Prop cavitation

C

camman73

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May 23, 2005
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17493
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Have a 4 blade stainless prop on Bravo II drive. This previous season noticed some cavitation on the some of the front sides of the blades. I finally got around to cleaning them thoroughly, it seems that there is cavitation only on the front side of 2 adjoining blades, none on the other 2 blades or any on the back sides of the blades.

What could be possible causes? Improper balance of the propeller? Prop shaft? Shaft was tested 2 years ago for runout and was within specifications. Where to start diagnosing what the cause is?
 
"Where to start diagnosing what the cause is?"

Take the prop to propscan and have it scanned. If the prop is well within tolerance, they may have a reason. If it is not within tolerance, then that alone may be the cause.
 
Are you pretty sure it is cavitation pitting, rather than possibly tiny marine life calcium deposits on 2 blades left in the water when tilted up, or same for possible electrolysis pitting effects ?

I have no idea if the same 2 blades could more often end up being ones still wet.
 
quote:
Originally posted by Radioactive

"Where to start diagnosing what the cause is?"

Take the prop to propscan and have it scanned. If the prop is well within tolerance, they may have a reason. If it is not within tolerance, then that alone may be the cause.

Click to expand...


Thanks. Bringing it in to a local shop with Propscan later today. Having my usual mechanic check the run out of the prop shaft as well again. I'm thinking that since this is only showing up 2 on blades that it must be the prop or shaft causing this. If the cavitation was coming from the hull somehow it would be on all blades.
 
quote:
Originally posted by Sandy

Are you pretty sure it is cavitation pitting, rather than possibly tiny marine life calcium deposits on 2 blades left in the water when tilted up, or same for possible electrolysis pitting effects ?

I have no idea if the same 2 blades could more often end up being ones still wet.

Click to expand...


Definitely not calcium deposits, cleaned thoroughly with toilet bowl cleaner. Bravo II is left straight down with all 4 blades fully submerged in the slip. Fresh water.

Never thought of electrolysis, but not sure how that would only affect 2 of 4 blades and only the front side.
 
These are cast blades. There may have been micro porosity or casting debris caught in the mold during the casting process. Cleaning and polishing the the blade may have covered the pits which got exposed during use.

How big of an area and a specific shape are the defects?
 
It's really difficult to actually get "Cavitation" on an I/O or outboard prop. Maybe "ventilation", sucking air into the prop from the surface, but that would be on all 4 blades equally. There is definitely something wrong with the 2 blades on your prop. Possibly the shaft, but you really need to get that prop checked. "Prop Scan" or something, someone who knows what they are doing.
 
Cavitation has to be caused by something. A nick on the leading edge or a slight concavity on the leading surface. You should be able to feel it.
 
It will be interesting to see what the Propscan "before" scan results may indicate for each blade and how they match , and whether the prop meets ISO 1 specs for conformity to original overall pitch specs..

Cavitation is usually the result of frequent severe throttling up or an over-pitched prop not allowing well into the spec'd RPM range at WOT. Tubing , water skiing and other abnormal loads on the props can influence this as well.

But it certainly shouldn't just be evident on only 2 of 4 blades. So if the scan checks out nicely , Charlie's suspicion could be on the mark.

I'm thinking due to possible previous strike aberration, the 2 blades may be carrying more of the load than the other 2 and the scan would reveal that..
 
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