Solved part of the mystery, the prop says 10x6 but it actually measures about 8 1/2". It's either mislabled or expertly trimmed down. Both tachs I have are industrial type digital tachs with an input shaft and various rubber tips that you physically push against whatever you measure. I used a concave shaped tip that fit snug against the end of the aluminum prop nut. I'll try to upload a pic , put a real 10x6 on it, and take some more tach readings. I've never uploaded a video but I'm going to try to video me taking the tach readings and maybe someone can see what is going on cause I myself don't believe 18500 even with an 8" prop. Will probably tommorrow before I can do this.