I have an Alumacraft 1860 (non pad style hull). Just put a new Mercury 90 4 stroke on it. 6” jackplate. Came with a 21p Laser II 3 blade prop. Can only hit 5400 rpms at 42mph. I have gradually raised jackplate to halfway to get to this point. Any suggestions? Should I drop down to a 19p prop? Max for this motor is 6000 rpm.