Question

A new vessel exhibits severe aft end vibration

As a Second Engineer Officer, outline a procedure to investigate and identify the source of vibration.

Suggest possible remedies to obviate / reduce ant end vibration.

Answer 1

Part (a)

Procedure to investigate and identify the source of vibration:

Engine Performance Check – Record main engine performance parameters and ensure there is no power imbalance between units.
Crankshaft Deflection – Measure crankshaft deflections and compare results with sea trial values to detect any misalignment or deformation.
Bearing Clearances – Measure and verify correct clearances for all units’ main bearings, crankpin bearings, crosshead bearings, and thrust bearing.
Foundation & Tie Rods – Check all engine foundation bolts and tie rods for correct tightening as per maker’s specifications; ensure none are slack.
Top Bracing Arrangement – Inspect the engine top bracing to ensure it is in good condition and correctly adjusted.
Propeller & Hull Condition – Arrange for an underwater survey to check the propeller and hull for:
- Damage to propeller blades (indentation, cracks, or blade loss)
- Deformation or fouling on hull near aft end
Tail Shaft Coupling Bolts – Inspect tail end shaft coupling bolts for any slackness or wear.
Vibration Source Correlation – Check for possible resonance where propeller-induced vibration and hull-induced vibration coincide, e.g., M/E units at TDC position while a propeller blade enters the wake.
Propeller Position & Balance – Verify correct positioning of the propeller and ensure it is dynamically balanced.
Tip Clearance – Ensure propeller tip-to-hull clearance is as per design and not reduced due to propeller shift or structural deformation.
Tunnel Shaft Bearings – Check tunnel shaft bearings for any damage or excessive wear.
Ballast Condition Effect – If in ballast condition, confirm that the propeller remains fully immersed; partial immersion can cause vibration.

Part (b)

Possible Remedies:

Trim Adjustment – Trim the vessel by filling the aft peak tank to fully immerse the propeller and reduce cavitation.
Barred Range Avoidance – Avoid running the main engine continuously in its barred range to prevent resonance build-up.
Foundation & Tie Rod Tightening – Tighten any loose foundation bolts or tie rods to the correct maker’s specified torque.
Coupling Bolt Tightening – Secure any loose tail end shaft coupling bolts.
Bearing Clearance Correction – Replace or adjust bearings to restore correct clearances if found incorrect.
Power Imbalance Correction – Overhaul or adjust units to eliminate any detected power imbalance.
Propeller/Hull Repairs – If propeller or hull damage is detected (e.g., blade cracks, deformation, shearing), arrange dry docking for repair or replacement.

✓ Verified Model Answer (Text Solution)

Procedure to investigate and identify the source of vibration:

Possible Remedies: