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Just took this prop off for inspection. It's from the 1979 Montauk 17 I bought this fall. The prop would be in decent condition except for the gash I'm showing in the photos I will attach. Thinking I'll keep this as a spare but was wondering what would cause this damage? From what I can read online repairing this would cost nearly as much as a new one. PN:389949 / 13.75 x 15 - 13 spline. Would you use the prop in this condition? http://s1146.photobucket.com/user/rjw...amp;page=1
rj: looks like cavitation damage to me. The interesting thing is that it is on the aft face (pressure side of the blade). Normally cavitation happens on the forward side (suction or low pressure side). Cavitation is water which is vaporized due to local low pressure. When the vapor collapses as it hits an area in the flow with higher pressure, the collapse causes erosion damage. A good analogy is lightning, when the heated air from the lightning collapses, it causes the loud clap we hear as thunder. Lots of energy being released upon such collapses. Cavitation can erode metal little by little. There are lots of articles on cavitation, just google it to read more.
Now the question is why on your prop and on that side. I am guessing that either the blade is mis-shapen (unlikely since there appears no damage) or (more likely) there is something irregular on the leading edge which is causing a disturbance in the smooth flow of water. If it were my prop, I would file the leading edge and make sure there are no casting defects or roughness directly in front of the cavitation scar. Check this blade against the others, assuming they don't have the same damage.
As far as using the prop, there does not seem to be loss of a significant amount of metal that would significantly weaken the blade. However, it is hard to tell from the pictures. The prop would be fine to use as an emergency spare, or, if you boat in shallow water from time to time, use this prop...if you hit bottom and damage it, you at least did not damage a better prop!
Al
I had a cavitation scar years ago on an OMC i/o that was not caused by a deformity of the stainless blade, but an dent on the trailing edge of the lower unit skeg.
Fixed the skeg and had the prop welded and balanced, used it for 3 years after without issue.
