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12.4 Propulsive coefficients

The propulsive coefficients of the shipperformance form the essential link betweenthe effective power required to drive thevessel, obtained from the product of resistanceand ship speed, and the power delivered from

the engine to the propeller.The powerabsorbed by and delivered to the propeller PDin order to drive the ship at a given speed VSis PD = 2nQ (12.36)

where n and Q are the rotational speed andtorque at the propeller. Nowthe torquerequired to drive the propeller Q can beexpressed for a propeller working behind thevessel as

Q =KQbn2D5 (12.37)

Where KQb is the torque coefficient of thepropeller when working in the wake fieldbehind the vessel at a mean advancecoefficient J . By combining equations (12.36)and (12.37) the delivered power can beexpressed as

PD=2KQb n3D5 (12.38)

If the propeller were operating in open waterat the same mean advance coefficient J theopen water torque coefficient KQo would befound to vary slightly from that measuredbehind the ship model. As such the ratio

KQo/KQb is known as the relative rotativeefficiency

r= KQbKQo (12.39)

this being the definition stated in Chapter 6.

Hence, equation (12.38) can then be expressedin terms of the relative rotative efficiency as

PD=2r

KQo

n3D5 (12.40)

12.4 s

H s y ca cht lng hnh dng tu lbn cht gia hiu qu nng lng cn thit iu khin tu c ly t lc cn v tc tu v nng lng c cp t ng c nchn vt. Cng sut tiu th v cung cp cho

chn vt PD li con tu ti mt tc Snht nh l

PD=2nQ (1236)

Trong n v Q l tc quay v m-menxon ca chn vt. Lc ny yu cu m-menxon li chn vt Q c th c th hincho mt chn vt lm vic pha sau tu nh

Q =KQbn2D5

KQb l h s m-men xon ca chn vt khilm vic phm vi sau ln tu vo thi imh s sm trung bnh J. S kt hp giaphng trnh (12.36) v (12.37) nng lngcung cp c th c th hin nh sau:

PD=2KQb n3D5 (12.38)

Nu cc chn vt ang c hot ng trongmt thong nc trc cng vi h s smtrung bnh J h s m-men mt thong ncKQo s c xy dng t thay i t phpo tu mu.

Nh vy t l r= KQoKQb c bit nh lhiu qu quay tng i

r=KQb

KQo(12.39)

y l nh ngha nu trong Chng 6.

Do phng trnh (12.38) c th sau c th hin v hiu qu quay tng i nh

PD=2r

KQo

n3D5 (12.40)

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Now the effective power PE is defined as

PE=RVS=PDQPC where the QPCis termedthe quasi-propulsive coefficient.

Hence, from the above, in association withequation (12.40),

RVS=PDQPC=2r

KQo

n3D5QPC QPC

which implies that

Now the resistance of the vessel R can beexpressed in terms of the propeller thrust T as

R=T(1-t),where t is the thrust deductionfactor as explained later. Also from Chapter 5the ship speed Vs can be defined in terms ofthe mean speed of advance Va as

Va =Vs(1-wt), where wt is the meanTaylor wake fraction. Furthermore, since

the open water thrust coefficient KTo isexpressed as To= KTon

2D4, with To being the

open water propeller thrust at the meanadvance coefficient J ,

ToK

To= n2D4 and the QPC can be expressed

from the above asoQot

rToao

nDTKw

KVtTQPC

2)1(

)1(

which reduces to QPC= )1

1(

tw

t

0r

since, from equation (6.8), 0=Qo

To

K

Kj

2

The quantity (1 t)/(1 wt) is termed the hulleffi-

ciency h and hence the QPC is defined as

QPC = h0r (12.41)

By gi hiu sut nng lng c nh nghal

PE=RVS=PDQPC

QPC y c gi l h s y quasi

Do t trn kt hp vi phng trnh

(12.40)

RVS=PDQPC=2r

KQo

n3D5QPC

H qu:

By gi lc cn R ca tu c th c th hintrong iu kin ca lc y chn vt T nhR=T(1-t) trong t l h s khu tr lc yc gii thch sau. Cng t Chng 5 tc tu S c th c xc nh trong cc iukin tc trung bnh ca a trc v

a = s(1-wt) trong wt l gi trtrungbnhphnln tu Taylor. Hn na k t khih s lc y nc mt thong KTo c th

hin nh To= KTon2

D4

vi To l lc y ccchn vt nc mt thong tc ng vo h strung bnh J

ToK

To= n2D4 v QPC c th cth hint

trn nhoQot

rToao

nDTKw

KVtTQPC

2)1(

)1(

iunyrtgn QPC= )11

(tw

t

0r

T t phngtrnh (6.8) 0=Qo

To

K

Kj

2

Gi tr (1-t)(1-wt) c gi l nng sut thntu h v t QPC c xc nh

QPC= h0r (12.41)

