ShipPropulsion MAN

8/12/2019 ShipPropulsion MAN

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Contents:

Basic Principles of Ship Propulsion

Page

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Scope of this Paper . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Chapter 1Ship Definitions and Hull Resistance. . . . . . . . . . . . . . . . . . 4

Ship types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

A ships load lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Indication of a ships size . . . . . . . . . . . . . . . . . . . . . . . . 5

Description of hull forms . . . . . . . . . . . . . . . . . . . . . . . . 5

Ships resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Chapter 2Propeller Propulsion. . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Propeller types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Flow conditions around the propeller . . . . . . . . . . . . . . . . . . 11

Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Propeller dimensions . . . . . . . . . . . . . . . . . . . . . . 13

O perating conditions of a propeller . . . . . . . . . . . . . . . . . . . 15

Chapter 3Engine Layout and Load Diagrams . . . . . . . . . . . . . . . . . . 20

Power functions and logarithmic scales . . . . . . . . . . . . . . . . . 20

P ropulsion and engine running points . . . . . . . . . . . . . . . . . . 20

Engine layout diagram . . . . . . . . . . . . . . . . . . . . . . . . . 22

Load diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Use of layout and load diagrams examples . . . . . . . . . . . . . . 25

Influence on engine running ofdifferent types of ship resistance summary . . . . . . . . . . . . . . 27

Closing Remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29


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Introduction

For the purpose of this paper, the termship is used to denote a vehicle em-ployed to transport goods and personsfrom one point to another over water.Ship propulsion normally occurs withthe help of a propeller, which is theterm most widely used in English,although the word screw is some-times seen, inter alia in combinationssuch as a twin-screw propulsion plant.

Today, the primary source of propellerpower is the dieselengine, and the powerrequirement and rate of revolution verymuch depend on the ships hull formand the propeller design. Therefore, inorder to arrive at a solution that is asoptimal as possible, some generalknowledge is essential as to the princi-pal ship and diesel engine parametersthat influence the propulsion system.

This paper will, in particular, attempt toexplain some of the most elementaryterms used regarding ship types,ships dimensions and hull forms andclarify some of the parameters pertain-ing to hull resistance, propeller condi-tions and the diesel engines loaddiagram.

O n the other hand, it is considered be-yond the scope of this publication togive an explanation of how propulsioncalculations as such are carried out, asthe calculation procedure is extremelycomplex. T he reader is referred to thespecialised literature on this subject, forexample as stated in References.

Scope of this Paper

Thispaper is divided into three chapterswhich, in principle, maybe considered asthree separate papers but which also,with advantage, maybe read in closeconnection to each other. Therefore,some important information mentioned inone chaptermaywell appear in anotherchapter, too.

C hapter 1, describes the most elemen-

tary terms used to define ship sizesand hull forms such as, for example,the ships displacement, deadweight,design draught, length between per-pendiculars, block coefficient, etc.O ther ship terms described include theeffective towing resistance, consistingof frictional, residual and air resistance,and the influence of these resistancesin service.

C hapter 2, deals with ship propulsionand the flow conditions around the pro-peller(s). In this connection, the wakefraction coefficient and thrust deduc-tion coefficient, etc. are mentioned.

The total power needed for the propel-ler is found based on the above effec-tive towing resistance and variouspropeller and hull dependent efficien-cies which are also described. A sum-mary of the propulsion theory is shownin Fig. 6.

The operating conditions of a propelleraccording to the propeller law valid fora propeller with fixed pitch are describedfor free sailing in calm weather, and

followed up by the relative heavy/lightrunning conditions which apply whenthe ship is sailing and subject to differenttypes of extra resistance, like fouling,heavy sea against, etc.

C hapter 3, elucidates the importanceof choosing the correct specified M C Rand optimising point of the mainengine,and thereby the engines load diagramin consideration to the propellers designpoint. The construction of the relevantload diagram lines is described in detailby means of several examples. Fig. 24shows, for a ship with fixed pitch pro-peller, by means of a load diagram, theimportant influence of different types ofship resistance on the engines contin-uous service rating.

3

Basic Principles of Ship Propulsion


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Ship Definitions and HullResistance

Ship types

Depending on the nature of their cargo,and sometimes also the way the cargois loaded/unloaded, ships can be dividedinto different categories, classes, and

types, some of which are mentioned inTable 1.

The three largest categories of shipsare container ships, bulk carriers (forbulk goods such as grain, coal, ores,etc.) and tankers, which again can bedivided into more precisely definedclasses and types. Thus, tankers canbe divided into oil tankers, gas tankersand chemical tankers, but there arealso combinations, e.g. oil/chemicaltankers.

Table 1 provides only a rough outline.In reality there are many other combi-nations, such as M ulti-purpose bulkcontainer carriers, to mention just oneexample.

A ships load lines

Painted halfway along the ships sideis the P limsoll M ark, see Fig. 1. T helines and letters of the Plimsoll M ark,which conform to the freeboard ruleslaid down by the IM O (International

M aritime O rganisation) and local au-thorities, indicate the depth to whichthe vessel may be safely loaded (thedepth varies according to the seasonand the salinity of the water).

There are, e.g. load lines for sailing infreshwater and seawater, respectively,with further divisions for tropical condi-tions and summer and winter sailing.According to the international freeboardrules, the summer freeboard draughtfor seawater is equal to the Scantlingdraught, which is the term applied to

the ship s draught when dimensioningthe hull.

The winter freeboard draught is lessthan that valid for summer because of

the risk of bad weather whereas, on theother hand, the freeboard draught for

tropical seas is somewhat higher thanthe summer freeboard draught.

4

C ategory C lass Type

TankerO il tanker

G as tanker

C hemical tanker

O BO

C rude (oil) Carrier

Very Large C rude Carrier

Ultra Large Crude C arrier

Product Tanker

Liquefied Natural Gas carrier

Liquefied P etroleum G as carrier

O il/Bulk/O re carrier

C C

VLCC

ULCC

LN G

LP G

O BO

Bulk carrier Bulk carrier

Container ship Container shipC ontainer carrier

R oll O n-R oll O ff Ro-Ro

G eneral cargo shipG eneral cargo

C oaster

Reefer R eefer R efigerated cargo vessel

Passenger shipFerry

C ruise vessel

Table 1: Examp les of ship types

T Tropical

S Summer

W Winter

WN A Winter - the North Atlantic

D L

D: Freeboard draught

SeawaterFreshwater

Danish load mark

TF

F

D

Freeboard deck

Fig. 1 : Load lines freeboard draught


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Indication of a ships size

Displac ement and dea dweightWhen a ship in loaded condition floatsatan arbitrarywater line, itsdisplacement isequal to the relevant mass of water dis-placed bythe ship. Displacement is thusequal to the total weight, all told, of therelevant loaded ship, normally in seawa-ter with a mass density of 1.025 t/m

3.

D isplacement comprises the ships

light weight and its deadweight, wherethe deadweight is equal to the shipsloaded capacity, including bunkers andother supplies necessary for the shipspropulsion. The deadweight at any timethus represents the difference betweenthe actual displacement and the shipslight weight, all given in tons:

deadweight = displacement light weight.

Incidentally, the word tondoes notalways express the same amount ofweight. B esides the metric ton (1,000kg), there is the English ton (1,016 kg),which is also called the long ton. Ashort tonis approx. 907 kg.

The light weight of a ship isnot normallyused to indicate the size of a ship,whereas the deadweight tonnage(dwt), based on the ships loading ca-pacity, including fuel and lube oils etc.for operation of the ship, measured intons at scantling draught, often is.

Sometimes, the deadweight tonnagemay also refer to the design draught ofthe ship but, if so, thiswill be mentioned.Table 2 indicates the rule-of-thumb rela-tionship between the ships displacement,deadweight tonnage (summer freeboard/scantling draught) and light weight.

A ships displacement can also be ex-pressed as the volume of displacedwater, i.e. in m

3.

G ros s register tonsWithout going into detail, it should bementioned that there are also suchmeasurements as G ross Register Tons(G RT), and Net Register Tons (NR T)where 1 register ton = 100 English cubicfeet, or 2.83 m

3.

These measurements express the sizeof the internal volume of the ship in ac-cordance with the given rules for suchmeasurements, and are extensivelyused for calculating harbour and canaldues/charges.

Description of hull forms

It is evident that the part of the shipwhich is of significance for itspropulsion

is the part of the ships hull which isunder the water line. The dimensionsbelow describing the hull form referto the design draught, which is lessthan, or equal to, the scantling

draught. T he choice of the designdraught depends on the degree ofload, i.e. whether, in service, the shipwill be lightly or heavily loaded. G en-erally, the most frequently occurringdraught between the fully-loaded andthe ballast draught is used.

Ships lengths LOA, L

WL, and L

PP

The overall length of the ship LOA

isnormally of no consequence whencalculating the hulls water resistance.The factors used are the length of thewaterline L

WLand the so-called length

between perpendiculars LPP. T he di-mensions referred to are shown inFig. 2.

