Oil lubricated bearings

Unlike for the water lubricated stern tube a shaft liner is unnecessary. Generally a small one is fitted in way of the aft seal bolted on to the propeller boss. In this way it excludes sea water contact with the main shaft and provides an easily replaceable rubbing surface for the seal. Lined with white metal are to have a bearing length so as not to exceed a bearing pressure from the weight of the shaft and propeller of 5 kg/cm2. The limitations of a bearing are the load it can withstand without metal cracking or squeezing out and the temperature it can withstand without melting. Length of bearing not less than 2 Н D in any case. Cast iron and bronze bearings must have a bearing length not less than 4D. Lubrication system must be capable of maintaining oil tightness despite varying temperature. Gravity tanks fitted with low level alarms, Usual for aft peak to be filled with water to provide cooling low suction valve to be fitted to be locked shut.
Wear down for the white metal should not exceed 2mm to avoid hammering out and the period for inspection is 6 years. A highly resilient reinforced plastic may be used in place of the white metal. It is claimed to have greater load carrying capacity, high resistance to fatigue and shock loading, with good lubrication properties. Ceramic liners can also be used.

This system depends upon Hydrostatic lubrication stern tube oil charge remaining in stern tube until pressure test is carried put to ensure that oil supply line is not blocked. This is done by manipulation of valves at header tank and operation of pump which slightly over pressurises stern tube. Oil returning to tank indicating clear oil lines. Top half of white metal bearing is usually machined to give a left hand and right hand helix, this gives a small pumping pressure forward to aft to provide lubrication and to assist in maintaining oil tightness of the oil seals.

If outboard seal leaks, the following steps are to be taken
1. Fresh water in gravity tank to emulsify and coagulate it, oil pumped around system to seal and lubricated.
2. Recharge with high viscosity oil
3. disconnect oil supply line and reconnect to 45 gallon drum which is supported by block and tackle in order to give a variable head. By raising and lowering the drum the oil pressure in the system can be made to match the water pressure from outside (taking into account the difference in gravities.

When large propellers are fitted the heavy overhanging weight greatly increases the load at the after end of the stern tube breaking down the hydrostatic lubrication causing metal to metal contact and seizure towards the aft end of bearing. To obviate this it is usual to angle the shaft downwards for about 8mm over 100m length thus attempting to ensure than the weight of the bearing is taken on the full length of the bearing. It is good practice to leave the oil tank open to the stern tube when in port with machinery stopped, this prevents sea water leaking into the system. However, water has been known to contaminate lubricating oil systems causing rusting of tail shaft particularly when shaft is stopped for periods long enough for water to settle in bottom of bearing. Fit only water seperator I,e, a coalescer or cyclonic or osmosis system.
In ships with large changes in draught it is usual to fit two gravity tanks. The upper tank is used when fully loaded or there is water leaking in.
Water based oil replacements
Available are water based sterntube lubricants having the advantages of oil but with a more eco-friendly face. These lubricants must have an adequate viscosity, resistance to sea water contamination as well as biodegradability.

They typically have a water content greater than 90% and are highly soluble. Friction is reduced in comparison to equivalent mineral oil/white metal bearing.

Other benefits include increased heat transfer rates and better protection against galvanic corrosion of dissimilar metals found in the shaft/prop arrangement.The fluid has no measurable flash point.