Prediction and model experiments on speed loss of a ship in waves

Prediction and Model Experiments on Speed Loss of a Ship in Waves

Kunihiro Ikegami*

Yoshiteru Imaizumi*

For the prèdiction of seakeeping qualities of a ship and the establishment of design criteria for wave loads, ir is very important to estimate the ship's speed in rough seas. Speed loss caused by added resistance dùe ro wind and waves is cal/ed "nominal speed

loss, ' and its amount is a/so affected by the main engine characteristics.

In the present study, the prediction method of nominal speed loss was developed on the basis of ordinary power estimation method for propulsive performance of ships in calm water taking into account added resistance in waves propel/er characteristics

self-propulsion fac tôrs, and the main engine characteristics.

An engine-simulated se/f-propulsion apparatus was newly developed for model experiment in place of an ordinary propulsion motor By use of this apparatus model experiments in regular and irregular waves were carried Out for a high speed container ship model and a large full tanker model and their speed loss was measured for differing modes of engine characteristics under shaft

torqúe, power, or propeller revOlUtion ipeed kept constant

Experimental results were compared with the predictions, and the validity of the present prediction method was confirmed.

Remaining problems were pointed Out which require further investigations.

1. IntrodUction

In navigation at sea, the speed of a sNip decreases due to effects of wind and waves.

-When navigating in rough sea with constantengine pow er, the speed necessarily drops because of the added resist-ance and decreased propulsive efficiency due to winds and

waves. This is called "nominal speed loss." Then, in

navi-gating through very rough seas; there might occur deck

wet-ness, slamming and propeller racing with the increase of ship motion. io avoid them, the power is deliberately low ered through judgernent by the captain to reduce the ship

speed. lt is called "deliberate speed loss." The speed loss

directly affects not only various seakeeping qualities such

as ship motion and acceleration bUt also the design criteria

from the viewpoint of the wave loads. Therefore, accurate prediction of the' speed loss is essential for ship form and hull structure design and navigation method of weather

routing in broader sense.

As is well known, the amount of nominal speed loss is affected not only by external disturbances such as winds

and waves but also by main engine tharacteristics12 In

theoretical prediction and model tests for the speed loss in

waves, thereföre, it is essential to consider main engine

characteristics.

In the present study, a prediction method was derived of nominal speed loss considering main engine

character-istics on the basis of researches on the added resistance in

waves and propulsive pérfòrmance. By using a newly

de-signed engine-simulated self-propulsion apparatus, model tests were carried out on the speed loss in waves to examine

validity of the prediction method.

Nagasaki Technical Institute, Technical Headquarters

2. Prediction method of the nominal speed Io in wäves

Major factors which are related to the cause of the nOmi-nal speed loss are the added resistance due to waves and winds, the changes of self-propulsion factors and propeller characteristics in waves, and main engine characteristics. To predict the speed loss in waves accurately, it is necessary to'

clarify these factors and introduce them in the prediction

method.

2.1 Prediction of the added resistance due to waves The added resistance, RAW ,

of a ship hull in oblique

regular waves Vs given by Maruo151 as follows.

RAW 4

rçal Ci (m+K02)2(mkcosu)

-L J_o., Ja2 i

J(ni + K0ì)4

K02rn2

jH1 (m)I2dm (1) where, k = 2ir/X K0 = glU2

= UIwj/g

Wek(C

Ucosp)

C2 = gik

+22±'/1 4&2)

H1(m) :. a function determined by the singularity distribu-tion

X : wave length

g :. gravitational acceleration U : advanôe speed of ship

We : circular frequency of encounter

ji

: wave direction (head waves, 1800)

p : density of water

Fär Ñ1m), the. ship hull is assumed tóbe â slender body. And, the âppfôxrmatiôn is made that the source a(x) which has a strength proportional to the relative vertical speed at

each section of the hull to the wave is concentrated at a Specific depth Z = C1,. d (Cup: vertical prismatic

co-efficient and d: draft) and distributed over sh.ip length.

H1m) is thus given as(6) H1(m)

=f

(o - io5)

1.

exp1

k

z +.,mxjdx

(2) where,

cos Wet+ 0s Sm Wet

1. a a

=-- (-- U)z,(x) . y

(x)}

4

at

ax

Zr)ZXG)Oib ...

Zr(X) : relative vertical displacement to the wave of the ship hull atèaäh longitudinal cross section

y(x) : brèadth of ship huli at each croSs section z : heaving displacement, z z cos (wet + O : pitch.ing angle, O O cos (wet +

x; xc :

coordinate, along the ship length, and that of

the center of gravity

aib elevation of wave subsurface

sub hAe_kzacos(kx cosj.z wet) hA : wave amplitude

By closely satisfying the boundary conditions on the

hull surface of bôw añd then determining H1(m), the added resistance due 'to wäve can be. obtäined from Eq. (1). lt is

very difficult to determine H1 (m) strictly, and the quantity from Eq. (1) by the approximations of Eqs. (2) and (3) i's

mainly the còrlipohent due to ship motion. This is denoted asR4w (0).

