Changing prop pitch vs diameter

[XcellR8]

I'm trying to gain a better understanding of the effect of changing the prop pitch vs the diameter. From what I've been told decreasing the prop pitch by 1" or decreasing the diameter by 1/4" will result in an 200 rpm increase at WOT. Having said that a 12 3/4" x 21 prop should run at the same WOT rpm as a 13 1/4" x 19. How differently would these two props perform? Is it better to change the pitch or diameter to raise rpms?

 

[Silvertip]

Re: Changing prop pitch vs diameter

It is better to change PITCH to change rpm up or down. Changing prop diameter by 1/4 inch will not change rpm by 200 rpm. Changing pitch by 1-inch will generally change rpm by 200 rpm (give or take a little). Even that statement is not always true because a 21P prop from manufacturer "A" may be more or less efficient than a 21P prop from manufacturer "B" and therefore rpm increase or decrease may be more or less than 200 RPM. If you look props sold here on the iBoats site you will notice (generally) that as prop pitch increases, diameter decreases slightly and vice-versa. Outboards and I/O's are designed to accommodate a specific diameter prop with about 3/4-inch or so variation. So you couldn't put a 15 inch prop on an engine that normally requires a 13-1/2 to 13-3/4 inch prop as it would hit the AV plate or the torque tab. Read this:

http://forums.iboats.com/showthread.php?t=158083

 

[XcellR8]

Re: Changing prop pitch vs diameter

Thanx ... that was good information! Is there a minimum amount of clearance you should have between the tip of the blade and the A/V plate? The reason I'm asking is I picked up a 14" x 19 OMC aluminum 3 blade prop for use as a spare for my 1994 140 HP Johnson V4 but I'm now wondering if the diameter is a bit too large?

The prop it came with is a 12 3/4" x 21 and the "stock" prop for this motor as listed in the manual is a 13" x 19 (the boat is a 94 Glastron SSV170 which weighs about 1900 lbs). The tach reads about 5000 rpm at WOT with the 12 3/4" x 21 prop but that wasn't with the motor trimmed all the way out (trimmed all the way out I'm guesiing it might hit 5200). I was looking to raise the R's a bit which is why I went with a 19 pitch but I didn't realize that the diameter was an 1 1/4" larger until I got it home.

 

[studlymandingo]

Re: Changing prop pitch vs diameter

As long as you have at least 1/4" of clearance with the A/V you should be fine.​

 

[XcellR8]

Re: Changing prop pitch vs diameter

Thanks! 1/4" is the amout clearance I have.