what is the mathamatical formula for finding out the amount of slipping you have

I use this one.

that will work thanks

Scooter, the props are 32 pitch and the gear ratio is 1.48:1, if your calculating Scoop's slip.

If you don't have the above site handy. This is the formula:

rpm x pitch
------------------= speed (no slip)
1056 x gear ratio

speed (no slip) - actual speed divided by speed (no slip) then press % button on calculator.
My boat 5500 r's, 26 pitch, 1.36 drive, GPS speed = 89.7 mph.

5500 x 26
----------------= 99.57
1056 x 1.36

99.57 - 89.7 = 9.87 divided by 99.57 = 9.91%