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DYNAMIC POSITIONING CONFERENCE 

October 13-14, 2009 

Thrusters 

An Environmentally Preferable Lubricant 

for Tunnel and Azimuth Thrusters 

Hiroyuki Sada 1, David Hawkins 2, Paul Erikson 2, and Yancey Wisenbaker 2, 

1: Kobelco Eagle Marine Engineering Co. Ltd., Takasago, Japan 

2: Kobelco Eagle Marine Inc., New York, Ny 
Hiroyuki Sada Thrusters Environmentally Preferable Lubricant 

- 2 -

An Environmentally Preferable Lubricant for Tunnel and 

Azimuth Thrusters

Hiroyuki Sada1, David Hawkins2, Paul Erikson2and Yancey Wisenbaker2

1: Kobelco Eagle Marine Engineering Co. Ltd., Takasago, Japan

2: Kobelco Eagle Marine Inc., New York, NY

ABSTRACT

Ocean-going vessels use oil lubrication in the tunnel thrusters. Offshore vessels also use oil 

lubrication in the azimuth thrusters. Oil leakage from the thrusters is a serious environmental 

issue in these vessels. In this paper, a water-soluble, environmentally preferable lubricant newly 

developed for such thrusters is outlined. Properties required for a thruster lubricant and a 

suitable base fluid are discussed. Environmental compatibility (biodegradability, low toxicity 

and no sheen or sludge formation), viscosity, lubricity, oxidation stability, rust prevention and seal 

compatibility of the new lubricant are described. These properties show that for tunnel and 

azimuth thrusters the new lubricant can improve not only their environmental compatibility but 

also their seawater tolerance. Field experience of the thruster lubricant is also presented.

1. INTRODUCTION

Medium and large ocean-going vessels use oil 

lubrication in the propulsors for the sterntube bearings 

and the tunnel thrusters. Offshore vessels also use oil 

lubrication in the propulsors for the azimuth thrusters. 

Recently the requirements for the environmental 

compatibility of vessels have increased. 

Governments around the world are looking at 

vessel pollution sources with an eye towards increasing 

regulations and penalties. The public supports 

increased regulations of vessels operating in their 

waters. In 2009, in the United States, the US 

Environmental Protection Agency published the 

National Pollution Discharge Elimination System 

Vessel General Permit rules. Propulsion equipment 

was specifically stated in the rules in section 2.2.9:

“The protective seals on controllable pitch 

propellers, azimuth thrusters, propulsion pods, rudder 

bearings, or any other oil to sea interfaces must be 

maintained in good operating order to minimize the 

leaking of hydraulic oil or other oils. The vessel 

owner/operator must not discharge oil in quantities that 

may be harmful as defined in 40 CFR Part 110 from 

any oil to sea interface.” [US EPA – NPDES VGP 

2009]

Severe crackdowns against vessels with oil leakage 

have been carried out. Oil leakage can result in heavy 

fines or imprisonment. Therefore, oil leakage from

the propulsors is a serious environmental issue for these 

vessels.

Tunnel and azimuth thrusters have a drive shaft, a 

propeller shaft, gears transmitting power from the drive 

shaft to the propeller shaft, rolling bearings supporting 

the shafts, an oil lubrication of the shafts, the bearings 

and the gears, and a propeller shaft seal preventing oil 

leakage. The lubricant presently used is industrial

gear oil with high lubricity. A typical tunnel thruster 

is schematically outlined in Figure 1. The seal used is 

Figure 1 Typical Tunnel Thruster
Hiroyuki Sada Thrusters Environmentally Preferable Lubricant 

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a basic design because the thruster does not have 

enough space available for a more sophisticated design. 

Therefore, it is very difficult to seal the thruster oil 

perfectly.

One of the solutions that is attracting keen interest 

in minimizing oil leakage issues are biodegradable oils. 

Biodegradable oils, which are decomposed rapidly to 

carbon dioxide and water in the marine environment, 

are environmentally friendly. Some biodegradable 

oils have already been used in sterntube applications 

[Wholley 2005]. Existing biodegradable oils, however, 

do not satisfy all the properties required for propulsor 

lubricants.

