Experiments in the lithgow propeller tunnel

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EXPERIMENTS IN THE

LITHGOW PROPELLER TUNNEL

B

ARNOLD EMERSON, M.Sc., Associate Member

& L. W. BERRY

A Paper read before the North East Coast Institution of Engineers and Shipbuilders in. Newcastle upon Tyne

on the 18th April, 1947, with the discussion and correspondence upon it, and the Authors' reply thereto. (Excerpt from the Institution Transactions, Vol. 63.)

NEWCASTLE JPON TYNE

PUBLISHED BY THE NORTH EAST COAST INSTITUTION 01' ENGINEERS AND SHIPBUILDERS, BOLBEC HALL

LONDON

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THE INSTITUTION IS NOT RESPONSIBLE FOR THE STATEMENTS MADE, NOR FOR THE OPINIONS EXPRESSED, IN THIS PAPER, DISCUSSION AND AUTHORS' REPLY

MADE AND PRINTED IN GREAT BRITAIN

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EXPERIMENTS IN THE LITHGOW

PROPELLER TUNNEL

By A. EMERSON, M.Sc.; AsociatE Member, imd L" W. BERRY.

(Communication from the National Physical Laboratory) 18th April, 1947

SyiropsIs.This paper describes experiments made with five 'four-bladed propellers of 0 80 face-pitch ratio Comparative resultc are given at different

Rej'noldr numbers, and for Tunnel and "open water" teits. The effect f.

variation of static pressure head is shown by thrust and torque curves 'to a base

of cavitation number and the early stages of cavitation are illustrated

'by photographs.

Introduction

THE

vatiorial experiments and in particular to show the degree of cavitationLithgow Propeller Tunnel has been used for a variety of

obser-present on propellers under various conditions. But the. tunnel allows a much greater range of speed for model-propeller experiments than

the standard "open water" tests in' the Tank and offers a quicker and

more economial method of testing.

Before using the Tunnel for this purpose certain modifications, described in a recent paper, were made, and it was decided that comparative tests should be made. with a series of model propellers.

The "open water"

thrust, and torque measuring gear is related to the speeds and forces obtained during model self-propulsion experiments; the Tunnel measuring gear is designed for much larger forces and the tests are made at two or three times

the normal open-water test' speeds. The comparison raised the whole question

of scale effect on model propellers.

The experiments were designed 'to show the order Of accuracy of tunnel tests, the comparison of tunnel and "open - water" results, the variation in, propeller thrust- and torque characteristics with Reynolds number, - and the variation in, cavitation phenomena with change in blade area and in type

of section. ' '

2. Description o.f the e.cpérimen;

The tests were made with five four-bladed. propellers, each of 8 in:. diameier

and 6 40 in. face pitch over the outer half, of the blade. Screws N.2, N.7,' and N.4, were made to Troost'st B design with blade-area ratios 040, 055, 070 respectively; N.5 and N.6 are similar to N.7, but N.5' has "aerofoil" sections at all radii, and N 6 circular-back sections, (N 7 has circular-back sections at the tip Sand aerofoil sections at the root). The model propellers were cast from the same pitch block but th last screw made, N.7;has. a pitch % too high over the outside in. of blade.. This screw wal made to replace an earlier. model, N.3, which measurements showed to have more, wash back than designed. A,.drawingof propeller N.5 is ,shown in' Fig.. 1.

* For list of references see the end of the' paper...

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334 EXPERIMENTS IN THE LITHGOW PROPELLER. TUNNEL -.

Each propeller was tested: (a) in." open water" in the New Tank at 540 revolutions per minute and at 1,200 revolutions per minute, theVvariation

of slip being obtained by varying the speed of advance; (b) in the Lithgow Tunnel at 600 revolutions per minute, 1,200 revolutions per minute, and over part of the speed rahge at 1,800 revblutions per minute; and (c) Id the Lithgow Tunnel at varying .pressures at 1,200 and 1,800 revolutions per

minute. V

The experiments were made at 54O revolutions per minute becaUse

this gave the same Reynolds number as Troost's' experiments. At 1,200 revolutions, per minute it was just possibleEto. reach zero thrust speed in the tank without swamping the gear and 1,800 revolutions per minute in the

tunnel allowed a sufficient' range of testing before the forces caused deflection

of the propeller blads. Additional experiments were carried out at various

revolutions V per minute near zero, thrust point. V

V

The results of the. experiments in the new tafik are. given in Fig.. 2 as thrust

coefficient K and torque coefficientKQ to a base of J. The results obtained in the Tunnel at atmospheric pressure. are plotted in the same form in Fig. 3, add the comparative efficie'ncl( curves are shown in Fig. 4. The reduced-pressure tunnel results ar, given in Figs. 5 and, 6, as thrust and torVque co-

-efficients to a base of cavitation number The measured speed of water

in V the V tunnel has been cOrrected for the constraint of tunnel, walls.'

The . symboh used are defluied as follows

:-T=Thrust in lb.

p = static' pressure in lb./sq. ft.

at shaft axis.

Q = Torque in lb. ft..

e = water-vapour pressure.

D = Propeller diameter in ft

VA relative velocity of section at

V V

Ø7 tip radius.

V = Speed of advance. V

n.. = Propeller ±evolutions /5cc. . = v' + (O.7nnD)'

p = water density

1 938 lb. /cub. ft. (fresh water) Thrust coefficient _{KT --} _p7ZD4 Q pn'D5 V nD . PVA2

3. of. lesults at Atmospheric Pressure

The three points to be examined were :-( 1) the accuracy. of the: tunnel measurements ;' (2) the variation of thrust and torque With Reynolds number or speed of testing; and (3) the comparison of tunnel and open-water' results. The first of these is primarily a matter of care in calibration and in experi-mental observatiofis. The consistency of V measurement was checked by "mied" testing of the five model propellers so that the values plotted for the 1200 'revolutions per thihute curve were obtained on two or three

separate'days' testing V

V

Considering' next they results obtained at different speeds of testing, Figs.

