36Marine Equipment

Tribology

36.1 Introduction36.2 Marine Oil Properties and Chemistry

Fuel and Oil Rheology and Chemistry • Environmental Concerns

36.3 Diesel Engine LubricationSlow-Speed Diesel Engines (<250 rpm) • Medium-Speed Diesel Engines (250 to 1000 rpm) • High-Speed Diesel Engines (>1000 rpm)

36.4 Steam and Gas Turbines36.5 Ancillary Equipment

Stern Tubes • Rigging Equipment • Pumps • Oil Maintenance Equipment

36.1 Introduction

Marine equipment presents an extremely demanding environment to tribologists. While the machineelements and lubricants used are substantially the same as in other applications, some fundamentaldifferences regarding the operating environment play a substantial role in their success. Chief amongthese are the chemical nature of the fuels used, the operating temperatures achieved, and the speed rangescommonly 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 equipmentscale. Small ships can use diesel motors of only a few horsepower compared to the largest aircraft carrierswith nuclear reactors of sufficient capacity to power a medium-sized city. Very large thrust loads developedby propeller shafts must be efficiently transferred to the ship structure (and present perhaps the mostcommon 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 machineelements is covered in other sections of this handbook and in a number of excellent reference texts, suchas 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 adetailed investigation into individual machine elements or applications. Instead, this chapter focuses onthe 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 ChemistryIn the past 15 years, diesel engines have exerted a dominance over other power sources in commercialships and have displaced steam engines and steam and gas turbine engines to minor roles. Steam and

Steven R. SchmidUniversity of Notre Dame

Karl J. SchmidJohn Deere Corporation

gas turbine engines are still very popular for warships, however. For economic reasons, ships are routinelypowered by fuels with rather high viscosity in order to lower the fuel cost by reducing refining require-ments. 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 temper-atures, and are therefore usually preheated to up to 130°C. The goal is to achieve a viscosity value near20 cSt (Pevzner, 1998a).

The drive toward high-viscosity fuels is an economic one because the thicker fluids undergo less refiningand are therefore less expensive. There has been a considerable increase in the viscosity of fuels used in thepast decade or so. As a comparison, Wilkison’s (1983) excellent overview of marine tribology restricted attentionto 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 increasedacidity result. While sulfur is intentionally used in low concentrations as an extreme pressure additive forautomotive lubricants, sulfur concentrations are sufficiently high in fuels to make sulfur corrosion an impor-tant concern. Sulfur content in some fuels can be over 4%,* leading to significant SO2 and SO3 combustionproducts, and a very acidic environment. Also, carbon particles can be suspended in the fuel, leading tofouling of cylinders and causing ring and piston sticking. In addition, abrasive particles are conveyed intothe 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 naphthenicoils were dominant prior to the early 1980s, paraffinic base oils and synthetics are now totally dominantin the industry, mainly due to the need for higher viscosity indices (VI) associated with high operatingtemperatures. Oils with VIs in the 100 to 150 range are used, although specific values are manufacturerspecific. 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 byapplication. While specific values depend on application, the TBN of low-speed diesels is roughly 70; thatof medium-speed diesel engines is roughly 30; and that of high-speed engines is comparable to automotiveoils (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, defoa-mants, 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 contami-nants, 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,” whilethose with low sulfur content are “sweet crudes,” and fuels are sour or sweet depending on their source. This has ledto 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 withlow 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.

about the lubricant itself involve disposal. Air pollution is mostly due to the chemical nature of the fueloil 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:

(36.1)

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 theweight percent of sulfur in the fuel; and V is the vanadium content and Na is the sodium content, bothin parts per million.

In 1992, the International Maritime Organization (IMO), a United Nations organization, identifiedtwo pollutants (NOx and SOx) for control by the year 2000 (Bastenhof, 1992). However, no specificrequirements or procedures for achieving emissions reductions have been mandated. It is entirely possiblethat 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

TABLE 36.1 Properties of Selected Marine Application Low- and Medium-Speed Diesel Engine Lubricants

Manufacturer Designation SAE No.

