Unusual phenomenon at the stern of full ship models

SER. 1S6

ARCHIEF

SYMPOSIUM ON

"HYDRODYNAMICS OF SHIP AND OFFSHORE PROPULSION SYSTEMS"

HØVIK OUTSIDE OSLO, MARCH 20. 25., 1977

"UNUSUAL PHENOMENON AT THE STERN OF FULL SHIP MODELS"

By

K. Watanabe and H. Tanibayashi Nagasaki Technical Institute

Mitsubishi Heavy Industries, Ltd.

Nagasaki, Japan

Ref.: PAPER 7/5 - SESSION 1

()

Lab.

y. Scheepsbotiwbnd

Techn;sce Hogsh00l

unusual Phenomenon at the Stern of Full Ship Models

By K. Watanahe Dr. Eng.

H. Tanibayashi

Abstract

It has been reported that test results of some full ship models show peculiar characteristics which are called unusual phenomenon; propulsion tests resulting in unusually high values of wake fraction and manoeuvring tests showing unusual stability on course for a range of small rudder angles.

This paper describes the investigations undertaken in Nagasaki Experimental Tank to make clear the unusual phenomenon

in propulsion tests since the discovery in 1960 and the recent research on the unusual phenomenon in manoeuvring motions. And

it is shown that both unusual phenomena are closely related as chnracterized by a triggering action of propeller to the change of flow at the stern.

There remains, however, much room for further investigation into this phenomenon, and therefore some comments for the future study are added.

*

Vice Manager, Nagasaki Technical Institute, Mitsubishi Heavy Industries, ltd.

**

1. Introduction

(inusual phenomenon described in this paper means a phenome-non particular to testing models with high block coefficient, which can not he explained within the scope of the existing data.

In propulsion tests it has been reported that some models show a

very large wake fraction as a result of slight change in fullness of afterhody, shape of frame line, loading condition, position of propeller and so on _{Later, reports have been published}

describing unusually stable characteristics on course at small rudder angles which were observed on free running models with

high block coefficient2

Such an unusual phenomenon causes a serious problem in model testing, since the existing model-ship correlation data will not he applicable to such unusual model test results and comparison of model data without special attention to such a phenomenon would lead us to an erroneous conclusion.

It _{is as early as in 1960 that Nagasaki Experimental Tank}

discovered an unusual phenomenon in propulsion tests on a model of supertanker. _{Since then extensive investigations have been} made to elucidate the phenomenon :nd to establish a method to cope with it. _{Some of the results of them have been published} under a title with 'unstable phenomenon'

34x5)

_{because the} unusual phenomenon in those days occurred in a very unstable manner. _{With further increase in fullness of ship model, there}

appeared those on which the unusual phenomenon occurred _{in a} rather stable manner, and in the meantime publications came to hand informing of unusual phenomenon in _{course stability of full} ship models.

Based on the consideration that there are common character-istics in the both unusual phenomena, investigations were made

into the mnoeuvring motions of _{a model on which the unusual}

h;il i hi i a (Jo'e relat tonshp between the both unusual

hrrir,;i ru;,

r;u t (rIZ(:d ;J

a change ir, flow

_around

the stern induced hy act ion of propeller.

lt is hard to say, however, that the phenomenon has been completely elucidated, but there is much room for further in-vestigations. This paper outlines the investigations carried

out in Nagasaki Experimental Tank and some comments on the study

to be made in the future. It is hoped that this paper will help to understand the phenomenon in testing full ship models and to find where to lay emphasis in the future investigations.

2. Unusual Phenomenon in Propulsion Tests

2.1 Discovery of the Phenomenon

In the late 1950's efforts were concentrated in Nagasaki Experimental Tank on improving the reliability of self-propulsion tests, to cope with the trend of increasing fullness of super-tankers; investigations were made into the effect of variation in

advance speed of ship model, method of adjusting revolution of

propeller and so on. Tn spite of such efforts, there arose cases in which scatter of measured data could not he explained by

measurement errors. Fig. 1 shows an example of them. A number

of test runs were repeated especially in the range of 0.17 - 0.21

in Proude number, but the measurements resulted in considerable

scatter. _{Eventually it was discovered that thrust and torque of}

propeller changed from one set of values to another during a

single run, and besides disturbance of water surface at the stern changed, though very slightly, with the change of thrust and

torque. _{Measurement of side forces acting on the fore and aft}

guides showed that the direction of the force on the aft guide changed simultaneously with thrust and torque as shown in Fig. 2.

Such a discrete change suggested to us the existence of two kinds of flow.