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or, in terms of the effective and deliveredpowers, PE = PDQPC

that is,

PE=PDh0r (12.42)

12.4.1 Relative rotative efficienc

The relative rotative efficiency (r) as definedby equation (12.39), accounts for thedifferences in torque absorption characteristicsof a propeller when operating in mixed wakeand open water flows. In many cases the valueof r lies close to unity and is generallywithinthe range 0.95 r 1.05

In a relatively few cases it lies outside this

range. Holtrop (Reference 39) gives thefollowing statistical relationships for itsestimation:

For conventional stern single-screw ships:

For twin-screw ships

If resistance and propulsion model tests areperformed then the relative rotative efficiencyis determined at model scale from themeasurements of thrust Tm and torque Qmwith the propeller operating behind the model.Using the non-dimensional thrust coefficient

KTm as input data the values J and KQo areread off from the open water curve of the

model propeller used in the propulsion test.The torque coefficient of the propellerworking behind the model is derived from

Hence the relative rotative efficiency iscalculated as:

Hoc trong cc iu kin ca hiu qu v cungcp nng lng

P E = PDQPC

l

PE=PDh0r (12.42)

12.4.1.iu qu qua tng i

Hiu qu quay tng i (r) c xc nhnh phng trnh (12.39) iukhc bit trongm-men xon hp th c im ca mt chnvt khi hot ng trong vng ln tu v ccdng nc mt thong. Trong nhiu trnghp gi tr ca rl khng thay i v thng

trong khong .

Trong mt s t trng hp n nm ngoiphm vi ny.Holtrop(tham kho 39) cung cpcho cc thng k sau y nh gi n:

i vi cc tu thng thng

i vi tu hai chn vt

Nu sc cn v m hnh th nghimlc yc th nghim sau hiu sut quay tngi c xc nh phm vi m hnht phpo ca lc y Tmv m-men Qm vi chn vtc hot ng sau m hnh. S dng h slc y khng th nguyn KTmcung cp sliu nh gi J v KQo c ra t ng cong

nc mt thong ca m hnh chn vt lc thnghim y tu. H s m-men ca chn vtlm vic m hnh c ly t

Do hiu qu quay tng i c tnh :

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The relative rotative efficiency is assumed tobe scale independent.

12.4.2 Thrust deduction factor

When water flows around the hull of a ship

which is being towed and does not have apropeller fitted a certain pressure field is set upwhich is dependent on the hull form. If thesame ship is now fitted with a propeller and ispropelled at the same speed the pressure fieldaround the hull changes due to the action ofthe propeller. The propeller increases thevelocities of the flow over the hull surface andhence reduces the local pressure field over the

after part of the hull surface. This has theeffect of increasing, or augmenting, theresistance of the vessel from that which wasmeasured in the towed resistance case and thischange can be expressed as

T=R(1+r) (12.44)

where T is the required propeller thrust and aris the resistance augmentation factor. Analternative way of expressing equation (12.44)

is to consider the deduction in propellereffective thrust which is caused by the changein pressure field around the hull. In this casethe relationship

R=T(1-t) (12.45)

applies, in which t is the thrust deductionfactor. The correspondence between the thrustdeduction factor and the resistance

augmentation factor can be derived fromequations (12.44) and (12.45) as being

If a resistance and propulsion model test hasbeen performed then the thrust deductionfactor can be readily calculated from therelationship defined in the 1987

Hiu qu quay tng i c gi thit lphm vi c lp.

12.4.2. H s quyt nh lc

Khi nc chy xung quanh thn ca con tu

ang c ko v khng c mt chn vt nothch hp vimt khong p sut nht nhm c trang b ph thuc vo hnh dngthn tu.Nu trn mt con tu c trang bmt chn vt v c y i vi tc tngng vi phm vi p sut quanh nhng thayi ca thn tu do s hot ng ca chn vt.Chn vt lm tng vn tc dng chy trn bmt v tu v do lm gim phm vi p sut

cc b trn phn sau ca b mt thn tu. iuny hiu qu c tng ln hoc lm tngsc cn ca tu t c xc nh theo cctrng hp lc cn v thay i ny c thc th hin:

T=R(1+r) (12.44)

Trong T l lc y cn thit ca chn vtv l mt yu t lm tng thm sc cn. Mtcch khc th hin phng trnh (12.44)

nh gi quyt nh hiu qu lc y chn vtl do thay i vng p lc xung quanh thntu. Trong trng hp ny mi lin h cp dng

R=T(1-t) (12.45)

t l yu t khu tr lc y. S tng quangia cc yu t khu trlc y v lm tngthm yu t sc cn c th c ly ra t

phng trnh (12.44) v (12.45) nh sau:

Nu sc cn v th nghim m hnh lc y c thc hin sau cc yu t khu trlc y c th d dng tnh ton t mi lin h vch r nm 1987.