In the case of a large full ship, the added résistañce due to vave reflection at bow appears to be remarkable at

shorter wave length range. So, this component is denoted as

RAw (1). and thus the added resistance ÂAW in regular

waves is given approximately as folldws.

RAW = RAW (o) + RAW (1)

Added resistance due to wave reflection at bow can be

estimated by approximate formula (6) after Havelock s

calculation method, of drifting fòrcè7.

RAW _Cil =a1 (1

+2)jpghA2Bsin23

(6)

B

2

f B/2

2ßdy

Fig. i Blùntness cóefficient

(3)

Rair = k0C0 +PaA T 0r2

Fig. 2 Wind.direction effect coefficient above

(8)

where,

2J2/(.2I2+

_{K12) :}

_{a corréction coeffcièñt fOr}

fi-nite dräft effect

= 5/

:, an empirical cörrection coefficient for

ad-vance speed effect

4

Ì (kd), K1 = K1(kd) : mddifièd Bessel function

sin2ß =-j

_{B_812}

sin2ßdy : bluhtness coefficient

Definition of ß etc. in the bluntness coefficient are

shown in Fig. 1'.

According to MàFu'o8, the added isistarie ii irregular waves is obtained by the reponse fùnction of the added

resistañce in regular waves and the irregular wav spectrum based on the linear superpositiòn theor'y as follows:

RAW

=2f,"[f(W)]2dw...(7

where,

RAW : mean added resistance in irregular waves

w wave circular frequency

[f(w)]

2 _{: wave spectrum}

2 2 Prediction of added resistance due to winds

In navigation thròügh stormy weather, there is ádded

resistance due to winds as well as that due. to waves. The added resistance due to winds is givén as1

where,

-

ka: wind direction effect coefficient in the

relativè wind direction

C«

ahead wind resistance coefficient

Pa density of the àir

AT: transverse projected area of the hul water line

Ur relàtive wind velocity

1.5

1.0

0.5

o

30 60 90 120 l5O 18O

Relative wind direction

0.5

1.0

Air resistance

Thrust dèdu6tion frtiä

A&iáiide spêéd of ropeIler j

Relative rotative efficiency

Changing ship speed No

1.2 LO E-. 0.8 0.6 0.4 0.2 O 04 06 2

HL. Mean height of longitudinal projected area

Fig. 3 Ahead wind resistance coefficient

Wind direction effect coefficient ka is obtained from the

standard curve9> in the standard speed trial analysis method

of the Japan Towing Tank Conferen.ce. And, ahead wind resistance

coefficient Cxo

is obtained from Wagner's

chart1. These are shown in Figs. 2 and 3.

2.3 Propeller characteristics and selfpropulsiòn factors in waves

There remain number of problems still unsolved in

pro-peller characteristics and self-propulsion factors in waves. Taniguchi10 described that the propeller characteristics

and self-propulsion factors in waves on a time average re-main approximately the same as those in calm water. Then,

Resist, in calm water

Propeller opn 6hiidteiistids

Thrust in waves

T/J=f/uD

s. or

t

J. e,

Shaft horse power

p(in wave) =P(in calm water

(Powér coñ5tant)

Yes

Nominal speed toss

08 10 1.2 14 1.6

Revolution cons t)

(Torque coast)

Ship motion in regular waves Resist increase in regular waves

Torque

Effectiive horse power

Number 6f revolution

Resist in6rease

Q(in wave)'

,Ç(in calm water I

fyes

Irregular wávé speátruñi

regu lar waves

n (in wave) = n(in calm water)

Yes

w

o 'o_w

w

Fig. 4 Flòw chart of calculation of nomiñal speed loss in waves

Nakamura et al.'1

studied ¡n detail the self-propulsion factors ¡n waves; however, they are still under qualitative

study.

In the following, the prediction method of nominal

speed loss is dèrived under assumptions as follows:

Propeller open characteristics in waves remain the same as in calm water.

Self-propulsion factors inwaves are the same values as in calm water.

Correlation factor is the same value as in calm water. 2.4 Predictión method of nominal speed loss

To derive the prediction method of nominal speed loss

in waves, there is the problem of treating main engine char-acteristics besides the treatment of factors described above. Main engines of actual ships can be grouped in the two;

the steam turbines and Diesel engines. Characteristics of

these engines are studied extensively by actual ship meas urement and simulatiôn calculation. They are, howeve, not

simple, and some approximations will be made. Generally

speaking, the characteristics of the steam turbine engine can be regarded as that of constant power, and the Diesel engine that of constant torque. In the present study, therefore, the prediction method öf nominal speed loss in waves is derived

taking three cases: constant power. constant torque, and

3

MTB 137 NovernIr 1979

Principal particulars of ship: SÑp speed in calm water

Wave condition

Hull efficiency

Propulsive efficiency eÏivered horse porer Stern tube friction loss

constant revolution, as fundamental cases for the engine

characteristics.

A block diagram of the calculation method is shown in

Fig. 4. That is, a prediction method Of: the power in waves

is given on the basis of the actual ship power estimation method by thrust idèntity using resistance and selfpropul-sion test results in calm water, actuàl ship propeller char-acteristics and correlation factors'2 When the conditions

of ship speed, wave, and wind are given, the corresponding total resistance of ship hull can be given; and therefore, the

thrust of propeller necessary to run the ship at a speed is

obtained through the thrUst deduction fraction t. And,

using selfpropulsion factors w, er, and propeller open

char-ácteristics, the speed of propeller revolution, the power,

and the torque are obtainable. A ship speed is obtained by iterative calculation such that either of the propeller

revolu-tion, the power, or the torque will be equal to that in calm

water depending on the conditions of engine characteristics. The computer program was developed fOr consistent

pre-diction of the. nominal speed loss in waves, ship motion,

added resistance and power.

3. Engine-simulated selfpropulsion

apparatus and test

method of speed loss in waves

3.1 Need for engine-simulated self-propulsion apparatus Electric motor generally used for self-propulsion tests in calm water has such characteristics ¿s to keep the propeller

revolution constant during the test. In most cases, same

motor are used also for tests in waves. In this case, the

pro-peller rotative speed is adjusted to attain prescribed ship speed in waves, and the corresponding thrust increase in waves is obtained.

On the other hand, at ship's main engines, turbine or Diesel, the characteristics are different from that of a

self-propulsion motor for model tests. Such model

seif-propul-sion motor, therefore, is not suitable for tests simulating

the speed loss of actual ships. And moreover, the variations

of propeller thrust, torque, and rotative speed at model

tests do not correspond to those atan actual ship.

In case simple self-propulsion motor is used, test can be performed by repeating it for differingship speed, and then interpolations are made to obtain nominal speed lOss in ref-èrence to the given.main engine characteristics. This meth-od, however, appears to be extremely laborious. When the engine simulated self propulsion apparatus is available the méasuremeñt of the speed loss and the seakeeping qualities

are straight-forward. And moreover, engine-simulated dy-namic test condition must be essential for studiès of such

phenomena as propeller racing.

3.2 Characteristics of the engine-simulated

self-propul-sion apparatus

In design of engine-simulated self-propulsion apparatus,

it may be essential to clarify qualitatively and

quantitative-ly static/dynamic characteristics of an actual ship's main

en-gine änd thereby consider its respective elements. In the

designed apparatus, however, assumption was made that the

actual ship's main engine can be simulated if only the

torque/speed relation is in principle similar to that of an

actual ship's main engine.

As fundamental functions of the engine-simulated self-propulsion apparatus, the functions are provided of

stant power, constant torque, and constant revolutiOn con-trol for basic research.

For control method of apparatus, indirect control on the

current and voltage of a self-propulsion motor is used in-stead of using direct control based On torque and rotative

speed signals of a self-propulsion dynamometer. That is, the

current and voltage are detected of a servomotor used as the self-pôpulsion motor, and control was mad for con-stant voltage, concon-stant current, or concon-stant power in

ac-cordance with a selectiOn. In this way, motor characteristics

corresponding to the constant revolution, constant torque,

or constant power are obtained.

Actual main engine characteristics are not of simple

con-stant torque, or concon-stant power(13 (14) _Therefore,

con-stant voltage pre-set signal and concon-stant current/voltage control signals are introduced in parallel ma comparative

Circuit, so that a circuit can work for the intermediate

char-Voltage

setting

(I) Constant trqùe contrOl

Constant power control intermediate control

& Torque (current)

(a) Conceptual diagram of control mode Cur eut detection f Gain Voltage deidn Main control Cancel circuit Time cons't setting Multiplier Control block diagram

Conotant revolution control

C,apacities and characteristic vaInes

Fig. 5 Characteristics of engine-simulated self-propulsion

apparatus

Seit probù.I

sion motor PrOpeller

Fig. 6 EngiflesimuIated self-propulsion apparatus

4

Control mode Design condition Time constant Control acëuracy Constant torque 1.5kg -cm O.Bs 0.1% Constant revolution 2Orps O.2s 0.5% Constant power 100W 0.5s 0.3% .

acteristics betwèen those for constant revolution and con-stant torque/power. Then, assuming that the system of an

actual ship propeller can be approximated by first order lag

time constant adjusting circuit by first order lag circuit is

added, to simUlate the time constantof response in the pro-peller rotative system.

In Fig 5 are showñ the concept òf contrOl mode änd

block diagram of the apparatus and its capacities and char-acteristic values Fig. 6 shows photograph ofthe control

powér source and self-propulsion motor.

Fig. 7 shows records of thrust, torque and revolution in

speed loss tests by use of the apparatus. In Fig. 7 it is-sêen

thát the variation of. propeller revolution frequency dIffers

with the control modes. In the case of constant torque con-trol, there are still fluctuations though the average torque is

fairly close to that in calm water. Thts fact may be due to relative insufficiency of constant torque control

perform-ance arisen from large time constant having similar order to

the wave period of encounter, as seen in

characteristic-values of Fig. 5(c).

When average values only are dealt with such as ship speed loss, present control performance of the apparatus will be adequáte Hôwevèr, there is need fòr further

im-provement when variation and dynamic characteristic as in propeller racing are to be dealt with.

3.3 Method of speed loss test in waves

Speed loss test with the engine-simulated self-propulsion

apparatus is madé in the Seakeeping and Manoeuvring

Basin, as follows. A model ship is self-propelled at a speci-fied speed, and made to encounter the waves generated in

the Basin The number of propeller revolutions, torque, or power in calm water is memorized in the contról power source, and the control is made to keep them constant in

Constant revolution control

Torque

Thrust

Revolution frequeny Constant power control

Torque

Thrust

Revolution trequeiìy Constant torque control

Torquè

Thrust

-RevoIutionrequenc

RUnning io calm water

I' 2s

ps

Running in waves

Fig. 7 Example of monitor records of torque, thrust and revolution

OE2

- Calculated value of eed loss for

Lm=175rn full-scale ship

- Calculated value of eed loss in model point for = 3.5 model ship

In calm water F, =0.25

AIL

Fig. 8 Effect of skin friction correction on nominal speed loss

the waves. And subsequently the ship speed begin.sto drop in waves. When the average speed is fOund to be substan tially constant, measurements are started of ship.speed and ship motions.

The speed of a model ship in waves is indirectly obtained

from the speed of the towihg carriage running with the

model sh:i p. For the speed in irregular waves, however, two methods are used for comparison; i.e. the speed of a tow-ing carriage and the calculated speed obtained from runntow-ing distance and time.

In such speéd loss tests- as mentioned above, the skin

friction correction -is extremely difficult; and therefore, the

test

is made at model point.. However, the correlation

should be clarified of the results of a speed loss test at

model point-and the speed loss of the full-scale ship. For example, Fig. 8 gives comparison of the calculated speed loss of the full-scale ship and of a 1/50 scale model, having the length = 3.5m for a single screw container ship of = 175m. In contrast to the speed V5 = 20.1 kn in calm water, the wave conditions: X/L = 1.0 and

h/L

1/50, gave risè to the drop of the speed down to V,= 14.5 kn for model point, while estimated speed for ship point is

V 12.1 kn: therefore, the test in model point gives a

'speed loss about 2.4 kn less. Thus, when the speed loss of a full-scale ship is to be predicted-accurately, the correction for the experimental result is necessary.

For the purpose of examining variatioñs Of seàkeeping qualities with ship speed, loss, the speed loss test at modél

point is usefül. It should be remembered, however, that

there is the difference in speed loss with model scale, ship form, and wave conditions as shown in the above example.

When validity of the prediction methods of seakeeping

qualities including the speed loss have been confirmed by model test, it would be easy-to estimate seakeeping quali-ties of full-scale ships.

4. Model test on the-nominal speed loss

In order to confirm validity of the prediction method of nominal speed loss, model test in regular and irregular waves were carried out for a high-speed contaiñer ship at

full load- and ballast condition, and for an ULCC at full load

condition, by using the engine-simulated self-propulsion apparatus. MTB 137 Novem\ber '1979 Torque constant hw/L' 1150 5 1.0 1.5 20 10 0.1 o o o Wave leogth A/L= 1.0 Wave direction =180, 02kg cm 0.3 20

6

Table 2 Irregular Wave characteristics

4.1 Modèl ships and test conditions

In Table 1 are shown principal particulars of the

high-speed contaiñer ship môdel and the ULCC modél, and the test conditions.

The waves for test are regular. and irregular waves; the irregular wave is based on ISSC wave spectrum. Irregular sea state is expressed by Beaufort scäle, as shown in Table 2, where significant wave height and mean wave period are the

representative parameters. In speed loss test, fairly long

rurming distance is necessary. So, arnodel ship is made to

run in longitudinal direction of the Basin, the directions of the waves are then head wave and beam wave, using the

wave generators on two adjacent sides of the Basin.

Control modes adopted in the test by use of the engine-simulated self-propulsion apparatus are constant torque and

constant revolution, and some additional tests were made

also fôr constant power.

