Propeller cavitation: some observations from 16 in. propeller tests in the new King's College cavitation tunnel

(1)

PROPELLER CAVITATION: SOME OBSERVATIONS

FROM 16 IN. PROPELLER TESTS IN THE NEW

KING'S COLLEGE CAVITATION TUNNEL

By PROFESSOR LENNARD C. BUBJULL, M.Sc., Ph.D.,. Vice-President

and

ARNOLD EMERSON, M.Sc., Member

13th November, 1953

Svr.iopsis.The paper discusses the new King's College Cavitation Tunnel

and gives the reyults of tests On propellers of moderately high blade-area ratiO and also on a series of merchant ship propellers having different types of blade

sections. _{Some observations on the singing of propellers are also given, based on}

experiments made in the Tunnel Ond the paper concludes with a discussion on the use of the Cavitation TunnEl in studying ship propellers

1.Introduction

THE

King's College was completed in December 1949.

'cavitation tunnel in the Naval Architecture Department

_{At the end}

at

of the war, in accordance with a new policy of developing

research facilities within the department, the Council of the College

was

asked to approve the acquisition and installation of a small cavitation

tunnel in which systematic-series experiments into the conditions

governing the cavitation of tharine propellers might be carried out.

The installation of the larger tunnel finally erected was only made

pos-sible by the vigorous suppOrt given to the project by the Admiralty and

by the Department of Scientific and Industrial Research, in shipping

the material from which it was constructed from Germany to this country

and placing it at the disposal Of the College, and also by the interest

shown in it by the principal propeller manufacturers (Manganese Bronze

and Brass Company Limited, J Stone and Company Limited, and Bulls

Metal and Marine Co. Ltd.) who contributed most. generously towards

the erection costs.

The construction of the tunnel was carried out,under a special arrangement, by Vickers-Armstrongs Ltd., Elswick Works, who were responsible not only for all the structural alterations made but also for the electrical installation instrumentation, and the model propeller drive. Special tribute must be paid

to Lord Eustace Percy (now Baron Percy of Newcastle) who as Vine-Chancellor of the University, sponsored the new project with enthUsiasm and made potaible the erection of the tunnel in an existing building at the College, and also to Mr. G. McCloghrie, Assistant (nOw Deputy) Director' of Naval Construction,. and

Dr. R W. L. Gawn,

. Superintendent of the Admiralty Experiment Work,

Hasiar; who gave every possible assistance from their own experience, in

con-nexion with the design of the tunnel and it 'equipmeiit. V

,when the tunnel was. fin11y taken over from Vickers-Armstrongs Ltd.,, in

December 1949, the circuit was completed and it was generally in full working

A CHEF

LaS,

v. Sdeurnde

(2)

122 ?ROPELLER CAVITATION

order, but there were still some small air leaks in several of the joints, and the new model-propeller drive had not been tested. To carry out satisfactory

model-propeller experiments it was first necessary to determine the hydrodynamic performance of the tunnel as erected, With the view of absorbing as economically

as possible the power available; to calibrate the water speeds obtainable at various impeller revolutions, to study and improve the distribution of velocity across the measuring section, and generally to calibrate the thrust, torque and

revolutions gear, in order to ensure that ac urate measurements could be made. To this eid, and to take charge of the running of the tunnel and the development of other research facilities, Mr. Emerson was appointed to the staff of the Depart-ment in February 1950; together with one research assistant, one draughtsrnan and a mechanic. This constituted the staff working on the tunnel until December 1950, by vhich time a considerable amount of testing had been carried out and

the initial calibration period could be considered to be completed.

Since that time, in the light of experience gained from this first year of testing,

important modifications have been made to the model-propeller drive; the driving unit and the thrust- and torque-meastiring gear have been re-designed and mounted on a single base to improve the alignment, the stroboscopic and

flash liglting units have been made more effective, and the technique of

photo-graphy in the tunnel has been developed to a satisfactory level. Apart from

these improvements, most of the available time has been spent on four

model-propeller series. One series is concerned with three-bladed Admiralty-type

propellers having segmental blade sections; the second, with four-bladed

merchant ship propellers designed with sections of uniform-velocity type, while

the third, which covers three important blade-area ratios (namely P85, 70 and

55), is concerned mainly with the effect of thickness changes, pitch-dintribution

variations and blade shape, and the fourth is made up from models for which interesting full-scale results are available, with the object of correlating the

observed phenomena in the tunnel with the results Of particular experience on ships.

These series have been designated KCA, KCB, KCC and KCD, respectively,

and it is hoped that they Will provide valuable information for the design of propellers covering a wide range of types, working under both cavitating and

non-cavitating conditions. The KCA series propellers have been designed

generally in accordance with the requirements of Dr. R. W. L. Gawn, and the

KCB series, which was initiated under a research grant from the Department of Scientific and Industrial Research, was drawn up in consultation with Dr. J. F.

Allan, Superintendent of the Ship DivisiOn, NP.L. The KCC and KCD series

have been developed in collaboration with a small Advisory Committee,

includ-ing representatives from the marine-propeller industry. It will be some time

before these research programmes ate completed, but it is hoped that this work will eventually provide new basic design data and with the necessary theoretical

study, will lead to closer correspondence between calculated and measured

model-propeller forces, and, in particular, of calculated local pressures with the observed cavitation patterns.

In the, meantime, and in accordance with the condition made by the Council

of King's College that the Tunnel should be mainly used for fundamental

research, the results of which should be freely published, it is felt that two groups oftests will be of immediate interest, and these are dealt with in the present paper. In connexiOn with the series of propellers suitable for liners or heavily loaded single-screw applications, the programme included three-bladed propellers for

high blade-area ratios and four-bladed propellers at lower blade-area ratios. At

an intermediate or change-over blade-area ratio, results with three- and four-bladed propellers have been supplemented by tests with a five-four-bladed and a second three-bladed propeller, all of the same general design and these are

(3)

?ROPELLE1 CAVITATION ₁₂₃

Before starting the main series of sthgle-screw-shi propellers, preliminary tests were made with four propellers differing only in type of blade section, and

these are reproduced in Section 3c. During some of these tests "singing" was

heard under various conditions in the tunneL This has been given further- study,

and some preliminary results and conclusions are given in Section .4. Finally,

the various doubts and approximations in applying, cavitation tunnel results

to the ship have received some consideration and these are discussed briefly in Section 5.

