On the importance of rudder and hull influence at cavitation tests of high speed propellers

MEDDELANDEN FRAN

STATENS SKEPPSPROVNINGSANSTALT

(PUBLICATIONS OF THE SWEDISH MARITIME RESEARCH CENTRE, SSPA)

Nr 88

GoTEBORG

1980

ON THE IMPORTANCE OF RUDDER AND

HULL INFLUENCE AT CAVITATION TESTS

OF HIGH SPEED PROPELLERS

by

OLLE RUTGERSSON

Paper presented at the

High-Speed Surface Craft Conference

Distributed by:

Liber Distribution :-S-162 89, VALLINOBi'

ISBN 91-305977-6 ISSN 0373=47i4.

CONTENTS PAGE SUMMARY 2 INTRODUCTION 3 NOTATION 3 WORKING CONDITIONS 4 3.1 WAKE FIELD 5 3.2 CAVITATION NUMBER 8 3.3 PROPELLER LOADING 10 3.4 CAVITATION DEVELOPMENT 11

INTERFERENCE BETWEEN PROPELLER AND HULL 16

4.1 INFLUENCE OF VERTICAL CLEARANCE 16

4.2 INFLUENCE OF CLEARANCE TO RUDDER 17

4.3 INTEGRATED EFFECT OF RUDDER AND HULL 19

4.4 PROPELLER-INDUCED PRESSURE FLUCTUATIONS 20

SSPA LARGE CAVITATION TUNNEL 23

5.1 DESCRIPTION OF THE TUNNEL 23

5.2 TEST TECHNIQUE 25

5.3 RESULTS OBTAINED IN THE TUNNEL 26

5.4 ROOT EROSION 26

CONCLUSIONS 29

ACKNOWLEDGEMENTS 30

REFERENCES 30

2

SUMMARY

Models of high-speed displacement ships and matching propellers have been tested in towing tank and cavitation tunnel. The re-sults are used to define the working conditions for high-speed propellers in quantities as wake, cavitation number and propeller

loading.

The typical cavitation development on a high-speed propeller is also discussed and compared with the cavitation development on a container ship propeller.

Results from cavitation tests with high-speed propellers working behind first simple plate arrangements and then complete ship models are given. The influence of propeller hull clearance on

the propeller characteristics and the pressure fluctuations indu-ced on the hull is discussed.

Finally it is concluded that the complete propeller arrangement with the correct clearances to hull and rudders should be used at cavitation tests of high-speed propellers in order to ensure re-liable predictions of prototype performance. Equally important are realistic estimates of the input parameters cavitation num-ber and propeller loading.

1. INTRODUCTION

Water propellers used for propulsion of high-speed surface craft are usually arranged with:

shafts inclined relative to the flow propellers rather close to the hull rudders behind or beside the propellers

These deviations from homogeneous flow represent interference sources which influence the propeller performance. In spite of this fact cavitation tunnel tests for high-speed craft are often carried out in homogeneous flow without hull and rudder and with assumed values of propulsion parameters such as wake fraction and cavitation number.

It is the purpose of this paper to discuss deviations in results obtained at cavitation tests with a very simple model and tests with a more complete model with rudder and hull and with

propul-sion parameters estimated by measurements.

2.

NOTATION

IC Ap/0.5pV2 = pressure coefficient

propeller diameter (m) vapour pressure (Pa)

FN V

/,45E

= Froude number

acceleration of gravity

0 draught of propeller centre at zero speed (m)

VA/nD = advance ratio of propeller

0 advance ratio at shock-free entrance

2Ap/pD2n2 = amplitude coefficient, pressure fluctuations

KT T/pD4n2 = propeller thrust coefficient

ship length between perpendiculars

4

n. number of revs- (r/S)

static pressure at propeller centre v(Pa)

P0 static pressure in undisturbed flow (Pa)

Ap p - 130 = static pressure difference (Pa)

.Ap single amplitude, peak value, of pressure fluctuations (Pa)

radius of. propeller blade section 010 B/2 (m)

T' propeller thrust (N)

ATA sinkage of ship stern (m)

VA advance speed of propeller (m/s)

ship speed (m/s) S

Vx V /I +A (xTr./J)2 F.- inflow velocity for blade section at

xR

(m/s)

(Vs- - VA)/Vs = wake fraction

WT mean wake obtained by the method of thrust identity

r/R

0 blade position angle (degrees)

density of water (kg/m3)

aVA (p - e)/0.5pVA2 = cavitation number

ax (p - e)/0.5pVx2'= cavitation number

3.

