The seawater-lubricated propeller shaft

  • Use of seawater lubrication dates back to 1843
  • The original bearing was Lignum Vitae - a very dense oily wood noted for its strength
  • Seawater lubricated systems are less complicated and time consuming
  • Zero pollution equals no criminal penalties or fines from illegal oil leaks
  • Potential fuel savings can be achieved

Despite the introduction of legislation to reduce shipping’s impact on the oceans, together with shipowners’ ever-present need to reduce operational costs, the majority of commercial ocean-going vessels continue to use a system that is increasing risk and is costly to operate, Thordon Bearing notes.

The paper explains that, while the low capital expenditure of an oil-lubricated system is an obvious attraction, any financial advantage is completely lost once the vessel enters the water. This, cites Thordon Bearings, is due to the costs associated with purchasing lubricating oils, the regular maintenance and unscheduled drydockings required to repair or replace faulty shaft seals.

It is quite staggering that over 95% of all new commercial ships continue to be built with oil lubricated propeller shafts – a system that is not only operationally expensive but environmentally questionable. Our Future, Our Ocean paper presents the case for water lubrication to shipowners and shipbuilders as a commercially and technically viable way of increasing profits while achieving corporate sustainability goals,

...said Craig Carter, Thordon Bearing’s Director of Marketing and Customer Service.

Environmental benefits

  • No oil expenditures (acquisition, storage & disposal costs)
  • Eliminates aft seal repair and emergency drydocking costs
  • Zero pollution - stops environmental fines and penalties
  • Installation costs recouped
  • Bearing wear-life guarantee for newbuilds

While emergency seal repairs alone can cost anywhere from US$150,000 to US$300,000, excluding drydock costs, the paper furthers that the constant topping up of an oil-lubricated system, combined with the regularity of aft seal failure, can cost shipowners in excess of US$6.5 billion over a 25-year period.

The paper reveals that for a typical single screw bulk carrier turning a 650mm shaft, the total cost of operating an oil-lubricated system over 25 years (including capital investment) is US$605,925 against $370,000 for a seawater-lubricated arrangement.

Although part of the cost of operating an oil-lubricated system is dependent on oil price fluctuations and does not take into account the use of more expensive EALs, the use of seawater as the lubricating medium is obviously free of cost.

While the environmental impact of oil leaking from sterntubes has been well documented, the commercial impact on the shipowners bottom line has not been addressed. This study intends to raise awareness of the commercial disadvantages from the continued use of oil lubricated propeller shaft bearing systems, while offering a more cost-effective and more environmentally acceptable solution.

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