Who can tell me the formula used to determine prop slip percentage.

http://www.go-fast.com/Prop_Slip_Calculator.htm

Here is another one I found.
http://www.rbbi.com/folders/prop/propcalc.htm

Take your prop pitch X rpm and divide by 1584 = 100% efficiency. If you figure you have 10% slip, mutiply the total by .9 and that will get you close..
play with the numbers and you can figure it out.. Or go to the sites.. This is for a 1.5:1 drive ratio.

For a 1.36 drive, divide by 1436.16....
For a 1.25 drive, divide by 1320...

Hope this helps.
Dick

Using the equations you get 'calculated mph' assuming no slip. To find % slip use:

[(calculated mph - actual mph)/calculated mph] X 100 = percent slip

Where 'actual mph' is gps verified.