Prop Cavitation

Dazzajohm

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Just started cleaning one of my props and noticed some cavitation damage.

Should (can?) this be repaired or is minor damage ok to just live with?

I’m not entirely sure whether this is recent or old. The props were covered in scale and there was no indication of it until it was polished. Surely if it happened recently it would have been noticeable before cleaning, as how can the metal of the blade be damaged without effecting the scale too?

Whilst I understand what cavitation is, what causes this to happen? No issues with performance of the boat or anything like that to indicate something with the props is awry.

I’ll try and attach before and after pics. 5F67E305-F166-4811-BA47-56A124DEBDFA.jpg89AA58B9-CEE1-4B63-8BCF-16E7458863D2.jpg

Cheers
D
 

BruceK

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Look for something forward of the blades that might be causing an obstruction to the water flow. Otherwise the blade(s) may have a too high a curvature / camber at that particular point. Is it only on the one blade?
 

QBhoy

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I’m only looking on my tiny screen of the phone here, but are you sure it’s from cavitation and not been from a rope round it ? Any signs on the shaft gear, p bracket etc ?
 

QBhoy

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Oh. I just read that those are before and after pics. I see what you mean now. Is it the white coloured bits you mean ?
 

jfm

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Your two pictures show opposite sides of the prop and cavitation damage appears usually on the hp side not lp side. Assuming it's a right handed prop then your shiny picture looks like cavitation damage. Your dirty picture should never have cavitation on the face showing in your picture

What to do? If there is nothing obstructing flow and you don't want to run slower your only choice is to have the prop fettled, or get a new prop of similar dimensions, and hope that changes to the flow eliminate the cavitation. Or just live with it and keep checking the props. It can get worse/snowball as the prop pitting induces more cavitation. But yours doesn't look particularly bad so I wouldn't lose any sleep over it.

QB it's the pitting in the shiny pic, not the white bits in second pic.
 

MapisM

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Assuming it's a right handed prop then your shiny picture looks like cavitation damage.
Your dirty picture should never have cavitation on the face showing in your picture
Reasonable assumption ref. the shiny pic, considering that there is "RH" stamped on the hub... :)
Btw, even if to a lesser extent, that pic shows signs of cavitation also at the blade's root.
But all considered, I'd agree with your conclusions.

I'm a bit puzzled by the other pic though, because the blades shape suggest that it's a LH prop - i.e., not the same one as in the shiny pic.
And if that would be true, the pic would show the HP side potentially prone to cavitation, even if it might well be visible only after a good clean.
 

Dazzajohm

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I’m only looking on my tiny screen of the phone here, but are you sure it’s from cavitation and not been from a rope round it ? Any signs on the shaft gear, p bracket etc ?

It's funny you should say that, but when the boat was lifted for survey late last year, there was some rope wrapped around the shaft between the prop and the P bracket on that side. But the surveyor reported no damage or corrosion to either prop. I'm due down the boatyard again tomorrow so I'll have to check the P bracket for any sign of corrosion
 

Dazzajohm

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Sorry guys, I should just clear something up about the pictures I posted - the clean one is the RH prop and the dirty one is the LH prop. I never took a picture of the clean one before I started work on it, but essentially as they were both in the same condition when they came off the boat I just used that one to demonstrate what they looked like dirty.

I haven't started working on the LH one yet, but have just been assuming that will show signs of cavitation once I do - but from what you guys are saying I guess it's possible it could just be limited to that RH prop. I'll clean up one of the blades on the LH one this evening after work and report back.

Another thing to add which has just come to mind is the surveyor reported an issue with the electrical connection of the anode on the shaft of the prop in question. Apparently the resistance should read 1ohm but was registering 13.4ohm. Could it be galvanic corrosion instead of cavitation? The report gives a timeframe of 6 months to repair (which is being done while the boat is out the water).

Cheers
D
 
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jfm

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Ok that clears up the left/right hand props.

