg difference in props so long as they're all running in the recommended rpm range. There's about a hundred other things that affect fuel use more than your prop.

If you really want to chase that demon, reducing prop slip will have the biggest impact. Usually a higher pitched prop will sllip less but not always.

 

[steelespike]

Re: Better gas mileage

Maybe I missed something. Is it a 120 Force? and what year? Do you know the gear ratio?
Not sure of the gear ratio and it's difficult getting an accurate result with a spread of 5200=5400 rpm.
Using 5400 I get 6-12% slip depending on the gear ratio, acceptable numbers.
At wot a 120 would use about 12 gph. Water sports would use a lot of fuel.
Some things to check; Does the boat respond to up trim?
What is the compression?
A bad fuel pump diaphram will leak gas into the crank case.
Is the timing set right?
Is the bottom clean and smooth?
Is the antivent plate(just above the prop) about even with the bottom of the transom?

 

[Frank Acampora]

Re: Better gas mileage



At about 5400 RPM at full throttle, you are propped correctly. Your Force will run quite nicely at high cruise. When you bang open the throttle, you will get a lot more noise, use a lot more fuel, and go only a couple of MPH faster. Back off WOT if you want fuel economy. This 55 eats 6 gallons of gas an hour. The 125 (badged 85) sucks down 15 gallons of gas an hour at WOT and 60 MPH. They would do significantly better if I backed off a bit, but that is not what I use these boats for.

To see best cruise position, take off the cover. Look at the cam on the bottom carb. Note the "finger." If you set the throttle to the point where the finger just starts to move the carb roller, the timing will be advanced almost all the way but the carbs will be open only about 1/4 of the way, giving best fuel economy.

 

[jerryjerry05]

Re: Better gas mileage

Like Frank says at the rpm's your getting your prop is the one you need. Just stay off the throttle. I run about 4100 27mph and that gives about the best speed and performance for my boat. I can run it at 5500 and only get 5mph more and burn way more gas.
I go about 12-18 miles out in the ocean and cruise around another 5+mi. looking for fish and come back. I use about 20 maybe 25 gal a trip.

 

[ENSIGN]

Re: Better gas mileage

All you have to do is slow down a little.

 

[Btrent1999@gmail.com]

Re: Better gas mileage

Thanks guys I keep the rpms down just seems to burn a lot of gas I guess that's part of the game I,m not in a big hurry just fishing