9 or 10 inch pitch?

[JC9.9]

Can anyone tell me what the best prop pitch is for my motor. Mercury 9.9 2 stroke on a 16 ft aluminum boat. Used primarily for duck hunting carrying 3 people, decoys, etc.. Im looking at the 9 and 10 inch pitch which would be better? Any comments on those or a different pitch will be appreciated. Im currently running a 7 inch pitch.

 

[BoobieBouncer]

Re: 9 or 10 inch pitch?

You might get a better response by posting in this forum:

Prop Questions and Topics

 

[steelespike]

Re: 9 or 10 inch pitch?

Please read the stickies at the top of the page.
we need base line performance.Present prop size,max rpm and speed.
A tinytach is reasonable,easy to install,gps excellent for speed.You shouldn't make prop changes without a tach.
hwsiii has a prop program he will need boat length,width, weight, motor gear ratio,typical load etc.

 

[hwsiii]

Re: 9 or 10 inch pitch?

How fast are you going now and do you have any idea of your RPM.
Thank you Steele.

H

 

[JC9.9]

Re: 9 or 10 inch pitch?

I currently have a tach and gps for speed. Max rmp's im reaching are 5400-5500, the motor is rated to 6000 max rpms max speed is 17mph. My boat is 16 ft aluminum 48 inches wide typically carrying 3 people with an extra 200 lb load. gear ratio is 2.33:1.

thanks for the response

 

[hwsiii]

Re: 9 or 10 inch pitch?

JC, something is wrong with the information you gave me, if I use those numbers it shows a -11% prop slip and that is not possible.

Prop Slip

H

 

[JC9.9]

Re: 9 or 10 inch pitch?

it is possible my gear ratio is wrong im still waiting on my manual to come in on my motor I was going by what someone had told me they thought my ratio was Ill check back when I get it figured out for sure
thanks for the info

 

[hwsiii]

Re: 9 or 10 inch pitch?

Yes, it is very possible. We will be waiting.


H

 

[Wee Hooker]

Re: 9 or 10 inch pitch?

Somebody jump on me if I'm missing something but if he's actually getting only 5500RPm on a motor rated @ 6000, I'm thinking he's got the green light to drop 2" from a 7 to a 5 pitch prop (assuming they make one that flat.) That should allow the motor to run much closer to factory spec/max output.

 

[hwsiii]

Re: 9 or 10 inch pitch?

Wee, if he changed from a 7" pitch to a 5" pitch he would gain 2,200 RPM and that would put him at 7,700 RPM. In lower pitches the differences in RPM changes gets very dramatic. Most people don't know that, so don't feel lonely.

Prop Change

H

 

[Wee Hooker]

Re: 9 or 10 inch pitch?

"Most people don't know that, so don't feel lonely"

Thanks for the info. I'd always read that 200 RPM/1" was the rule of thumb. Guess it makes sense that that # would vary at the extremes of functional prop range.

 

[hwsiii]

Re: 9 or 10 inch pitch?

Wee, that is what I was told since I was a kid too, I grew up with that knowledge. The actual formula is Present RPM * (Present Prop Pitch / New Prop Pitch) = New RPM.


H