- The stern tube is normally constructed of cast iron slightly larger at the forward end to ease removal. The forward end is flanged and bolted to a doubler plate stiffened aft peak bulkhead. The forward end is supplied with a stuffing box and gland, the after end with a bearing comprising lignum vitae or similar, the wood is dove tailed into a brass bush, the wood is machined and cut on end grain. Can be lined with Lignum Vitae , rubber composition (cutlass rubber) or an approved plastic material (Certain plastics possess good bearing properties being inert and very tolerant of slow speed boundary lubrication conditions. Cresylic resin bonded asbestos such as Railco WA80H give good results in condition of heavy water contamination in the lubricating oil of almost 100%)For water lubricated bearing not less than 4 x the diameter of the steel shaft. If the bearing is over 380mm diameter forced water lubrication must be used, a circulating pump or other source with a water flow indicator.
The shaft is withdrawn for examination every 3 years.
Modern Water tolerant oil lubricated stern Tube
With the increase in size of VLCC's shipping companies required a stern tube bearing capable of operating with high degrees of water comtamination. The alloys in white metal tend to oxidise and the clearance is removed leading to seizure. In addition as shaft revs reduced in search of improved propeller efficiency the hydrodynamic forces available become limitedfor oil film generation. For this reason Railco WA80H bearings where developed.These contained a phenolic resin impregnated asbestos yarn. The next generation contained non-asbestos material. This material tended to be tainted due a series of overheating problems. (later found due to the combination of stiff high power transfer shafts and flexible hull design).The modern material is called SternSafe and comprises an inner bearing surface with an overwound outerlayer. This has greater tolerance to overheating and reduced swell in the event of water contamination. The latter allows for reduced running clearance and thereby greater control of the shaft position reducing oil loss, seal damage and water ingress.
A wear gauge is incorporated into the bearing as our temperature sensors.