What's your WOT RPM's now? A 21p or even a 19P seems like a big prop for a 90HP motor. My 01 Merc 90 HP 4 stroke max RPM's was 6000. I ran a 13p prop on a 19 foot aluminum boat. I had a 18 foot Wellcraft with a F115 Yamaha, and only ran a 13 1/4 x 17 prop on it.