I've been playing around with prop options for my 20' SF and found this:
http://www.mercurymarine.com/everyth...ut_propellers6
It's a tutorial on props. There are lots of variables to consider. Most folks talk about pitch, but diameter, number of blades, cup, rake, and blade area all affect efficiency and how a prop will handle. It's worth checking out if you want to learn more about props.
I started out with a 19p that ventilated bad at high engine trim and in turns with a WOT RPM of 5300 and 41 MPH and ended up with a 22P with WOT RPM at 5500 and 49mph. The difference was slightly less blade area, more cup, and more rake. I was also able to raise the engine one hole once I was running the right prop.