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MARINE DIESEL POWER PLANTSand Ship Propulsion

By PETER KYROPOULOS

THE revolutions per minute of marine propellers isdetermined by hydrodynamic considerations. Thetype of engine furnishing the propeller with thenecessary power does not affect the speed for best pro-peller efficiency. It is the engine designer's task toprovide the propeller with the power and speed re-quired by the naval architect. Besides, the power plantis to weigh as little as possible and to take up a mini-mum of space in th e vessel.I n order to obtain an idea of the order of magnitudeof the speed for maximum propeller efficiency as affectedby forward speed and shaft horsepower, average valuesfo r a series of commercial vessels have been collectedand plotted. A plot of such data is shown in Fig. No. 1.It is seen that the required propeller speed becomesquite low, especially at low speeds and large power.If the engine is directly coupled to the propeller shaft,the engine size will be excessive, unless the power issubdivided into two engines and propellers. A definitionof propeller efficiency and its relation to ship propulsionis given in the appendix. Since the initial cost of aninstallation is proportional to its weight, direct drivewith low engine speed will be both heavy and expensive.DIRECT AND GEARED DRIVE

The difficulty of direct drive described above has ledto the application of gear transmissions to marine Dieseldrives. This allows the propeller to operate at its bestrate of speed without imposing limitations on the en-

gine speed. I t furthermore permits the subdivision ofthe engine into several independent units, all gearedto the same shaft through the transmission. It is usualto gear two parallel engines to one propeller shaft.However, there are installations which have four en-gines driving a single shaft. The gear transmissionenables the engine speed to be selected within widelimits. As a result of high engine speed a considerablegain in weight and space is obtained. The cost of theinstallation is decreased in proportion to the decreasei n weight.

To illustrate the saving in weight and space for agiven vessel. Fig. No. 2 shows sections of a cargo ves-sel with a displacement of 8,000 tons and 2,500 shaft-horsepower (shp). Installation A represents a directdrive Diesel engine. Propeller and engine speed areequal (80 revolutions per minute 1 . The weight of theinstallation is 242 pounds per shaft-horsepower. Tn-stallation B shows two parallel engines geared to thepropel ler shaft through a transmission. Propeller speedis 80 revolutions per minute, as before, and the enginesare running at 250 revolutions per minute. Weight ofthe twin engine installation is 116 pounds per shaft-horsepower. The saving in weight and space is appre-ciable.In common Diesel engine terminology such an iq -stallation would be called a hieh speed installation. Tshould be kept in mind that this term is applied to anvDiesel engine running at or above about 250 revolutions

10 12 14 16 8 20 22SPEED (KNOTS)

per minute.As mentioned above, directdrive limits the number of en-gines to the number of the pro-peller shafts. An example willillustrate the effect of this limita-tion on cruisins; economy: Agiven vessel is designed to runat 15 knots at rated full power.Cruising sneed, at which most ofthe operation will be maintained.is 11 knots (i.e., 73 per cent offull speed). For this sneed, only40 per cent of rated full power isrequired. Fig. No. 3 shows acurve of specific fuel consumw-tion for a typical two-stroke ma-rine Diesel engine plotted againstload. The consumption at fullpower is 0.361 ~ o u n d sof fuelper brake power hour. At 40 per

AT LEFT:FIG. I-Propeller revolutionsper minute for maximum pro-peller efficiency vs. speed fo rdifferent shaft horsepower.'

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cent power it i s 0.361 pounds of fuel per brake powerhour. The geared twin engine drive would reducepower for cruising by disengaging one engine com-pletely. The remaining engine would then run at 80per cent of its ful l power with a specific fuel consump-tion of 0.349 pounds of fuel per brake power hour.It has been assumed in this example that the variationof fuel consumption with load i s the same fo r theindividual engine of the twin installation and for thesingle engine of twice the power. This is not neces-sarilv true. However. f or engines above a certain powerthis assumption is permissible.

