WMTC 2009 Carter Page number 1

Elimination of a Ship Source Pollutant - 

STOP (Stern Tube Oil Pollution) 

C. D. Carter, MBA (Director of Marketing & Customer Service, Thordon Bearings Inc., Canada) 

This document reports the mechanism relating to stern tube oil pollution from ships and its impact and extent of 

stern tube oil pollution on the seas. The paper also looks at a proven, available technology that eliminates stern 

tube oil pollution by focusing on the performance of seawater lubricated stern tube bearings based on ship 

owner experience. 

KEY WORDS 

Stern tube oil pollution; water lubricated bearings; stern tube 

bearings; propeller shaft bearings; ship source pollution. 

INTRODUCTION

During the last few decades, the pollution of the world’s oceans 

has become a matter of increasing international concern. Zero 

tolerance for any kind of ship source pollution is now becoming 

the norm and international regulations are becoming more and 

more stringent. Since 1998, almost US $140m in criminal fines 

have been levied by US Courts against shipping companies 

found guilty of polluting US waters. 

Currently, the majority of commercial ocean-going ships operate 

with a propulsion system using a propeller shaft typically 

supported by oil lubricated metal bearings with the oil contained 

in the stern tube by forward and aft shaft seals. According to 

seal manufacturers, the seal must leak (aft-into the sea or 

forward-into the ships bilge) at the shaft/seal interface in order 

for the seal to function properly. As well, simple fishing net or 

rope caught on a ships rotating shaft can also damage the aft seal 

allowing stern tube oil to flow out into the sea. According to 

seal repairers, this occurs on a frequent basis. Typical oceangoing ship stern tubes contain 1500L (396 US gal) of oil. Even 

at a conservative leakage rate of 6L/day (from LR Class 

Society Seal Type Approvals), the amount of stern tube oil 

pollution is estimated to be over 80 million litres (21m US 

gal) annually from normal operations. In comparison, oil 

pollution from the Exxon Valdez oil tanker spill was 41.6m 

litres (11m US gal). 

Using a proven, available technology, there is an alternative to 

an oil lubricated sealed system that completely eliminates stern 

tube oil pollution. A seawater lubricated open system uses 

seawater as the lubrication medium in place of oil. The seawater 

is taken from the sea, pumped through non-metallic shaft 

bearings and returns to the sea. No stern tube oil is needed. 

Proven materials and new designs of non-metallic bearings now 

offer performance similar to metal shaft bearings. One 

significant difference, however, is that the mild steel propeller 

shaft requires corrosion protection from the seawater. This may 

mean a higher up-front cost for the stern tube bearing system. 

However, with the elimination of aft seal maintenance, the upfront costs are recouped in lower in-service costs along with no 

aft seal damage worries or oil pollution risk (fines).

HISTORY

Over fifty year ago, propeller shafts were supported by lignum 

vitae bearings made typically from a dense wood from South 

America and lubricated with seawater in what was called an 

“open system”. The constant flow of water from the sea 

lubricated the stern tube bearings and then flowed back into the 

sea. There was only one shaft seal in an “open system” and this 

prevented seawater from coming into the ship. 

However, in that era, seawater lubricated bearings did not have 

reliable wear life limits much beyond 5 years which meant 

withdrawing the shaft and replacing the bearings; an expensive 

task. In addition, most shaft seals of the day were packed 

stuffing boxes and these tended to score the bronze shaft liner in 

way of the packing. That meant skimming or replacement of the 

packing liner at the same time, which was another additional 

cost. 

Improvements in sealing technology in the 1950’s encouraged 

the move to an oil lubricated propeller shaft. White metal 

bearings offered a proven technology, which, in a sealed oil 

system, provided for predictable and controlled wear life and 

reduced maintenance of stern tube bearings. There were some 

issues with this oil system. Although stern tube bearing 

maintenance was reduced, the two shaft seals required frequent 

maintenance or oil would leak into the sea or into the ship. 

Some stern tube seal oil leakage was considered “normal 

operational consumption” and an acceptable practice. Oil in 

larger quantities also leaked to the sea if the seal was 

accidentally damaged due to a rope or fishing net caught on the 

ships rotating shaft or with propeller impact. 

. 

WHAT IS STERN TUBE OIL POLLUTION?

In a typical sealed oil system, the bearings supporting the shaft 

are mounted inside a hollow tube that is sealed with a lip type 

seal at each end and filled with mineral oil as shown in Figure 1. 