One of the authors presented a newly developed, 

environmentally preferable sterntube lubricant, which is 

based on polyethylene glycol, a type of polyalkylene 

glycol [Sada 2006]. The lubricant is water-soluble, 

and forms no sheen on seawater surface or sludge 

below seawater surface. From the viewpoints of 

marine lubrication applications and rust prevention, the 

lubricant can tolerate 10% seawater, while conventional 

mineral based lubricants cannot tolerate 5% seawater. 

Application in marine hydraulic systems, such as 

controllable pitch propellers and stabilizers, is another 

promising area for this lubricant [Sada 2008]. The 

new sterntube lubricant, however, does not provide the 

high lubricity required in thruster applications.

In this paper, a polyethylene-glycol-based, 

water-soluble, environmentally preferable lubricant for 

tunnel and azimuth thrusters is outlined. The 

environmental compatibility and the viscosity are 

described. Concerning the lubricity, the scuffing load 

capacity in FZG gear test, the extreme-pressure 

parameters in four-ball test, and the wear property in 

translatory oscillation test are described. Additionally, 

the oxidation stability, the rust prevention, and the seal 

compatibility are described. Finally, field experience 

is presented.

2. NEW THRUSTER LUBRICANT

2.1. Base Fluid

The following properties are required for 

environmentally preferable propulsor lubricants:

- No sheen or sludge formation 

- Biodegradability

- Low toxicity

- High viscosity

- Water tolerance

- Seal compatibility

Triglyceride and synthetic ester form surface sheen 

because they are not very soluble. At high 

temperature, hydrolysis can occur in these fluids with 

water contamination. Polyalkylene glycol forms no 

surface sheen or sludge and tolerates water well 

because of its water-solubility. 

From the viewpoints of biodegradability, low 

toxicity and high viscosity, triglyceride, synthetic 

ester and polyalkylene glycol were selected as 

potential base fluids. Triglyceride is natural 

ester, and includes vegetable oils. A 

comparison of these fluids is given in Table 1.

The TH-100 fluid has a minor drawback in that it 

causes excessive swelling at high temperatures of the 

most common seal material, a fluoro-elastomer. Poor 

solubility and hydrolysis are essential fluid properties, 

and they cannot be easily changed. The seal 

compatibility, however, can be solved by modifying the 

seal material. In conclusion, polyalkylene glycol is 

the most suitable base fluid for a propulsor lubricant.

2.2. New Thruster Lubricant

As a sterntube lubricant, turbine oil of a viscosity 

grade of 68 or 100 is generally used. As a thruster 

lubricant, industrial gear oil of a viscosity grade of 100 

or 150 is generally used. Their base fluid is mineral

oil. For thruster lubrication, higher viscosity and 

lubricity are required.

An environmentally preferable thruster lubricant 

has been developed. Its base fluid is polyethylene 

glycol, a type of polyalkylene glycol, which is the most 

suitable base fluid for propulsor lubrication. Its 

viscosity grade is the same as that of standard thruster 

-------------------------------------------------------------------------------------

Presented at the 12th Propellers/Shafting Symposium of the Society of 

Naval Architects and Marine Engineers (SNAME) in 2009.

No Sheen or

Sludge Formation

Water

Tolerance

Seal

Compatibility

Triglyceride Poor Weak, hydrolysis

can occur.

Fair

Synthetic Ester Poor Weak, hydrolysis

can occur.

Fair

Polyalkylene

Glycol Good Good Poor,high swelling

Table 1 Comparison of Potential Base Fluids

Type Base Fluid Viscosity @40oC

mm2/s

Specific Gravity

@15oC

TH Thruster

Lubricant

Polyethylene

Glycol 99 1.12

ST Sterntube

Lubricant

Polyethylene

Glycol 76 1.12

MG Gear Oil

(AGMA 3EP) Mineral Oil 99 0.89

MT Turbine Oil Mineral Oil 69 0.88

Table 2 Tested Lubricants
Hiroyuki Sada Thrusters Environmentally Preferable Lubricant 

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oil. The lubrication performance is equivalent to that 

of standard thruster oil.

2.3. Comparison of Properties

In this paper, the properties of the new thruster 

lubricant (TH) are compared to those of three lubricants, 

polyethylene-glycol-based sterntube lubricant (ST), 

mineral-oil-based industrial gear oil (MG) and 

mineral-oil-based turbine oil (MT). MG is standard 

oil for thrusters and MT for stern tubes. The 

lubricants tested are outlined in Table 2.