5

Ship typedwt/light

weight ratio

Displ./dwt

ratio

Tanker and

Bulk carrier6 1.17

C ontainer ship 2.5-3.0 1.33-1.4

Table 2: Examples of relationship between dis-

placement, deadweight tonnage and light weight

L

L

L

PP

WL

OA

A M

DA

B WL

D F

D

Length between perpendiculars: L

Length on waterline: L

Length o : LBreadth on waterline: B

Draught: D = 1/2 (D +D )

M idship section area: A

PP

WL

OA

WL

m

F A

verall

Fig. 2 : Hull dimensions


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The length between perpendiculars isthe length between the foremost per-pendicular, i.e. usually a vertical linethrough the stems intersection withthe waterline, and the aftmost perpen-dicular which, normally, coincides withthe rudder axis. G enerally, this length isslightly less than the waterline length,and is often expressed as:

LPP

= 0.97 LWL

Draught DThe ships draught D(often Tis used inliterature) is defined as the vertical dis-tance from the waterline to that point ofthe hull which is deepest in the water,see Figs. 2 and 3. The foremost draughtD

Fand aftmost draught D

Aare normally

the same when the ship is in the loadedcondition.

Breadth on waterline BWLAnother important factor is the hullslargest breadth on the waterline B

WL,

see Figs. 2 and 3.

B loc k co efficient CB

Various form coefficients are used toexpress the shape of the hull. The mostimportant of these coefficients is theblock coefficient C

B, which is defined

as the ratio between the displacementvolume and the volume of a box withdimensionsL

WL B

WL D, see Fig. 3, i.e.:

CL B D

B

WL WL

=

In the case cited above, the block co-efficient refers to the length on water-line L

WL. However, shipbuilders often use

block coefficient CB, PP

based on thelength between perpendiculars, L

PP, in

which case the block coefficient will, as arule, be slightlylarger because, as previ-ouslymentioned, L

PPis normallyslightly

lessthan LWL.

CL B D

B PP

PP WL

, =

A small block coefficient means less re-sistance and, consequently, the possibil-ityof attaining higher speeds.

Table 3 shows some examplesof blockcoefficient sizes, and the pertaining

service speeds, on different types ofships. It shows that large block coeffi-cients correspond to low speeds andvice versa.

Ship type

Block

coefficient

C B

Approxi-

mate ship

speed Vin knots

Lighter 0.90 5 10

Bulk carrier 0.80 0.85 12 17

Tanker 0.80 0.85 12 16

G eneral cargo 0.55 0.75 13 22

C ontainer ship 0.50 0.70 14 26

Ferry boat 0.50 0.70 15 26

Table 3: Examples of block coefficients

Wa ter pla ne a rea c oefficient CWL

The water plane area coefficient CWLexpresses the ratio between the ves-sels waterline area A

WLand the product

of the length LWL

and the breadthBWL

ofthe ship on the waterline, see Fig. 3, i.e.:

CA

L BWLWL

WL WL

=

G enerally, the waterplane area coeffi-cient is some 0.10 higher than the blockcoefficient, i.e.:

CWL

CB

+ 0.10.

This difference will be slightly larger onfast vessels with smallblock coefficientswhere the stern is also partly immersedin the water and thus becomes part ofthe waterplanearea.

Mids hip se ction c oefficient CM

A further description of the hull form isprovided by the midship section coeffi-cient C

M, which expressesthe ratio be-

tween the immersed midship sectionarea A

M(midwaybetween the foremost

and the aftmost perpendiculars) and theproduct of the ships breadth B

WLand

draught D, see Fig. 3, i.e.:

CA

B DMM

WL

=

6

LWL

A WL

BWL

DA MWaterline plane

LPP

,

M idship section coefficient

Volume of displacement

Waterline area

Block coefficient L based

Waterplane area coefficient

WL

Longitudinal prismatic coefficient

:

:

: =

: =

: =

: =

A

C

C

C

C

WL

B

WL

M

P

L BWL x x DWL

B x DWL

A x LM WL

L BWL WLxAWL

A M

Fig. 3: Hull coefficients of a ship


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For bulkers and tankers, this coefficientis in the order of 0.98-0.99, and forcontainer ships in the orderof 0.97-0.98.

Longitudinal prisma tic co efficient CP

The longitudinal prismatic coefficientC

Pexpresses the ratio between dis-

placement volume and the productof the midship frame section area A

M

and the length of the waterline LWL

,see also Fig. 3, i.e.:

CA L C B D L

CCp

M WL M WL WL

B

M

=

=

=

As can be seen, CP

is not an independ-ent form coefficient, but is entirely de-pendent on the block coefficient C

B

and the midship section coefficient CM.

Longitudinal Centre of Buoyancy LCBThe Longitudinal C entre of Buoyancy(LC B) expresses the position of thecentre of buoyancy and is defined asthe distance between the centre ofbuoyancy and the mid-point betweenthe ships foremost and aftmost perpen-diculars. The distance is normallystatedas a percentage of the length betweenthe perpendiculars, and is positive ifthe centre of buoyancy is located tothe fore of the mid-point between theperpendiculars, and negative if locatedto the aft of the mid-point. For a shipdesigned for high speeds, e.g. containerships, the LC B will, normally, be nega-tive, whereas for slow-speed ships,such as tankers and bulk carriers, it willnormally be positive. The LC B is gener-ally between -3% and +3% .

Finenes s ratio CLD

The length/displacement ratio or fine-ness ratio, C

LD, is defined as the ratio

between the ships waterline length LWL

,and the length of a cube with a volumeequal to the displacement volume, i.e.:

CL

LD

WL=

3

Ships resistance

To move a ship, it is first necessarytoovercome resistance, i.e. the force work-ing against itspropulsion. The calculationof this resistanceRplays a significant role

in the selectionof the correct propellerandin the subsequent choice of main engine.

GeneralA ships resistance is particularly influ-enced by its speed, displacement, andhull form. T he total resistance R

T, con-

sists of many source-resistances Rwhich can be divided into three maingroups, viz.:

1) Frictional resistance

2) R esidual resistance3) A ir resistance

The influence of frictional and residualresistances depends on how much ofthe hull is below the waterline, while theinfluence of air resistance depends onhow much of the ship is above the wa-terline. In view of this, air resistance willhave a certain effect on container shipswhich carry a large number of contain-ers on the deck.

Water with a speed ofVand a densityof has a dynamic pressure of:

V2(Bernoullis law)

Thus, if water is being completelystopped by a body, the water will reacton the surface of the body with the dy-namic pressure, resulting in a dynamicforce on the body.

This relationship is used as a basiswhen calculating or measuring thesource-resistances Rof a ships hull,by means of dimensionless resistancecoefficients

C. T hus

Care related to

the reference force K, defined as theforce which the dynamic pressure ofwater with the ships speed Vexerts ona surface which is equal to the hullswetted area A

S. The rudders surface is

also included in the wetted area. T hegeneral data for resistance calculationsis thus:

Reference force: K= V2

AS

and source resistances: R= C K

O n the basis of many experimentaltank tests, and with the help of pertain-ing dimensionless hull parameters,some of which have already been dis-cussed, methods have been estab-lished for calculating all the necessary

resistance coefficients Cand, thus, thepertaining source-resistancesR. Inpractice, the calculation of a particularships resistance can be verified bytesting a model of the relevant ship ina towing tank.

Frictiona l res ista nce RF

The frictional resistance RF

of the hulldepends on the size of the hulls wet-ted area A

S, and on the specific fric-

tional resistance coefficient CF. The

friction increases with fouling of thehull, i.e. by the growth of, i.a. algae,sea grass and barnacles.

An attempt to avoid fouling is made bythe use of anti-fouling hull paints toprevent the hull from becominglong-haired , i.e. these paints reducethe possibility of the hull becomingfouled by living organisms. The paintscontaining TBT (tributyl tin) as theirprincipal biocide, which is very toxic,have dominated the market for decades,but the IM O ban of TBT for new appli-cations from 1 January, 2003, and afull ban from 1 January, 2008, may in-volve the use of new (and maybe notas effective) alternatives, probably cop-per-based anti-fouling paints.

When the ship is propelled through thewater, the frictional resistance increasesat a rate that is virtually equal to thesquare of the vessels speed.

Frictional resistance represents a con-siderable part of the ships resistance,often some 70-90% of the ships totalresistance for low-speed ships (bulkcarriers and tankers), and sometimesless than 40% for high-speed ships(cruise liners and passenger ships) [4]. Thefrictional resistance is found as follows:

RF

= CF

K

Residual resistanc e RR

Residual resistance RR

comprises waveresistance and eddy resistance. Waveresistance refers to the energy losscaused by waves created by the vesselduring its propulsion through the water,while eddy resistance refers to the losscaused by flow separation which cre-ates eddies, particularly at the aft endof the ship.

7


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Wave resistance at low speeds is pro-portional to the square of the speed,but increases much faster at higherspeeds. In principle, this means that aspeed barrier is imposed, so that a fur-ther increase of the ships propulsionpower will not result in a higher speedas all the power will be converted intowave energy. T he residual resistancenormally represents 8-25% of the totalresistance for low-speed ships, and upto 40-60% for high-speed ships [4].

Incidentally, shallow waters can alsohave great influence on the residualresistance, as the displaced water un-der the ship will have greater difficultyin moving aftwards.

The procedure for calculating the spe-cific residual resistance coefficient C

Ris

described in specialised literature [2]and the residual resistance is found asfollows:

RR

= CR

K

Air resistance RA

In calm weather, air resistance is, in prin-ciple, proportional to the square of theships speed, and proportional to thecross-sectional area of the ship above thewaterline. A ir resistance normally repre-sents about 2% of the total resistance.

