Propeller cavitation tests at atmospheric pressure

ARCHIEF

SYMPOSIUM ON

"HYDRODYNAMICS OF SHIP AND OFFSHOREPROPULSION SYSTEMS"

HØVIK OUTSIDE OSLO, MARCH 20. - 25.,1977

"PROPELLER CAVITATION TESTS ATATMOSPHERIC PRESSURE"

By

A. Ruiz-Fornells and G. Pérez Goméz Astilleros Espânoles, S.A., Madrid

SPONSOR: DET NORSKE VERITAS

«j

Lib. y. Scheepsbouwkuncle

t

II

,echnische riogeschool

SYPNOS1S

The authors, ôn the ocassion of the annual meeting of the Asociaòi6ñ Española de Ingenieros Navales at Bilbao (Spain) in 1974, put forward the idea of carrying out cavitation tests at atmospheric pressure by means of towing a completely Sub-merged hull. rndel with its propeller, ma conventional towing

tank.

The theoretical basis for this procedure have been generally

accepted already (A. Gorshkof f, ref.clJ, mentioned this

proce-dureon the l4thI.T.T.C. See alsö discussion on ref.L2). It

was,, however, thought that serious practical difficulties

could arisé at the time of òarrying out the tests.

In order to verify ¿he feasibility of the proposed method, Astilleros Españoles, S.A. ordered several

investigation

programs both from S.S.P.A., Gothenburg, (ref.c3J) and from Canal de Experiencias Hídrodinmicas de El Pardo (this last investigation program i still in the development stage). The results obtained so far are very satisfactory, as it has been

verified that with the proposed method an excellent flow

simu-lation is produced and that the cavitation field which dvelo,

ed over the propeller was quite stable and showed a. good corre lation with the rea]. cavitation phenomeña.

tests, whereas the new proposed procedure is dealt with in

Chapter two.

Chapter three contains the results of the investigation.

1. Arguments on sorné of the existing procedures for carrying

out cavitation tests

1.1. Theoretical eaurIn!nS_tO obtain a cornalete similarity

betwéen test and real Ehenomena ..

if only the. influence is considered, which the following physical variables have on cavitation: the acceleration

due to gravity (g), the f lui4 velocity relative to the

propeller (V), the propeller revolutions (n), its diameter

(D), the density of the fluid its capilarity tension (f0)., itS kinematic viscosity (i), the fluid existinq

pressure C?) añd the fluid

vapour

saturation pressure (Pu), It ca be concluded that tests have to be carried out in such a way that the

nendithensiönal

parameters

listed be

low reach the

sarne values

respectively tö those correspon ing t° the real propeller (reet2), Chapter 3):

- Advance coefficient of propeller, J. - Froude Number of the propeller,!.

- Cavitation index, p

P Pv

- Ratio of the absolute pressure to the vapour saturation

pressure of the f lujd,

2

- Weber Number,

2L

fo,

- Reynolds Number of Propéller,

In order to obtain a complete physical similarity, it. is necessary that the veloòity distribution aròund the.

real propeller be similar to that of the model, and

hence equal Froude and Reynolds Numbers should have to be

kept for both the real and model hulls.

1.2. Considerations on the non.dimensional Earalneters which overn.. the phenomenon of cavitation and söme_proce6ures

.sd_t carr

out cavitation tests

To try to obtain in the tests the values corresponding

t the real propeller for all of the mentioned para-.

meters, would lead to contradictory conclusions (see

ref.E4],

Chapter XVII) and it is, therefore, required

,to.make

_a

.seiectior of the.conditionsöf similarity which

have

a greater effeòt On the development of

sa-.

vitation. Ïn

this

respect it can be pointed out the

following:. ..

1.2.1. Hull ProudeNuxnber

up

to the time when the modern vacuum tanks and. free surface circulation tunnels came into use (N.S.N.B.,

Kriloff, S.R.l. Berlin), the hull Froude Number was completely ignored when carrying out cavitation tests.

Dr. Van Manen showed great foresight at the 11th I.T,T.C.

(ref.

Ej)

when he predicted this type of installations

desined so that cavitation tests could be carried out under a tridimensional flow as similar as possible to the flow xisting around the real propeller, trying to correct the errors which were being made at that time in the prediction of the speeds at which.cavita-tioninceptionappears, as well as in the prediction

of its amplitude and distribution.