3. ENVIRONMENTAL COMPATIBILITY

3.1. Biodegradability

Biodegradability means that a lubricant is 

decomposed rapidly to carbon dioxide and water in the 

marine environment. The biodegradation process is 

shown in Figure 2.

For biodegradation testing, the CO2 evolution test 

[OECD 301B] and the O2 uptake test [OECD 301C] are 

commonly applied. For the development of new 

lubricant, the O2 uptake test was applied, because it is 

more stringent and severe than the CO2 evolution test 

[Batterby 2005]. When a test substance has greater 

than 60% biodegradation after 28 days in the test, it is 

defined as readily biodegradable.

Figure 3 shows the biodegradation curves of TH, 

ST, MG and MT. The biodegradation of TH and ST 

reached the pass level of 60% after 14 days. The 

biodegradation after 28 days for TH and ST was 69% 

and 83%, respectively. These lubricants are readily 

biodegradable. TH is less biodegradable than ST, 

because TH is thicker than ST. MG and MT had 

less than 20% biodegradation after 28 days.

MG and MT were tested with emulsification by 

ultrasonication because of its poor solubility. 

Biodegradation of poorly soluble substances is very 

dependent on the emulsification level applied in the test. 

In one case, a poorly soluble substance had 88% 

biodegradation with full emulsification and 18% 

biodegradation with no emulsification [Batterby 2000]. 

Because TH and ST are water-soluble, they need no 

emulsification in the biodegradation test. The 

biodegradability of TH and ST in the field is expected 

to be much higher than that of a poorly soluble 

lubricant even if these lubricants have the same 

biodegradation in the O2 uptake test.

3.2. Low Toxicity

Polyethylene glycol, which is the base fluid of TH 

and ST, is a well-known non-toxic material. It is 

generally used as a base material for cosmetics and 

medicines, often noted as PEG in the ingredient list. 

The toxicity of TH and ST was evaluated by the acute 

fish toxicity test [OECD 203]. The results for 50% 

lethal concentration after 96 hours, 96h-LC50, are 

given in Table 3. The toxicity parameter for TH and 

ST exceeded 100 ppm. Both TH and ST are classified 

as practically non-toxic according to the United Nations 

definition [UN GHS].

3.3. No Sheen or Sludge Formation

In the USA, the Code of Federal Regulations state:

“§ 110.3 Discharge of oil in such quantities as „„may be 

harmful‟‟ pursuant to section 311 (b) (4) of the Act 

(Clean Water Act). For purposes of section 311(b)(4) 

of the Act, discharges of oil in such quantities that the 

Administrator has determined may be harmful to the 

public health or welfare or the environment of the 

United States include discharges of oil that: 

(a) Violate applicable water quality standards; or 

(b) Cause a film or sheen upon or dis-coloration of 

the surface of the water or adjoining shorelines or 

cause a sludge or emulsion to be deposited beneath 

C, H + O2 CO2 + H2O

Bacteria

Water 0

20

40

60

80

100

0 7 14 21 28

Time [days]

Biodegradation [%]

Pass Level

MT

MG

ST

TH

Figure 2 Biodegradation Process

Figure 3 Biodegradation vs. Time

Table 3 Acute Toxicity to Fish

96h-LC50

ppm

TH > 100

ST > 100
Hiroyuki Sada Thrusters Environmentally Preferable Lubricant 

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the surface of the water or upon adjoining 

shorelines. ” (40CFR Ch. 1)

The base fluid of TH and ST, polyethylene glycol, is 

water-soluble. Since the additives, such as the rust 

inhibitor, are also water-soluble, TH and ST are 

completely water-soluble. The environmental impact 

of surface sheen and sludge cannot be neglected. Both 

TH and ST form no sheen on seawater surface or sludge 

under seawater surface because of their water-solubility. 

They are more environmentally friendly than poorly 

soluble, biodegradable lubricants.

4. VISCOSITY

4.1. Viscosity-Temperature Relationship

Figure 4 shows the relationships of kinematic 

viscosity and temperature for TH, ST, MG and MT. 

At 40 oC TH is as thick as MG, but the curve of TH is 

flatter. The curve of ST is also flatter than that of MT. 

