36
Marine Equipment
Tribology
36.1
36.2
Introduction
Marine Oil Properties and Chemistry
Fuel and Oil Rheology and Chemistry • Environmental
Concerns
36.3
Steven R. Schmid
University of Notre Dame
Karl J. Schmid
John Deere Corporation
Diesel Engine Lubrication
Slow-Speed Diesel Engines (<250 rpm) • Medium-Speed
Diesel Engines (250 to 1000 rpm) • High-Speed Diesel
Engines (>1000 rpm)
36.4
36.5
Steam and Gas Turbines
Ancillary Equipment
Stern Tubes • Rigging Equipment • Pumps • Oil
Maintenance Equipment
36.1 Introduction
Marine equipment presents an extremely demanding environment to tribologists. While the machine
elements and lubricants used are substantially the same as in other applications, some fundamental
differences regarding the operating environment play a substantial role in their success. Chief among
these are the chemical nature of the fuels used, the operating temperatures achieved, and the speed ranges
commonly encountered. Also, a ship at sea may need to be totally self-sufficient for weeks at a time.
One of the most demanding and exciting aspects of marine equipment design is the huge equipment
scale. Small ships can use diesel motors of only a few horsepower compared to the largest aircraft carriers
with nuclear reactors of sufficient capacity to power a medium-sized city. Very large thrust loads developed
by propeller shafts must be efficiently transferred to the ship structure (and present perhaps the most
common application of hydrodynamic thrust bearings). Cams, gears, chains, journal bearings, pistons,
rolling element bearings, etc., are all found in marine applications. The basic theory for these machine
elements is covered in other sections of this handbook and in a number of excellent reference texts, such
as Hamrock et al. (1999), Shigley et al. (1989), and Juvinall et al. (1991).
This chapter is intended as a broad overview of marine equipment tribology. It is not intended as a
detailed investigation into individual machine elements or applications. Instead, this chapter focuses on
the aspects of marine tribology that differentiate it from other applications.
36.2 Marine Oil Properties and Chemistry
36.2.1 Fuel and Oil Rheology and Chemistry
In the past 15 years, diesel engines have exerted a dominance over other power sources in commercial
ships and have displaced steam engines and steam and gas turbine engines to minor roles. Steam and
© 2001 by CRC Press LLC
gas turbine engines are still very popular for warships, however. For economic reasons, ships are routinely
powered by fuels with rather high viscosity in order to lower the fuel cost by reducing refining requirements. Pevzner (1998a) gives the general fuel oil types and their viscosities as follows:
•
•
•
•
Bunker (marine) fuel oils: viscosities between 380 and 700 cSt at 50°C
Intermediate fuel oils: viscosities between 30 and 380 cSt at 50°C
Marine diesel oils: viscosities between 11 and 14 cSt at 40°C
Gas oil: viscosity less than 6 cSt at 40°C
The thicker fuels will not be easily pumped or atomized in the combustion chamber at ambient temperatures, and are therefore usually preheated to up to 130°C. The goal is to achieve a viscosity value near
20 cSt (Pevzner, 1998a).
The drive toward high-viscosity fuels is an economic one because the thicker fluids undergo less refining
and are therefore less expensive. There has been a considerable increase in the viscosity of fuels used in the
past decade or so. As a comparison, Wilkison’s (1983) excellent overview of marine tribology restricted attention
to fuel blends with viscosities up to 180 cSt at 50°C, while much more viscous fuels are routinely used today.
The trend of ever-higher viscosity fuels has a serious shortcoming in that contaminants and increased
acidity result. While sulfur is intentionally used in low concentrations as an extreme pressure additive for
automotive lubricants, sulfur concentrations are sufficiently high in fuels to make sulfur corrosion an important concern. Sulfur content in some fuels can be over 4%,* leading to significant SO2 and SO3 combustion
products, and a very acidic environment. Also, carbon particles can be suspended in the fuel, leading to
fouling of cylinders and causing ring and piston sticking. In addition, abrasive particles are conveyed into
the engine by the lubricant, so that abrasive wear rates are much higher than in other applications. Further,
the thick fuels result in operating temperatures somewhat higher than normally encountered.
