MEDDELANDENFRAN

STATENSSKEPPSPR:OVNI:NGS.ANSTALT(PUBLICATIONS OF TEE SWEDISH STATE SHIPBUILDING EXPERIMENTAL TA?K)

Nr63 GUTEBORG 1968

AN EXPERThIENTAL VERIFICATION

OF A DESIGN METHOD FOR DUCTED

PROPELLERS

BY

GILBERT DYNE

Paper to be presented for the American Society of Mechanical EngineersPhiladeJphia May 1968. Symposium on

"Pumping Macbinery for Marine Propulsion".

SCANDINAVIAN UNIVERSITY BOOKSAKADEMIF8RLAGET.GUMPERTS . GÔTEBORG

SCANDINAVIAN UNIVERSITY BOOKS

Denmark: MUNXSQAABD, Copenhagen

Norway: VlivEBsITETsFoBL&aET, Oslo, Bergen

Sweden: AZADEMU'ORLAGET-GVMPERTS, Oöteborg

SVENSK4t BOKPOBLAGET/NOrstedtsBOflmerS, Stockholm

PRINTED IN SWEDEN BYELANDERS BOKTRYCKERI AKTIEBOLAG, GUTEBORO 1908

Summary

A method for the design of ducted propellers has been developed atthe Swedish State Shipbuilding ExperimentalT a n k (SSPA). Starting from known values of. total thrust, numberof revolutions, duct vorticity etc. the method: determines ductedpropeller efficiency, duct thrust, shape of the: duct and. the propelleretc. The distribution of, blade circulation is arbitrary and the numberof propeller blades finite., To simplify the calculations a combinationof the method of singularities and the momentum theorem is: used.

In order to obtain an experimental verification a series of. openwater tests with four heavily loaded ducted propellers has beenôarried out in the SSPA cavitation tunnel: At the advance ratio

J=0.412

the non dimensional total' thtust was. ' '

K.7.=0 278

The theoretical thrust of the duct KTD (ie. the duct vorticity) wasvaried systematically within the range,,

0.01 KTfl/KTT 0.4$.

The blade form and the distribution of blade circulation were thesame as for a conventional propeller.' '.:

The thrust and torque measurements': indicated, a good generalagreement between the experimental and theoretical values of totalthrust, duct thrust and. propeller efficiency 'especially for. three ofthe four ducted propellers tested. The experimental duct thrust was,however, somewhat larger than' the theoretical, when the duct vorti-city was small and vice versa. " .' . . ., ,: ...........

The most extreme .duoted propeller. which theoretically, shouldhave, given' a duct thrust' of 'KTDtKTT=O.45 suffered from flowseparation'insid.e'the rear' part Of: the duct which, decreased KTD/KTTto 0 29 The separation was detected m a series of flow visuahzationstudies which were carried out using a quarzline lamp ffluminatingsmall air bubbles, in the flow: through a.narrosv slit,; ,. '.

4

At the design total load K/J2= 1.63 the ducted propellerstested1 had an efficiency of maximum 53%. The corresponding valuefor a conventional propeller was 46%, i.e. a power decrease of about

To nvestigate the sensivity of the co-operation between duct andpropeler, some of the ducts were also tested together with propellersorigmlly designed for other ducts At the design KTT/J2 both the ductthrust1 and the propeller efficiency were decreased considerably if thepitch ratio of the propeller was lower and the camber of the bladesectioii higher than the design values In one case of two this was alsotrue when the pitch ratio was higher and the camber lower than thedesign) values. The flow visualization studies indicated that theprobable reason was a flow separation inside the rear part of theduct.

Plow separation outside the ducts was recorded orily at very lowvalues of -

Studies of cavitation inception showed that none of the ductedpropellers tested suffered from suction side bubble cavitation at thedesign point In some cases, however, hunted cavitation was observedm the gap between the blade tips and the duct No significant differ-ences in the cavitation characteristics were found between the ductedpropellers and a conventional propeller.

1. Introduction

A method for the design of ducted propellers has been developedat SSPA [1]') Startmg from known values of total thrust, numberof revolutions, propeller diameter, blade number and form, distributionof blade circulation, duct vorticity, minimum cavitation margin,etc the method determines propeller efficiency, blade area, ductthrtist, shape of duct and pitch, camber and thickness of propellerblade sectiOns. .: H:.:..