Dr. Van Manen indicated also the little credibility of

the wake distribution curves which are usually

obtain-ed from wake tests, specially those obtainobtain-ed from models in which the measured speeds are below 0.7 mts. per

second.

q

There is no doubt that this difficulty must be taken into account when using partial flow simulators such

as mesh simulators, diffusors and even duiiuny models.

A very attractive aspect of the new installations mentioned

above is

that by using a similar modélto

the ship in order to simulate the flow, not only are the traditional problems of reproducing the flow

eli-minated, but also the suction

effects originated from the propeller are reproduced automatically with great

er possibilities of credibility, provided the

propel-ler model is tested at the right running point and th hull model Reynolds Number is sufficiently high.

stern area and specially in order to find out the risk Of appearance of propeller-hull vortex cavitation

(ref.[63).

It must be pointed out now, although this point shall be

,deal.t with later on, that the large cavitation tunnels

without free surface such as that of Gthenburg also have the advantages mentioned above, even though lacking the simulation of the free surface effect (thus not taking intoaccount the wake components due to wave formation). This is not essentiäl, however, since the components of thewake due to the formation of waves; both in model and

ship, has relatively little importance.

In any case, the obtention Of a good thrèe-dimensionai similarity of the potential and frictional components of the wake, will, always be better than a deficient two-dimensional reproduction of the nominal values o the. aia1 components of the wake speeds, which is what has generally been done until now, more so when n thé'

majority of cases not only Such two-dimensional reprodc' tion does not resemble the axial distribution of the ship

(as the proper corrections are not normàlly made), but it is' neither a faithfull reproduction of the phenomena

related with the model itself, due to defficiencies

in the checking measurements and also due to the uncon-trolled effects of the propeller suction on the f low'S coming out from the simulators.

which .result in an unnecessarily low hull Reynolds Number,' thus impairIng the Simulation of he wake frictional component.

1.2.2. Froude's law of Similarity for the propeller

The compliance with the Froude's law of similarity

be-tween the real propeller, and its model in the

cavita-tioñ tests (assuming that an acceptable reproduction of

the f lOw in the Stern area has also been reached) has a double:, objective:

To guarantee that the cavitation indexes

correspond-ing to homologous points on both propellers are the

same. (see item. 3.1 of ref.[2J), making the total phenomenon of cav-itatïon developed in the propéller

mödél to be valid for any area afected by cavitation.

It is worthwhile to mention however, that in those

..cases.where the Froude's law of similarity is not

followed, there exists the alternative of simulatiig during the tests various cavitation indexes, depend

ing on the area of the propeller disc'under obser_: vation.. This procedure

has

been fully recognise4 bythe I.T.T.C. (see ref.[iJ).

'To satisfy a necessary but not, sufficient requiremént för a complete physical similarity 'to exist' between

the working conditions of the real propeller and its

tests in such a way that both Froude and Reynold Numbers

corresponding to the real propeller are approached

simultaneously. . .

It must be pointed out that in the case of large free surface vacuum tanks and tunnels, the Froude numbers corresponding to thé hull and to the propeller cannot be Obtained simultaneously düring tests, either self-propélling or cavitation, due to the well known scale

efsfect existing in the effective wake coefficient (alf ferent propeller and hull Reynolds Numbers) and

there-fore it is not possible to carry out selfpropulsion and

cavitatIon tests simultaneously, in these tanks and tunnels.

There seems to be little logic in trying to carry out cavitation tests so that the real propeller Froude

Number is obtained through a partiál dynamic similarity.,

Since it IS more practial to make it a Condition that

the propeller model in thê testS should wörk with the.

same cOefficient Kt Corresponding to the real pröpel

1er,

because

as. lt is well kñown in this manner

we shall

benearer

to

the desired siEilar1ty conditions.

But in this,

case, i.e. when carrying out cavitatIon

tests at the same K-coefficient, it' is not possible either to carry out selfpropelllng and cavitation tests simultaneously, because of similar arguments as

to be carried out at a low Reynolds Number, with the

cons,equent loss of physical similarity between the

be-haviour of the real propeller and its model.

Given the above reasons, the normal practice is to car-ry out cavitation tests under identical Kt and at suf-ficient high revolutions to ensure that the Reynolds number of the propeller is higher than the critical.

1.2.3 Hull Reynolds Number.

This parameter has been ignored up to date by the great majority of the investigation centres when carrying out

cavitation tests.