The twin-engine drive also adds to the safety ofoperation. In case of fai lure of one engine, thetrip can be continued on the functioning engine.Often, repairs can be made during the trip, thusmaterially reducing the period for which the vesselis tied up for overhaul.A NOTE ON RATED ENGINE POWER

In connection with Fig. No. 3 a remark aboutrated engine power is in order. It will be noticed

AT RIGHT:FIG. 3- pecific fuel consumptionof a typical marine diesel engine(4000 brake horsepower two-stroke).

SCALE (FEET)

FIG. 2-Space required for directdrive (A)and geared (B) diesel en-gine installation. 8000-ton freighterwith 2500 shaft horsepower at 80propeller revolutions per minute.

that the curve of specific fuel consumption has not yetreached its minimum value at 4/4 load, that is, at fullrated power. For the engine from which Fig. No. 3was taken, the rated full power was 3,750 brake power.110 per cent overload, or 4,125 brake power were listedas allowable continuous overload. The maximum powerobtained in the acceptance test was 5,300 brake power,or 1 41 per cent of rated power. This represents a caseof conservative rating. It is clear that such a ratingllets the engine show up poorly as far as weight perhorsepower is concerned. From a point of view ofmaintenance and engine life, however, the practice of

LOADPage 5EBRUARY. 1944

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conservative rating, sometimes called "rating down,"pays by reducing engine wear and failure due to over-load. Manufacturers are inclined to present maximumpowers in order to make them compare favorably withother power plants.In a similar fashion, weight figures are often opti-mistic, since they are based on an arbitra ry list of part sassumed to constitute an engine.TWO- AND FOUR-STROKE ENGINES

For a given power and speed, weight and space re-quired of the power plant will also depend on the typeof engine chosen. To illustrate this, Table No. I showsa comparison of three engines having the same powerand speed. Engine A is a single acting two-stroke en-gine, B a four-stroke engine with exhaust turbo super-charger, C an unsupercharged four-stroke engine.It is seen that the two-stroke engine is both lighterand more compact than the two other types, a reasonwhy two-stroke engines have gained in popularity dur-ing recent years. The advent of exhaust turbo super-charging has again put two- and-four-stroke engines side by side,at least as far as weight is con-cerned. The exhaust superchargedfour-stroke engine is seen to beonly 7.3 per cent heavier thanthe two-stroke engine, and slightlylarger in size. The unsuper-charged engine is 25 per centheavier than the two-stroke en-gine and considerably longer.EFFICIENCY OF GEARTRANSMISSIONS

The foregoing examples haveshown the desirability of gearingan engine with relatively highspeed to the propeller shaft. Asa result, gear transmissions areextensively used, usually in con-nection with mechanical or hy-

AT LEFT:FIG. 4-Transmission efficiency vs.shaft horsepower for differenttypes of power transmission.Curves (A): Gear transmission;Curves (B): Electric transmission(A.C.); Curves (C): Electric trans-mission (D.C.). (The circle indicatesa point for diesel-electric drivefrom Fig. No. 5).

draulic clutches and coupline-s. This-form of power transmission is, of course,equally applicable to steam turbines andDiesel engines as prime movers. Thetransmission efficiency (see appendix fordefinition) of gear transmissions is high,as shown by curves A of Fig. No. 4. Theupper curve represents single, the lowerdouble reduction gears.Although high, the initial cost of re-duction gears is rarely a deciding fac-tor for or against gear transmissions incomparison with electric power trans-mission at least on large installations.Considerations of safety, simplicity of design, main-tenance, noise, and vibration and their evaluation asdesign factors are subject to personal preferences andallow no general statements.