WORKING CONDITIONS

The environmental conditions influencing'the mirk of:the-propel-lers can be discussed under the following subheadings.:

wake field

cavitation number propeller loading cavitation development

3.1

WAKE FIELD

Due to the presence of the hull the water speed at the propeller disk is not usually constant and has not the same mean value as the ship speed. The relation between the speed at the propeller disk and the ship speed is defined by the following equation:

VA ,

= - w

Vs

(1)

When analysing the influence on propeller performance it is con-venient to split up the wake field into:

circumferential variations of axial wake mean value of axial wake

mean inflow angle relative to propeller shaft

The viscosity has a dominant influence on the wake field of full ship forms. On these ships the propeller works partly inside the boundary layer, which causes two characteristic effects (see for example Johnsson & SOntvedt [1]):

large variations in the circumferential wake field water speed always considerably lower than Ship speed

On a high-speed craft the propeller normally works outside the hull boundary layer. Disturbances from inclined propeller shafts and brackets will, however, create a peak in the circumferential wake distribution, as shown in Fig 1. As the'wake peak usually is very narrow the mean wake is mainly determined by the potential flow and is considered to be low

(VA

a

V) on high speed ships.

In Fig 2 the effective mean wake for a twin screw patrol craft is given as a function of ship speed. It is shown that the assump-tion VA = VS is not valid for the speed range 0.3 < FN < 0.9

where the speed of advance is considerably (up to 12%) higher than

the ship speed. That is we have a negative wake fraction. Negative mean wakes have also been shown by Hadler & Cheng [2].

The wake of a high-speed craft is sensitive not only to

6 " Radius .055rn Radiuso035ti 1-W1.= VA/Vs 115 , -Water spired 7.0m/s Starboard OS .0 7 Fthude number Na -21L V-91 09

influence of ship speed on effective

meAn.wake-lee 270 .BtadepasiVan

Top Outward

err 180.1

-Fig 1 Axial wake measured in cavitation -t'Unnel..

110

105

-1.10 1.0 11 9 7

er

/

:)'

\o/

IrShaft inclination

,by relative to ship

c7/

-0 2 4 6

Trim angle (degrees)

Fig Influence of trim on axial wake and inflow angle

tions in speed but also to trim angles, as shown in Fig .3. A larger trim by the stern gives higher speeds at the propeller disk and also a larger inflow angle relative to the propeller shaft. The measurements were

carriO:a

out on a model of a:high-speed displacement ship at FN = 0.55. The inflOW angle measured by a 5-hole Pitot tube is shown to be larger than eight degrees, which is the angle between ship bottom and propeller shaft. When the ship is trimmed by the stern the inflow angle increases

by

about one third of the rate of the increase in trim angle.

The characteristic features of the wake field of a high-speed craft seem to be:

o the mean wake is often negative (1 - w > 1)

6 the mean wake is sensitive to speed and trim variations

The performanCe. of the

is highly dependent of peller. The basis of a type Mutt therefore be

number:

PO e

0/21,y

'The-static_ pressure "at the propeller Centre is ususlly;calcUlated

as the sumCfl:thepxessure from thepropeiler,centre up tothe

free surface and the atmospheric pressure- The wave pattern

_in

tne.case

of

a

high-speed craft, however, makes the estimation of

the static pressure more complex. In first place it is not

obvious which surface should be used for the calculations Sec-ondlY the potential

flow

_{:induces pressures, which in some cases}

-0.05

Trim angle (degrees)

2 4

A P P-1:10

W

Po

4MM

Fig Influence- of trim on static pressure

3,2_

CAVITATION NUMBER

propulsion

system

Of a-high-speed draft the development Of cavitation _On the pro good correlation between model and proto-a reproto-alistic estimproto-ation of. the cproto-avitproto-ation

Approximations

&WO.