To vrifr the design method a series of testsin homogeneous flowwith four different heavily loaded propellers has been carried out.The experiments which comprised measurements of thrust and tor-que, determination Of incipient cavitation and flow visualizationstudies are described ui. the 'following. In the .beginnig of the papera short describtion of the design method is given

1) The iumbers within brackets refer to the list of rferenoés in Section 10.

2. List of SymbolS

A0 = - = propeller disk area

AD = developed blade area

coefficients for the distribution of blade circulation

D = propeller diameter/ = camber of blade sections

GL - _____ - non-dimensional blade circulation coefficientrDVAVJ = advance ratio of ducted propellernD

= = torque coeffióient

T= = duct thrust coefficientpD'n2

KTP - = propeller thrust coefficient

TKTT = i-- total thrust coefficient

pD4n2

1 length of blade sectionL = length of ductn = number of revolutionsPfD = propeller pitch ratio in ideal flowp = static pressure

= vapour pressureQ =torquer = radius

D

2

O.7= Reynolds number. For propellers R5 = -V

LVFor ductsR=-4a = maximum thickness of blade section

= duct thrust= propeller thrust

Tr = total thrustV4 = advance velocity of ducted propeller

rr= non-dimensional hub radius

y = axial coordinate

5

+ (O.7Dn 2

z

r

V

p

a

= number of propeller blades= blade circulation= duct vorticity= duoted propeller efficiency= kinematic viscosity= mass density of water

ppt, -.= _____ - test:cavitation numberp VA2

3. Design Method

In the main part of the calculations the propeller is replaced by anequivalent infinite-bladed propeller represented by continuous radialdistributions of semi-infinite tubes of ringvortices and rectilinearvortices. The strength of the ringvortices is determined by the bladec]rdulatlon and the velocities m the ultimate wake The duct is repre-sented by systems of ringvortices and ringsources which simulate theflow acceleration (deceleration) and the thickness of the duct respec-tively. The hub is replaced by a source distribution along the axis.If the hub is cylindrical and the thickness distribution of the duct isprescribed, the strength of the source distributions of the duct andthe hub is determined by the local axial velocities.

The definite strength of the different singularities is calculated inan iteration process described as follows:--1 From the immediately preceding iteration approximate values

of the following quantities are obtained -

blade area from which the blade section length.is calculated (theblade form is given),total axial velocity at the propeller disk and along the hub andthe. duct,tangential velocity at the propeller disk,blade circulation.

The strength of the source distributions of the hub and the duct iscalculated from the law of continuity.The ideal total thrust determined by the momentum theorem iscompared with the total thrust desired increased by the axialcomponents of the drag caused by the propeller blades and theduct.If the two values of ideal total thrust differ the blade circulationis corrected. Then the total axial velocities at the propeller diskand along the hub and the duct are determined. The velocities

7

induced by the propeller, duct and hub are thereby calculatediyuse of the law of BlOT-SAVAnT, see [1]. The calculations are repeatedfrom point 1 until convergence is obtained.When this is true the thrust of the propeller is determined as theideal thrust calculated by the law of BERNOULLI with the axialcomponent of the drag of the propeller blades deducted.A strength calculation according to the rules of D e t N o r $ k eV e r i t a s gives the thickness of the blade sections.The local cavitation number is determined at various radii alongthe propeller blades. Thereby also the pressure change caused bythe duct is considered. Comparing the local cavitation numberwith the critical cavitation number the cavitation margin is deter-mined. If the margin differs from the prescribed minimum valuethe blade area is corrected and the calculations are returned topoint 1 above. The iterations are continued until convergenceis obtained.In order to determine the shape of the duct the axial velocities

induced by the singularities of the propeller, duct and hub arecalculated by the law of BlOT-SAVAnT at different radii along theduct. The shape of the internal surface of the duct is then obtainedapplyiig. the law of continuity. From a practical point of view it isdesirable that the duct is cylindrical at the propeller disk. To obtainthis it is often necessary to complete the original duct vortex distribu-tion with an additional distribution. If the propeller is located at themidcord of the duct and an uxisymmetrical duct vortex distribution isintroduced the velocities at the propeller disk and the thrust of theduct are only slightly influenced. The calculations, described aboveneed therefore hardly be repeated. The duct vortices added can re-distribute the curvature of the duct profile and as a result an amountof duct thrust is shifted from the rear part of the duct to the forepart or vice versa.

