What to check on a Stern Tube LO Pump.
This pump circulates lubricating oil through the stern tube bearing that supports the propeller shaft where it passes out through the hull. Unlike most lubricating oil pumps on a ship, which serve an engine that can simply be stopped if oil pressure is lost, the stern tube bearing supports a shaft that is often still turning under way, driven by the propeller's momentum even after the pump stops. That makes redundancy and reliable low-flow alarms more important here than in most other lubrication circuits.
What makes this type
This pump circulates lubricating oil through the stern tube bearing that supports the propeller shaft where it passes out through the hull. Unlike most lubricating oil pumps on a ship, which serve an engine that can simply be stopped if oil pressure is lost, the stern tube bearing supports a shaft that is often still turning under way, driven by the propeller's momentum even after the pump stops. That makes redundancy and reliable low-flow alarms more important here than in most other lubrication circuits.
Main components
Pump unit
Usually a small positive displacement or centrifugal pump, often supplied in a duplicate arrangement with a standby unit on automatic changeover.
Header tank
A gravity tank mounted above the waterline that maintains a positive pressure at the stern tube seal, preventing seawater ingress even if the pump stops, on oil-lubricated stern tube arrangements.
Cooler
A small heat exchanger, often seawater-cooled, keeping oil temperature within the bearing's design range under continuous circulation.
Filters
Fine filtration protecting the bearing surface from any debris in the oil circuit, usually duplex to allow cleaning without stopping flow.
Seal arrangement
Forward and aft seals at the stern tube, whose condition depends directly on the pump maintaining correct oil pressure relative to sea pressure outside the seal.
Selection / sizing
- Bearing type: oil-lubricated white metal bearings need continuous forced circulation; water-lubricated arrangements do not use this pump at all, which is the main dividing line in this category.
- Required flow rate against shaft speed and bearing length, set by the bearing manufacturer.
- Header tank height above the waterline, fixed by the need to keep seal pressure positive relative to sea pressure at the draft the ship normally operates at.
- Standby pump arrangement and automatic changeover logic.
Regulations / Class
Class rules require duplication or an equivalent standby means of lubrication for the stern tube bearing on most vessels, along with a low-pressure or low-flow alarm at a manned station. MARPOL Annex I governs stern tube lubricants where an oil-to-sea interface exists, and many owners now fit environmentally acceptable lubricants in the stern tube specifically to reduce pollution risk from that interface, which class societies recognise with dedicated notations in some cases.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Loss of pump flow | Electrical fault, blocked suction, standby pump not started in time | Bearing runs dry under load, rapid wear or scoring of the shaft liner |
| Header tank low level | Slow oil loss through the aft seal not topped up | Seal pressure falls below sea pressure, risk of water ingress |
| Cooler fouling | Seawater side scaling or biofouling | Rising oil temperature, reduced film strength at the bearing |
| Water in oil | Seal wear allowing seawater past the aft seal | Emulsified oil loses lubricating properties, accelerates bearing wear |
What to look for in a supplier
- Pump duty point matched precisely to the bearing manufacturer's flow requirement, not an oversized generic unit.
- Reliable automatic changeover logic between duty and standby pump, tested during commissioning.
- Compatibility with environmentally acceptable lubricants if the vessel is fitted or being retrofitted with them, since viscosity and seal material compatibility differ from mineral oil.
Check header tank level and pump changeover alarm function at every engine room round. This is one circuit where the consequence of a missed low-level alarm is a shaft that has to come out for repair, not a part that can be swapped at sea.