STERN TUBE ARRANGEMENTS ON SHIPS

The ship propeller isattached at the aft and linked to a crankshaft  issuing  from the main engine. This is so that the spinning motion of the Main Engine can be changed into thrust that propels the ship. The propeller shaft or tail shaft is being supported by a bearing arrangement which functions as an intermediate between the sea and the ship.

The stern tube is actually hollow tube that passes at the lower stern component of the ship carrying tail shaft and joining it to the propeller out at sea, bearing for the tail shaft, lubrication arrangement and most outstandingly the sealing arrangements.

The stern tube bearing arrangement and sealing carries out a vital part in ship’s operation and pollution prevention.

The two vital purpose of the stern tube bearing are:

Withstand load

The propeller that hangs at the aft end exerts load on the shaft, which is keptin position 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 turn smoothly in the bearing area for uninterrupted propulsion.

Sealing

The stern tube bearing comprise of sealing arrangement to stop ingress of water and to prevent the lubricating oil escaping into the sea.

What is sealing arrangement

The lubrication system for some ships with variable draught (due to loading and unloading of cargo) comprise of header tanks found at around 2 to 3 meters above the water line so that the differential pressure allows no water ingress.

Different sealing arrangements are used to stop water ingress and oil leakage:

  • Stuffing boxes comprising of packing material.
  • Lip seals in contact with shaft to stop passage of oil or water along the shaft.
  • Radial face seals supported with springs fitted radially around the shaft, aft bulkheads and the after end of the stern tube.