Bravo III Prop Pitting - what is causing this?


FTL_Alan

New member
2008 350 with original stainless steel props. I bought it in 2015. Props were good at time of purchase. Over last year, I've noticed some minor pitting on both rear props, but now I see that one of the props has lost metal, about the size of a magic marker tip. Props are 28-P. I don't run them hard. Boat had 300 hours when purchased in 2015, now has 540. Most of those hours were slow cruising on the intracoastal. I keep the boat on a lift at my dock. I will be taking the rear props off and taking them to a prop shop for refurbishing. Is this likely due to normal corrosion? I keep the props clean, wash them down after every outting. I have used 50/50 mix (water, muriatic acid) maybe three times to get stains off when boat has sat in brackish water at the marina. I don't think the acid caused the issue because its only on the rear props. I read something about cavitation may have caused this, when trimming the props up a little at cruising speed. Could that contribute to the issue?

Here's the 3 worst blades among the 6 blades on the two rear props. The last photo (3rd) shows the blade where metal is actually gone. The other two blades don't have missing metal yet, but have rough spots near the same area, near the blade's outer edge.

20180115_100108.jpg 20180115_095920.jpg 20180115_095940.jpg
 
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Appears to me that the props are not tuned and air bubbles are burning the blades. Take them to a prop shop and ask. I would suggest bblades and while they are on the high end, they are good
 
I'm certainly no expert but looks like cavitation burn, not corrosion, to me also. Since it's happening to both rear props, I think over trimming could be the problem.
 
I agree excessive cavitation from ovwr trimming, its burning the tips of the blades. Do you get a porpous effect while running with the boat trimmed out?
 
Thanks for the validation that this is a cavitation problem. Apparently it does not take very long at all before damage occurs. In the roughly 200 hours I've had the boat, I've been on plane maybe 10 of those 200 hours, and never more than an hour at a time, i.e. short runs on the ocean or on the intracoastal. Regarding trim level, I never exceed the "1" mark on the guage. I bump the trim up once which puts it at the 1 mark. I don't experience any porpous effect while running with trim at the 1 mark.

Curious what others are doing regarding trim level. What mark are you at on the trim gauge, i.e. less than 1?

I will be taking the props to a shop for refurbishing.
 
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Over here in the Gulf I rarely use my trim tabs, unless the boat is filled with passengers. I have had them at five but usually when on plane between 0 and 2.
 
Not to get sticky, but trimming up to high causes ventilation, cavitation is different. Below is a link where bblades explains it and have posted his comment

https://www.offshoreonly.com/forums/prop-talk/177617-what-causes-cavitation-pitting-prop-blades.html

True cavitation burn is cause by a pressure differential on the face of the blades. This pressure differential can be caused by a variety of things. Some examples are propeller LE damage that causes the water to tumble. Poor propeller geometry from damage. Bubble trails off gear cases, transducers, etc. Boat bottom configuration in connection with drive placement.

Cavitation is so violent against the face of the blades that it litterally wears or burns the metal away.

Most propellers are 15-5 stainless. They will simply wear away with time also.
 
I don't think it's cavitation. I have two sets of wheels, both tuned identical. The originals had this phenomenon from the beginning, and were replaced. Manufacturer just gave me the old ones. I took them to my prop shop, and he said it's fairly common,but they could not be repaired. He further added that they could still be used without concern. I've been doing so every other year, as I swap wheels each season, have the "removed" pair tuned/balanced, and anti-fouling re-applied. The second pair has never exibited this effect...
 
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