Oil-Lubricated Stern Bearing System

The primary method for preventing seawater ingress into an oil-lubricated stern bearing system is a combination of mechanical seals and maintaining a balanced oil pressure. The system uses lip seals to contain the lubricating oil within the stern tube. An oil header tank ensures the oil pressure inside the stern tube is approximately equal to the surrounding seawater pressure. This balanced pressure prevents seawater from entering the stern tube.

Corrective Actions While Afloat

If the stern bearing system fails and seawater begins to ingress, the following temporary corrective actions can be taken while the vessel is still afloat:

• Switch to High-Viscosity Oil: The system can be recharged with a higher-viscosity oil. This thicker oil is less likely to leak past the seals, reducing the rate of seawater ingress.
• Install a Temporary Header Tank: Disconnect the regular oil supply line and connect a 45-gallon drum. This drum, supported by a block and tackle, acts as a temporary header tank with a variable head. The height of the drum can be adjusted by raising or lowering it to match the seawater pressure, ensuring the correct pressure balance is maintained.

Why Two Stern Bearing Oil Header Tanks Are Fitted

In some cases, two stern bearing oil header tanks are fitted, especially on vessels that experience large variations in draft, such as tankers. The two tanks are installed at different heights to accommodate these draft changes.

• The purpose is to match the oil pressure to the changing seawater pressure as the vessel's draft changes.
• By having tanks at different heights, the crew can switch between them to maintain the necessary differential pressure to keep seawater out of the stern tube. The maximum allowable pressure difference between the seawater and the oil is typically 0.3 bar.
• For example, the changeover between the tanks is often done at a specific draft, such as 11.7 meters.

Modern ships often use a single header tank with an air pneumatic system. This system automatically adjusts the oil pressure to match the seawater pressure based on the vessel's draft, eliminating the need for manual checks and tank changes.