Table i Principal particulars of tested models

4.2 Test results

4.2.1 Nominal speed loss

In Figs 9 and 10 are shown the experimental results of

nominal speed loss of a container ship model in comparison

with the calculated ones The results in regular waves are

based on wave length, and those in irreúlar Waveson

signi-ficant wave height. The agreement between exprimental

value and calculàtèd 0e is generally good.

In ballast condition, however, there is sorne problem

re-sulting from direct use of the prediction method

In this

connection, calculation is made under the assumption that

the propeller openwater characteristics are equivaldnt to the steady characteristics av propeller immersion: = 0.559

in calm water. r

Fig. il shows the propeller open-water test results

ôb-tamed for various propeller immersions, used for

calculati-ons. The thrust coefficient Kr and the torque cofficient

KQ are found to decrease almost linearly with

decreasi-ng ¡ID startidecreasi-ng from the vicinity of immersion ¡ID = 0 7

For speed loss in beam waves1 the experimental1value is

somewhat larger than calculated one. This is possibly be-cause the model was running with transverse drift:vith the

model's heading kept constant at 90 degrees to the ,ave by

an auto-pilot system and there were also effects of rolling and steering.

The speed loss in regular waves is large at aboih X/L

1.0 where the ship motiôn is considerable and the added

Container ship Tanker

Scale ratio _(liS) 1/50 1/88.3838

Load côndition Full load Ballast, Full Joad

Length (L) 3.500 m 35ô0 m 3.960 m Brêàdth (8) 508.0 mm 508.0 mm 792.58 mm Draft (d) 190.0mm 125.4mm 256.0mm Trim (t) 0 0.985% 0 Displacement (Ao) 193.57 kg 116.8 kg 656.49 kg Block coefficient (Cb) 05725 0.5275 0.8167

Midship sectiOn coefficient (Cm) 0.9700 0.9546 0.9991

Center of gravity (Xe) -50.9 mm -68.4 mm 1353 mm

Center of gravity (KG) 190.3mm 160.0mm 177.2mm

Metacéntric height (GM) 19.6 mm 55.5 mìn 153.9 mm

Longitudinal gyradius

(K/L)

0.24 0.256 0.230

Tränsverse gyradius

(K/B)

0.34 0.30 0.33

Nàtuîai rOil periòd

(T)

2.62 s 1.51 s 1.44 s L

Propeller diameter (D) 130.00mm 105.51 mm

Pitch (P) 130.00 mm _{79.53 ''}

L

Expanded area ratio Ae/Ad) 0.652 0.6817

Number of blades (Z) 5 5

Beaufort

ale

Significant wave height

Hw(l,a) Mean wave periodTw

BFT 6 3.Om 7.Os

-7 4.0 8.8

8 5.5 10.7

9 7.0 12.8

E 20 15 20 15 OES LO-0.5

(a) In regular Waves

E 1.5 1.0 In calm water V,=1.465m/s

4_

_N0

1.5- In calm water V,,, = 1.465m/s o fl11 1113) (mm) O -HlrO/3) (m( I I I I S 6 7 8 9 10 BFT

o,,

Constant torauel Constañt revbtutinnl 1.0 1.5 2.0 AlL In calm waterV,,,1.465m/s A Constant torque In calm water V, 1.465m/s Cons tant revolution

(Model ship)

000 200

resistance is large. In full load condition, under constant torque còntrol, the ship speed at wave height h/L = 1/50,

is about 75% of that in calm water. The speed loss in

irreg-ular waves becomes the larger, the severer the sea state. Under the sea state equivalent to Beaufort 10 (BFT1O) at full load côndition under constant torque control, the ship

speed is about 80% that of in calm water.

Concerning the effect of load condition, the speed loss is

about 10% larger in ballast condition than in full load con-d.ition.

As to the effects of main engine characteristics for

con-stant torqùe control, the speed loss becomes larger than that for constant révolution control because the propeller

E

1.0

t0 I

05 (a) In regular waves

o MTB 137 November 1979 In calm water V,,, 1.465m/s Al L Constant torque In cain, water V,,,=1.465m1s

___=,,=A

Constant revnlution' 1.0 1.5 2.0 In caIrn water V,,=1.465m/s o Cnstunt torque A Constant revolun (Model ship) loO 200 1114(1/3) (mm) o 5 10

!IW)I13) (m) (FulIscale ship)

I I I I I J

5 6 7 8 9 10

BFT

revolution decreases in waves.

In Fig. 12 are shown the experimental results of speed

loss of a tanker model in full load condition compared with the calculated ònes. In regular head waves at wave height

h/L = 1/100, the agreement between calculated value and experimental one is generally good. Then, at wave height h/L = 1/50, the calculated value is higher, although

calcu-lated and experimeñtäl values show the same tendency. In regular head waves at about X/L 1.0, the speed loss

be-comes large for any case of the contrôl modes. That is, at wave.height h/L = 1/50, the ship speed ¡s about 60% of that in calm water. In effect of the main engine

character-istics, the amount of nominal speed loss is large in the ordér

trolL CaIcu. Eupri.