Viscosity Viscosity Index

TBNSulfated

AshcSt at 40°C cSt at 100°C (mg KOH/g) (wt%)

Low-speed diesel engines — crankcaseMobil Mobilgard 312

Mobilgard 412Mobilgard 512

304050

103141199

11.414.418.4

100100100

151515

2.12.12.1

Texaco DORO AR 30TARO 16 XD 30TARO 16 XD 40

303040

11198

139

11.91114

969797

61616

0.751.951.95

Low-speed diesel engines — cylinderChevron Delo Cyloil Extra

Delo Cyloil SpecialDelo Cyloil Heavy

605050

320214244

14.818.018.1

95101100

857050

Mobil Mobilgard 570 50 247 21 100 70Shell Alexia D

Alexia Xb

5050

260211

19.019.5

8095

70100

2.02.0

Texaco TARO 70TARO 85

5060

225320

1925

9595

7085

911.00

Medium-speed diesel enginesChevron Delo 1000 30

Delo 1000 40Delo 1000 50Delo 6170 CFOa

30405040

110145227144

11.914.419.414.8

979897

101

12121217

1.671.671.672.0

Mobil Mobilgard 330Mobilgard 430Mobilgard 450a

304040

106143145

11.013.514.5

99100

98

303013.5

441.65

Shell Argina S 30Argina S 40Argina T 30Caprinus HPD40a

30403040

104139104160

11.3914.1311.814.5

102102102

98

20203013.0 1.5

Texaco TARO 20 DP 30TARO 20 DP 40TARO 40 XL 40

304040

95135139

111414

100100

97

202040

2.52.54.9

a Zinc-free formulation.b For fuels with sulfur content greater than 3.5%.

PHR S

V Nc

a

∝( )

( )

1 7

0 3

.

.

cost of fuel. The results of his model are given in Table 36.2. Lanz also mentioned that an alternativeapproach is the treatment of exhaust gases, which will represent a fuel cost of about $25 per tonne withfuel consumption increased by 3%.

A number of countries enforce emissions regulations in coastal waters, which can extend a few hundredmiles from the shoreline. However, the economic reasons for using bunker oil as fuel are so compellingthat a common practice is to use bunker oils in international waters and more refined fuels (i.e., reduced-sulfur fuels) along coastlines. This has a dramatic influence on acid rain because the vast majority ofsulfur emissions are not deposited on land, but at sea. The sulfur content of seawater is 1015 tonnes, whileall fossil fuels combined would contribute only 1011 tonnes, which is accepted as producing no deleteriouseffect (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 eliminatemost of the sulfur content, then the need for increased alkalinity in the lubricant would be removed. Infact, lubricants would need to be reformulated, in that sulfur when present in small amounts is anextremely useful EP additive. Thus, as discussed by Golothan (1976), using fuels without sulfur canactually 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 retardingfuel injection timing, albeit at a loss in specific fuel consumption of about 10% (Hold, 1993). Retardingtiming increases soot loading in the cylinder lubricant oil, requiring reformulation, higher cylinder oilflow rates, or reduced oil change intervals, all of which have their own environmental concerns. For largeremission reductions, some type of catalytic reduction is necessary. This will necessitate reformulation oflubricants because popular extreme pressure (EP) additives such as zinc dithiophosphate are incompatiblewith NOx-reducing catalysts.

The sources of particulate emissions in the exhaust consist mostly of the sulfur and carbon particlesin the fuel. However, some lubricant is always combusted and can contribute to the gas and particleemissions. High TBN oils contain up to 12% sulfated ash, which during combustion is converted intoinorganic ash particles. In addition, some attention has been paid to reducing particulate emissions ofdiesel engines, although regulatory agencies have pursued the marine industry much less arduously thanother industries to date.

Particulates are also a concern. Fuel oil and lubricants contain metals such as calcium (provided asCaOH) to improve alkalinity. The metal ions form ashes with sulfur (e.g., CaS), which is then a potentialparticulate exhaust. Carbon particles (soot) are also exhausted. Gros (1990) reports that typical medium-speed 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 thissection. However, a few distinctions must be made. Many of the issues result from the acidity of combustionproducts as discussed above. However, there are other peculiarities of diesel engines in marine applications.

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($)

3.53.01.5

51052

15–30—

46–58

After Lanz, R. (1995), The Motor Ship, 5, 22-23.

36.3.1 Slow-Speed Diesel Engines (<250 rpm)

Slow-speed diesel engines are large, two-stroke engines with crosshead construction (Figure 36.1), andusually run between 90 and 250 rpm. These engines use a diaphragm and stuffing boxes to separate thecylinder and crankcase, allowing each to be lubricated independently. Slow-speed diesels are the largestengines, with bore sizes around 1 m and strokes of over 3 m. Piston speeds are around 7 to 8 m/s, andpower output for the largest engines is over 65,000 kW. These engines are primarily used in large tankersand 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 characteristicconcerns. Excellent summaries of the issues and concerns in low-speed diesel engine tribology includethe 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 thatseparate lubricant supplies and preventive maintenance schedules must be maintained. It is beneficial inthat the special environment presented by the cylinder can be treated without adversely affecting othercomponents. This design was conceived for this reason.

