I cannot figure out where I am going wrong on this.
MY RPM at WOT is 5400
Ratio on the etec is 2:1
Prop is an 18" SS
so using the calculation according to Downhilldai...
5800 engine rpm / 2 = 2700 prop rpm
2700 prop rpm x 18 inch pitch = 48600 inches per minute forward travel speed
As there are 63360 inches in a mile (according to my grandfather!)
48600/63360 = 0.767 miles per minute
0.767 x 60 = 46mph
(theoretical mph with your set up at 5400 RPM, assuming nil prop slip.)
Yet my speed according to GPS and Speedo = 47 mph (40.5 knots)
Even if I add a couple of inches to prop size to allow for cupped SS blades then I should only be getting without slip 51mph
So I appear to be getting negative slip or virtually no slip
What all this about then
I am getting my speed by running both with and against tide and then averaging the two readings. I am definate about the prop size I am running. the Tacho appears to be reading correctly and I have checked it against a timing gun with tacho.