The base stocks used in the lubricants have seen a drastic change in recent years. While naphthenic
oils were dominant prior to the early 1980s, paraffinic base oils and synthetics are now totally dominant
in the industry, mainly due to the need for higher viscosity indices (VI) associated with high operating
temperatures. Oils with VIs in the 100 to 150 range are used, although specific values are manufacturer
specific. A summary of selected available lubricants is given in Table 36.1.
The chemical considerations of the fuel are overcome by formulating lubricants with specific additives,
both to compensate for the acidic nature of the fuel and combustion products and to aid in lubrication.
Alkalinity is reported in terms of a total base number (TBN) of milligrams KOH per gram and varies by
application. While specific values depend on application, the TBN of low-speed diesels is roughly 70; that
of medium-speed diesel engines is roughly 30; and that of high-speed engines is comparable to automotive
oils (5 TBN or so). TBN plays a large role in a lubricant’s success. For example, Hellingman et al. (1982)
showed that lubricants with TBNs of 100 reduce cylinder liner and top ring wear in low-speed diesel engines.
Other additives include the typical blend of boundary additives, extreme pressure additives, defoamants, detergents, etc., but in higher concentrations than in automotive applications (Woodyard, 1990).
Additive functions are discussed in the specific chapter sections for different classes of equipment.
36.2.2 Environmental Concerns
Most of the harmful emissions from marine equipment arise from combustion products and contaminants, mostly sulfur in the fuels. While some lubricant is inevitably combusted,** the major concerns
*Fuel sulfur content varies with the fuel source. Fuels with high sulfur content are known as “sour crudes,” while
those with low sulfur content are “sweet crudes,” and fuels are sour or sweet depending on their source. This has led
to quite a few detective quandaries, where cylinder liner wear will appear and disappear at apparently random times,
when often they are attributable to the fuel used. Also, because the advantage of using high TBN oils vanishes with
low sulfur content in the fuels, the lubricant TBN can be tailored to anticipated fuel quality.
**Note that low-speed diesel engine cylinder lubricant is totally combusted, and contributes significantly to metal
(Ca) emissions.
© 2001 by CRC Press LLC
TABLE 36.1
Properties of Selected Marine Application Low- and Medium-Speed Diesel Engine Lubricants
Viscosity
Manufacturer
Designation
Low-speed diesel engines — crankcase
Mobil
Mobilgard 312
Mobilgard 412
Mobilgard 512
Texaco
DORO AR 30
TARO 16 XD 30
TARO 16 XD 40
Low-speed diesel engines — cylinder
Chevron
Delo Cyloil Extra
Delo Cyloil Special
Delo Cyloil Heavy
Mobil
Mobilgard 570
Shell
Alexia D
Alexia Xb
Texaco
TARO 70
TARO 85
Medium-speed diesel engines
Chevron
Delo 1000 30
Delo 1000 40
Delo 1000 50
Delo 6170 CFOa
Mobil
Mobilgard 330
Mobilgard 430
Mobilgard 450a
Shell
Argina S 30
Argina S 40
Argina T 30
Caprinus HPD40a
Texaco
TARO 20 DP 30
TARO 20 DP 40
TARO 40 XL 40
a
b
TBN
(mg KOH/g)
Sulfated
Ash
(wt%)
SAE No.
cSt at 40°C
cSt at 100°C
Viscosity
Index
30
40
50
30
30
40
103
141
199
111
98
139
11.4
14.4
18.4
11.9
11
14
100
100
100
96
97
97
15
15
15
6
16
16
2.1
2.1
2.1
0.75
1.95
1.95
60
50
50
50
50
50
50
60
320
214
244
247
260
211
225
320
14.8
18.0
18.1
21
19.0
19.5
19
25
95
101
100
100
80
95
95
95
85
70
50
70
70
100
70
85
2.0
2.0
9
11.00
30
40
50
40
30
40
40
30
40
30
40
30
40
40
110
145
227
144
106
143
145
104
139
104
160
95
135
139
11.9
14.4
19.4
14.8
11.0
13.5
14.5
11.39
14.13
11.8
14.5
11
14
14
97
98
97
101
99
100
98
102
102
102
98
100
100
97
12
12
12
17
30
30
13.5
20
20
30
13.0
20
20
40
1.67
1.67
1.67
2.0
4
4
1.65
1.5
2.5
2.5
4.9
Zinc-free formulation.