In the propeller calculations the number of blades is assumed to befinite. Starting from the propeller thrust as obtained above thepropeller induced velocities are determined by an ordinary lifting linemethod assuming the pitch of the helical vortices to be determined bythe velocities in the ultimate wake 'Due to the relatively large bladewidths generally used for ship propellers, the axial variation of theinduced velocities make camber and pitch corrections necessary.Beyond the ordinary corrections calculated by some lifting surfacemethod additional corrections have to be introduced primarily

8

due to the vorticity of the duct. Pitch corrections due to viscosityand blade thickness are determined in the same way as for a con-ventional propeller.

In the actual investigation some deviations from the above men-tioned design method have been made. Thus,

the propeller efficiency is determined assuming the number of bladesto be infinite,the source distribution of the duct is prescribed while the resultingthickness of the duct profile is calculated,the influence of hub is not considered,when the shape of the duct is determined, the velocities induced bythe propeller are calculated using the blade circulation obtainedfrom the infinite blade number calculations.

The influence of the deviations mentioned in point 1 and 2 above arediscussed in Sections 5 and 4. The deviations in point 3 and 4 areconsidered to be of less importance.

4. Ducted Propellers Investigated

Four different ducted propellers were calculated, manufactured andtested. The load was chosen to permit application to a 150000 TDWtanker with speed 16.6 knots, number of revolutions 105 r/m andpropeller diameter 7.0 m.

Data for this case were as follows: -Total thrust KTT=O.278Design advance ratio J_-0.412Hub diameter XH=O.186Number of blades z=5Blade form and blade section thickness, see Fig. 1Mean line of blade sections: NACA a=0.8Thickness distribution of blade sections: NACA 16Cavitation margin c=30%.

The distribution of blade circulation was normal with zero circula-tion at the hub and at the duct

GL= const(xxH)(1 x)(1 +bx) [a+(1 ±XH x)2]

where a and b are constants which determine the fullness and theposition of the maximum of the distribution, see Fig. 2. In the actualinvestigation a=0.3 and b=1.0.

aoig

G/GL max.1.0

0.8

06

040.2

0

Fig. 1; Outline of propeller P1315.

The efficiency of a ducted propeller is a function of the th±ust ofthe duct.. In the present investigation therefore the duct thrustKTD/KTT has been varied systematically, as shown in the table below.

The corresponding duct vortex distributions are shown in Fig. 3.For the ducts D5D7 the original vortex distributions have beenmodified to make the duCts cylindrical at the propeller, compareSection 3.

0.........Q2.. .0.4. -06.Fig. 2. Distribution of blade circulation.

0.8 x 0

9

rP I/O L'OI.0

9

0.8

0.7

05

0.502660.0.3004 45

0.3 0.E3

035

- .aA

o:co;b:a-co;b=1a 03;

0

// b =1 (actualdistributicn)

-I.

. LI

10

In a uniform flow with small induced velocities the source distribu-tion used for ducts D5D7 would have given a NACA 0015-profilewhile the thickness of duct D4 would have been one third of a NACA0015-profile. Due to the large and varying induced velocities alongthe duct in the present cases, however, the maximum thickness ofthe duct profile was decreased and moved aft.

3

A

IAIL.-0.4 -02 Q2 0.4 Y/R

-1

Fig. 3. Duct vortex distributions.

Propeller DuctCalculated

KTD/KTT (VDIVA)p=o I AD/AO

P1313 D4 0.01 0 0.68

P1314 D5 0.15 0.43 0.65P1315 D6 0.30 1.20 0.64

P1316 D7 0.45 2.26 0.69

L=O5D.

Fig. 4. The shape of the ducts.

11

12

The shapes of the ducts are given in Fig. 4. An increase in thethrust of the duct is obtained if the camber of the duct profile is m-creased and the geometriöal angle of incidence of the profile is dec-reased slightly.

The pitch ratio P/D and geometrical camber of the propellerblades f/i are shown in the diagrams in Figs. 5 and 6.

The propeller and duct models were manufactured in white metal(a special tin alloy) and in bronze respectively The propeller diameterwas 181.5 mm and the radial clearence between propeller bladetips and duct was 0.3-0.5 mm.

