Is this prop cavitation damage?
- Thread starter Don S/V ILLusion
- Start date
A quick search showed most images of cavitation damage at the prop tips and edges not at the hub. This makes sense because the tip speed is greater than the hub speed and the damage is caused by the water boiling to a vapor.
It's in the wrong place for cavitation but it does look serious. If it's a hollow in the casting, how far does it extend ?
I replaced the prop and just sent it to a metallurgist for examination out of curiosity. His only comment thus far is “there Is no one area of a prop more prone to cav damage than any other area”. if Anyone cares, I’ll post the result of his assessment.
Please do post. I can remember my studies of cavitation way back when and they sure as hell don't agree with this guys thoughts. The actual cause of the erosion of the metal is the rapid implosion of small bubbles (caused by low pressure) as they leave the low pressure area and enter a higher pressure area. One of the most evident cases of cavitation is found in a pronounced clicking sound from inside the casing of a centrifugal pump operating at low suction pressure.
You sure as hell are not generating nor collapsing any low pressure vapor bubbles where you show the defect in your prop. The speed is simply too slow at that point. I'd ask for my money back based on his statement.
Take a quick look here for examples of corrosion :
propeller cavitation - Google Search
www.google.com
I realize it is the internet, but look at the many photographs and let them speak as to where you are likely to find cavitation erosion on a prop.
I agree that this is a casting problem, not damage from use. I should imagine a good prop shop could easily fix and rebalance the prop.
Oh dear.
He is a metallurgist, not a fluid flow guy (chemical engineer that has seen hundreds of cases of cavitation damage). Yes, the area of damage can vary quite a bit, depending on the specific case, but there are patters, and the nature of the pitting is also characterisic.
Casting flaw. I've seen that on pump impellers too.
Unfortunately, I can’t award a prize to whoever guesses correctly but I appreciate the comments.
The guy to whom I sent the prop works for the NTSB and has a career worth of experience diagnosing similar metal failures in boats.
NIB - before anyone gets catatonic about government misappropriation, they use these type of failures in training.
The guy to whom I sent the prop works for the NTSB and has a career worth of experience diagnosing similar metal failures in boats.
NIB - before anyone gets catatonic about government misappropriation, they use these type of failures in training.
I take it all back.
I looked through a bunch of old inspection images of mine, and a few had cavitation damage along the hub that looked very much like that, but they so had severe damamge in other areas and they werre SS, so not one-to-one. I still say it is very unusual to have damage there and not also at the tips. But after reviewing my images, I think it is probably not a casting flaw, and I withdraw my first comment.
Now, working out the cause is going to be more dificult. I would guess there is something on the shaft that is upsetting the flow.
This video would suggest that this is not universally correct. It may be true for a properly designed prop but you can get cavitation at the root if the root has too much depth or sweep which will lower the pressure on the upstream side.
Cavitation has very distinct morphology. Un fortunately your photo does not have good enough resolution to really determine if it is, but from what I can see it does not look like cavitation. I would need better images to give you an opinion. But would very much love to look at the report you get.
For sure share if possible.
dj
For sure share if possible.
dj