Stern Tube

The stern tube is a watertight assembly penetrating the after end of the hull that supports and seals the propeller shaft as it exits the ship into open water. It consists of a structural tube welded into the hull, forward and aft bearings supporting the shaft, and a stern tube seal assembly at the aft end preventing seawater ingress and lubricant loss. Traditional water-lubricated white metal bearings have largely been replaced on modern vessels by oil-lubricated designs, which require a pressurised oil seal system and strict monitoring of oil levels and consumption to detect seal deterioration. Stern tube oil consumption and losses must be recorded in the Oil Record Book Part I under MARPOL Annex I because lubricating oil lost through a failing aft seal is discharged directly into the sea. Classification societies prescribe inspection intervals for the stern tube seals and bearing clearances, often during drydock surveys every two to five years. Excessive vibration or bearing temperature rise on the shaft revolution counter are early warnings of bearing failure or shaft misalignment that engineers must investigate immediately to prevent catastrophic damage to the propulsion system.