Re: Correct speed
Props work in a liquid so there is some slip. A 10" pitch will theoretically move 10" forward in one revolution.Thus if there is 10%
slip It will actually move about 9". Slip varies depending on rpm ,weight, prop design, and hull type.
Reasonable slip numbers of typical planing hulls would be about 5% to about 12% Could be more but not likely less.
A tach that errors on the high side will give a false indication of high slip.
A tach that reads low could indicate exceptionally low slip.A prop calculator will figure performance the
5 variables that help identify prop performance. Rpm,gear ratio, prop size,slip and speed.
Any 4 accurately measured will give the 5th variable.Rpm should be easy but many times is suspect. Prop size is easy,speed is easy with a gps,usually the ratio can be found on the net.
with these we can get the slip.Slip is the trickiest to get. You can best guess some results based on logic of how a boat should perform. And you develop a feel for what numbers don't make sense.