All tests were carried out during 1967 in the SSPA cavitationtunnel. The test section was 0.5.m X 0.5 m and the water velocity

Propel/er Na

P1313P1314P13/5P1316

0 02 0.4 0.6 08 ,c 1.0

Fig 5. Geometric camber of the blade profiles.

004

003

002

001

Propeller No.P1313P1314P1315P1316

13

0.2 04 06 0.8 x 1.0

Fig. 6. Pitch ratio curves..

generally 2.5-'3 rn/s. At the design advance ratio and VA= 3 rn/sReynolds number was

R=76x 1O' for the propeller blade sections at x=O.7 andB=2.4x 1O for the duct.

5. Thrust and Torque Measurements

Test Equipment. Accuracy of Measurements

The duct was mOunted on the propeller shaft via three arms, ahub and radial and thrust plain bearings as shown in Fig. 7. A strutprevented the rotation of the duct. A conventional mechanicalbalance described in [2] measured the total thrust and the torque.The thrust of the duct was determined by the use of an annularstraingauge balance located between the duct hub and the propellerhub, seeFig. 7.

The water velocity in the tunnel was measured in accordancewith the venturuneter prmciple, i e the fall m pressure m the nozzleupstream of the test section was measured by a manometer. Walleffect corrections of velocity and static pressure were calculatedaccording to equations given by GraurnT [3]..

£2

PIDf.0

08

05

04

Q2

00

Strut

Ball bearing

Duct

Propeller

Arm

14

Axial plainbearingsStrain gougebalance

Radial plainbearings

-..-i

Fig. 7. Test arrangement.

During a test series the uncorrected water velocity VA was main-tained constant while the number of revolutions, n, was varied insteps in the range

0.3 :J= - O.8.

To decrease the errors in measurements several test series atdifferent speeds were carried out for every ducted propeller tested.The resulting curves were faired. The maximum errors at the designpoint were estimated as follows:

Total thrust AKTTIKTT= 0.01Ducted thrust LIKTD/KTT== 0.01Torque LKQ/KQ = 0.02Efficiency = 0.03

Results

The test results expressed in nondimensional form as

K TTD4fl2

pD4n,2

QKQ = pD5n2

J K7..7o -

are presented in Fig. 8 as functions of the advance ratio

Dn

for the four ducted propellers P1313 D4, P1314 D5, P1315 D6 andP1316 D7 designed according to the method described in Section 3.The results of the conventional propeller P1313 are also given.

A comparison between theory and experiment is also given in Figs.9-12 and following table valid for the design value of KTT/J2.

It is interesting to note that the experimental values of duct thrustare larger than the theoretical values when the theoretical strengthof the duct vortices is small. When this strength is increased above acertain value the conditions are reversed. This effect can possiblybe explained as follows:

When calculating the ducted propeller the ringvortex tubes rep-resenting the duct and the slipstream of the propeller are assumedto be cylindrical with constant diameter. In reality the ringvortices

15

Propeller DuctKTDIKTT

theory exp. theory exp. theory exp.

P1313 - - - 0.416 45.9P1313 D4 0.01 0.11 0.412 0.414 49.1 47.7P1314 D5 0.15 0.20 0.412 0.406 52.0 49.9P1315 D6 0.30 0.29 0.412 0.422 55.2 52.9P1316 D7 0.45 0.29 0.412 0.443 58.0 53.0

16

0.3

P1313 Conventional propellerP1313 04 )--- P13/4 05 j Ducted

-- P13/5 06 [ propellersP13/6, 07 J

0.4

\08

Fig. 8. Results from open water tests.

