Thursday, July 23, 2020

Propeller slip

  • When the vessel is moving ahead the propeller exerts pressure on the water to create the forward motion. 
  • Propeller slip occurs because water is not a solid medium and there is some slip related to it.
  • Slip may be considered as the difference between the speed of the vessel and the speed of the engine. 
  • It is always expressed as a percentage.
Propeller Slip = Actual forward speed/ Theoretical forward speed.
  • The calculated value of slip will be increased when the wind and sea are ahead and if the vessel has a fouled bottom. 
  • The differing values of slip are especially noticeable after a vessel has been cleaned in drydock.
  • Theoretically a vessel should never have negative slip, but this may occur in one or more of the following conditions:--  A strong following sea.
                                                                         A following current or a strong following wind

How to find the Propeller Slip
Mean Apparent slip = Distance run by propeller - distance run by ship per day
                                                             Distance run by propeller

Distance run by propeller in nautical miles = Pitch(m) x total engine revolution per day

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