A basis with diesel engines; they operate best if set up correctly. One should use a prop that allows the engine to reach "max rated rpm", plus 200 (or so) rpm to allow for accumulating "stuff" in the boat.
On my R21EC, I was never able to reach max rated rpm; it was about 200 rpm less. Talked with Ranger folks, they offered to swap out props. The general conversation on this board, at the time, was that all of us R21EC folks were getting different max rated rpm, all using the same prop. I felt it made no sense to mess around with props. Perhaps the differences were the tachs.
I saw slightly more than 10 knots (GPS) surfing down wind in big waves. Usually, max speed was around 8 to 9 knots, depending on wind and waves.
gene