Modifying prop

[jeffkrol]

I have 2 props for our 20' pontoon. 1) 14-11 3 blade and 2) 13.25-13 4 blade prop.
RPM (from dash tach, so take it for what that's worth) w/ the 3 blade is about 5300.
from the 4 blade about 4700
Engine range is 4500-5500 so both seem to be within range. Optimum is 5000.

Now I called our local prop shop and apparently they suggest a different prop if I wanted to change anything. I have read that props can be modified but unsure (these are aluminum btw)

NOW IF I wanted to attempt it and could find someone to do it would it be possible to modify either the pitch or the diameter of the 4 blade (Solus Rubex if that matters) I like the cruising range response of the 4 over the 3 though this could be psychological, and it does seem more fuel efficient..

One problem common to both but seemingly more pronounced on the 4 is ventilation at full throttle (seems mostly trim related though but I'm still in the playing stage of finding out the cause).

I'd prefer zero over-speed/lost power issues w/ full throttle but the engine is as deep as it can go and don't have anywhere to start on solving this issue anymore. I was actually hoping the smaller dia. 4 blade would have fixed this (it didn't). Just replaced the trim gauge as well so unfortunately I can't compare old blade trim position to new blade trim position.. can't be sure the gauges read the same.

Back to the orig. question: My gut feeling is I could reduce the dia of the 4 blade to say 13".. Or is it best to change the pitch to 12? Shooting in the dark here so any suggestions based on what I wrote would be appreciated.

 

[craze1cars]

Re: Modifying prop

A good prop shop can change pitch. Usually up to 2 inches either direction, maybe less. Random link...I do NOT know if this is a "good" prop shop, but they discuss the concept: http://www.rundquist.com/how_change_pitch.htm

Is it worth it on an aluminum unit? They're comparatively cheap to buy. I suppose that all depends on what your prop shop charges for the repitch service.

Did you not ask your prop shop if they could do this? If they say, "no", then I submit this is not a good prop shop...a good prop shop is HAPPY to repitch, and does it regularly. But if they simply SUGGEST no, it might be more or equally economical to just buy another aluminum prop to replace your current aluminum unit, then their opinion may have financial merit in your favor.

Recent personal example: I had 2 identical stainless 4 blade props...a 21 and a 19. The 21 spun 4800 rpm, the 19 spun 5200 rpm. I wanted my motor to spin 5,000.

So I had my 21 repitched to a 20, and happily I found it now spins exactly 5000...and handles/grips the water BEAUTIFULLY with very very low slip numbers. While it was in, I paid for a few nicks to be cleaned up, and the additional upcharge from my local prop shop was $10 per blade for a repitch....so I paid $40 to have mine adjusted.

So yes, repitching can work, and I often recommend it...for stainless props...because they're expensive to buy.

I'm not sure about changing diameter....I have never heard of this being done. Most prop shops just use pitch blocks to beat the pitch in a different direction, and leave the diameter alone...