Cavitation Burn

[fishdady]

I just bought a Turning Point aluminum prop (LE1423) for my Johnson 150. 4th time out, I noticed dime sized spots in the middle of each blade. After calling TP tech line, I was told it was cavitation burn. The skeg has taken a pretty good hit (previous owner) and I cleaned it up, best I could, impeller and complete seal replacement, and repainted. My question is, cavitation burn is something I don't know. I don't doubt the tech. just wonder how it peels the powder coat on the blades. He did explain bubbles from the skeg, just don't quite grasp it. An explanation, from the Gurus on here, would be appreciated. Thanks in advance.

 

[Scott Danforth]

cavitation burn is an issue, those of use with DP drives know it well. cavitation is bad no matter if you are talking props or hydraulics. cavitation is caused from such a drop in pressure that the water boils and forms air bubbles. this isnt bad, however the bubbles then create the issue.

basically think of an air bubble hitting your prop - the air bubble collapses and it implodes when it hits. when this happens, it basically explodes little pieces of your prop with it.

 

[fishdady]

So this is something that needs to be addressed as soon as possible, otherwise, my prop is toast, correct? Thanks for the quick response!

 

[walleyehed]

Yes, and occasionally the wrong design of prop will show similar symptoms. Cavitation "can" eventually erode through the blade. Is it still doing this after fixing the skeg?

 

[steelespike]

I have 4 al. props from a 65 Merc 500 on a 57 18 ft Crestliner utility These are all merc props 11 and 12" pitch. The oldest prop is white
to give yopu an idea how long it has been around this setup. It was used for years and it has fairly severe cavitation burn. Narrow strips of missing paint
And fairly severe erosion of the metal.No structural issues just a cosmetic issue The other 3 props by age start from burn marks slightly less than the oldest prop ranging down to one single dot in the paint of the least used prop. The pattern developed exactly the same from least to most.
I feel that cavitation burn is a cosmetic issue and I was told years ago that it can be an indication that a prop is very efficient.

 

[fishdady]

Didn't mean to mislead you guys, but my plan was to get a skeg guard to cover it up, guess what, they don't make one for this engine, 1980 johnnie 150, so I didn't fix the skeg. Next question, I can get a skeg pro which will clean up the leading edge. Now the TP tech said the leading edge is the culprit of these bubbles. Do you think that will work. Guess you should know, this engine is on a 17 1/2 Hydra Sport bassboat. Also, these spots didn't show up until the 4th run on the prop, but, I was by myself and it ran the fastest so I was pretty much just trying to see how fast she'd go. Again, thanks for your help!

 

[Outsider]

I feel that cavitation burn is a cosmetic issue and I was told years ago that it can be an indication that a prop is very efficient.

That's like saying galvanic corrosion is cosmetic. Disturbed paint is cosmetic, metal erosion is quite a bit different ...

 

[fishdady]

Okay, appreciate your help. Think I'll let it go until next winter and tear this L/U down and have a new skeg put on. Thanks, again, all, Fishdady.

 

[Chris1956]

Fish, I had a new skeg welded on my Merc, without tearing it down. The welder was familiar with that particular kind of welding issue and was careful to limit the heat. I never had an issue after having it welded.

I run a TP on my '98 Johnny 150HPV6. I never have any cavitation issues, although my skeg is intact, and the boat is much lower performance than your bass boat. Let me know how it works out with the cavitation. Do you hear the motor speed up, when it cavitates?

 

[steelespike]

I feel that cavitation burn is a cosmetic issue and I was told years ago that it can be an indication that a prop is very efficient.

That's like saying galvanic corrosion is cosmetic. Disturbed paint is cosmetic, metal erosion is quite a bit different ...

Galvanic corrosion is much more sinister. The 150 might be able to generate enough erosion to ruin a prop But will it be in a season or 10 years later when the
prop may need replacement anyway.Is this erosion on the working side of the blade or the underside?
It will be interesting to see if the skeg repair solves the issue.

 

[fishdady]

I should have fixed it while I had it down, my bad, but, you learn from mistakes, right? The whole thing was, I bought this engine last year, naturally did all the right checks, compression, spark, fuel. Could only get 4600 rpm's out of it. Decarb got up to 5000. Carbs rebuilt, new base gasket, L/U reseal, naturally impeller. Had an old OMC 21" prop that had a crack at the base. Put on the TP 23" now I'm up to 5300 RPM @ tad over 54(GPS) TP seemed to wake the old girl up. Now this. My concern was flex in the aluminum prop, making powder coat flake. Now I'm into implosion, corrosion, erosion.(LOL) Anyways, it's the working side that spots are on and will probably fix the skeg when I go on vacation( next month I'll take it off and to the weld shop) I'll keep y'all in mind and let you know what happens. Almost forgot, with the OMC on, cavitation would start not long after I started to trim her up, with the TP, she grabs pretty good almost all the way up. I'm sure I could make her brake, but, haven't been there yet. Rambled enough, you guys are the greatest and really appreciate your help. Thanks again!

 

[H20Rat]

I
I feel that cavitation burn is a cosmetic issue and I was told years ago that it can be an indication that a prop is very efficient.

no, and no... I've seen props destroyed by cavitation in a season. And cavitation is actually a huge waste of power! It is an indication you have a serious problem with the entire system. Creating hundreds/thousands of vacuum bubbles per second takes a lot of energy, energy that is NOT being used to move you forwards.

 

[Frank Acampora]

How high are you running the engine or how much trim are you using? Excessive height or trim can lead to ventilation ( which is different than cavitation) but can itself lead to cavitation.

I have a 10 foot tunnel hull with a 55 engine on it. I am running it high to maximize speed. After a few runs with a 3 blade aluminum prop, I saw quarter sized spots of cavitation burn on the blade faces. I could sometimes hear the prop ventilating. Switching to a different prop with a lot more cup solved the problem.

 

[Mi duckdown]

Pics of burnt prop would help>

 

[Texasmark]

I had a '72 Chrysler 85 with the stock alum prop which had a funny reverse curve near the front of the root. This prop had the same thing right at the reverse turn. You could see the alum eroding away like you poured acid on it or something. Frank A. shows pictures of his props from time to time and one of his props is of the same design. Over time it would have eaten through the blade; no doubt.

Mark

 

[fishdady]

No Title

 

[fishdady]

Pics of burnt prop would help>

hope pic helps

 

[Scott Danforth]

how bad is your skeg? many good repair places can repair it in about 2 hours.

 

[fishdady]

Pretty bad. PO hit something hard. Since I've had this engine, found more and more signs of neglect. Just about have everything back to what it should. Took the engine off of a procraft F&S I bought for $1200. Put a '78 70hp johnnie on the back and resold the boat for 2200. 70 hp had give to me, needed a timer base and stator.

 

[Scott Danforth]

If it is "pretty bad" then I would definitely spend the Benjamin on getting it fixed.