Emersons' paper on the 13th I.T.T.C. (ref.[8J) is special ly convincing in this respect. He separates the answers obtained from 34 investigation centres Into two catego-ries: those in which they simulate only the axial còmpa nents of ehe wake speeds, and those In which they have tried to reproduce the transverse components by means óf a whole or partial model of tle stern section of the

ship. He clearly points out that none of the answers mentioned the intention of correcting the model flow due to the difference between the model and ships

Fortunately there is at present a great interest on this

subject and even a cavitation

tunnel

has been built (N.S.F.I.) specially designed to carry out experiments in the presencè of a corrected axial wake field.

If the fact that flow is not properly simulated during cavitatiön tests should be a source of worries for Shin yards and Owners, it is really alarming to read the

conclusions

of G.G. COX's paper at the 13th I.T.T.C.

(ref.[9J).To the question of hbw is obtained the Ship's wake field at the propeller position, half the centres answered that the propeller project was carried

out by adapting it only to the model wake field.

It seems oportune to denounce;this pròcedure, and state that the only way to remedy this situation is that the clients of these expezimental centres be more andmore demanding and criticize the large differences existing

between the, bright conclusions of those research projects

carried out by the investigation departments of some

testing centres and the daily work of those same centres.

It is the Authors'

opinion

that cavitation tests should

e carÌed oit se hat the three-dimensional flow

around the propeller model is as similar as póssible

to the real

one.

Taking into account that today the general interest

also in its effect On the exciting fOrces which at

ove the ster zone of the hull, and that therefore thèse.forces are to be predicted, it will be neces-sary to ensure that the relative dynamic

conditions

f the pröpeller model and the. pressure sensors are as similar as. possible to reality.

To achievé this double objective there exist the

fol-lowing possibïlities:

a) Tests with a hull model totally Similar to the ship in a free surface towing tank or circulation

tunnel

and in such

_a

way that the shIp's. Froude Number is obtained.

The Authors think that even wIth 12 in long models (1.5 times larger than normal), the Reynolds Number

is not as high as they would wish, or at least is

smaller than can be btained with Other procedures

tO

be mentiöied later

On. .

this

respect some

textuai.references are given; .herebelöw:

S.G. Bmndel (ref.LlOj): . . .

"among the parameters of similitude, the Reynolds Number seems to be the most important. It is

there-fore recommended to operate at velocities as high

als possible on models of dimensions

as

large as poS sible".

H.G. Lïndgren (Ref.L:

"Contrary to the vacuum towing tank and the

cavita-tion tunnel with free water surface, this method (ca

vitation tunnel without free surface), offers a

pos-sibility to carry out tests at a high Reynolds Number

not limited by Froude Number similarity".

A.S. Gorskof f (Ref.

makes similar remarks to

those of Lindgren. Also, in Ref.ElJthe same Gorskoff

mentions that the great majority of the

investi-gation centres recognise the existance of

unaccept-able deviations in the results due to the non

com-pliance with the Reynolds similarity law during the

tests.

It must .be concluded that the hull Reynolds Number

is an extraordinarily influential parameter on the

cavitation developed over the propeller, and it is

therefore reasonable to use very long models, so

that this parameter be as near as possible to that

of the

hip.

As far as the Authors know, thé N.S.M.B. is at

pre-sent actively working in the development of model

botndary layer suction techniques, in order to try

to approximate more to reality the cavitation

pheno-mena developed over the propeller model.

b) Tests with a model hull completely similar tothe ship ih a cavitation tunnel without fre surface,

and with a high Reynolds Number.

It is weil known that in the majority of ships., he wàke component due to wave making is insignificant.

compared with thé frictional component, so that it is preferable to simulate the later in a mOre real

way, although this involves to ignore the first.,

Most of the flow arriving at the propeller disc is of a viscous nature, and the fluid cannot, there-fore, be considered as ideal. The outside flow with in which the fluid behaves as ideal (including the

surface deformation óf the water due to waves) is

only of interest as far as its direct influence on

the boundary conditions or integration of the boundary layer equation is concerned.(before flow separation), or in other words, because of its iñ-direct Influence on the distribution of the fluid velocities through the propeller disc (this distri

bution being equally dependent, on the propeller hull

interaction). . .

.

When the fluid velocity is increased above that cor-responding to the hull Froude Number, in order tà

increase the Reynolds Number, the pressure

itri-bution in the outside flow will not surely be simi

the boundary conditions of the boundary layer and

of the. wake are changed so that the zone. in which

the fluid acts as viscous narrows itself and, as it will be seen later on from the results of the re-search tests conducted by the Authors, everything

seems to converge reasonably well towards the obten-tion of a viscous flow very similar to the full

scale f 1w. .

