The estimation methods for ship added resistance and propulsive characteristics in seaway

V

_ARCHIEF

9cgbliotheek van de

)n4erafdeling der ScheePsb0UW1«

Technische Hogeschool, Deift

:'

Ç(3!V-DOtUMENTAT

D A)T U M:

- I

yternationa1 Symposium on the Dynamics Marine Vehicles _{and Structures in Waves}

PER 40

JSTtMATIoN METHODS _{FOR, SHIP} RESISTANCE AND PROPULSIVE

HARÁCTERIspIg IN

AWAY MEl 1974

oíLab.

y.

Scheepsbouwkunde

Technische Hogeschool

DeIft

by

V.M. IlyinX, V.5.

Shpakofr aosc.9C A.I. SmoròdinM.

Suary

In this paper the

_question

_{connected with estimation}

_ot

added res.stance

_{and propulsive}

_{characteristics of}

_{a ship}

in a seaway

are

_considerea.

_{.Whe data on the}

oean wave sta-..

tistj

in the regions

of

actie shipping

_are

_{given, which}

confirm practica]. importance of the

_reliable

_{prognosing of,}

the ship service.

_{speed in actual}

_{metéoro1ogjc.}

_coitjou

cperimentaJ..\ results and.

_{calculations are used}

_for

estima...

tion the influence on

_{screw propeller}

flydroay2lamjc

charac-teristics

of

such factors as pitching atad orbital

wàve

mo-tions of

water. The results

_{of free-running}

se]f...prpe],led

model tests in

waves

are

_described,

whichjndjcate the

im.-portance

_{of further study of}

_{screw-hull interaction}

charac-teristics varying due

_{to the ship}

_motions.

I. Introduction

CoRtenlporary design _{methods for}

_merchant

ships are based on the

_requirments

to provide them the designated speed in still

water

_Meanwhile

most of the operative

_time

_{at sea the} ships ar

_moving

in waves of definite intensity, which results

in decreasing of

the

service speed.

In the last few

_{years the systematized}

_{data on wave}

static.-tics were published

_{being based}

_on

the materials of special

observations and.

_{instrumenta], measurements on. 'board of}

oceano-graphic

_and

_other

_ships,

which include more

than

_IOOOOO

thetè.-orologica]. cards obtained

_{for the Norlh}

_{Pacific in 1954.-63}

/1/

x,

,

m Krylov Ship

_{Research IflSt1tt,}

and approximately 1000000 similar observations made in the

Atlantic ana

Indian Oceans /2/. These data

_ard

some others /3/ are illustrated in Pigs.I-5 and show first of all that still water conditions are negligibie _{in tota), balance o ship ser-.} vice time and secondly that the sea states of small and zaèdi-.m wave intensity should. be considered _{most important} förpra-ctical purposes _{The data ment iàned confirm also that wind ana.} wave distribution characteristics are close enough in

diffe-rent regions of snipping, _{which permits to derive rather}

uni-versal generalized characteristics similar tö that shown in Pig.5.

he pössibility

of reliable prognosis Ö± _{service speed}

de-crease in a seaway nas

_undoubtly

_{practical importance and may} effect upon the ship main _{elements selectiOn at initia], design} stages as well, as the optimal routing of ships with conside-ration given to weather forecasts.

Instead of well known approximate methods of estimation of ship speed losses in waves using the values of its additional

resistance, more correct method maybe _{considered which leads}

to the solution of

both

_{the differential equation. of ship} mo-tion, as a body affected by resistance and thrust forces, sr4 the one for screw-engine interaction:

(i

-=

(V, n,'C)

_-

R

(V, t)

21TIi2

_{Me(n) - .N.(v, n)}

where ?e(V,n,t)

= (1-t)P(V,n,t)

= (-w)v

In addition,

it

_{is supposed that there}

exists definite rela-tion between the number of revolurela-tions and torque output of

ship propulsion plant.

The solution of the _{system ( I ) requires knowledge o}

instantaneous

values of

resistance

_{and screw}

_thrust.

_{The most}

difficulties in

practical usage. of such _{a method are connected}

with determination of the screw hydrodynazzuc _{characteristics} affected by pitching and waves and especially of thè screw hull interaction coefficients.