For container ships in head wind, theair resistance can be as much as 10% .The air resistance can, similar to theforegoing resistances, be expressed asR

A= C

A K, but is sometimes based

on 90% of the dynamic pressure of airwith a speed ofV, i.e.:

RA

= 0.90 air

V2

Aair

where

airis the density of the air, and

Aair

is the cross-sectional area of thevessel above the water [4].

Tow ing resista nce RT

a nd e ffec tive (tow ing) pow er PE

The ships total towing resistance RT

isthus found as:

RT=RF

+RR

+RA

The corresponding effective (towing)power, P

E, necessary to move the ship

through the water, i.e. to tow the shipat the speed V, is then:

PE

= V RT

The power delivered to the propeller,P

D, in order to move the ship at speed

Vis, however, somewhat larger. T his isdue, in particular, to the flow conditionsaround the propeller and the propellerefficiency itself, the influences of whichare discussed in the next chapter

which deals with Propeller P ropulsion.

Tota l ship res ista nce in ge neralWhen dividing the residual resistanceinto wave and eddyresistance, as earlierdescribed, the distributionof the totalshiptowing resistance R

Tcould also, as a

guideline, be stated asshown in Fig. 4.

The right column is valid for low-speedships like bulk carriers and tankers, andthe left column is valid forveryhigh-speedships like cruise liners and ferries. C on-tainer ships may be placed in betweenthe two columns.

T he main reason for the differencebetween the two columns is, as earliermentioned, the wave resistance. Thus,in general all the resistances are pro-portional to the square of the speed,but for higher speeds the wave resis-tance increases much faster, involvinga higher part of the total resistance.

This tendency is also shown in Fig. 5for a 600 teu container ship, originallydesigned for the ship speed of 15 knots.Without any change to the hull design,

8

R F

V

R A

V

R W

R E

Ship speed V

% of R TType of resistance

R A

R W

R E

R F = Friction

= Wave

= Eddy

= Air

High

speedship

Low

speedship

45 - 90

40 - 5

5 - 3

10 - 2

Fig. 4: Total ship tow ing resistance RT= RF+ RW+ RE+ RA


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the ship speed for a sister ship was re-quested to be increased to about 17.6knots. However, this would lead to arelatively high wave resistance, requir-ing a doubling of the necessary propul-sion power.

A further increase of the propulsionpower may only result in a minor shipspeed increase, as most of the extrapower will be converted into wave en-

ergy, i.e. a ship speed barrier valid forthe given hull design is imposed bywhat we could call a wave wall , seeFig. 5. A modification of the hull lines,suiting the higher ship speed, is neces-sary.

Increase of ship resistance in service,R ef. [1], page 244During the operation of the ship, thepaint film on the hull will break down.Erosion will start, and marine plantsand barnacles, etc. will grow on thesurface of the hull. B ad weather, per-

haps in connection with an inappropri-ate distribution of the cargo, can be areason for buckled bottom plates. Thehull has been fouled and will no longerhave a technically smoothsurface,

which means that the frictional resist-ance will be greater. It must also beconsidered that the propeller surfacecan become rough and fouled. The to-tal resistance, caused by fouling, mayincrease by 25-50% throughout thelifetime of a ship.

Resistance will also increase becauseof sea, wind, and current. T he resis-tance when navigating in head-on sea

could, in general, increase by as muchas 50-100% of the total ship resistancein calm weather.

Estimates of average increase in

resistance for ships navigating the

main routes:

North Atlantic route,

navigation westward 25-35%

North Atlantic route,

navigation eastward 20-25%

Europe-Australia 20-25%

Europe-East A sia 20-25%

The Pacific routes 20-30%

O n the North Atlantic routes, the firstpercentage corresponds to summernavigation and the second percentageto winter navigation.

However, analysis of trading conditionsfor a typical 140,000 dwt bulk carriershows that on some routes, especiallyJapan-Canada when loaded, the in-creased resistance (sea margin) canreach extreme values up to 220% , withan average of about 100% .

Unfortunately, no data have been pub-lished on increased resistance as afunction of type and size of vessel. Thelarger the ship, the less the relative in-crease of resistance due to the sea.O n the other hand, the frictional resis-tance of the large, full-bodied ships willvery easily be changed in the course oftime because of fouling.

In practice, the increase of resistancecaused by heavy weather depends onthe current, the wind, as well as thewave size, where the latter factor mayhave great influence. Thus, if the wavesize is relatively high, the ship speedwill be somewhat reduced even whensailing in fair seas.

In principle, the increased resistancecaused by heavy weather could berelated to:

a) wind and current against, andb) heavy waves,

but in practice it will be difficult to dis-

tinguish between these factors.

9

P ower and speed relationship for a 600 TEU container ship

20 knots

6,000

Normal service point

Ship speed

Propulsion power

10 15

4,000

2,000

0

8,000

"Wave wall"

New service point

kW

Fig. 5 : The wave wall ship sp eed barrier


9/28

Chapter 2

Propeller Propulsion

The traditional agent employed tomove a ship is a propeller, sometimestwo and, in very rare cases, more thantwo. The necessary propeller thrust Trequired to move the ship at speed Vis normally greater than the pertainingtowing resistanceR

T, and the flow-related

reasons are, amongst other reasons,

explained in thischapter. See also Fig. 6,where all relevant velocity, force, powerand efficiency parameters are shown.

Propeller types

Propellers maybe divided into the follow-ing two main groups, see also Fig. 7:

Fixed pitch propeller (FP -propeller)

C ontrollable pitch propeller(C P-propeller)

P ropellers of the FP-type are cast inone block and normallymade of a copperalloy. The position of the blades, andthereby the propeller pitch, is once andfor all fixed, with a given pitch that can-not be changed in operation. Thismeans that when operating in, for ex-ample, heavy weather conditions, thepropeller performance curves, i.e. thecombination of power and speed(r/min) points, will change according tothe physical laws, and the actual pro-peller curve cannot be changed by the

crew. M ost ships which do not need aparticularly good manoeuvrability areequipped with an FP -propeller.

P ropellers of the CP-type have a rela-tively larger hub compared with theFP -propellers because the hub has tohave space for a hydraulically activatedmechanism for control of the pitch (an-gle) of the blades. The CP-propeller isrelatively expensive, maybe up to 3-4times as expensive as a correspondingFP -propeller. Furthermore, because ofthe relatively larger hub, the propeller

efficiency is slightly lower.

C P-propellers are mostly used forR o-Ro ships, shuttle tankers and simi-lar ships that require a high degree of

10

Efficiencies

1 t

1 w

Relative rotative efficiency :

P ropeller efficiency - open water :

Propeller efficiency - behind hull : =

Propulsive efficiency : =

Shaft efficiency :

Total efficiency :

_

_

x

x

Velocities

Ships speed : V

Arriving water velocity to propeller : V

Effective wak e velocity : V = V_ V

A

W A

Forces

Towing resistance : R

Thrust force : T

Thrust deduction fraction : F = T

_

RT _ R

T

T

T

T

Power

Effective (Towing) power : P = R V

by the propeller to water : P = P /

Power delivered to propeller : P = P /

B rake power of main engine : P = P /

E T

T E

D T

B D

x

Thrust deduction coefficient : t =

Hull efficiency : =

Thrust power delivered

H

H

B

D

S

T B

S

B

0 R

T

---- ---- x ---- x ---- x x x x x= = = =P P P PB T D B

P P P PE E T D

H

S H 0 R SH

VA

V

VW

P D P EP T P B

V

T

R TF

Wake fraction coefficient : w =

R

0

B

(Speed of advance of propeller)

V _ V

V

A

Fig. 6 : The propulsion of a ship theory

Controllable pitch propeller (CP-Propeller)Fixed pitch propeller (FP-Propeller)

M onobloc with fixed

propeller blades

(copper alloy)

Hub with a mechanism

for control of the pitch

of the blades

(hydraulically activated)

Fig. 7 : Propeller types


10/28

manoeuvrability. For ordinary ships likecontainer ships, bulk carriers and crudeoil tankers sailing for a long time in nor-mal sea service at a given ship speed,it will, in general, be a waste of moneyto install an expensive C P-propeller in-stead of an FP-propeller. Furthermore, aC P-propeller is more complicated, invol-ving a higherrisk of problemsin service.

Flowconditions aroundthepropeller

Wake fraction coefficient wWhen the ship is moving, the friction ofthe hull will create a so-called frictionbelt or boundary layer of water aroundthe hull. In this friction belt the velocityof the water on the surface of the hull isequal to that of the ship, but is reducedwith its distance from the surface of thehull. A t a certain distance from the hulland, per definition, equal to the outersurfaceof the friction belt, the watervelocity is equal to zero.

The thickness of the frictionbelt increaseswith its distance from the fore end ofthe hull. T he friction belt is thereforethickest at the aft end of the hull andthis thickness is nearly proportional tothe length of the ship, Ref. [3]. Thismeans that there will be a certain wakevelocity caused by the friction along thesides of the hull. Additionally, the shipsdisplacement of water will also causewake waves both fore and aft. All thisinvolves that the propeller behind thehull will be working in a wake field.