2.Description of Tunnel

The tunnel was made from parts of a horizontal closed-circuit flow_{tank built}

in Germany for acoustic tests on underwater

_{weapons and scheduled for}

destruction in 1945. This tank was 70 ft. long by 20 ft wide, and it included

in the circuit a rectangular reservoir

_{15 ft by 13 ft, in which the acoustic}

experiments were made. As shown by the outline drawing reproduced_{in Fig.}

I, this apparatus, which was made for a different purpose, and with no provision for-propeller drive, bears little resemblance to a propeller cavitation tunnel, but use of parts of this material, and alterations to others, allowed the construction of

a cavitation tunnel big enough for tests on large model propellers, at low first cost, and without undue delay. _{It imposed considerable limitations in design,} and, because so much of the circuit has only a small increase in area from the

measuring section area, the flow- losses are correspondingly high. The surface

finish and matching of the joints were of low standard fOr a cavitation tunnel working under reduced pressures, and consequently considerable delays have occurred in eliminating small air leaks. These two disadvantages are, hOwever,

more than offset by the size of the measuring section and by the advantage of being able to circulate the water in both directions at almost fill power, ie. in the usual way for propeller-cavitation tunnels, so that the test corresponds to the propeller advancing into '-' open water," -and in the _{opposite direction, so.} that the water approaches the propeller along the driving -shaft as it does

"behind" the ship.

-An outline general arrangement of the tunnel, which is_{approximately 40 ft.} long by 35 ft. high, and now stands in the Vertical direction as is usual for

propeller-cavitation tunnels, is given in Fig.- 2.

In the "open" direction, and

starting from the large section at the top left hand corner, the water passes

through a 4" grid honeycomb 18" long and through a 5 tO I Venturi contraOtion

mto the measuring section which is rectangular in cross section 40" deep by

32" wide and 12 ft. long, with rounded corners, having an internal cross-sectiOnal

area of 87 sq. ft.

At the corner behind the -model propeller, the section is split_{internally into}

two parts, and the model propeller shaft leaves- the -tunnel at the breeching piece;

The water is guided round this corner by five curved plates. _{In the downward}

leg the two channels are rejOined following a long expansiOn formed by internal

plates fitted in this vertical limb, and the water is then turned by three curved

plates into the lower limb. _{Here there is a change of section from rectangular}

to circular to accommodate the-impeller.- -

-In the- last part of the circuit alterations have been made since erection.

Behind the impeller there were two sets of contra-blades forming _{a support for}

the impeller shaft and starting to tuin the water round the corner; In the rising

leg there was a rapid expansion to the top corner with the _{three splitter plates.} Initial tests showed that, the resulting flow through the_{measuring section was} unsteady, and that there was about 15 per cent. increase in_{velocity fEom the} middle to the top of the tunnel (see Fig. 3a). _{This was eliminated by cutting}

away parts of the contra-vanes behind the impeller, adding a curved plate at the

bottom corner, and subdividing the expansion

_{in the rising limb into eight}

compartments by a coarse honeycomb, or "egg box." - -

(4)

-14

PkOt'ELLER CAVITATION

In the "open" direction the maximum speed is 24 ft. /sec.; this value could

perhaps be increased to 25 ft. /sec. by increasing theimpeller pitch setting and

accepting cavitation on the impeller blades at low cavitation numbers, or by making a new impeller.

In the "behind" direction the present maximum

speed is 17 ft. /sec.the difference is due to the great energy loss in the expansion through the venturi at the end of the measuring section. The velocity

distribu-tion in the measuring secdistribu-tion in "open" and "behind" directions is shown in

Fig. 3d and e. Recently, plates have been added on the vertical centre-line above

the model propeller shaft, joining the shaft strut to the breeching piece. This

will be a permanent structure, and, for" behind" tests,extensions and additions

will be made to these plates to obtain a range ofcharacteristic ship wake patterns.

The tunnel is lined with a bitumen composition to cover the uneven steel

surface. Once this lining is damaged thrre is a tendency for it to flake off, and

sodium nitrite (1 per cent.) is thórefore added to the water to

act as a rust

inhibitor. To obtain a high degree of clarity the water is continuously filtered.

The use of the sodium nitrite to prevent the rustingof exposed metal surfaces, and of the Metafiltration filter to clear the water and to keep it sparkling, has

removed the serious problem of having to change the water several times a year. The water in the tunnel has a density of 1 950slUgs/cub. ft. and a surface tension

about 10 per cent. less than tap water.

The power for the model propeller drive is supplied by a 100-h.p. electric

motor. The motor drives one end of a

variable-speed gear hydraulic pump;

the other end of the pump drives the propeller shaft through an epicycic gear

box. This system makes available practically the full horse-power over a wide

range of revolutions, in both directions of rotation; The torque reaction on

the pinions of the epicycic gear box is measured by weights and balances at the

end of lever arms. The torque due to the model propeller is the measured torque reaction corrected for secondary gear losses and for the shaft friction losses.

The secondary gearing losses were measured by fitting a lorry brake drum on the shaft in place of the propeller (see Fig. 4) and comparing the torque' on the brake

diiim with the torque on the pinions. The frictional torque depends on the

condition of the bearings, on the viscosity of the oil and on the shaft revolutions. The variations have been calibrated in terms of bearing-oil temperature and shaft

revolutions. Measurements are taken each time the model screw ischanged,

i.e. about once a week. The variation in friction torque is small. Thrust is

measured through a lever system connected to the shaft thrustsleeve.

Speed is obtained by reading, on a water U-tube, the differencebetween the pressure at a static hole in the large section forward of the measuring section

and. the pressure at a static hole three feet forward of the model propeller. The

use of water in the manometer eliminates errors due to inaccuracy in reading, because for the usual 18 ft./sec. water speed the difference in head is nearly five feet and 1/10 inch represents about 1/10 of one per cent. of speed. The venturi pressure drop was calibrated as a speed indicator by tests with a Pitot static tube which had been tested in Ship Division at the National Physical Laboratory. A volume mean velocity for a 16 inch diameter circle about the shaft centre happens to be the same as the velocity at the tunnel centre; so no correction is necessary for variation in velocity in the tunnel,

in the "open"

directiOn. It may be of interest to note' that the calibration of Pitot static against Venturi difference was not affected by varying the model screw loading from zero thrust to 40 per cent. slip.

Static pressure at the shaft centre is obtained by connecting the static hole

in the measuring section to one leg of 'a mercury U-tube, the other leg of which is' connected to a large jar of water having a free surface open to atmosphere at

tunnel centre-line level. This static pressure reading is corrected to the value

obtained 8y calibration against the static pressure from Pitot static tube.

RevolutiOns are measured by a Dawe high-speed counter with either one or

(5)

I'ROPELLER CAVITATION 125

The air content of the water is measured by the Van Slyke method, but the

apparatus made for this purpose takes a large sample of water (200 c.c.), instead

of the very sthail quantity used by the standard instrument. This apparatus

can be seen in the photograph of the measuring section (Fig. 5).

The measuring instruments and controls are grouped so that from one position an experimenter can set, read, and. record water speed, tunnel pressure, thrust,

torque; revolutions, and bearing temperatures of the model-propeller shaft,

while a second experimenter sketches and photographs cavitation and analyses the results.

3.Model Propeller Experiments

(a) Model Propellers

The 16-inch-dialñeter manganese-bronze propellers used fOr tunnel

experi-ments are made by The Manganese Bronze and Brass Company _{The onginal}

mtention was that these propellers would be made with a positive tolerance and that equipmónt would be developed at King's College to execute the last stages

of measuring, hand finishing and polishing. With the demand for considerablC

numbers of small propellers required for the variOus research programmes, the

Company have established a separate department to deal with them. Thanks

to this development work the models are supplied to a tOlerance of within

+0O05 inch and can be tested with6ut further treatment. As a result, the

consideration of methods to obtain a finished propeller correct to 000l inch have

been postponed, although it is realized that this accuracy may be required for

particular investigations.

(b Tests with three, four- and five-biaded propellers

The model propellers KCD 3, KCD 4, and KCD 5, have the same total blade area (namely BAR -60) distributed on three-, four- and five-blades respectively.

Model KCD 3(N) has three blades each blade having.the same outline_{as those}

of KCD 4, so that KCD (N) has three-quarters of the blade area of the other

propellers. (Note: The actual numbering of these propellers has been altered

for this paper to avoid confusion of screw number and number of blades). The

propellers, which are designed for the same loading, are shown in Fig. 6.

Each propeller was tested at constant speed (18 ft. /sec.) at air-content ratio

= 02 to 03 (see Appendix for nOmenclature) and at four cavitation

numbers = 6 3, 3O, 20 and 1

The measured speeds are corrected for

tunnel-wall interference to equivalent unrestricted openwater speeds of advance

using the momentum method described by Wood and Harris in R & M 662.

The results are given in Table 1 al values of thrust coefficient Kr, torque

coefficient K0, efficiency y, and corrected advance coefficient .1 at intervals of

()05 in I of the uncorrected curves. The corrected results are plotted in Figs. 7, 8, 9, 10, together with sketches of the cavitation phenomena.

Tests with four-bladed propellers having different blade sections

The four propellers KCB 1, 2, 3, 4 of 0-50 blade-area ratio and unity pitch

ratio are shown in Fig. 11. They have the same blade outline and were designed

with a uniform face pitch so that the different blade sections could be measured

and shaped from a completely, machined face surface. 'Model KCB 2 has

sections similar to the Troost " B" series of propellers; KCB 3 and KCB 4 have

respectively, N.A.C.A. 16 series and N.A.C.A. 66 series sections;. KCB I was

designed using the Ludweig and Ginzel camber correction and has a hollow face

section near the blade tip. The results of experiments with these four propellers

are given in Table 2 and in Figs. 12, 13, 14 and 15. Discussion of Results

The cavitatiOn number o- is the ratio: (Static pressure at shaft centre-line, less

vapour pressure) to dynamic head corresponding to speed of advance of the propeller through the water. Very roughly the values 1 P25, 2O, 30 'and 6.3

correspond to merchant ship -propellers having a speed of advance 027 Idiots,

(6)

-126 PROPELLER CAVITATION

In eneia1 the propellers of the types considered will be designed to work at .1

values be1ween 07 and 08. The model propellers have been tested over a

wide: rang of o and 1: the approximations mentioned above indicate, the parts

of the range of importance for practical design at the moment.

The dee1opment of cavitation follows the same general cycle in each case.

Starting at the right-hand side of the-diagrams (7-10) and (12-15) in the region

of zero thrust, there is sheet cavitation extending from the leading edge on the

face of theiblade. - With increasing revolutions and constant speed (J decreasing) the extent of face cavitation decreases and cavitation appears on the back of the

blade. There are two main forms of back cavitation shown in the various

sketches: sheet cavitation from the leading edge spreading down the blade from

the tip voitex and bubble cavitation occurring near the maximum thickness of

the blade sections. Atvery high slip (low J) and low pressure the cavity covering

the back is perhaps 1 inch thick, and extends well beyond the trailing edge and

is followed by a diffuse tip vortex and strong trathng vortices nearhalf radius

Taking a ection through the sketches in the other direction at constant J the

different aj,pearance of cavitation gives some idea of the local pressures existing

at different parts of the blade.

One use of propellers of the KCD 3,4, 5 type is as liner propellers working at cavitation numbers between 20 and 3O and .1 values approximately 075. Inspection of the results in this region shows that increasing the number of

blades reduces the strength of the tip vortex and the associated sheet cavitation,

but increases the extent of bubble cavitation on the back and also the extent of

face cavitation. In efficiency, and in delay of thrust breakdown, the three-bladed

propeller shows a small advantage.

