Aux 2 Unit 17 Misc parts Q2 Answer

(a) List THREE defects that may be found during an inspection of a fixed bladed propeller, stating the possible cause of EACH.(6)
(b) State the likely consequences if the defects stated in part (a) are not rectified.(4)

Defect	Description / Appearance	Possible Cause
1. Cavitation erosion or pitting	Localised pitting or rough, honeycombed areas on the pressure or suction face of the blade, often near the tips.	Caused by cavitation — the formation and collapse of vapour bubbles due to low pressure behind the blade when operating at high speed or poor flow conditions (e.g., propeller imbalance or fouled hull).
2. Blade bending or distortion	Blade tips or edges bent out of shape or uneven pitch across blades.	Caused by impact with floating debris, grounding, or incorrect handling during maintenance or drydocking.
3. Surface cracks	Fine cracks usually found near the root, leading edge, or fillet radius.	Caused by fatigue stresses from vibration, overloading, or sudden impact.
Alternative acceptable defects: Scoring from contact with rope or netting, galvanic corrosion (if dissimilar metals), or dezincification (for bronze propellers).

(1 mark per defect + 1 mark per cause = 6 marks)

Loss of propeller efficiency
- Cavitation damage and rough surfaces increase drag, causing loss of thrust and higher fuel consumption.
Increased vibration and noise
- Distorted or cracked blades cause propeller imbalance, leading to vibration throughout the shafting system.
Damage to shaft bearings and stern gear
- Vibration and imbalance place excessive loads on stern tube bearings, thrust blocks, and couplings, leading to premature wear or failure.
Risk of blade failure or detachment
- Fatigue cracks may propagate, resulting in fracture of a blade section, possibly causing severe hull vibration or damage to the rudder and structure.

(1 mark per valid consequence = 4 marks)