Cp=0,w=0 Cp =O. w=0 w=0. Cp ace. to Fig.4 w acc. to Fig.3 -'-2.0 1. 05 2 4

Trim angle (degrees)

Static pressure

Wake

Fig 5. Influence of trim, static pressure and wake on cavi-tation number at the propeller centre

cause important changes of the cavitation number. Figs 4 and 5 show some measurements and calculations carried out for a high-speed displacement ship. Fig 4 gives the pressure difference between static pressure at the propeller centre and the free stream pressure at the same depth. This difference is shown to increase with increased trim. The absolute static pressure at the propeller centre is obtained by adding Ap of Fig 4 to the atmospheric pressure and the pressure of the water column from the propeller centre to the undisturbed water surface. In Fig 5

different ways to calculate the cavitation number at the propel-ler centre are compared. _{Following expression and methods are}

101300 + pg(H0 + ATA)

Cp aVA

-p/2V 2(1 - W)2 (1 - 41) 2

Method No 1: In this very simple method Ho (= depth to the pro-peller centre at Vs = 0) is used for calculating

the static pressure. w is assumed to be zero. Method No 2: As method No 1 with trim change at the stern ATA

added to H0.

Method No 3: As method No 2 but with static pressure corrected according to Fig 4.

Method No 4: As method No 3 but with wake fractions according

to Fig 3.

The differences between the cavitation numbers shown in Fig 5 are large enough to give considerable differences in results at cavitation tests. Further, different tendencies are obtained in the variation of cavitation number with change of trim by the simplest input methods and the more rigorous method.

3.3

PROPELLER LOADING

The propeller loading is also a fundamental parameter for the cavitation test. The estimation of the propeller loading in-cludes all the traditional towing tank problems:

towing tests for measurements of resistance of hull and appendages

self propulsion tests for determination of the thrust deduction factor and wake fraction

calculation of propeller loadings by the use of scale factors and correlation factors, empirically estimated on the basis of earlier experience n-p2vA 2 " 10

For most merchant ships the cavitation does not develop so far as to influence the propeller characteristics. The self

propul-sion tests in the towing tank can therefore form the basis of the prediction of power and number of revolutions at different speeds. The purpose of the cavitation tests is for most merchant

ships to check the erosion properties of the propeller and to measure the vibration excitation forces and the noise generated by the propeller.

The characteristics of a high-speed propeller are, however, very much influenced by cavitation. Further, this influence is differ-ent in constant flow and when the propeller is working behind the hull (as showed later on). Thus both propulsion tests and cavi-tation tests are necessary for a reliable power prediction. In the latter case the tests have to be carried out in behind

con-dition.

The purposes of the different tests are:

open water tests in towing tank give the relation between thrust and advance coefficients and form the basis of cal-culation of effective wake fraction

propulsion tests in towing tank give propeller thrust and wake for different ship speeds

cavitation tests in behind condition give the relation be-tween thrust, torque, efficiency and number of revs at cavitating conditions. Input parameters are the cavitation number according to Eq (3) and the propeller loading

accord-ing to Eq (4)

3.4

CAVITATION DEVELOPMENT

The dynamic behaviour of propeller cavitation is responsible for many propulsion problems experienced on all types of ships. Erosion, vibrations and noise are found on both merchant ships and high-speed craft. The problems are the same, but the methods of solving them are often different, depending on differences in

12

0-90 180 270° Top

--gottom\

z

OCn° DIUUC 4UU position Top 360° Blade position

Fig 6. Comparison of profile angle of attack for the pro-pellers on a container ship and a patrol craft

cavitation development. In Fig 6 the working conditions are com-pared for the propellers of a 30 knots container ship and a 30 knots patrol craft. The diagram in Fig 6 shows the variation in profile angle of attack versus blade position for two different blade sections (blade tip and root). The angles have been

cal-culated by the method used in [3] starting from measured wake fields and propeller characteristics. For the container ship the variation in angle of attack for both tip and root is very

simi-lar to the circumferential wake variation.

Also at the blade tip for the patrol craft the variation is simi-lar to the wake variation, that is, very small amplitudes. The variation at the blade root is, however, very much larger. This

is due to the oblique flow, which gives this somewhat surprising effect, earlier explained by Rader [4] among others.

