Class 2 (2nd Engineer) MEKG 📅 Jul 2022

Exam Question

(a) State why fixed pitch propellers have a poor efficiency when going astern.

(b) With reference to controllable pitch propellers state:

(i) Why is it preferable that the main servomoter be housed in the propeller hub rather than in the shafting forward of the propeller shaft?

(ii) What regular maintenance and checks should be carried out to ensure maximum reliability of the gear at all times?

Reference Answer

(a) Fixed pitch propellers are less efficient in the astern direction due to:
When moving astern, the angle of attack on the back of the blade is high, causing significant eddying and turbulence on the blade's surface, which reduces efficiency.
Thrust generation primarily depends on the high pressure exerted on the face of the blade. During astern movement, this thrust diminishes significantly, leading to a loss in propeller efficiency.
The thrust acting on the back of the blade during reverse motion is less effective because the blade curvature reduces the area available for thrust production.

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