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.

Comments

Manganese bronze is not typically used as a propeller material .Why

1. High cost: Manganese bronze is a costly alloy, making it less economical for large propellers. 2. Low strength-to-weight ratio: Compared to other propeller materials like nickel-aluminum bronze or stainless steel, manganese bronze has a lower strength-to-weight ratio. 3. Susceptible to corrosion: Manganese bronze can corrode in seawater, especially when exposed to high velocities and turbulence. 4. Poor cavitation resistance: Manganese bronze is more prone to cavitation damage than other materials. 5. Difficult to cast and machine: Manganese bronze is challenging to cast and machine, making it less desirable for complex propeller geometries. 6. Limited weldability: Manganese bronze has poor weldability, making repairs and modifications difficult. Nickel-aluminum bronze or stainless steel are commonly used for propellers due to their: - High strength and durability - Excellent corrosion resistance - Good cavitation resistance - Ease of casting and machining - Weldability

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