Boat is a 22' Prokat Catamaran, Yamaha F115's counter rotating, 2.15 gear ratio. I have calculated at 4500 RPM, I am getting 25 MPH, which equates to 27% slip. I have stainless props that are 17" pitch, I do not know the diameter, I would guess 13.25". At top end 5800 rpm I am getting 38 MPH...