For fuels with sulfur content greater than 3.5%.
about the lubricant itself involve disposal. Air pollution is mostly due to the chemical nature of the fuel
oil used, but any changes in the fuel oil have far-reaching implications on lubricant formulation.
Fuel quality is the most important variable in controlling emissions. For example, Thomas (1992)
suggests that carbon (soot) emission can be correlated to the fuel quality by:
(HR) S
P ∝
V (N )
1.7
c
0.3
(36.1)
a
where Pc is the particulate carbon emissions (excluding ash) at 4% excess oxygen in percent of fuel weight;
HR is a measure of the carbon-forming tendency of the fuel measured chromatographically; S is the
weight percent of sulfur in the fuel; and V is the vanadium content and Na is the sodium content, both
in parts per million.
In 1992, the International Maritime Organization (IMO), a United Nations organization, identified
two pollutants (NOx and SOx) for control by the year 2000 (Bastenhof, 1992). However, no specific
requirements or procedures for achieving emissions reductions have been mandated. It is entirely possible
that sulfur may in the future be removed from fuels through a costly refining process (dehydrosulfurization).
Lanz (1995) has calculated SOx emission rates as a function of fuel sulfur content, along with the increased
© 2001 by CRC Press LLC
TABLE 36.2
Effect of Fuel Sulfur Content on SOx Emissions and Fuel Cost
Fuel Sulfur Content
(%)
Reduction in SOx Emissions
(%)
Increase in Fuel Cost per Tonne
($)
3.5
3.0
1.5
5
10
52
15–30
—
46–58
After Lanz, R. (1995), The Motor Ship, 5, 22-23.
cost of fuel. The results of his model are given in Table 36.2. Lanz also mentioned that an alternative
approach is the treatment of exhaust gases, which will represent a fuel cost of about $25 per tonne with
fuel consumption increased by 3%.
A number of countries enforce emissions regulations in coastal waters, which can extend a few hundred
miles from the shoreline. However, the economic reasons for using bunker oil as fuel are so compelling
that a common practice is to use bunker oils in international waters and more refined fuels (i.e., reducedsulfur fuels) along coastlines. This has a dramatic influence on acid rain because the vast majority of
sulfur emissions are not deposited on land, but at sea. The sulfur content of seawater is 1015 tonnes, while
all fossil fuels combined would contribute only 1011 tonnes, which is accepted as producing no deleterious
effect (Lanz, 1995). Lanz also noted that the total sulfur emissions from all ships (105 tonnes per year)
is roughly the same as that produced by a single coal-fired power station.
The implications of sulfur reductions are very far reaching. If fuels were refined enough to eliminate
most of the sulfur content, then the need for increased alkalinity in the lubricant would be removed. In
fact, lubricants would need to be reformulated, in that sulfur when present in small amounts is an
extremely useful EP additive. Thus, as discussed by Golothan (1976), using fuels without sulfur can
actually lead to an increase in engine wear. Also, important tertiary functions (oxidation inhibition,
detergency, etc.) aided by high alkalinity would need to be achieved with different lubricant additives.
Nitrous oxide emissions are also a concern, and a number of approaches can be used to reduce NOx
emissions. For example, NOx emissions can be reduced by 50% from uncontrolled levels by retarding
fuel injection timing, albeit at a loss in specific fuel consumption of about 10% (Hold, 1993). Retarding
timing increases soot loading in the cylinder lubricant oil, requiring reformulation, higher cylinder oil
flow rates, or reduced oil change intervals, all of which have their own environmental concerns. For larger
emission reductions, some type of catalytic reduction is necessary. This will necessitate reformulation of
lubricants because popular extreme pressure (EP) additives such as zinc dithiophosphate are incompatible
with NOx-reducing catalysts.
The sources of particulate emissions in the exhaust consist mostly of the sulfur and carbon particles
in the fuel. However, some lubricant is always combusted and can contribute to the gas and particle
emissions. High TBN oils contain up to 12% sulfated ash, which during combustion is converted into
inorganic ash particles. In addition, some attention has been paid to reducing particulate emissions of
diesel engines, although regulatory agencies have pursued the marine industry much less arduously than
other industries to date.