\

\

Container ship

/

/

\

I

/

Bade_mat

\

\

II

_/

\

_/

\\1,31/tde

tip

A

//

/ \

... Bla

...i---

e tip

Blade root / Patrol 90°

craft propeller

18 270" 0

.2

a 4; 0

-2

4 2

Profile angle of attack (degrees)

Fig 7, Cavitation "buckets' and profile loading lines for a

, container ship propeller

-Cp min and (Tx -Cpmin and Tx 13 1.5 Blade root 11-03 ir1180° 910° to 270°

1111111i1111M

11111111.

IN

WM 14'

=0

180° -,

B 4 2 0 -2 -A 0.75 0.5 Blade root

0° MIMI

, 270°a-=0.3 P=90° 0.25 :lade tip 900 4 f=0° 180° 4 2 0 -2

Profile angle of attack (degrees)

Pig 8. Cavitation "buckets" and profile loading lines for a patrol craft propeller

The cavitation development caused by these profile angles can be understood by the use of Figs 7 and 8. Here the pressure minimum for the profiles of tip and root of the propellers considered are given for different angles of attack. The curves are reproduced from Valentine [5]. The angles of attack from Fig 6 are then given at the local cavitation numbers in question. By the use of the following simple relation (scale effects are not considered) cavitation inception can then be predicted:

-C

_{p min}

.=a

14

(5)

This means cavitation-free zones inside the "buckets". Eq (5) gives for the container ship propeller a completely

cavitation-free blade root and a very dynamic cavitation at the outer part of the blade. The cavitation at this part of the blade changes, as shown in Fig 9, from heavy back cavitation to face cavitation through almost cavitation-free positions. Erosion-,vibration- and noise problems are of course caused by this dynamic cavitation on the outer part. Efforts to reduce these problems can be made on the ship lines (in order to reduce the wake variations) or on the propeller design (unloading of blade tip or blade skew, see

Johnsson [6]).

The cavitation of the patrol craft propeller has a completely different appearance. Fig 8 shows that the blade root has a dynamic cavitation (similar to that of the blade tip of the con-tainer ship propeller). On the blade tip a stable cavitation with a moderate variation in extension with blade position develops

(see Fig 10). For the patrol craft propeller the risk of erosion is obviously at the blade root. Vibrations and noise are induced mainly by the blade tips (as they are close to the hull). This

stable cavitation at the tips will, however, not cause as large cavitation amplification of the pressure fluctuations as the cavitation of a merchant ship propeller.

Face cavitation Face cavitation 270° 0' 180° 90°

Fig

10. Cavitation pattern. Patrol craft propeller at

V = 30 knots 15 ship -propeller at Cavitation pattern. V = 30 knots Container

Fig 9.

4, INTERFERENCE BETWEEN PROPELLER AND HULL

On most ships propeller, rudder and hull are close together. Accordingly interference effects are unavoidable. A survey of the

classical interference factors as thrust deduction and wake frac-tion for various types of high-speed craft is given by Wilson [7]. The interference effects that will be discussed here are:

influence of clearances to rudder and hull on the propeller characteristics of a cavitating propeller

influence of vertical clearance on the propeller-induced pressure fluctuations

4,1 INFLUENCE OF VERTICAL CLEARANCE

In the high-speed test section of the large cavitation tunnel at SSPA systematic tests have been carried out in order to investi-gate the influence of vertical clearance on propeller thrust and

torque.

The propeller being located in the ordinary right-angle dynamo-meter (can be tilted to shaft inclinations - 2° < a < + 15°) a horizontal plate arrangement with pressure transducers was moun-ted at different clearances relative to the propeller. Results from tests with a supercavitating propeller using this set-up have earlier been given by Rutgersson [8]. They are reproduced in Fig 11 together with results for a wide-bladed conventional propeller. The influence on thrust and torque at clearances about az/D =

0.2 - 0.3 is shown to be of the same order of magnitude for the two propellers. When the clearance is increased the influence is reduced more rapidly for the conventional propeller. This is prob-ably due to the smaller extent of cavitation on the conventional than on the supercavitating propeller. The effective wake at these tests was calculated with the use of the thrust identity of the cavitation-free characteristics. This means two assumptions:

no effect of clearance at the cavitation-free conditions no effect of cavitation on the effective wake

KTo KTo

0.9

Vertical clearance az/D

K CIO

as

-Supercavitating propeller AD/A0=0.6 40=0.85

--- Conventional propeller

AD/A0=1.05 3/40=0.85

Fig 11. Influence of vertical clearance on measured thrust and torque at J/Jo = 0.85

Fig 11 shows that thrust and torque are reduced by about the same magnitude. The influence of clearance on the efficiency is there-fore rather small. These factors are, however, very important for the choice of correct propeller pitch. If the influence of clear-ance is neglected the propeller is most likely to be underpitched. This is also the full scale experience of Blount & Hankley [9], who have given empirically derived corrections for two different propeller types.