'2Vand 43 'show the. marked'increase with speed, of thrust coefficient values Vat

the .sameiJ'or 'speed/revolutions ratio. For each propeller the thrust co

effiëient.at'.any fa1ue of J depends. on the lift coefficients of' the blade .sections, variation-in' drag having little effect. Table: 1 gives the approximate 'Reynolds

number at whichT the.'propeller sections are working.at .J = 040. . (Except for the' root sections. thevariationVwith J is small). V

V

'Torque' coefficient R

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TABLE 1. t,t "1 z- -4'I, z -4 -4 0 C 0 t!t

r

-4 C z z Reynolds Number x. 106 Thickness Ratio Model Propeller. N.2 N.5, N.6, N.7 N.4 N.2 N.5 N.4

Revolutions per minute

540 1,200 1,800 540 1,200 1,800 540 1,200 1,800 Section radius! Tip radius

-02

0-05

010

0-15

007

014

0'21 008 018

027

0222 0162 0127 0-5 0-13

028

0-42 0-19

038

056

0-24 0-48

074

0-112 0-082 0065

07

0-18

040

0-60 0-25

055

083

0-32 0-70 105 0073 0-053 0042

09

017 038 057

023

052

078.

029

066

0-99 0-045 0033 0026

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336' EXPERIMENTS IN THE LITHGOW" PROPELLER TtJ1'NEL

Lift and drag coefficients for circular back sections at various values of Reynolds number R have been obtained recently4 in the compressed-air tunnel at N.P;L. These show a very steep rise of lift coefficient with Reynolds

nunber up to R: 03 x 106, the increase depending on the thickness ratio of the secthin. For the model screw with circular back -sections, N6,. all,

sections are working at speeds below R = 0'3 x 106 at 540 revolutions per, minuie and so the lift coefficients are. low; at 1,200 revolutions per minute all the outer half of the blade is working at the high value, and at 1,800'revo-lutions per minute only the actual root section is giving "reduced" lift. So the thrust coefficients obtained with N.6, with. a substantial increase from 540 to 1,200 revolutions per minute and a fOrther small increase up to 1,800 revOlutions per minute show general agreement with the results obtained with sections at different speeds. The other screws show the same type of variation, the differences being most marked with screw N.2 and least with the thin widd screw' N.4.

The comparison of' tunnel and' "open water" results shows that about the same thrust coefficient, curves are obtained but the tunnel torque values are higher, the curves diverging froth the value at. zero thrust. Variation

in tor4ue for the same thrust must be due to variation in drag coefficient of the blade sections. The turbulence in the tunnel flow is much greater than that for the open-water tests in which the model propeller advances into still water. A movement of the' transition point on the blades from the position found by Dr.. G. S. Baker' in his analysis of open-water results, to

the leading edge of the blades would account for an increase in drag coefficient

corresponding to the difference betweentunnel and open-water torque values. One object. of this investigation .was to consider the us,e of the Tunnel in replacing "open-water tests" in the New Tank. The general use of such open-water experiments is to obtain wake fraction, open and behind screw 'efficiency, and relative rotative efficiency. As Dr. Baker pOints out, the' open-water results themselves' contain many anomalies but the Clements of

propulsive efficiency obtained frOm the comparison of self-propulsion and

open-water results are still more difficult to interpret. The model self-propulsion results are reasonab1y consistent.; an empirical formula forpro-pulsive coefficients of single-screw models' fits most cases if allowance is 'made for size ofmodel propellers and for immersion; axd thrust-deduction factors appear to vary in. a rational manner. -It is possible that the degree of turbulence in the tunnel agrees more closely with the self-propulsion conditions and would give, better comparative data. But the alternative use of open water or tunnel is part ofthe larger question of interpretation of

model-screw results.

4. Experiments at Reduced Pressure

For the ordinary model-propeller experiments the dimensions 'and

con-ditions applying to the ship propeller are reproduced to scale wth the exception,

of the absolute pressures. If the ship propeller is 18, ft. diameter and the

immersion of the shaft centre is 12. feet, the static pressure at the- shaft centre

is 2,110 lb. /square foot, atmospheric pressure + (12 x 64 = 784) lb. /square foot water head - 2,894 lb. /square foot. For a model propeller 8 in. in diameter the static pressure at the shaft centre is 2, 1l0 lb. /square foot atmos-pheric pressure ± 29 lb. /square foot water head = 2,139 lb. /square foot

2,894 .

instead of

27 - 107 lb. /square foot, the- correct scale pressure. In the tunnel the pressure is reduced so thai the pressure conditions on the model propeller correspond with the full-scale pressure conditions, i.e. the same,. ratio of static head to dynamic head, for ship and model, as defined earlier in this paper. , .

The results obtained are plotted as thrust and torque coefficients to a base of cavitation number for a number of values of / or in other words each thrust.

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The, photographs were taken by Mr. A. I.' Williams, Assistant Experimental Officer; Ship

-Division, NP.L. . ' .

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EXPERIMENTS IN THE LrrHGOW PROPELLER TIThWEL 337 and torque curve is at constant slip and varying pressure. At the high slip,

J

0 5 the blade sections a±e at a small positive angle of incidence. As

the pressure is reduced from atmospheric pressure a vacuum forms in the' low-pressure core of the tip vortex. There is, of course, little difference

between the five screws in the pressure at which the tip vortex becomes visible

and there is no change in thrust or torcue. As the pressure is reduced, cavitation appears on the back, either starting from the leading edge or from the maximum thickness of the section. In the first case the tip vortex seems to spread down the blade from the tips and becomes more like a ribbon than a tube, the cavities in the blade consisting of streaks from the leading edge. In the second case small bubbles appear and collapse on the back in the area between maximum thickness and trailing edge. As the pressure is further reduced the bubbles increase n size and collapse further across the blade.

The initial stage of back cavitation is difficult to observe, appearing sometimes

as an increase in brightness of the propeller surface. The decrease in thrust and torque for these screws Qccurs at the beginning of back cavitation. (It should perhaps be noted here that for screws with back cavitation starting at the leading edge, the thrust, torque and efficiency curves rise before the rapid fall at still lower pressures). This brief description is illustrated by photographs' of the five screws at the same J and the same cavitation number. The photograph of N.5 shows the effect of a minute speck of dirt poised on the. leading edge of the 'section.

At the intermediate slip values there is less thrust, less circulation and the tip vortex, becomes visible at lower pressures; the incidence of the ,blade sections is reduced and the cavitation forms on the back from the maximum thickness line.

At low slip the incidences of the blade sections are negative; smallnegative

angles at the tip and large at the root. Face cavitation is the first visible phenomenon as the pressure is reduced. For these screws the cavitation appeared as a blister at the leading edge near the root and spread outwards along the leading edge and backwards across the bladç. With propeller N.6 having circular-back sections at negative incidence the face cavitation is very marked. By comparing the cavitation number at which cavitation appears on the screw sections with the results obtained on circular-back sections by Walchner8 and Martyrer5 the incidence of the blade sections may be estimated at slips fOr which face cavitation is the first formof breakdown. The angles of incidence obtained from this comparison suggest that the angles of incidence calculated from ProfessorL. C. Burrill's paper7 are over-estimated

at the root and possibly slightly under-estimated near

the tip; but at the.

moment the allowance to be made fOr scale effect on a section in a cascade makes a direct check impossible.

