I had to change outboard units. I have a 165 6 cyl mercruiser. The outboard was a 165hp. I had and installed a 120/145hp outboard. The gear ratio is definately different. I went from 4300 rpm WOT to past 5000 rpm. Planing rpms went from 3200 to 4300. Both outboards have a 19 pitch 3 blade prop. Will a 21 pitch prop correct this?