In the range of J between 075 and 080

there is a balanced choice between a four-bláded propeller free from cavitation

except for ithe tip vortex and a three-bladed propeller having a rather higher

efficiency, but with more strongly developed sheet cavitatiOn near the blade tip.

For J valub less than 075 the advantage seems to lie more definitely with the three-b1add propeller. 'The narrow three-bladed propeller KCD 3(N) shows

serious cavitation in the .1 region considered, and also shows the gain in efficiency

obtained b' reduction in blade area. Another use of the four- and five-bladed propellers of these dimensions and characteristics would be for large high-speed

tàñkers, with the propeller working at

60 and J = 0.75. From the results

given in the paper it would appear to be necessary to balance the higher efficiency of the fOurLbladed propeller and the possible face cavitation of the five-bladed,

against the danger ássociated with the stronger tip vortex on the four-bladed screw working behind the ship In heavy weather conditions, with increased

propeller slip, it would appear that the advantage of the four-bladed propeller with the same blade area, would become increasingly important, because the

thrust and torque breakdown occurs earlier (at a lower thrust and higher I

value) with the 'five-bladed screw than for the four-bladed screw, as the u value (cavitation number) is decreased.

The KCB series screws, in the practical range / = 075 and o

60, show

no significant differences. Apart from this interesting though negative result,

attention may be drawn to the variation in slope of KT curves, to the increased strength of 'tip vortex with KCB 1 and to the bubble cavitation on KCB 2

This discussion of results is necessarily brief and general because it lacks the

weight and balance which will be provided by the completed series of

experiments.

4.Model Propeller Noise

Early this year,. steps were taken to keep the noise from the propeller drive Out of the control and observation sections of the tunnel. During the next

set of experiments, with the KCB series propellers, it was found that high-pitched

sounds or whistles were heard. These occurred when different models were being testçd and at different test conditiors. In two cases, lines of bubbles on

(7)

PROPELLER CAVITATION 127

the blade back near the tip parallel to the leading edge and behind the blade

parallel to the trailing edge (see Fig 17c) appeared when the high pitched sounds

were heard. The lines of bubbles on the back seemed to be shaken off the

exten-sion down the leading edge of the tip vortex cavity. This showed that the blade

tip was vibrating and the estimated frequency, obtained by dividing the interval between lines into the relative water speed, was of the same order as the audible

frequency. During a test condition, when the high-pitched sound was present,

a solid particle stuck to the leading edge of a blade about 1 inch from the tip (7 inch radius); the sound stopped immediately but started again when the particle

was washed off (This sensitiveness to dirt on the leading edge has been repeated

in a number of cases and one screw, KCB 2, which was accidentally nicked on the

leading edge and repaired, gave a different range of singing before and after).

The observations showed that the sounds in these cases were directly related to

the vibration of the model-propeller blades. It hasP been assumed that all similar

high-pitched sounds occurring during model tests, were due to propeller vibra-tion.

The range of frequency of the audible sounds was roughly 1,000 to 5,000

cycles/sec. Assuming a linear-scale relation for freqtiency, the corresponding

range for 16-ft. diameter ship propellers would be 80 to 400 cycles/second.

The sounds weremade by some of the model propellers working in uniform flow, representing the mean service conditions of ship propellers in the variable wake

behind a ship. It seems reasonable to relate the model propeller sounds to ' singing" propellers.

It is known that propellers can be made to vibrate, the mode of vibration

depending on the exciting frequency. Each propeller model produces the same

characteristic sound at different water speeds, propeller revolutions, and

pres-sure. Sounds covering a wide range of frequency have been produced by

differ-ent propellers. There does not seem to be any possibility of excitation by

resonance with frequencies occurring in the tunnel flow or propeller drive.

The first observations of singing were made during the standard tests with

model propellerssets of experiments, each at one cavitation number were

made at constant water speed with measurement of thrust and torque and

observation of cavitation phenomena at different revolutions. It was noticed

that the conditions under which singing occurred could be deflited by the advance

coefficient J and the cavitation number o.

If a propeller sang at I = 090 and

a- = 40 when tested at 18 feet/second (and 900 r.p.m.) it also sang at the same I value when tested at 12 ft./sev. (600 r.p.m.) if the pressure was adjusted to

give a- = 4-0. The singing at the lower speed seemed to have the same sound

characteristics, but possibly less volume. As there does not seem to be any

simpler parameter to deflie a particular singing condition, results are presented

in diagrams (Fig. 16) having cavitation number as base and I as ordmate. In

each diagram the limits of visible cavitation are shown i.e. the line connecting J

and a- values above which face cavitation is visible and the line below which

back cavitation starting with the tip vortex is visible.

The "singing" diagrams show a band of singing associated with appearance of the tip-vortex, or of leading-edge, back cavitation. In KCB I the singing reaches maximum loudness and then stops as the tip vortex becomes visible. The occurrence .of maximum amplitude just as the tip vortex cavity appears seems to apply also to KGB 2, KGB 3, KGB 4, KCD 4 and KCD 5, but the

singing persists after the cavitation is visible and in the cases of KGB 3, KCD 4,

and KCD 5, there is a separate band with appreciable blade cavitation and a

well-developed tip vortex. KGB 2 and KGB 3 show a band associated with the

visible face-cavitation line. KGB 3, KCD 4, and to a less extent KGB 4, sing

in a considerable part of the non-cavitating area. Some of the propellers

also "sing" when at rest with the water movingcorresponding to a ship

"stopping."

In the case of KGB 2, the extension of the tip vortex down the leading edge

(8)

128 PROPELLER CAVITATION

in which the cavity forms, and either by its formation or by upsetting the separa-tion at the trailing edge, deflects the blade; as a result the incidence to' the flow is

reduced and the vortex no longer cavitates. This cycle could also occur when

the cavittiOn was incipient, and probably not visible, and if the forces were created at any antinode of the modes of vibration of the blade, singing may be

present. This could be accepted as a possible explanation of the tip-vortex

band of singing; or of the face cavitation singing, but cannot apply to the

middle of (the non-cavitating zone nor to the singing with the screw stationary.