Typical stern tubes contain 1500L (396 US gal) of oil. Stern 

tube propeller shaft seals are the only barrier between the oil and 
WMTC 2009 Carter Page number 2

the sea, and over time, the seals can become damaged or worn 

allowing oil leakage to occur into the sea, into an onboard 

containment system or to the engine room bilge. 

Figure 1. A typical oil lubricated stern tube bearing system 

A propeller shaft sealing system is designed to prevent the 

ingress of water into the stern tube where it could damage the 

bearings. The seal is also designed to prevent the leakage of 

stern tube oil into the sea or into the engine room bilge. These 

sealing systems are costly to maintain because they are normally 

designed to withstand extreme conditions such as axial and 

radial shaft displacement, ship vibrations and operating periods 

of up to five years. The presence of sea water coming into the 

stern tube can also indicate that there is a good chance that oil is 

escaping out and polluting the seas. 

WORDS FROM THE SHAFT SEAL 

MANUFACTURES/REPAIRERS AND OIL 

INDUSTRY 

At the 2003 RINA International Conference for the Design 

and Operation of Container Ships, one of the largest shaft seal 

manufacturers, Wartsila Propulsion (Bearings & Seals) UK, 

stated that, “Consumption from the seal is controlled to the 

minimum acceptable to maintain an acceptable life, but by 

design it is essential to have oil at the mating surfaces. Oil 

consumption is always lost direct to the sea so contaminating 

the environment. The best estimates that can be made can 

only be based on seals in a lab condition running in clean and 

controlled environments but even this indicates an excess of 

10,000,000 litres/year of oil is lost. Many organizations 

predict far higher in reality but it is difficult to prove with 

facts.” 

 

At the 11th Shafting Symposium of the Society of Naval 

Architects and Marine Engineers in 2006, another large shaft 

seal manufacturer, Kobelco Eagle Marine Engineering Co. 

Ltd., Japan, stated that, “In stern tube bearings, the radial 

movement of the shaft is considerably larger than that of 

bearings for general industrial application. In addition, 

external disturbances such as rough seas and vibration are 

considerable. It is practically impossible to seal the stern tube 

oil perfectly. Therefore, one of the serious environmental 

issues in medium and large commercial vessels is stern tube 

oil leakage.” Dr. David Gilmour of BP-Castrol Marine 

Lubricants, a large supplier of stern tube oils, stated that 

"Environmental legislation can only get tougher, but even well 

maintained and managed ships will still leak oil." 

 

In 2007, a major stern tube seal repairer, Simplex Americas 

LLC, repaired a large tanker stern tube seal that was damaged 

due to fishing line. This was the 21st stern tube seal repair job 

completed afloat by Simplex Americas LLC during 2007, 

allowing ship owners to renew their stern tube seals quickly on 

the spot. The various customer vessels included bulkers, tankers, 

and containerships. This statistic represented one seal repair 

company in North America, but it certainly gives an idea of the 

scope of problems that occur with stern tube seals. 

 

In 2001, the European Commission DG Joint Research Centre 

reported on ship pollution totally based on spaceborne SAR 

remote sensing. It revealed for the first time the dramatic 

dimension of shipping pollution in the Mediterranean Sea, not as 

a result of accidents, but from routine unauthorized operational 

discharges. 

In the commercial shipping industry, this type of "operational 

discharges" of oil is considered normal, but based on these 

leakage rates of ships trading in international waters, the amount 

of oil pollution from stern tubes may have a large detrimental 

impact on birds, fish and the marine environment given the 

following: 

Assume: 45,000 commercial ships (>1000 DWT) presently in 

operation worldwide (source: LR-Fairplay PC Register) 

Assume: 90% of these ships operate with oil lubricated stern 

tubes 

Assume conservative number: average oil leakage = 6 litres/day 

(source: Lloyd's Register Type Approvals for stern tube seals) 

Assume: 330 working days/year (according to IMO) 

A conservative estimate would be: 

45,000 ships x .90 x 6 litres x 330 days = 80.19 million 

litres/year of stern tube oil from operational discharge 

Thus, an approximation of annual stern tube oil loss from 

the commercial shipping fleet is approximately 80 million 

litres per year (21 million US gal) and this does not include 

oil lost from damaged stern tube seals. 

PROVEN AVAILABLE TECHNOLOGY TO 

ELIMINATE STERN TUBE OIL POLLUTION 

Using a proven, available technology, stern tube oil pollution 

can be eliminated from the world’s oceans and seas. Two 

“conventional” alternatives currently in use claim to be 

environmentally friendly: 1) Seal manufacturers have developed 

more sophisticated multi lip seals which reduce the amount of 

oil that escapes, but shaft seals can still be damaged, and oil can 

still escape into the sea. 2) Biodegradable oils are also available, 

but they are still an “oil” and leave a sheen which is considered 

pollution. 