IOKQ 06

0.5

0.4

03

0.2

QI

/

---C-

Duct D7Duct D6Duct 05Duct D4

17

fr0 Duct and propeller designed togetherx Duct and propeller not designed together

I -iP1313 P1314 P1315 P1316

Propeller No.Fig. 9. Thrust of the duct for the different ducted propellers.

should be placed on the real streamtubes. If the duct vortex strengthis small a contraction of these streamtubes not considered in thedesign method occurs behind the propeller For a certam value of theduct vorticity this contraction can be neutralized and a better agree-ment between the assumed and the real vortex systems is obtained.In the actual case this should occur at KrD/KTT 0.25 where asshown in the table above a good agreement between the theoreticaland experimental values of KTD/KTT can be expected,

If the theoretical duct vorticity is mcreased too much the thffusor

a4A'TD

,rTT

03

0.2

0.1

0

18

0.45

.1

0.40

0.35

Duct D7Duct 06Duct 05Duct 04

o Duct and propeller designed togetherx Duct and propeller not designed together

I I I I

P1313 P1314 P1315 P1316

Propeller No.Fig. 10. Advance ratio at the design value of K./J2.

angle of the trailing edge of the duct is so large that flow separationoccurs and the duct thrust is decreased. Thus the flow visualizationstudies described in Section 6 indicate a flow separation inside therear part of the duct of ducteci propeller P1316 D7. In the actualinvestigation the maximum duct thrust obtained at the design valueof KTT/J2=l.63 is KTD/KTT=O.29.

The agreement between the experimental and theoretical advanceratio J is extremely good in all cases except P1316 P7 where as men-tioned separation occurs, see Fig. 10.

The experimental values of efficiency are all lower than thetheoretical values. Since is a function of duct thrust the differenceis best studied in a diagram where i is plotted against KTDIKTT,see Fig. 12. As shown ezp is 3-6% lower than theory The mainreason for this is that the number of propeller blades when calcula-ting the efficiency was assumed to be infinite.

To investigate the sensivity of the co-operation between ductand propeller ducts Nos. D5, D6 and D7 have also been tested together

,,

C

Duct 07Duct D6Duct 05Duct 04

o Duct and propeller designed togetherDuct and propeller not designed together

P1313 P 1314 P1315 P1316

Propeller No.Fig. 11. Efficiency of the different ducted propellers at the design value of KTT/J2.

with propellers originally designed for other ducts. The results areshown in Figs. 9-11 where duct thrust KTD/KTT, propeller efficiency

and advance ratio J valid for the design value of KTTIJ2 have beenplotted against the different propellers tested.

In five cases of six a considerable decrease both in propeller ef-ficiency and in duct thrust was recorded if the pitch ratio of thepropeller and the camber of the blade sections deviated from thedesign values. In one case was decreased from 53% to 46% (P1315D6P1313 D6). Only for P1315 D5, where the pitch ratio was higherand the camber of the blades lower than the design values, a slightincrease in 'lo was observed.

19

0.60

0.55

0.50

0.45

'20

Conventional propeller0 Duct and propeller designed togetherx Duct and propeller not designed together

0

g. 12. Ef iciency versus duct thrust for the ducted propellers at the design va1ue ofKTTIJ2.

6. Flow Visualization StudiesTeét Equipment

The fkw vivaliation studies were carried Out to clarify some ofthe questions posed by the thrust and torque measurements.

During the tests small air bubbles were forced mto the main flowthrough an inlet in the diffusor of the tunnel The air bubbles wereilluminatd by a special light source which produced a narrow beamof light 'ciith the cross section 300 x 5 mm2 The hght source whichwasdesignedandmanufacturedbySvenska Flygmotor AB,Troliliattan [4], was made up of a quarzhne lamp, mirrors, twoadjustable slits and a cylindripal. lens. The streampaths of the airbubbks rere recorded photographically To get maximum reflectionfrom the bubbles an angle of about 1100 between the optical axisof the camera and the incoming light was chosen.

I ..02 Q3

"To

055

0.50

Q.45

r -x

00x

xx

xx

Fig. 13. Flow pictures. Ducted propeller P13 16 D7 at KTT/JZ 1,6 and 0.6.

Fig. 15. Flow pictures. Ducted propeller P1314 D5 and P13 13 D5 at the design value of KTTIJ2.

Fig. 16. Flow pictures. Ducted propeller P1315 D6 and P1316 D7 at the design value of KTTIJ2.

INo separation at

jDesi9nDuct 07-

Duct D5DuctD4

exterior surface of duct

I.1

Separation at exterior surface of ducto_ I I I

P13/3 P1314 P1315 P1316

Propel/er No.Fig. 14. Limits for. separtidn at the exterior surface of the duct.