Experiments have been started to determine the vibratioh patterns

corres-ponding t the recorded frequencies. Some points of interest may be mentioned

at this stage. The frequencies recorded are those corresponding to combined

modes and not to the simple flexural or torsional modes; the modes of vibration

occur amdng those produced by excitation at arbitrary frequency, eg by dr CO2 on the blade; small pieces of plasticine on the leading edge reduce the amplitude 'of vibration if placed at antinodes

These observations may perhaps be summarized as

f011ows:-Theinging of the model propellers reproduced so many of the

character-istics Of singing ship propellers that it is reasonable to use models to

examine the nature of singing.

Theie is clearly one form of singing which is associated with the formation of the tip-vortex cavity.

The singing consists at any particular condition of a combination of. I, 2, or 3 frequencies rather than of a pure note

Theie is evidence to suggest that in some cases only one blade is vibrating

or eciting vibrations.

The sound is affected by fine particles on the.leading edge.

It is also damped, or not excited, when there is considerable cavitation on the blade.

6.Conclusion

The paper has described the Cavitation Tunnel at King's College, and its'

use in ship propeller problems. As in many cases of practical hydrodynamics,

which do not permit a' complete theoretical treatment, model experiments can serve two purposes: they can be used to obtain an approximate solution to a simplified fom of the problem, or they can be .used to represent the fullscale

system as cldsely as possible and the observations or measurements on the model extrapolated 'y some means. The emphasis in the work so far described has been on the first purpose, i.e. to obtain consistent measurements on model propellers in a uniform 'stream, so that using model-scale data it will be possible to develop

an approximte solution of the local conditions on the model propellers.

The 16-inch-diameter prOpellers are large enough to be manufactured

accu-rately. For this size of propeller, and the usual water speeds, the

propeller-blade sections operate under conditions clear of the major irregularities of 'scale effect" which occur at lower Reynolds Numbers, and it has been found

that experiments at constant speed and varying revolutions defiuie the same curve

as experiments at fixed revolutions and varying speed. The air content of the

water can sometimes affect the results at reduced pressure. For the 16-inch-diameter proellers so far examined there is no measurable variation in thrust-torque coefficients with variatiofi in air-content ratio from about 05 (at which value the molel cannot be seen through the air bubbles in the flow) to 02 at which the strehm is clear. Within the range of conditions under which the tunnel is operated th results form a reliable basis for checking approximate theoretical

solutions. ' .

This programme of tests will continue, but ,other experiments are in hand

which come into the second category The full scale propeller operates in the

(9)

- COefflcie'zts: - T/pn2JY KQ Q/pn2D5

./

v/nD KT/KQX J/27T cr

(pe)/q

-.

I- ...

Thrust coefficient Torque coefficient Advance coefficient Propeller efficiency Cavitation number Mr c ntent number PROPELLER' CAVITATION 129

screw works in a uniform pipe flow By running the tunnel in reverse, the model

screw can be placed in a "behind ship" position with the water approaching

the propeller along the shaft.

(The photographs illustrating, cavitation in

model propellers working in the Tunnel, Fig 17 show in the behind direction

the de,yeloped boss vortex cavity which is absent in the normal open water

tests) The plank shown in the tunnel outline joins on to the breechmg

piecO in the thunel sO that in effect it extends from just forward of the propeller

round the top corner and half way dOwn the vertihal limb. It is intended that

this plank

or 'deadwood

will be modified until characteristic ship-wake

variations are obtained Just as the first method of using the tunnel is a corn

bination of theory and experimental results, so the Second method is a

combina-tion of model experiments with full scale data Provision has been made for

testing in the tunnel mOdels for which cavitation erosion histories are available. The early results of these comparisOns are very encouraging. KCD 4 is a model

of a twin-screw liner propeller with service conditions

= 25 and / = O75.

After a number of years in service there has been a slight roughening of the face on a small area of abOut 2 inches by 5 inches, on a total blade area Of 187 sq. ft., situated near the leading edge at 75R and in the tunnel cavitation (see Fig 17d) persisted in this region on the model when the rest of the blade was completely

clear. The second comparison is indirect. Heavily loaded single-screw ship

propellers not dissimilar to KCD 4 working in the region o = 6 0 J = 0 75 tend to roughen on the back near the tips The open water experiments show

the tip vortex cavity only. The wake variation is necessary to give the more accurate pictures on the model scale It is felt that during the next few years

the emphasis will move on to tests in the behind ship condition

SYMBOLS

D Diameter Of propeller

T Thrust of propeller

Q Torque of propeller

V Speed of advance of Propeller

n Rate of revolutions of propeller

g Acceleration due to gravity

w Specific weight

p =

- Specific mass

V Kinematic viscosity

Static pressure at axis of propeller

e Vapour pressure of water

Air content of water

Air coiiteflt Of saturated distilled water at atmospheric pressure and

(10)

Model KCD.4 TABLE 1 Model KCD 3. CavitatlonNo.