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ITTC proceedings in which TM and FD aredefined previously and Rc is the resistancecorrected for differences in temperaturebetween the resistance and propulsion tests:

where CFMC is the frictional resistancecoefficient at the temperature of the self-propulsion test.

In the absence of model tests an estimate ofthe thrust deduction factor can be obtainedfrom the work of Holtrop (Reference 39) andHarvald (Reference 17). In the Holtropapproach the following regression-based

formulas are given:

In equation (12.46) the value of the parameterCstern is found from Table 12.5. Thealternative approach of Harvald to thecalculation of the thrust deduction factor is toassume that it comprises three separatecomponents as follows:

t = t1 + t2 + t3 (12.47)

in which t1, t2 and t3 are basic values derivedfrom hull from parameters, a hull form

correction and a propeller diameter correction,respectively. The values of these parametersfor single-screw ships are reproduced inFigure 12.26.

12.4.3 ull efficienc

The hull efficiency can readily be determinedonce the thrust deduction and mean wakefraction are known

Cc bo co ITTC trong TM v FD cxc nh trc y v RC l sc cn iu chnhcho s khc bit nhit gia sc cn v thnghim sc y:

CFMC l h s ma st sc cn ti nhit tkim tra lc y.

Trong trng hp khng c m hnh thnghim mt nh gi yu t khu tr lc yc th clyt thnghimcaHoltrop (thamkho 39) v Harvald(tham kho 17). Trongcch tip cn Holtrop s hi quy sau y davo cng thc c a ra:

Trong phng trnh (12.46) gi tr ca tham sCsternc tm thy t bng 12.5.Phng phptip cn thay th ca Harvald tnh ton ccyu t khu tr lc y tha nhn rng n baogm ba thnh phn ring bit nh sau:

t = t1 + t2 + t3 (12.47)

Trong t1, t2 v t3 l gi tr c bn c lyt cc thng s ca thn tu chnh sa hnhdng thn tu v chnh sa ng knh chn

vt tng ng. Cc gi tr ca cc thng s nycho tu chn vt n c m phng Hnh12.26.

12.4.3. iu sut thn tu

Hiu sut thn tu c th d dng c xcnh mt khi lc y quyt nh v thnh phnsau c bit n

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However, because of the pronounced scaleeffect of the wake fraction there is a differencebetween the full-scale ship and model values.In general, because the ship wake fraction issmaller than the corresponding model value,due to Reynolds effects, the full-scaleefficiency will also be smaller.

12.4.4 Quasi-propulsive coefficient

It can be deduced fromequation (12.41) thatthe value of the QPC is dependent upon theship speed pressure field around the hullthewake field presented to the propeller andthe intimate details of the propeller designsuch as diameter, rate of rotation, radial loaddistribution, amount of cavitation on the blade

surfaces, etc. As a consequence, the QPCshould be calculated from the threecomponent efficiencies given in equation(12.41) and not globally estimated. Ofparticular interest when considering generaltrends is the effect that propeller diameter canhave on the QPC; as the diameter increases,assuming the rotational speed is permitted tofall to its optimumvalue, the propellerefficiency will increase and hence for a givenhull from the QPC will tend to rise. In thisinstance the effect of propeller efficiencydominates over the hull and relative rotativeefficiency effects.

Tuy nhin do phm vi tc ng r rt chothy phn khc bit gia quy m y contu v nh gi qua m hnh. Nhn chung biv phn ln tu nh hn so vi gi tr m hnhtng ng do hiu ng Reynolds hiu sutton b phm vi cng s nh hn.

12.4.4. s uasi

N c th c rt ra t phng trnh(12.41)gi tr ca QPC ph thuc vo tc tumin p sut xung quanh thn tu khu vc lntu n chn vt v nhng thng s ring cathit k chn vt nh ng knh tc quayphn phi ti hng tm lng to bt trn b

mt cnh. Nh mt kt qu l QPC nnc tnh ton t ba thnh phn hiu sutc a ra trongphngtrnh(12.41) vkhng nhgi trn ton b.

c bit l theo xu hng chung th nhhngca ng knh chn vt c th c trnQPC; nh tng ng knh gi thit rng tc quay c php gim n gi tr ti u can hiu qu chn vt s tng ln v do thn

tu t QPC s c xu hng tng.Trong trnghp ny tc dng ca hiu qu chn vt chimu th hn thn tu v c tc dng quay tngi hiu qu.

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12.5 The influence of rough water

The discussion so far has centred on theresistance and propulsion of vessels in calmwater or ideal conditions. Clearly the effect ofbad weather is either to slow the vessel downfor a given power absorption or, conversely,an additional input of power to the propeller inorder to maintain the same ship speed.