Photographs of N.4 have been selected to show the variation in appearance with pressure and slip variations. The comparative results are shown in Figs. 5 and 6, the most interesting points being the effect of area in delaying

back cavitation and the extent of face cavitation with the sharp-edged root sections of N.6..

The effect of specks on the leading edge of the model propeller has been illustrated (the corresponding case for ship is a damaged or rough blade edge)

and the diagrams a10 show the effect on cavitation breakdown of the speed of testing, Ond for N.2, of dissolved air in the water. It should be added that the tunnel experiments correspond to ideal conditionsuniform wake and constant- speed of rotation. The values to avOid ,cavitation require the ship service conditions to be taken into account

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338 EXPERIMENTS IN THE LITIIGOW PROPPIT 9'

The work described above has been carried out as part of the research programme of the National Physical Laboratory, and this paper is published by permission of the Director of the Laboratory.

REFERENCES

I. "The Lithgow Propeller Water Tunnel," by A. Emerson and L. W.

Berry. Vol. 90. Inst. E. S. in Scot., 1947.

"Open Water Test Series with Modern Propeller Forms," by L. Troost.

Vol. 54. N.E.C.Inst., 1938.

"Elements of Aerofoil and Airscrew Theory," page 222, by H. Glauert.

Cambridge University Press, 1926.

"Tests on Four Circular-back Aerofoils in the Compressed Air Tunnel,"

by D. H. Williams, A. F. Brown and C. S. W. Miles.

"The Efficiency of Marine Propellers and the Drag Coefficient, by

G. S. Baker. Vol. 61. N.E.C.Inst., 1945.

"Resistance of Hulls of Varying Beams," by A. Emerson. Vol. 85.

I.N.A., 1943.

"Calculation of Marine Propeller Performance Characteristics," by

L. C. Burrill.Vol. 60. N.E.C. Inst., 1943.

"Profilimessungen bei Kavitation!'

Otto Walchner. _{Hydromechanische}

10. "Kraftmessungen an Widerstandskorpen ,- Probleme des

Schiffans-and Flugelprofilen im Wasserstrom bei trisbes, Hamburg, 1932.

Kavitation." E. Martyrer. )

Fzg. 2Thrust and Torque Coefficients to a Base of J obtained in New Tank

.

aU____

-0-0--o..

_{__}

- uiu

_,m___

o.

uuiuwuu___

..

U. _____UUUUUUk

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EXPERIMENTS IN THE LITHGOW PROPELLER TUNNEL 339

Fig. 1

Fig. 3Thrust and Torque

Coefficients to

a Base of J obtained in

Lithgow Tunnel

°:

024

-. 0 .

= :

-0-G-Q-002 5.20 2

-T

o 02 0

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340 EXPERIMENTS IN TUE LITHGOW PROPELLER TUNNEL VMUS P02 2150 TK5UGI. TUNTICL. NE TMM 0 50 M.4 300 600 RLVS.?ERM500 - 1200 N. -I-.

.,

_I-.

REVS.PORfIN. -3O0 600 300 1000 .11.2 N.7. - N 4 .:. 0-70 ..50 010 43

'

'\

_'\

-30 CJRVE5 OF SCRCI LIFICIETICY ON BS.SL 7. _t

0 0 5c226/s MC. 0.7, TI 4. 2 " -:0 IIFCECMr,G I OZO TLMN6L 1202RPM

_{000RPM. ---}

II _I ii 010 TRW 1NF. ZOO RPM. -... - I I I 0 540 RPM I . _I 1 1 0 0-To _N.5. _hA,. -20 .10 -EFFICIENCY ON B?St SCREWS 11.5, N.E (CHANGE IN GOCTIO1ISFROM TUNNEl. IWO RPM. . G0OP.PR---MtW INK ZOO RPM-7 N. 0-30 0 5.0-RPM--- -050 070 J 000 0 50 0.40 050 O 070 010

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Fig. 5 0.0.00 0 0IO

u:

--'L

1

-1lSCREW F1.Z. SCREW 1.7 0000.?!! $00

-0.?*.

-ll

1

SCREW ft4. 000 L0!0 - 000 0!?.

-,I

10000!!.--:;.

Ill

I

/

Ifl

I I

-

IF

5.040 .O.10 $

IU1ll

l,

--ViUUUl

O

O2

I I

MIiEE

_iD

If

-IZIU

!177A1

1 Ill

0 0! 00 0! 00 05 04. $(MI OF 0, 00A1 Ot0. SCM.0 OF C4.SITATIOH 05000* 0. .0 .0 0I 0.I K-7 0.! 0 0 0.00 .000 .011 .0! 00 01 0! ₀₀₀ .015 00. .004 0.1$ .10 .14 I0 K 7 .0I .54 0

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342 EXPERIMENTS q THE LITHGOW PRDPELER TC -- 2.0.00 051 7.1110

(14UU

11111

dl

Vff4

.

UU

wri

0,'

VI,.--.

SCREW N 5. ScREw N.6.

4 liPlill

II

0SL

0l.

0_

z_ -

'

-

0440

ii

--

fJ

DI 00 00 04 0-I 00 04 00011 07 CAVIT010II 0014110 Fig. 6 0I IC Q .010 .000 006 .14 K .00 .04 00 0 001 .020 020

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2

EXPERIMENTS IN THE LIT}!GOW PROPELLER TUNNEL 343

Fzg. A-N2. J

525

- =

187

FigB-N7

J =

25

- =

187

Fig. G-N4.

.1 = 522

o- = 190

Fig. D-N5. J =

523

- =

176

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344 EXPERIMENTS IN THE LITHGOW PROPELLER TUNNEL

Fig. G-N4. J =

625

- =

189

Fig. H-N4. J =

672

=

178

Fig. I-N4. J = 522

- =

150

Fig. J-N4. J

572

- =

141

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DISCUSSION ON ".EXPERIMENTS IN THE

LITHGOW. PROPELLER TUNNEL" *

Prof. L. C. BUIRRILL, Member of. Council:

In the first place, I think the Authors

are to be congratulated on haviii chosen

the B 440 screw as the basis of these'new

tests, since this B 440 screw is well known to propellei designers. The

systematic series data published by Dr. L.

Troost (i.e. B 44O, B 455 and B 335,

B 350 series), is being used by a number

of desigsiers in this country at the present

time, and the pforrnance at

sea of

propellers of this type is quite. well known..

