Koninklijke Vereniging - Société Royale



Our FUTURE - Our OCEAN : Environmental & Commercial Benefits of SEAWATER LUBRICATED PROPELLER SHAFTS

A White Paper Published by Thordon Bearings Inc.


Despite the introduction of legislation to reduce shipping’s impact on the oceans, together with shipowners’ ever-present need to reduce operational costs; 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.

While the low capital expenditure of this technology is an obvious attraction to shipyards and shipowners, this financial advantage is lost on operation. Firstly, the aft seal of an oil lubricated propeller shaft system is inherently prone to failure, resulting in a number of ships having to pay for drydock or in situ emergency repairs and seal replacements. And although it is difficult to put a definitive figure on the total number of vessels affected, regular reports in the international maritime press of commercial vessels undergoing propeller shaft seal repair is indicative of the scale of the problem.1 Keeping an oil lubricated system in good order means high operating costs spent on aft seal maintenance and replenishing the lubricant.

Indeed, according to a Lloyd’s Register Classification Society report: “Defect statistics over the last 20 years indicate that the aft stern bush represents 10% of all shaft line failures, with the forward stern bush representing 4% of total failures. Interestingly, the aft stern gland (seal) and forward stern gland (seal) represent 43% and 24% of failures respectively. 2

While the repeated failure of this component is an obvious boon for ship repair yards and underwater hull maintenance companies, for the shipowner such unscheduled and unbudgeted drydocking fees and maintenance, along with any financial loss from vessel downtime and potential fines including financial impact on reputation, can render any savings gained from the initial expenditure of an oil lubricated propeller shaft as immaterial. An additional operating expense that shipowners need to budget for when selecting an oil lubricated propeller shaft bearing system is the amount of oil required to carry onboard to replenish the system as oil discharges occur.

A typical ‘sealed’ oil system, where the bearings supporting the shaft are mounted inside a hollow tube sealed with a lip type seal at each end, is filled with about 1500 litres (396 U.S. gallons) of mineral or synthetic oil. But rather than being considered a closed system, as it is designed to be, lubricating oil is commonly viewed as a consumable, with some shipowners accepting propeller shaft seal oil leakage as “normal operational consumption” and acceptable practice.

In the past, some Classification Societies provided Type Approvals that listed oil discharge rates of between 6 to 12 litre (1.6 to 3.2 U.S. gallons) per day3, which adds up to a colossal 244 million litres (64.5 U.S. gallons) of operational oil being discharged by commercial ships annually. That is equivalent to about five Exxon Valdez events every year. In 1989, the tanker spilled 41.6 million litres (11 million U.S. gallons) of oil into the Alaskan environment.

Aside from the environmental impact of this currently “legal and acceptable practice”, which will be covered further in this paper, the cost of constantly topping up an oil lubricated system combined with the regularity of aft seal failure is estimated to cost the shipping industry in excess of US$6.5 billion over a twenty-five-year period. Such needless expenses can be completely negated or re-invested in the purchase and installation of, say, a couple of ballast water treatment systems or, indeed, a new asset for the ship owner.

The intention of this White Paper is not only to raise awareness of the commercial disadvantages and environmental damage created from the continued use of oil lubricated propeller shaft bearing systems, but also to offer a more cost-effective and more environmentally acceptable solution:



With more than 90% of world trade carried by about 50,000 ocean­going ships of 1000dwt or more, the threat of oil pollution is a real and growing international concern. However, in the past ten years, there has been a seismic shift in attitudes towards our environment and in some parts of the world, stricter legislation is intended to reduce the impact of shipping on the marine environment.

Legislation is now in force to reduce ship-to-air and ship-to-sea emissions, yet while the discharge of, say, oily bilge waters has been heavily regulated, operational oil leakage remains ‘legal’, despite some 244 million litres (64.5 million U.S. gallons) of oil being discharged annually from ships operating oil lubricated propeller shaft systems .4 It is widely believed that this type of pollution will also fall under the regulatory gavel. A prime mover in reducing the impact of operational oil on the marine environment was the U.S Environmental Protection Agency, which in 2013 mandated that all vessels trading in U.S waters use an environmentally acceptable lubricant (EAL) in all oil-to-sea interfaces. However, as many seal manufacturers attest, it is not a case of simply replacing mineral oil with an oil-based EAL.