180 1/50 (hw7.Ocm) 1/70 (hir5,Ocm) 1/100(#w=3,5cm) O A o

---

--90 1/50 (hv'=7.Ocm) A

0.1 yelL Calcu. Capen.

180 1/30 (hiç'ii.lcm) 1/50 (hv'=7.0c,,) 1/70 11ir=5.Ocm( o/loolvw=3.scn,)

-

--

-,-O O A C yo i/so )!vv7Ocml ---- A LI Calco. Exper). 080 O 90 A 0.1 Calcu. Eaperi. l80 O

90 --

A

(b) In irregular waves (b) In irregular waves

Fig. 9 Nominal speed loss of container ship model in full Fig. 10 Nominal speed loss of container ship model

load condition among waves in ballast condition among waves

2.0 1.5 2.0 1.5 o o o 1.5 E 1.0 T o 1.5 1.0 0.5 LS E 1.0 1.5 E 1.0 T 10 (Fall-scale ship) In calm water V,=1.465m/s AO

06

0.5

0.4

0.2

0.1

Propefler revolution frequency = 13.0ps

KQ s,7

e s=Q.6

/

$ = 0.7 ¡ID s = 0.5 s OE4 K s0.7 s=06

/

s=Q.5

/

s =0.4 0.5 1.0

Fig. 11 Propeller open-water test result in vaious

propeller immersion

of constant revolution,, constant power, and constant

torque; however, their differences are fairly small.

42 2 Self propulsion factors

In Fig. 13 the results are shown of selfpropulsion fac-tors for a container ship model at full load condition, ob-tained from the measured results of torque thrust and pro

peller revolution in speèd loss test. The results of analysis are shown against wave length, or significant wave height. The solid and broken lines are values of self propulsion fac tors ìn calm water at same ship speeds as those at experi-mental points.

The relative rotätive efficiency e,. varies little with wave

length and ship speed. The values are somewhat smaller

than in calm water and they are close to unity.

In full load condition, the wake fraction Wm is similar to that in calm water. In ballast condition, however, the

value is somewhat smaller than in calm water;

The thrust déduction fraction t appears to vary with

wave length, a little more than other self-propulsion factors.

4.2.3 Ship mótiöfl

In Figs. 14 and 15 are-shown the experimental results of ship motion for a' container ship model in regular and irreg-ular Waves in comparison with the calculated ones.

Calcula-tiOns of the ship motion were made for the same ship

speeds -as measured ones. In all aspects of ship motion,

fáirly good agréêment is òbtáinèd betweén calcúläted valuès and experimental ortes. Thréfore, the seákeeping qualities such as ship motion with, speed loss may be predicted with

reasonable accuracy. 8 E 0.5 E 0.5 LO E ! 0.5 10 01

io-

In calm water V5=0.935ni1s Qonstant power ànstint torque Constant revolution 0.5 In regular waves In irregular waves

Fig. 12 Nominal speed loss of tanker model in full load

conditión

4.-3 Discussion

It -is shown that the. effect of the main engine character-ist iòs on the nominal speed loss is the smallest at constant

revolution, and the largest at constant torque. conditions.

This tendency is especially remarkable for a container ship model;- however, the différence is rather small for a tanker

modél. In Table -3 is shown comparatively the propulsive

performance of a container ship model and a tanker model in calm water. In cairn water V,.=0.935m1s

--A A Constan power Calcu. Ex pe ri. Constant revolutión

lo calm water V,=O.935in1s

o

In cairn water V, =0.935mIs

-o o 1.0 _ - 1.5 - AIL In calm water V,,0.935m/s aT i 0-- - -- 50 100 He'u/3 (mm) 10 I I I 6 -7

89

10 OFT 2.0 (Model shiip)

/- hwIL Cáicù. Experi.

i80 1150 (7.9cm) 11100(4.0cm) 0 A ___L. 1150 (7.9cm)

---

O - - 1/100(4.0cm) .- O -180 90 o A 2'O 1,0 15 E 10 05 20 15 HW(]13) (m) (Full-scale ship)

1.0 0.5 0-. 1.5 1.0 0.5 o Heaving Pitching H,v1i131 (mm) A 5 10 H,y11131 (m) -I -I- I J 5

67

8 9 10 OFT (b) In irregular waves

Fig. 13 Self-propulsion factors of container ship model

in full load condition

(Full-scale. ship) Constant torque u Calcu. Experi. Surging 0.5 1.0 1.5 2.0 AI L

Fig. 14 Motions of container ship model in full load

condition among regular waves

In the case of a container ship model, the change of

propeller slip with change of the revolution and the change

of thrust with change. of the propeller slip are much larger than in the case of a tanker model. So, the effect of main

engine characteristics is evident for a container ship model.