Furthermore, said process of convergence, happens in a natural way under the dependence on the hull form, sinçe one of the. boundary conditions Qf the viscous

flow (the solid wall) keeps itself completely slid-l:arto that of the ship. Therefore it should not cause surprise that the aforesaid convergence be

really good.;

H. Takaháshi (réf.[133) shows how the progress made

in the study of viscosity has thade it possible to

relate cavitation inception with the existence of flow separation and 0f turbulence. Furthermore, Ara Kerl (ref. [14]) has investigated the dependence of

cavitation inception on various possible types of

viscous flow.

Once the advantages to be gained from carrying out

tests t a high Reynolds Number are accepted, it should be endeavoured to make it as high as possible

within the limits of the cavitation tunnels,

It is fair to remernber here that cavitation tunnels

offer a very comfortable viewing of the phomena anda long observation time of same (this is parti-. cularly interesting in erosion tests), but on the other hand they have the following disadvantages, as

compared with vacuum tañks:

i) The. water moves in respect of the model, which

is inconvenient for the obtention of a good

dis-tribution of velocities and of a good cavitation. stability.

There exists an effective bloòkage effect.

The reflection of the pressure waves on the

tun-nel walls may influence the magnitude of the pres

sure fluctuations to be recorded during the tests.

c) . Tests with a deformed hull model (dummy model) fitted

with a special appendage, in order to reproduce the axial cOmponents Of the ship's wake field.

it.Is well known that n Ships with high aft end pris-matic coefficient, the influence of the böundry layer

on the wake axial veløcity components and. hence on

the. cavitatIon developed over the propeller, is more. Important (see ref. l5J). This fact has caused some investigators such as Sasajima (rèf. Dyne.

(ref.. c17J ).and HOekstra (ref. [18]), to develo extrem

ly ingeneous procedures to transform the wake axial

velocity components of the model into those of the.

Judging from the references at hand, said methods have proved to be reasonably good to repro4uce in the propeller model cavitation phenomena near to

reality.

Although these procedures may be advantageous to re produce the nominal full-Scale wake field when

comparèd with the other methods described above, since a high waterspeed is not required to get such

reasonable reproduction, there exists in our opinion, the following disadvantages:

i)The simulated velocity field of the ship is based on the wake velocities measured in the model

field, when towing the hull model at a Froude Number corresponding to the ship, with the grave

inconvenience, therefore, that the measured velocities &re not very exact because of their

low values.

The procedures mentioned above are only par.tiar: ly approximate.

The hull-propeller

interaction

in the dummy model

cannot be similar to the full-scale one, as there is no similarity between the nominal wake

poten-tia].

components

of the model and the ship.

Xf measurements are to be carried out of the pres sure fluctuations produced by the cavitation de-veloped over the propeller, the results may have

little similarity with reality as the arrangemeflt

of the pressure transducers relative to the propel 1er, is different in the dummy model as compared

5) The measured values may have been altered due to the reflection of the pressure waves on the

tun-nei walls.

d) Procedure developed by the Authors as described in the following chapter and which may be considered to be an improved combination of the procedures des

cribed in a) and b) above.

1.2.4. Propeller Reynolds Number.

The great influence of this parameter on the propeller behaviour has been known for a long time. At present

a large number of investigation centres accept the fact that cavitation tests should be carried out so that the value of this parameter is not less than 5 x 1O

(ref.[i

Obviously, the higher the propeller Reynolds Number during the cavitation test, the smaller the scale ef-fect associated with the non-compliance with the propel

1er Reynolds law1

Due to limitations in the available installations, the value of the Reynolds Number for the propeller model

is always kept under a certain top value, and the re-suits of the tests will therefore be always influenced by some scale effect. Nevertheless, this scale effect will not be very important if the correlation between

the power-rpm-speed-thrust curves for the ship aid for the model is carried out properly, and the cavitatibn tests àre carried out at K identity conditions.

1.2.5. K coefficient.

According to the points made in 1.2.2., it is impossible

tó achieve a total mechanical similarity during the.

cavitation tests, thereby the advance coefficient of the propeller and the Froude's similarity law are

nor-mally ignored, and it is generally aditiitted that the

maximum mechanical similarity which may be achieved during the tests (arguments related to flow simulation excluded) is to happen when the propeller model works at the same coefficient corresponding to the real propeller

Since the value of the Kt coefficient, at which the cavitation tests are to be carried out, depends on the correlation procedure model-ship tobe used, the

pre-dictions of the cavitation inception and of the amplitude of Its development will be subject to the influence of any

error inherent to the correlatioi procedure.

