what happens after dropping 2 sizes in pitch?

GlasV162

Petty Officer 1st Class
Joined
Jul 27, 2006
Messages
302
This may be a dumb question, but are there any symptoms (other than running at higher rpms) that a motor will show after switching to a lower pitch prop? I just switched from a prop that BARELY got my motor (99 Merc 150 efi) to the low end of the WOT range, to a prop that allows it to reach the high end (5600 rpms) very easily. Since then the motor has intermittently given off excessive smoke in the exhaust for a minite or so, and then clears up. I should also tell you that at the same time, I also ran a dose of Quickleen through the motor. Thanks!!
John
 

Bondo

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Apr 17, 2002
Messages
70,468
Re: what happens after dropping 2 sizes in pitch?

This may be a dumb question, but are there any symptoms (other than running at higher rpms) that a motor will show after switching to a lower pitch prop? I just switched from a prop that BARELY got my motor (99 Merc 150 efi) to the low end of the WOT range, to a prop that allows it to reach the high end (5600 rpms) very easily. Since then the motor has intermittently given off excessive smoke in the exhaust for a minite or so, and then clears up. I should also tell you that at the same time, I also ran a dose of Quickleen through the motor. Thanks!!
John

Ayuh,... The prop change is irrelevant to the smoke...
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: what happens after dropping 2 sizes in pitch?

Poor scientific method: You made two changes at the same time (prop and Quickleen). Thus, it is not possible to attribute the results to either, but usually a prop change would not result in smoking.
 

GlasV162

Petty Officer 1st Class
Joined
Jul 27, 2006
Messages
302
Re: what happens after dropping 2 sizes in pitch?

Yes, I agree--2 changes at once is not good. I'm still trying to figure out which prop to go with: the one that came with the boat when we bought it (SS 13x21) or the one we tried last week (SS14x17). The 21" pitch barely gets the motor to the low end of WOT, and the 17" pitch gets it to the high end of WOT range very easily. I could've easily hit 5600 rpm but didn't want to push it. In both cases, it was how the boat is used most of the time--my wife and me. Based on what I've read here, the 21" pitch is borderline "lugging" the motor, but the 17" pitch has the potential to over-rev. Any advice you have would be much appreciated. Thanks!!
 

Texasmark

Supreme Mariner
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Dec 20, 2005
Messages
14,544
Re: what happens after dropping 2 sizes in pitch?

I am going to slightly disagree with the above. You were on the verge of lugging your engine which can build up carbon deposits in the combustion chambers. By doing what you did, you unloaded your engine, hence cleaner running and you added a cleaner so you were seeing the effects of BOTH simultaneously. That's the way I see it. But I "ain't" the expert; just a long time casual user.

But I do agree that usually you do one thing at a time and get the results before moving on.

Mark
 

BoatDrinksQ5

Petty Officer 1st Class
Joined
Feb 11, 2012
Messages
377
Re: what happens after dropping 2 sizes in pitch?

Yeah i would think pretty similar - you got two things going on now. A prop that is letting the motor operate properly and that combined with something that cleans your engine up...will probably make it run a little smokey for a outing or two.

For the best i would think. If the smoke persists past the tank...i would wonder.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: what happens after dropping 2 sizes in pitch?

So the real question should be what happened to hole shot and top end speed with the change in pitch and the decarb operation.
 

GlasV162

Petty Officer 1st Class
Joined
Jul 27, 2006
Messages
302
Re: what happens after dropping 2 sizes in pitch?

Well, the hole shot was never an issue with the 21" pitch. It was always pretty quick, and pulled me at 180 lbs. up on 1 ski with no problem. I didn't ski with the 17" pitch, but I can only imagine it will be better all the way around. On top end speed, I lost about 8 mph, topping out at 50. With the 21" pitch, I could get up to about 58-59, but the boat would start to "chine walk" and I don't like that. I would do it just to get the rpms up, but that would only be for about 30 seconds and then I would back off before losing control. But even at that, I could barely get the rpms up to 5000-5100, with just me in the boat. The 17" SS prop I used was a loaner prop from our boatyard. I returned it today and gave them my findings and also told them I'd like to switch to an aluminum prop. They recommended a 14.5 x 19" pitch aluminum, which I purchased. The mechanic said the 19" aluminum prop should easily get me within the WOT range; the boat goes back in the water Friday, so I'll find out then. I'd rather use an aluminum for price and safety--if I hit a rock, I'd rather ruin a prop than the entire gearcase. Any comments / recommendations you have would be much appreciated. Thanks again.
John
 

stnick

Seaman Apprentice
Joined
Aug 27, 2005
Messages
44
Re: what happens after dropping 2 sizes in pitch?