-2-Analysis of the test results of Fig. I was made very

care-I y by ca leu lat i ng i SOt uf sel f-prujuilSion 'act ors br each

test run,

hut

t ;ik ing simiO y t he mean curves through t lie measured

values. Attention was paid especially to those runs i n which a

discrete change in thrust and torque occurred; for them two sets of self-propulsion factors were calculated for higher and lower values of thrust and torque. As a result the apparently

scatter-ing data were reduced to the two sets of wake fraction as shown in Fig. 3, thus indicating the exist,ence of the two kinds of

flow.

One th i ng to he noted is t hat t h rust deduct i on fract i on i n

the above results might he affected by acceleration of the model duc to change in thrust. An attempt was made therefore to make clear this effect by a jropulsion test on the model captured by resistance dynamometer, and the results were almost the same as those obtained by the self-propelled model. lt is supposed from this comparison that resistance as well as thrust changes with the kind of flow at the stern.

2.2 _{Further Evidences for the Existence of Two Kinds of Flow}

in the course of continual effort to develop more economical ship forms of tankers and hulk carriers, more evidences for the cxi stence of two kinds of flow have been found, among which _some typical examples are shown in Figs. 4 - 8.

Figs. 4 and 5 show an cxampI in which the two sets of experimental values are shown more clearly than the case in Figs.

1 - 3.

Fig. 6 shows another example showing how the phenomenon is sensitive to variation of circumstances. In this figure it is

shown that in 65% load condition two kinds of wake fraction were obtained at propeller position A, while at propeller position B, 15mm forward of A (for 6m model) the wake fraction lies on a

i ng I e cii rye which co i nc i des nearly w i

tlì t he h i gher val ties

oht i

i n

-od w it li P rope I i i' pos i t I un A - Ou t he ot lier mind

;itch a

lion did not

appear i n ful

load

cond i t ion; wake fract i oui iena i

the same irrespective of propeller position.

In general there is certainly a tendency that wake fraction increases with a forward shift of propeller, hut the difference in 65% load condition is too large compared with the existing data and with the results of the same model in full load

condi-tion. If we assune that the unusually high wake fraction result-ed from separation of flow which is not suppressresult-ed by suction due

to propeller, influence of the propeller on the flow field would

he

larger when positioned at B than when positioned at A rearward of B, which however is contrary

to

the experimental results.

It

i s supposed therefore that the difference may he ascribed to a substant ial change of flow field triggered by the act ion of

propeller.

Figs. 7 and 8 show the test data of systematically varied hull forms, which were obtained by the investigation conducted as a program of SR 61 Committee of Japan Shipbuilding Research

Association. In 65% load condition (Fig. 7) many models show two kinds of wake fraction as denoted by unstable phenomenon. By

combining either higher or lower values, two sets of curves can

he drawn in an acceptable manner. In full load condition (Fig. 8)

each model has a single value of wake fraction, but it would be

reasonable to arrange the plots in two groups as shown by two curves in the figure.

With suieh evidences for thc existence of

two kinds of flow,

further invest igat ions into the unusual phenomenon have been conducted. In 12th ITTC it was proposed to call the flow giving unusually high wake fraction S type, while the one giving normal

lower values F

type6

Since then these notat ions have been used for classification of type of flow at the stern.

. 3 Ident i ficat ion of the Unusual Phenomenon

E f t he unusua ¡ phenomenon observed on a mode i appc'a rs on t ht

ship as well , there i s litt le to hot her t hose who a ic eivacd i n

model test ing in experimental tanks. lt is not probable, however,

that the unusual phenomenon appears on ships. It is therefore a serious problem in model tests to identify the unusual ithenomenon. For example, for the two sets of results shown in Figs. 4 and 5, difference in power amounts to 10% and accordingly difference in revolutions of propeller to 3%. If two kinds of flow appear in the tests, the identification would be rather easy, but in

general it should he assumed that either of the two appears. in the beginning of the study on the unusual phenomenon, an

attempt was niade to measure the side force act ing on the tow i ng

guides, since side force at the aft guide directed toward star-hoard as shown in Fig. 2 in case of flow type S.

Later it was found to he more convenient to conduct propul siofl test with propeller loading varying from zero thrust to model point of propulsion. Fig. 9 shows an example of the test results in which it can he seen wake fraction for F type

decreases remarkably with increasing propeller loading, but wake fraction for S type flow remains almost unchanged against the propeller loading. Now it is the practice in Nagasaki

Experi-mental Tank to carry out such propeller loading tests for ship models with block coefficient over 0.75 for each displacement and

t r i n at one speed, usual ly the one correspond i ng to serv ice speed

ut t he ship.

Variation of wake fraction in F type flow with propeller loading almost coincides with thai of nominal wake calculated by

taking the effect of propeller suction on wake into considerationY Therefore F type may be regarded as a normal type flow, while S

type seems to involve peculiar phenomenon.

