What is the Ship Slip?

Ship slip is the difference between the speed of the Engine and the actual observed speed of the ship.

SLIP
Slip = 100% – Efficiency

Where,

Efficiency = observed distance covered or engine speed.

Positive slip is influenced by various reasons such as fouled bottom or hull part which offers resistance to the movement of the ship, environmental factors such as water current and wind against the ship direction.

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%

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