What causes cavitation pitting on prop blades?
 

Below is a couple photos of the very small pitting on my 28P stock Bravo 1 props with 100 hours fresh water run time. They are off my 31ss sonic that I just purchased this fall from LOTO. They did have some calcium build up on the backside of blades and barrels of props which I could easily cleaned off. Both props have this pitting only on the entire front (preasure) side of the blades. I measured the X dimension which appeared to be about 6.5 inches. I have twin stock 496 HO's that turn a max of 4800 at wot. The previous owner always left the k-planes in the up (#7) position and just used the outdrive trim. I have put about 10 hours on the boat and it has always jumped right on plane without any problem with drives down no matter where the k-planes are positioned.

What is the possible cause?
Is this common?
Is it hurting performance?

Its hard to tell by the picture. If you drag your fingernail across leading edge is it smooth? Has the paint been worn off your skeg from running thru shallow water? If some one cleaned them with a strong hull cleaner and left it on too long it could have etched the finish. It doesn't appear to be a cavitation burn as that is usually localized and trails behind a nick in the forward edge of the blade.

Leading edges are very smooth, not a nick or scratch anywhere. The outdrives still look like new paint, even the bottom of the skeg still has paint. I did use a hull cleaner to remove the calcuim buildup on the backside of the blades but it would seem the cleaner should have etched the entire prop not just the leading surfaces.

Good read. I'm wondering about this last sentence under "THE BAD" heading. "If the propeller is in good condition and the problem still persists, the cause may be a hull appendage (water intake, strut, etc) up stream of the propeller that is causing the problem."

With the boat on the trailer I can't see any scratches or damage to the hull, but I did notice the first time I seen the boat it was up on a fork lift (or whatever you call those tractors used for in/out service) that the bottom had some marring from the trailer bunks which I suspect may be from power loading onto the trailer at LOTO. I wouldn't have guessed that this could be causing cavitation pits...

After reading your suggestions... I wonder if power loading the boat onto the trailer could have caused this? I would never power load a boat due to the risk of marring the hull and MI ramps strictly prohibit power loading but we also have docks.
The original owner used to power load the last 6 or so feet onto the trailer from the one time I seen him load the boat.

I have power loaded boats on trailers for years. Never had this problem. The bottom would have to have things hanging down like transducers or intakes and still find your problem resulting from that unlikely given all blades pitted evenly. Scratches in the hull won't do that. I would think its related to the cleaner chemically etching it.

It's hard to see the pitting in the photos. True cavitation burn is cause by a pressure dufferential 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, tranducers, etc. Boat bottom configuration in conection 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.