>iJih t

ed , hut Wake d i st r i hut i on in t he p lane of prope li er

I i I

ii, I r,

ç J a s j fi cat i on o f i he flow type w i li he made by

comparison of effective wake fraction obtained by propulsion test and nominal wake fraction calculated from the results of wake

survey. This is based on the consideration that the nominal wake fraction generally agrees well with the effective wake fraction obtained by the propeller loading test at zero thrust, therefore wake survey may be a substitute for propeller loading tests.

This method is useful especially when analyzing the data obtained

before the Present practice of propeller loading tests was established. Fig. 10 shows the correlation between effective wake and nominal wake fraction. It may well be assumed that the group denoted by 'full ship' corresponds to F type flow and the one denoted by 'extremely full ship' to S type.

3. tinusual Phenomenon in Manoeuvring Motions of Models

Despite the common tendency that the directional stability of full ships decreases with increasing fullness of afterhody, sorne full ship models show unusually stable characteristics on course in a range of smal[ rudder angles. With the background of the extensive investigations into the unusual phenomenon in

propulsion tests, it was considered that the types of flow around stern play a significant role in the manoeuvring tests as well.

Especially the lateral force found in propulsion tests in S type

flow suggested a close relationship with the unusual phenomenon in manoeuvring tests.

As a first step, therefore, a 5 meters model which is

geo-metrically similar to M.1592 (6m in length) mentioned in the

()

previous chapter (Figs. 6 and 9) was chosen for manoeuvring tests;

the length of the model was reduced for convenience of handling a

free running model. _{The model was fitted with a normal rudder} with an area of 1/70 of lateral area of the model. The rudder

-6-area ratio was chosen rather small to facilitate the appraisal of

characteristics of the hull.

Previous to the manoeuvring tests, propulsion tests were made to identify the type of flow around the stern, in the tests unstable phenomenon occurred showing the instability between S and F type of flow. it may he supposed, however, that with slight drift angle which is almost always present in manoeuvring motions, the type of flow will tend solely to S type. As an attempt to change the flow type, a pair of stern fins were there-fore fitted. The test results are summarized in terms of wake fraction as shown in Fig. 11.

For both of these stern configurations of the model,

manocuvring tests were carried out. Results of turning tests arc shown in Fig. 12. lt is clear that without the stern fins the rate of turn is extremely small for the rudder angles smaller than 8 degrees. On the other hand with the stern fins the un-usually stable zone was reduced to about 1/3.

In order to find the cause of such a difference, captive

model tests at drifting angles were conducted with the propeller

running. The turning moments N (total-less-rudder moment) measur-ed in the tests are plottmeasur-ed to the base of drift angle as shown in Fig. 13, where N and being defined to be positive for turn-ing starboard. Since the tests were conducted with a left hand propeller, the turning moment at = O results in a positive value, which corresponds to the results shown in Fig. 2 with right handed propeller. lt should be noted that without fins there appears a discontinuity of turning moment at 1.5°,

wth

while this discontinuity disappears when fittedAthe stern fins. The unusually stable characteristics on course as shown in

Fig. 12 may he explained as resulting from the unusual turning moment with the discontinuity in Fig. 13. Namely, for small negative ¿ , when turning to port, the rate of turn is suppressed

by resist ing moment N> O until the drift angle ' reaches -l5° where the re i St i ng TfloIflCflt di mi n i shes. On t he other hand for

small positive , when turning to starboard, the rate ei turn

increases in the initial stage by the help of positive turning

moment. However, with increasing drift angle the turning moment becomes negative, thus increase in the rate of turn is suppressed. A more detailed analysis based on the equations of motions

endorses the relation between the turning test results and the

captive model test results.8

The cause of such a peculiar behaviour of the turning moment was investigated by measurement of forces normal to rudder,

propeller bearing force and pressure on the hull near the stern.

As a result it was found that the turning moment comes mainly

from the difference of pressure distribution on both sides of the hull which is induced by action of Propeller. Fig. 14 shows an example of difference of pressure coefficient on both sidos. Mean values of pressure differences at the points indicated by

black mark in Fig. 14 were taken as the representatives, and the

difference between those obtained with and without propeller was plotted against This is shown in Fig. 15 in contrast to the difference of turning moment N measured with and without

propeller. From the similarity of both curves it may he conclud-ed that the unusually stabilizing moment is due to pressure

dis-tribution over hull near the stern induced by propeller operating

in S type flow.

