Re: Correct prop...
Figuring 150-200 rpm change per inch of pitch; take the prop off and determine the pitch (stamped somewhere on the prop hub), then make your adjustment in a prop of less pitch to get you up to at least 5500 rpm.
Your numbers indicate that you are running about a 14 pitch now; but we'll have to wait and see what you find.
Mercury's engine tests of the 50 bigfoot on like size pontoon boats indicates that a 14 X 11 might be the right size prop. Those 50's were turning about 5700 rpm @ 20-21 mph.
