Over the years sailing OPBs, most folks ended up cruising at around 75% of max RPM or whatever got them to hull speed, whichever was lower.
My 30' Islander Bahama goes about 6.5 knots at 2700 RPM, which is 75% of my 2GMF's 3600 max RPM. My hull speed is 6.64 knots, so that's about right.
Those who do not know their hull speed should check out this
Sailcalc page. It shows you all kinds of interesting statistics about your brand, size, and model boat (assuming it is on the list). I have no idea how accurate it is overall, but it was pretty close when I spot-checked a couple of its statistics for my boat using the original formulae.
Tom