A third, less conventional, but simple, option uses proven stern 

tube bearing technology from Thordon Bearings. The simplest 

way to completely eliminate oil from the stern tube is to use 
WMTC 2009 Carter Page number 3

seawater as the lubrication medium and Thordon non-metallic 

bearings in place of oil and white metal bearings. The seawater 

is taken from the sea, pumped through the bearings and returns 

to the sea. The seawater enters the forward section of the stern 

tube just aft of the seal. The seawater passes through the forward 

and then aft bearing prior to re-entering the sea as shown in 

Figure 2. The quality of the seawater supplied to the bearing is 

critical in ensuring long wear life. To ensure that abrasives are 

removed from the seawater supply, a water quality package is 

used. This package uses centrifugal forces to remove particulate 

from the sea water stream, then collects it and discharges it 

through a blow down line. Use of seawater lubricated bearings 

eliminates the aft seal, as well as the storage, sampling and 

disposal of oil. The potential impact of stern tube oil pollution is 

zero. 

Figure 2. Thordon COMPAC seawater lubricated stern tube bearings – 

no oil required 

Research presented at the 2007 RINA (Royal Institute of Naval 

Architects) conference noted the benefits of water lubricated 

propeller shaft bearings. The presenters from Fincantieri, a large 

shipyard in Italy, stated that, “Traditionally, the shaft line is oil 

lubricated, and located inside the tube case with a diameter 

larger than the shaft itself. An alternative solution is presented 

with water lubrication, which offers some consequent benefits. 

First of all, the inflow water meets a smaller diameter and so the 

wake peaks on the propeller plane are reduced. Furthermore, the 

water through frictional effect is trailed in rotation towards the 

propeller with a significant benefit for propulsion efficiency 

(about 2%). The water lubricated shaft line is also practically 

maintenance free and represents a ‘green’ solution as the risk of 

oil leakage is avoided.” 

An additional design consideration with seawater lubricated 

bearings is that the propeller shaft requires corrosion protection 

from the seawater. For economic and technical reasons, the 

shaft is typically steel which will corrode in (salt) seawater. To 

prevent this corrosion, bronze or stainless steel liners are 

generally placed over the shaft in way of the bearings. Between 

the liners a waterproof flexible coating is typically fitted to 

protect the shaft. Thor-Coat, a new 2-part epoxy shaft coating 

developed by Thordon Bearings, was specifically developed to 

complement seawater lubricated propeller shaft bearing systems 

with the objective of providing corrosion protection for a 

minimum ten-year period in service. Thor-Coat addresses the 

issue of current shaft coating systems not being sufficiently 

reliable to allow extension of shaft withdrawal periods beyond 

five years. 

Currently there are over 500 commercial ships operating with 

Thordon seawater lubricated propeller shaft bearings with the 

first commercial installation in 1983. Some references/orders 

are presented below. 

• Carnival Corp. (U.S.A.) - 9 cruise ships (108,000 to 115,000 

GT) first delivered in 1998; 5 on order 

• Flinter Groningen BV (Netherlands) - 19 dry cargo ships 

(4,000 to 9,000 DWT) first delivered in 2002; 6 on order 

• Chemikalien Seetransport (Germany) - Cinderella (14,000 

DWT) LNG Carrier since 1995 

• BP Shipping (U.K.) – 4 tankers (185,000 DWT) since 2004 

• Scanscot Shipping (Germany) – Four heavy lift cargo ships 

(9500 DWT) on order 

• Upper Lakes Group Corp. (Canada) 2 bulkers (27,500 DWT) 

since 1999; 1 on order 

• Interlake Steamships (U.S.A.) 4 bulkers (2-25,900 & 2-62,000 

DWT) since 1999 

• Palmali Group (Turkey) - 3 product tankers (8,000 DWT) 

since 1999 

• Gypsum Transportation (U.S.A.) - Gypsum Centennial bulker 

(50,000 DWT) since 2001 and sister ship on order 

• New York City Staten Island Ferries (U.S.A.) - 3 ships (5,900 

GT) since 2004 

• COSCO - China Ocean Shipping Group (China) - 4 car 

carriers (56,000 GT) and 1 bulker (32,000 DWT) on order 

• The U.S., Canadian and Indian Coast Guard (oil pollution 

enforcement agencies) also use Thordon seawater lubricated 

stern tube bearings on over 65 of their ships with the first 

installation in 1983. 