4.2

INFLUENCE OF CLEARANCE TO RUDDER

The propeller and rudder have a mutual interaction, the propeller influence on the rudder characteristics being the most well-known of these. When the rudder is mounted close to the propeller its influence on the propeller characteristics is, however,

consider-17 0=118

/

/

al

0.6 G;a8

//

N '

//

Vr

/

'

c as

/

10 0.5 1.0 Vertical clearance 0.5

18 1.15 KT with rudder KT without rudder 5=0.6 0..0.8 ern Ka with rudder Ka without rudder -110 with rudder 110 without rudder 1.0 1.5 1.0 1.5 1.0 1.5 d/D d/D d/D Rudder clearance

Fig 12. Influence of rudder clearance on thrust, torque and efficiency. Reproduced from [8]

able. In Fig 12 some results from systematic tests are shown, the rudder being mounted on the plate behind the supercavitating pro-peller. The rudder was fairly large, having a height roughly equal to the propeller diameter, a mean chord of about 1.7 times the_ propeller diameter and a thickness/chord ratio of about 10 per cent. As shown in Fig 12, the rudder caused an increase of both

thrust and torque. This effect is more pronounced when the rudder is close to the propeller and when the propeller is cavitating. All tests showed a small increase in efficiency due to the

pres-ence of the rudder, the reason probably being a reduction of the rotational losses in the slipstream.

Tests with two rudders located behind the propeller but outside the slipstream gave results quite different from those discussed above. In this case thrust and torque as well as efficiency of the propeller were reduced by a few per cent.

SHIP A

SHIP B

Fig 13. Influence of arrangement on centre propeller thrust. Reproduced from [8].

4.3

INTEGRATED EFFECT OF RUDDER AND HULL

In order to illustrate the effect of shaft inclination, vertical clearance and rudder arrangement, results of two projects, ship A

and ship B, are given in Fig 13. Both projects were tested in homogeneous flow only. For ship A the reduction of thrust caused by shaft inclination and clearance to bottom is compensated by an

increase caused by the rudder. The results from the simple cavi-tation tests in homogeneous flow therefore seem to agree very well with prototype tests. For ship B, on the other hand, the rudders caused a reduction of thrust, the result being 20% lower thrust

for the prototype than indicated by the simple cavitation tests. These examples show that the complete propeller arrangement with hull, brackets and rudders must be present at the cavitation tests

in order to achieve reliable results.

19

SHAFHNCL HULL RUDDERS TOTAL

100% 100% 100% 100%

98% 92% 109% 98%

4.4

PROPELLER-INDUCED PRESSURE FLUCTUATIONS

The pressure fluctuations induced around the propeller are trans-mitted through the water, thereby affecting the pressure field

around the hull. The mean value of the integrated pressure fluc-tuations on the hull causes the thrust deduction (horizontal in-tegration) and trim changes (vertical inin-tegration). Theeffects of the amplitudes of the pressure fluctuations are, however, vibra-tions and noise. On high-speed craft the pressure fluctuavibra-tions are dominated by the blade frequency. The amplitudes are also fairly stable from one blade passage to another, which _{is not} usually the case for merchant ships, as shown in [3]. The dis-cussion will therefore here be confined to blade frequency mean

amplitudes.

In Fig 14 the pressure fluctuations induced by various 3-bladed high-speed propellers at cavitation-free conditions are shown. The dimensionless pressure amplitude Kp has been related to the

thrust coefficient KT' giving the parameter

KP 2ApD2

KT T

One of the benefits of this parameter is that it appears to give levels almost independent of the advance ratio, when using it for pressure amplitudes for non cavitating propellers. (See for

example the theoretical curve in Fig 15).