DIESEL ELECTRIC DRIVEBecause of the twofold power transmission, the Dieselelectric drive has a lower transmission efficiency thanthe gear drive, as shown by curves B of Fig. No. 4, rep-

resenting A.C. installations and curves C for D.C.Since Diesel fuel and fuel oil are comparative in costand heating value, fuel consumption is a suitable basisfor comparing the various Diesel drives with each otherand with steam turb ine drive. Table No. I1 shows sucha comparison. It is seen that the Diesel drive has theadvantage. For a 10,000 shaft-horsepower installationthe fuel saving of the Diesel drive as compared withthe steam turb ine would be 2,200 pounds of fuel perhour. I n making this comparison, it should be kept inmind that Diesel power plants are not suitable for veryTABLE I-COMPARISON OF WEIGHT AND SIZE OF DIESEL ENGINESOF DIFFERENT TYPES AND EQUAL POWER

I single acting exhaust- unsuper-Type of engine: engine supercharged charged........................................................................hp 510

Engine rpm .......................................................... 350Weight of engine (Ibs.) .................................. 30,500

............................eight of auxil iaries (Ibs.) 4,750Weight of coupling and transmission (1:2)

................................................................lhs.) 6,400......................eight of installation per bhp 82..................verall length of engine (inches) 134

................verall height of engine (inches) 92Increase in wei ght hhp over engine ( A )........................................................per cent)

x .

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AT RIGHT:FIG. 5-Power vs. transmission ef-ficiency vs. speed for diesel-elec-tric (D.C.) drive of an 8000-tonice breaker. Curve (A): Enginebrake horsepower; Curve (B): Shafthorsepower; Curve (C): Transmis-

sion efficiency.

For powers above 50,000isFig. No. 5 shows a plot of power and

(D.C.) installation of anbrake power engines. Onef th is plot at V=15 knots and

8 10 12 14 16

SPEED (KNOTS )

A great advantage of the electrical installa tion i s the DIESEL AND ELECTRIC DRIVE AT OVERLOADThe geared Diesel engine is essentially a constantof the ~ropellermotor. This eliminates 10%' torque drive. As long as the fuel cut-off ratio of theThe engine remains unchanged, the torque developed ,,,illcan, therefore, dispose more freely of the remain regardless of speed.The electric propeller motor is a constant powerPropeller rotation is reversed electrically, eliminating machine; i.e., the electric power supplied by the ~ i ~

generator remains constant [he torque in the pro-=. Besides, the drive is colltrolled directly f rom peller shaft increases as the propeller speed decreasesdue to increased propeller load. Such an increase inload may result from increased wave or wind resistanceC. OR A. C. DRIVE or increased draft. The speed loss will be smaller in

Direct current drive is desirable because of its con- case of the constant power electr ic drive. On the otherspeed control. I ts disadvantage lies in a lower hand, the increase in torque is accompanied by an in-4 ) , greater weight and crease in stress in the propeller shaft and blades, re-quiring larger shaft diameters and heavier blades.Summarizing, it should be kept in mind that there isnot one ideal type of drive applicable to all problemsof ship propulsion. The present survey points outsome of the main considerations leading to adoption ofA;C. drives lack the flexible speed control of D.C. one type or the other.

the power It is also well to note the limitations of generalbe handled. Simple construction and the studies such as the one presented here. It shows trends,of wing high voltaees are decided advan- illustrated by specific examples. In the case of a defi-Use is also made of the synchronous driving nite design, a detailed study of all questions must bemade. Appendix

PROPULSIVE COEFFICIENTAND EFFICIENCYTABLE 11-COMPARISON OF SPECIFIC FUEL CONSUMPTIONS OFDIFFERENT PROPULSIVE SYSTEMS (AVERAGE VALUES) In order to show the relationbetween the power developed by

....................................................................m i , a g drives0 . 6 6

At the coupling or brake the(Turn t o Page 10)

Specific fuelType of powerplant consumption,Ibs./shp.-hr.

Diesel, gear transmission .................................................................................. 0.39-0.42..............................................................................iesel, electric transmission 0.44-0.46

Diesel, average, all drives .................................................................................... 0.44

the engine and the speed of theship, some definitions are pre-sented here.The power delivered by theworkingA ubstance to the pistonof the Diesel engine is the indi-dicated power, ihp, as found fromthe indicator n-v-diagram.