Typically one size down ( 23 to 21 ) pitch will raise rpm by 3-400... 23 to 19 = 6-800 and on and on.
Stainless props are thinner blades and are much heavier than aluminum.
One inch of diameter, up or down equal 150 rpm.

Nick
 

NHGuy

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May 21, 2009
Messages
3,631
Re: what happens after dropping 2 sizes in pitch?

The 19 should be right at the proper WOT. Bet you are fine.
 

GlasV162

Petty Officer 1st Class
Joined
Jul 27, 2006
Messages
302
Re: what happens after dropping 2 sizes in pitch?

14.5 x 19" pitch is right on the money. Great out of the hole, 5500-5600 rpms at WOT and approx 55 mph. It's strange but it "feels" better running with the ability to hit the high end of WOT. Thanks guys.
 

mommicked

Lieutenant Commander
Joined
Dec 15, 2009
Messages
1,700
Re: what happens after dropping 2 sizes in pitch?

I'm sure your motor would say it "feels" better too, if it could talk :D
 

tysdad2010

Petty Officer 3rd Class
Joined
Apr 2, 2012
Messages
88
Re: what happens after dropping 2 sizes in pitch?

I got a 1985 Blackmax 150 on a 1986 18ft Ranger bassboat running a SS 23p prop 3 blade Im am thinking bout trying out a 21p to see if I will get a better hole shot and maybe better top end...right now I am running 49MPH at WOT with just me in it...What do yalll think a
21p should get me where I need to be? what about Diameter of prop


Im running around 5650 RPM's at WOT maybe even 5800
 

lil buggy

Senior Chief Petty Officer
Joined
Sep 24, 2011
Messages
766
Re: what happens after dropping 2 sizes in pitch?

ysdad2010,
the only way that going down 2 on pitch that would increase speed is if your rpms couldnt reach wot with the higher pitched prop.
that being said i think when you change the diameter and rake of the blade you could tweak more out of it,,,,, also having said that going down 2pitch will give you a quicker hole shot if all other things are equal,,,, rake diameter blades etc,,,,but you will lose top speed.
but im by no means a pro.
iboats has a prop selecter tool that could help maybe

http://boatpropellers.iboats.com/propeller-tutorials/rotation/?*******=274942970

and the part about how pitch changes rpms
http://boatpropellers.iboats.com/propeller-tutorials/rpm/?*******=274942970
Determining RPM's
Manufacturer's Recommended RPM (see image 1)
Check your owner's manual or contact your dealer to find the manufacturer's recommended RPM range.

Wide Open Throttle (WOT) RPM
For safety and efficient performance, make sure that your engine is running within the manufacturer's recommended RPM range. Matching the right prop for the load is the most significant factor of RPM adjustment.
info-rpm2.gif
Image 1
Effect of Prop Pitch on RPM (see image 2)
Changing the boat propeller pitch will increase or decrease the RPM's. Adjust your prop pitch so the RPM's fall within the recommended range. A 2" pitch increase (21" to 23" for example) acts like shifting a car gears up. It typically results in a 300-400 RPM decrease.
info-rpm1.gif
Image 2
Test for Maximum RPM (see image 3)
Using your existing boat propeller or a new propeller, make test runs to determine the maximum RPM and boat speed. Vary the trim angle for optimum performance.
info-rpm3.gif
Image 3
RPM Higher Than Recommended (see image 4)
If the actual WOT RPM's are above the recommended RPM range, install the next larger pitch propeller to decrease your WOT RPM. Re-test the WOT RPM.
info-rpm4.gif
Image 4
RPM Lower Than Recommended (see image 5)
If the actual WOT RPM range is below the recommended range, install the next smaller pitch propeller to increase your WOT RPM. Re-test the WOT RPM.

When you combine all these factors, you have the information you need to select the correct propeller for maximum performance, safety, and fuel efficiency.
info-rpm5.gif
Image 5

hope it helps
 
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