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RAKE and SKEW of propeller

 

RAKE:

  • When the propeller is viewed from sideways, we see that the blades of the propeller are not perpendicular to the surface of the hub. It is ‘tilted’ at an angle either towards the fore-end or the aft end of the ship. This is termed as Rake. 
  • One of the biggest reasons for the rake is allowing higher clearance between the blades and the vicinal hull surface. If the blade tip happens to be very close to the hull plating (which is very prone in case of no or little raked ones), there is a chance of induced vibration due to ship propeller action. 
  • When the blade is raked forward, that is in the direction of the shaft axis towards fore-end, it is called Negative Rake. Conversely, when it is raked aft, it is called Positive Rake 

SKEW: 

  • When we look at the ship propeller surface from behind). The blade appears to be ‘Skewed’, that is, bent or twisted sideways. 
  • After years of experimentation, analysis and sea trials, it was observed that aptly skewing a ship propeller nullifies or considerably minimises the extent of unsteady hydrodynamic loading in this flow field. This indirectly has positive effects in reducing resistance due to viscous ‘drag’ effects. It also reduces propeller-induced vibrations. 
  • Reduction in unsteady bearing forces and moments. 
  • Reduction in unsteady pressure forces.
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