Particulates are also a concern. Fuel oil and lubricants contain metals such as calcium (provided as
CaOH) to improve alkalinity. The metal ions form ashes with sulfur (e.g., CaS), which is then a potential
particulate exhaust. Carbon particles (soot) are also exhausted. Gros (1990) reports that typical mediumspeed diesel engine exhaust pollutants are about 66% metals (oxides and sulfides), 25% carbon, and 10%
hydrocarbons from fuel and lubricating oil.
36.3 Diesel Engine Lubrication
Many of the concerns of diesel engine lubrication are identical to the issues discussed in Chapter 33 of this
section. However, a few distinctions must be made. Many of the issues result from the acidity of combustion
products as discussed above. However, there are other peculiarities of diesel engines in marine applications.
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Exhaust valve
Exhaust gas manifold
Cooling jacket
Piston
Air cooler
Scavenging ports (quills)
Cylinder liner
Scavenge air collector
Piston rod
Cam shaft
Crankshaft
Connecting rod
Bed plate
FIGURE 36.1 Schematic illustration of cross-head diesel engine. (Adapted from Pevzner, L.A. (1998a), Aspects of
marine low-speed, cross-head diesel engine lubrication, Lubr. Eng., 54, 16-21.)
36.3.1 Slow-Speed Diesel Engines (<250 rpm)
Slow-speed diesel engines are large, two-stroke engines with crosshead construction (Figure 36.1), and
usually run between 90 and 250 rpm. These engines use a diaphragm and stuffing boxes to separate the
cylinder and crankcase, allowing each to be lubricated independently. Slow-speed diesels are the largest
engines, with bore sizes around 1 m and strokes of over 3 m. Piston speeds are around 7 to 8 m/s, and
power output for the largest engines is over 65,000 kW. These engines are primarily used in large tankers
and passenger liners. The motor directly powers the propeller shaft without a reducing gear or clutch.
These engines are very fuel efficient because of long combustion times and low break-specific friction,
but as will be seen, this has its drawbacks as well.
Slow-speed diesel engine tribology is itself a unique specialization with its own very characteristic
concerns. Excellent summaries of the issues and concerns in low-speed diesel engine tribology include
the papers by Pevzner (1998a), Lane et al. (1987), Langer et al. (1987), and Hold (1993).
36.3.1.1 Cylinder Lubrication
The fact that the cylinder is separated from the crankcase is both a boon and a curse. It is a curse in that
separate lubricant supplies and preventive maintenance schedules must be maintained. It is beneficial in
that the special environment presented by the cylinder can be treated without adversely affecting other
components. This design was conceived for this reason.
Because the fuels and combustion products are acidic, isolating them from as much of the engine as
possible is the best approach to preventing corrosion and corrosion-assisted fatigue and wear. For the
© 2001 by CRC Press LLC
cylinder itself, the unalterable presence of corrosive elements requires pursuit of unique tribological
solutions. These are divided into lubricant chemistry and coatings efforts.
The lubricant is applied by cam-actuated reciprocating pumps through a number of ports or quills
(usually 4 to 8) positioned around the cylinder liner. The cylinder oil must be thin enough to spread
quickly, but must also allow formation of a hydrodynamic film at operating temperatures. Cylinder
lubricants are most commonly SAE 50 or SAE 60 to ensure the desired viscosity at the cylinder liner
operating temperature of around 200°C, with TBN values of 50 to 70. In addition, EP additives and
detergents are especially important for these lubricants.
Many problems associated with lubrication arise from too low or too high an application rate. If the
application rate is too high, overlubrication can cause fouling of the cylinder or can even cause postcylinder fires in the engine, especially the turbocharger. Further, overlubrication adds significantly to
maintenance cost in a highly competitive industry. If the application rate is too low, then the alkaline
nature of the lubricant is neutralized before it spreads over the entire cylinder. This usually means that
the oil applied just below the quill will neutralize the sulfuric acid but will become increasingly ineffective
as the distance from the quill increases. This leads to classic periodic wear patterns on cylinder liners,
called “clover leafing,” where significant corrosion fatigue wear and corrosion-assisted abrasive wear
occurs between quills, but almost none occurs near the quills. An example of clover leafing is shown in
Figure 36.2.
