Question

(a) Describe the effect of cavitation on (6)

(i) The thrust and torque

(ii) The propeller blades

(b) A ship of 355190 tonne displacement is 325 m long, 56 m wide and floats in sea water of density 1025 kg/m3 at a draught of 22.4 m. The propeller has a diameter of 7.4 m, a pitch ratio of 0.85, and when rotating at 1.5 rev/s the real slip is 48.88% and the fuel consumption is 165 tonne per day. The Taylor wake fraction Wt, Wt=0.5Cb-0.05. Calculate: (10)

(i) The ship speed in knots

(ii) The reduced speed at which the ship should travel if the fuel consumption on a voyage is to be halved

(iii) The length of the voyage if the extra time on passage is six days when travelling at the reduced speed

(iv) The amount of fuel required on board before commencing on the voyage at the reduced speed.

Answer 1

Part (a)

(ii) Effect of cavitation on propeller blades:

Erosion:

Cavitation causes the formation and collapse of vapor bubbles on the propeller blade surface.
The collapse of these bubbles produces high-pressure shockwaves and microjets that erode the blade material, leading to surface pitting and progressive damage.

Vibration:

Uneven cavitation across the blades leads to imbalanced forces, causing vibrations in the propeller and the ship.
These vibrations can reduce the comfort of passengers and crew and stress the ship's structural components.

Noise:

The collapse of vapor bubbles generates loud noise, which can interfere with onboard communication and underwater sonar systems.
This noise is a significant concern for naval vessels and marine life.

Reduced Performance:

Cavitation reduces the efficiency of the propeller by causing loss of thrust and torque.
The presence of cavitation decreases the propeller’s ability to convert rotational energy into forward motion, lowering the ship's speed and increasing fuel consumption.

✓ Verified Model Answer (Text Solution)

(ii) Effect of cavitation on propeller blades: