Stern Tube Seals
A ship requires vast quantities of water to float and sail, yet this very sea water should not enter the ship from any openings or leaks. Of course the ship is made of strong material but certain points are vulnerable and could act as entry points for sea water. Just imagine the thick propeller shaft protruding out of the ship’s hull and connecting the propeller to the main engine inside the engine room. How do we ensure that water from outside does not enter the ship through this opening.
A stern tube, which is present at the bottommost part of the ship, connects the engine, inside of the ship to the propeller outside. A propeller shaft passes through this stern-tube and is used to connect the propeller to the engine. Stern tube bearings, generally oil lubrication glands, are used to prevent the ingress of sea water inside the engine room and also to support the weight of the propeller. But this is not it, the whole stern tube arrangement is packed and sealed from both the sides for additional protection of the stern tube. The packing material used for these sealing arrangement are known as stern tube seals.
Thus, these specially made seals, fitted at the inboard and outboard ends of the tail shaft are used for two main functions :
- To provide extra protection for preventing the entry of sea water
- To prevent loss of lubricating oil from the stern tube bearings.
Types of Stern Tube Seals
In the older ships, where the timber bearings with sea water lubrication systems were used, conventional stuffing box and glands were used at the aft bulkhead of the ship. However, the modern ships which uses oil lubricated stern bearings mainly uses two types of seals :
- Lip seals
- Face seals
The subsequent sections describe both these types of seals with the help of extensive diagrams which would help the reader to clearly understand how the arrangement works in both types of seals.
Lip seals are seals made of special kind of materials (elastic) , having a projecting lip or edge. This peculiar shaped ring is held in close contact with the shaft and the stern tube to prevent oil leakage or water entry. The shape, size and number of lip seals depends on the size and type of application.
Face Seals are seals made in pairs. The seals consists of two mating radial faces, which are forced against each other to prevent any kind of leakage. Out of the two faces, one is rotating and the other is stationary. The rotating seal is attached to the propeller boss and the stationary seal is attached at the aft bulkhead. The arrangement also consists of a spring which forces the two faces, one stationary and other rotating towards each other in order to provide the adequate seal.