63

30

I

20

125 Uncorrected J

J

KT KQ ,

J

K K0 ,

j

K K0 i

j

K K0 50 V '576 0226 o::o357 578 0'579 0201 0.0334 0556 0581 '0153

00264 0538

0586 0104

00195 0499

65

629 0324 623

0629 0198

00324 0612 631 162 0272 600. 638 02O1 55T

70

681 177 0290 661 -682 164 0276 648 686 116 -0208 611

75

733 152 0258 690 V 736 I21 0215

80

785 128 0225 714 78T 121 0216 704 85 839 104 0192 V 722 V

90

.95 892 .944 ()Ø056 0160 0127 661712

-V V V 105 105099 P032

.7

00940059 .549 211 V

.V1066

0 0 CavitationNo. V

63

30

V

20 I25

Uncorrected.! .1

KTIKQ

0265 00415 . -480

J

0479 K 0172 K0, 00287 0456

.VJ

0484 KT 0126

x

.JKT

K0

50

471 00223

0435 0490

0062. O01'29 0374 55 524 0 256 0 0394 541 529 0 185 0 0306 509 532 0 135 0 0238 480 538 0 084 0 0161 447 .575 0231 00358 590 578 0196 00316 570 582 0142 00251 525 586 0096 00178 505

65

629

0204 0O323

V .633 629 0198

003i6

P626

633 0i50

00262 V577

638 0100

00186 546

70'

V P681 0.179 00290 668 681 Ol78 00290 665

682 ..0160

00272 639 689

O100.00190

573 .75, .733

0153 .00258

692

.733 .0153

00258 P692

739 0101 00194

611

80. .7850.128

00226 707 . 788 0109 O0205 .666

85

840 0103 00194 710 . 840 0103 O0I94 7I

90

891 0077 00160 682 . -892 0.076 00I57 687. .95 100 I 05 .945 998 1.050 1.056 0053 0028 0003 0 O0124 00084 00039

-648 529 0 V . V V 945 I -999.

-l'o30

ooso

0021. 0 0:0116,

0007'

--

648498 0

(11)

Model KCD 3N.

TABLE

Model KCD 5.

CavitaiionNo.

63

95

'946 0038 00113 '506 '946 0028 0'OlOO 422 949 0007 00072 '147 I'OO '998 0005 0'0019 '115

-

-F007

0

-

0 989 0

-

0 '958 0 0

Cavitation No.

63

30

2'O 125

0 C'avllatiOANo. 6'3

30

20

125 Uncorrected J

J

KT KQ

J

K'

KQ' i

I

KT ! KQ

I

K KQ 50

0480 0156 00266 0447

.55 :

0523 026O 00398

0547

530 O169

00285 .499 0'534 0125

00222 0479

0538 O082 00157

0448

.575 0'236 00358 605 580 0180 00300 .555 P584 0131 0'0231 526 589 0087 00166 0491' 65

628 0208

00321 652 630 0'187 00306 614 633 0140 00242 584 P639

009i

00I75 0528 .70 681

0182 00287686

68i 0'182 00295 666 684 0148 00254 .634 P689 0095 00I83

0569.

.75 .80 .734786

0130

0155 0025500224 710 726 .734 0135 0'0257 703L 734 0151 00260 684 ! .739789. 0098 0102 00191,00198 06040647. 85

840 0105

00191 .735 84O

0099 00194 0681

90 .892

0080 00156

729 892 0078

00156 0710'

.95

945 0054 00120

.676 .945 0048 001l6 0623 100 997 0027 00080 .535

999 O014

0'0067 0329

i046.

0

-.

0 1019 0

-

0

(13)

Model KCB 4.

CavitallonNo.

₆₃

₃₀

₂₀

_i'25

Uncorrected I

I

_KT K0

i

I

K K0

I

_KT K0

I

K'

K0 _t

'50₅₅ O'473

_{524 0250}

0'244 O'0383 O479 O'480 0'153 00256 0'457

0485. 0'116

0'0206

0435 0490 0066

TABLE 2

(14)

Fig. 1Outline arrangement of Original Flow Tank

'ItOPELLBt CAVITAtION

,

I f

flufl

(15)

n

AIN

TAiIICS.

Fig. 2Outline drawing of Cavitation Tunnel

PLANK ON

cNywu

LINa.

V

(16)

136

oi.ou 105

va0cti1 TRWIUSE àt'e,i' DIECTI0N

ntI $350.

PROPELLER CAVITATION T09

lERSE

4.T2V5S5

'oniA' DscTI0N 3Etu&V DIECfloW

3v1.Y 1950. 7u1.V i950.

p7g. 3a, b, c Velocity distribution on vertical line

through shaft centre.

To?

,1

(17)

M

ii

'ass

I

riuvir

ThI!AIiiIII!

V

e G 4 2 4

STAOAD.

POQT.

CONYOU5 :0F

5PEED.

Fig. 3dVelocity distribution in "open" direction

PiTOT T?AVESC

(18)

-4 PORT.

CONTOLJ5 OF SPEED.

Fig. 3e Velocity distribution in " behind" direction

4:' I0

u..u,1u.aIUI4

IUIUI__

₄

"ir,i__tiara

arwivFI__taaia

iuI

man

(/l___

1I\ SUf/L__I. '

aauauaam

_-..aa'aa

aa

-aI

UUUIUURIIUUI

N 138 PROPELLER CAVITATION

PITOT TAVQ5E BEHIND.

(19)

Fig. 4Torque Absorption Dynamo-meter in position on prop1Ier shaft

(20)

&3 O571 o-s "I'

ii

dl

B,.'

a,

c,.0

a

I

a

.0* 5

°

C.-003 K

02

020 010 0 050 0(,0

070J

080 090 tOO

Fig. 7Curves of Kr, KQ and

and Cavitation Sketches for KCD 3

0 70 040 030 J 10 ,

-Z0

IZ5'

KCD3.

3B.ADES. BkR 140 PROPELLER CAVITATION 0 (0 17 050

(21)

i1 400' 4 8o (cc2, :w.Y) kC . 4 SEcTIONS. SACK FACE OIDWJATBS. C-I P(cW !'014 CO.5. CII000.

kC D. 5 5EcTIo.JS. PACE 0105519ArES.

ws) Cuew I'CD. 3 5ECTIOIJS. FACE DI9IATE5. (M0) P47911 FOl co 3 & 2t4). Ia5. 54! I64&

16 DIAMETEQ MODEL PROPELLES. KC 0.

KCD 3N.[] KCD. 4fA] KCD. 5[3]

Fig. 6-Propeller Drawings of KCD 3, 4, 5, 3(N)