In order to gain some general idea of the effectof weather on ship performance appeal can be

made to the NSMB Trial Allowances 1976(Reference 42). These allowances were basedon the trial results of 378 vessels and formedan extension to the 1965 and 1969 diagrams.Figure 12.27 shows the allowances forshipswith a trial displacement between 1000and 320 000 tonnes based on the Froudeextrapolation method and coefficients.Analysis of the data upon which this diagramwas based showed that the most significantvariables were the displacement, Beaufortwind force, model scale and the lengthbetween perpendiculars. As a consequence aregression formula was suggested as follows:

trial allowance = 5.75 0.793_1/3 + 12.3Bn

+(0.0129LPP 1.864Bn)1/3 (12.48)

12.5 Snh hng ca nc

Cc tho lun cho n nay tp trung vosc cn v sc y ca tu trong iu kinnc tnh hoc iu kinl tng. R rngnh hng ca thi tit xu lm chm tugim cng sut ca tu hoc ngc li cn bsung cng sut chochn vt duy tr tc ca tu.

t c mt s kin tng qut ca nhhng ca thi tit ln nng sut con tu c

thc thc hin cho cc NSMB TrialAllowances 1976 (ti liu tham kho 42) . Cc tng c da trn kt qu th nghim ca378 con tu v hnh thnh mrng n 1965v 1969 s . Hnh 12.27 th hinr cho tuth nghim di chuyn gia 1000 v 320 000tn da vo phng php ngoi suy v h sFroude. Phn tch s liu ca s ny l cscho thy bin sng k nht c thayth, sc gi Beaufort, chiu di gia ccng vung gc.l mt kt qu c xut nh l mt cng thc hi quysau:

Th nghim gii hn= 5.75 0.793_1/3+12.3Bn +(0.0129LPP 1.864Bn)1/3

(12.48)

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where Bnand are the Beaufort number andthe model scale, respectively. Apart fromglobal indicators and correction factors suchas Figure 12.27 or equation (12.48)considerable work has been undertaken inrecent years to establish methods by which theadded resistance due to weather can be

calculated for a particular hull form. Latterlyparticular attention has been paid to the effectsof diffraction in shortwaveswhich is aparticularly difficult area. In generalestimation methods range from those whichwork on data bases for standard series hullforms whose main parameter have beensystematically varied to those where thecalculation is approached from fundamental

considerations. In itsmost simplified formtheadded resistance calculation is of the form

RTW=RTC(1 + R) (12.49)

where RTW and RTC are the resistances of thevessel in waves and calmwater, respectively,and Ris the added resistance coefficientbased on the ship form parameters, speed andirregular sea state. Typical of results ofcalculation procedures of this type are the

results shown in Figure 12.28 for a containership operating in different significant waveheights HS and a range of heading anglesfrom directly ahead ( =0) to directly astern( =180).

Shintani and Inoue (Reference 43) haveestablished charts for estimating the addedresistance in waves of ships based on a studyof the Series 60models.This data takes into

account various values of CB, B/T, L/B andl.c.b. position and allows interpolation to therequired value for a particular design. In thiswork the compiled results have beenempirically corrected by comparison withmodel test data in order to enhance theprediction process.

Bn v l s Beaufort v quy m m hnhtng ng. Ngoi cc ch tiu ton phn vyu t hiu chnh nh l hnh 12.27 hocphng trnh (1248) . Mt cng vic ln c thc hin trong nhng nm gn y thit lp phng php b sung sc cn v thitit c th tnh cho hnh dng thn tu c

th.Sau ny ch ring c a ra chonh hng ca nhiu xca nhng con sngngnl lnh vc c bit kh khn. Nhnchungphng php c lng xp loi lmvic trn c sd liu cho lot cc hnh dngthn tu.Nhng thng s chnh c hthng khc nhau m tnh ton c tip cn tnghin cu n c bn. Trong nhng hnhdng n gin nht ca n tnh ton them sc

cn ca hnh dngRTW=RTC(1 + R) (12.49)

Trong : RTW v RTC l sc cn gy ra dosng v yn tnh ca sng tng ng,

v R l h stng khng da trn cc tham skhch thc tu, tc v cc sc cn bt

thng. Cc kt qu tnh ton ca lo ny thhin trong hnh 12,28 cho mt tu containeriu hnh

chiu cao sng ng k khc nhau HS v ccnhm gc t( = 0 .180 ).

Shintani v Inoue ( Tham kho 43 ) c thitlp biu nh gi sc cn tng a vosng ca con tu da trn nghin cu vi 60m hnh khc nhau. S liu ny tnh n s

khc nhau ca CB, B / T, L / B v l. c. b.v chophp bsung gi tr cn tm t bn thit kring. Kt qu ca vic ny c chnh satheo kinh nghim bng cch so snh vi sliu thtrn m hnh nng cao quy trnh don.