A. Emerson (refs.c,8]and[19J) and Bindel (ref.E1OJ) have

consistently reported that full scale observations show ed the cavitation developed over the real propeller to be more extensive than that which appeared over the

mode 1, and also a premature

inception

of same (f re-.

quently associated with cavitation i the pressure face). Both cases are dangerous from: the points of

view of propeller erosion and risk Of vibration

exctation.

The differences between the amplitudes of cavitation at full scale and in the model field may be justified

by.the influence of the Reynolds Nuntber and the flow

simùlator that have been used, but the deviations of

the

inception

point may be mainly due to the fact that

the value of 'Kt used in the tests was not cOrrect.

Ïtmust be pöïnted out that when calculating the

coefficient. (with the help of the própeller open water

crves), some cent-res use a ficticious wake effective coefficjent (mean value between the wake effective coef ficients at equal torque and thrust), thus obtaining an:optimist Kt coefficient, frOm the point of view of

"cavitation on the

.ptessure side.

1.2.6. Cavitation Index.

As it is weil known, this parameter has a very sensi-tive influence on the cavitation phenomena, and

there-fore all tests

must be

carried out so that the value corespond1ng to the real propeller isachieved (ac-cording to the area where cavitationmay appear: see

1.2.7. Weber Number.

Inview of the fact that capillary forces are of import ance only in the cases of severe cavitation, this para meter is normally ignored when carrying out cavitation

tests.

1.2.8. Ratio of the absolute pressure to that of the saturated

vapour of the liquid.

This parameter is also ignored, normally.

1.2.9. Influence of other physical parameters of the fluid.

At the inception and development of cavitation a very important roll is played by the amount of undissolved

-

gas or air existing within the fluid, and therefore

the scale effect associated with these parameters should be reduced to a minimum, it being necessary to carry but statistical testing of sea water properties

along various navigation routes and during the ¿if fexent

yearly seasOns, with the purpose of reproducing similar cönditions during cavitation tests

2. Basic:ideas about the proposed new procedure

2.1 DescriEt ion

The. Authors gave ample considerations on the practïcal

application possibilities of the proposed new proçedure

in ref.[2Jo

In this chapter a brief resume f the fundamentals is

giveñ, which will allow an evaluation.of the advantageS

of this new procedure.

When trying to equalize the. cavitation. indexes correspond

ing to model and ship, it was traditionally thought that the model cavitation index denominator being smaller than that of the ship cavitatiOn index, the only

possible way to achieve this would be to lower the model,

cavitation index numerator as well. In order to carry out this idea .in practice, it was necessary to make a

vacuum in the testing facility, thus increasing

cönsider-ably the cost of .the installations.

It wás shown in the previous chapter that the requirement

of a good flow similarity brought 'about the need of

arrying out 'the cavitation tests with a large hull modef, thus' making the installations capable of carrying put

Faced with this state of the art, the Authors started to wonder whether it were really indispensable to carry out cavitation tests with any vacuum at all.

In 1951 Castagneto (ref.E20J) published the results obtain ed during cavitation tests of propellers in homogeneous flow, in a towing tank at atmospheric pressure, equalling the real propeller cavitation index by means of increasing the speed of the propeller carrier (increasing the

denominator of the model cavitation index instead of lower

Ing its numerator).

In 1969 Peterson (ref.[2121) studied the development of

the cavitation present on a solid of revolution when towed totally submerged in a towing tank at atmospheric

pressure.

The idea proposed by the Authors is to tow a totally

sub--

merged hull-propeller model in a towing tank at such à speed that tbe cavitation index over the required area of observation in the propeller model ii the same as for the real propeller, the propeller revolutions being adjusted so that the real propeller Kt coefficient is obtained

simultaneously.

'tern 4.2 of ref.E2J shows that this is perfectly feasible, f rom a theoretical point of view, under the condition th&t

In this procedure the wake components due tò wave making are completely ignored, as in order to sepárate the

flows under and above the theoretical free waterplane.

corresponding to the tested loading condition, a flat plate is placed to materialize said water plane, similar to that used in the cavitation tunnels (see fig.l.]).

It is to be noted that the upper part of the hull model has a streamlined cover with the purpose of reducing the

towing resistance.

\

As the time available to observe the developed cavitation is short, the direct visual inspection of the phenomenon would be very inconvenient, and therefore the observation

should have to be indirect through a T.V. camera, fitted

with recording devices.

Another possibility would be to use a high velocity movie

camera.