Because the fuels and combustion products are acidic, isolating them from as much of the engine aspossible is the best approach to preventing corrosion and corrosion-assisted fatigue and wear. For the

FIGURE 36.1 Schematic illustration of cross-head diesel engine. (Adapted from Pevzner, L.A. (1998a), Aspects ofmarine low-speed, cross-head diesel engine lubrication, Lubr. Eng., 54, 16-21.)

Exhaust valve

Cooling jacket

Piston

Cylinder liner

Scavenging ports (quills)

Piston rod

Exhaust gas manifold

Connecting rod

Cam shaft

Crankshaft

Bed plate

Scavenge air collector

Air cooler

cylinder itself, the unalterable presence of corrosive elements requires pursuit of unique tribologicalsolutions. 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 spreadquickly, but must also allow formation of a hydrodynamic film at operating temperatures. Cylinderlubricants are most commonly SAE 50 or SAE 60 to ensure the desired viscosity at the cylinder lineroperating temperature of around 200°C, with TBN values of 50 to 70. In addition, EP additives anddetergents are especially important for these lubricants.

Many problems associated with lubrication arise from too low or too high an application rate. If theapplication rate is too high, overlubrication can cause fouling of the cylinder or can even cause post-cylinder fires in the engine, especially the turbocharger. Further, overlubrication adds significantly tomaintenance cost in a highly competitive industry. If the application rate is too low, then the alkalinenature of the lubricant is neutralized before it spreads over the entire cylinder. This usually means thatthe oil applied just below the quill will neutralize the sulfuric acid but will become increasingly ineffectiveas 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 wearoccurs between quills, but almost none occurs near the quills. An example of clover leafing is shown inFigure 36.2.

Pevzner (1998b) investigated the effect of oil feed rate into the cylinder and its effect on cylinder linertemperature and liner wear rate. His results are shown in Figure 36.3 for a number of different powerlevels 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 wearpatterns 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.)

to decrease with lubricant feed rate above a certain threshold. Also, Pevzner noted that ash depositsformed on supercharger turbine blades and on the piston crown at high feed rates. Pevzner conductedsea trials of the result for verification. The optimal feed rate changed with power rate of the engine, butthere 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 isin the 2 to 3% range, then a TBN of 60 or so is sufficient. High-performance lubricants, which operatesuccessfully for fuels with over 3% sulfur, can have TBN values of 70 or more. Obviously, the higher thesulfur 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 ringsexposed to the highly acidic environment. Liners are commonly chrome electroplated, and rings arechrome 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 turbo-charger. 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 introducecylinder liner wear debris as well as the acid combustion products mentioned above. Seawater contam-ination is also a concern. All of these contaminants can facilitate corrosion and rusting of journal bearingsand white metals, and require special lubricant formulation to obtain good product life.

FIGURE 36.3 Effect of oil feed rate on cylinder liner: (a) wear rate vs. oil feed rate; (b) liner temperature vs. oilfeed rate. (Data from Pevzner, L.A. (1998a), Aspects of marine low-speed, cross-head diesel engine lubrication, Lubr.Eng., 54, 16-21.)

100%80%64%50%

0

50

100

150

200

250

300

350

0.4 0.8 1.2 1.6 2.0

Oil feed rate, g/kWhr

Lin

er w

ear

rate

, mg/

hr

150

140

130

120

110

100

900.4 0.8 1.2 1.6 2.0

100%80%64%50%

Cyl

inde

r lin

er te

mpe

ratu

re, °

C

Oil feed rate, g/kWhr

(a)

(b)

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 pistonengine 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 theoil 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 featuresof 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 fromthe oil; hence, additives that limit the volume fraction of water which can be dispersed or emulsifiedin 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 acidiccontaminants, these are also additives that inhibit corrosive interactions with surfaces.

5. Antifoaming (A). Antifoaming additives are added to ensure proper flooding of contacts and pumpoperation.

6. Detergency (A). Because of the possibility of carbon- or ash-based soot, deposits can foul enginecomponents and inhibit efficient operation.

