Stern Tube Lubricants — Choices for efficiency and environment
Proper lubrication of the propeller shaft bearings is essential to the efficiency of the ship. The weight of the propeller and shaft must be supported in bearings which also play a role in the proper alignment of the shaft. Without adequate lubrication much of the energy of the shaft would be lost to friction which in turn would mean potential for damage and also increased fuel consumption.
The choice of lubricant will depend to some extent on the bearing and sealing arrangements but also on local regulation. The latter point was much less of an issue until 2013 when the US authorities introduced the Vessel General Permit (VGP) covering permissible discharges of pollutants in US waters.
For the greatest part of the history of propeller driven ships, the bearing material of choice was Lignum Vitae which is still in use in some older ships today. The bearings comprised of several staves of the wood inserted longitudinally into the perimeter of a bronze sleeve surrounding the propeller shaft. Lignum Vitae is more than adequately lubricated by seawater and to prevent ingress into the ship a stuffing box filled with packing material was located immediately forward of the Lignum Vitae bearing.
As the use of wooden bearings declined in the second half of the 20th century, they were replaced by white metal bearings which required oil lubrication. The lubricant products used then were all mineral oil based and today the largest proportion of stern tube bearings are lubricated using mineral oil-based products. To prevent excessive loss of oil and ingress of seawater, seals were needed at either end of the stern tube replacing the stuffing box. Even with seals, wear and damage would mean that some leaking was inevitable.
Some ships did still use water lubricated bearings replacing the lignum vitae with rubber or man-made materials but in general the practice declined in favour of the metal bearings.
Bio-degradable lubricants a new step for ships
Lubricant manufacturers produce a variety of different lubricants for stern tube bearings. Until Vickers produced the first bio-degradable oil in 2002, all of the lubricants for metal bearings were mineral oil-based as are most available today. The most commonly used oils are of the SAE 30 or SAE 40 types. One leading seal maker lists around 150 different mineral oil products from almost 30 manufacturers as being compatible with its seals and bearings.
The fact that the first commercially available biodegradable oil preceded the US VGP by more than a decade highlights the fact that the polluting effect of leaking stern tube lubricants was already becoming to be seen as unacceptable. Under MARPOL, operational leakage from stern tubes is permitted to the extent of normal loss from seals but excessive leakage would be considered as a violation of MARPOL Annex 1. Almost certainly there are ships in operation which do suffer from excess leakage but there appear to be very few prosecutions – probably because perpetrators conceal the fact with false oil record book entries and if questioned can claim that a seal must have become damaged just prior to discovery.
Take up for biodegradable lubricants was not high but has been much boosted by the US VGP rules. Since December 2013 all vessels in US waters must use environmentally acceptable lubricants (EALs) in all oil-to-sea interfaces, unless technically not feasible.
There are no exemptions under the rule and the technically not feasible get out applies only if there are no EALs specified by bearing and seal manufacturers as being compatible with the system on board the ship. However, it is not a long-term reason and the systems must be changed to ones that can work with EALs at the next drydocking.
Shipowners must document in their Annual Reports the specific EALs used for each oil-to-sea interface or reasons why an EAL was not used. It should be noted that use of an EAL does not prevent a violation of the VGP. The vessel owner and/or operator must not discharge oil (including EALs) in quantities that may be harmful to the environment as defined in 40 CFR Part 110.
EALs are usually formulated on a base oil of vegetable origin, a synthetic ester or polyalkylene glycol (PAG). The different properties of different types of EALs can make ensuring a compatible mix difficult if the type being used becomes unavailable and a substitute must be used for topping up. PAGs can also exhibit different characteristics within the group.
Some are soluble in water and some in oil. Water-soluble variants are not compatible with mineral oils and therefore must be handled and disposed of separately. If mixed with mineral oils the result is a gelatinous compound that not only does not lubricate but actually causes more friction and problems. Some EALs can also react with and damage non-metal components of seals.
In the period shortly after the VGP required ships to operate with EALs, there were very few products available. That position has changed but the number of products is still far fewer than mineral oil lubricants. The seal maker mentioned earlier lists less than 50 products as compatible with its range of seals – about one third of the number of mineral oil products.
Back to basics with a modern approach
The alternative to mineral oil and EALs is to use seawater although making use of new bearing materials rather than a return to the days of Lignum Vitae. Modern water lubricated bearings are manufactured by a number of suppliers from various materials. Polymers and composites are the most common. In general, EALs are more expensive than mineral oils and their longevity has recently come into question with suggestions by some class societies and insurers that their use has been linked to a higher incidence of shaft and bearing failures.
The bearings can be produced in different ways; as full sleeves, staves or half sleeves. Water lubricated bearings are not without issues of their own although in the main, the early teething troubles have been overcome by improvements in design and materials. The problems included wear caused by grit and sand in the seawater which has been addressed by incorporating grooves in the upper half of the bearing to transport particles away from the load-bearing lower half of the bearing. Some materials used were found to be subject to absorbing water and swelling with the potential for increased friction or even seizing. Material technology has mostly eliminated this fault.
Water-lubricated bearings have some obvious advantages when compared to the more conventional solutions in use. Firstly, and most obviously, the lubricant – seawater – is effectively free and the method for automatic replenishment in the bearing means that little maintenance is required. Secondly, there is no question of contravening any of the pollution regulations either from MARPOL, the US VGP or any other local law that may exist. There is also no risk of the lubricant being contaminated by use of an alternative product.
The choice of lubricant is a personal one for any shipowner but the combination of the VGP and budgetary constraints does appear to have increased the number of ships operating with water lubricated bearings.