21P Mirage Copy/250 XS Merc

[THEFERMANATOR]

I'm wondering if you don't have a 2:1 lower unit ratio in it. I know I hit 53 with a 17 pitch MIRAGE copy on my HYDRA-SPORTS which was only about 3-4% prop slip. If your running even a stock 1.86:1 lower unit ratio, then your prop slip even on the 21 at 60 must be around 18% or so which is on the high side(5-8% is just about ideal at WOT, with 12-15% considered ideal in teh cruise range). if you had a 2:1 lower unit, that would explain your high RPM's with a 21 pitch prop. I know my flats boat with stock 1.86:1 lower unit hit 60 with a 21 pitch TROPHY plus, but I had it jacked up high and was still only turning 5600 with it slipping and blowing out all over the place.