Results

The visualization method used was particularly suitable to discoverflow separation at the exterior surface of the duct. This kind ofseparation was however observed only at propeller loads less than thedesign value, see Fig 13 The critical values of K/J2 are shown inFig 14 for all ducted propellers tested If the propeller load wasdecreased below the critical value the region Of separatIon wasgradually increased No sudden changes in the flow were recorded

Separation which occured inside the duct was difficult to detectmainly due to the fact that the duct was untransparent. Behind theduct, however, a region of unsteady flow was observed This flow wasmade up of the normal unsteady flow induced by the propellerthe boundary layer of the duct and possible separation. The thicknessof thö usteady flow region at the trailing edge of the duct was aboutthe same for ducted propeller P1313 D4, P1314 D5 and P1315 D6

21

o Duct and propeller designed togetherx Duct and propel/er not designed together3

Krr

2

I

22

If, however, duct D5 and P6 were tested together with propellerswith a pitch ratio lower and a camber of blade sections higher thanthe design values a considerable increase in the thickness was observed,see Fig. 15. There is much to indicate that the thickening of theunsteady flow region as well as corresponding decrease in efficiency,see Fig. 11, were caused by a separation inside the rear part of theduct or at the tips of the propeller blades.

Also for duct P7 a rather thick region of unsteady flow was observedbehind the duct. To make a more detailed examination possible thenarrow beam of light was in some tests directed obliquely into the ductfrom behind. The resulting flow pictures for ducted propeller P1315P6 and P1316 P7 are reproduced in Fig. 16. The pictures indicatea flow separation or a tremendous increase in the thickness of theboundary layer at the rear part of duct P7.

7. Incipient CavitationTest Procedure

During the cavitation tests the cavitation number

pPD0 =PVA2

and the uncorrected velocity of the water V4 were maintained cons-tant while the number of revolutions was varied. The uncorrectedwater velocity was in all tests VA = 3 rn/s. The cavitation number

was varied in the region4c22.

The limits for the different types of cavitation were determined byvisual means. Partly due to the observing method used and partlydue to unknown factors such as the amount of air bubbles in thewater, the location of the cavitation limits was somewhat uncer-tain. Thus, when repeating the cavitation tests with one of the ductedpropellers after hail a year differences in inception cavitation of aboutEiK TT/J 0.2 at the same cavitation number were obtained. A certaincaution is therefore recommended when examining the result.

Results

The results of the incipient cavitation tests with the conventionalpropeller P1313 and the ducted propellers P1313 P4, P1314 P5,

20

15

10

5

0p0..)

No cavitation

5JGtic?bbIe51de.to°GII

Design/point

F

;4./',,/_

1/ .-; / __..'

/,,!

/

/

//

// o''---: ,d

., p'.

23

F--F

.1F-

4.

P1313 Cony, propellerP1313 04 '1

P1314 D5 I DuctedP1315 06 (propellersP1316 07 J

0 Q5 1.0 1.5 2.0 2.5' KTTIJ2

Fig. 17. Incipient cavitation curves.

P1315 D6 and. P1316 D7 are presented in Fig. 17 where the curvesfor incipient cavitation are plotted in a K/J2_a_diagram. Theincipient of tip vortex cavitation and suction side bubble cavitationcan also be studied in Figs. 18 and 19 respectively. In these figuresalso the results for other combinations of ducts and propellers aregiven.

The propellers were designed to have a margin of 30% to suctionside bubble cavitation at the design point K/J2= 1.63. Since thedesign cavitation number was a= 16 this implies that theoreticallythe propellers should cavitate when a was lower than 11.2. As shownin Fig. 19 almost all propellers tested had a cavitation margin largerthan 30%.

In the case of the conventional propeller the tip vortex cavitationhad a helical form and was visible all the way from the blade tips

Tip vortex. covitatios rct 124Duct 05Duct b6Duct 07

I I

P1313 P1313

Conypropeller

JNo tip vortex cavitation

Conventional prope!/ero Duct and propeller designed togetherx Duct and propeller not designed together

-I- I - I

P1314 P1315 P1316

Duc ted propellers

Fig. 18. Limits for incipient tip vortex cavitation at the design cavitation number.

to the tiltimate wake. For the ducted propellers on the other handthe tip rortex cavitation appeared only in the gap between the bladetips an4 the duct In no case this cavitation was observed behind theduct, pobably due to the flow equalizing effect of the duct Bothfor the conventional propeller and the ducted propellers the tipvortex cavitation continuously changed into suction side sheetcavitaton for mcreasing values of KTT/J2

Pressire side cavitation was observed only on the conventionalpropeller.