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In general the majority of the practicalestimation methods are based in some way onmodel test data: either for deriving regressionequations or empirical correction factors. Inthe case of theoretical methods to estimate theadded resistance and power requirements inwaves, methods based on linear potential

theory tend to under predict the addedresistance when compared to equivalent modeltests. In recent years some non-linear analysismethods have appeared which indicate that ifthe water surface due to the complete non-linear flow is used as the steady wave surfaceprofile then the accuracy of the addedresistance calculation can be improvedsignificantly (References 56 and 57).

Although CFD analyses are relatively limited,those published so far show encouragingresults when compared to measured results,for example Reference 58.

Ni chung phn ln phng php c lungthc tda theo mt s cchtrn m hnh thnghim: hoc l thu c tphng trnh hiquy hoc cc yu t hiu chnh t thcnghim. Trong trng hp cc phng phpl thuyt nh gi thm sc cn v yu cunng lng trong sngphng php da trnl thuyt tuyn tnh c khnng don sccn tng khi em so snh vi m hnh thnghim.Trong nhng nm gn y mt sphng php phn tch khng tuyn tnh c xut,ch ra rng nu mt nc dodng chy phi tuyn tnh hon ton cdng lm mt ct mt sng n nh nhvy chnh xc ca tnh ton sc cn cng c thc ci thin ng k ( Tham kho 56 v 57

). Mc d CFD phn tch tng i hn ch,nhng nhng ngi xut cho ta thy ktqung khch l khi so vi o kt qu, v dtham kho 58.

Hnh dng 12.28. c tnh nng lngtng ln duy tr tc tu khc bit trng thibin cho tu container

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In the context of added resistance numericalcomputations have suggested that the formofthe bowabove the calm water surface can havea significant influence on the added resistancein waves. Such findings have also beenconfirmed experimentally and have shown thata blunt-bow ship could have its added

resistance reduced by as much as 20 to 30 percent while having minimal influence on thecalm water resistance.

12.6 Restricted water effects

Restricted water effects derive essentiallyfrom two sources. These are first a limitedamount of water under the keel and secondly,a limitation in the width of water each side of

the vessel which may or may not be inassociation with a depth restriction. In order toassess the effects of restricted water operation,these being particularly complex to definemath ematically, the ITTC (Reference 32)have expressed typical influencing parameters.These are as follows:

1. An influence exists on the wave resistancefor values of the Froude depth number Fnh in

excess of 0.7. The Froude depth number isgiven by

where h is the water depth of the channel.2. The flow around the hull is influenced bythe channel boundaries if the water depth todraught ratio (h/T) is less than 4.This effect is

independent of the Froude depth numbereffect.3. There is an influence of the bow wavereflection from the lateral boundary on thestern flow if either the water width to beamratio (W/B) is less than 4 or the water width tolength ratio (W/L) is less than unity.

Trong trng hp b sung thm sc cn cho thy rng hnh dng ca mi tu trn mtnc tnh c th c mt nh hng ng ktng thm sc cn trong sng. Nhng phthin ny cng c xc nhn bng thcnghim v ch ra rng vng cung ca tu cthgia tng sc cn ca n nhiu nh 20 n30% trong khi c nh hng rt t sc cnnc tnh

12.6 Hn chnc hiu qu

Tc dng nc hn ch ch yu t hai ngun.Trc tin hn chnc di sng thuyn vth hai gii hn trong chiu rng ca nc

mi bn ca tu c th hoc khng da vochiu trm ca tu. nh gi cc tc ngca hot ng hn chnc n c bit phctp hiu r ton hc, ITTC ( Tham kho32) da tham snh hng in hnh.

Nh sau :

1. nh hng ca sc cn sng cho cc gi trFnh su Froude vt qu 0.7.

Froude c trao cho bng

Trong : h l su ca nc ca knh

2. Cc dng chy xung quanh thn tu bnhhng bi knh ranh gii nu su nctheo t l (h / T) nhhn 4 v n khng lthuc vo tc dng s chiu su Froude.

3. C mt nh hng ca s phn xsng mit phn bn trn dng ui tu nu chiu rngnc ( W / B ) nhhn 4 hoc chiu rngnc t s bdi ( W L ) t hn tnh nnht.

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4. If the ratio of the area of the channel cross-section to that of the mid-ship section(Ac/AM) is less than 15, then a generalrestriction of the waterway will start to occur.In the case of the last ratio it is necessary tospecify at least two of the followingparameters: width of water, water depth or the

shape of the canal section because a singleparameter cannot identify unconditionally arestriction on the water flow.

