Stern Tube Lubrication, Sealing & Bearing Systems

Stern Tube in Marine Propulsion Systems

The stern tube is a key structural and functional part of a vessel's propulsion system. It houses and supports the propeller shaft where it passes through the aft hull, forming the transition between the inboard shaft line and the outboard propeller.

Its main purpose is to provide bearing support, maintain watertight integrity, and help ensure correct shaft alignment under both static and dynamic operating conditions.

Structurally, the stern tube consists of a robust outer shell, usually made from steel and integrated into the hull structure. Inside the tube, one or more bearings support the rotating shaft and transfer loads from the shaft and propeller into the vessel structure. These bearings may be white-metal, composite, or elastomer-based depending on the lubrication system and vessel design.

Seal assemblies are installed at the inboard and outboard ends of the stern tube to separate the sea from the vessel's machinery spaces and to control lubricant leakage or water ingress. In oil-lubricated systems, multi-lip or face seal arrangements are commonly used to retain oil and prevent seawater contamination. In water-lubricated systems, the design may use composite or elastomer bearings with simpler sealing arrangements.

Correct stern tube alignment is essential for reliable shaft operation. Misalignment can increase bearing pressure, accelerate wear, and contribute to vibration, noise, and fatigue in the shaft line and connected machinery. For this reason, stern tube installation and maintenance often include careful measurement, alignment checks, and inspection of bearing clearances.

The stern tube also plays an important role in vibration control and long-term propulsion reliability. Proper bearing spacing, lubrication, and sealing performance help reduce friction, limit shaft movement, and maintain stable operation throughout the propulsion train.

Because the stern tube forms a critical boundary between seawater and the vessel's internal machinery spaces, regular inspection of seals, bearings, lubricant condition, and system alignment is an important part of propulsion system maintenance.

Functional Applications of Modern Stern Tubes

A stern tube supports the propeller shaft, provides bearing support, and creates a controlled environment for lubrication and sealing where the shaft passes through the vessel's aft hull.

The design of the stern tube system — including lubrication type, seal arrangement, and bearing material — has a direct influence on reliability, energy efficiency, lifecycle cost, and environmental compliance.

Modern stern tube systems are typically designed around three key technical areas:

- Lubrication system

- Seal arrangement

- Bearing type

Together, these choices determine how the system manages friction, wear, water ingress, oil leakage, vibration, and long-term propulsion reliability.

Common Stern Tube Seal Arrangements

Stern tube systems can be configured with different sealing arrangements depending on vessel type, shaft design, lubrication system, environmental requirements, and maintenance philosophy.

The most common areas to consider are the aft seal, the forward seal, and the stern tube bearing section. Together, these components help control lubricant retention, seawater exclusion, shaft support, and long-term sealing stability.

Aft seal arrangements

The aft seal is located at the outboard end of the stern tube, close to the propeller. Its main function is to retain the stern tube lubricant and prevent seawater from entering the system.

Because the aft seal operates directly at the oil-to-sea interface, it is one of the most critical areas for oil leakage, water ingress, seal wear, and environmental exposure.

Forward seal arrangements

The forward seal is located at the inboard end of the stern tube, towards the machinery space. Its function is to retain lubricant inside the stern tube system and help protect the vessel's internal machinery areas from leakage.

Forward seal condition is important for system cleanliness, oil level stability, and controlled stern tube operation.

Lip seal systems

Lip seal systems are widely used in stern tube applications. They use one or more flexible sealing lips to control oil leakage and seawater ingress around the rotating shaft.

Seal lip wear, shaft surface condition, oil compatibility, and pressure balance are important factors in long-term performance.

Face seal and Cedervall-type systems

Face seal arrangements use sealing faces rather than flexible lips to create the sealing interface. Cedervall-type systems are commonly referenced in relation to stern tube sealing and may be used where robust mechanical sealing performance is required.

These systems can offer stable sealing performance, but condition, alignment, lubrication, and operating profile still influence leakage risk and service life.

Air seal systems

Air seal systems use a controlled air barrier between seawater and the lubricant side of the stern tube system. This can help reduce direct oil-to-sea exposure and support improved leakage control.

Air seal arrangements are more complex, but they are often used where environmental protection, seal monitoring, and reduced leakage risk are important.

CPP-related sealing arrangements

For vessels with controllable pitch propellers, the stern tube and shaft line may interact with additional oil-filled or hydraulically influenced components. This can make sealing conditions more complex.

In these cases, seal arrangement, oil type, pressure balance, leakage behaviour, and system layout should be reviewed carefully before any treatment or application is recommended.

Lubrication Systems

Oil-lubricated stern tubes

Oil-lubricated stern tubes use mineral oil or Environmentally Acceptable Lubricants (EALs) in a closed or semi-closed system. The oil helps create a stable lubrication film between the shaft and bearings, while also supporting corrosion protection and vibration damping.

Oil-lubricated systems are commonly used where high load capacity, stable bearing performance, and predictable maintenance intervals are important. They can offer low friction and long bearing life when oil condition, alignment, and sealing performance are well maintained.

