Answer
If heavy vibration appears immediately after coming out of dry dock, first suspect something that was disturbed, repaired, removed/refitted, painted, or aligned during docking, especially around the propeller–shaft–rudder system.
The most likely causes are:
- Propeller imbalance or damage — blade bent, unequal blade repair, excessive grinding/build-up, uneven polishing, or one blade damaged during docking. An unequal blade mass or geometry creates a strong once-per-revolution excitation.
- Propeller incorrectly refitted on the taper — poor contact, wrong push-up, key/keyway problem, or propeller not seated properly. Your handout stresses that final propeller fit on the tailshaft taper should be checked properly, including contact on the taper.
- Shaft alignment disturbed — if tailshaft, intermediate shaft, bearings, engine chocks, or stern-tube bearings were worked on, bearing offsets may have changed.
- Stern-tube bearing problem — incorrect clearance, damaged bearing, poor lubrication, or localized bearing contact after refitting.
- Rope guard / fairing / stern-tube seal component touching the shaft after incorrect assembly.
- Rudder or rudder bearing problem — abnormal clearance, looseness, or rudder/propeller interaction after repair.
- CPP-specific issue — unequal blade pitch, one blade not synchronized, hydraulic fault, or wrong pitch setting can cause severe cyclic thrust.
- Loose foundation or bearing holding-down bolts after work.
- Propeller cavitation / disturbed wake due to altered hull surface, wrongly fitted appendage, anode, rope guard, or damaged propeller surface.
Diagnostic Checklist for Chief Engineers
Vibration Frequency Analysis: Determine if vibration frequency corresponds to shaft RPM (1×RPM = mechanical unbalance/shafting) or blade passing frequency (No. of Blades×RPM = hydrodynamic/cavitation).
Bearing Load Verification: Check intermediate bearing (Plummer block) temperatures and perform jack-up tests to verify actual bearing loads match classification society curves.
Propeller Pitch Verification: On CPP vessels, record pitch vs. power feedback to rule out asymmetric blade angle offsets.
Holding-Down Bolt & Chock Inspection: Check torque on main engine foundation bolts and inspect top bracing units (hydraulic/friction type).