Themost obvious sign of a ship entering intoshallow water is an increase in the height ofthe wave system in addition to a change in theships vibration characteristics. As aconsequence of the increase in the height ofthe wave systemthe assumption of small wave

height, and consequently small wave slopes,cannot be used for restricted water analysis.This, therefore, implies a limitation to the useof linearized wave theory for this purpose; asa consequence higher-order theoreticalmethods need to be sought. Currently severalresearchers areworking in this field andendeavouring to enhance the correlationbetween theory and experiment. Barrass

(Reference 44) suggests the depth/draughtratio atwhich shallowwater just begins to havean effect is given by the equation

in which the Cw is the water-plane coefficient.Alternatively, Schneekluth (Reference 45)provides a set of curves based on Lackenby swork (Figure 12.29) to enable the estimationof the speed loss of a vessel from deep to

shallow water. The curves are plotted on abasis of the square of Froude depth number tothe ratio

AM/h. Beyond data of this type there is littleelse currently available with which to readilyestimate the added resistance in shallow water

4. Nu t l gia vng knh mt ct ngangphn gia tu (Ac AM) l t hn 15 sau nhn chng hn ch ca ng thy s bt uxy ra. Cc trng hp t s cn thit xcnh t nht hai trong cc thng s sau: chiurng ca nc su ca nc hoc hnhdng ca cc on knh v mt tham s duynht khng thxc nh khng iu kin mthn chdng nc chy.

Du hiu r rng nht ca mt con tu mccn nc l mt gia tng chiu cao ca hthng sng ngoi v sthay i vc tnhrung ng ca con tu. Hu qu ca stngln chiu cao ca h thng ct sng lm tngthm sthay i c tnh rung ng ca contu. Nh hu qu ca stng thm chiu caoca h thng sng t nhng con sng nh vdo nhng con sng nhc xp li khngthdng cho vng nc phn tch b hn ch.

ng Barrass xy dng (tham kho 44) chothy su / d tho t l m ti nhhng ca nc nng c cho bi phngtrnh

Trong : Cw l h s mt nc ( Tham kho45 ) cung cp hng cong da trn ( Hnh12.29 ) cho php nh gi ca tn tht tc.

ng cong l s trn csca hnh

vung ca s chiu su Froude. iu ny gyra hin tng tu chi ui gy ra p lc lntu gim p lc ny ng Barrass a ramt bn phc tho pht trin mt mi quan hcho tu chi ui

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One further effect of shallow water is thephenomenon of ship squat. This is caused by aventure effect between the bottom of thevessel and the bottom of the seaway whichcauses a reduction of pressure to occur. Thisreduction of pressure then induces the ship toincrease its draught in order to maintain

equilibrium. Barras developed a relationshipfor ship squat by analysing the results fromdifferent ships and model tests with blockcoefficients in the range 0.5 to 0.9 for bothopen water and in restricted channelconditions. In his analysis the restrictedchannel conditions were defined in terms ofh/T ratios in the range 1.1 to 1.5. For theconditions of unrestricted water in the lateraln

direction such that the effective width of thewater way in which the ship is travelling mustbe greater than

bng cch phn tch cc kt qu t cc tukhc nhau v m hnh th nghim vi h skhi, trong khong 05 n 0,9 cho c haivng nc ngoi khi v a vo iu kinlng dn hp. Trong phn tch cc iu kinknh b hn chquy nh v t s h / T trongkhong 11 n 1,5.

Gii hn tu chi ui c cho bi gii:

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12.7 High-speed hull form resistance

In the case of a conventional displacementship the coefficient of wave making resistanceincreases with the Froude number based onwaterline length until a value of Fn 0.5 isreached.After this point it tends to reduce in

value such that at high Froude numbers, inexcess of 1.5, the wave making resistancebecomes a small component of the totalresistance. The viscous resistance, however,increases due to its dependence on the squareof the ship speed; this is despite the value ofCF reducing with Froude number. As aconsequence of this rise in the viscousresistance a conventional displacement hullrequires excessive power at high speed andother hull forms and modes of support requireto be introduced. Such forms are the planinghull form, the hydrofoil and the hovercraft.The underlying principle of high-speedplaning craft resistance and propulsion havebeen treated by several authors: for example,DuCane (Reference 46) andClay- ton andBishop (Reference 47). These authors not onlyexamine high-speed displacement and planing

craft but also hydrofoils and hovercraft. As aconsequence for the detailed principles oftheir motion reference can be made to theseworks. The forces acting on a planing hull areshown by Figure 12.30 in which the forcesshown as W, Fp, Fn Fs and T are defined asfollows:

W is the weight of the craft;

Fp is the net force resulting from the variationof pressure over the wetted surface of the hull;

Fh is the hydrostatic force acting at the centreof pressure on the hull;

Fs is the net skin friction force acting on thehull;

T is the thrust of the propulsor.