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AT LEFT:

Fig. 3. (Continued)

arrangement , shows a marked separat ion of the fluidand a considerable disturbance of the flow.In some pieces of equipment i t may be highly desir-abl e to induce eddying flow. Figs. No. 3-P a nd 3-Q showtwo different types of cyl inder arrangements: eachmight represe nt a bank of boile r tubes. Th e flow inFig. No. 3-Q i s probably more des i rab le as far as heattransfer is concerned.USE OF APPARATUS

For low veloci t ies the streamlines approximate thetwo-dimensional flow of a frictionless incompressiblefluid. In som e problems studies of this type of flow

ar e important , whereas in o ther prob-lems it is necessary to investigate three-dimensional flow.The apparatus cannot be operatedcontinuously for a long t ime because of

the clogging of sma ll passages and th edepositing of white particles on theglass plates. Thu s frequ ent cleaningis necessary for continued operat ion.It is necessary to make sui table ar-rangements for exhaus t ing the mixtureof a i r and smoke so that i t canno ti r r i t a te the human body or corrodesteel surfaces. Pip ing the exhaust out-s ide to the open a tmosphere is usual lya sat isfactory arrangement .Placing an open dish of am m on i umhydroxide ups t ream from the smoketube makes the smoke mo re dense. Tin

tetrachloride can be used instead oft i tanium tetrachloride. Dr y ice mig htbe used for generat ing smoke, but i toffers some disadvantages as far as con-t inuous handl ing and cont ro l are con-cerned . Kerosene vapor forms a g o o d dense smoke, buti t is difficul t to control and generate without igni t ing.Baffles and screens can be arra nged at different placesin the a i r s t ream to cont ro l the f low and make i t un i -form. Fo r tes t purposes the g lass p la tes can be arrangedin a horizontal plane with the top plate s imply rest ingon the model a nd the gaskets along the edges. Modelscan be changed easi ly with this arrangement . F o r i n -struct ional purposes the glass plates can be clampedtogether along the edges and mounted in a vert ical plane.A series of l ights and a reflector along the top edge ofthe apparatus can be provided for showing the flow pat-terns to l arge groups .

Ma ri ne Power Plants(Continue d from Page 7 )

engine del ivers the brake power, bhp, which is equal tothe indicated power minus the mechanical losses in theb h ~engine. From this mecha nical efficiency = --i h p '

From the coupling to the stern tube bearing, poweris lost in bearing frict ion, and, i f a t ransmission ispresent , in frict ion in the t ransmission. The r a t i o ofpower actual ly del ivered to the propel ler, divided bythe brake power del ivered by the engine, is cal led t rans-mission efficiency.The power required to overcome the resistance of theship at a given speed is cal led the effect ive power, ehp,an d is equal to the product of speed and resistance. Therat io of effect ive power over propel ler power, ehp/php,is ca lled propulsiv e efficiency. Prop ulsive efficiency canbe subdivided into propeller efficiency, hull efficiencyand relative rotative efficiency, which represents the effectof the wake on the propel ler. It is seen that al l sourcesof loss between th e combust ion cham ber of th e engineand the power represented by the motion of the ship arePage 10

accounted for. The product of the above terms is cal ledpropulsive coefficient and can be wri t ten:Pro pul sive coefficient=In connection with Diesel drives, propulsive coefficient isoften used to denote the rat io 9.b h pW e then have the re la t ion

mechanicali h p efficiency.

For electric drive the above equat ions remain unal tered.Only the t ransm ission efficiency is modified to accountfor the losses in electrical t ransmission.References.Rossel, H. E. and C hapm an , L. B. Princip les of NavalArchitecture, Vol. 11. The Society of Naval Architectsand Marine Engineers , New York , 1939.Magg, J. Dieselmaschinen, V.D.I. Berlin, 1928.

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