What an amazing coincidence, lol. I was turned on to trying out 4 blade props from one of my best buds that was seeing exactly what you are seeing here. It was suggested to him that there was something causing a slight air trail that was slamming shut at the tips of his blades causing this to happen. The suggestion was to try a 4 blade configuration to see if a different water capture trail would make any difference. Well, because none of his rpm/speed numbers were really out of whack he went out and put a equally matched pitch 4 blade Mercury Spitfire prop on the thing. It's only been since the beginning of this season and there is zero marks in those areas of the prop, but because he was so impressed with the increased speed, faster hole shot, and fuel efficiency from that Spitfire, he talked me into trying it on my little old 14' runabout with great success. But, there are far more talented brains about props in here than me. It was just a funny coincidence that my friend was having the exact same issue on the same motor!! Good luck