While compliant with revised Vessel General Permit (VGP) rules, a major concern is that not all oil-based EALs are compatible with traditional propeller shaft seals. Some EALs can be used based upon the seal supplier’s recommendation, but it is often the case that shipowners need to upgrade the sealing rings or the seal itself. And if an alternative seal is required, then third party certification is recommended to determine oil-tight integrity which, of course, further adds to the cost of an oil lubricated propeller shaft system.

Another concern is that oil-based EALs absorb a major amount of water, which can place the longevity of the EAL into question. It could also affect the performance of system components, resulting in greater wear and tear due to corrosion. A question raised by one shipowner was “if an equipment [seal] manufacturer indicates that the working life of a conventional seal is reduced when used with EALs, is it possible to use a non-EAL lubricant based on technical infeasibility?” The EPA says no .5

From an environmental perspective, the effect of these new EALs on marine life and birds must be considered the same as mineral oil-based lubricants in that their chemical composition can be toxic, with a viscosity that coats the feathers and fur of marine life causing hypothermia and death.6

What’s more, the use of an EAL in a conventional oil lubricated propeller shaft system is not a cure-all to meeting corporate social responsibility goals, since the system’s aft seal – the only barrier between the oil and the sea – can be damaged allowing lubricant discharges, if rope or fishing line wraps around the rotating shaft or if the propeller itself is affected by ice impact or grounding.

Not only can major seal damage cause the entire shaft lubricating oil (whether mineral or vegetable) to leak and pollute, but seawater ingress can corrode the shaft and bearings. Seawater dilutes the lubricant, reduces the viscosity and may cause damage to the bearing and possibly the shaft. It could also result in civil or criminal lawsuits being issued for unlawful pollution and an increase in insurance premiums.

It was reported in Britannia P&I Club’s RiskWatch7 magazine that Brazil’s Federal Prosecution Office is now treating even the slightest oil spill as seriously as larger pollution incidents, with the Club warning that “small spills do not necessarily correspond to small [insurance] claims”.

In the article, Brazilian law firm Siano & Martins Advogados Associados advised shipowners to treat even a small spill with care, since ‘contamination of habitats or organisms’ can result in civil lawsuits and significant punitive damages. Britannia also reported that there could be no time limit on civil public actions due to the long-term effects of oil pollution on the environment.



To prevent oil pollution, reduce ship operational costs, and to simplify installation and maintenance procedures, shipowners can now adopt an alternative oil/EAL-free shaft bearing lubricating system that uses seawater as the lubricating medium. The technology, common place in ships prior to the introduction of oil lubricated systems in the 1950s, uses seawater pumped through non-metallic polymer bearings before discharge back into the sea.


Non-metallic bearings are at the heart of the award-winning8 seawater lubricated propeller shaft bearing system. The polymer alloy bearing is homogeneous (there are no layers of differing materials) with properties that are consistent throughout the wall thickness. The polymer alloy bearing is also highly abrasion resistant and designed with no grooves in the bottom half of the bearing to reduce running friction and improve low speed hydrodynamic film development. And although start-up friction is initially higher than an oil system, at rated shaft speeds, viscous friction acting on the rotating shaft is lower with seawater than with oil. This has been shown to lead to fuel savings9 and thus reduced emissions. A key benefit of the seawater system is that it negates the need for a damage-prone aft seal, while new developments in forward seals provides a level of system redundancy up to this time unknown with seawater lubricated bearing systems.



A key difference between the oil and seawater lubricated system is that the latter technology requires a means to protect the propeller shaft against seawater corrosion. Fortunately, shaft coating technologies have significantly improved since the early days of water lubricated bearings. And today, two-part polymer coatings have been developed to keep the propeller shaft corrosion free. Typically, bronze liners, InconelTM or welded cladding are used in way of the bearings.

New developments of shaft coatings for seawater lubricated propeller shafts prevent completely the cracking of shaft coatings and subsequent saltwater corrosion. A homogenous shaft coating offers a much higher level of corrosion protection and is three to nine times more flexible than traditional wrapped propeller shaft coatings. The effectiveness of the coating system was confirmed in extensive tests subjected to a harsh 180 degree bend. It failed to crack. The development in new shaft coatings was a major factor in the decision by Classification Societies to extend shaft withdrawal periods of seawater lubricated propeller shafts making them technically equivalent to oil lubricated propeller shafts.

