Propeller shaft and stern tube with the tube filled with oil (oil-lubricated stern tube)

Sketch: the propeller shaft (tailshaft/line shaft) passes through the stern tube, which is fitted in the stern frame. The stern tube is filled with lubricating oil and sealed at both ends by oil seals - a stern (inboard) seal near the flange and an outboard (aft) seal at the propeller end. Bearing/lining (white metal or controlling-fit) is attached to the tube, and the shaft runs on it in oil. The oil is circulated/pressurised by a small lub oil pump from an expansion/header tank, or a gravity system, so that the tube is always full of oil. A line diagram shows: stern tube -> oil-return and oil-supply pipes -> expansion tank (with level glass) -> lub oil pump -> seals/rings -> shaft.

• Maintain the correct oil level in the expansion/header tank - top up as necessary so the tube stays full.
• Regularly observe the oil sight glass / level and the oil pressure if a circulation pump is fitted; ensure the lub oil pump is running and delivering.
• Check the sterntube oil temperature - it should be steady; a rise indicates a bearing/alignment or lubrication problem.
• Watch for oil leakage - oil dripping from the outboard/inboard seals, or a falling tank level, indicates seal wear/blow-by; note the amount of leakage (a small weep is sometimes normal, but an increase warns of seal failure).
• Periodically sample the sterntube oil for the laboratory for water content/contamination and metal wear particles, as an indicator of bearing and seal condition.
• Monitor for vibration/noise from the shaft indicating a worn bearing or misalignment.
• In port/lying idle, keep the shaft turned occasionally (if required) and check the oil system.

Repair method: dry-dock the vessel, withdraw the propeller shaft (tailshaft) from the stern tube inboard, and examine the oil seals and the bearing. Renew the worn oil seals with new seal/insert rings; the cutters/seal carrier/springs and the shaft's wearing surface (the ground/plated area) are inspected. If the shaft surface is worn/grooved at the seal land, it is built up by metallising (flame-spray/thermal spraying) and re-machined to size, or the shaft is renewed, so a new seal runs on a good surface. Renew the white-metal bearing lining if worn, and remake the bearing to size. Re-assemble, refit the shaft, set the seals, and refill the tube with fresh lubricating oil; run and check for leaks/temperature.

Defects to look for when the shaft is withdrawn in dry-dock

• Worn, scoured or grooved shaft surface at the seal and lining (out-of-roundness, battering of the lands).
• Corrosion pitting/staining on the shaft and at the coupling.
• Leaked oil stain/grease on the hull and shaft - indicates the seal failed and sea water/corrosion may have entered.
• Damaged or distorted seals, carriers, springs and counterfaces.
• Bearing/lining wear - scored, overheated (discoloured) white-metal, wear particles; check the clearance.
• Evidence of water ingress (emulsified/water-contaminated oil, corrosion of the tube) indicating seal or bearing damage.
• Cracks/defects in the stern tube, coupling flange, and the shaft bend/bow (run-out) at the bearing; damage to the propeller cone/threads.