However, oil-lubricated systems also require attention to oil sampling, seal condition, oil changes, and leakage risk. If the aft seal wears or becomes damaged, oil may escape to sea or seawater may enter the system.

For vessels operating in regulated waters, oil-to-sea interfaces may require the use of EALs, depending on applicable rules and technical feasibility. EALs can reduce environmental impact, but they may also require separate compatibility review due to differences in base oil chemistry, viscosity behaviour, and ageing characteristics.

Water-lubricated stern tubes

Water-lubricated stern tubes use seawater or fresh water as the lubricant, typically together with composite or elastomer bearing materials. Because there is no stern tube oil in the system, the risk of oil leakage to sea is largely removed.

Water-lubricated systems are often chosen where environmental simplicity, reduced oil handling, and lower system complexity are important. Modern composite and elastomer bearings are designed to operate reliably with water flow, cooling, and debris flushing.

Compared with optimized oil-lubricated systems, friction may be slightly higher in some operating conditions, especially during low-speed or boundary lubrication situations. However, water-lubricated designs can offer strong lifecycle advantages where environmental compliance and system simplicity are key priorities.

Sealing Technologies

Single seals

Single lip or face seals provide one sealing barrier between the stern tube system and seawater. They are simple, cost-effective, and commonly used in applications where system complexity must be kept low.

In oil-lubricated systems, single seals require careful monitoring, because seal wear can lead to oil leakage or seawater ingress.

Double seals

Double seal arrangements use two sealing barriers, often with an intermediate chamber that can be filled, monitored, or pressurized. This provides greater redundancy and improved protection if one sealing element starts to wear.

Double seals are more complex than single seals, but they can reduce the risk of serious contamination, oil loss, and unplanned maintenance.

Air seal systems

Air seal systems use a controlled air barrier between the lubricant and seawater, often combined with multiple lip or face seals. The air barrier helps reduce direct contact between seawater and oil, supporting improved sealing stability and early detection of seal issues.

Air seal systems are more complex, but they are often selected for vessels with higher environmental requirements or where reduced oil-to-sea leakage risk is a priority.

Bearing Types

The bearing design is one of the most important factors in stern tube performance. Stern tube bearings support the propeller shaft, carry radial loads, help maintain alignment, and influence friction, wear, vibration, and long-term reliability.

The most common bearing types are white metal bearings, composite bearings, and elastomer bearings.

White metal bearings

White metal, also known as babbitt, is traditionally used in oil-lubricated stern tube systems. A soft bearing layer is bonded to a stronger backing structure, creating a low-friction bearing surface when operating with clean oil and correct alignment.

White metal bearings can provide excellent hydrodynamic performance and long service life in well-maintained oil systems. They are commonly used where high load capacity, stable oil film formation, and predictable operation are important.

However, they depend heavily on oil cleanliness, lubrication stability, and correct shaft alignment. Water contamination, particles, oil degradation, or prolonged boundary lubrication can increase wear and may lead to bearing damage.

Composite bearings

Composite bearings are widely used in water-lubricated stern tube systems. They are typically made from engineered polymer or fibre-reinforced materials designed to operate with water as the lubricant.

These bearings can offer good wear resistance, corrosion resistance, and stable performance in marine environments. Grooved bearing designs help promote water flow, cooling, and debris removal during operation.

Composite bearings are often selected where environmental simplicity, reduced oil handling, and resistance to seawater exposure are important. Their performance depends on proper water flow, alignment, material selection, and operating conditions.

Elastomer bearings

Elastomer bearings, often based on rubber-type materials, are also used in water-lubricated stern tube systems. They are known for good vibration damping, tolerance to shock loads, and ability to handle some degree of shaft movement or misalignment.

They are commonly used in workboats, offshore vessels, and other applications exposed to variable loads, low-speed manoeuvring, or abrasive water conditions.

Elastomer bearings are generally simple and robust, but wear rates can increase if water flow is poor, contamination levels are high, or the system operates for long periods under heavy boundary lubrication conditions.

Why Bearing Condition Matters

For oil-lubricated stern tube systems, bearing performance depends strongly on lubricant condition, seal stability, contamination control, and shaft alignment. If the oil film becomes unstable, or if water and particles enter the system, friction and wear can increase over time.

This may lead to higher operating temperatures, reduced bearing protection, seal stress, and increased risk of leakage or water ingress.

Proactive lubrication support and leakage control can therefore play an important role in maintaining stern tube reliability between planned service intervals.

Emerging Developments in Stern Tube Technology

Stern tube technology continues to evolve in response to increasing operational, technical, and environmental demands. Across the industry, greater attention is being placed on sealing stability, wear control, lubricant performance, condition monitoring, and long-term reliability.

For vessel operators and technical teams, stern tube performance is no longer assessed only by basic function. It is increasingly connected to maintenance planning, operational continuity, environmental responsibility, and lifecycle cost.

Addvation follows these developments closely, with a practical focus on sealing behaviour, wear management, and system stability in oil-lubricated stern tube applications where reliability is critical and service access may be limited.

At the same time, we continue to monitor material developments, lubricant technologies, and alternative technical approaches that may become relevant to future sealing and lubrication-related applications.