Having chosen the .B 440 propeller as

their basis scew, they are to be ëon-gratulated also on having made a ne*

systematic series which will, be of

con-siderable value to us in extending the

previous data. In designing the new

screw N 4, (i.e: B 4.70) they have adopted

the same methods as Dr. Troost and the

increase' in surface area has bee:n \Eadë by

increasing the helical widths in the same

proportion, so that the B 4 4Q, B 4:55

and B 470 screws all have the same radial

distribution of surface and the same type of section at each corresponding radius.

This is much better than basing the expansion of surface on the developed outline, since the screws then become,

wider in the tip as the surface is increased,

and this is undesirable for purposes of

comparison.

The other variation they have tested is

also of considerable interest. The B 440 screw has "round-back" outer sections

and aerofoil inner sections, this

arrange-ment having been chosen by Troost

because excessive vibration and" singing"

had been noted with propellers having

aerofoil sections throughout. The new

propellers N5 and N5, introduced .by the

Authors, are generally similar to the B 440 screw, but N5 has acrofoil sections throughout, while N6 has round-back

sections throughout. It will be very

interesting indeed to study the results

that have been obtained in the tunnel for this systematic 'variation in blade-section

shape.

The information regarding the test

results 'which the Authors have given in the paper is 'very complete, but I think we must say at this stage that it will require very close study and com-parison with other work before we can fully appreciate its true value, and it is

unfortunate that the Authors have not been able to carry out very much of this analysis

f" Developments in Propeller Design and

* Paper by Arnold Emerson, M.Sc., Associate Manufacture for Merchant Ships," Trans. I.

Membe,, and L. W. Berry. Mar.. E., VoL 55, 1943.

30

work themselves. For example, I should

have liked to see a comparison of the.

present results with the standard Troost

data. Can Mr. Emerson tell us whether the results he has obtained in open water

compare well with the standaid data

published by Troost and now in general

use ? How do the tunnel results with these screws compare with the

open-water tests ?

Since the Authors have included

experi-ments on propellers having 40%, 55% and 70% disc-area ratio, they should,

perhaps, be able to give us some tentative recommendations regarding the choice

of blade area in terms of propeller loading

to a base of o. Under what conditions

do they suggest that we should change

from a B 44O to B 455 screw, for

example ?

I am very glad to know that in

deter-mining the value of. their cavitation numeral the Authors have decided to relate

the value of p v_s to the relative velocity

at 7 radius. Iti my opinion, this is

much more reasonable. than relating the

value of r to 'the forward speed of the

propeller alone. It is, in fact, in line

with a suggestion I made in a recent paper before the Institute of Marine Engineers±,

and the value of o given in this paper should, therefore, be capable of direct

comparison with the actual ship-propeller

results on which the cavitation diagram

given in that paper was based.

I do

not know whether Mi. Emerson has already

made any comparisons of this kind; if so, perhaps he will tell us what results

he has obtained.

The use of the tunnel for open-water

tests appears to me to be sound. I have

always had it in mind that if it is not

possible to obtain similar results in open water and in the tunnel under full

atmos-pheric conditions, then the tunnel experiments do not start off from the right point. In fact, when the pressure head

over the screw is similar, we should be

ableto run any screw in thetunnel without

cavitation and obtain exactly the same

result as in open water. If this is correct,

then it should also be possible to use the tunnel as an alternative to the open-water

tests, if and when it is convenient to do so.

This is, of course, oitly an opinion We

have now obtained a number of. results, and whether these will substantiate that opinion remains to be seen.

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D210 ERIMENTS IN THE LITHGOW 'PROPELLER TUNNEL

For example, I find the results in Fig. 4

to be rather disturbing.

This Fig. 4

shows the efficiency curves for the various

screws to a base of J=V/nd, and it tells

us that if the- speed of advance is doubled, and the corresponding revolutions fOr the

same slip rise to 1,200 r.p.m., then the

maximum efficiency with' the same screw

rises to 66%. With a further increase

in speed of advance, such that the

corres-ponding revolutions for the same slip

rise to 1,800 r.p.m., an efficiency of 73% can be obtained. There is a very wide

range from 62% to 73%, and this makes one doubt whether the, actual speeds of revolution of model propellers, as tested in open water or in the behind condition, are sufficiently high to avoid scale effect.

It 'certainly appears that this questiOn

must be looked into much more closely,

since it may give an explanation for some

rather curious results we have had from time to time, when propellers of the same

type and design have been tested at different speeds of advance, and have-given

quite different efficiency values. It appears

also that the effect of Reynolds number

(or r.p.m. in this case) is different for

aerofoil and round-backed screws

respectively.

I have said that these results are rather

disturbing. It is now up to Mr. Emerson

to say that ht knows a little more than this,

and that he can tell us what is the correct

way to deal with such results, so that

these inconsistencies are brought into

line. Perhaps he could relate the

maxi-mum efficiency obtainable to propeller r.p.m. for a given size of screw, and also prescribe the lowest limits of revolutions

and' speed of advance for satisfactory

results with propellers of, say, 7 in.,

8 in., 9 in. and 10 in. diameter.

Fig. 5, which shows the thrust and

torque coefficients to a base of a' the cavitation numeral, is very interesting and instructive. In each of the several

dia-grams we, can see the thrust and torque

coefficients for given slip plotting on a

horizontal line, and as: the pressure in the tunnel is reduced, a point is reached where

the thrust and' tor4ue begin to fall off.

In nining the KQ diagrams, I note

that these points for the N2, N7, and N4

screws which represent 40, '55 & 70

disc-area ratio respectively, do not run in a systematic manner. For example,

for the '71 value of J the breakdown

occurs at a' '25 for the first screw N2,

but then it occurs at about '18 for both

of the other two screws.- That is to say, the 55 and '70 disc-area results are

ch5se 'together, while the 'other curve is

quite appreciably different.

On the other hand, in the lowerKT

dia-grams, if we examine -the '655 J line,

where this is a definite line in the diagram

for the limit of back cavitation, this is

found to occur at, '23 for the first screw

N2, 22 for the second screw N7, and

'then moves over - to '17 for the third

screw N4. That is to say, instead of say

25, '21, '17, as one might expect, the

two screws having the largest surface are close together when dealing in terms of

torque, and in the second case, when enrnined in terms of thrust, the two

smaller surfaces are grouped together

and the largest surface is markedly different. Furthermore, I observe that

the breakdown which is represented by the rate of drop of these curves, appears

to be much more rapid in N2 'and N4 than in N7; for the N7 screw these curves are much more rounded.