Therefore, and mainly originating fromthe friction wake, the water at the pro-peller will have an effective wake veloc-ityV

wwhich has the same direction as

the ships speed V, see Fig. 6. Thismeans that the velocityof arriving waterV

Aat the propeller, (equal to the speed

of advance of the propeller) given asthe average velocityover the propellersdisk area is V

wlower than the ships

speed V.

The effective wake velocity at the pro-peller is therefore equal to V

w= V V

A

and may be expressed in dimensionlessform by means of the wake fractioncoefficient w. The normally used wak efraction coefficient w given by Taylor isdefined as:

wV

V

V V

V

you getV

Vw

W A

A

= =

= ( )1

The value of the wake fraction coefficientdepends largely on the shape of thehull, but also on the propellers locationand size, and has great influence onthe propellers efficiency.

The propeller diameter or, even better,the ratio between the propeller diameterdand the ships length L

WLhas some

influence on the wake fraction coeffi-cient, as d/L

WLgives a rough indication

of the degree to which the propellerworks in the hulls wake field. Thus, thelarger the ratio d/ L

WL, the lower wwill

be. T he wake fraction coefficient win-creases when the hull is fouled.

For ships with one propeller, the wakefraction coefficient wis normally in theregion of 0.20 to 0.45, correspondingto a flow velocity to the propeller

VAof

0.80 to 0.55 of the ships speed V. Thelarger the block coefficient, the larger isthe wake fraction coefficient. O n shipswith two propellers and a conventionalaftbody form of the hull, the propellerswill normally be positioned outside thefriction belt, for which reason the wakefraction coefficient wwill, in this case,be a great deal lower. H owever, for atwin-skeg ship with two propellers, thecoefficient wwill be almost unchanged(or maybe slightly lower) comparedwith the single-propeller case.

Incidentally, a large wake fraction co-efficient increases the risk of propellercavitation, as the distribution of thewater velocity around the propeller isgenerally very inhomogeneous undersuch conditions.

A more homogeneous wake field forthe propeller, also involving a higherspeed of advance V

Aof the propeller,

may sometimes be needed and can beobtained in several ways, e.g. by hav-ing the propellers arranged in nozzles,below shields, etc. O bviously, the bestmethod is to ensure, already at the de-sign stage, that the aft end of the hull isshaped in such a way that the opti-mum wake field is obtained.

Thrust de duc tion coe ffic ient tThe rotation of the propeller causes thewater in front of it to be suckedbacktowards the propeller. T his results in anextra resistance on the hull normallycalled augment of resistanceor, if re-lated to the total required thrust force Ton the propeller, thrust deduction frac-tion F, see Fig. 6. This means that thethrust force Ton the propeller has toovercome both the ships resistance R

T

and this loss of thrust F.

The thrust deduction fraction Fmay beexpressed in dimensionless form bymeans of the thrust deduction coeffi-cient t, which is defined as:

tF

T

T R

T

you get R

Tt

T

T

= =

= ( )1

The thrust deduction coefficient tcanbe calculated by using calculationmodels set up on the basis of researchcarried out on different models.

In general, the size of the thrust deduc-tion coefficient tincreases when thewake fraction coefficient wincreases.The shape of the hull may have a sig-nificant influence, e.g. a bulbous stemcan, under certain circumstances (lowship speeds), reduce t.

The size of the thrust deduction coeffi-cient tfor a ship with one propeller is,normally, in the range of 0.12 to 0.30,as a ship with a large block coefficienthas a large thrust deduction coefficient.For ships with two propellers and aconventional aftbody form of the hull,the thrust deduction coefficient twill bemuch less as the propellers suckingoccurs further away from the hull.However, for a twin-skeg ship with twopropellers, the coefficient twill be almostunchanged (or maybe slightly lower)compared with the single-propeller case.

Efficiencies

Hull efficiency

H

The hull efficiency

His defined as the

ratio between the effective (towing)powerP

E= R

T V, and the thrust power

11


11/28


12/28

As can be seen, the propulsive efficiency

Dis equal to the product of the hull

efficiency

H, the open water propeller

efficiency O, and the relative rotative

efficiency

R, although the latter has

less significance.

In this connection, one can be led tobelieve that a hull form giving a highwake fraction coefficient w, and hencea high hullefficiency

H, will also provide

the best propulsive efficiency

D.

However, as the open water propellerefficiency

Ois also greatly dependent

on the speed of advance VA, cf. Fig. 8,

that is decreasing with increased w,the propulsive efficiency

Dwill not,

generally, improve with increasing w,quite often the opposite effect is obtained.

G enerally, the best propulsive efficiencyis achieved when the propeller works ina homogeneous wake field.

Shaft efficiency S

The shaft efficiency S

depends, i.a. onthe alignment and lubrication of theshaft bearings, and on the reductiongear, if installed.

Shaft efficiency is equal to the ratio be-tween the power P

Ddelivered to the

propeller and the brake power PB

deliv-ered by the main engine, i.e.

S

D

B

P

P

The shaft efficiency is normally around0.985, but can vary between 0.96 and0.995.

Tota l efficienc y

T

The total efficiency T, which is equal to

the ratio between the effective (towing)power P

E, and the necessary brake

power PB

delivered by the main engine,can be expressed thus:

T

P

P

P

P

P

PE

B

E

D

D

B

= =

=

D

S =

H

O

R

S

Propeller dimensions

P ropeller diamete r dWith a view to obtaining the highestpossible propulsive efficiency

D, the

largest possible propeller diameter dwill, normally, be preferred. There are,however, special conditions to be con-sidered. For one thing, the aftbody formof the hull can varygreatlydepending ontype of ship and ship design, for another,the necessary clearance between the

tip of the propeller and the hull will de-pend on the type of propeller.

For bulkers and tankers, which are oftensailing in ballast condition, there arefrequent demands that the propellershall be fully immersed also in this con-dition, giving some limitation to the pro-peller size. This propeller size limitationis not particularly valid for containerships as they rarely sail in ballast condi-tion. All the above factors mean that anexact propeller diameter/design draughtratio d/Dcannot be given here but, asa rule-of-thumb, the below mentionedapproximations of the diameter/designdraught ratio d/Dcan be presented,and a large diameter dwill, normally,result in a low rate of revolution n.

Bulk carrier and tanker:

d/D< approximately 0.65

C ontainer ship:

d/D< approximately 0.74

For strength and production reasons,the propeller diameter will generally notexceed 10.0 metres and a power out-put of about 90,000 kW. T he largest-diameter propeller manufactured so faris of 11.0 metres and has four propellerblades.

Numbe r of propeller blad esPropellers can be manufactured with 2,3, 4, 5 or 6 blades. The fewer the num-ber of blades, the higher the propellerefficiency will be. H owever, for reasonsof strength, propellers which are to besubjected to heavy loads cannot be

manufactured with only two or threeblades.

Two-bladed propellers are used onsmall ships, and 4, 5 and 6-bladedpropellers are used on large ships.Ships using the M AN B&W two-strokeengines are normally large-type vesselswhich use 4-bladed propellers. Shipswith a relativelylarge powerrequirementand heavily loaded propellers, e.g. con-tainer ships, may need 5 or 6-bladedpropellers. For vibrational reasons, pro-pellers with certain numbers of bladesmay be avoided in individual cases inorder not to give rise to the excitationof natural frequencies in the ships hullor superstructure, Ref. [3].

Disk a rea c oefficientThe disk area coefficient referred to inolder literature as expanded blade arearatio defines the developed surfacearea of the propeller in relation to itsdisk area. A factor of 0.55 is consideredasbeing good. The disk area coefficientof traditional 4-bladed propellers is oflittle significance, as a higher value willonly lead to extra resistance on thepropeller itself and, thus, have little ef-fect on the final result.

For ships with particularly heavy-loadedpropellers, often 5 and 6-bladed pro-pellers, the coefficient may have ahigher value. O n warships it can be ashigh as 1.2.

Pitch diameter ratio p/dThe pitch diameter ratio p/ d, expressesthe ratio between the propellers pitchpand its diameter d, see Fig. 10. Thepitch pis the distance the propellerscrewsitself forward through the wa-ter per revolution, providing that thereis no slip see also the next sectionand Fig. 10. As the pitch can varyalong the blades radius, the ratio isnormally related to the pitch at 0.7 r,where r= d/2 is the propellers radius.

To achieve the best propulsive efficiencyfor a givenpropellerdiameter, an optimumpitch/diameter ratio is to be found,which again corresponds to a particu-lar design rate of revolution. If, forinstance, a lower design rate of revolutionis desired, the pitch/diameter ratio has

to be increased, and vice versa, at thecost of efficiency. O n the other hand, ifa lower design rate of revolution is de-sired, and the ships draught permits,the choice of a larger propeller diame-

13


13/28

ter may permit such a lower design rateof revolution and even, at the same time,increase the propulsive efficiency.

Propeller coefficients J, KT

and KQ

Propeller theory is based on modelsbut, to facilitate the general use of thistheory, certain dimensionless propellercoefficients have been introduced in re-lation to the diameter d, the rate of rev-olution n, and the waters mass density

. The three most important of thesecoefficients are mentioned below.

The advance number of the propeller Jis, asearliermentioned, a dimensionlessexpression of the propellers speed ofadvance V

A.

