PropSlipCalc

Is it important to know your prop’s slip? Only if you care about insignificant, little items like thrust. 

Slip is the most misunderstood of all propeller terms, probably because it sounds like something undesirable. Slip is not a measure of propeller efficiency. Rather, slip is the difference between actual and theoretical travel resulting from a necessary propeller blade angle of attack. If the blade had no angle of attack, there would be no slip; but, of course, there would be no positive and negative pressure created on the blades and, therefore, there would be no thrust. The Mercury Racing Prop Slip Calculator (below) lets you quickly determine propeller slip.