1

An Environmentally Compatible Lubricant for Sterntubes

and Marine Hydraulic Systems

Hiroyuki Sada

1

, Seiji Yamajo

1

and David W. Hawkins

2

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

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

ABSTRACT

Medium and large ocean-going vessels use oil-lubricated shafting and bearings in the sterntube.

Oil leakage from the sterntube is a serious environmental issue in these vessels. In this paper, a

newly developed, environmentally compatible lubricant for such sterntube systems is outlined.

Properties required for a sterntube lubricant and a suitable base fluid are discussed.

Environmental properties, viscosity, corrosion prevention, water contamination lubrication, seal

compatibility and field experience of the new lubricant are described. Application in marine

hydraulic systems is also discussed.

1. INTRODUCTION

Medium and large ocean-going vessels use an oillubricated sterntube system, which consists of white

metal bearings supporting the propeller shaft, oil

lubrication of the shaft and bearings, and seals

preventing oil leakage. In sterntube 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 very difficult to seal

the sterntube oil perfectly and oil leakage can be an

issue. Another area of concern is oil leakage from

below water hydraulic systems, controllable pitch

propeller hydraulic systems and thruster drives.

Therefore, a few of the serious environmental issues in

medium and large commercial vessels are sterntube,

thruster and controllable pitch propeller oil leakage.

In order to solve the oil leakage issue from

sterntubes, considerable efforts are being made to

improve seal performance. The introduction of highly

reliable seals with an air chamber between the oil and

the seawater, so-called air seals [Shiomi 1987; Yamajo

2003], has made a big improvement in the

environmental compatibility of sterntubes. The air

seals have a drawback in that they require the

installation of piping system in existing vessels. On

the other hand, the requirements for the environmental

compatibility of vessels are being increased. Recently

severe crackdowns against vessels with oil leakage

have been carried out. Oil leakage can result in heavy

fines or imprisonment.

Biodegradable lubricant is attracting keen interest

as a solution for the oil leakage issue. Biodegradable

lubricants, which are decomposed quickly to carbon

dioxide and water in the natural environment, are

environmentally friendly. Such lubricants can be

easily applied to existing vessels by simply replacing

the conventional mineral oil. Some biodegradable

lubricants have already been used in sterntube

applications [Wholley 2005]. Existing biodegradable

lubricants, however, do not satisfy all the properties

required for sterntube lubricants.

In this paper, a newly developed, environmentally

compatible lubricant for sterntube shafting and bearing

systems is outlined. Properties required for sterntube

lubricants and base fluids suitable for sterntube

application are discussed. Environmental properties,

viscosity, and corrosion prevention related to the new

lubricant, based on the most suitable fluid, are

described. Concerning lubrication under water

contamination, which is the most important property of

a sterntube lubricant, the results for translatory

oscillation and journal bearing tests are also described.

In addition, the seal compatibility and field experience

are described. Finally the application of the sterntube

lubricant in marine hydraulic systems is discussed.

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Presented at the Advanced Naval Propulsion Symposium 2008 of the

American Society of Naval Engineers (ASNE) in Arlington, VA on

December 15-16, 2008.
2

2. BASE FLUID

Base fluid suitable for an environmentally

compatible sterntube lubricant is discussed.

2.1. Properties Required for Sterntube Lubricants

The following environmental properties are

required for sterntube lubricants:

- Biodegradability

- Low toxicity

- Non-sheening

Biodegradability means that a lubricant is

decomposed quickly to carbon dioxide and water in the

marine environment. Low toxicity means that a

lubricant is practically non-toxic to aquatic life. Nonsheening means that a lubricant forms no sheen on

seawater surface. The impact of sheen on marine

environment cannot be ignored.

In addition to these properties, the following

properties are required:

- High viscosity

- Water tolerance

- Seal compatibility

Sterntube lubricants generally have a viscosity

grade of 68 or 100. The base fluid needs to have an

equally high viscosity. Seawater ingress into the

sterntube is also a possibility. The base fluid needs to

absorb this seawater. As a seal for sterntubes, a radial

lip seal made of fluoro-elastomer (FKM) is the most

common. The base fluid needs to be compatible with

the seal material.

2.2. Selection of Base Fluid

From the viewpoints of biodegradability, low

toxicity and high viscosity, triglyceride, synthetic ester

and polyethylene glycol were selected as potential base

fluids. Triglyceride is a natural oil derived from

animals or plants. Synthetic ester is made of natural

oil or petrochemical oil. Polyethylene glycol is a

petrochemical lubricant. A comparison of these fluids

is given in Table 1.

