Exam Question

A modern oil-lubricated stern tube sealing arrangement is designed to perform two critical functions: prevent the leakage of lubricating oil from the stern tube bearing into the sea, and prevent the ingress of seawater into the stern tube. The system typically consists of two sets of seals: an aft seal assembly located outboard to seal against seawater, and a forward seal assembly located inboard to seal against the engine room bilge.
Each assembly contains multiple lip seals (typically 3 to 4) made of an elastomeric material, which run on a dedicated, highly polished chrome-steel liner fitted over the propeller shaft. The seals are energised by garter springs to ensure constant contact pressure against the liner. The space between the seals is filled with lubricating oil, which is circulated, cooled, and supplied from a header tank to maintain a slight positive pressure over the external seawater pressure.
### (a) Common Forms of Seal Failure (6 Marks)
Seal failure can occur due to a combination of operational stresses, material degradation, and mechanical damage. The most common forms are: