Class 2 (2nd Engineer) MEKG 📅 Jul 2023

Exam Question

(a) State why fixed pitch propellers have a poor efficiency when going astern. (8)

(b) With reference to controllable pitch propellers state:

(i) Why is it preferable that the main servomotor be housed in the propeller hub rather than in the shafting forward of the propeller shaft? (4)

(ii) What regular maintenance and checks should be carried out to ensure maximum reliability of the gear at all times? (4)

Reference Answer

### (a) Inefficiency of Fixed Pitch Propellers (FPP) in Astern Operation (8 Marks)
A Fixed Pitch Propeller's inefficiency in astern operation is a direct consequence of its blade design, which is highly optimized for thrust generation in the ahead direction only. The blades are shaped as hydrofoils, and reversing their direction of rotation fundamentally compromises their hydrodynamic performance.
1. **Optimized Hydrofoil Design for Ahead Thrust:**
* **Blade Profile:** FPP blades have a specific airfoil or hydrofoil cross-section. The forward-facing side, known as the 'pressure face', is typically concave. The aft-facing side, the 'suction back', is convex and cambered.

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