_{Owing to}

_{this, the development} of a general method of _{evaluation of ship servicespeed}

in ac-tua], sea state should _{be preceeded by thorough}

both, theoreti-ça]. and experimental study of the effect of ractors mentioned

above upon the ship _{propulsive characteristics,. In}

the experi-. mental part, such a study should be based on a method of repre-sentation of irx'egular _{waves as harmonic wave series rather} widely used at present. _{The determinatjön of mean values accor} ding tO the methods of random process theory gives the initial data, which may be used for ship speed and _power characterl-sties prediction in irregular seas. It is _{bvious that the} nexb step should be determination of mean values of ship ser-vice speed. on deriite routes taking into account the

conside-red above dataon _{ocean wave statistics in different}

regions

of shipping.

2. Additional resistance of ship in a seaway

The total resistance _{of a ship R(Vt) in}

a seaway may be con-sidered as a sum o± three components

(v)

=0(v)

_{+ LR<v) + 5R(Vt)}

.

. (

fïrst two of which

_(R0(v)

_-.

towing resistance in stili water, mean value of additional _{resistance in. waves )}

&re the object of measurements in conventional _{seakeeping model tests} and are traditionally used for rough _{estimation, of}

_ship

speed losses' in waves, and _{ÖR(V,r) represents the variable}

component of additional resistance caused by the _{ship motions,} d±frac-tion of waves and a periodical changes of the hull wetted

_sur-

-face _{As ship speed Íluctuàtions during the}

r

according to the _{experiments], data}

are small, i _{may bé taken:}

R0

(y) _{R0(y0) and}

AR (y)

= R

_{(V0) (}

where v..

- mean value of speed in the waves of definite in-tensity.

The results of measurements of the forces _{acting on an} un-moving model in regular oncoming

waves may be used to estimate the variable component

_5RV,r)

_{of additional}

esistance

inwa-ves. The results _{mentioned show that it}

may be derived, in àuch a way:

dR(v,r)

5R0(v)in(o,)

where ô'1 _{- actual wave frequency}

and..

- phase angle.

In Fig.6 one can see, that _{, is rather close}

to tbe pit-.

ching phase angle and that the _{main part of}

_5R(v,r)

in the

wa-ves of significant length _O.

) may be related to the model weight projection on the _{instantanéôu}

waterpjane (Fig.7 The mean characteristics or _{additional resistance ¡(v)}

in irregular waves may be round _{/,6/ using the}

following integ-. rai relation:

=

2JP(6s(ú)«

₍₃₎

whére

=

- may be Considered _{as some form of}

¡'j

¼'

coI.jtjonal responce functio. of

ad-ditjonaj, _resistance

_AR(6)

in regular wáves _{Witb, height}

h

,

S()

_-

_{two-dimensional moael of sea}

wave spectrum.

In

practice, the

_diflicuities

in the _{experimental determi}

nation

_{or responce functjòn}

_F(G)

should be related _{to the} ne-cessary measurements of a model

additional resistance _{R in} the wave lengtii _{- ship length ratio}

range exceeding technical Possibilities of towing

tanks ('/L« 0,5).

_{For example, most}

extrapolation of ?(G,)

_{curves into th}

short-wave region (dotted line in Fig.8) may lead _{to the 20-30% error in the}

es-timation o± & _{for the}

_sea

.3.

Pitch and. 'wave effect upon propeller

operation

The motions of ship

and

additional velocity components due

to waves may sufficiently effect the screw working conditions,

resulting in a periodical fluctuations of the instántaneousX

thrust and torque values which consequently lead. to the change

in the number of revolutions.

The most simple way to, evaluate separately the influence

0±'

te factors xuentioned, if there are no air suction to the

screw

blades òr their emergence, is to use the hypothesis of

a pro-.

cess being quasi-stationary. One can find the justification of

such an assumption in the isolated. screw model test resul's

with hariaonicaly varying nwnber of revolution in the frequency

range

from

0.7 to 2.0 Hz ( Fig.9 ).

Furthermore, additional velocities. related. to the screw mo-tions

mean

over the screw disk may be

considered. Heaving

ef-fects being neglected and velocity field disturbances caused

by hull motions not taken into account,

_{one can. derive (Fig.IQ):}

=

+h5_(i..cosP)vo

-

[C(G+K)Cos' _Sn(C+K)Sn'']

(4)

AVE = V0 Sin'V M'

. -

Cos(«t

1<

Sin1

where

P= '4'oSin(Ç4E)

-

angle o

pitching,

- actual and, apparent wave

frequency.