I do

not know whether there is an explanation for this, but I am quite sure that many of us will be puzzled by these results as we

analyse the diagrams. Perhaps Mr.

Emerson can make some suggestions in

this connexion. Towards tIle end of

the paper, the Authors make a comparison

with the angles of incidence given by the calculation method described in my 1943

paper to the Institution.

It is very interesting ideed to find that

as a result of these tests the Authors

suggest that the angles of incidence

calculated in accordance with that paper

are over-estimated at the root and may

be slightly under-estimated at the tip. If this is correct, then it means that the calculations which I put forward in my

a very big need in the

shipbuilding " Some Notes on Propeller Theory," Inst.

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and propeller-designing world. I think

they are to. be congratulated on bringing this work to fruition, and alo for having

given us this first paper on systematic

cavitation experiments published in this

Country.

EXPERIMENTS IN THE LITHGOW- PROPELLER TUNNEL D21 1

Mr. J. LENAGHAN, Member:

Mr. Berry in reading the paper

des-cribed the Tunnel, and I would suggest,

for the benefit of those members who have not seen this apparatus,

that a

sketch and description of the Tunnel be

included as an appendix to the present

paper.

The real value of the "open water test" on propellers is, presumably, to check the general characteristics of the proposed propeller and, no dOubt, the investigations being undthaken by the

Authors will, in time,, substitute the

Tunnel test for the "open water" test,

as part of everyday model tests. Nothing is said about the constraint from the

tunnel sides and its effect on the propeller disc. _{Perhaps the Authors would say}

what correction is nade for this.

Fig. DN.5. is interesting. I wonder what a photograph would show if a

pro-peller resembling some of those seen when ships are in dry dock were tested

in the tunnel, i e tip of blade slightly bent,

tip of blade missing, or tip perforated like

a honeycomb.

I hope the, Authors will continue their. experiments and let this Institution have

the results. On the many occasions I

have visited

_{the Tank I formed the}

opinion that this apparatus was redundant;

it is gratifying to know from this, and _a

_{In Figs. 2 aid 3 thrust add}

torque coefficients are given, and in Fig. 4 we have a direct comparison between tunnel and open-water results in the form

of efficiency curves, with which we older members are more fimil mr

With the screws running at the same

r.p.m. there is an 'appreciable difference

in the efficiencies obtained from the

tunnel when compared with the similar figures deduced from open-water results. Also, the effective pitch, deduced from the J value for no thrust, is higher in the

open-water curves. Certain remarks on

p. 336 suggest that the Authors think that the tunnel results may more

accu-rately represent self-propulsion conditions

and that the turbulence in the tunnel may more closely resemble conditions in the

ship. But are we sure that the turbulence

in the tunnel is of the same character as

that produced by the ship ? May not

the reduction in efficiency be due tO some

effect of the tunnel walls, or to rotation in the water stream?.

'ersonally, I feel that considerably

more evidence is required before we are

justified in abandoning open-water

ex-periments and relying, entirely upon

results obtained in a tunnel.

Regarding the experiments at reduced pressures, it would be interesting to learn whether any comparison has been made between. the results obtained and those

published by Lerb's in 1936 of

experi-ments . made in the Hamburg Tunnek Lerb's screws had. ogial cross sections

which may limit the comparison to No.

6 only.

VOTE OF THANKS

The CHAIRMAN (Mr. J. RAMSAY

GEBBIE, O.B.E., Past-President): I would like to take the. opportunity of saying how much we are indebted to you two gentlemen for coming here to-night

and reading us this most interesting paper

and one which, 1 am sure, all will make

it their business to take. an interest in.

They will fitid it very valuable.

I am at the moment a member of the

Froude Ship Research Committee and I

have seen a great deal of the Tank in

recent years. The Ship Department of

the N.P.L. has a tremendous lot of work on its shoulders, and is very much

under-staffed. These gentlemen are each doing

the work of two or three men. Mr.

Emerson is smiling, ,but I am sure he

dOes so feelingly. It is quite remarkable that they should find time to write papers

and come out into the districts to read them, especially when one knows the

(18)

1)212 EXPERIMENTS n.i LITHGOW PROPELLER TUNNEL

great amount of work they have waiting fcr them when they get back. It is very

much appreciated by all who know the

circumstances.

In the past you have pefhaps suffered

from lack of money, apparatus and

facilities, but from what I have seen now on the Froude Committee I think a real

endeavour will be made to give you

everything you need just as soon as it can be made available. Certainly the shipbuilders and engineers will back you

up to the fullest extent. It is not so easy

to back you up with staff because the

shipbuilders themselves are short of staff,

but if you persevere and keep smiling as you are, you wiil no doubt struggle

through in the end.

(19)

Dr. G. S. BAKER, O.B.E., Honorary Fellow

The paper contains, a great deal of test

data having a general bearing on the application of screw theory in practice,

and is mainly concerned with the effects

produced by R and in the, comparisofi of open tank and tunnel' data. The

general comparison has been given by the Authors, and broadly one accepts

this. There are only two points. on this part of the paper on which I should like to comment. First, it is clear from the

diagrams that the 540 revolutions on an 8-inch .screw ar'e a bit low. The tunnel

was designed for 9-10 inch screws, so why not use them and avoid this worst

effect ? Second, in Section 3 the Authors

speak of "marked" increase of KT.With

speed. Th not "marked" a little

ex-aggerated, except possibly for the

circular-back screw N.6 ? I deal with this in

my next paragraph, but the difference at

working slips only varies between nil and 4%.

Coming to details, and particularly to

comparison of results, to keep my remarks

within 'bounds I have taken one screw,

N.7, and analysed its working at different

speeds, in open water and in tunnel. This is in effect a Troost B, 55 screw. First, as regards the KT/J curve, if a mean straight line is drawn to average

the KTcurvefrorn slip 35 to lO the value

of-' i.e. the slant of this curve, per blade

is 119 in the tunnel and in open water, and this value falls' right on the "master

curve." for thrut of a screw given in my paper° on thrust.-. There is a small

movement of the KT line consisting of a slight increase of Jo or the effective pitch as speed is increased, ,and in going from

open water to tunnel, but there is no

change in growth of thrust with slip. In

the tunnel the value of Jo varies some

4% for the R covered, but, ignoring the low revolistiok, thi variation is reduced

to less than 2%. The mean J is some 0867 and in open water for the 1,200

revolutiOns it 'is practically the same.

