Guide 06 Marine engineering and ship repair
Stern tube seals and bearings: types and wear
The stern tube carries a ship's propeller shaft through the hull at the stern, and its bearings and seals are what keep the shaft running true and the sea out. Seal type, lubrication system and bearing wear are checked at every dry docking and at each classification survey.
- Topic
- Marine engineering and ship repair
- Checked
- 23 September 2026
- Sources
- 4
- Questions
- 6
01
What the stern tube does
The stern tube is the tube through the hull at the stern that carries the propeller shaft from inside the ship out to the propeller. It houses the stern bearings that support the shaft and propeller weight, and it is filled with oil, grease or water as lubricant depending on the system fitted.
Seals are fitted at each end of the tube: the forward seal keeps lubricant out of the shaft tunnel or engine room, and the aft seal keeps seawater out of the tube. Both are wearing items, inspected whenever the shaft is exposed.
02
Seal types: lip seals and face seals
Lip seals use several elastomer rings, usually fluoroelastomer (FKM), running against a chrome-steel liner on the shaft, with the oil spaces between the rings pressurised from header tanks above the waterline. They are the most common arrangement on oil-lubricated stern tubes.
Face seals work differently: they seal axially between a rotating face and a stationary face rather than radially against the shaft, and are often used with water-lubricated bearings, where a lip running directly on a wet shaft would wear faster.
03
Oil-lubricated and water-lubricated systems
Oil-lubricated stern tubes give low friction and long bearing life, sealed by lip or face seals, but carry a risk of small, steady oil loss to sea, which is why environmentally acceptable lubricants are increasingly specified. Water-lubricated systems use seawater or clean fresh water instead, removing that pollution risk.
IACS UR Z21 sets different tailshaft survey methods for the two systems, reflecting the different wear items and inspection access each one has, so the lubrication system fitted is one of the things that decides which survey method a ship follows.
04
Measuring wear: weardown and propeller drop
Bearing weardown is measured with a poker gauge or a fixed wear-down gauge, taken from the same reference point on the seal housing with the shaft in the same position each time. The first reading is taken at build, and what matters afterwards is the change between readings against the maximum weardown approved for that bearing.
Propeller drop is the related measurement: the downward movement of the shaft at the aft stern tube bearing as that bearing wears, taken in dry dock with a poker or feeler gauge between a fixed reference and the shaft. A rising trend in either reading signals bearing wear and is treated as an early warning of possible seal damage.
05
Why a stern tube seal leaks
Common causes of oil leakage are a grooved or worn seal liner, hardened or cut seal lips, fishing line or rope fouled in the aft seal, excessive bearing weardown letting the shaft run off-centre, incorrect header tank pressure, and misalignment carried over from the shaft line.
A small, steady loss of lubricant is treated as normal running wear rather than a fault; it is a sustained or growing leak, or a leak accompanied by rising weardown, that points to a seal or bearing problem needing attention.
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Questions people also ask
Stern tube seals and bearings: types and wear
What is a stern tube on a ship?
The stern tube is the tube through the hull at the stern that carries the propeller shaft from inside the ship to the propeller. It houses the stern bearings that carry the shaft and propeller weight, is filled with oil, grease or water as lubricant, and has seals at each end to keep the water out.
What is the difference between a lip seal and a face seal?
Lip seals use several elastomer rings, usually fluoroelastomer, running on a chrome-steel liner, pressurised from header tanks, and are the common choice for oil-lubricated stern tubes. Face seals seal axially between a rotating and a stationary face and are often used instead where the bearing is water-lubricated.
What is the difference between oil-lubricated and water-lubricated stern tubes?
Oil-lubricated stern tubes use lip or face seals, give low friction and long bearing life, but can leak small amounts of oil to sea. Water-lubricated systems use seawater or clean fresh water, removing that pollution risk. IACS UR Z21 sets different tailshaft survey methods for the two systems.
How is stern tube bearing weardown measured?
Weardown is measured with a poker gauge or a fixed wear-down gauge, from the same reference point on the seal housing with the shaft in the same position each time. The first reading is taken at build; what matters afterwards is the change against the maximum weardown approved for that bearing.
What is propeller drop measurement?
Propeller drop is the downward movement of the propeller shaft at the aft stern tube bearing as that bearing wears. It is measured in dry dock with a poker or feeler gauge between a fixed reference and the shaft, and compared with earlier readings; a rising trend signals bearing wear.
Why does a stern tube seal leak oil?
Common causes are a grooved or worn seal liner, hardened or cut seal lips, fishing line or rope fouled in the aft seal, excessive bearing weardown letting the shaft run off-centre, wrong header tank pressure, and misalignment. A small, steady loss is normal running wear rather than a fault.
Checked against
Sources
- 01Stern tube sealing system, TriboNet
- 02Oil-lubricated seals, Wärtsilä
- 03UR Z21 Rev.4, Surveys of Propeller Shafts and Tube Shafts, International Association of Classification Societies, hosted by Türk Loydu
- 04About IRClass, Indian Register of Shipping
This guide explains how the system works in general, as the documents above describe it, and was last checked on 23 September 2026. Rules and thresholds change: the governing document is the authority, not this page.
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