Plastic prop issue - SailNet Community

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  #1  
Old 05-07-2009
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Plastic prop issue

I have a customer with a Catalina 320. We replaced his Martec folder with a 2-blade, 15X11 Perfect Pitch plastic prop. When the owner took the boat out for a test putt, he noticed a loud "thrumming" resonance seeming to come through the transom at a certain RPM range. Inspection underway indicated no problems, nothing loose or vibrating in the drive train or stern of the boat. Thinking it may have been a cutless problem, he hauled the boat and had the yard manager look it over. Everything seemed fine. On the assumption that the problem was a tip clearance/vortex issue, the owner bought the "Extendo" version of the same prop (see photo), which moves the blades further aft and away from the hull. I installed it yesterday and to the owner's great dismay, the same problem occurred, but at a different RPM range and much louder now.

Talking to a local prop shop owner, he suggested it may be a harmonic problem or something similar (although he did admit to never having come across this issue with the Perfect Pitch props) and suggested a fix might be to file the leading edge of the blades down.

Has anyone else experienced anything like this with these props?

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Old 05-08-2009
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" a fix might be to file the leading edge of the blades down."
I'd be scared to do that, outright scared. If the prop maker did ANYTHING right, the odds are that they also put some effort into how they mold or shape the edges, and randomly attacking them with a file will just ensure the maker won't take them back. And no one else will really want them.

Thrumming resonance? Well, yeah, pulses of water shooting off the prop and hitting the hull. Heard of that with many "wrong" prop installations, either a matter of clearance, or balance, or always something very basic but simply wrong for that combination or prop and hull.

Has Perfect Pitch made any comment?
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Old 05-08-2009
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Quote:
Originally Posted by hellosailor View Post
Has Perfect Pitch made any comment?
Yes, they suggested he buy the "Extendo" model.
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Old 05-08-2009
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Old 05-08-2009
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How much clearance does the prop have on the boat??? Do you have a photo of it in-situ???
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Old 05-08-2009
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Why was the original prop changed, did it make the noise too?

If nothing else was changed but the props then I think that RULE 1 applies here and for some reason these props are just not suitable for this application.
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Old 05-08-2009
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Quote:
Originally Posted by sailingdog View Post
How much clearance does the prop have on the boat??? Do you have a photo of it in-situ???
No pix. The first plastic prop had about 3 1/2 fingers clearance. Didn't check the second one, but it's more than that.
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Old 05-08-2009
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Quote:
Originally Posted by timebandit View Post
Why was the original prop changed, did it make the noise too?
The original prop was a Martec folder. It was replaced because it was undersized and also not the correct pitch for the boat. Before the introduction of these plastic props, this issue did not exist on this boat.

It has been suggested to me that the plastic blades may be changing shape under load and causing the harmonics or resonance or whatever it is.
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Old 05-08-2009
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How is the alignment at the drive coupling?...

Before filing anything down, I would consider unbolting the coupling, turning the propshaft 1/3 or 1/2 a turn relative to the drive side of the coupling, and re-test. If the symptom changes, it's not the prop.

I suspect that the alignment of propshaft may be out.

Did you get paid to swap props twice?
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Old 05-08-2009
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Quote:
Originally Posted by eherlihy View Post
How is the alignment at the drive coupling?...

Before filing anything down, I would consider unbolting the coupling, turning the propshaft 1/3 or 1/2 a turn relative to the drive side of the coupling, and re-test. If the symptom changes, it's not the prop. ?
I will suggest this to the owner, but if the alignment was not as issue with previous props, why would it be now?

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Did you get paid to swap props twice?
Of course. And I'll get paid again to effect whatever remedy the owner decides upon. I don't work for free.
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