JV

n dA

=

The thrust force T, is expresseddimensionless, with the help of thethrust coefficient K

T, as

KT

n dT =

2 4

and the propeller torque

QP

nD

=2

is expressed dimensionless with thehelp of the torque coefficient K

Q, as

K Qn dQ

=

2 5

The propeller efficiency

Ocan be cal-

culated with the help of the above-men-tioned coefficients, because, as previouslymentioned, the propeller efficiency

Ois

defined as:

= =

=

P

P

T V

Q n

K

K

JT

D

A T

Q2 2

With the help of special and very com-plicated propeller diagrams, whichcontain, i.a. J, K

Tand K

Qcurves, it is

possible to find/calculate the propellersdimensions, efficiency, thrust, power, etc.

Manufac turing ac curac y of the propellerBeforethemanufacturing of thepropeller,the desired accuracy class standard ofthe propeller must be chosen by thecustomer. Such a standard is, for ex-ample, ISO 484/1 1981 (C E), whichhas four different Accuracy classes ,see Table 4.

Each of the classes, among other de-tails, specifies the maximum allowable

tolerance on the mean design pitch ofthe manufactured propeller, andtherebythetolerance on the correspond-ing propeller speed (rate of revolution).

The price of the propeller, of course,depends on the selected accuracyclass, with the lowest price for class III.However, it is not recommended touse class III, as this class has a toohigh tolerance. T his again means thatthe mean pitch tolerance should nor-mally be less than +/ 1.0 % .

The manufacturing accuracy tolerancecorresponds to a propeller speed toler-ance of max. +/1.0 % . When also in-corporating the influence of the toleranceon the wake field of the hull, the totalpropeller tolerance on the rate of revo-lution can be up to +/2.0 % . T his tol-erance has also to be borne in mindwhen considering the operating condi-tions of the propeller in heavy weather.

Influence of propeller diameter andpitch/dia mete r ra tio o n propulsiveefficiency D.As already mentioned, the highest pos-

sible propulsive efficiency required to

provide a given ship speed is obtained

with the largest possible propeller dia-meter d, in combination with the corre-sponding, optimum pitch/diameter ra-

tio p/d.

14

ISO 484/1 1981 (CE)

C lassM anufacturing

accuracy

M ean pitch

for propel-

ler

S

I

II

III

Very high accuracy

High accuracy

M edium accuracy

Wide tolerances

+/0.5 %

+/0.75 %

+/1.00 %

+/3.00 %

Table 4: Manu facturing accuracy classes

of a propeller

110 120100 r/min130

Shaft power

80 90

8,800

70

8,700

8,900

9,100

8,600

8,500

9,400

0.95

9,200

9,300

9,000

d = P ropeller diameter

p/d = Pitch/diameter ratio

Power and speed curve

for various propeller

diameters d with

optimum p/dPropeller speed

Power and speed curve

for the given propeller

diameter d = 7.2 m with

different p/d

80,000 dwt crude oil tanker

Design draught = 12.2 m

Ship speed = 14.5 kn9,500

0.90

0.85

0.80

0.71

1.00

0.60

0.75

d

0.65

0.55

6.8 m

p/d

0.67

7.2 m

6.6 m

7.4 m

7.0 m

0.70

p/d

0.68

0.69

0.50

p/d

p/d

d

kW

Fig. 9 : Propeller design influence o f diameter and pitch


14/28

As an example for an 80,000 dwt crudeoil tanker, with a service ship speed of14.5 knots and a maximum possiblepropeller diameter of 7.2 m, this influenceis shown in Fig. 9.

According to the blue curve, the maxi-mum possible propeller diameter of 7.2m may have the optimum pitch/diame-ter ratio of 0.70, and the lowest possi-ble shaft power of 8,820 kW at 100r/min. If the pitch for this diameter ischanged, the propulsive efficiency willbe reduced, i.e. the necessary shaftpower will increase, see the red curve.

The blue curve shows that if a biggerpropeller diameter of 7.4 m is possible,the necessary shaft power will be re-duced to 8,690 kW at 94 r/min, i.e. thebigger the propeller, the lower the opti-mum propeller speed.

The red curve also shows that propul-sion-wise it will always be an advan-tage to choose the largest possiblepropeller diameter, even though theoptimum pitch/diameter ratio wouldinvolve a too low propeller speed (in rela-tion to the required main engine speed).Thus, when using a somewhat lowerpitch/diameter ratio, compared with theoptimum ratio, the propeller/ enginespeed may be increased and will onlycause a minor extra power increase.

Operating conditions of a propeller

S lip ra tio SI f the propeller had no slip, i .e. i f thewater which the propeller screwsi tself through did not yield ( i.e. if thewater did not accelerate aft), the pro-peller would move forward at a speedofV = p n, where n is the propellersrate of revolution, see Fig. 10.

The similar situation is shown in Fig. 11for a cork screw, and because the corkis a solid material, the slip is zero and,therefore, the cork screw always movesforward at a speed ofV= p n. How-ever, as the water is a fluid and doesyield (i.e. accelerate aft), the propellersapparent speed forward decreaseswith its slip and becomes equal to theships speed V, and its apparent slipcan thus be expressed as p n V.

The apparent slip ratio SA, which is

dimensionless, is defined as:

Sp n V

p n

V

p nA =

=

1

The apparent slip ratio SA, which is cal-

culated by the crew, provides usefulknowledge as it gives an impression ofthe loads applied to the propeller underdifferent operating conditions. T he ap-parent slip ratio increases when the

15

S x p x nV or VA

P itch p

n

0.7 x r

r

d

p x n

Slip

The apparent slip ratio : S = = 1A_

The real slip ratio : S = = 1R_p x n

_ V V

p x n p x nA A

p x n _ V V

p x n p x n

Fig. 10: M ovement of a ship s p ropeller, w ith pitch p and slip ratio S

Velocity of corkscrew: V = p x nP itch p

Wine bottleC orkscrew C ork

V

n

Fig. 11: Movement o f a corkscrew, without slip


15/28

vessel sails against the wind or waves,in shallow waters, when the hull isfouled, and when the ship accelerates.Under increased resistance, this in-volves that the propeller speed (rate ofrevolution) has to be increased in orderto maintain the required ship speed.

The real slip ratio will be greater thanthe apparent slip ratio because the realspeed of advance V

Aof the propeller is,

as previously mentioned, less than theships speed

V.

The real slip ratio SR, which gives a truer

picture of the propellers function, is:

SV

p n

V w

p nRA

=

=

1 1

1( )

At quay trials where the ships speed isV= 0, both slip ratiosare 1.0. Incidentally,slip ratios are often given in percentages.

P ropeller la w in ge neral

As discussed in C hapter 1, the resis-tance Rfor lower ship speeds is pro-portional to the square of the ship sspeed V, i.e.:

R = c V2

where cis a constant. T he necessarypower requirement Pis thus propor-tional to the speed Vto the power ofthree, thus:

P= R V= c V3

For a ship equipped with a fixed pitchpropeller, i.e. a propeller with unchange-able pitch, the ship speedVwill be pro-portional to the rate of revolutionn, thus:

P= c n3

which precisely expresses the propellerlaw, which states that the necessarypower delivered to the propeller is pro-portional to the rate of revolution to thepower of three.

Actual measurements show that the

power and engine speed relationshipfor a given weather condition is fairlyreasonable, whereas the power andship speed relationship is often seenwith a higher power than three. A rea-

sonable relationship to be used for esti-mations in the normal ship speed rangecould be as follows:

For large high-speed ships like con-tainer vessels: P= c V4.5

For medium-sized, medium-speedships like feeder container ships,reefers, RoRo ships, etc.: P= c V4.0

For low-speed ships like tankers and

bulk carriers, and small feeder con-tainer ships, etc.: P= c V3.5

P ropeller law for heavy running propellerThe propeller law, of course, can onlybe applied to identical ship runningconditions. When, for example, theships hull after some time in servicehas become fouled and thus becomemore rough, the wake field will be differentfrom that of the smooth ship (clean hull)valid at trial trip conditions.

A ship with a fouled hull will, conse-

quently, be subject to extra resistancewhich will give rise to a heavypropellercondition, i.e. at the same propellerpower, the rate of revolution will be lower.

The propeller law now applies to an-other and heavierpropeller curvethan that applying to the clean hull,propeller curve, R ef. [1], page 243.

The same relative considerations applywhen the ship is sailing in a heavy seaagainst the current, a strong wind, andheavy waves, where also the heavywaves in tail wind may give rise to aheavier propeller running than whenrunning in calm weather. O n the otherhand, if the ship is sailing in ballastcondition, i.e. with a lower displace-ment, the propeller law now applies toa lighterpropeller curve, i.e. at thesame propeller power, the propellerrate of revolution will be higher.

As mentioned previously, for ships witha fixed pitch propeller, the propeller lawis extensively used at part load running.It is therefore also used in M AN B& WDiesels engine layout and load diagramsto specify the engines operationalcurves for light running conditions (i.e.clean hull and calm weather) and heavyrunning conditions (i.e. for fouled hull

and heavyweather). These diagrams us-ing logarithmic scales and straight linesare described in detail in C hapter3.