Results from theoretical calculations according to a method given by Johnsson [10] and experimental results with both simple models and complete ship models are given in Fig 14. All results are

shown to fall on the same curve when _{K' 'Y} and the vertical

clear-P

ance ratio az/D are used as parameters. Clearance ratio and thrust coefficient are obviously the most important parameters for the pressure fluctuations at lower speeds. One of the practical as-pects of this finding is that increasing the clearance by decreas-ing the propeller diameter is of very little help, when trydecreas-ing to reduce the pressure fluctuations. The reason is that the thrust coefficient KT and thereby also the pressure coefficient Kp is in-creased when the diameter is reduced.

20

Kp 2ap D2 KT T to 0.8 0.6 0.4 0.2 02 OA 06

Vertical clearance az/b

Fig 14. Influence of clearance on pressure fluctuations at cavitation-free conditions

The curve given in Fig 14 also shows that the clearance is a very efficient way to control the pressure amplitude. The effect on the pressure amplitudes is also shown to be larger the smaller the clearance. This is illustrated by the following example. Consider

a ship with a clearance ratio of about 0.2. If the clearance is increased by 50% to 0.3 this will give a reduction of the press-ure amplitudes of about 50%. If, on the other hand, the clearance is reduced to 0.1 this will cause an increase of about 100% of the pressure amplitudes.

In Fig 15 the influence of cavitation on the parameter Kp/KT is shown at a clearance ratio of about 0.2. The theoretical curve and the experimental results obtained with a simple plate

ar-21 0

Theoretical calculations conventional propeller Measurements simple arr. conventional propeller Measurements simple arr. supercavitating propeller Measurements on complete shipmodel conventional propellers a A II a Theoretical curve

06 22 08 0.2 Kp 2p D2 Go6 0

--Measurements on Measurements Measurements Theoretical calculations prototype Oz/D=0.23

on complete shipmodel az/13:0.23 on simple arrang. az/D=0.20

0z/I)=0.20 (r. 0\E\1

\

N...

GIos

N.

. all., Ii: 1.0i- ,

\

-... -...-___. Theoretical (No cavitation) curve 011.8 -06 07 08 1 LI ₀

Fig 15. Influence of cavitation on the pressure fluctuations

rangement show that the amplification due to cavitation is very dependent on the angles of attack of the profiles. At advance coefficients close to the design point (.70) the amplification is very small. At larger angles of

attack-(J/J0 a

0.7) the amplifi-cation is up to 100% for a = 0.6.

In Fig 15 these results are compared with results from tests of

almost the same propeller behind a complete ship model in the large test section. The results obtained with the complete ship model agree very well with results from prototype tests also shown in Fig 15. The differences between the amplitudes measured with these test arrangements are howewer large. The

amplifica-tion is due to the cavitaamplifica-tion being larger and rather complex for

the complete model.. The plate arrangement is obviously to simple

to be used for measuring the total influence of cavitation on the pressure fluctuations.

5. SSPA LARGE CAVITATION TUNNEL

From comparisons of results obtained with the plate arrangement in the large cavitation tunnel with results from prototype tests the following conclusions were drawn:

influence of clearances to rudder and hull is important for the estimation of correct pitch and power for a cavitating propeller

influence of axial wake field and flow incidence relative to the shaft is important for the prediction of the risk of erosion on a high-speed propeller

influence of wake field and vertical clearance is important for the estimation of the pressure fluctuations

These conclusions led up to the decision to manufacture an insert to the low-speed test section of the large cavitation tunnel

1978. Thereby it has been possible to apply a technique for tes-ting high speed craft similar to that developed for testes-ting

mer-chant ships.

5.1 DESCRIPTION OF THE TUNNEL

The tunnel is fitted with two interchangeable test sections, i e

one circular, high-speed test section

one rectangular, low-speed test section, large enough for tests with combinations of propellers and complete ship models

A sketch of the tunnel with the large test section in place is given in Fig 16. The most important data of the test sections are given in Table 1.