Triglyceride and synthetic ester form surface sheen

because they are not very soluble and lighter than

seawater. At high temperatures, hydrolysis can occur

in these fluids with water contamination.

Polyethylene glycol forms no surface sheen and

tolerates water well because of its water-solubility.

The fluid has a drawback in that it causes excessive

swelling of the seal material (FKM). Poor solubility,

specific gravity and hydrolysis are essential fluid

properties, and they cannot be easily changed.

However, the seal compatibility problem can be solved

by modifying the seal material. In conclusion,

polyethylene glycol is the most suitable base fluid for a

sterntube lubricant.

3. ENVIRONMENTAL PROPERTIES

An environmentally compatible sterntube lubricant,

KEMEL ST-77, has been newly developed. Its base

fluid is polyethylene glycol. The ST-77 environmental

properties, biodegradability, low toxicity and nonsheening, are described below.

3.1. Biodegradability

For biodegradation testing, the CO2 evolution test

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

commonly applied. For the development of ST-77, the

O2 uptake test was applied, because it is more stringent

and severe than the CO2 evolution test [Batterby 2005].

The biodegradation of ST-77 in the O2 uptake test was

83% after 28 days. When a test substance has greater

than 60% biodegradation after 28 days in the O2 uptake

test, it is defined as readily biodegradable. Thus, ST77 is highly biodegradable. Figure 1 shows the

biodegradation curve of ST-77 for the O2 uptake test.

The biodegradation reached the pass level of 60% after

14 days. The biodegradation curve of a mineral oil,

which is commonly applied in sterntubes, is also shown

in Fig. 1. The mineral oil had less than 20%

Non-Sheening

Water

Tolerance

Seal

Compatibility

Triglyceride Poor

Weak,

Hydrolysis can

occur.

OK

Synthetic

Ester

Poor

Weak,

Hydrolysis can

occur.

OK

Polyethylene

Glycol

Good Good

High swelling

effect on FKM

Table 1 Comparison of Potential Base Fluids

Fig.1 Biodegradation vs. Time

0

25

50

75

100

0 7 14 21 28

Time (days)

Biodegradation(%)

Mineral

ST-77

Pass Level
3

biodegradation after 28 days.

The mineral oil was tested with emulsification by

ultrasonication, because it is poorly soluble.

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 ST-77 is water-soluble, it needs no

emulsification in the biodegradation test. The

biodegradability of ST-77 in the field is expected to be

much higher than that of a poorly soluble lubricant even

if both lubricants have the same biodegradation in the

O2 uptake test.

3.2. Low Toxicity

Polyethylene glycol, which is the base fluid of ST77, is a well-known non-toxic material. It is generally

applied in cosmetics and medicines. The toxicity of

ST-77 was evaluated by the acute fish toxicity test

[OECD 203]. The result for 50% lethal concentration

after 96 hours, 96h-LC50, exceeded 100 mg/l. ST-77 is

defined as practically non-toxic according to the

classification defined by the United Nations [UN GHS].

3.3. No Sheen Formation

The base fluid of ST-77, polyethylene glycol, is

water-soluble. Since the additives, such as the

corrosion inhibitor, are also water-soluble, ST-77 is

completely water-soluble. The environmental impact

of surface sheen cannot be neglected. ST-77 forms no

surface sheen because of its solubility, and is more

environmentally friendly than poorly soluble,

biodegradable oils.

4. VISCOSITY

The ST-77 viscosity characteristics, related to

temperature and water contamination, are described

below.

4.1. Viscosity-Temperature Relationship

The relationship of kinematic viscosity and

temperature is shown in Fig. 2. The typical value of

kinematic viscosity is 77 mm

2

/s at 40

oC, and 13 mm

2

/s

at 100

oC. Standard sterntube mineral oil has a

viscosity grade of 68 (VG68) or a viscosity grade of

100 (VG100). In the figure, the relationship between

kinematic viscosity and temperature is also shown. As

shown in the figure, ST-77 is as thick as the mineral oils,

but the curve of ST-77 is flatter. ST-77 has superior

temperature-related viscosity characteristics. The

specific gravity of ST-77 at 15

oC is 1.12.

4.2. Water Contamination Viscosity

Because mineral oil is poorly water-soluble, water

contaminated in the mineral oil becomes free water.

The free water causes a considerable decrease in

lubrication performance. On the other hand, water

contaminated in ST-77 does not become free water

because ST-77 is water-soluble. Therefore, the

lubrication performance under water contamination

does not decrease dramatically. For ST-77, the

relationship of kinematic viscosity at 40

oC and water

content is shown in Fig. 3. The viscosity decreases as

the water content increases, but the relationship is linear.

No sudden drop occurs.