For the above analysis I have used a Thrust of a Marine Screw Propeller." Inst. M.E. Vol. 155, 1946.

straight line Kr whichignores change at quite small slip. This change is due to

the large effect which small variations in

slip angle, arising from shape of blade, have upon the mean thrust and the real

mean slip. It has no importance in any

practical calculation. ' It shows most

with circular-back screws (as for example N.6 here) and with screws Of heavy mean camber. It. does not exist with wide-,

bladed screws.

-Passing ow,to the question of friction

coefficient, for, N.7, at 7 radius the

Reynolds numbers are 25 and .55, >,

10° for 540 arid .1,200 revolutions per

minute and fOr the mean section the drag 'coefficients are

:-O147 and, Ol23, assuming turbulent

flow

and at 1,200 revolutions in partial laminal

flow is 0085. In my paper on the

efficiency of marine propellers before, this

Institution, I gave a method of obtaining

a factor a from the curve of- efficiency

which could be regarded as a measure of

the loss due to friction. This factor a

is given in the formula

S(1S'l

a+bS

APplying this method to the efficiency

curves given for screw Ni, the following

results are obtained

:-Tunnel 1,200 revolutions per minute

a= 082.-'b.= 8l

Tunnel 540 revolutions per minute

a= l07 b= .55

Open Tank 1,200 revolutiOns per mm.

a= '068 b= 81

First, it is seen that the general inflow factor b is quite unchangad passing from

open *ater to tunnel and taken in con-junction with the being' the same,

changes in results are clearly really due to drag and not to any tunnel-wall effect.

Second, the coefficients

"a" when

corrected for small changes due to effectiv

pitch ratio, should vary directly with the drag coefficients given above, assuming

turbulent flow in the tunnel and partial

laniinal in the tank.

. drag coefft

These 'ratios of _{a value} are

f" Efficiency of Marific Propellers," Vol. 61.

-' Revolutions

1,200 tunnel 540 tunnel 1,200 tank

0l23 ' , ' , , 0085

.082

- 15

,

107.-3% -

142

0068 -

125

EPERIMBNTS fl's THE LITHGOW PROPELLER TUEL D213

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r>214 EXPERIMENTS IN THE LITHGOW PROPELLER TUNNEL

The Authors' assumption of turbulent

flow in the tunnel appears to be about

right, but the drag coefficient in the open

water should be some 0008 above that

given in my paper to raise the above 125 to the 14 region, or the partially laminar flow for open water screws is not

quite as much as shown in my paper. The conclusion from all this detail is that the tunnel can be used for open-screw testing, provided, as the Authors

suggest, that the drag coefflcie}it for fully

turbulent flow is used in any analysis.

Finally, I should like to congratulate Mr. Emerson first for the improvement

he has made in the tunnel by re-designing

the impeller, and second for this very

fine paper Also Mr. Williams on his

lighting efforts. I am looking forward

to seeing one thy a cinema film of bubbles

forming and breaking on the blades.

Dr. J. F. C. CONN, Member:

The paper begins with the relatively startling claim that the tunnel offers an

alternative method of carrying out Open-water tests. In view of the demand for

tank te%ts this claim, if substantiated, should help to relieve the tank of

open-water tests.

The evidence adduced in the paper is

not entirely convincing. Dealing first

with the New Tank Tunnel comparison,

the curves of efficiency in Fig. 4 show

considerable differences in tests on similar propellers. The Authors point out that the degree of turbulence in the tunnel may agree more closely with

self-pro-pulsion conditions than the common

open-water tests, but surely, as they

themselves state, "the alternative use of open-water or tunnel is part of the larger question of interpretation of model-screw

results." What steps are being taken to elucidate this larger problem ? The effect

of turbulence in itself may have to be

explored as well as the effeët of Reynolds number. A beginning might well be made

with the measurement of turbulence in

both open-water and tunnel.

The Authors are obviously conscious of the fact that heavily loaded propellers may

deflect under load, and if the deflections

include twisting, the performance may alter appreciably so far as cavitation is

concerned. It is difficult to measure the

deflection under load but there are several

possible methods. Have the Authors

tried to measure angular distortion of

propeller blades ?

My view is that in order to forecast the incidence of cavitation, it is necessary to

amplify the usual blale element

alcii-lations (as given in reference 7) by the

inclusion of cavitation data obtained from

tests on blade sections both singly and in

cascade. I, therefore, hope that the

Authors will be in a p.osition to undertake

such experiments in the not too distant future, although I appreciate that the

tunnel corrections necessary will add to their troubles.

Since constant-velocity sections offer

one means of avoiding cavitation, tests of

an additional propeller with

constant-velocity sections but otherwise similar to

screw N.7 would be most informative.

A minor point is that the units of p

are not quoted correctly. This quantity is generally termed specific density and

represents the density in lb. per cubic foot divided by the acceleration due to

gravity in ft./sec;2

Prof. A. M. ROBB, Member:

The results presented in this paper are

very disturbing. We have all been brought

up in the belief that the slip, or the

advance constant alone, is a satisfactory basis for the plotting of propeller results, whatever be the rotary speed or the speed

of advance; and we have been taught. that it does not matter whether slip be imposed

by variation in speed of advance with

rotary speed constant or by variation in

rotary _{speed with speed of advance} constant. In effect, we have been taught

that variation in Reynolds number is

practically unimportant. On this matter it has been stated that if the product

ND2, in nun. Sand ft. units, exceeds 200

there should be no large error involved

in expanding the results for a model

pro-peller to the figures for a full-size propro-peller. For the model propellers tested in the tank

at 540 r.p.m. the value of ND2 is 240,

and for the corresponding full-size

pro-peller with a dimension ratio of 25 the

value of ND2 is 30,000. Thus common

belief is that there is no material difference

in result for values of ND' between 240 and 30,000 or more. But the efficiency curves in Fig. 4 show marked differences for rotary speeds of 540 r.p.m. and 1,200 rp.m.,. the corresponding values of ND2

being 240 and 533. If a mere increase in the value of ND2 from 240 to 533 leads

to the differences shown in Fig. 4 the

increase from 240 to, say, 30,000 ought to lead to much larger differences. The results in Fig. 4 seem, therefore, to show

that the ND' criterion is completely invalid.

On the basis of Fig. 4 it would appear that if importance is to be attached only to a Reynolds number, or ND2, criterion

the efficiencies of full-size propellers

should be very much higher than the

efficiencies of model propellers. But that consideration is qualified by

considéra-tion of the results in Fig. 5, and it would

appear that the increase of efficiency

associated with increase of Reynolds

number should be offset by the loss due to reduction in absolute pressure for the

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full-size propellers. If this be indeed a

valid general conclusion it would follow

that open-water experiments on model

propellers are

of no

value for the prediction of results for full-size propellers

and that only experiments in tunnels can

give figures which can be expanded to those for full-size propellers; it would

follow also that all "standardized"

pro-peller results are not reliable. Wherefore

the initial statement that the results in this paper are very disturbing.