TH and ST have superior temperature-related viscosity 

characteristics.

4.2. Water Contamination Viscosity

Because mineral oil is poorly water-soluble, water 

contamination in mineral oil becomes free water. The 

free water causes a considerable decrease in lubrication 

performance. On the other hand, water contaminated 

in TH and ST does not become free water because they 

are water-soluble. Therefore, the lubricity under water 

contamination does not decrease dramatically.

For TH and ST, the relationships of kinematic 

viscosity at 40 oC and water content are shown in 

Figure 5. The viscosity decreases as the water content 

increases, but the relationship is nearly linear. No 

sudden drop occurs.

5. LUBRICITY

5.1. FZG Gear Test

The scuffing load capacity was evaluated through 

the FZG gear test [DIN 51354]. The test rig is shown 

in Figure 6. Test gears of type A with high sliding at 

the pinion tooth tip were used. A pitch line velocity of 

8.3 m/s and a lubricant temperature of 90 oC were 

applied. Loads were increased in stages 1 to 12. For 

industrial gear oil, scuffing load stage of 12 or more is 

required [AGMA 9005].

The scuffing load stage results are given in Table 4. 

At the maximum load stage of 12, no scuffing occurred 

for both TH and MG. For ST and MT, scuffing 

occurred at stage 9 and stage 8, respectively. TH 

satisfies the scuffing performance required for 

industrial gear oil.

Figure 7 shows the torque applied at the scuffing 

load stages. The scuffing torque was greater than 535 

N·m for TH and MG, while it was 302 N·m for ST and 

239 N·m for MT. Concerning scuffing load, the 

capacity of TH and MG is at least 1.8 times larger than 

that of ST.

5.2. Four-Ball EP Test

The four-ball extreme-pressure (EP) test [ASTM 

D2783] was carried out to evaluate the EP parameters. 

The test method is schematically outlined in Figure 8. 

Bearing steel balls (diameter: 12.7 mm, material: JIS 

SUJ2) were immersed in a test lubricant. A ball 

rotation speed of 1760 rpm, a test temperature of 25 oC, 

and a test period of 10 seconds were applied. The 

following EP parameters were derived from the test:

- Weld load (WL)

0

100

200

300

400

500

600

TH ST MG MT

Scuffing Torque [N.m]

Table 4 Scuffing Load Capacity

TH ST MG MT

Scuffing Load Stage

(A/8.3/90) 12+ 9 12+ 8

Figure 6 FZG Gear Test

Figure 7 Scuffing Torque
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- Last non-scoring load (LNL)

- Load wear index (LWI)

For TH, ST, MG and MT, the relationships of wear 

scar diameter and normal load are shown in Figure 9. 

The wear scar diameters at LNL to WL are shown. 

The curve of TH is considerably different from that of 

MG. TH has much higher LNL. The curve of ST is 

similar to that of MT.

Figure 10 shows the comparison of EP parameters. 

Compared to MG, TH had lower WL, and higher LNL 

and LWI. The difference in WL was very small. The 

EP parameters for ST are similar to those for MT. TH 

has high EP performance equivalent to standard thruster 

oil.

100

1000

10000

TH ST MG MT

WL, LNL, LWI [N]

W L

LNL

LWI

Applied Load

Rotating Ball

Fixed Balls

0

1

2

3

4

5

100 1000 10000

Normal Load [N]

Wear Scar Diameter [mm]

MT

MG

ST

TH

Figure 8 Four-Ball EP Test

Figure 9 Wear Scar Diameter vs. Normal Load

Figure 10 EP Performance

100

1000

10000

0 5 10 15 20

Water Content [%]

WL, LNL, LWI [N]

W L

LNL

LWI

Figure 11 EP Parameters vs. Water Content

Figure 12 Translatory Oscillation Test

Oscillation f10 Ball

f24 x 7.85 Disk

Test Fluid

Normal Load

0

0.3

0.6

0.9

1.2

0 100 200 300 400

Normal Load [N]

Wear Scar Diameter [mm]

Herzian Contact

MG

TH

Figure 13 Anti-Wear Performance
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For TH, the lubricity under water contamination 

was evaluated through the four-ball EP test. The 

relationship of EP parameters and water content is 

shown in Figure 11. The EP parameters decreased as 

the water content increased, but the EP parameters for 

TH with 5% water are the same as those for neat TH. 