The Authors are not implying that this idea could be

carried out iii all of the now existing towing tanks, due

to the relative high velocities at which the carrier

-must travel during the ests and the fact that the tank

length limits the time of observation.

It is in those towing tanks having lengths in excess of

300 m. and test carriages with speeds up to 10 rn/sec.

where it is believed this procedure could be carried out

ThOse organizatioÌs. which áre at present planning the construction of a towing. tank should take intö account

that with a length of about 400 m.. and a ca.riage speed of up to 15 rn/sec, they would have a very attractive

tank, 'since it would be possible to carry out conventional

towing tests, self-propulsion tests, open water test, etc. and also cavitation tests with the best hopes of

r success.

Should a large number of tests have to be carried out, another aúxilary conventiònal test carriage could be built (with lower speed) and divide the tank into two

haifa to carry out any type of test other than cavitation.

22.

Advantages and disadvantages.

2.2.1. Simulation of the wake component due to wave making..'

The proposed procedure does not ffer the sairtepossibilitie as the vacuum tanks, but It is rather analogous in this respect

to the cavitation tunnels

withOut free surface.

22.-2.

hull Reynolds Number.

The proposed procedure is advatageos in this respect when compared to the vacuum towing tanks and to the cavita

tion tunnels ,with free surface, because without vacuum the'

speeds which have to

be

obtained are about five times high er than those corresponding to the Froude's similarity law,

The proposed procedure is also bettér than the cavitation tünnels without free surface, as here again a lower

circulation speed is to be obtaird under vacut (all present

tunnels have been designed to work normally undr vacuum).

FurthermOre there is the additional advantage in respect of tunnels of having a moving model where the flow condi

tions are more stable and similar to reality, without the

high wall effects that. may exist in the cavitation

tunnels both on the f lôw simulation and on the pressure fluctuation measurements.

2.2.3. Propeller Reynolds Number.

The hull model speed relative to the water being higher, the propeller model has to run alSO at higher r.p.m with the purpose of reaching the Same Kt coefficient correspond

ing

tothe real

propeller, making thuS the propeller

working conditions much better than those associated with all other

existing

procedures.

2.2,4. Test visualisation.

Both the vacuum towing tanks and the proposed procedure

aré subjeàt to uncomfortable visualisation methods,

s

are repeated, which on the other hand is no .great inconvenience (erosion tests excepted)

2.2.5. Watr preparation

SInce the proposed procedure does not require vacuum,

the water contents of non-dissolved air and gas are better controlled, and their influence on cavitation inception

building Experimental Tank (S.S.P.A.).

3.1. DescriEtion of the test program.

In order to check by means of experiments the feasibir

lity of the proposed: procedure, Astilleros Españoles S.A. ordered from S..S.P.A. in October 1974 a seriés of

tests with the purpose of studying the influence of the degree of vacuum and of the circulating water speed

in the tunnel on the cavitation phenomena developing

over the propeller. The changes experienced by the

no-minal wake speeds in the station of the propeller disc

were also studied, when the hull, Reynolds Number was

gradually increased until the value was réached for which the pressure inside the tunnel required to obtain

the propeller cavitation index was equal to atmospheric.

To carry out the tests a model of an AESA tanker proto

type of 236000 TDW and 313 m LBP (see f ig.[2J) was used.

The propeller had six blades and a disc-area ratio of

O.74. The chosen scale factor was 45.

Figs. [3j and

_t31

show the re1atve position of the model inside the No. 2 cavitation tunnel of S.S.P.A.

A full load

condition

was adopted to carry out the tests, because the lines were fuller in this condition and consequently the influence on the flow of th

contraction of the boundary layer when. increasing the

hull Reynolds Number, were expected to be more

The wake speeds at the propeller disc were obtained

in the towing tank by means of a five-hole Pitot tube.

The chosen speed corresponded to a full scale speed of

16 knots. Figs. [4] and [4a] show the results obtained.

In addition complementary measurements were taken with a

Prandtl tube. Fig. [5] shows a comparison made be-tween both measurement results (axial components).

The nominal wake field inside the tunnel was obtained

at speeds of 2, 4 and 6 rn/sec (figs.[6J,[7J,[83 and[9J). Fig. [ioj shows the wake distribution curves (axial

compónents) and fig.[11] includes the nominal

velo-city transverse components.

Furthermore, an estïmate of the ship's wake axial ve-locity distribution was made according to the proce-dure set out by Dr. G. Dyne (ref.[17]), and in order to obtain the theoretical potential distribution Hess and Smith's procedure was used (ref.[22J). Calculation

results are shown in figs. [7-1oJ.

