mate,
you need to consider how much weight is in the boat aswell,
maybe the prop pitch may be another option.
I had a 15hp on my 3.60 tiny (which was very light) I had 2 props for it.
The speed i got was unbelievable from one of them. On my own in a river id pass other boats @ WOT
Id get some strange facial expressions.
Like a few have already mentioned, you need to know what rpm @ WOT
Huey may be the man to speak to.
Mork calling Huey come in Huey
cheers Zed