In accordance with hypothesis. that the

_{process is}

quasi-sta-1ionary, one can assume that the screw blade t1yctrodynamic

cha-racteristics are fully determined by the in.tantaneous value

of advance coefficient

_{calculated in presumption that the.}

presence of athwart velocIties in the screw disk is equlvaleùt

to the variation in its rotational velocity for

a dei'i.nite

x/

By the terni "instantaneous" here the

sean values of the

thrust and torque for one screw revolution sould be meant.

-6

radius R0 1 5 I, i.e. :

=

:3 where: Vo _'E

_Cs

a

R0=e,

_=Tr(n'e+.),

_{L=OD1,2,...-1.} Then the thru$t( _{or torque )}

coefficients of the i-th screw

blade

_{will be determined by the following}

expressjon

K(e)

(i...4))a

(5')

where K)

_{- appropriate coefficient}

value correspOnding _to the screw action in still water

With small

_{(3_ .3e)}

_values:

(T)=

K(j+

₍₆₎

where

is derived for advance coefficient value

j.;

Then, after taking integral ₍

_{5 )}

for one revolution period

and. suznmin.g _{up the corresponding}

values for all the _blades, the screw

_K(t)

instantaneous values may be calculated:

+1T

K()

=

_Z.

_{(t

+'+'t)t{K( )....

(7)

then ( 7 ) _{may be transformed into the following}

expression:

As _{.AV =t'J iv(Çr+.1)}

and

_{4&\IØ n(6#E),}

K ('t)

= K ()

₊

A4'ii(t+ ;)+ A21

-(s)

where

,

A[K(0)_J]IEo)

Thus, the presence of additional _velocity

component AV

leads to the thrust

and

_{torque fluctuations with frequency}

Ç,

meanwhile

the athwart _{velocity cornpoñent}

causes the fluc-tuations mentioned with double _frequency.

device( Fig..LI ) enabling to

_simuìaite

_{the screw àperation}

Condit10

_{corresponjngo ship pitching.}

Whe main purpose of the tests

_was

_{to prove the validity of}

the

initial assum-ptions. Here the values of velocity _{components were} characte-rised. by the foLLowing:

A'IXO j)0

« k Cos

=

_c

J

(y1)a'

Cos(Ç'1'+

63)

As the separation of periodical _{cOmponents ( 8 ) having}

the

same

order

of magnitude ( due to the actual _{relations between} for conventional

ships ) is rather difficult, the Second.

_harmonic

_{was damped}

down

by

_means

_{of increase up}

tOOe45

Cornparjsonof the

_experimental

results and the data

calàu-lated. using (

8 ),

_{Fig.12, shows}

satisfactory agreement be;-ween the Corresponding _values.

ig.I2 gives also _{A. .values for} steady isolated _{screw mod.eJ. motion in}

regular head waves

_when

additional velocity Components were:

AV0

=J'ÇeCos(ç+.x)

=

The

_{joint pitch and wave eftect may be}

taken into account using superposition method

assuming that interaction of these is rather

_{weak (}

_{by the experimental data,}

F±g.13 ). S

a r_

sult, one

can

_receive:

K.,Çz)=

=

Ki()±tj(AJ+

J)

<_2.

()

11(V,n,t)

K2(j)+ ì#4)

J 3=3e

(9,)

x/ The

harmonic

analysis

_{of experimental}

data showed: that _in

all

cases _{2/4 ratio did not}

wbere

I. 'J.'8aznanouchi

2. i.Hogben

2

and A'4 _{mean values of pitch}

and wave-.induced velocities over screw disk.

It may be of interest to

compare the calculated results

ba-sed on. the _{use of (}

_{9 )}

and the

corresponding experimental da-ta of Self-propelled

free-running ship model in regular _seas ( Fig.14 ). The main conclusion from this _{Comparison is that} for relatively _{long waves, ¼y}

O 8, the

propulsive qualities of a ship in a seaway are detennjned

fully enough by taking

into accomt the _{joint effect of pitching and}

waves upon the screw hydrod.yn.j0

characteristics. However returning to the ocean wave statistics data, such _{a wave regime is}

rather be-.

yorid the limits' _{of. practical.}

interest in the problem of ship service speed prognQsis. On the

contrary, the changes in screw operation conditjon. _{behind the hull of a ship moving}

in re-latively short waves _{< 0.8 ) are much}

nra significan.