Pevzner (1998b) investigated the effect of oil feed rate into the cylinder and its effect on cylinder liner
temperature and liner wear rate. His results are shown in Figure 36.3 for a number of different power
levels for the low-speed diesel engine examined. Pevzner noted that wear and temperature do not continue
FIGURE 36.2 An example of severe clover leafing on a chrome-plated cylinder liner. Note the position of the wear
patterns relative to the quills. (From Wilkison, J.L. (1983), Marine equipment, in CRC Handbook of Lubrication,
Vol. I. Booser, E.R. (Ed.), CRC Press, Boca Raton, FL, 227-248. With permission.)
© 2001 by CRC Press LLC
350
Liner wear rate, mg/hr
300
100%
80%
64%
50%
250
200
150
100
50
0
0.4
0.8
1.2
1.6
2.0
Oil feed rate, g/kWhr
(a)
Cylinder liner temperature, °C
150
100%
80%
64%
50%
140
130
120
110
100
90
0.4
0.8
1.2
1.6
2.0
Oil feed rate, g/kWhr
(b)
FIGURE 36.3 Effect of oil feed rate on cylinder liner: (a) wear rate vs. oil feed rate; (b) liner temperature vs. oil
feed rate. (Data from Pevzner, L.A. (1998a), Aspects of marine low-speed, cross-head diesel engine lubrication, Lubr.
Eng., 54, 16-21.)
to decrease with lubricant feed rate above a certain threshold. Also, Pevzner noted that ash deposits
formed on supercharger turbine blades and on the piston crown at high feed rates. Pevzner conducted
sea trials of the result for verification. The optimal feed rate changed with power rate of the engine, but
there really is no rationale for overlubricating an engine.
The level of alkalinity required depends primarily on the fuel’s sulfur content. If the sulfur content is
in the 2 to 3% range, then a TBN of 60 or so is sufficient. High-performance lubricants, which operate
successfully for fuels with over 3% sulfur, can have TBN values of 70 or more. Obviously, the higher the
sulfur content, the more rapidly the TBN is consumed in the engine.
Coatings are commonly used in low-speed diesel engines, especially for cylinder liners and piston rings
exposed to the highly acidic environment. Liners are commonly chrome electroplated, and rings are
chrome plated or plasma sprayed to reduce wear rates.
36.3.1.2 Crankcase Lubrication
The crankcase oil lubricates the gearing, bearings, and other engine components, including the turbocharger. A chief concern with low-speed diesel engines is the isolation of crankcase and cylinder oils,
although some migration of oils through the piston rod diaphragm is inevitable. This can introduce
cylinder liner wear debris as well as the acid combustion products mentioned above. Seawater contamination is also a concern. All of these contaminants can facilitate corrosion and rusting of journal bearings
and white metals, and require special lubricant formulation to obtain good product life.
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Three crankcase oil types are used in marine service (Pevzner 1998a):
• Rust and oxidation oil types (for older engine designs)
• Low alkalinity (TBN = 4–6) for water-cooled piston engines
• Medium alkalinity (TBN = 8–12) for oil-cooled piston engines or when a water-cooled piston
engine uses the same oil for main and oil-cooled auxiliary engines
Given the adverse affects of contaminants, lubricant additives are especially important. Wilkison (1983)
gives the following requirements for crankcase lubricants, which are either base oil (B) or additive (A)
determined properties:
1. Sufficient viscosity (B), especially at the operating temperatures. This is ensured by producing the
oil from highly refined medium- or high-viscosity index base oils or from synthetic blends.
2. Oxidation and thermal stability (B, A). Piston temperatures can be very high, and these features
of a lubricant are required to prevent a loss of viscosity (breakdown).
3. Demulsibility (A). It is advantageous if any seawater contamination can be quickly separated from
the oil; hence, additives that limit the volume fraction of water which can be dispersed or emulsified
in the oil. This is also referred to as a “water-shedding” capability.
4. Rust and corrosion prevention (A). Often in the form of alkaline additives to quickly suppress acidic
contaminants, these are also additives that inhibit corrosive interactions with surfaces.
