Developments in Seawater Lubricated Propeller Shaft Bearings for Commercial Ships

Early propeller shafts operated in water lubricated lignum vitae bearings and used stuffing boxes as seals at hull penetrations. The bearings, however, required regular replacement, and the stuffing boxes not ideal as seals. Around 60 years ago, new seals were developed that allowed the use of white metal bearings for propeller shafts. The technology offered a controlled environment for the bearings, which extended bearing lifespan. The weakness of the system, though, has always been the effectiveness of the seals, which must leak some amount in operation for cooling. Yet leakage into the stern tube causes oil emulsion and often catastrophic bearing failure, while leakage from the stern tube results in pollution. With even the latest sealing technologies, that weakness continues; sea pollution remains a possibility and is becoming ever-more intolerable, as witnessed by increased environmental awareness and legislation. Improvements in water lubricated material technologies have resulted in greater choice of materials, which can now offer improved and predictable bearing wear life. Progression of sealing technology has also replaced the previous troublesome packing glands. Classification Societies Rules related to water lubricated propeller shaft bearings are also undergoing revision and extended shaft withdrawal is now possible.