Eric's Big Adventure All the research I did indicated that it should be raised one inch for every foot of setback. The cavitation plate was 3" above the bottom of the hull and it was perfect.
I believe I settled on a 16 X 18 Mercury prop (it's been a couple of years and the boat now has a new owner). I tried a few including a four blade. The highest speed I ever saw was 36mph (~31 knots).