5. Antifoaming (A). Antifoaming additives are added to ensure proper flooding of contacts and pump
operation.
6. Detergency (A). Because of the possibility of carbon- or ash-based soot, deposits can foul engine
components and inhibit efficient operation.
7. Extreme pressure (EP) performance (A). Most contacts are highly loaded at operating temperature
and will use extreme-pressure additives to reduce friction and wear.
8. Biocides (A). Biological attack of the base oil and additives can reduce lubricant effectiveness.
Many of these properties are of inherent benefit or requirement to tribologists. A few special considerations should be addressed, however. White metal babbits (tin-based alloys) are commonly used as a
bearing material in slow-speed engines. Saltwater contamination can allow galvanic corrosion of the tin,
forming a black oxide layer. This layer can cause increased interference between bearing and journal, and
can also flake off, resulting in a three-body wear condition. The obvious solution is to prevent seawater
contamination, but when this is not practicable, effective demulsifiers are essential.
In many low-speed diesel engines, up to 200 L of oil will flow through the engine per minute (a few
hundred liters per day will be consumed), and the lubricant will collect contaminants and combustion
by-products. Ideally, the oil would be discarded after one pass through the engine, but this is obviously
a wasteful and expensive proposition. Instead, effective detergent additives are included in the lubricant
formulation to prevent fouling of machine elements by these contaminants. Large ships typically have
equipment to centrifugally water separate, filter, and condition lubricants.
Biological attack of lubricant has been experienced. However, because biological organisms require
water to survive, this problem can be eliminated by preventing seawater contamination and by removing
water from the oil (greatly assisted by demulsifiers). Various biocides can be added to oils and fuels to
prevent microbial growth; otherwise, the oil can be heated to a temperature sufficient to kill the microbes.
Microbes are of special concern because of their tendency to plug filters.
Turbochargers present special problems in that heat must be quickly removed. Traditionally, this has
been done with separate low-viscosity oils that allowed for large volume flow rates and easy circulation;
but more recently, SAE 30 oil has become the norm.
36.3.2 Medium-Speed Diesel Engines (250 to 1000 rpm)
Medium-speed diesel engines are usually four-stroke engines and have a slightly higher power-to-weight
ratio than slow-speed diesel engines. They are commonly used for ferries, container ships, and cruise
© 2001 by CRC Press LLC
Total base number - mg/KOH/g
30
25
Low sulfur fuel and high oil consumption
20
15
10
High sulfur fuel and low oil consumption
5
0
0
1000
2000
3000
Hours of operation
4000
FIGURE 36.4 Drop in total base number (TBN) as a function of time and operating conditions. (From Wilkison,
J.L. (1983), Marine equipment, in CRC Handbook of Lubrication, Vol. I. Booser, E.R. (Ed.), CRC Press, Boca Raton,
FL, 227-248. With permission.)
ships. Maximum power delivered is around 1500 kW per cylinder, or 27,000 kW from an 18-cylinder
engine.
Medium-speed diesel engines usually use the same fuels as slow-speed engines, although sometimes
use lower viscosity fuel blends for improved performance. However, the same oil usually lubricates both
cylinder and crankcase, so the lubricant must have the ability to deal with acidity, soot, and other
combustion products. Some medium-speed diesels will have separate forced feed cylinder lubrication,
which allows direct application of fresh, fully alkaline oil to the cylinders. The oil is usually taken from
the crankcase, because there is inevitably some mixing, but this arrangement keeps most contaminants
out of the crankcase.
As can be seen from Table 36.1, the TBN level of medium-speed engine oils is not nearly as high as
for slow-speed diesel engine cylinder oils, but these lubricants do have many of the same requirements.
These include proper viscosity and alkalinity, oxidation and thermal stability, corrosion prevention,
antifoaming capability, detergency ability, and extreme pressure (EP) additives for wear prevention.
A difficult maintenance task is establishing intervals for lubricant maintenance. Oil must be periodically
added to the crankcase because it is tapped to provide cylinder lubrication. However, as the alkalinity
drops, the corrosion prevention capability of the oil is seriously compromised. Therefore, oil exchange
is a fairly complicated function of the fuel sulfur content and the rate at which oil is consumed and
replenished. Wilkison (1983) compared the change of TBN in crankcase oil, and typical results are shown
in Figure 36.4. Most lubricant suppliers provide a chemical oil analysis service to assist operators in
determining proper oil change intervals.