E 1.0 o 3.0 2.0 1.0 O O E E SO Heaving o BFT

Fig. 15 Motions of container ship model in full load

coñdition among irregular waves

As thus seen, the effect of main engine characteristics on nominal speed loss differs largely with ship form and

pro-peller characteristics.

Comparing the speed loss in waves, between calculation

and experiment, the calculated value tends-to be large at

higher wave heights. In Fig. 16 is shown the effect of wave height on speed loss for a container ship model. The

calcu-lated nominal speed loss increases with the square of the

wave height. Such apparent discrepancy between calculated value and experimental one may have been caused by the

relative inaccuracy of the added resistance due to waves. In Fig. 17 is shown the calculated total resistance in waves for a container ship model. The larger the wave

height and the lower the ship speed, the larger becomes the

percentage of added resistance due to waves in the total

resistance. And, the larger the percentage of added resist-ance due to waves, the wórse becomes the accuracy in

pre-diction of the nominal speed loss. lt is thus seen that the

prediction accuracy of nominal speed loss depends largely MTB 137 November 1979

Constant toj1

200 (Mndel ship)

9

I.L Calcu. Experi.

180 o

9« -- A

o O_ll 1.0 L5 2.0 -AIL (a) In regular waves

1.0

0.5

o o o o o o

Censtant torque

Wave direc. In waves In calm wáter

180 o O -Ii 100 200 (Model ship) e, 1.0 0.5 o A Constant torque

Wave height Wave direct. In waves In calm water

hw/L =Í/50 180

o!

90 4.0 E r-E 1; 50 3.0 2.0 5.0

-

100 Surging o 4.0-(FuIIscale ship) 100 iisj (mm) 0 5 10 HW( I/SI () I I J I I s 6 7 8 9 10 loo. o 90 A

Table 3 Comparison of propulsive performance between container ship model and tanker model 10 In calm water Vm= 1.465 rn/s 1.5 E 1.0 0.5

-N

I Container ship I

Wave directionu=180ÌConstant

torqì'\

wave height (cm)

Fig. 16 Effect of wave height oh nominal speed loss

in regular waves

on the accuracy of the prediction of added resistance due to waves.

Theoreticàlly the calculated added resistance due to waves are proportional to the square of the. wave height.

However, according to the experimental results reported in SR12514'51 there appears non-linearlity, so that there can be seen the case that the value is not necessarily

propor-tional to the square of the wave height. In this respect, further study may be necessary. At high ship speeds, the

àddèd resistance calculated by the method described is gen-erally smaller than the experimental one. Then, at low ship speeds near F =0.15 and smaller,thé calculated value tends

to be larger than the experimental one. These tendéncies may partly have caused relatively larger calculated speed

loss.

In the effects of self-propulsion fàctors on the nominal

speed loss, the. thrust deduction fraction has the largest in-fluence. For a tanker model, the nominal speed loss at dif-ferent thrust deduction fractions was calculated. By

chang-ing the fraction by 25%, the resulted change in the speed loss is about 10%-15%. As described already of the self-propulsion faôtors, ship motion has thé greatest influence on the thrust deduction fractiOn. Then, quantitative esti-mation of the effect will be difficult. Nevertheless in order

to improve the accuracy in prediction of the nominal speed

loss, further studies both in theoretical and experimental

lo 05_e Container ship RAw

---Resistanòe in calm water I I I 1.1 1.2 1.3 1.4 1.5

Fig. 17 Comparison between total resisnce and added resistance in regùlär waves

aspect may be necessary.

5. Concluding remarks

The prediction method of nominal speed loss in waves was derived. Then, to confirm validity of the method, ship

speed loss test was carried out with engine-simulated self propulsion apparatüs. Obtáined results maybe summarized as follows:

The nominal speed loss in waves increases in the order

of constant revolution, constant pOwer and constant torque control. So, the speed loss is influenced by the

main engine characteristics of à ship. Ìhe èffe1ct,

hovv-ever, differs appreciably with ship fòm and

ropellér

characteristics. In the case of a container ship the dif-ference in nominal speed loss with main engine

char-acteristics is remarkable; however, in the case ofa tanker the difference is very little:

In efféct of the ship form upon the nominal speed loss

in waves, the speed loss is larger for a tanker tlan for a container ship. As to the load condition for a container

ship, for example, the nominal speed loss in ballast conk

ditiòn is somewhat larger than in full load condiion. By the pred.iction method of nominal speed loss in waves, the calculated value tends to be larger han the experimental one at larger wave height. At sm'all wave - heights, however, the value is in good agreement with

the experimental one. Though the accuracy of predic-. tion differs with ship form, the prediction method shows

Factors Ship speed

in calm water ke . rac ion Propeller revolution

frequency Slip ratio

Change of slip ratio vs. change of

revolution frequency

Change o thrust

vsJ

change of slip ratio

Ship'type Wm o S0 (aS/aI7)=

(aKr/as)55

Con mer ship model

e. .rn

1.465 rn/s . 0.285 13.7 rps 0.413 0.046 1/rps 0.4k Tanker model . 35m/s 0.550 19.4 rps 0.721 0.013 l/rps

Lpp3.96m

I . --Wave direc. Wave Ievgth Wave height S mbol 18Ö A/L-1.o hw/L hw/1. l/loO

AIL Calcu. Engen.

