I'm getting 44 mph on my homebuilt Rascal using a new 10R17 (Michigan Wheel) on a '90 Merc 60hp, WOT 5500 rpm. I'm pretty happy with this performance, except for the fact that the designer pushed the prototype (in 1989) to 52 mph with essentially the same engine (merc 60hp), same prop (10R17) but with a "super cup", running at slightly lower RPM (I think he came down to 5200rpm).

Conventional wisdom says, since I'm at the upper range of RPM, I should try the next higher pitch. But this is not so easy to do, "10R19" is not an easy prop to find!

I don't know what the rule of thumb is about jumping up in diameter, and where then to go with pitch. Thoughts?

My suggestion would be to take your prop to a prop shop and have the cup increased, you could then probably raise your engine slightly.