24 0.5 0.025 20 15.3 -=1_190'1 .1

Fig 16. The SSPA large cavitation tunnel for tests of complete ship models

High-speed Low-speed section

section

original with insert

Length (m) 2.5 9.6 9.6

Area B x H (m2) diam 1 m 2.6 x 1.5 2.6 x 1.15

Max speed (m/s) 23 6.9 8.8

Min cav number 0.06* 1.45 0.50

*Empty tunnel. At propeller tests cavitation on right-angle gear dynamometer sets a = 0.15 as the lower limit.

Table 1. Main data of test sections of cavitation tunnel 2

The low-speed test section is covered by a recess, in which the ship model is placed. The model is the one used for self propul-sion tests in the towing tank and is usually made of fibre-glass, when testing high-speed craft. The model is placed in the tunnel

with the correct draught at the stem and with the same trim as at the self propulsion test. Individually cut wooden plates are then fitted to simulate a flat free surface and the test section and the recess are completely filled with water.

To drive the propeller models one AC electric motor for each pro-peller is used. Strain gauge dynamometers for measuring thrust and torque are placed in the shafts, close to the propellers.

The operating range covers ship speeds up to 30 knots for the original test section. With the insert the range of operation has been extended up to about 45 knots.

A more thorough description of the tunnel and its background is given in [6].

5.2

TEST TECHNIQUE

The test technique used for cavitation tests of merchant _ships has been found to give reasonable agreement with measurements on full scale ships [1, 3]. This technique had, however, _{to be} some-what modified to the special problems connected with cavitation

tests of high-speed vessels. The main modifications of the tech-nique are:

o the cavitation number is corrected for differences _{in static} pressure and mean wake due to the wave pattern being elimin-ated in the cavitation tunnel

the propeller loading is used as input parameter at the cavi-tation tests instead of the thrust and advance coefficients

o _{the thrust, torque and advance coefficients measured at the}

cavitation tests are used for power prediction for high-speed craft

5.3

RESULTS OBTAINED LN THE TUNNEL,

Tests of high-speed propellers were up to 1978 only carried out in the high-speed test section.. Efforts have been made in the

following areas:

systematic tests of conventional wide-bladed propellers systematic tests of supercayitating propellers [8]

systematic tests of the risk of erosion in Oblique: flow (some results presented by Johnsson [11])

systematic tests of the influence of clearances to rudder and hull (some results shown in this paper and in [8])

Since 1978 several high-speed craft have been tested in the large test section. For one of these projects extensive prototype tests have also been carried out.. The main results of the model

-prototype comparison are:

good .agreement for speed - power - rpm prediction

reasonable agreement for the estimation of pressure fluctua-tions as shown in Fig 15

the risk Of root erosion indicated

by

the model- tests was

con-firmed on the prototype

514 ROOT EROSION

The risk of root erosion on high-speed propellers working on in-clined shafts is very large. This is experienced by most people dealing with high-speed propellers. The reasons .for the erosion are well understood, the primary parameters being the cavitation number and the shaft inclination, as was shown earlier in this paper. A number of ideas of how to reduce this risk have been presented in the literature [12, 13, 14, 15]. Some of these, ideas have been tested at SSPA, e g

Original blade Slightly eroded area

Leading edge Leading edge Modification Nol No erosion Leading edge Modification No2

Fig 17. Root erosion and influence of modifications. Conven-tional 3-bladed propeller, AD/A0 = 1.05

larger hub conical hub

larger fillet radius

with rather disappointing results. Two things were, however, found to have beneficial effects:

reduction of the fillet radius to zero at the leading edge short propeller sections at the root [11] (root erosion is

seldom found on supercavitating propellers or on controllable

pitch propellers)

In order to reduce the risk of erosion on wide-bladed propellers the modifications shown in Figs 17 and 18 have been found

28

Lead! ng

Original blade _edge

Erosion tree

Leading

Modified blade edge

Fig 18. Root erosion and influence of blade modification. Con-ventional 3-bladed propeller, AD/A0 = 0.75

clent. For propellers with a blade area ratio of about 100% the erosion is usually located in the middle of the back side of the root section. A hole drilled through the blade

in

front of the erosion, will reduce and sometimes eliminate the damage. A second

hole, as shown in Fig 17.., is rather hazardous from the fatigue point of view.

Propellers with more moderate blade area ratios _usually get the erosion close to the trailing edge. A cut-away,. as shown in Fig

6.