From pic, I would be very confident that is cavitation not galvanic corrosion. It's on the hp side, with a cavitation-consistent pattern. It won't necessarily appear on the other prop. It's not a bad amount but is worth fixing. I'd give the props to a prop specialist and ask them to tweak. Depending on whether you feel slightly long or short ask them to reduce/increase pitch or cupping very slightly (often, after a while owning a boat and filling with clobber, your props feel long).

Also you can buy plastic tabs to glue on blades to help on this. Never done it myself but might be easy fix. Google them maybe.

You need to do something to alter slightly the water flow.
 

Dazzajohm

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Ok that clears up the left/right hand props.

From pic, I would be very confident that is cavitation not galvanic corrosion. It's on the hp side, with a cavitation-consistent pattern. It won't necessarily appear on the other prop. It's not a bad amount but is worth fixing. I'd give the props to a prop specialist and ask them to tweak. Depending on whether you feel slightly long or short ask them to reduce/increase pitch or cupping very slightly (often, after a while owning a boat and filling with clobber, your props feel long).

Also you can buy plastic tabs to glue on blades to help on this. Never done it myself but might be easy fix. Google them maybe.

You need to do something to alter slightly the water flow.

After doing some more cleaning just now, I believe it is cavitation too. Not only is the evidence of it on the other side of the prop too, now I’ve cleaned it up but I also cleaned off a blade on the other prop to have a look and that’s the same too.

Just going to do some googling now and see where I might send them how much it might cost for a re-pitching or whatever it is that’s required.

Cheers
D
 

BruceK

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Thanks for this. Any idea how much a re pitch is likely to cost?

I dont know. Best to call. It was a bit of an education walking into their workshop. They do a lot of commercial I gather because it was row upon row of some pretty impressive sized propellers. Any way they repaired 4 x stainless steel props for me that I had grounded out like a right idiot and rolled the blade tips like butter. Bill worked out at 200 per prop (including rehubbing) and when they came back they looked new. I really was taken back by just how good they looked because when I first saw the damage I thought bin job and went and bought new for a 3k pop. Damn I kicked my heels.
 

Dazzajohm

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I dont know. Best to call. It was a bit of an education walking into their workshop. They do a lot of commercial I gather because it was row upon row of some pretty impressive sized propellers. Any way they repaired 4 x stainless steel props for me that I had grounded out like a right idiot and rolled the blade tips like butter. Bill worked out at 200 per prop (including rehubbing) and when they came back they looked new. I really was taken back by just how good they looked because when I first saw the damage I thought bin job and went and bought new for a 3k pop. Damn I kicked my heels.

Wow, £3k for one new prop! That’s quite something.

I’m just trying to do some reasearch online and I think my 20x25’s are the wrong prop for my Fairline Corniche although from what I’ve read there are a range of other sizes people are using. I’m going to have to speak to my engineer tomorrow as it seems pointless to just repair the props only for the same thing to happen again. Having said that I’ve not had any issues with the performance of the boat so if it’s gping to cost me a small fortune for new ones I might just repair what I’ve got and shove them back on !
 

MapisM

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After doing some more cleaning just now, I believe it is cavitation too. Not only is the evidence of it on the other side of the prop too, now I’ve cleaned it up but I also cleaned off a blade on the other prop to have a look and that’s the same too.
Hang on, do you mean that you've got a similar pattern also on the opposite side of the blades?
Normally, cavitation affects only the high pressure side (that is, the aft side).
After cleaning the LH prop, did you find a similar pattern? And on which side(s)?
 

Portofino

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Hang on, do you mean that you've got a similar pattern also on the opposite side of the blades?
Normally, cavitation affects only the high pressure side (that is, the aft side).
After cleaning the LH prop, did you find a similar pattern? And on which side(s)?

Cavitation is a LOW pressure thing .Normally occurs on the back side which is low .It can occur on the front ( shaft side ) but it’s not as common .
As the pressure lowered the water boils and the bubbles of gas implode and the released energy takes chunks out the metal surface .
 
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