Dealer reccomends 17 or 19" prop. Which one for best MPG???

[mmurray70]

I have a 19.5' fiberglass boat (probably around 2300lb with motor gear and passengers) with a 19" pitch prop on a Yamaha F90. The manufacturer says I can use a 17" or 19" prop on this combo but he usually uses 17".

Im running at 5400 rpm top speed right now and am happy with performance. Just wondering how the 17" would compare. Espesially in terms of gas mileage, would the 19 be any better or worse then the 17"?

 

[steelespike]

Re: Dealer reccomends 17 or 19" prop. Which one for best MPG???

At wot your motor will use about 9 gph (gallons per hour). Whether pushing a houseboat or a runabout.Mileage is achieved by the speed of the boat.
To make it very basic the best speed at the least throttle while not forcing the motor out of its rpm range will get the best mileage.I believe your 19 produces the best wot speed for your present setup.Do you have a gps speed reading?Assuming the 17 is the same model prop
it will have a quicker hole shot will likely stay on plane a little better with less fiddling.
Slightly slower at wot.Being slower at wot doesn't necessarily mean less mpg at other slower throttle settings.
Measuring the difference for mpg would likely require a flow meter.
The best practical cruising speed will usually be somewhere just on plane to about 1000 rpm higher.Again a flow meter would likely be required to measure the difference

 

[Silvertip]

Re: Dealer reccomends 17 or 19" prop. Which one for best MPG???

Fuel economy on a small boat is sort of like a dog chasing it's tail -- what's the point? You will likely not save enough money over the life of the boat to pay for the new prop. So here are a few things to consider. Without looking at the Yamaha specs, I think that Yamaha would recommend propping the engine so you see 6000 RPM at wide open throttle rpm. Fuel consumption is based on throttle opening. While the 19P may deliver slightly better economy once you are on plane, getting on plane would take longer and use more throttle opening to do so than with the 17P and with that prop, you would see closer to the upper end of the RPM band and hence less engine lugging (engine-speak for stress). So the gain is offset a bit by hole shot. Next is the load you normally carry. You may find that on plane, the 17P may require LESS throttle opening at cruise than the 19P just because the engine loading is less with the 17P. While this is all wrapped around "physics" the only way you can determine what situation is best is, as was suggested, is with a fuel flow rate monitor. And even then, how you use the boat "most of the time" is what you need to prop for.

 

[John_S]

Re: Dealer reccomends 17 or 19" prop. Which one for best MPG???

Agree with Steelespike. Hooking up a flow meter will show at what rpm for each prop where the ideal rpm/mpg range is. While the mpg may be close between the two props, in general, the higher pitch will be at a higher speed. So, at approximatly the same MPG, you get there a little sooner.

If you don't already have a spare, get a 17P, and will have the option to run that on "heavy" load days.

PS: I didn't check your particular engine, so assume you are in the correct wot rpm range.

 

[Master Chief 09]

Re: Dealer reccomends 17 or 19" prop. Which one for best MPG???

If I have read correctly the recommended rpm for your motor is 5-6000 rpm. Although you are in the range it is on the low side of the spectrum. I would verify that the motor is mounted high enough on the hull so that you are not getting any loss due to drag through the water. Once you have accomplished that I would change props until I was able to reach very close to the top rpm listed for my motor when trimmed out and running WOT, I would also do this with my boat loaded as close to where I would be normally or most of the time.

When you reach the optimum WOT RPM loaded the way you normally will, you are allowing the motor to be a efficient as possible throughout all RPM ranges and will get longevity and performance and probably the best MPG as well. Lots of testing and tuning to fine tune an outboard to it's potential, but when you do reach that, it will be much easier to drive and much more cost effective and the motor will last longer since it will not be lugging and building up carbon deposits from not burning all the fuel.

 

[BaileysBoat]

Re: Dealer reccomends 17 or 19" prop. Which one for best MPG???

5400 is a bit low. Going to make oil. 17P would probably be best.

 

[mommicked]

Re: Dealer reccomends 17 or 19" prop. Which one for best MPG???

I agree w BaileysBoat. I have a carbed F90 and it WILL make oil at prolonged lower RPMs W any pitch prop. You need to be able to turn it near 6000 to help it make less oil or get it out of the crankcase. I have a 17 and it turns over 6000, but my boat is half the weight of yours. You may find you need a 15 to get the RPMs closer to 6000. But im no expert. Check Yamahas outboard websight, Performance bullitins have all kinds of boat/prop combinations w the performance specs. of different props. It may list your boat or a similar one.