I have a Zodiac 285s and Tohatsu 9.8 hp.
With the standard prop l get 18-20mph at WOT as above.
RPM is 5500 (WOT range is given as 5-6,000)
I tried a higher pitch prop as the boat is so small. The max speed was the same, but at 5,000rpm.
I am not sure if this is better? It takes longer to get on the plane with two on board (with the standard prop there's no delay at all) but is running 500rpm lower at any given planing speed.
I've currently got the original prop back on.