Alkalinity has another beneficial effect: there is a latent detergency associated with high TBN. This is
beneficial if there are insoluble contaminants present in the oil. A high TBN keeps the soot suspended
and prevents deposit formation and sticking rings. Obviously, the rate at which particulate contaminants
build up in the oil is a function of the fuel quality, and is inversely related to oil consumption. Also, large
amounts of particulate matter can drastically increase the viscosity of the oil (see Wilson et al. (1993) for
a summary of solid-phase concentration effects on viscosity). Regardless, continuous centrifuging of the
crankcase oil is recommended by engine manufacturers to keep insolubles below a desired threshold;
Wilkison (1983) suggests 3% concentration.
Fuel quality plays an important role in the success or failure of a lubricant system. More highly refined
fuels will have lower sulfur content, such as distillate fuel with a sulfur content of 0.2 to 1.3% by weight,
but usually less than 0.5% for American fuels (Wilkison, 1983). Medium-speed diesel engines powered
by distillate fuels are relatively rare, but these engines place a much lower burden on the lubricating oil
and require oils with lower TBN.
© 2001 by CRC Press LLC
It should be mentioned that many manufacturers have specific requirements for their engines. For
example, some General Motors two-stroke engines use a silver piston pin bushing, which then necessitates
the use of zinc-free oils to prevent damage to the bushings.
36.3.3 High-Speed Diesel Engines (>1000 rpm)
High-speed diesel engines are the engines of choice for pleasure craft as well as special applications on
larger vessels such as power for winch operation, pumping engines, electric power generation, etc. Many
of the design challenges with high-speed diesel engines are identical to those faced by automotive diesel
engines discussed in Chapters 32 and 33. For these engines, only the highest quality distillate fuels are
used, and heavy duty automotive type diesel oils are used as lubricants.
36.4 Steam and Gas Turbines
Steam and gas turbine engines are used for very large ocean-going vessels and for many warships. Nuclearpowered craft use a steam turbine engine for propulsion. In general, turbine engines have the following
advantages over comparably sized diesel engines:
•
•
•
•
•
Higher efficiency at high-speed operation
Relatively small and light, requiring smaller foundations and deck space than diesel engines
Lubricant consumption is low and the exhaust is relatively free of oil
Less vibration due to the absence of reciprocating parts
For large sizes, initial and maintenance costs are lower than for diesel engines
Because turbine engines are efficient at high speeds (4000 rpm and higher), and propellers are limited
to low speeds (100 rpm approx.), a large speed reduction is needed. This is usually accomplished with
reduction gearing.
Usually, a turbine-powered ship will have a high-pressure (HP) turbine with the exhaust fed into a
second, low-pressure (LP) turbine. On some ships, an intermediate-pressure (IP) turbine will be used,
with the HP exhaust heated before being fed into the IP turbine. Figure 36.5 depicts a typical arrangement
of a twin turbine geared propulsion unit.
Nested double
reduction gear
Reduction gear
radial bearings
Low pressure turbine
Astern
element
Line Shaft
Packing
Radial bearings
Packing
Bearing
Solid
coupling
Reduction gear
radial bearings
FIGURE 36.5
Main thrust
bearing
High pressure turbine
Typical arrangement of geared-turbine propulsion unit.
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Turbine
thrust bearings
It should be noted that turbine engines are not easily reversible; geared systems as shown are arranged
to provide most of the power in the forward direction. Astern (rearward) propulsion is achieved by
arranging a few rows of blades in the LP turbine appropriately, giving low-power propulsion astern.
The tribological issues with turbine engines are the journal and thrust bearings and the associated
large hydrodynamic film which is desired, and the gears in the speed reduction units, which require
effective EP additives for wear resistance.
36.5 Ancillary Equipment
36.5.1 Stern Tubes
The stern tube supports the propeller shaft; it must also effectively isolate the ship, drive components,
and engine from the water. The propeller shaft thrust is absorbed by the hydrodynamic thrust bearing,
but stern tube bearings are needed to support the weight of the propeller shaft (Figure 36.6). Historically,
stern tube bearings were constructed of water-lubricated lignum vitae (Latin for “the wood of life”), a
tropical wood, but today are oil-lubricated rolling element bearings.