P ropeller performanc e in general atincreas ed s hip resista nceThe difference between the above-men-tioned light and heavyrunning propellercurves maybe explained byan exam-ple, see Fig. 12, for a ship using, asref-erence, 15 knots and 100% propulsionpower when running with a clean hull incalm weatherconditions. With 15% morepower, the corresponding ship speedmayincrease from 15.0 to 15.6 knots.

As described in C hapter 3, and com-pared with the calm weather conditions,it is normal to incorporate an extrapower margin, the so-called sea mar-gin, which is often chosen to be 15% .This power margin corresponds to ex-tra resistance on the ship caused bythe weather conditions. H owever, forvery rough weather conditions the influ-ence may be much greater, as de-scribed in C hapter 1.

In Fig. 12a, the propulsion power isshown as a function of the ship speed.When the resistance increases to alevel which requires 15% extra powerto maintain a ship speed of 15 knots,the operating point A will move towardspoint B.

In Fig. 12b the propulsion power isnow shownasa functionof the propellerspeed. As a first guess it will often be as-sumed that pointA will move towardsBbecause an unchanged propeller speedimplies that, with unchanged pitch, thepropellerwill move through the waterat an unchanged speed.

If the propellerwasa corkscrew movingthrough cork, this assumption wouldbe correct. H owever, water is not solidas cork but will yield, and the propellerwill have a slip that will increase with in-creased thrust caused by increasedhull resistance. T herefore, point A willmove towards Bwhich, in fact, is veryclose to the propeller curve through A.Point Bwill now be positioned on apropeller curve which is slightly heavyrunning compared with the clean hulland calm weather propeller curve.

16


16/28

Sometimes, for instance when the hullis fouled and the ship is sailing in heavyseas in a head wind, the increase inresistance may be much greater, cor-responding to an extra power demandof the magnitude of 100% oreven higher.An example isshown in Fig. 12c.

In this example, where 100% powerwill give a ship speed of 15.0 knots,point A, a ship speed of, for instance,12.3 knots at clean hull and in calmweather conditions, pointC, will require

about 50% propulsion power but, atthe above-mentioned heavy runningconditions, it might only be possible toobtain the 12.3 knots by100% propulsionpower, i.e. for 100% power going frompoint A to D. Running point Dmaynowbe placed relatively far to the left of pointA, i.e. very heavy running. Such a situ-ation must be considered when laying-out the main engine in relation to thelayout of the propeller, as described inC hapter 3.

A scewed propeller (with bent blade

tips) is more sensitive to heavy runningthan a normal propeller, because thepropeller is able to absorb a highertorque in heavy running conditions. For

a ducted propeller, the opposite effectis obtained.

He avy w ave s and sea and w ind ag a ins tWhen sailing in heavy sea against, withheavy wave resistance, the propeller

can be up to 7-8% heavier runningthan in calm weather, i.e. at the samepropeller power, the rate of revolutionmay be 7-8% lower. An example validfor a smaller container ship is shown inFig. 13. T he service data is measured

17

(Logarithmic scales)

Power

Propeller speed

15.0 knots

100% power

P ropeller curve

for clean hull and

calm weather

Propeller

curve for

fouled hull

and heavy

seas

LR

Slip

HR HR = Heavy running

LR = Light running

D A

C

12.3 knots

100% power

12.3 knots

50% power

10.0 knots

50% power

D

Fig. 12c : Propeller speed performance atlarge extra ship resistance


Power

Propeller speed

15.6 knots

115% power

15.0 knots

100% power

15%

Sea

margin

Slip

Propeller curve for clean

hull and calm weather

15.0 knots

115% power

B

A

B

Fig. 12b: Propeller speed performance at15% sea margin

BHP

21,000

18,000

15,000

12,000

9,000

6,000

76 80 9284 9688 100

Shipspeedknots

Shaft power

C lean hull and draught D

D = 6.50 m

D = 5.25 m

D = 7.75 m

Source: Lloyd's Register

MEAN

F

A

Average weather 3%

Extremely good weather 0%

Extremelybad weather 6%

Propeller speed

Apparentslip

10%6%

Heavyrunning

2%

-2%

13

16

19

22

CB

A

C

B

A

r/min

Fig. 13 : Service data over a period of a year returned from a single screw container ship


15.6 knots

115% power

15.0 knots

100% power

15%

Sea

margin

Power

Ship speed

Propeller curve for clean

hull and calm weather

15.0 knots

115% power

B

A

Fig. 12a: Ship speed performance at 15%sea margin


17/28

over a period of one year and onlyincludes the influence of weather con-

ditions! T he measuring points havebeen reduced to three average weatherconditions, and show an averageheavy running of 6% , and therefore, inpractice, the heavy running has provedto be even greater.

In order to avoid slamming of the ship,and thereby damage to the stem andracing of the propeller, the ship speedwill normally be reduced bythe navigat-ing officer on watch.

Another measured example is shown

in Fig. 14, and is valid for a reefer shipduring its sea trial. Even though thewind velocity is relatively low, only 2.5m/s, and the wave height is 4 m, themeasurements indicate approx. 1.5%

heavy running when sailing in headwind out, compared with when sailing

in tail wind on return.

S hip a cc elerationWhen the ship accelerates, the propel-ler will be subjected to an even largerload than during free sailing. The powerrequired for the propeller, therefore, willbe relatively higher than for free sailing,and the engines operating point will beheavy running, as it takes some timebefore the propeller speed has reachedits new and higher level. An examplewith two different accelerations isshown in Fig. 15. The load diagram is

described in C hapter 3.

Shallow watersWhen sailing in shallow waters, the re-sidual resistance of the ship may be in-

creased and, in the same way as whenthe ship accelerates, the propeller willbe subjected to a larger load than dur-ing free sailing, and the propeller will beheavy running.

Influence of displac ementWhen the ship is sailing in the loadedcondition, the ships displacement vol-ume may, for example, be 10% higheror lower than for the displacement validfor the average loaded condition. This,of course, hasan influence on the shipsresistance, and the required propellerpower, but only a minor influence onthe propeller curve.

O n the other hand, when the ship issailing in the ballast condition, the dis-placement volume, compared to theloaded condition, can be much lower,and the corresponding propeller curvemayapply to, for example, a 2% lighterpropeller curve, i.e. for the same powerto the propeller, the rate of revolutionwill be 2% higher.

P a ra meters ca using hea vy runningpropellerTogether with the previously describedoperating parameters which cause aheavy running propeller, the parame-ters summarised below may give an in-dication of the risk/sensitivity of gettinga heavy running propeller when sailingin heavy weather and rough seas:

1 Relatively small ships (


18/28

4 Ships w ith a flat stemmaybe slowed down faster bywavesthan a ship with a sharpstem.Thus an axe-shaped upper bow maybetter cut the waves and thereby

reduce the heavy running tendency.

5 Fouling of the hull and propellerwill increase both hull resistance andpropeller torque. P olishing the pro-peller (especially the tips) as often aspossible (also when in water) has apositive effect. T he use of effectiveanti-fouling paints will prevent foulingcaused by living organisms.

6 Ship accelerationwill increase the propeller torque,and thus give a temporarily heavy

running propeller.

7 Sailing in shallow watersincreases the hull resistance and re-duces the ships directional stability.

8 Ships with scewed propellerare able to absorb a higher torqueunder heavy running conditions.

Manoeuvring speed

Below a certain ship speed, called themanoeuvring speed, the manoeuvra-bility of the rudder is insufficient be-cause of a too low velocity of the waterarriving at the rudder. It is rather difficultto give an exact figure for an adequatemanoeuvring speed of the ship as thevelocity of the water arriving at the rud-der depends on the propellers slipstream.

O ften a manoeuvring speed of themagnitude of 3.5-4.5 knots is men-tioned. According to the propeller law,

a correspondingly low propulsionpower will be needed but, of course,this will be higher for running in heavyweather with increased resistance onthe ship.

Direction of propeller rota tion (side thrust)When a ship is sailing, the propellerblades bite more in their lowermost po-sition than in their uppermost position.The resulting side-thrust effect is largerthe more shallow the water is as, forexample, during harbour manoeuvres.

Therefore, a clockwise (look ing from aftto fore) rotating propeller will tend topush the ships stern in the starboarddirection, i.e. pushing the ships stemto port, during normal ahead running.This has to be counteracted by therudder.

When reversing the propeller to asternrunning as, for example, when berthingalongside the quay, the side-thrust ef-fect is also reversed and becomes fur-ther pronounced as the ships speeddecreases. Awareness of this behav-iour is very important in critical situa-tions and during harbour manoeuvres.

According to R ef. [3], page 15-3, thereal reason for the appearance of theside thrust during reversing of the pro-peller is that the upper part of the pro-pellers slip stream, which is rotative,strikes the aftbody of the ship.

Thus, also the pilot has to know pre-cisely how the ship reacts in a givensituation. It is therefore an unwrittenlaw that on a ship fitted with a fixedpitch propeller, the propeller is alwaysdesigned for clockwiserotation whensailing ahead. A direct coupled mainengine, of course, will have the samerotation.

In order to obtain the same side-thrust

effect, when reversing to astern, on

ships fitted with a controllable pitchpropeller, C P-propellers are designedfor anti-clockwiserotation when sailingahead.

