We all know that the propeller is powered by the ship's main engine. The propeller pushes the water behind resulting in moving the ship forward. Under ideal condition of weather and sea the propeller's rated capacity would result in ship traveling a certain measured distance in 24 hours theoretically. However, it normally does not happen because the actual distance covered would be different than the rated as the ship might be subjected to weather, current and wind forces.

Slip is calculated in percentage by subtracting the Actual distance covered by ship from the Engine distance dividing the whole thing by engine distance and multiplying the whole thing by 100 as below -

Slip % = ((Engine distance - Ship Distance ) / Engine Distance ) * 100

Example
During 24 hrs of a period ship’s propeller shaft was observed to turn 87 RPM. the pitch of the propeller was 3.8 m. the observed ships speed over ground was 10 knts. Calculate the value of the propeller slip during this period. ( A nautical mile equals 1852 m.)
Soln: Slip (percent) = Engine distance – ship’s distance/ Engine’s distance X 100.
Engine distance = Pitch X RPM X 60 X 24 / 1852.
= (3.8 X 87 X 60 X 24)/1852
=257.054.
Ship’s distance = 24 X 10
= 240.
Therefore slip = (257.054-240)/257.054 x 100
= +6.6%