For obvious reasons, effective sealing of the stern tube is of great concern, especially because the
lubricating oil for the stern tube is often tapped from the main propulsion engine. Any contamination
of this oil has extremely serious consequences. To promote effective sealing and to prevent water seepage
into the stern tube, a static pressure is placed on the oil. This can result in excessive oil seepage through
the stern tube seals in the case of seal wear or other unusual circumstances. Seal wear is of special concern
with shallow-draft vessels and riverboats because of the abrasive particles (sand) in suspension in the
water. Worn seals result in oil slicks around the sterns of vessels, which is obviously undesirable from an
environmental standpoint.
When a seal is worn and excessive oil leakage occurs, the only long-term solution is to replace the seal.
In the meantime, a common practice is to use higher weight oils to reduce oil leakage. For example, one
manufacturer recommends oil up to 275 cSt at 40°C in such circumstances (Wilkison, 1985).
A separate issue is the support of a kort nozzle, commonly used in cargo vessels, which encloses the
ship’s propeller and is slewable. A kort nozzle (sketched in Figure 36.7) improves propeller efficiency and
allows pitching of ships for greater maneuverability. The rudder shaft bearing shown must carry the
entire weight of the rudder, and also carries a large axial load during pitching of the ship. The stern
lubrication system does not include the kort nozzle, which instead is isolated with seals, and water
contamination of the bearings is a possibility.
Oil head tank
Oil level gage
Cooler
Oil vent (stern tube)
Oil inlet (stern tube)
Grease connection
(backup)
Pump
Filter
Drain line
Pressure gage
Temperature gage
FIGURE 36.6
Schematic illustration of a typical stern tube lubrication system.
© 2001 by CRC Press LLC
Oil drain tank
Top rudder
shaft bearing
Bottom rudder
shaft bearing
Kort nozzle
Rudder
Propeller
(a)
(b)
FIGURE 36.7 Schematic illustration of a kort nozzle: (a) general view of ship stern with kort nozzle; (b) detail of
rudder shaft bearing.
36.5.2 Rigging Equipment
Winches and capstans are devices used with wire rope under tension. Wire rope is commonly used for
rigging and hoisting and is also run between two ships at sea. These are usually electric, diesel, or
hydraulically powered and equipped with gear reducers. The enclosed gears require a good-quality gear
oil, and hydraulic equipment of course needs high-quality working fluids. The wire rope is lubricated
with thick greases to reduce friction between wires, thereby increasing the fatigue life of the wire (Hamrock et al., 1999).
36.5.3 Pumps
Pumps are used to move ballast water, transport fuel, circulate engine coolant, and transfer liquid cargoes.
Centrifugal pumps operate at high speed and usually do not use a gear reducer. These use light turbine
oils as lubricants and grease-packed bearings. Positive displacement pumps operate at lower speeds and
use a gear reducer, which requires a good-quality gear oil. Often, lubrication is performed by the fluid
being pumped.
36.5.4 Oil Maintenance Equipment
Given the large number of contaminants that can be present in a lubricant, there is a need for treatment
of used lubricant before it is recirculated. One important device is a centrifuge, shown in Figure 36.8,
run either as a separator/purifier or as a clarifier. The distinction between these two modes is that
separator/purifiers are arranged to remove any entrained water, while the clarifier attempts to remove
soot and sediment from the oil. To decrease the viscosity of the oil and the centrifuge efficiency, oils are
commonly preheated before centrifuging.
Filters are commonly used upstream of the oil pump to remove fine debris. For smaller vessels, where
centrifuging is not economically viable, filters are used to remove entrained particulates.
© 2001 by CRC Press LLC
Oil outlet
Oil outlet
Water outlet
Sediment
Used oil inlet
Used oil inlet
Separator
Clarifier
(a)
Used oil inlet
Used oil inlet
Oil outlet
Oil outlet
Water outlet
Separator
Clarifier
(b)
FIGURE 36.8
type.
Schematic illustration of lubricant centrifuges in service in marine applications: (a) tube type; (b) disk
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© 2001 by CRC Press LLC