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Cavitation burns

 
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Marco Flamingo



Joined: 09 Jul 2015
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City/Region: Seattle
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C-Dory Year: 2004
C-Dory Model: 16 Cruiser
Vessel Name: Limpet
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PostPosted: Mon Oct 23, 2017 7:20 pm    Post subject: Cavitation burns Reply with quote

These showed up right after I put on my new prop, and never got any worse, so I just let it be. But I'm wondering if it might be the symptom of something I should correct. It's mostly just the paint that's missing, but the underlying aluminum is rough, so it must have taken (or is taking) some of the metal off.

[img]http://www.c-brats.com/modules.php?full=1&set_albumName=Limpet&id=102_0156&op=modload&name=gallery&file=index&include=view_photo.php[/img]

I still have never successfully placed a picture in a post on this site. That has to be a record. It's the last picture in the Limpet photo album.

Mark[/img]
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Robert H. Wilkinson



Joined: 26 Jan 2011
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City/Region: Port Ryerse
State or Province: ON
Vessel Name: Romakeme IV
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PostPosted: Mon Oct 23, 2017 7:54 pm    Post subject: Reply with quote


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Da Nag



Joined: 24 Oct 2003
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City/Region: Port Angeles
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C-Dory Year: 2008
C-Dory Model: 16 Angler
Vessel Name: Taco
Photos: <a>Da Boats</a>
PostPosted: Mon Oct 23, 2017 9:34 pm    Post subject: Re: Cavitation burns Reply with quote

Marco Flamingo wrote:
I still have never successfully placed a picture in a post on this site. That has to be a record. It's the last picture in the Limpet photo album.


https://www.youtube.com/watch?v=IjPmIRvAPww

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thataway



Joined: 02 Nov 2003
Posts: 20779
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: thataway
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PostPosted: Mon Oct 23, 2017 10:41 pm    Post subject: Re: Cavitation burns Reply with quote

Marco Flamingo wrote:
These showed up right after I put on my new prop, and never got any worse, so I just let it be. But I'm wondering if it might be the symptom of something I should correct. It's mostly just the paint that's missing, but the underlying aluminum is rough, so it must have taken (or is taking) some of the metal off.
Mark


Mark--the change you made is the new prop... What is the difference from the old prop? Several things involved--turbulence from a transducer ahead of the prop, different configuration (cupping or rake) in the prop, prop too high, running at constant RPM where the cavitation is worse.

Here is a quote from Tom Clarke (Whose opinion I greatly respect) on THT
Quote:
"Let me straighten a few things out about cavitation and ventilation, which are two distinct phenomena.

Cavitation is NOT the formation of air bubbles. The bubbles formed are vacuums, or more precisely water vapor, not air. Cavitation bubbles are formed when the pressure is so low the water goes from the liquid phase to the vapor phase. Remember how water boils at lower temperatures at higher elevations? ....

The damage to a propeller blade occurs when these bubbles collapse actually eroding the stainless steel (or other metal) of the blade.

Ventilation is the sucking in of air from the surface or exhaust gases from the exhaust into the propeller and the dilution of the solidity of the water around the propeller with these gases."


So will it eventually damage the blade? Yes. Is it doing any harm to the boat or engine? Most likely not.

What can be done? Change props. Take the prop to a really good Prop Scan Shop, and see if they can change the cup....but this will most likely cost more than another prop--down the line.
You might try running at different RPM's than you are most of the time now--higher or lower.
If the engine is performing correctly--I would not change the height. But in some cases that may help.
If there is anything close to the prop causing turbulence, that can be moved further outboard--consider that.
What is the WOT RPM and GPS speed?
Personally if the boat is reaching its rated RPM at WOT, then I might see if changing the trim of the motor makes significant difference--but probably run with it, and buy a new prop down the line...The next prop may be fine...

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Bob Austin
Thataway
Thataway (Ex Seaweed) 2007 25 C Dory May 2018 to Oct. 2021
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Barry Rietz



Joined: 21 Jun 2006
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City/Region: Sierra Vista
State or Province: AZ
PostPosted: Mon Oct 23, 2017 11:48 pm    Post subject: Cavitation Burns Reply with quote

These props are cast from molten aluminum. The molten solution could have had some impureties that collected at the outer edge of that blade. Another possibility is the prop could have been removed from the tool before it was fully cured. Unless it occurs on the remaing blades, I would just keep an eye on it and not worry.
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Barry Rietz
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