5. CORROSION PREVENTION

Seawater ingress into the sterntube is possible.

Corrosion prevention is a key lubricant property for

sterntube shafting and bearing systems.

The corrosion prevention under seawater

contamination of ST-77 was evaluated according to the

JIS K2510 test method. The test metal was carbon

steel (S20C). The test temperature and the test period

were 60

oC and 72 hours, respectively. For

0

20

40

60

80

100

0 5 10 15 20 25 30

Water Content (%)

Kinematic Viscosity at 40

oC(mm

2

/s)

Fig.2 Viscosity vs. Temperature Fig.3 Viscosity vs. Water Content

10

100

1000

0 20 40 60 80 100

Temperature (

oC)

Kinematic Vis cosity (mm

2

/s)

Mineral VG68

Mineral VG100

ST-77
4

comparison, a mineral oil commonly used in sterntubes

was also tested. In total, eight fluids were tested:

1) ST-77 + 5% seawater

2) ST-77 + 10% seawater

3) ST-77 + 15% seawater

4) ST-77 + 20% seawater

5) Mineral oil + 5% seawater

6) Mineral oil + 10% seawater

7) Mineral oil + 15% seawater

8) Mineral oil + 20% seawater

The test results are given in Table 2. Carbon steel

was not corroded in ST-77 with 15% seawater, but was

corroded in the mineral oil with 10% seawater. ST-77

has sufficient corrosion protection performance as a

sterntube lubricant.

6. LUBRICATION UNDER WATER

CONTAMINATION

The lubrication under water contamination of ST77 was compared with a VG68 mineral oil, which is

commonly applied in sterntubes, through translatory

oscillation and journal bearing tests. The translatory

oscillation test and the journal bearing test are for

evaluating the anti-wear performance and the antiscoring performance, respectively.

6.1. Translatory Oscillation Test

The translatory oscillation test [DIN 51834], a type

of ball-on-plate sliding test, was carried out. As the

test specimens, a bearing steel ball (diameter: 10 mm,

material: SUJ2) and a bearing steel disk (diameter: 24

mm, thickness: 7.85 mm, material: SUJ2) were used.

The lubricants of 0.03 cc were supplied at the contact

point of the ball and the disk. The test method is

schematically outlined in Fig. 4. A frequency of 50 Hz,

an amplitude of 2 mm, an ambient temperature of 40

oC,

a test period of 10 minutes, and a normal load of 100 N

were applied. The maximum Herzian stress at the

contact point was 2.16 GPa. In total, six fluids were

tested:

1) Neat ST-77

2) ST-77 + 5% water

3) ST-77 + 10% water

4) ST-77 + 15% water

5) ST-77 + 20% water

6) Neat mineral oil

Figure 5 shows the diameters of the wear scars on

the balls. The wear scar diameters of ST-77 with a

water content of 15% or less were nearly the same, and

were less than that of the neat mineral oil. The antiwear performance of ST-77 with a water content of

15% or less is superior to that of a neat mineral oil.

6.2. Journal Bearing Test

Using a small journal bearing system with the same

materials used in an actual sterntube system, the antiscoring performance of ST-77 was tested. As the test

specimens, a carbon steel shaft (diameter: 80 mm,

surface roughness: 1.5 μm Ra, material: S45C) and a

tin-based white metal bearing (length: 20 mm, radial

clearance: 0.11 mm, surface roughness: 1.2 μm Ra,

material: WJ2) were used. The bearing test specimens

are shown in Fig. 6.

Oscillation

φ10 Ball

φ24 x 7.85 Disk

Test Fluid

Normal Load

Fig.4 Translatory Oscillation Test

Fig.5 Wear Results in Translatory Oscillation Test

0.36 0.37 0.38 0.39 0.4 0.41 0.42

Neat

ST-77

ST-77

+ 5% Water

ST-77

+ 10% Water

ST-77

+ 15% Water

ST-77

+ 20% Water

Neat

Mineral

Diameter of Wear Scar on Ball (mm)

Table 2 Corrosion Test Results

ST-77 Mineral Oil

Lubricant

+ 5% Seawater

Corrosion-free Corrosion-free

Lubricant

+ 10% Seawater

Corrosion-free Corroded

Lubricant

+ 15% Seawater

Corrosion-free Corroded

Lubricant

+ 20% Seawater

Corroded Corroded
5

The lubricants were fully mixed in the tank during

the test at a given temperature, and were supplied at a

flow rate of 5 cc/s from the bearing top center. A shaft

revolution of 360 rpm (peripheral velocity: 1.51 m/s),

and a lubricant temperature of 50

oC were used. The

shaft and bearing were soaked at 50

oC until the start of

the test. Bearing pressure (=W/(DL), W: bearing load,

D: shaft diameter, L: bearing length) was increased with

a load step of 0.1 MPa and a time interval of 1 minute

up to the bearing scoring. In total, five fluids were

tested:

1) Neat ST-77

2) ST-77 + 10% water

3) Neat mineral oil

4) Mineral oil + 5% water

5) Mineral oil + 10% water

The absolute viscosity at 50

oC of these fluids is

shown in Fig. 7. As described above, the viscosity of

ST-77 decreases with increasing water content. In

contrast, the viscosity of the mineral oil increases.

