Cavitation is the formation and collapse of vapour bubbles on the propeller blade when the local pressure falls below the vapour pressure of water. Effects:

• Erosion/pitting: the collapse of the bubbles on the blade surface produces high local pressures that erode the blade material, causing pitting, loss of material and reduced blade life.
• Loss of thrust and efficiency: the vapour bubbles reduce the effective blade area and the density of the fluid, so the propeller produces less thrust for the same power and the efficiency falls.
• Vibration and noise: the collapse of bubbles produces noise and vibration, which can be transmitted to the hull and cause discomfort and structural fatigue.
• Reduced performance: cavitation limits the maximum thrust and speed, and can cause the propeller to "race" and lose grip.

Cavitation is controlled by using a larger blade area (higher blade area ratio), a lower blade loading, a suitable pitch distribution, and by avoiding excessive speed and loading; the design should keep the cavitation number above the critical value.

Ship 15,000 t displacement, Admiralty coefficient (based on shaft power) = 420. Mechanical efficiency 83%, shaft losses 6%, propeller efficiency 65%, QPC 0.71. At a particular speed the thrust power is 2550 kW.

(i) Indicated power.

Propeller efficiency = thrust power/delivered power, so delivered power = 2550/0.65 = 3923 kW.

Shaft power = delivered power/(1 - shaft losses) = 3923/0.94 = 4173 kW.

Indicated power = shaft power/mechanical efficiency = 4173/0.83 = 5028 kW.

(ii) Effective power.

QPC = effective power/delivered power, so effective power = 3923 x 0.71 = 2785 kW.

(iii) Ship speed.

Admiralty coefficient C = Delta^(2/3) V^3 / P_shaft. Delta^(2/3) = 15000^(2/3) = 608.

V^3 = C x P_shaft/Delta^(2/3) = 420 x 4173/608 = 1,752,660/608 = 2882.7.

V = 2882.7^(1/3) = 14.23 knots.

Answer: indicated power about 5028 kW; effective power about 2785 kW; ship speed about 14.2 knots.