Corrosion protection may mean higher up-front costs than an oil lubricated system but the elimination of the aft seal and associated maintenance, together with the obsolescence of oil/EAL and the requisite storage, sampling and disposal, these costs are recouped with a three to five year return on investment.


The quality of the seawater supplied to the shaft bearings is critical in ensuring long predictable bearing wear life. With this in mind, a self-contained supply, conditioning and monitoring package is available to ensure that clean water is consistently delivered to both the forward seal and the bearings.

Designed for full integration into the ship's control and monitoring systems, the Water Quality Package uses centrifugal forces to remove particulate from the water stream. Suspended solids with a specific gravity of 1.2 or higher and greater than 100μm are removed.

The system also incorporates a flow alarm to alert the operator of low water flow. The Water Quality Package supplies seawater to the propeller shaft bearings for lubrication and cooling at a minimum flow rate of 0.15 litres/minute/mm (1 USGPM/inch) of shaft diameter. This conditioning and monitoring package controls the bearing environment similar to an oil lubricated system.



In a seawater lubricated system, there is only one seal, to prevent seawater from entering the engine room. New forward seals are designed for a maintenance free life meaning that they are able to provide no visible or minimum leakage, long life and high operational reliability. Seal faces of silicon carbide are intended to last the life of the vessel. If there is a face failure, a Safe Return to Port Emergency Seal can be engaged. The seal is not meant to be completely water tight, but unlike other “maintenance seals”, the shaft can be rotated and the vessel can proceed to a repair facility on its own power, at 50% of shaft rpm.



In an open seawater lubricated system, Class rules may require a bearing monitoring system that enables real time observation of propeller shaft bearing behaviour by measuring the displacement of the shaft position in time due to bearing wear, and comparing it with the initial value at installation. The measured data is then displayed on the Control Panel located remotely in the vessel’s engine room or bridge.

There has been substantial growth in the newbuild and retrofit market for seawater lubricated propeller shaft bearings, with references on more than 2000 vessels, ranging from cruiseships, tankers and containerships to naval vessels and workboats. Yet still the vast majority of the global commercial fleet continues to operate with older technology that is costly to maintain and operate and detrimental to the environment and corporate reputations.

One of the primary reasons for the industry’s slow adoption lies, unfortunately, with the shipbuilder. While shipowners drive any equipment decision, shipbuilders’ unfamiliarity with the technology is placing a premium on the newbuild or retrofit cost of a seawater lubricated alternative. In fact, when shipowners have inquired about it, many have been discouraged by the shipbuilders unfamiliarity of a new open seawater lubricated propeller shaft system that ultimately is less complex and time-consuming to install than the oil-based system. The industry will need to provide further education to the shipyards that building a ship with seawater lubricated propeller shafts actually takes less time to build and has fewer components than a ship with an oil lubricated system.


While seawater lubricated propeller shaft bearings have a long history in the shipping industry, the problems associated with traditional seawater lubricated bearings had created a barrier to the wider adoption of seawater lubricated systems, despite the development of new bearing materials, shaft coatings and water conditioning technologies.

Today, a seawater lubricated propeller shaft bearing system offers considerable advantages to shipowners and shipbuilders, not only in bearing wear life predictability and reliability, but they are also much cheaper to maintain and operate.

The equipment cost to build a new ship with an open seawater lubricated bearing system is typically 15-20% higher than an oil lubricated system. The higher cost is related to the corrosion protection of the shaft and stern tube. Shaft liners are typically an expensive part of a shipowner‘s decision to use a seawater lubricated system at the newbuild stage. Bronze liners are used in way of the bearings although welded cladding and InconelTM have also been used.

To operate a seawater lubricated bearing system, there is no oil/EAL replenishment required; no AFT seal lip replacement; no AFT liner adjustments, machining or replacement; no oil sampling, processing or record keeping; and no oil or oily water disposal. The concerns of oil discharges and any pollution risk are eliminated. The water lubricated shaft line is a simple solution and not complicated by the oil system’s compressed air multi-lip seal system with different storage tanks for offshore disposal of oil.

