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Cavitation of propellor and cavitation number

Pressure distribution in a blade element is shown below

As the pressure on the back of a propeller falls lower and lower with the increase in a propeller’s n, the absolute pressure at the back of the propeller will eventually become low enough for the water to vaporize and local cavities form. This phenomenon is known as cavitation. (     , vapor pressure of water

 Cavitation on a propeller

  • Lower the thrust of the propeller, & thus decrease its efficiency
  • Cause vibration of hull & the propeller and generate uncomfortable noise, &cause erosion of the propeller blade.

Cavitation number 

The cavitation is most likely to occur at the tips of blades where the relative velocity is the largest and the hydro-static pressure is the lowest when blades rotate to the highest position. 
It can also occur near the roots where blades join the boss of a propeller because the attack angle is the largest.
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