There then arizes the question whether the results for the propellers in the tunnel

are completely reliable. The marked

differences in efficiency for thhk and tunnel

results in Fig. 4 seem much larger than

can be explained by a minor factor such as turbulence. Is it not possible that

all the tunnel tests are prejudiced by the

proximity of. the propeller to the bend

in the tunnel ? In spite of the presence of guide vanes in the bend, the presence

of the bend entails the application of

sonic force to change the direction of the

water, and the application of this force may have some connexion with the re-duction of efficiency in the tunnel as

compared with the efficiency in the tank. Incidentally there appears

to be an

appreciable difference between the tank

results for N2 and N? propellers and the results for Troost's B.40 and E55.

propellers. A rough plotting indicates that the efficiency curves for the latter

propellers lie, approximately, between

the two top curves for N2 and N7 pro-pellers in Fig. 4. It might be useful if

the Authors could give a complete

com-parison of the Troost results and the tank and tunnel results. As a minor point the Authors say that 540 r.p.m.

was chosen as one rotary speed because

it gave the same value of Reynolds number

as that in . Troost's experiments. But whereas for. the Troost B.40 propeller the

rotary speed was 450 r.p.m., for the B.55

propeller the rotary speed was reduced

to 378 r.p.m. because of the greater,width

of the blade. On the same basis, with 540 r.p.m. as the rotary speed for N2 propeller, the rotary speed for N7

pro-peller should have been only about 399

r.p.m., and that for N4 propeller about

308 r.p.m.

Dr. Jr. W. P. A. VAN LAMMEREN:

I quite agree with the Authors of this

very interesting paper that the initial

stage of back cavitation is difficult to

observe, appearing sometimes as an

increase in brightness of the propeller

surface. But in- this stage I never

ob-served the beginning of decrease in thrust

and torque. - On the contrary, the de-crease in thrust and torque is starting

only if about one third of the blade

surface is covered by the tip vortex

(laminar cavitation), or if the burbling

cavitation is rather serious.

In the case of aerofoil propellers, with

back cavitatiOn starting at the leading edge (laminar cavitation) the decrease in torque

may occur slightly earlier than the

de-crease in thrust, this causing a small rise

in the efficiency curve. It may occur, too,

that both the thrust and torque curves rise before the rapid fall at still higher slips, the rise of the former occurring

slightly earlier than the rise of the latter,

which, again, results in a nall rise in the

efficiency curve. This phenomenon can

be explained by the fact that under

certain circumstances the laminar-vortex sheet leads to a virtual lengthening of the

blade sections, resulting in an- improvement of the profile characteristics.

It 'is the practice of the Wageningen Model Basin to plot the results of the

reduced pressure experiments in the form

OfKT,KQ and ip curves on a base of speed

constant J= where v is the measured

speed of water in the tunnel in place of

the propeller. In definlng the cavitation

number o we use this speed of advance

v instead of the relative velocity VA of section at 07 tip radius.

pe

So o

_{= p v'}

pc

= PVA'

whereas, according to the

Authors,

With normal, propeller work the

re-duced-pressure experiments are carried out at constant speed of water and varying

number of revolutions. The great

ad-vantage of this method of defining the

cavitation number o is that the pressure

in the tunnel needs not to be changed

during the test. In this way the time for

testing is reduced considerably. A- draw-back to this method, however, is the

variation with the number of revolutions

- and consequently with the slip of the Reynolds number and the cavitation number cr, which, indeed, is a better parameter for analysing the cavitation

characteristics of the propeller at varying

slip. On the other hand, one should bear

in mind that with the actual propeller every different slip-ratio corresponds to a certain

cavitation number Cr or i. So with the

curves of Kr, KQ and jp plotted over

o for constant J as well as with the same curves plotted over J for constant Cr or

Cr1 there is only one relation between

J and ti or o', for each curve that

corre-sponds to a certain condition of the actual

propeller. Why should we use, therefore,

with normal propeller work, the more

cumbrous method of testing the propellers

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- at constant instead of constant - o?

We only calculate the cavitation number,

a for the actual propeller-service

con-ditiOn in order, to be able to compare the

results of Lthe cavitation, tests with those

given in' Professor Btirxil's diagrarn.* Very interesting is Professor Lewis's method of plotting the reduced pressure results. He uses two cavitation numbers

pe

= andu,

-between both, expressions is

0.1

₌

nJY

I

-, or 0.1=

(for J=.I,o1=0.)

During the tests both the' pressure p - e and the number of revolutions are

/

The relation very. simple,

'I

Trans. Inn, of Mar.E., 1943. --

-kept constant whereas the speed of the

water is varied. So o is kept constant.

The results are plotted in the form of

KT,KQ and p curves over J for constant

o as well as constant o. This method

looks v&y attractive but can only be applied

if the tests are carried out for a series of

cavitation numb,ers.

'The methods Of carrying i5ut cavitation

,test in the various prOpell& tunnel's and

the methods of plotting the results appear to be enti±ery different, even in the same

cOuntry. I wonder whether it would be

possible to bring the various methods into

line,and I shouldbevery pleased to have

the Authors' opinion on this point. A

possibly forthcoming international con-feiene 'of tanksuperinten4ents might gie

a good opportunity to deal with' this

important question.

[AUTHORS' REPLY

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EXPERIMENTS IN THE. LITHGOW PROPELLER TUNNEL D217

in reply to 'ProfrisorBurrill, if account is

taken of the. boss correction to. thrust,

and the :diiference in Reynolds number of

the 'B4/55: tests,. the comparison with

Troost results is good. The KT and KQ

values...obtained with N-2and N 7 are a little less than the . Troost values and the

curves, for N. 2'. are straighter.

The comparison of the screws under

reduced pressure is made mOst easily by

considering the curves of KT to a base of u at the beginning of cavitation, i.e.

approxi-mately, the purves marked "bacit

cavita-tion" For o=025 'cavitation begins for

N 4 at'KT=Ol8 forN 7 at Kr=0 14, and

for. N .2 at Kr=0 13. The difference

be-tween 0,40 and O55 disc area ratios is

surprisingly small.. It would be premature

to attempt to,. explain the results without more exact theory or more data.

The difference in radial variation of

angle of incidence between the calculated

values and those inferred from

experi-mental observation is not accounted for by. the amended contraction correction It is hoped to discuss this in 'more' detail with

PrOfeisor BürriU preferab1y after making ad hOc expennients.