The water contamination allowable for standard thruster 

oil is very low. It is generally recommended as 0.1%. 

TH can tolerate much higher water contamination.

5.3. Translatory Oscillation Wear Test

The translatory oscillation test [DIN 51834], a type 

of ball-on-plate sliding test, was carried out to evaluate 

the anti-wear performance. A bearing steel ball 

(diameter: 10 mm, material: JIS SUJ2) and a bearing

steel disk (diameter: 24 mm, thickness: 7.85 mm, 

material: JIS SUJ2) were used. The lubricants of 0.03 

mL were supplied at the contact point of the ball and 

the disk. The test method is schematically outlined in 

Figure 12. A frequency of 50 Hz, an amplitude of 2 

mm, an ambient temperature of 80 oC, a test period of 

10 minutes were applied. The normal loads applied 

were 20 N, 50 N, 100 N, 200 N and 400 N.

Figure 13 shows the diameters of the wear scars on 

the balls for TH and MG. Contact diameters derived 

from Herzian theory are also shown in the figure. The 

contact diameters are baseline to evaluate anti-wear 

performance for this test. The wear scar diameters 

increased with increasing normal load. The diameters 

for TH were nearly equal to those for MG at the normal 

loads of 100 N or less, but they were smaller than those 

for MG at the normal loads of 200 N or more. The 

curve for TH is nearly parallel to the baseline, even 

when the load is high. TH has excellent anti-wear 

performance.

6. OXIDATION STABILITY

For TH and MG, the oxidation stability was 

evaluated through the standard test for industrial gear 

oil [ASTM D2893]. A test temperature of 121 oC, a 

test period of 312 hours (13 days) were applied. Dry 

air with a flow rate of 10 L/h was delivered to a test 

lubricant of 300 mL during the test. For industrial 

gear oil, viscosity change at 100 oC is required to be 6% 

or less [AGMA 9005].

The viscosity change results are shown in Figure 

14. The viscosity of TH decreased, but the viscosity 

of MG increased. The changes in TH and MG were 

approximately 5%, and were less than the requirement 

of 6%. TH has sufficient anti-oxidation performance 

as a thruster lubricant.

-10

-5

0

5

10

TH MG

Viscosity Change [%]

40°C

100°C

Seawater %

TH ST MG MT

5 rust-free rust-free rust-free rust-free

10 rust-free rust-free rust-free rusted

15 rust-free rust-free rust-free rusted

20 rust-free rusted rusted rusted

Table 5 Anti-Rust Performance

Figure 14 Anti-Oxidation Performance

FEPM

-(CF2-CF2-CH-CH2)-

 |

 CH3

FKM

-(CF2-CH2)-(CF-CF2)-

 |

 CF3

Figure 15 Fluoro-Elastomers

0

5

10

15

FEPM FKM

Volume Increase [%]

TH

ST

Figure 16 Swelling of Fluoro-Elastomers
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7. RUST PREVENTION

For TH, ST, MG and MT, the rust prevention under 

seawater contamination was evaluated according to the 

anti-rust test method [JIS K2510]. The test metal was 

0.2% carbon steel (JIS S20C). The test temperature

and the test period were 60 oC and 72 hours (3 days), 

respectively. Synthetic seawater with 3.5% sodium 

chloride was used.

The test results are given in Table 5. The test 

metal was not rusted in TH with 20% seawater, but was 

rusted in MG with 20% seawater. In ST with 15% 

seawater the test metal was not rusted, but in MT with 

10% seawater the test metal was rusted. TH has 

sufficient anti-rust performance as a thruster lubricant.

8. SEAL COMPATIBILITY

A new seal material was developed for 

polyethylene-glycol-based lubricants, because 

polyethylene glycol, which is the base fluid of TH and 

ST, causes excessive swelling in commonly used seal 

material, such as a fluoro-elastomer of vinylidene 

fluoride and hexafluoro-propylene (FKM), under high 

temperature condition. The new seal material is a 

fluoro-elastomer of tetrafluoro-ethylene and propylene 

(FEPM). FEPM has excellent resistance to chemicals. 

The structures of FEPM and FKM are shown in Figure 

15.