The world’s largest cruise ship operator, Carnival Corporation 

through its operator Princess Cruises, has COMPAC seawater 

lubricated stern tube bearings currently installed on nine of their 

ships with five more on order. Mr. Chris Joly, Principal 

Manager, Marine Engineering for Carnival Corporation recently 

stated, “Seawater lubricated bearings are the present for many of 

our ships; I would like to see them for all of our future ships. In 

the Thordon COMPAC bearing, clean seawater is pumped to the 

‘A’ bracket and the aft stern boss bearings as the lubricating 

medium and it flows through the bearings to the sea. No aft oil 

seal and no oil means lower in-service costs of the shaft line 

and, as important, no pollution worries. The added advantage is 

that the COMPAC tapered key system allows inspection of the 

bearing and shaft journal without pulling the shaft. To date, our 

experience is that COMPAC bearings with Inconel journals will 

mean worry free shaft lines for the life of the vessel.” 

BEARING WEAR LIFE PERFORMANCE 

EXPECTATIONS 

Figures 3 and 4 are bearing wear life charts showing actual and 

expected wear life for seawater lubricated propeller shaft 
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bearings. Based on these actual results, seawater lubricated 

Thordon bearings can be expected to last for 15 to 20 years. 

Figure 3. Seawater Lubricated Bearing Wear for Disney 

Magic after 10 years 

Figure 4. Seawater Lubricated Bearing Wear for Grand Princess 

after 9 years 

LOWER FRICTION THAN OIL 

Thordon COMPAC stern tube bearings have been designed to 

reduce running friction and improve low speed hydrodynamic 

film development. The lower (loaded) portion of the COMPAC 

bearing is smooth and the upper half is designed with water 

grooves for lubrication and cooling. Although start-up friction 

is initially higher, at rated shaft speeds drag on the rotating shaft 

resulting from the viscosity of the lubricating fluid is lower with 

water than with oil (see Figure 5), resulting in potential fuel 

savings. 

Figure 5. Crossover Friction Curve – Water vs. Oil

LIFE CYCLE COSTS 

Non-metallic, seawater lubricated stern tube bearings typically 

may mean a higher up-front cost initially than comparable oil 

lubricated white metal stern tube bearing systems due to the 

need for corrosion protection of the shaft and addition of bronze 

liners. However, with the elimination of aft seal maintenance, 

the up-front costs are recouped with lower in-service costs. 

Based on existing user experience of COMPAC seawater 

lubricated propeller shaft bearings, the overall life cycle costs 

have been reduced for these ship owners. 

CONCLUSION 

Ships trading in the world’s oceans and seas can now eliminate 

both operational and accidental stern tube oil pollution while 

reducing ship owner maintenance costs and saving money over 

the service life of the ship. The improvements using proven 

bearing designs and new technologies from Thordon Bearings 

have resulted in Thordon seawater lubricated stern tube bearings 

offering improved wear life, fitting and monitoring methods to 

meet Class Society approvals. The performance of seawater 

lubricated COMPAC bearings to date has been comparable to 

oil lubricated white metal stern tube bearings. It also eliminates 

any risk of criminal, civil and administrative penalties and other 

adverse reactions such as bad public relations for the ship owner 

that may occur from oil leaking from the stern tube. 

 

REFERENCES

A&A Thorpe, U.K., Ship Repair Newsletter, Issue No: 1108, 

30th November 2007. 

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. 

Gilmour, Dr. David, BP-Castrol Marine Lubricants, 

Biodegradable Range, Seatrade, Jan./Feb. 2008, pp. 91 

Higgenbottom, Adrian. Wartsila Propulsion (Bearings & Seals) 

UK, Coastguard Non-Polluting Stern tube Sealing System, 

presented at the RINA International Conference for the Design 

and Operation of Container Ships, 23-24 April 2003, London, 

UK, pp 53-60. 

Pavlakis, Petros, Dario Tarchi and Alios J. Sieber. EC DG Joint 

Research Centre, On the Monitoring of Illicit Vessel Discharges 

- A Reconnaissance Study in the Mediterranean Sea 2001, pp 

13. 

Sada, Hiroyuki, Seiji Yamajo, David W. Hawkins and Tsuyoshi 

Kawazoe. Kobelco Eagle Marine Engineering Co. Ltd., Japan, 

An Environmentally Compatible Lubricant for Stern Tube 

Shafting and Bearing Systems, presented at the 11th Shafting 

Symposium of the Society of Naval Architects and Marine 

Engineers, Sept. 12-12 2006, Williamsburg, VA, U.S.A., pp 1.