Tailshaft Surveys

Answer

To maintain structural integrity, ensure watertightness, and prevent catastrophic failures, classification societies (IACS rules) and flag state administrations require mandatory Tailshaft Surveys.

1. Types & Intervals of Tailshaft Surveys

Classification society survey intervals depend on the shaft connection type, corrosion protection, and propulsion arrangement (e.g., oil-lubricated vs. water-lubricated). Under modern IACS frameworks (UR Z21), survey regimes include:

A. Complete Survey (5-Year Intervals)

The shaft is drawn completely inward or outward for detailed examination.

Interval: Standard 5 years (can be extended up to 15 years under Condition Monitoring schemes).
Scope:
- Complete withdrawal of the tailshaft into the engine room or aft peak.
- Removal of the propeller to expose the taper, keyway (if fitted), and thread areas.
- Thorough cleaning and NDT (Non-Destructive Testing via Magnetic Particle Inspection or Dye Penetrant) on the forward portion of the taper (top 1/3 of the taper length), keyway, and flange fillet radii.
- Visual inspection of shaft surface for scoring, corrosion, pitting, or fretting.
- Calibration of stern tube bearings (aft and forward) and tailshaft journal diameter using micrometers to evaluate clearance/ovality.
- Overhaul/replacement of forward and aft stern tube seals (lip seals/face seals) and inspection of the chrome-steel liner.
- Examination of coupling bolts and flange radii using NDT.

B. Modified / Partial Survey

Carried out when full withdrawal is not due, but partial inspection is required to maintain class validity or investigate defects.

Scope:
- Partial withdrawal of the shaft sufficient to examine the aft bearing and shaft journal in way.
- Backing off the propeller (for keyed shafts) to expose the top taper section for NDT crack detection.
- Check stern tube bearing wear-down/clearance using a poker gauge/feeler gauge.
- Verification and renewal of outer shaft seals as necessary without full shaft removal.

C. Tailshaft Condition Monitoring (TCM / SCM) Program

Vessels equipped with oil-lubricated stern tubes and approved seal arrangements can extend the complete survey interval up to 15 years, subject to annual data verification:

Requirements for TCM Notation:
- Monthly/quarterly stern tube lube oil analysis (checking for water ingress, chloride content, iron/bronze wear particles, and oil oxidation/aging).
- Daily records of forward and aft stern tube bearing temperatures.
- Lube oil consumption monitoring.
- Annual wear-down measurements (poker gauge readings).
- Operational shaft grounding system to prevent spark erosion/galvanic pitting across the bearing film.

2. Preparations & Procedure for Shaft Removal

When carrying out a Complete Tailshaft Survey during drydocking:

Pre-Survey Preparations:
- Ensure the engine room overhead crane, chain blocks, special lifting beams, and rigging gear are inspected, tested, and certified.
- Mark and record baseline alignment reference marks on the tailshaft and intermediate shaft coupling.
- Perform an initial Poker Gauge Reading (propeller drop) before dismantling to record baseline wear-down.
Dismantling Steps:
- Drain Oil / Disassemble Seals: Drain the stern tube lube oil, remove the rope guard, and dismantle the aft seal assembly and chrome liner.
- Uncouple Shaft: Disconnect the intermediate shaft coupling bolts, hydraulically push back the intermediate shaft, or remove the spool piece.
- Remove Propeller:
  - Back off the propeller nut using hydraulic jacks/Pilgrim nut.
  - Use hydraulic oil injection on keyless tapers (or heating/jacking gear for keyed tapers) to break the taper contact.
  - Secure the propeller with dock cranes/chain blocks and shift it clear of the shaft line.
- Draw the Shaft: Attach extension running pipes/rigging gear to the forward coupling flange and carefully draw the tailshaft inward into the engine room tunnel/shaft space.

3. Key Inspections & Surveyor Checks

During the survey, the attending classification surveyor inspects the following key parameters:

Inspection Area	Specific Check & Methodology	Acceptance Criteria / Rule Limits
Shaft Taper & Keyway	Visual & NDT (MPI / DPI) on the forward 1/3 length of the shaft taper and keyway radii.	Zero cracks permitted. Slight surface corrosion/defects may be ground out, provided rule shaft diameter drule is not reduced by more than 3%.
Propeller-to-Shaft Bedding	Blue matching test (Prussian Blue) applied between the female propeller hub bore and male shaft taper.	Minimum 70% to 80% uniform surface contact required across the taper length.
Stern Tube Bearings	Micrometer calibration of white metal bearing ID or synthetic bush ID; check for wiping, fatigue cracking, or oil channel blockages.	Max allowed clearance depends on shaft diameter (typically 0.1% to 0.2% of shaft diameter, or as per manufacturer limit).
Chrome Liner	Check for grooving/scoring at lip seal contact positions; check for slackness via hammer test.	Machining/skimming permitted up to manufacturer limit (e.g., max 48 mm reduction on 1000 mm dia liner). Garter springs shortened accordingly (approx. 3 mm per 1 mm diameter reduction).
Coupling Bolts	NDT (MPI) on fitted coupling bolts and flange transition radii.	Must be free of fatigue cracking, fretting, or severe distortion.
Seals & Glands	Visual check of rubber lip seals (Viton/NBR) for hardening, cracking, or wear; verify garter spring tension.	Lips replaced at every complete survey; sealing arrangement leak-tested after reassembly.

4. Reassembly, Alignment & Post-Survey Verification

Re-installation & Bedding:
- Shaft drawn back into position, ensuring white metal bearings are lubricated with clean oil prior to entry.
- Re-verify propeller shaft bedding with Prussian Blue.
Propeller Mounting:
- For keyless propellers, drive-up distance and oil injection pressure are controlled according to manufacturer's temperature-corrected drive-up charts to ensure correct friction grip without over-stressing the hub.
- Secure the propeller nut and lock it with the keeper plate/grub screws.
Stern Tube Leak Testing & Wear-Down:
- Measure final poker gauge (wear-down) reading at top and bottom positions and record in the engine logbook/survey report.
- Fill the stern tube with fresh lube oil and perform a static head pressure leak test on both forward and aft seals for at least 1–2 hours.
Shaft Alignment Checks:
- Perform a Jack-Up Test on the aftmost intermediate bearing to measure static bearing loads and verify shaft alignment limits.