The higher up-front costs are recouped, as the maintenance costs to operate a seawater lubricated bearing system are much lower than an oil system along with being compliant with all pollution regulations such as the U.S. Vessel General Permit (VGP).




No Oil Leakage - Zero Pollution Risk

A seawater lubricated propeller shaft bearing system eliminates oil, as the lubricant is seawater. There is no aft seal, no storage of oil, no sampling of oil, no disposal of oil or costly EALs and no worry of seawater ingress contaminating the oil.










Controlled Bearing Environment




Advances in seawater lubricated shaft bearing technology have resulted in an adequate supply of clean water consistently delivered to both the forward seal and the propeller shaft bearings. A controlled supply of clean water allows long predictable bearing wear life that is guaranteed for 15 years.


No Expensive Seal Repairs

A seawater lubricated shaft bearing system eliminates expensive aft seal repairs resulting from a damaged aft seal. Emergency seal repairs can cost anywhere from US$150,000 to US$300,000 or may even require a drydocking.

Future Compliance

A seawater lubricated propeller shaft bearing system meets current and anticipated environmental requirements. It is more economical to adopt the technology at the newbuild stage rather than having to convert your vessel mid-life when more stringent regulations enter into force.


Shaft Withdrawal Periods

Most Class Societies consider a seawater lubricated propeller shaft bearing system to be technically equivalent to an oil lubricated system. Based on monitoring criteria, shaft withdrawal periods for seawater lubricated systems can now be extended similar to oil lubricated systems.


If a serious bearing failure occurs with a white metal or reinforced plastic bearing, significant heat is often produced and there can be damage to the shaft as well as the bearing. A seawater lubricated polymer bearing softens at a lower temperature than metallic or other non-metallic bearing materials, excessive amounts of heat are not produced in failure mode and shaft damage is avoided.


Globally Supported

In-house design and engineering support is available for seawater lubricated propeller shafts systems with a global after-sales service capable of providing on-site installation and technical support wherever it is required.

Classification Society Approvals
ABS (American Bureau of Shipping) BKI (Bureau Klasifikasi Indonesia) BV (Bureau Veritas)
CCS (China Classification Society) CRS (Croatian Register of Shipping) DNV GL
IRS (Indian Register of Shipping) KRS (Korean Register of Shipping) LR (Lloyd's Register)
RINA (Registro Italiano Navale) RRS (Russian Register of Shipping)























  1. “Commercial Ship AFT Seal Oil Discharges - Factual Accounts of Oil Leaking from Propeller Shaft Stern Tubes Due to AFT Seal Damage” Thordon Bearings Inc. Feb. 10, 2017. Retrieved from Web 24 Feb. 2017
  2. Smith, Andrew. Lloyd’s Register Global Technology Leader-Engineering Systems. “Shaft Alignment Problems Analyzed,” Marine Engineers Review, April 2009, pg.16
  3. Lloyds Register Type Approvals Part lA Subject: Main Propulsion Shaft Line Product: Sterntube Seals LR 2005. Retrieved from Web June 8, 2006
  4. Etkin, Dagmar Schmidt. Environmental Research Consulting. (2010). “Worldwide Analysis of In-Port Vessel Operational Lubricant Discharges and Leakages” pg 14.
  5. U.S. EPA National Pollutant Discharge Elimination System (NPDES). Vessels - Frequent Questions: If an equipment manufacturer indicates that the working life of a conventional seal is reduced when used with EALs, is it possible to use a non-EAL lubricant based on technical infeasibility? Retrieved from Web Jan. 10, 2017
  6. Environment and Climate Change Canada. Birds Oiled at Sea - A Chronic But Mostly Hidden Problem Retrieved from Web Jan. 12, 2017
  7. Britannia P&I Club RiskWatch, January 2017.
  8. The International Tanker Owners Pollution Federation (ITOPF) Environmental Award won by ‘oil-free’ propeller shaft. 26th Nov. 2015
  9. G. Lavini & L.Pedone, Fincantieri Cantieri Navali Italiani SpA, Italy, Improvement on Hydrodynamic Performance of Conventional Passenger Ship Hulls Through RANSE Codes Application” presented at RINA’s Design and Operation of Passenger Ships, part of Cruise & Ferry Conference, April 25, 2007.



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