4. Some Notes on Further lnvestigatiDns into the Unusual Phenomenon

In concluding this review on the investigations undertaken in Nagasaki ExperimQntal Tank, some comments and recommendat ions

based on our experiences are g i ven in the fol lowi ng, which

are

considered to he worth attention for the future works on I i i s

-8-'1. Iiindarrìrnta Study on the Phenomenon

The character of the unusual phenomenon has been made clear to a considerable extent, hut fundamental studies are necessary to reach the essence of the phenomenon. Useful information will

he obtained by detailed measurements of the flow field around the

stern. The measurements carried out recently suggests that attention should be paid to a region above propeller as well as

forward proximity of propeller9

4. 2 Unusual phenomenon in Various Model Test s

lt is considered that the unusual phenomenon first discover-ed in propulsion tests appears also in other kinds of model tests.

For manoeuvring tests investigations were made on one model at one load condition at which a typical unusual phenomenon appeared. Further collation of such data is necessary to draw a conclusion regarding the relation with propulsion test results.

It is also probable that the phenomenon appears in measure-ments of propeller vibratory forces and propulsion tests in waves.

Careful examination of the measured data will he necessary for

those kinds of tests on which effect of type of flow has not been investigated.

4.3 Model-Ship Correlation

Prediction of full scale performance from model test results with unusual phenomenon is a serious problem. Various approaches to this problem are made including improvement in model testing techniques; a proposal is made to apply nominal wake fraction with the effect of propeller suction instead of effective wake fraction, and modification of type o flow by fitting a pair of

b

i mure d i rc:ct approach can he made, however, i f ship dt

:ihl e (,fl

whrr;e

model the unusual phenomenon appeared.

Recent experience on a IJLCC result i ng i n propeller revolution

much higher than cstìmated may be an example of

thcm0

on which

it is advisable to make detailed analysis and further examination. Collation of such data and analysis of them are planned also as a program of SR 159 Committee of Japan Shipbuilding Research

Association.

5. Acknowledgement

It is about 15 years ago that the studies on the unusual phe-nomenon were initiated in Nagasaki Ixperimental Tank. Since the discovery of the phenomenon the problem has been examined with growing interest in accordance with the requirement to develop more economical ship form. For the investigations introduced in

this paper the authors are indebted to all the members of the

Nagasaki Experimental Tank to their contributions made in each field of activities.

References

I )

K. Watanahe,

"Unstable Phenomenon in the

Self-pr I s i on Test s of Ful i Ship Models," (in .Japanesc) Mit subi shi

Technical Review, Vol.4, No.4, 1967.

K. Nomoto, "Unusual Scale Effect on the Manoeuvrabi1ity of Ship with Blunt Bodies," Proc. of 11th TTTC, 1966.

K. Watanahe, "Unstable Phenomenon in the Self-propulsion Tests of Full Ship Form Models," (in Japanese), Journal of S.N.A. of Japan, No.126, 1969.

K. Watanahe, "unstable Phenomenon in the

Self-propus lion Tests of kill Ship Form Models," (in Japanese , Proc.

of 12th [FTC, 1969.

K. 'l'amura, "Speed and Power Prediction Techniques for High Block Ships Applied in Nagasaki Experimental Tank" STAR Symposium, SNAME, 1975.

K. Taniguchi, "Three Types of Flow Pattern at Sterns of Ship Models," Introductory Statement in Group Discussion, 12th

ITTC, 1969.

T. Nagamatsu and T. Sasajima, "Effect of Propeller suction on Wake," Journal of S.N.A. of Japan, No.137, 1975.

F1. Tagano and S. Asai, "On the Unusual Phenomena in

Manueuvri ng Mot ions of a Full Ship Model ," Nlitsubi shi Tec hni cal

Bulletin (to he published).

G. Dyne, "A Study of the Scale Effect on Wake,

Propeller Cavitation and Vibratory Pressure at Hull of 'Iwo Tanker Models," Trans. of SNAME, 1974.

G. Niisson and K. Restad, "Problems in Full Scale Propulsion from a Shipbuilder's Viewpoint," 3rd Lips Symposium,

Io

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Self-propulsion factors (Model-A)

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0.150

0.175

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Self-propulsion factors (ModeL-B )

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Fig. 6

Scattering of effective wake obtained

from repeated self-propuLsion test

(M. 1592)

0.14

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018

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0.22

0.70

0.60

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c

i

by dotted line show the

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occurrence of unstable p

o

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WTjE=o.ie

Two values of Wm connected

henomenon

1753

0.60

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Lpp

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Fig. 7

Relation between Effective wake fraction ( Wm)

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1658 60

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I

CB Series

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59'

o

o

0.60

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8

_{ReLation between Effective wake fraction (Wm)}

and Fullness of Aft- body

L/B Se ries

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I I I I

o

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i

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4

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Fig. IO

Relation between effective wake

and nominal wake

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I I

Comparison of

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With fins

Without fins

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With fin

Fig. 13

_{Turning Moment Measured in Captive Tests}

with LH PropeLler at

I

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Difference of Pressure Distribution between

Port and

Starboard Side

(with LH PropeU.er

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I 5

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