What is slipping
Slip of the propeller is called the actual speed difference from the theoretical, in percentage.
slipping due to the fact that the propeller does not "work" within a solid object (like a screw in wood), but within a liquid mass , in which, and slides. So at every turn of the boat propeller does not "go" step in the prop but a distance smaller step, called progression board.
one = 1.15077945 mph Knot
Knot an = 72913 inches Sources