This is a tricky question -- so many variables. Length, displacement , rig, crew, windspeed, upwind, downwind, etc.
There's a fairly large speed difference between a typical 30 footer and the typical 40 footer. I would say Charlie's estimate is pretty high for a 30 footer, but in the ballpark for a well-crewed performance oriented 40 footer.
I'm wondering what you plan to use this average speed figure for? Is it for planning a passage? IF for that purpose, I would use a figure closer to 4.5 knots for the 30 footer and maybe 5.5 knots for the 40 footer.
If you are trying to compare the speeds of displacement vs. semi-planing hulls, that's another conversation. A bit more info, perhaps?
P.S. Welcome to Sailnet!