This is because the emulsified water in the mineral oil

creates higher viscosity.

The Stribeck curves, friction coefficient vs. duty

parameter, of ST-77 and the mineral oil are shown in

Figs. 8 and 9, respectively. The duty parameter is

defined as ηN/p (η: absolute viscosity in Pa

.

s, N: shaft

revolution in 1/s, p: bearing pressure in Pa).

Comparing the fluids with water contamination, ST-77

with 10% water formed a hydrodynamic film up to the

scoring, and the mineral oil with 10% water formed no

hydrodynamic film. In the mineral oil with 5% water,

the friction coefficient dramatically increased when the

duty parameter was 3x10

-7

or less. The lubrication

was switched from hydrodynamic lubrication to mixed

lubrication at a duty parameter of 3x10

-7

. ST-77 with

10% water provided excellent lubrication, but the

mineral oil with 5% water did not. It is recommended

by a ship classification society that the alarm criterion

of 2% water contamination be applied for conventional

mineral oil in sterntubes [NK GSTL]. The value of

2% is considered valid from the viewpoint of

20

30

φ110 φ80.22

Lubricant Inlet

Fig.6 Bearing Specimens used in Journal Bearing Test

Fig.7 Viscosity of Fluids used in Journal Bearing Test

0

0.01

0.02

0.03

0.04

0.05

0.06

0 2 4 6 8 10 12

Water Content, %

Absolute Viscosity at 50°C, Pa.s

ST-77

Mineral

Fig.8 ST-77 Stribeck Curves

0.001

0.01

0.1

0.E+00 2.E-07 4.E-07 6.E-07 8.E-07 1.E-06

Duty Parameter G

Friction Coefficient μ

Neat ST-77

ST-77 + 10% Water

Fig.9 Mineral Oil Stribeck Curves

0.001

0.01

0.1

0.E+00 2.E-07 4.E-07 6.E-07 8.E-07 1.E-06

Duty Parameter G

Friction Coefficient μ

Neat Mineral

Mineral + 5% Water

Mineral + 10% Water
6

lubrication.

As shown in Fig. 7, the viscosity of the mineral oil

with 10% water is greater than that of ST-77 with 10%

water. However, the mineral oil with 10% water

provided no hydrodynamic lubrication. It is noted that

the lubrication performance of a poorly soluble

lubricant with water contamination cannot be evaluated

with its physical property of viscosity. For the

evaluation, functional test such as journal bearing test is

required.

The bearing pressures at scoring are shown in Fig.

10. In the figure, the results for the mineral oil with

5% and the mineral oil with 10% water, which had

lubrication problems, are excluded. The bearing

pressure at scoring of ST-77 with 10% water was less

than that of neat ST-77, and was greater than that of

neat mineral oil. Even when ST-77 contains 10%

water, it provides a higher scoring load than VG68 neat

mineral oil.

In order to compare the lubricants in anti-scoring

performance, the effect of viscosity differences need to

be excluded. The anti-scoring performance of fluids is

evaluated using a duty parameter at scoring. A smaller

duty parameter at scoring means a higher anti-scoring

performance. The duty parameters at scoring are

shown in Fig. 11. The duty parameters at scoring of

the three fluids were nearly the same. Even when ST77 has 10% water, it provides the same anti-scoring

performance as mineral oil with no water

contamination.

7. SEAL COMPATIBILITY

A new seal material was developed for ST-77,

because, as mentioned above, polyethylene glycol,

which is the base fluid of ST-77, causes extensive

swelling in commonly used seal material, fluoroelastomer of vinylidene fluoride and hexafluoropropylene (FKM), under high temperature conditions.

The new seal material is a fluoro-elastomer of

tetrafluoro-ethylene and propylene (FEPM). FEPM

has excellent resistance to chemicals. The structure of

FEPM and FKM is shown in Fig. 12.