19

80 100 10585

50

7565 90 9560

60

70

80

90

mep

110%

Engine speed, % A

40

A = M100

Engine shaft power, % A

100%

90%

80%

70%

60%

O

A 100% reference pointM Specified engine M C RO O ptimising point

110

(Logarithmic scales)70

Fig. 15: Load diagram acceleration


19/28

Engine Layout andLoad Diagrams

Power functions and logarithmicscales

As is well-known, the effective brakepower P

Bof a diesel engine is propor-

tional to the mean effective pressure

(mep) peand engine speed (rate of rev-olution) n. When using cas a constant,P

Bmay then be expressed as follows:

PB= c p

e n

or, in other words, for constant mepthe power is proportional to the speed:

PB

= c n1

(for constant mep)

As already mentioned when runningwith a fixed pitch propeller the powermay, according to the propeller law, be

expressed as:

PB

= c n3

(propeller law)

Thus, for the above examples, the brakepowerP

Bmaybe expressed asa func-

tion of the speed nto the power ofi, i.e.

PB

= c ni

Fig. 16 shows the relationship betweenthe linear functions, y= ax+ b, see (A),using linear scales and the power func-tions P

B= c n

i, see (B), using logarith-

mic scales.

The power functions will be linear whenusing logarithmic scales, as:

log (PB) = i log (n) + log (c)

which is equivalent to: y= ax+ b

Thus, propeller curves will be parallel tolines having the inclination i= 3, andlines with constant mep will be parallelto lines with the inclination i= 1.

Therefore, in the layout and load diagramsfor diesel engines, as described in thefollowing, logarithmic scales are used,making simple diagrams with straightlines.

Propulsion and engine runningpoints

P ropeller de sign po int PDNormally, estimations of the necessarypropeller power and speed are basedon theoretical calculations for loadedship, and often experimental tank tests,both assuming optimum operatingconditions, i.e. a clean hull and goodweather. The combination of speedand power obtained may be called the

ships propeller design point PDplacedon the light running propeller curve 6,

see Fig. 17. O n the other hand, someshipyards and/or propeller manufactur-ers sometimes use a propeller designpoint PD that incorporates all or partof the so-called sea margin describedbelow.

Fouled hullWhen the ship has been sailing forsome time, the hull and propeller be-come fouled and the hulls resistancewill increase. C onsequently, the shipspeed will be reduced unless the enginedelivers more power to the propeller, i.e.the propellerwill be further loaded andwill become heavyrunning HR.

Furthermore, newer high-efficiency shiptypes have a relatively high ship speed,and a very smooth hull and propellersurface (at sea trial) when the ship isdelivered. This means that the inevitablebuild-up of the surface roughness onthe hulland propeller during sea serviceafter seatrial may result in a relativelyheavier running propeller, comparedwith older shipsborn with a more roughhull surface.

Heavy wea ther and sea margin usedfor la yout o f engineIf, at the same time, the weather isbad, with head winds, the ships resis-tance may increase much more, andlead to even heavier running.

When determining the necessary en-gine power, it is normal practice to addan extra power margin, the so-calledsea margin, which is traditionally about15% of the propeller design PDpower.However, for large container ships,20-30% may sometimes be used.

When determining the necessary en-gine speed, for layout of the engine, itis recommended compared with theclean hull and calm weather propellercurve 6 to choose the heavier propel-ler curve 2, see Fig. 17, correspondingto curve 6 having a 3-7% higher rate ofrevolution than curve 2, and in generalwith 5% as a good choice.

Note that the chosen sea power mar-

gin does not equalise the chosenheavy engine propeller curve.

20

A. Straight lines in linear scales

a

2

X

1 20

1

0

b

y = ax + b

B. Power function curvesinlogarithmic scales

P

P

B

B

= engine brake power

c = constant

n = engine speed

log( ) = i x log(n) + log(c)

y = ax + bP = c x ni

i = 1

i = 2

i = 3

y = log (P )B

i = 0

x = log (n)

y = log (P ) = log (c x n )Bi

y

Fig. 16: Relationship between linear functionsusing linear scales and power func tionsusing logarithmic scales


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Continuous service propulsion point SPThe resulting speed and power combi-nation when including heavy propellerrunning and sea margin is called thecontinuous service rating for propulsionSPfor fouled hull and heavy weather.The heavy propeller curve, curve 2, forfouled hull and heavy weather will nor-

mally be used as the basis for the en-gine operating curve in service, and thepropeller curve for clean hull and calmweather, curve 6, is said to represent alight running LRpropeller.

Co ntinuous se rvice rating SThe continuous service rating is thepower at which the engine, includingthe sea margin, is assumed to operate,and point Sis identical to the servicepropulsion point SPunless a main en-gine driven shaft generator is installed.

Light running factor fLRThe heavy propeller curve for a fouledhull and heavy weather, and if no shaftgenerator is installed may, as mentionedabove be used as the design basis for

the engine operating curve in service,curve 2, whereas the light propellercurve for clean hull and calm weather,curve 6, may be valid for running con-ditions with new ships, and equal tothe layout/design curve of the propel-ler. Therefore, the light propeller curvefor clean hull and calm weather is saidto represent a light running LRpro-peller and will be related to the heavypropeller curve for fouled hull andheavy weather condition by means of alight running factor

fLR, which, for the

same power to the propeller, is definedas the percentage increase of the rateof revolution n, compared to the rate ofrevolution for heavy running, i.e.

fn n

nLRlight heavy

heavy

=

100%

Engine ma rginBesides the sea margin, a so-calledengine marginof some 10-15% isfrequently added as an operationalmargin for the engine. T he correspond-

ing point is called the specified M C Rfor propulsion MP, see Fig. 17, andrefers to the fact that the power forpoint SPis 10-15% lower than forpoint MP, i.e. equal to 90-85% ofMP.

Specified MCR MThe engines specified M C R pointMisthe maximum rating required by theyard or owner for continuous operationof the engine. Point Mis identical to thespecified propulsion M C R point MPun-less a main engine driven shaft genera-tor is installed. In such a case, the extra

power demand of the shaft generatormust also be considered.

Note:

Light/heavy running, fouling and seamargin are overlapp ing terms.Light/heavy running of the propeller re-fers to hull and p ropeller deterioration,and bad weather, and sea margin, i.e.extra power to the propeller, refers tothe influence of the w ind and the sea.

Based on feedback from service, itseems reasonable to design the pro-

peller for 3-7% light running. The de-gree of light running must be decidedupon, based on experience from theactual trade and hull design, but 5%is often a good choice.

21

LR(5%)

Engine speed

Power

MP

Sea margin(15% of P D)

2 6

S P

2 Heavy propeller curve fouled hull and heavy weather

6 Light propeller curve clean hull and calm weather

M P : Specified propulsion point

SP : Service propulsion point

PD: P ropeller design point

PD`: Alternative propeller design point

LR : Light running factor

HR : Heavy running

_

_

HR

P D

P D

Engine margin(10% of MP )

Fig. 17: Ship propulsion running points and engine layout


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Engine layout diagram

An engines layout diagram is limited bytwo constant mean effective pressure(mep) lines L

1-L

3and L

2-L

4, and by two

constant engine speed lines L1-L

2and

L3-L

4, see Fig. 17. T he L

1point refers to

the engines nominal maximum contin-uous rating. Within the layout areathere is full freedom to select the en-gines specified M C R point Mand rele-vant optimising point O, see below,

which is optimum for the ship and theoperating profile. P lease note that thelowest specific fuel oil consumption fora given optimising point Owill be ob-tained at 80% of point Os power.

Based on the propulsion and enginerunning points, as previously found, thelayout diagram of a relevant main en-gine may be drawn-in. The specifiedM C R point Mmust be inside the limita-tion lines of the layout diagram; if it is

not, the propeller speed will have to bechanged or another main engine typemust be chosen. Yet, in special cases,point Mmay be located to the right ofline L

1-L

2, see O ptimising Pointbelow.

Optimis ing point OThe O ptimising point Oor, better,the layout point of the engine is therating at which the turbocharger ismatched, and at which the engine tim-ing and compression ratio are adjusted.

As mentioned below, under Load dia-gram , the optimising point Ois placedon line 1 (layout curve of engine) of theload diagram, and the optimised powercan be from 85 to 100% of point Mspower, when turbocharger(s) and en-gine timing are taken into consideration.When optimising between 93.5% and85% of point Ms power, overload run-ning will still be possible (110% ofMspower), aslong asconsiderationto thescavenge air pressure hasbeen taken.

The optimising pointO

is to be placedinside the layout diagram. In fact, thespecified M C R point Mcan be placedoutside the layout diagram, but only byexceeding line L

1-L

2, and, of course,

only provided that the optimising pointOis located inside the layout diagram.

It should be noted that engines withoutVIT (variable injection timing) fuelpumpscannot be optimised at part-load. There-fore, these engines are always optimisedin pointA, i.e. having point Ms power.

Load diagram

DefinitionsThe load diagram (Fig. 18) defines thepower and speed limits for continuousas well as overload operation of an in-stalled engine which has an optimisingpoint Oand a specified M C R point Mthat conforms to the ships specification.

Point A is a 100% speed and powerreference point of the load diagram,and is defined as the point on the pro-peller curve (line 1) the layout curve ofthe engine through the optimising point

O, having the specified M C R power.