Cavitation tests were carried out simulating average

service

conditions (+

20%) in full load. The effect

on propeller rpm due to a dirty hull was also taken

into account by increasing the effective wake coef-f icient at thrust identity ocoef-f the ship by 0.1. The re-suiting value of the coefficient was 0.207.

Fig. (12) shows the change in the cavitation maximum

extension when varying the water speed.

3.2. Conclusions f rom the investiation.

a) There exists a

significant

_{influence of the hull} Reynolds Number over the boundary layer thickness

developed over the hull model.

'b) The distribution of nominal wakes converges asympto tically with that of the ship, when increasing the Reynolds Number of the hull Model.

c) The measurements of torque and thrust carried _out in the cavitation tunnel at different speeds have shown that the effective wake coefficient also

varies 'appreciably when increasing the Reynolds

Number of the hull model, so that it converges to that of the ship as well.

tation index was kept constant and equal to 2.55.

The circulaing water speeds (V0), and the ratios

be-tween the pressure at the shafting centre line _(po) and the atmospheric pressure _{were as follows:}

V0 (rn/sec).

3.0 0.24

4.1 0.536'

4.7 0.743

water speeds, but their dúration and stability increased

with the. circulating water speed.

S.S.P.A. holds the view that these facts are not only due to changes made to the wake

distribu-tion by the thickness reducdistribu-tion of the boundary

layer of the hull model, but also to. the increased

turbulence of the flow whïch develops over the

blades of the propeller model and to the, reduction

.of theii on'boundary layers when increasing the Rènolds Number of the propeller mdel.

The final conclusion in H. Lindgren and E. Bjäerne's repOrt is that by

increasing

the velocities, the results f the cavitatIon tests will show the best coincidence with the full scale performance.

It ShOuld be

pointed out that the

hull model waS.

built of omal wax

and

'it did 'not suffer any

4aage at all

during' the

tests,

and also that there

were no

abnormal vibrations due to

the high

water

4. Final Note:

The Canal de Experiencias Hidrodinamicas de El Pardo has decided to incorporate in its standard test list the experimental procedure proposed by the Authors, taking advantage of the fact that shortly a very modern test

carriage capable of speeds up to 10 rn/sec shall be installed at El Pardo facilities.

At the time of writing this paper, the visualization equipment has already been ordered and it is hoped that when this paper is read, additional information may be

Gorshkof f A., Cavitation Committee 14th .I..T.T.C. Appendix 8 "Standard, for Cavitation Tests".

Ruiz-Fórnel].s R., Pérez Gómez G. and Vivanco J. "Cavitaci6n

en propulsores marinos. Procedimientos existentes para su

detecci6ri y eliminación. Propuesta de una nueva técnica de experimentación". ingeniería Naval. Octubre 1974.

Lindgren H. "he Cavitation Laboratory, of the Swedish State Ship Büilding Experimental Tank". Report ño. 43 S.S.P.A

Fernández Avila A. Apuntes de Hidrodinántica.Escuela

Técni-ca Superior de Ingenieros Navales. Madrid.

'Van Manen, Cavitation .Committeé 11th I.T.TSC. Appendix II, "The Ef fect of', Non-Uniform Flow on Cavitation of Propellers".

6'. Husd E. "Propeller-Hull Vortex Cavitation". Report from

N.S.M.E.T.

ader H.P., Vitt1on Committee 12th

Ï.T.T.C.

Appendix

tÏ1

"Cavitation Phenomena in NonUniform Flows".

Emerson A., Própeller Committee. 13th I.T.T.C. Appendix IX. "cavitatIon Erosion. Model-Ship Comparsonü.

Cox. G.G., Propeller Committee. 13th I.T.T.C. Appendix I..

li. _{Lindgren H.B., Contribution to the Cavitation Committee.}

11th I.T.T.C. "On Experimental Procedures for Determining

the Cavitation Phenomena on Propel1er is Non-Uniform Flow".

12.. _{Gorshkoff A.S.,' Cavitation Committee. 13th I.T.T.C.}

Ap-pendix IV. "interaction Between

_a

_{Cavitating Propeller and.}

a Ship

Takahashi H., Cavitation Committee. 14th 'I.T.T.C."Viscous

Effects on Cavitation".