12.7 Sc cn hnh thc v tu cao tc

Trong trng hp ca con tu thng thngh s ca sc cn ch to sng tng viFroude s da trn chiu di ng nc chon khi mt gi tr Fn 05 l t. Sau thiim ny n c xu hng gim

trong gi trnh vy m sFroude cao vtqu 1,5, th sc cn sng trthnh thnh phnnh ca sc cn tng. Sc cn tnh nht, Tuynhin, sc cn do tnh nht ca cht lng tngln v n ph thuc vo bnh phng tc tu, mc d gi tr ca CF gim theo h sFroude. Nh mt h qu ca stng sc cndo tnh nhtca cht lng, mt quochuyn ng ca thn tucn qu mc nng

lng tc cao v cc dng thn tu khcv cc hnh thc h trc gii thiu.Nh vy cc dng l dng planning hull,tu cnh ngm v tu m kh. Cc nguyn lc bn ca sc cn ca planing craft cao tcva thit by c mt s tc giiuchnh: v d, DuCane (tham kho 46) vClayton v Bishop (tham kho 47).Cc tc gikhng ch nghin cu displacement v planingcraft tc cao m cn tu cnh ngm v tum kh.Nh mt h qu cho cc nguyn lchi titca cc ti liu tham kho v chuynng ca chng c thc p dng cho cctc phm ny. Cc lc tc dng trn mtplaning hull c hin th trong hnh 12.30.Trong cc lc c hin thnh: W FpFnFs v T c nh ngha nh sau:

W l trng tm ca tu;

Fp l h lc hnh thnh do sthay i p suttrn b mt t ca thn tu;

Fn l lc thy tnh hot ng ti im trungtm ca p lc trn thn tu.

Fs l h lc ma st ngoi hot ng trn thntu.

T l lc y ca thit by.

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By the suitable resolution of these forces andnoting that for efficient planing the planingangle should be small it can be shown that thetotal resistance comprises three components:

RT = RI + RWV + RFS (12.50)

where RI is the induced resistance or dragderived fromthe inclination of Fp fromthevertical due to the trim angle of the craft; RWVis the derives from the wave making andviscous pressure resistance; RFS is the skinfriction resistance. At high speed the wavemaking resistance becomes small but thevessel encounters an induced drag com-ponent which is in contrast to the case forconventional displacement hulls operating atnormal speeds. To estimate the resistanceproperties of high-speed displacement andplaning craft use can bemade of either

standard series data orspecific model testresults.

12.7.1 Standard series data

A considerable amount of data is available bywhich an estimate of the resistance andpropulsion characteristics can be made. Table12.8 identifies some of the data published inthe open literature for this purpose.

Bng nhng phn tch hp l v cn ch ivi hiu quplaning gc planing nn nh,n c thc biu din bng tng lc cnvi ba thnh phn lc:

Ti: RIl lc cn gy rahoc lcko hnhthnh t nghing ca Fp tng vunggc do gc ct tu.RWV do sng to ra v lc cn p lc nht.RFS l lc ma st ngoi. tc caolc cn dosng to ra trnn nhnhng tu b mt lcko iu ny tri ngc vi cc trng hptu di ch hot ng tc thng.

nh gi cc vn v lc cn ca s dich v tu bo th cngcao tc thc thchin bng cch s dng cc ti liu theo tiuchun hoc l kt qu ca mt vi th nghimc th.

12.7.1 Cc ti liu c tiu chun ha.Mt slng ln cc ti liu c gi trc thc dng nh gi v lccn v cc c tnh ca thit by.Bng 128 a ra mt s cc tiliu c cng b cho mc chtrn.

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In addition to basic test data of this typevarious regression-based analysis are availableto help the designer in predicting theresistance characteristics of these craft; forexample, van Oortmerssen (Reference48) andMercier and Savitsky (Reference 49). Inaddition Savitsky and Ward Brown (Reference

50) offer procedures for the rough waterevaluation of planning hulls.

12.7.2 Model test data

In specific casesmodel test data is derived fora particular hull form. In these cases theprinciples for model testing outlined inReference 51 and the various ITTCproceedings should be adhered to in order to

achieve valid test results. Multi hull resistanceThe wave resistance of a multi-hull vessel iscommonly approximated by considering thewaves generated by each hull of the vesselacting in isolation to be super imposed oneach other (References 59 and 60). If thisapproach is followed through then anexpression for the wave resistance for a pair ofnon-staggered identical hulls takes the form

Where A()|SH2 refers to the amplitudefunction for the side hull and F() is a hullinterference function and is dependent on thehull separation, ship length and Froudenumber. However, it is important to phase thewaves generated by each hull correctly if theirtransverse components are to be

cancelled.This cancellation effect is a functionof the Froude number and the longitudinalrelative positions of the hulls. Moreover, thecancellation effect of the transverse waveswill be beneficial for a range of Froudenumbers around that for which thecancellation is designed to occur.

