The propeller of the ship is fitted at the aft and attached to a crankshaft coming from the main engine. This is done so that the rotating motion of the Main Engine can be converted into thrust to propel the ship. The propeller shaft or tail shaft is supported by a bearing arrangement which acts as an intermediate phase between the sea and the ship.
The stern tube is a hollow tube passing at the lower stern part of the ship carrying tail shaft and connecting it to the propeller out at sea, bearing for the tail shaft, lubrication arrangement and most importantly the sealing arrangements.
The stern tube bearing arrangement and sealing plays a vital part in ship’s operation and pollution prevention.
The two main purpose of the stern tube bearing are:
The propeller which hangs at the aft end exerts load on the shaft, which is supported and withstand by the stern bearing. The bearing is a cast iron bush lined with a white metal having excellent load handling and lubricating property.
The stern tube is fitted at the stern frame and internal framing of vessel’s hull at aft peak.
This allows the tail shaft to rotate smoothly in the bearing area for uninterrupted propulsion.
The stern tube bearing consists of sealing arrangement to prevent ingress of water and to avoid the lubricating oil to escape into the sea.
The lubrication system for ships with variable draught (due to loading and unloading of cargo) consists of header tanks located at around 2 to 3 meters above the water line so that the differential pressure ensures no water ingress.
Different sealing arrangements are used to prevent water ingress and oil leakage. They are as follows:
- Stuffing boxes consisting of packing material.
- Lip seals in contact with shaft to prevent passage of oil or water along the shaft.
- Radial face seals supported with springs fitted radially around the shaft, aft bulkheads and after end of the stern tube.
Out of these, the lip seal arrangement is most popularly used. Read more about Lip seals here.
You may also like to read-What is a Prop Guard?