7. Extreme pressure (EP) performance (A). Most contacts are highly loaded at operating temperatureand 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 consid-erations should be addressed, however. White metal babbits (tin-based alloys) are commonly used as abearing 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, andcan also flake off, resulting in a three-body wear condition. The obvious solution is to prevent seawatercontamination, 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 fewhundred liters per day will be consumed), and the lubricant will collect contaminants and combustionby-products. Ideally, the oil would be discarded after one pass through the engine, but this is obviouslya wasteful and expensive proposition. Instead, effective detergent additives are included in the lubricantformulation to prevent fouling of machine elements by these contaminants. Large ships typically haveequipment to centrifugally water separate, filter, and condition lubricants.

Biological attack of lubricant has been experienced. However, because biological organisms requirewater to survive, this problem can be eliminated by preventing seawater contamination and by removingwater from the oil (greatly assisted by demulsifiers). Various biocides can be added to oils and fuels toprevent 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 hasbeen 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-weightratio than slow-speed diesel engines. They are commonly used for ferries, container ships, and cruise

ships. Maximum power delivered is around 1500 kW per cylinder, or 27,000 kW from an 18-cylinderengine.

Medium-speed diesel engines usually use the same fuels as slow-speed engines, although sometimesuse lower viscosity fuel blends for improved performance. However, the same oil usually lubricates bothcylinder and crankcase, so the lubricant must have the ability to deal with acidity, soot, and othercombustion 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 fromthe crankcase, because there is inevitably some mixing, but this arrangement keeps most contaminantsout of the crankcase.

As can be seen from Table 36.1, the TBN level of medium-speed engine oils is not nearly as high asfor 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 periodicallyadded to the crankcase because it is tapped to provide cylinder lubrication. However, as the alkalinitydrops, the corrosion prevention capability of the oil is seriously compromised. Therefore, oil exchangeis a fairly complicated function of the fuel sulfur content and the rate at which oil is consumed andreplenished. Wilkison (1983) compared the change of TBN in crankcase oil, and typical results are shownin Figure 36.4. Most lubricant suppliers provide a chemical oil analysis service to assist operators indetermining proper oil change intervals.

Alkalinity has another beneficial effect: there is a latent detergency associated with high TBN. This isbeneficial if there are insoluble contaminants present in the oil. A high TBN keeps the soot suspendedand prevents deposit formation and sticking rings. Obviously, the rate at which particulate contaminantsbuild up in the oil is a function of the fuel quality, and is inversely related to oil consumption. Also, largeamounts of particulate matter can drastically increase the viscosity of the oil (see Wilson et al. (1993) fora summary of solid-phase concentration effects on viscosity). Regardless, continuous centrifuging of thecrankcase 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 refinedfuels 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 poweredby distillate fuels are relatively rare, but these engines place a much lower burden on the lubricating oiland require oils with lower TBN.

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.)

High sulfur fuel and low oil consumption

0

5

10

15

20

25

30

Tota

l bas

e nu

mbe

r -

mg/

KO

H/g

0 1000 2000 3000 4000Hours of operation

Low sulfur fuel and high oil consumption

It should be mentioned that many manufacturers have specific requirements for their engines. Forexample, some General Motors two-stroke engines use a silver piston pin bushing, which then necessitatesthe 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 onlarger vessels such as power for winch operation, pumping engines, electric power generation, etc. Manyof the design challenges with high-speed diesel engines are identical to those faced by automotive dieselengines discussed in Chapters 32 and 33. For these engines, only the highest quality distillate fuels areused, 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. Nuclear-powered craft use a steam turbine engine for propulsion. In general, turbine engines have the followingadvantages 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 limitedto low speeds (100 rpm approx.), a large speed reduction is needed. This is usually accomplished withreduction gearing.

Usually, a turbine-powered ship will have a high-pressure (HP) turbine with the exhaust fed into asecond, 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 arrangementof a twin turbine geared propulsion unit.

FIGURE 36.5 Typical arrangement of geared-turbine propulsion unit.

Reduction gearradial bearings

Line Shaft

Bearing

Solidcoupling

Reduction gearradial bearings

Nested doublereduction gear

Low pressure turbine

Asternelement

Turbinethrust bearings

PackingRadial bearings

Packing

High pressure turbineMain thrustbearing

It should be noted that turbine engines are not easily reversible; geared systems as shown are arrangedto provide most of the power in the forward direction. Astern (rearward) propulsion is achieved byarranging 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 associatedlarge hydrodynamic film which is desired, and the gears in the speed reduction units, which requireeffective 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”), atropical wood, but today are oil-lubricated rolling element bearings.