In this connection it should _{be noted that}

futthér develop...

ment of the

theoretical and. experirnentaj methods _{to facilitate} the study of velocity and _{pressure field, in the}

vicinity of oscillating ship hull seems to be of _{special interest. tJndoub-.} ted.ly, these _{would be useful tp make a more}

sound

_estimation

of insta..ntaneous _{and mean}

screw-huLL interaction _{coefficients.}

References

"Statistical diagrams on the winds a waves on the North _{Pacific. OCÒän'!,}

Toyo, 1970

"Ocean wave statisicsn, I?L,. _London, 1967

4. G.A..irsoff,

Â.1.Voznesseziy

J.I.Voitkunsky, R.J.Pershits, I .Á.Titoff J.Gerritsma, Van Den Bosch, W. Beukelman

regimes in

the

_{Oceans" (in Rtssian),}

Izd. Transpox, ₁₉₆₅

The, estimation _{method for a ship speed} losses in a seaway" _{(in..ussian),}

S±I Pubi., 1956

"Reference book on te ship theoi7"

(in Russian), _{Izd. Sudostrojenie,}

1973

"Propulsion in regular _{axid irregular} waves", ISP, vol.8, No 82, 96I.

170 F60 150 #VO 13Q 120 #10 loo 90 80 70 60 50 VO 30 20 lo Q 10 20 JO Yo 50 60 70 80 90 100 ff0 120 130 f10 150 f60 f70 180 #70 'N.11 t t I t t I I I I I I I I I I I I

II J_i_II

t I I I I I I I I I I

I,

170 160 ¡50 fl/C /30 120 1/0 /00 30 80 70 60 50 YO 30 ZO /0 0 10 20 30 YO 50 60 70 40 30 100 110 120 1.30 IvO 160 160 /70 /80 170

Pig. I

ZONES OP MEA8UR1TS

Fig. 2

L)NGPK

WAVE DI$TBUTION

L'o

20

io

Fig. 3

IG-'TERM WLV

DISTRIBUTION FOR THE ATI1AM C OCEAN

8 h1,mIÒ

Pige *

LONGTRM WAVE DISTRIBUTION

Fig.

5

THE GRAMZKD CHARACTERISTICS

OP LONG.J!iRM WAVE DISTRIBUTION

1N THE REGIONS OP ACTIVE SHIPPING

IO hw1m

L

I

5t1 O

R

50

Fig0 6

P}ILSE LNGL

POR THE PIIVWENG

AND ADDITIONAL RISTARCE

(hwfrI/5o

0,05

5k

D

o

e e _o

e o

e

Fig. 7 SPECIFIC ADDITIONAL

RII8TANCE

CWÂRJAOTXRISTIC

(hw,.

1/50, o o Xesi2tanee,

s s wéight projection.)

'o

2C

K,

10 K 14VO'

020

o e o

Fig. 9

SCR HYDRODYNAMIC CWLRACPZIBTICS

APFCTED B! TKE VARIATION OF NUMBER

.OP REVOLUTIONS

0,5

vo

Fig. IO

VTI

divè unit

Fig. II

TEE EXPERIMENTAL INSTALLLTION

SIMULATING A SCREW ACTION OCR-.

403-.-00

0,01

o

Fig. 12

THE CALCUI&TED VALUE8 OF TERUST,I,

AED TORQUE (2, magnified: by IO ).

COEFFICIENTS P ULTIONS AS A

FUNCTION

Expez'imental data for J=O.4:s

o o

stead$ motion in wavea

s a

pitchiig in still water

X

0,0

Pig. 13

8KPÀRLT AND JOINT icFiri«T OP PITCH

AND WAVE. VFOCITI

ON PEE SCREW THRUST

calculated:

i - pitching

2wave

3 - joint effect

0,1 5P

P

o

a

-O75

t2

Pig. 14

COMPARISON OP TEE PRNE-.RUI4NING

SE12PRT.TR1 M0DL

C

C?O.70 )

TEST RNSUThS AND CAISflJL&TED DATA

o o pitching, I-st haxionic only

c x

2M haz'mon.tc Only

G wave velocities

Ø Ø joint efZeOt

experiments.

1,05