C4449 kC D. 3(N) SECTIONS. PACE 91205147815 (J8) S C.B.S. A S C 11

12 FG Ii

J I 5-048 1.481 .090 .006 .980 540 503 .979 .099 .041 599 9 95201.199 .009 .079 -404 .140 .130 .449 .052 .09 -025 81 4579 4027 .981 .445 478 .816 588 807 447 .095 094 4 0400 9.407 .059 .189 -292 -312 .549 .309 .049 .159 09 3 5.440 2074 .040 -049 987 410 497 409 201 .198 .007 a 5-480 2048 480 .880 400 582 -00 810 -350 530 .07I 4 4.575 4.949 .049 .999 .595 434 479 .554 406 -089 .000

SC0R0YA BC 081 F12

14 J 1 3.054 1475 210 .040 .00050: .099 490 -09 542 09 O 5.926 2558 -09 .975 .404 .190 .110 III 064 09 005 5 9531 2289-031 119 178 .949 .288 -207 447 .005 .034 4 7082 3.705508 .140 285 442 .399 509 548 -11.0 .044 3 7139 9.745.042.913 .357 440 .437 .499 957 '455 .057 - 2 5719 5440 .095 .09 .486 457 .999 .910 -955 20.0 .051. -I 64008540 540 .393 .889 .034 .973 494 .406 .839 48_S I 418 C. 0 -81 F G w J 7 5-420 1.440 .020 .047 .008 .080 .089 .970 .054 .040 .09 9 3.494 4-444 495 -070 485 400 .495 .445 .09 557 583 9 3496 4407 .03: 441 477 .549 .987 .200 .446 .094 .094 4 4538 1404 .035 .494 .9_55 .341 .949 .599 -all .114 -045 3 4.9424490.949.811 .544 409 .489 -540 290 -IS? .005 8 4111 4299040 .007 495 .901 .351 409 .880 .09 .070 2 3.975 1441 .949 .395 .905 .434 .570 451 .494 .901 .004 YCOOOD Y A 5 C D 81 F 44 ,J 7 3043 1431 .09_S -098 .979 -052 .995 .047 .046 287 .09 4 5.983 4780 .010 054 .400 .449 -00 -13.4 .097 004 525 5 4478 2297 .290 498 -205 -545 -848 200 195 .409 -009 4 9.400 2.407 .044 -190 -094 -389 .375 .349 .545 .499 949 5 5.440 2o7f -945 .545 .587 471 385 .446 .395 530 .099 2 D02 2015 .045 .348 .449 .949 .952 .907 .49 .935 09 I 4.9791490.049 .979 .905 .792 .779 .914.449.309.049 sI41005 A 51 C D 11

F 4

.040 .022 .050 2 -.049 .00.0 -005 .040 478 .120 .10909s50 .014.400449.559

I

A S C 0 E P 4 008 .021 3 210 .932 09 6 -040 510 589.045 .079 ISO .113 .945 -954 254 l07 .l2l .902

5S2DY A S C 0 E F

'-I 1 1 9 5 -4 585549 3. .042 .954 -040 81 .049 .010 .917 .043 090.440 440 052 -007 --044 198 .157 -09

I A S C P B

F 4 1.1 J 7 S -a 4 --004-017 3. 010 .08l 289 2 -942 .040 -000 .040 .075 .409 .103.042.990 506.444.152.599 "Propeller Cavitation ".

Paper by PROFESSOR LENNARD C. BLJRRILL, M.SC., PH.D., Vice-President and ARNOLD EMERSON, MSC, Member

Plate I

50 O81DIMAIBS. (wo) SACK 012011447125 (ne) SACK CBD44ATES. (iis) C-. OODINA'rEs.

(22)

0

Fig. 8c'urves of Kr, KQ and i and cavitation Sketches for KCD ₄

0 070 a.'O 050 040 O.30 .3

0 '-:

KCD4.

4 BLADES. BAR. OO.

J!

41 __fl

4 ø./

iii_

B

TIr

D50 0-60

070 T

080 090 PROPELLER CAVITATION 141 0 0

(23)

0

A 0 azo KT'

00

0 /

/

\

KCD5,

5 BLADES .BAR 00.

\

I

1?

Fig. 9Curves ofKT, KQ and i and Cavitation Sketches for KCD 5

Q 5 0 4.-.

Jj

- - - _

I

1

050 060 070

080

090 )0O

£ 0 70 060 040

0

110 142 PROPELLER CAVITATION

I

030 I

o.so

(24)

KCD3(r.

3ES. B.A.R 045

Fig. 10Curves of K, KQ and _{and Sketches of C'avitatlOn for KCD 3(N)}

0 70 o.eo 47 050 940 O3O PROPELLER CAViTATION ₁₄₃

(25)

O3.

0-00 02.0 0 0

o-6-3

32

2.

5

S 00

-i

0104 0705 0414

Ii

VA(E

/

041 05% 050 0(,0 070

5ECTIONS USINC LUDWIEC.& )NZEL

CPsMBEK CORRECflON.

080

090

00

Fig. 12Curves

of KT, KQ and i and Cavitation Sketches for KCB I

'70 0 '50 040 030 110

II

'7

LEA%NG

KCBI.

4 BLADES. B,A.R. 050.

PESIC.NEO TO HAVE UNIFORM SIXTION

(26)

SECTIONS. ice'2 SCTIONS. CB SECTIONS. vc.4 sc IONS. 894

-

4.1 700 6.014. 014 2.0'. 14. 14 ISlo 1.511 L- -. .008' 'G 4 8 4.

I8LL ORDINATES N INC4E3.

-T

C

MA. 1141010NE89- -.o.p -50847.0. 48011. C,j08140 410. 0210 0701.4.

00 IcC01 I. BACK 01402JATRS.

Ill IbIc1AIBICI EI I6lIJ '4

4CR ORP1WAT5. BACK 0RDIsJA7414. 52cR 080NArWS 0140000 AS 5014 '4001 4. 144CR 01421144110$. 4010 00094A'rSS 15-IA_IRS

J6 DIMETE

MODEL POPELLES K-C B. 1, 2 3 &.4.

PITCk 16

B.A.. O--50

4

.

R BLADES.