B sung thm cc ti liu th nghim c bnca loi ny gip cc nh thit k doncc c tnh sc cn ca loi tu ny, v d,van Oortmerssen (Reference48) v Mercier vSavitsky (tham kho 49).Ngoi ra Savitsky vWard Brown (tham kho 50) a ra cc binphp nh gi th ca cc ng nc caplaning hulls.

12.7.2 D Liu v m hnh th nghim

Trong trng hp c th, m hnh th nghimc biu in cho mt loi thn tu c th.Trong nhng trng hp ny,cc nguyn tc th nghim m hnh c nu trong Tiliu tham kho 51 v cc bo co ITTC khcnhau nn c chp nhn t c kt qu

th nghim hp l. Sc cn vi tu nhiu thnSc cn sng ca tu nhiu thn thng ctnh xp x bng cch xt cc sng to ra bimi thn tu ca tu hot ng trong s tchbit v b chng ln nhau.(ti liu tham kho59 v 60). Cng thc s dng cho phngphp trn cho mt cp vtu khng ngnht.

cp n chc nng bin cho thn ph vF () l mt chc nng can thip thn

v ph thuc vo s chia tch thn tu, chiudi tu v sFroude. Tuy nhin iu quantrng l giai on sng c to ra bi mithn mt cch chnh xc nu thnh phn

ngang ca h b hy b. iu ny c hiu lchy b l mt chc nng ca s Froude vtheo chiu dc

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12.8 Air resistance

The prediction of the air resistance of a shipcan be evaluated in a variety of ways rangingfrom the extremely simple to undertaking acomplex series of model tests in a windtunnel. At its simplest the still air resistance

can be estimated as proposed by Holtrop(Reference 52) who followed the simpleapproach incorporated in the ITCC-1978method as follows:

in which VS is the ship speed, AT is thetransverse area of the ship and Cair is the airresistance coefficient taken as 0.8 for normalships and superstructures. The density of aira

is normally taken as 1.23 kg/m3. For moreadvanced analytical studies appeal can bemade to the works of van Berlekom(Reference 53) and Gould (Reference 54).Theapproach favoured byGould is to determinethe natural wind profile on a power law basisand select a reference height for the windspeed. The yawing moment centre is thendefined relative to the bow and the lateral andfrontal elevations of the hull andsuperstructure are subdivided into so-calleduniversal elements. In addition the effectivewind speed and directions are determinedfrom which the Cartesian forces together withthe yawing moment can be evaluated. Thedetermination of the air resistance fromwindtunnel measurement would only be undertakenin exceptional cases and would most probablybe associated with flow visualization studies

to, for example, design suitable locations forhelicopter landing and take-off platforms.Formore commercial applications the cost ofundertaking wind tunnel tests cannot bejustified since air resistance is by far thesmallest of the resistance components.

12.. c cn hng h

D on sc cn khng kh ca mt con tuc th c nh gi theo nhiu cch khcnhau t rt n gin n mt lot phc tpca m hnh th nghim trong ng hm gi. mc n gin sc cn khng kh vn c th

c c tnh theo xut ca Holtrop(thamkho 52) theo phng php tip cn n ginkt hp trong ITCC-1978 phng php nhsau:

Trong S l tc tu AT l din tch bngangca tu v Cairl h s sc cn khngkh ly l 08 vi tu bnh thng v kt cu

thng tng. Mt khng kh airthng l1,23kg/m3.

i vi nhiu nghin cu phn tch tin tinc th c thc hin cc cng trnh ca anBerlrkom(tham kho 53) v Gould(tham kho54). Cch tip cn thch hp bi Gould l xc nh bin dng gi t nhin vo mt nhl nng lng c bn v chn mt gi tr lmmc cho tc gi. Ti thi im tu i trch

ng sau c xc nh lin quan nmi tu sn v chiu cao pha mi thn tuv cu trc thng tng c chia nh rathnh nhng gi l thnh phn ph bin.

Ngoi ra hiu qu tc gi v cc hng vxc nh t h to -cc cng vi thiim tu i trch hng c th nh gi.

ic xc nh sc cn khng kh t vic olng ng hm gi s ch c thc hintrong cc trng hpc bit v c l s ckt hp vi cc nghin cu trc quan dngchy v d thit k ph hp vi a im chotu h b v ln .i vi nhiu ng dng ctnh thng mi chi ph cam kt kim trang hm gi khng th c k t khi sccn khng kh cho n nay l nh nht cathnh phn sc cn.