Normally, point Mis equal to point A,but in special cases, for example if a

22

Line 1: P ropeller curve through optimising point (O ) layout curve for engine

Line 2: H eavy propeller curve fouled hull and heavy seas

Line 3: Speed limit

Line 4: Torque/speed limit

Line 5: M ean effective pressure limit

Line 6: Light propeller curve clean hull and calm weather layout curve for propeller

Line 7: P ower limit for continuous running

Line 8: O verload limit

Line 9: Sea trial speed limit

Line 10: C onstant mean effective pressure (mep) lines

_

_

_ _

80 100 10585

50

70 7565 90 9560

60

70

80

90

mep

110%

Engine speed, % A

40

2

4

A= M

9

7

8

5100

Engine shaft power, % A

6100%

90%

80%

70%

60%

1

10

3

O

A 100% reference point

M Specified engine M C R

O O ptimising point

110

Fig. 18: Engine load diagram


22/28

shaft generator is installed, point Mmay be placed to the right of point Aon line 7. T he service points of the in-stalled engine incorporate the enginepower required for ship propulsion andfor the shaft generator, if installed.

During shoptest running, the engine willalways operate along curve 1, withpoint A as 100% M C R . If C P-propellerand constant speed operation is re-quired, the delivery test may be fin-ished with a constant speed test.

Limits to c ontinuous opera tionThe continuous service range is limitedby the four lines 4, 5, 7 and 3 (9), seeFig. 18:

Line 3 and line 9Line 3 represents the maximum accept-able speed for continuous operation, i.e.105% ofA, however, maximum 105% ofL

1. During sea trial conditions the maxi-

mum speed may be extended to 107%ofA, see line 9.

The above limits may, in general, beextended to 105% and, during sea trialconditions, to 107% of the nominalL

1

speed of the engine, provided the tor-sional vibration conditions permit.

The overspeed set-point is 109% ofthe speed in A, however, it may bemoved to 109% of the nominal speedin L

1, provided that torsional vibration

conditions permit.

Running at low load above 100% ofthe nominal L1speed of the engine is,however, to be avoided for extendedperiods.

Line 4:Represents the limit at which an ampleairsupplyis available for combustion andimposesa limitationon the maximumcombination of torqueand speed.

Line 5:Represents the maximum mean effec-tive pressure level (mep) which can beaccepted for continuous operation.

Line 7:Represents the maximum power forcontinuous operation.

Line 10:Represents the mean effective pressure(mep) lines. Line 5 isequal to the 100%mep-line. The mep-lines are also anexpression of the corresponding fuelindex of the engine.

Limits for overloa d opera tion

The overload service range is limited asfollows, see Fig. 18:

Line 8:Represents the overload operation limi-tations.

The area between lines 4, 5, 7 and the

dashed line 8 in Fig. 18 is available for

overload running for limited periods

only (1 hour per 12 hours).

23

Point A of load diagram

Line 1: P ropeller curve through optimising point (O )

Line 7: C onstant power line through specified M C R (M )Point A: Intersection between lines 1 and 7

Engine speed

Power

M : Specified M C R of engine

S: C ontinuous service rating of engine

O : O ptimising point of engine

A: Reference point of load diagram

A = M = M P

Propulsion and

engine service curve

for heavy running

7

S=SP

O

2

1

6

Fig. 19a: Example 1 with FPP engine layout without SG (normal case)

M : S pecified M C R of engine




Engine speed

A= M

Propulsion and engine service

curve for heavy running

3.3% A5

62

4

Power 1

S

7

O5

6

3

4 1

7

2

5% A

5% L1

Fig. 19b: Example 1 with FPPload diagram without SG (normal case)


23/28

Electronic governor with loa d limitationIn order to safeguard the diesel engineagainst thermaland mechanical overload,theapproved electronic governorsincludethe following two limiter functions:

Torq ue limiterThe purpose of the torque limiter isto ensure that the limitation lines ofthe load diagram are always observed.The torque limiter algorithm comparesthe calculated fuel pump index (fuel

amount) and the actually measuredengine speed with a reference limitercurve giving the maximum allowablefuel pump index at a given enginespeed. If the calculated fuel pumpindex is above this curve, the result-ing fuel pump index will be reducedcorrespondingly.

The reference limiter curve is to beadjusted so that it corresponds to thelimitation lines of the load diagram.

Scavenge air pressure limiterThe purpose of the scavenge air

pressure limiter is to ensure that theengine is not being overfuelled duringacceleration, as for example duringmanoeuvring.

The scavenge air pressure limiteralgorithm compares the calculatedfuel pump index and measuredscavenge air pressure with a refer-ence limiter curve giving the maxi-mum allowable fuel pump index at agiven scavenge air pressure. If thecalculated fuel pump index is abovethis curve, the resulting fuel pumpindex will be reduced correspondingly.

The reference limiter curve is to beadjusted to ensure that sufficient airwill always be available for a goodcombustion process.

RecommendationC ontinuous operation without a timelimitation is allowed only within the arealimited by lines 4, 5, 7 and 3 of theload diagram. For fixed pitch propelleroperation in calm weather with loaded

ship and clean hull, the propeller/enginemayrun along or close to the propellerdesign curve 6.

After some time in operation, the shipshull and propeller will become fouled,resulting in heavier running of the pro-peller, i.e. the propeller curve will moveto the left from line 6 towards line 2, andextra power will be required forpropulsionin order to maintain the ship speed.

At calm weather conditions the extentof heavy running of the propeller willindicate the need for cleaning the hulland, possibly, polishing the propeller.

The area between lines 4 and 1 is avail-

able for operation in shallow water,

heavy weather and during acceleration,

i.e. for non-steady operation without

any actual time limitation.

24

Point A of load diagram

Line 1: Propeller curve through optimising point (O )

Line 7: C onstant power line through specified M C R (M )Point A: Intersection between lines 1 and 7

Engine speed

M = M P

Propulsion and

engine service curve

for heavy running

7

S=SP

621

O

Power

A




A: R eference point of load diagram

Fig. 20a: Example 2 with FPP engine layout without SG (special case)

M

Propulsion and engine service

curve for heavy running

3.3% A5

62

4

1

S

7

5

6

3

4 1

7

2

5% A

5% L1

A

O

Engine speed

Power





Fig. 20b: Example 2 with FPP load diagram without SG (special case)


24/28


25/28

large wave resistance, it can, therefore,be an advantage to design/move theload diagram more towards the left.

The latter can be done by moving theengines optimising point Oand thusthe propeller curve 1 through the opti-mising point towards the left. H ow-ever, this will be at the expense of aslightly increased specific fuel oil con-sumption.

An example is shown in Figs. 20a and20b. A s will be seen in Fig. 20b, andcompared with the normal case shownin Example 1 (Fig. 19b), the left-handlimitation line 4 is moved to the left, giv-ing a wider margin between lines 2 and4, i.e. a larger light running factor hasbeen used in this example.

Examp le 3:Normal case, with shaft generator

In this example a shaft generator (SG )is installed, and therefore the servicepower of the engine also has to incor-porate the extra shaft power required

for the shaft generators electricalpower production.

In Fig. 21a, the engine service curveshown for heavy running incorporatesthis extra power.

The optimising point O, and therebytheengine layout curve 1, will normallybechosen on the propeller curve (~ en-gine service curve) through point M.

Point A is then found in the same wayas in example 1, and the load diagramcan be drawn as shown in Fig. 21b.

Examp le 4:Special case, with shaft generator

Also in this special case, a shaft gener-ator is installed but, unlike in Example3, now the specified M C R for propul-sion MPis placed at the top of the lay-out diagram, see Fig. 22a. This involvesthat the intended specified M C R of theengine (Point M) will be placed outsidethe top of the layout diagram.

O ne solution could be to choose adiesel engine with an extra cylinder,but another and cheaper solution is toreduce the electrical power productionof the shaft generator when running inthe upper propulsion power range.

If choosing the latter solution, the re-quired specified M C R power of the en-gine can be reduced from point MtopointMasshownin Fig. 22a. Therefore,when running in the upper propulsionpower range, a diesel generator has totake over all or part of the electricalpower production.

However, such a situation will seldomoccur, as ships rather infrequently op-erate in the upper propulsion powerrange. In the example, the optimisingpoint Ohas been chosen equal topoint S, and line 1 may be found.

Point A, having the highest possiblepower, is then found at the intersectionof line L

1-L

3with line 1, see Fig. 22a,

and the corresponding load diagram is

26

Shaft

gene

rato

r

Propulsion curve for heavy running

SG

O = S

61

7

2

M

Engine service curve

for heavy running

M P

SP

A

M

Engine speed

Power

M : S pecified M C R of engine




Point A and M of load diagramLine 1: P ropeller curve through optim ising point (O )

M : Located on constant power line 7 through point Aand at M P s speed

P oint A : Intersection between line 1 and line L - LPoint

1 3

Fig. 22a: Example 4 with FPP engine layout with SG (special case)





Shaft

generator

P ropulsion curve

for heavy running

3.3% A

SG

5

62

4

1

7

O = S

6

3

1

7

2

5% A

5% L1

M

Engine service curvefor heavy running

M P

SP

A

M

5

4

Engine speed

Power

Fig. 22b: Example 4 with FPPload diagram with SG (specialcase)

ShipPropulsion MAN

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