CONCLUSIONS

The working conditions of a high-speed propeller as well as the hydrodynamic parameters used at cavitation tests are shown to be more complex than they are usually considered:

the wake field (mean wake and inflow angle) is shown to be sensitive to speed and trim variations

the static pressure and cavitation number are changed at trim variations

the influence of clearances to hull and rudders on the pro-peller characteristics of a cavitating propro-peller is con-siderable

the cavitation amplification of the pressure fluctuations is shown to be rather complex

In order to improve the methods of predicting power, number of revs, erosion and pressure fluctuations for high-speed craft certain measures have been taken at SSPA:

the large cavitation tunnel has been modified so that complete models of high-speed craft can be tested

the testing technique has been modified to suit the special problems of these vessels

the calculation methods of the input parameters propeller loading and cavitation number have been developed

Comparisons between model and prototype tests show reasonable agreement using the described methods.

ACKNOWLEDGEMENTS

The author wishes to express his gratitude to the Naval Materiel Department of the Defence Materiel Administration of Sweden for sponsoring parts of the present investigation and to Dr Hans Edstrand, Director General of SSPA, for the opportunity to carry out this study. Thanks are also due to those members of the staff of SSPA who took part in the work.

REFERENCES

Johnsson, C-A, SOntvedt, T: Propeller Excitation and

Response of 230 000 dwt Tankers, Proc 9th ONR Symposium on Naval Hydrodynamics, Paris 1972. See also SSPA Publ No 70 and Report No 79 from Det norske Veritas

Hadler, J, Cheng, H: Analysis of Experimental Wake Data in Way of Propeller Plane of Single and Twin-Screw Ship

Models, Trans SNAME, Vol 73, 1965

Johnsson, C-A, Rutgersson, 0, Olsson, S, Bjorheden, 0: Vibration Excitation Forces From a Cavitating Propeller. Model and Full Scale Tests on a High Speed Container Ship, 11th ONR Symposium on Naval Hydrodynamics, London 1976. See also SSPA Publ No 78

Rader, H P: Cavitation Phenomena in Non-Uniform Flows, Appendix II Cavitation Session 12th ITTC, Rome 1969

Valentine, D: The Effect of Nose Radius on the Cavitation-Inception Characteristics of Two-Dimensional Hydrofoils, DTNSRDC Report 3813, 1974

Johnsson, C-A: Some Experiences from Excitation Tests in the SSPA Large Cavitation Tunnel, Symposium on Propeller Induced Ship Vibration, London 1979, Paper No 4.

Wilson, M: A Survey of Propulsion Vehicle Interactions on High-Performance Marine Craft, Proc 18th ATTC, 1977

Rutgersson, 0: Supercavitating Propeller Performance. Influence of Propeller Geometry and Interaction Between Propeller, Rudder and Hull, Joint Symposium on Design and Operation of Fluid Machinery, Colorado State University, Fort Collins, USA, 1978, Proceedings Vol II. See also SSPA Publ No 82

Blount, D, Hankley, D: Full Scale Trials and Analysis of High-Performance Planing Craft Data, Trans SNAME, Vol 84,

1976

Johnsson, C-A: Pressure Fluctuations Around a Marine Propeller. Results of Calculations and Comparison with Experiment, SSPA Publ No 69, 1971

Johnsson, C-A: Propeller Design Aspects of High-Powered Ships, Symposium on High Powered Propulsion of Large Ships, Wageningen 1974, Proceedings, Vol 1

Lindgren, H, Bjarne, E: Studies of Propeller Cavitation Erosion, Conf on Cavitation, I Mech E, Edinburgh 1974 Taniguchi, K, Tanibayashi, H: Root Erosion Experienced on the Propellers of a Destroyer, Journ of the Soc of Nay Arch of Japan, Vol 118, 1965, Dec

Maioli, P: Fillet Cavitation on Propellers, written contri-bution to Cavitation Session 11th ITTC, Tokyo 1966

Titoff, I, Rousetsky, A, Georgiyevskaya, E: Principles of Cavitating Propeller Design and Development on the Basis of Screw Propellers with Better Resistance to Erosion for Hydrofoil Vessels 'Raketa' and 'Meteor', Proc 7th ONR Symposium on Naval Hydrodynamics, Rome 1968