Arakeri V.H. "Viscous Effects in Inception and Development

of Cavitation on Axisyiiuuetric Bodies" C'.I.R. Report.. no

Eng. 188-1, '1973. _"

HuseE. "Cavitation Induced Hull Pressures, some Recent

Developments of Model Tèsting Techniques" N.S.F.I.

Publi-cation.

Sasajima H. and Tanaka Z.,' Resistance COmmittee. 11th Z.T.

T.C. "On the

Estimation of Wake of Ships".

Dyne G. "A study of. the Scale Effect on Wake Propeller Cavitation, and Vibratory Pressure at. Hull of two Tanker

Models". SNAME. November1974. .

Hoekstra M. "Prediction of Full Scale Wake Characteristics

Based in MOde]. Wake Survey. Symposium on High Powered Pro-pulsion of Larqe Ships". December 1974.. Wageningen.

Castagneto E. "Experimental Researches on Cavitation on Naval Propeller&'. La Marina Italiana, 1951.

Peterson F.B. "Water Tunnel-High Speed Basis Cavitation

Inception Comparisons" 12th I.T.T.C.

Hess J.L. and Smith A.M.O. "Calculation of Potential Flows about Arbitrary Bodies". Prog. in the Aeronautical

OBSERVATtON ZONE SUPPORT BASIN WATER SURFACE STREAMLINED COVER FI:gure

o: FREE HEIGHT REQUI:E RED FOR

THE PHENOMENA CAPTATION

\FLAT PLATE

IN tHEFLOATING WATERLINE

Looking forward

o

Speed of ship: 16 knots

..-I

n

-e.. -.. a

R(nm)

90

70

50

60

120 150 180 2 IO

240

21V

300

330 360

fl*rs-4à

AngIe,Pd.gr.s

I I I I i I I I I

-.90

/

-FigUre

5

1_.__ #

/

R(mm)

loo

90 80 70

60

50

40

30

I I I I L 210

240

270 300 330 360

Angle,

, degrees

60

Thick

lines indicales results from

5-hole Pitot tube measurements

¡ i

i.

Woke fraction, w LP .9 .8

7

.6

.5

.4 % 4snA in ccv. tunni 01. I f f I

0

30

60

90

120 t

\

0'

I I

F Igurs- 6

R(mm)

---30

330

Angle, V degrees I I I 180 210

240

270

300

ID Wake fractiofl,w

-

--I/i//I

R 50mm

v.6m* in ccv. tunnel

IS II II IS

vo4

° SS SS II

vo:I2m,in tang tank

E$imoted fufl icols woke

lds.l flOw wakø

N

\\

\

\%.',/

I

4... _#

/

FIgUre

-t t t t I t t t

t

'4

-Angis,

& j. I i - &-I

t

-I 30 90 120 I50 180 210

240.

270 300 330

Wake tra ctian1w

_to

Angle,

i

I 1 & I I i 'I t 30 60 90 120

l0

180 210 .240 270 300 330 'R '70 in m

o6mh icoVtu.nnøt

' i.

Ve2"

M U. H vol.2.m in towing lank.

Estimated túllicale woks

Ideal flow wake

/

'4

--A;=--.

Figur.

- 8

I /

1/

//

/1

\\

'I

\

/

i

.,I

'

/

1 I,

/

I,

'

/

/

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-,

I I

/

to .9 _.8 .7 _.6 .5 .4 .3 OL

---:EthflQtd full scali woks

'Ideal f low woke

I

R90mm

Fiur-9

Angle,

f,

I I

I

I i J I :120 150 180

20

24O 270 300 0 vo:6m,b ¡n cov.tunneî Woke froction1w

Vo4

H Si Vo :2 Ii IS SI IS

3

s..

Ç7íZIj

I 80°

Water velocity vo:6mh

0°

4Plp.

4

oie QQ 2 180° 150°

Figure- lO

27 120° 2

330°..

. 30

. -' -w4v

p,.

:'fj

oWAih,v

2 I0°iii:.I 50°

0

leo.

EstImated full scale

woke distrIbution 0°

3 '30°

,

POP4

4j!

'yØ

210

i [sI50°

leo.

120° 600 900 210° 150°

Water velocily

:2mh

WatOr veloCly v°4m,b

Water velOcity

y02m/s

300

Watervélocity v 4M

30° Watet y.: Gm/s 300 1800

AngIe, , degrees

330

300. 270

240

2O

360

L)

0.9 0.8 0.7

LO!'

:::

0.7:

09

O.8 0.7 180 150 120 Vo = 3.0 rn/s. Vo 0.9 Oß 0.7 LOV I

'Lo'

,

.*-.-ç__