Sketch: The outboard (aft) seal of an oil-filled stern tube is fitted at the aft end of the stern tube, where the tailshaft emerges into the water. It consists of:

• A seal housing (carrier) bolted to the stern frame/ stern tube aft end.
• A series of sealing rings (lip seals) - typically a set of elastomeric (nitrile/ polyurethane) lip seals, including a main sealing lip and a dirt/ water-excluding lip, arranged around the shaft.
• A sealing face/ liner (a hardened/ chrome-plated sleeve or the shaft itself) against which the lips bear.
• A spring (garter spring) that holds the lip in contact with the shaft.
• An oil connection/ drain to the seal housing (to supply oil to the seal and to drain any leakage).

The seal prevents sea water from entering the stern tube and prevents the stern tube oil from leaking out into the sea.

The lip seals are designed with a spring-loaded lip that maintains contact with the shaft/ liner even as the seal face wears. As the shaft/ liner wears, the garter spring keeps the lip pressed against the surface, and the elastomeric lip flexes to follow the surface, maintaining a sealing contact. The seal is also designed so that the oil pressure in the stern tube (maintained by the header tank) is slightly higher than the sea water pressure, so any leakage is oil outward rather than sea water inward. The multiple lips (a primary oil-sealing lip and a secondary water-excluding lip) provide redundancy; if the primary lip wears, the secondary lip still seals. The seal housing may also have a wear-adjustment/ spring arrangement to take up wear. Regular inspection and renewal of the lip seals at dry-dock maintains oil tightness.

• Effect: Sea water in the stern tube oil causes emulsification (oil becomes milky), corrosion of the shaft and bearing, loss of lubricating properties, and accelerated bearing/ seal wear. It can also cause the oil to become acidic and degrade the bearing material.
• Reducing the risk: Maintain the oil header tank level and the oil pressure slightly above sea water pressure; keep the seals in good condition (renew at dry-dock); drain and check the stern tube oil regularly for water (settling/ centrifuge test); use a water-detecting/ moisture sensor; and ensure the seal housing drains are clear. If contamination is detected, the oil is changed/ purified and the seals inspected.

• Good load-carrying capacity (to support the shaft and propeller loads).
• Low friction and good wear resistance (to minimise wear and power loss).
• Compatibility with the shaft material (to avoid galling/ seizure).
• Corrosion resistance (to sea water and oil).
• Good thermal conductivity (to dissipate heat).
• Ability to run with a thin oil film (good hydrodynamic/ boundary lubrication properties).
• Dimensional stability and resistance to swelling/ degradation in oil and sea water.
• Typical materials: white metal (babbit) on a bronze/ steel shell, or synthetic/ composite bearing materials (e.g. reinforced phenolic/ rubber-lined bearings) for water-lubricated types; for oil-filled, white-metal or bronze bearings are common.