Bert28 is on the money.
From some old 1976 charts, the Merc 233 engines had the following numbers:
3000 RPM 22 Kts 21 gph
3500 27 Kts 34 gph
3900 29.5Kts 40 gph
In a seaway (I'm talking 4-6'), you can drive her like a raceboat: left hand on the wheel, right hand on the throttles, feet planted & braced. You will be amazed at how much ground you can cover (if you want to).