Fig. 11-Propeller Drawings of KGB 1, 2, 3, 4

5947 A B C 0 14 . 50 I .) 7 .075 .052 .054 .008 070 .047 .050 .004 .006 3 .012 -044 .098 -I.e l4 .104 .04o .007 .007 B .040 090 -144 -740.150 484 .150 -100.078 4 .014 719 -005 -242 .250 .480 -19$-14$ 000 3 .054 -121 .477 .544 .349 446 .242 -150 -023 9 004 193 . .444 .436 .444 .304 .503 .005 I .404 -817 ... .544 444 .395 .470 -063 440.74 01 0 0 2 5 50 44 J 4 '7 '072 .034 044 047 -070 -027 .050 -048 030 -008 O -072 549 -091 - -114 -770 -204 .011 -005 007 O .010-100 .780 -19$ -799 -141 789 -729 090 .008 4 014 -440 .224 -244 -490 -070 -057 -17$ -407 -070 3 -440 -198 -245 -351 -244 .355 -372 -239 -770 010 3 .004 -250 047 -428 -408 430 387 -201-210 -044 7 028 -074 434 -503 -944 -503 -457 -949 -434 -040 0 p 00 7.4

j

7 '018 030 050 -087 .070 -044 -048 -031 -010 6 -010 089 .085 -110 -174 .724 .071 -047 .012 2 4 -04I.008 .290 .7550 -181 -346 .190 .085 -109 -440 -181 -220 -700 -205 099 -402 -044 039 3 440 -III 090 -644 -948 350 .024 -823 -048 3 o42 079 -394 -434 -450 -433 -349 -202 -244 I -3 -040 420 -520 -344 .919 -414-III -094 7 .oie .038 .059 000 -070 .029 -429 044 480 .004 0 -012 .039 .044 -lIe -114 -720 .094 .002 .044 -008 5 .012 -100 149 192 -492 -190 .104 -11$ -294 -074 4 074 14 -240 210 -200 402 234 -759 -ISO -420 3 .022 -104 -200 -395 .399 464 -540 004 -100 024 5 024 244 3$ 434 444 -423 -290 .079 -499 -059 I -025 .292 -494 -284 -844 -802 .440 -942 -244 --3927 A. 50 C -D B 4 '4 7 - .009 O -008 .009 5 .009 -002 - '004 -044 4 047 004 .005 -oIO .000 9 -040 .044 .003 .226 -014 080 074 2 -098 -041 -045 -003 015 .047 048 .192 I -143 -079 033 -058 .011 .020 .720 473 B C 01 14 F 50 74 .7 '4-1 .008 .005438 .004 0 043 .003 .204 - .008 9 .507 .240 .002 .000 .073 4 019 .018 .002 .006 -055 078 -038 042 024 001 .008 .074 .054035 .071 3 -082 030 .015 .010 -009 048 -079 -140 I -'SI. r08I 810 .003 .040 -067 .079 -709 -788 52474 50 C

0 OF 50

u-I J S 4 .002 -042 5 .027 -004 .047 -013 .082 9 .049 029 .009 .002 0119 447 .704 I .724 .078 037 .004 .002 .044 -077 .169 CV. A 50 C P -E 10 .12. .I4 ,J 4 7 - - .004 -007 .00 .010 .008 .007 .003 '007 -50 -004 0 002 .047 004 431 00I .003 -000 002 -087 4 .053 -009 009 AS - .004 -008 .050 -006 .090 3 017 009 .008 .000 .004 - -007 .070 5 .096 .545 -048 -00 .00 .009 .228 .035 -042 1 -108 .040 -019 .000 -008 -000 .001 .045 -704 C TSIICI4NISS OS C 37.10 RADII. 013. SACK 0140II.IATES.

"Propeller Cavitation ". Paper by PROFESSOR LENNARD C. BURRILL, M.SC., PH.D., Vice-President and ARNOLD EMERSON, M.SC., Member

Plate II

88077014 I4 7 .3 C071405 AS

IIiz

I Kc54.

(27)

0 050

00

LADlNG o.o KQ 002 02O

-K1 OlD PROPELLER CAVITATION

gI

Al_

I'-n.

_'U

Ii

ii

1,

491 0-54' 0 O-= (,3 -,

0

-0-1.71 01 -71.0 0711

'p

0-140 01 2I

I

070 080 0 90

i6o

Fig. 1 3Curves of KT, KQ and and Cavitation Sketches for KCB 2

145 I- 10 070 040 030

KCB2..

4 BLADES. B.kR.050.

TROOST & SERIES SECTIONS.

0O

I

(28)

050

00

070 0

Fig. 14Curves of KT, KQ and 7) and Cavitation Sketches for KCB 3

7O ).50 40 30

0

3

I0

-.

G.E

I

KCB 3.

4 B.ADES. BAR. 050. N.A.C.A.-I SECTIONS. ol AC 0

m:

AC 0123

3tIrlI

.

I

Q o r 0

cr'-'

-

-- Ky

__________________

46 PROPELLER CAViTATION El 0

(29)

0

D.0

0.z

0I

Fig. 15Curves of K', KQ and i and c'avitation Sketches Jbr KCB 4

070 040 030 .3

0 ?s

'p.,

Oc..O

-___ii_

_{t1__}

K C B

ADINCj DC.E

____

FACE 015 CT

iDj

I

0

-o.,10

V.

'a

8__$

_________ 04Z.

-'

T

-A

- - S

-PROPELLER CAVITATION 147. 0 50 0(,0 070 080 090 100 LI £ 0 -7 050

(30)

(31)

Fig. I 7(a)--KCD 4 in "Behind" Direction Shoving Root Vortex Cavity

Fig. 1 7(b)KcD 4 Showing Vortices Shed During Vibration

(32)

150 PROPELLER CAViTATrON

Fit. 17(g) Fig. 17h

fig.

1 7(d)(h) 3, 4 q,d 5 Bladed Propellers, in

Open Direction

Fig. 17(e) Fig. 17(f)

Fig. 17(c)KCD 4 Show- Fig. 17(d)

ing