Prop size question

[ponyman460]

Currently, the prop that is on the boat says 14X17 stamped on it.

I am looking on this site, and the choices are 14 1/4 to 15 1/2 inch diameter.

Is there any reason to increase the diameter of the prop? any performance gains? will it hurt performance by having too big of a prop?

OUtboard is 1989 Johnson 140 on an 18ft Robalo center console.

Thanks!

 

[Mi duckdown]

Re: Prop size question

Got to know your correct RPM's for that motor and gps speed before you do anything

 

[steelespike]

Re: Prop size question

Prop selection is made to get the best performance and remain within the recommended rpm range.
So your prop selection would depend on your present lightly loaded wot rpm and gps speed.
You can't just arbitrarily change prop size.Increasing pitch lessens power but if the motor can turn it within the range it
may be faster. Decreasing pitch increases power and rpm, result would depend on the previous performance.
Generally as pitch increases diameter goes down, as pitch decreases diameter goes up.
Diameter is a secondary consideration If the prop is intended for your motor and application the diameter should be right.
A 1" pitch reduction will add about 150-200 rpm.and increase power.I'm guessing the rpm range is 5000-6000.
It develops its power at 5500.You should try to confirm the numbers.

 

[ponyman460]

Re: Prop size question

Got to know your correct RPM's for that motor and gps speed before you do anything

before the prop damage, WOT was 5500 and GPS speed was 32-35mph.

Prop selection is made to get the best performance and remain within the recommended rpm range.
So your prop selection would depend on your present lightly loaded wot rpm and gps speed.
You can't just arbitrarily change prop size.Increasing pitch lessens power but if the motor can turn it within the range it
may be faster. Decreasing pitch increases power and rpm, result would depend on the previous performance.
Generally as pitch increases diameter goes down, as pitch decreases diameter goes up.
Diameter is a secondary consideration If the prop is intended for your motor and application the diameter should be right.
A 1" pitch reduction will add about 150-200 rpm.and increase power.I'm guessing the rpm range is 5000-6000.
It develops its power at 5500.You should try to confirm the numbers.

I am already at 5500, and these motors seem to like that rpm based on the reading that I have done.

Thank you!

 

[jestor68]

Re: Prop size question

I assuming your motor is the 25" shaft version(V-6 gear case, 15 tooth spline.

If you are happy with the performance of your 14 X 17 prop; you can certainly put another 17 pitch prop on it that is larger diameter, as long as it fits your motor.

If it's only slightly larger dia(1/4"), you won't notice any performance difference. Bear in mind that an inch of diameter is roughly equal to an inch of pitch. If you were to go to a 15 X 17 for example, you could expect to see your rpm drop the same as adding an inch of pitch(150-200 rpm).

Larger diameter equals more blade area. More blade area equals more thrust per revolution, and usually means stronger acceleration to plane and a little more speed, provided the motor has the power to turn it.

Since you're turning 5500 rpm, you could run a 15 X 17 on there and realize better over all performance.

Your motor's rpm range is 4500-5500; with the 140 hp @ 5000 rpm. So you can afford to drop a little rpm and still have full power to turn the larger diameter prop.

This all presupposes that you have the model with the V-6 gear case.

 

[steelespike]

Re: Prop size question

My 89 aftermarket service manual indicates 140 @ 5500. No range listed but typically 500 below and 500 above developed horsepower
rpm.

 

[ponyman460]

Re: Prop size question

I assuming your motor is the 25" shaft version(V-6 gear case, 15 tooth spline.

If you are happy with the performance of your 14 X 17 prop; you can certainly put another 17 pitch prop on it that is larger diameter, as long as it fits your motor.

If it's only slightly larger dia(1/4"), you won't notice any performance difference. Bear in mind that an inch of diameter is roughly equal to an inch of pitch. If you were to go to a 15 X 17 for example, you could expect to see your rpm drop the same as adding an inch of pitch(150-200 rpm).

Larger diameter equals more blade area. More blade area equals more thrust per revolution, and usually means stronger acceleration to plane and a little more speed, provided the motor has the power to turn it.

Since you're turning 5500 rpm, you could run a 15 X 17 on there and realize better over all performance.

Your motor's rpm range is 4500-5500; with the 140 hp @ 5000 rpm. So you can afford to drop a little rpm and still have full power to turn the larger diameter prop.

This all presupposes that you have the model with the V-6 gear case.

I have the v4 if that makes a difference

 

[jestor68]

Re: Prop size question

I am referring to the size of the gear case. The 20" shaft models are listed as using the "V-4" gear case, while the 25" shaft models use the larger "V-6" gear case with a different gear ratio.

The largest diameter props for the V-4 gear case would be 14"; while the larger V-6 gear case models can use props up to 16" diameter.

 

[jestor68]

Re: Prop size question

My 89 aftermarket service manual indicates 140 @ 5500. No range listed but typically 500 below and 500 above developed horsepower
rpm.

No WOT rpm range listed? A search has about as many citing 45K-55K as those saying 5K-6K. I found a 135 listed at 5000-6000, but the 140 wasn't on that list. Go figure.

It appears that 5500 is a good rpm to shoot for either way.

 

[jestor68]

Re: Prop size question

The only 14 X 17 aluminum prop I see is a Michigan listed for the V-4 gear case.

Evinrude's props are listed as 13- 1/4 X 17(V-4 gear case) and 14-7/8 X 17(V-6 gear case). Other makers's props are similar sizes.

Now I'm guessing that your motor is the 20" shaft model with the V-4 gear case. In that case, 14" is the largest diameter that will fit on that gear case.

 

[ponyman460]

Re: Prop size question

thanks a lot guys! I ended up picking up a used lower unit the other day that came with a Stainless 13X19 prop on it. I am going to try that (since it does not cost more) and if it does not work out I will go back. And yes, you are correct :v4 gearcase with 20" shaft