The volume change results for the seal

compatibility test are shown in Fig. 13. The seal

material test specimens were immersed in the test

lubricants. A test temperature of 150

oC, and a test

period of 70 hours were applied. In the figure, the

results for a standard sterntube mineral oil are also

FEPM

-(CF2-CF2-CH-CH2)-

|

CH3

FKM

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

|

CF3

Fig. 12 Structure of FEPM and FKM

Fig.10 Bearing Pressures at Scoring

0 1 2 3 4

Neat ST-77

ST-77

+ 10% Water

Neat Mineral

Bearing Pressure at Scoring, MPa

Fig.11 Duty Parameters at Scoring

0.0E+00 5.0E-08 1.0E-07 1.5E-07 2.0E-07

Neat ST-77

ST-77

+ 10% Water

Neat Mineral

Duty Parameter at Scoring

Fig.13 Volume Change Results in Seal Compatibility Test

0 5 10 15

New Seal

Material

(FEPM)

Existing Seal

Material

(FKM)

Volume Change (%)

Mineral

ST-77
7

shown. The results clearly show that ST-77 has a low

swelling effect on the new seal material. The results

also show that the new seal material is applicable for

conventional sterntube oil.

8. FIELD EXPERIENCE

The first trial in sterntube application began in

November 2005 on a fishing boat. By March 2006

this had extended to 4 vessels. No adverse indications

were given in these vessels, and in September 2006 ST77 was commercially launched as an environmentally

compatible sterntube lubricant. Totally the lubricant

has been applied to more than 30 vessels. This has

included newly built large containers. Used lubricant

analysis has shown the lubricant to be stable against

oxidation under normal operating conditions with no

indications of wear or corrosion. Used lubricant

analysis and lubricant consumption check have shown

that the new seal material works well in combination

with the lubricant.

9. APPLICATION TO HYDRAULIC SYSTEMS

Vessels use hydraulic systems under water surface

such as controllable pitch propeller and stabilizer. Oil

leakage from the marine hydraulic systems is also a

serious environmental issue. In the systems, mineral

oil is applied as a hydraulic fluid.

A comparison of ST-77 and typical hydraulic fluid

is given in Table 3. Except for pour point, the

performances of ST-77 are superior or similar to those

of typical hydraulic fluid. Pour point shows

application limit at low temperature. Fluid

temperature in marine hydraulic systems is usually

greater than 0

oC, because the systems are used under

water surface. The pour point of ST-77 is acceptable.

In conclusion, ST-77 is applicable to marine hydraulic

systems.

The first trial in marine hydraulic system

application started in July 2008 on a controllable pitch

propeller system of a vessel operating in an

environmentally sensitive region.

10. CONCLUSIONS

An environmentally compatible lubricant for

sterntube shafting and bearing systems, KEMEL ST-77,

of which the base fluid is polyethylene glycol, was

developed. The lubricant has excellent environmental

properties (biodegradability, low toxicity and no sheen

formation), and good water contamination lubrication

and corrosion protection properties, which are essential

for sterntube lubricants. From the viewpoints of

lubrication and corrosion prevention, the new lubricant

can tolerate a seawater content of 10%, while standard

mineral oil cannot tolerate a seawater content of 5%.

Application to marine hydraulic systems is another

promising area for this technology.

Table 3 Comparison of ST-77 and Typical Hydraulic Fluid

Properties Test Method ST-77

Typical Hydraulic Fluid

(Mineral Oil)

Viscosity @40

oC - 77 mm2

/s 68 mm

2

/s

Viscosity @100

oC - 13 mm2

/s 8.5 mm

2

/s

Viscosity Index - 175 95

Density @15

oC - 1.12 kg/L 0.87 kg/L

Pour Point ASTM D97 -10

o

C -30 oC

Flash Point ASTM D92 225

o

C 220

o

C

Anti-Rust for Steel

ASTM D665

seawater: 5, 10, 15, 20%

No rust @15% seawater No rust @5% seawater

Anti-Corrosion for Copper ASTM D130 1a 1b

Oxidation Stability

ASTM D2893

121

oC, 312 h, air: 10 L/h

Viscosity: -6% @100

oC Viscosity: +7% @100oC

Foam Suppression ASTM D892 I: 40/0, II: 20/0, III: 60/0 I: 20/0, II: 10/0, III: 20/0

Air Release ASTM D3427 6 minutes 4 minutes

Four-Ball Wear

ASTM D4172

40 kgf, 1200 rpm, 1 h, 75

oC

0.50 mm 0.55 mm

Scuffing Load Capacity

DIN 51354

FZG A/8.3/90

Fail Stage: 9 Fail Stage: 8
8

ACKNOWLEDGEMENTS

The authors gratefully acknowledge Sanyo

Chemical Industries, Ltd. for its support.

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