0.5

-.

o 1.0

---1.5

---

D 2.0 0 2.0 1.0

practically sufficient accuracy up to h/L = 1/30 and

Beaufort scale about ib for a container ship and up to

h/L = i/lob and Beaufort scale about 8 for a tanker.

The discrepancy between calculated value of nominal speed loss and experimental one at large wave heights may have been caused mainly by the inadequate

estima-tion accuracy of the added resistance due to waves. Therefore, to improve the accuracy of the prediction

method of nominal speed loss, it is necessary to improve

the calculation method of the added resistance due to waves especially at large wave heights.

In ballast condition, during navigatiOn in Waves, there occasionally occur slamming and propeller racing.

There-fore, in prediction of the nominâl speed loss, further

studies will be necessary on the estimation of the added

Nakamura S., Various Factors ofSeakeeping Qualities, The

So-ciety of Naval Architects of Japan, Symposium on Seakeeping

Öualities (1969)

Journée J.M.J., Prediction of Speed and Behaviour of a Ship ¡n

a Seaway, International Shipbuilding Progress, Vol. 23, No.265

(1976)

Nakamura S. and Naito S., Nominal Speed Loss and Propulsive

Performance of a Ship in Waves, Journal of The Kansai Society of Naval Architects, No. 166 (1977)

Nakarnura S. and Fujil H., Nomina! Speed Loss of Ships in Waves, Proceedings of the International Symposium on PRADS

(1977)

Maruo T., Resistance in Waves, Researches on Seakeeping

Qual-ities of Ships in Japan, The Society ofNaval Architects of.Japan, 60th AnniversarySeries, Vol. 8 (1963)

Fujii H. and Takahashi T., On the Increase in Resistance of a

Ship in Regular Head Sea Mitsubishi Juko Giho Vol 4 No 6

(1967)

Fujii H. and Takahashi T., Experimental Study on the Resist-ance Increase of a Large Full Ship in Regular Oblique Waves, Journal of The Society of Naval Architects of Japan. No. 137

(1975)

Maruo T., On the Increase of the Resistance of a Ship in Rough

Seas (Il). Journal of The Society of Naval Architects of Japan,

Refèrénces

MTB 137 November 1979

resistance and on the selfpropuIsion factors and propel-ler characteristics under such specific cases.

Among seakeeping qualities the item such as the ship motion under condition of nominal speed lOss in waves can be estimated by the presented method with

practic-ally sufficient accuracy so far as the prediction of nômi-nal speed loss is precise.

Acknowledgement

In the present study, experiment for a container ship model was partly made in the No. 161 Research Comit-tee, the Shipbuilding Research Association of Japan. The authors are deeply indebted to Professor Nakamura, the

chairman of the Committee, and all the members for valua

ble discussions.

No.108(1960)

Japan Towing Tank Conference, Draft Standard Speed Trial Method and Draft Standard Speed Trial Analysis Method,

Bul-letin of The Society of Naval Architects of Japan. No. 262

(1944)

Taniguchi K.. Propulsion of Ships in Waves. Bulletin of The Society of Naval Architects of Japan. No. 383 (1961)

Ñakamura 5., Hosoda R., Naito S. and Inoue M., Propulsive Performance of a Container Ship in Waves (4th Report), Journal

of The Kansai Society of Naval rchitects, No. 159 '(1975)

Taniguchi K., Model-Ship Correlation Method in the Mitsúbishi Experimental Tank, Journal of.The Society of Naval Architects

of Japan, No. 113 (1963)

Ogawara Y., Iwata S., Tsujita T. and Sasaki K., Governing Operation of the Diesel Engine for High Speed Ship in Rough Sea. Mitsubishi Juko Giho. Vol.8, No. 1(1971)

No. 125 Research Committee, The Shipbuilding Research Associätion of Japan, Studies on the Seakeeping Qualities of an Ultra-High Speed Container Ship. Research Report No. 157 (1972), No 211 (1975)

No. 125 Research Cimmittee, The Shipbuilding Research

Association of Japan Studies on the Seakeeping Qualities of an Ultra-High Speed Container Ship, Research Report No. 188

(1974)