What to check on a Stern Tube Seal.
The stern tube seal keeps seawater out of the stern tube and, on oil-lubricated systems, keeps lubricating oil from leaking into the sea. It sits where the rotating propeller shaft passes through the stationary stern tube, so it must seal a moving surface continuously, at whatever depth the propeller sits below the waterline, for years between dry dockings. Two designs dominate: lip-type seals, where several spring-loaded rubber lips ride against a shaft liner, and face-type (mechanical) seals, where a rotating and a stationary carbon or bronze face are held together…
What the stern tube seal actually does
The stern tube seal keeps seawater out of the stern tube and, on oil-lubricated systems, keeps lubricating oil from leaking into the sea. It sits where the rotating propeller shaft passes through the stationary stern tube, so it must seal a moving surface continuously, at whatever depth the propeller sits below the waterline, for years between dry dockings. Two designs dominate: lip-type seals, where several spring-loaded rubber lips ride against a shaft liner, and face-type (mechanical) seals, where a rotating and a stationary carbon or bronze face are held together by springs and hydraulic pressure. Face seals tolerate more shaft misalignment and are now standard on larger tonnage; lip seals remain common on smaller vessels for their lower cost.
Main components
Seal rings or lip elements
A lip-type unit typically has an aft set of lips facing the sea and a forward set facing the stern tube lubricant, with a drain between them to catch and monitor any leakage past the outboard lips before it becomes a pollution event.
Shaft liner or sleeve
The sealing surface itself, a bronze or stainless steel liner shrunk or bonded onto the shaft, must be smooth and free of pitting; the seal element wears against this liner, not against the shaft steel.
Housing and spring-loaded faces (face seals)
In a face-type seal the housing carries the stationary ring while a rotating ring turns with the shaft, held in contact by springs; a header tank supplies sealing oil at a small positive pressure above sea pressure so any leakage path runs oil outward, not seawater inward.
Selection and sizing
The seal is matched to the shaft, not chosen independently:
- Shaft diameter and the operating depth of the propeller, which sets the sea pressure the aft seal faces must hold.
- Lubrication type of the stern tube, oil or water, since seal materials and drain arrangements differ between the two.
- Maximum allowable shaft misalignment and whirl, particularly on ships with a long, flexible shaft line.
- Air seal option for dry-docking, allowing compressed air to be admitted between seal sets so the vessel can be surveyed afloat rather than always in dry dock.
Regulations and class
MARPOL Annex I addresses oil-lubricated stern tubes as a pollution source, which is one reason the industry has shifted toward water-lubricated systems or seal designs with a monitored oil-to-sea interface. Class rules require periodic survey of the stern tube seal and shaft, typically tied to a five-year special survey cycle, with a tailshaft survey interval that can be extended if continuous shaft condition monitoring is fitted and approved. Oil content in any overboard drain from the seal system is subject to the same discharge limits as other machinery space bilge water.
Typical faults
| Fault | Consequence |
|---|---|
| Worn or hardened lip elements | Increasing seawater ingress into the stern tube, diluting or contaminating the lubricant |
| Scored or pitted shaft liner | New seal lips wear rapidly against the damaged surface and leakage returns within months |
| Loss of sealing oil header pressure | Pressure balance reverses, allowing seawater to enter past the aft face seal |
| Shaft misalignment beyond seal tolerance | Uneven wear on the sealing faces and premature failure on one side only |
| Blocked drain between lip sets | Leakage is not detected early because the monitoring point itself is fouled |
What to look for in a supplier
- Exact liner and seal dimensions matched to the shaft as fitted, since aftermarket seals built to a slightly different tolerance shorten life significantly.
- Experience with underwater seal replacement by divers where dry-docking is not scheduled, if the vessel needs the work done afloat.
- Availability of the correct sealing oil grade and a supply plan for the header tank, not just the seal hardware itself.
- A record of class approval for the specific seal type, since an unapproved substitute can complicate the tailshaft survey.
Track the trend in stern tube oil consumption and water content over months, not just the pass or fail at survey; a slowly rising water percentage in the lubricant is the seal telling you it is wearing out long before it leaks visibly.