-The results given in the paper show that

small model-scre tests should not be

made with thick circular-back sections,

and that, if it is necessary-to. measure small

differences' in efficiency with change in

screw design, tests should first be made on several maxine-screw sections over the full

range,.of.Reynolds number ai4 theniodel

results corrected.

- In the first place the Authors would like

to 'thank Mr Gebbie for his introductory

remarks There is a great demand for

normal tank testing"of ship hulls and pro-peilers There has. been a nine-years gap

in research work. . Working at the Tank during this period it has been disturbing

to observe the variOus anomalies and small discrepancies in determining the ship

pOwer directly from model tests without being able to carry out the research work necessary to assess the errors involved.

Now the choice lies between ignoring the

errors in order to cope with the test work

or to avoid test work until a sufficient knowledge of scale. effect has been obtained.

Between these limits there is much

differ-ence of opinion in the correct use of staff but of the limited time spent on research

a very high proportion should be spent

on scale effect and the application of model' results. While it would be interesting and instructive to pursue the work on

cavitation futther,it is necessary first to know the accuracy with which the model results may be applied to full scale.

Before replying in detail to the

discus-sion 'there are two more experimentai 31

AUTHORS' REPLY

results which are relevant to the. points

raised in the. discussion. Screw N 6 was

roughened over the leading half of back and face and tested in the tunnel and. in

open wéter 'at 540 r.p.m. The tunnel

result was prñctically unaltered but in the

open water, torque increased and thrust

decreased so that for the '±onghend screw tunnel. and open water results agree.. The

experiments have not been made as yet at 1,200 r.p.m. . To show the scale effect of thick. sections. a four-blade screw N.52

having constant thickness ratio sections (15% circular back sections) was made

and tested in the tunnel. The value for zero thrust, i.e., the analytle pitch ratio varied from 085 at 300 r.p.m.. to098 at 1,500 r.p.m. These results will be

published later when complete; they are mentioned here because the discussion seems to show the need for emphasis on

the influence of the transition point or turbulence in model results and- of the scale effect on model screws.

In reply to Mr. Hinchliffe neither the open-water results nor the tunnel results

should be applied directly to the ship;

the suggestion on p. 336 in the paper is that the disturbed conditiOns behind the model hull may correspond more nearly to the tunnel flow thazt to the still water in which the open water tests' are made.

The variation from model to ship, is of

the type shown in Dr. 'Baker's 1945 paper (Ref. 5). It is not so much a question of

abandoning opeo-water experiments as of

interpreting the results.

As Mr. Hiichliffe and Mr. Lenaghan

point out, it is unfortunate that the

des--cription of the Twmel, which is really Part 1 of this paper, should have been

read elsewhere (Ref. 1) in the same

session. The correction for the con-,

straint of the tunnel walls is also discussed

in that paper; briefly, it is a calculated

correction to the water speed depending

on the screw diameter and thrust. For ship propellers approaching the cavitation

régime the. condition of the propeller blades is important; deep local erosion

has been observed on the blade back in

line with a damaged leading edge.

Dr. Conn raises a number of qilestions

which can only be answered in part. One

method of attacking the interpretation of

model-screw results would be to measure

the turbulence at the screw during self-propulsion experiments and to use only

blade sections with lift and drag

co-efficients obtained at the same turbulence

and Reynolds numbers as well as at the

ship Reynolds number. In the

open-water tests the open-water turbulence is

pre-sumably zero but the transition point on

(24)

D2l8 EXPERIMENTS IN THE. LITHGOW PROPELLER TUNNEL vibration of the carriage or measuring

gear, and on the surface and shape of the propeller; it .may'well. be extremely

sensitive. It .would appear to be safe to obtain. tim transition at the nose.

The distortion of the propellers has not

been. measured. A thrust of 45 lb. has

imen accepted as the upper limit.

Blade-element calculations using re-duced-pressure lift and drag characteristics

have been made for a circular back pro-peller.. .They show the same tendency as the. experimental results, .but . it is - not

possible to xake .a reliable comparison because. there is no data available on cascade effect with cavitation on the cascade.

The suggestion of a constant-velocity

sCctionpropelleris noted. The results may be -misleading: because of the turbulence

and lOw Reynolds number. As Dr. Coon

points Out, the definition of p is incorrect.

There are two main points in Professor

Robb's discussion.; he had been led to

believe (a) that above certain limits there was no scale effect on model propellers

and (1') that the tunnel results cannot be different merely because. of turbulence. These are separate points; the variation with Reynolds number is shown by the open-water results. Any arbitrary limit

based on propeller revolutions and diameter is misleading as an absolute

criterion. With reasonably thin b1ads

and the normal size and speed of test

propellers the sale effect is small over the

outer half of the blade and this part of the

blade develops, most of the thrust and

torque.. On the inner sections which are thicker and moving at slower speed - the

scale effect is large. Model propeller

results should not be applied directly to

the ship without correctiOn for scale., The influence of turbulence or transition

pont in the results is not a minor factor.

This point is dealt with in detail in Dr.

Baker's discussion and in lIis 1945 paper.

It is not clear how the flow round .the

bend can affect the to*que on -a model

propeller. - Professor Robb points out that

the correct comparison- of screws with

different areas should have been obtained

by varying the revolutions per minute

inversely as the blad /width.- These

models were tested at the same,revolutions

per minute to shorten and simplify the experimental work - of calibration and

checking gear friction.

In reply' to Dr. Baker, eight-inch-diameter screws were chosen to keep the

rh,innel speed correction small even at

high slip. Since anomalies and differences were expected, the diameter was chosen' to

avoid the possibility of error due to the

tunnel. The differences due to scale were deiberately Stressed because certain corn-parative- model-screw tests may be

mis-.-leading if applied directly to the ship.

The rest of Dr. Baker's discussion ii

complete in itself and requires- no corn-meat from the Authors- other than their

appreciation of this contribution.

Dr. van Lammeren's comparison of methods of presentation is of great interest.

We have no strong views on the subject and at present could easily change to an agreed form. We make our tests at

con--stant revolutions per -minute constant

speed and varying pressure and then

repeat at different . speeds. The- curves

dan be exprCssed. iii terms of o or o,

for each kTalue Of J and we- preferred 'to

use the one which gave the more useful

comparison. This is most certainly' the

time- to obtain an agreed presentation; there has been sufficient experience' to

show the. merits of the different methods and it would not -be too difficult to replot the present results if necessary.

-The Authofs 'would like to thahk those

who contributed tO the discussiOn for Their