For obvious reasons, effective sealing of the stern tube is of great concern, especially because thelubricating oil for the stern tube is often tapped from the main propulsion engine. Any contaminationof this oil has extremely serious consequences. To promote effective sealing and to prevent water seepageinto the stern tube, a static pressure is placed on the oil. This can result in excessive oil seepage throughthe stern tube seals in the case of seal wear or other unusual circumstances. Seal wear is of special concernwith shallow-draft vessels and riverboats because of the abrasive particles (sand) in suspension in thewater. Worn seals result in oil slicks around the sterns of vessels, which is obviously undesirable from anenvironmental 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, onemanufacturer 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 theship’s propeller and is slewable. A kort nozzle (sketched in Figure 36.7) improves propeller efficiency andallows pitching of ships for greater maneuverability. The rudder shaft bearing shown must carry theentire weight of the rudder, and also carries a large axial load during pitching of the ship. The sternlubrication system does not include the kort nozzle, which instead is isolated with seals, and watercontamination of the bearings is a possibility.

FIGURE 36.6 Schematic illustration of a typical stern tube lubrication system.

Oil head tank

Oil level gage

Oil inlet (stern tube)Oil vent (stern tube)

Pressure gage

Temperature gage Oil drain tank

Pump

Filter

Cooler

Drain line

Grease connection(backup)

36.5.2 Rigging Equipment

Winches and capstans are devices used with wire rope under tension. Wire rope is commonly used forrigging and hoisting and is also run between two ships at sea. These are usually electric, diesel, orhydraulically powered and equipped with gear reducers. The enclosed gears require a good-quality gearoil, and hydraulic equipment of course needs high-quality working fluids. The wire rope is lubricatedwith thick greases to reduce friction between wires, thereby increasing the fatigue life of the wire (Ham-rock 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 turbineoils as lubricants and grease-packed bearings. Positive displacement pumps operate at lower speeds anduse a gear reducer, which requires a good-quality gear oil. Often, lubrication is performed by the fluidbeing 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 treatmentof 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 thatseparator/purifiers are arranged to remove any entrained water, while the clarifier attempts to removesoot and sediment from the oil. To decrease the viscosity of the oil and the centrifuge efficiency, oils arecommonly preheated before centrifuging.

Filters are commonly used upstream of the oil pump to remove fine debris. For smaller vessels, wherecentrifuging is not economically viable, filters are used to remove entrained particulates.

FIGURE 36.7 Schematic illustration of a kort nozzle: (a) general view of ship stern with kort nozzle; (b) detail ofrudder shaft bearing.

(a) (b)

Kort nozzle

Rudder

Top ruddershaft bearing

Bottom ruddershaft bearing

Propeller

References

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Gros, S. (1990), Exhaust Gas Emissions in Marine Installations, Wartsila Diesel News and Views, 28-32.Hamrock, B.J., Jacobson, B.O., and Schmid, S.R. (1999), Fundamentals of Machine Elements, McGraw-

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FIGURE 36.8 Schematic illustration of lubricant centrifuges in service in marine applications: (a) tube type; (b) disktype.

Used oil inlet

Water outlet

Oil outlet

Used oil inlet

Oil outlet

Sediment

Separator Clarifier

Used oil inletOil outlet

Water outlet

Used oil inlet

Oil outlet

Separator Clarifier

(a)

(b)

Hellingman, G. J. and Barrow, S. (1982), Shipboard Investigations with Selected Fuels of Tomorrow,ASME Paper 82-BGP-9, American Society of Mechanical Engineers, New York.

Hold, G.E. (1993), Large-Bore Diesel Engines Require Different Lubrication, Past, Present and Future,ASME Paper No. 93-ICE-25, American Society of Mechanical Engineers, New York.

Juvinall, R.C. and Marshek, K.M. (1991), Fundamentals of Machine Component Design, John Wiley &Sons, New York.

Lane, G., Casale, P.G., and Chadwick, R.E. (1987), Development of Marine Lubricants for the FutureSlow and Medium Speed Engines, SAE Paper No. 871396, Society of Automotive Engineers,Warrendale, PA.

Langer, A.J. and Lim, K.C. (1987), Marine Cylinder Lubricants Development — Today and Tomorrow,Lubr. Eng., 43, 858-870.

Lanz, R. (1995), Sulfur sours emissions level agreement, The Motor Ship, 5, 22-23.Pevzner, L.A. (1998a), Aspects of marine low-speed, cross-head diesel engine lubrication, Lubr. Eng., 54,

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