At 1ow cavitation numbers and high load coefficients

KTT2.5

cavitation occured on the fore, part of the interior surface of duct D4tested together with propeller P13 13. In no other case duct cavitationwas observed.

0 K--Design --

24

3

2:

2

0

200

Design a-'.

15

No suction side bubble cavitation

,OI

.#-..x-,.--

Duct D4I DuctD5

I® DuctD6Duct D70Suction side bubble cavitation

II

I

I Conventional propellero Duct and propeller designed togetherx Duct and propeller not designed together

I I I

o_ i I I I I

P13/3 P1313 P1314 P1315 P1316

Conypropeller Ducted propellers

Fig. 19. Limits for incipient suction side bubble cavitation at the design value ofKrr/J2.

8. Conclusions

The experiments described were Carried out to get a verification of adesign method for ducted propellers developed at SSPA. Four heavilyloaded ducted propellers were designed to have the same total thrustwhile the thrust of the duct was varied systematically.

As long as no flow separation occured the agreement betweenthe theoretical and experimental values of total thrust at the designadvance ratio was extremely good. The duct thrust was found to beslightly too large for small duct vorticity while the opposite condition

-,-

25

I1 0/10

cavitationmargin

26

was valid when the theoretical vorticity of the duct was large. Theefficiency of the ducted propellers was somewhat lower than thatpredicted by the theory. Incipient cavitation tests showed that thecavitation qualities were comparable with those for a conventionalpropeller.

Hence the design method seems to function satisfactorily underthe conditions tested as long as no separation phenomena occur.

The efficiency of the ducted propeller is a function of the thrustof the duct. If the vorticity of the duct which determines this thrustis increased too much, however, the diffusor angle at the rear part ofthe duct internal surface becomes so large that the flow separates andthe efficiency decreases. For a given total thrust, therefore, thereexists a certain duct vorticity which gives maximum values of ductthrust and propeller efficiency. Factors which may influence themaximum duct thrust Obtainable are

the total load coefficient KTTIJ2,the vortex distribution of the duct,the length of the duct andthe distribution of propeller blade circulation.

The results of the experiments also indicated that the co-operationbetween duct and propeller was critical. Hence, if a duct was testedtogether with prOpellers originally designed for other ducts, generallylower efficiencies were recorded probably due to flow separationinside the duct. To obtain a good result, therefore, the duct and thepropeller must fit each other not only geometri6ally but also hydro-dynamically. The last requirement is probably the most important.

9. Acknowledgement

The author wishes to express his gratitude to the H u g o H a m-mar Foundation for Maritime Research, theHugo Hammar Foundation for InternationalMaritime Research and the Martina LundgrenFoundation for Maritime Research for sponsoringthe present investigation.

The author also wishes to thank Dr. HANS EDSTEAND, DirectorGeneral of the Swedish State Shipbuilding Ex-

e r i m e n t a 1 T a n k for the opportunity to carry out theinvestigation as well as for the interest he has shown.

10. References

Dxcs, G.: 'A Method for the Design of Ducted Propellers in a Uniform Flow". Pubi.No. 62 of the Swedish State Shipb. Exp. Tank, Goteborg, 1967.

LnWGRESt, H.: "The Cavitation Laboratory of the Swedish State ShipbuildingEperixnental Tank", Pubi. No. 43 of the Swedish State Shipb. Exp. Tank,Goteborg, 1958.

Gitraar, H.: "Wind-Tunnel Interference on Wings, Bodies and Air Screws".A.B.C. R.M. 1566, 1933.

Lonnnoxie, 0., R&w, J.: "Vattentunnel for aerodynamsins forsOk". TeknikTidakrift H34 (in Swedish), 1965.

Contents

Summary 3

1. Introduction . . . . . 4

2. List of Symbo1s . ................. 5

3. Design Method 6

4. ,Ducted Propellers Investigated . ............ 85. Thiiis1 and Torque Measurements 13

6. Flow Visualization Studies . . ........... 20

7. Incipient Cavitation 228. ConcMsions 259. Acknowledgement 26

10. References 27