The volume change results for the seal 

compatibility test are shown in Figure 16. The seal 

material test specimens were immersed in the test 

lubricants. The test temperature and the test period 

were 150 oC and 70 hours, respectively. The results 

clearly show that both TH and ST have a low swelling 

effect on the new seal material.

9. FIELD EXPERIENCE

The new sterntube lubricant was named KEMEL 

ST-77. The first trial in sterntube application began in 

November 2005 on a fishing boat. Totally the 

sterntube lubricant has been applied to more than 40 

vessels. This has included large container carriers.

In July 2008, ST-77 had been applied to a 

controllable pitch propeller system of a vessel operating 

in an environmentally sensitive area. This is the first 

trial in marine hydraulic system application.

The new thruster lubricant was named KEMEL 

TH-100. Firstly the lubricant had been applied to 

propeller shaft seals of azimuth thrusters of a tugboat in 

October 2008. For the lubrication of the gears and 

bearings, conventional gear oil had been applied. The 

application is schematically explained in Figure 17. 

The seal with TH-100 is a barrier to prevent the gear oil 

leakage.

Prior to the application, effect of mixture of 

mineral gear oil and TH-100 was evaluated through the 

FZG gear test aforementioned. In the test, MG and 

TH outlined in Table 2 were used. As given in Table 6, 

no scuffing occurred for MG with 10% TH at the stage 

12. Mineral gear oil provides good lubrication even 

when it contains 10% TH-100.

10. CONCLUSIONS

A water-soluble, environmentally preferable 

lubricant for tunnel and azimuth thrusters was 

developed. The lubricant has excellent environmental 

compatibility (biodegradability, low toxicity and no 

sheen or sludge formation), and high lubricity 

equivalent to industrial gear oil. In addition, it 

provides good water contamination lubrication, which 

is essential for thruster lubricants.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge Sanyo 

Chemical Industries, Ltd. for its support.

REFERENCES

AGMA 9005, Industrial Gear Lubrication - D94, 

American Gear Manufacturers Association

ASTM D2783, Standard Test Method for 

Extreme-Pressure Properties of Lubricating Fluids 

Table 6 Anti-Scuffing Performance

Figure 17 Barrier Thruster Seal

Scuffing Load Stage

(A/8.3/90)

Neat MG 12+

MG + 10% TH 12+

TH-100

Mineral 

Gear Oil
Hiroyuki Sada Thrusters Environmentally Preferable Lubricant 

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(Four-Ball Method)

ASTM D2893, Standard Test Method for Oxidation 

Characteristics of Extreme-Pressure Lubricating 

Oils

Batterby, N. S. “The Biodegradability and Microbial 

Toxicity Testing of Lubricants - Some 

Recommendations.” Chemosphere, 41-7 (2000)

Batterby, N. S. “Biodegradable Lubricants: What Does 

„Biodegradable‟ Really Mean?.” Journal of 

Synthetic Lubrication, 22-1 (2005)

DIN 51354, Testing of Lubricants – FZG Back-to-Back 

Gear Test Rig: Test Method A/8.3/90 for Gear 

Lubricants, Part 2

DIN 51834, Tribological Test in the Translatory 

Oscillation Apparatus

JIS K2510, Testing Method for Rust-Preventing 

Characteristics of Lubricating Oil

OECD 203, Test Guideline on Acute Fish Toxicity, 

Organization for Economic Co-operation and 

Development

OECD 301, Test Guideline on Ready Biodegradability, 

Organization for Economic Co-operation and 

Development

Sada, H., Yamajo, S., Hawkins, D. W., and Kawazoe, T. 

“An Environmentally Compatible Lubricant for 

Sterntube Shafting and Bearing Systems.” 11th 

Propellers/Shafting Symposium of the Society of 

Naval Architects and Marine Engineers, 

Williamsburg (2006)

Sada, H., Yamajo, S., and Hawkins, D. W. “An 

Environmentally Compatible Lubricant for 

Sterntubes and Marine Hydraulic Systems.”

Advanced Naval Propulsion Symposium of the 

American Society of Naval Engineers, Arlington 

(2008)

UN GHS, Globally Harmonized System of 

Classification and Labeling of Chemicals, United 

Nations

Wholley, C. “Development of an Environmentally 

Adapted Sterntube Lubricant.” 7th International 

Symposium on Marine Engineering, Tokyo (2005)

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