MARINTEKNISKA INSTITUTET SSPASWEDISH MARITIME RESEARCH CENTRE SSPA

GÖTEBORG

PUBLICATION NO 92 1982

MULTI-PURPOSE DUCTED CP-PROPELLERSFOR A SEMI-SUBMERSIBLE OFFSHORE

SUPPORT VESSEL

by

ERIC BJÄRNE and DAN MELITZ

Paper presented at the InternationalConference Offshore Göteborg 81

Göteborg, August 1981

'Pt

Distributed by:Liber Distribution

S-16289 STOCKHOLM SwedenISBN 91-38-06762-5

ISSN 0373-4714

Fig 1 Ship model 2119-A in SSPA cavitation tunnel No 2

I

I

3

ABSTRACT

A semi-submersible support vessel consisting of a platform connected bycolumns to two displacing pontoons was designed with two main fixed shaftCP propellers in partially steering ducts. These propulsion units had tofulfil certain requirements at different draughts and propeller loadings,which entailed a difficult compromise design. The main problem vas thereconcilation of the conflicting requirements for almost equal ahead andastern bollard pull thrust in the operating (deep draught) condition, withcavitation free operation at full speed in the transit (light draught) condi-t ion.

In the transit condition the ship speed had to be not less than 12.5 knotsat full power. This condition was found to be the most critical one fromthe cavitation point of view. Thrust reducing and eroding cavitation had tobe avoided or at least minimized.

In the dynamic positioning (bollard pull) astern condition at full operatingdraught and half power, the tow rope force had to be as near as possible tothe tow rope force ahead.

The side forces at a duct angle of 30 degrees, were expected to give addi-tional manoeuvring properties in combination with side thrusters.

At all conditions and pitch settings 220 RPM had to be constantly absorbedby the propellers.

Possibly the design requirements could have been solved more easily by theapplication of rotational thrusters with angular gears, but due to restricteddraught and other space considerations this solution could not be adopted.

In this paper three different propeller designs which were investigated aredescribed. The results of the model tests which were carried out in the SSPAcavitation laboratory, Fig 1, and in the towing tank have been analysed andare discussed.

DESIGN PROBLEMS

Propeller Design

The problems connected with the design of multipurpose CF propellers havebeen frequently discussed previously, for instance in connection with tugboats [il.

s

6

Bollard pullasternHalf power

Propeller model P1859,SMM design, =0.79-'i- P1851,SSRA -- -- =0.82-- P1821SMM -- -'- :0.78

Bollard pull,ahead TransitHalf power Full power

r

.. IVÍIIUIIlIV1VA

'LIiVL-1.5 -10 -0.5 O 05 1.0 1.5

Pitch ratio P/D

Fig 2 Pitch distributions at different loadings

The main parameters for obtaining desired properties are the propeller dia-meter andpitch The diameter was in this case restricted to 3 metres. Alarger diameter would have been advantageous for all conditions, but especial-

ly at the bollard pull conditions, at which a larger tow rope force at therequired power could have been obtained.

The desired rate of revolutions (220 RPM) could be constantly absorbed at thedifferent loadings by adjusting the pitch. Due to the fact that the requiredpitch is obtained by blade turning, this giving the same change of pitch anglefor all radii, the radial pitch and thus, the loading distribution will besignificantly deformed at off-design conditions, Fig 2. This fact disfavoursthe CF propellers in relation to FP propellers, but a better utilization ofthe machinery usually compensates for this disadvantage. The choice of the de-sign pitch is therefore important in order to obtain a good compromise pro-peller design which would fulfil the different requirements to the greatestextent.

Duct Design

In this case the propeller is placed inside a duct, the properties of whichare greatly dependant on the radial loading distribution of the propeller.The shape of the optimal duct is associated with a specific propeller havinga given radial circulation distribution. If the circulation distribution ischanged the duct shape must be correspondingly altered to obtain optimal per-formance. The shape of the duct must therefore also be a compromise.

1.Or

0.g

a0.8

0.7

0.6

0.5

0.4

0.3

Propeller model P1859 SMM design A0 IA0 0.79

Pl85lSSPA --, -'- 0.82- n P82lSMM e, - n nO.78

O 100 200Max. profile thickness S n mm

¿ 10 20 30 40Camber F ,mm

Fig 3 Propeller main data

3. PROPELLERS

111 the propellers tested had the Kaplan blade shape, Fig 3, ifl order toobtain acceptable cavitation properties at off-design conditions.

The propeller model P1821 had a constant pitch and was designed by StoneManganese Marine Ltd (SNM) for almost zero loading. This propeller gavesufficient backing power but unacceptable and even power-reducing cavitationat the transit condition, Figs i6, lT and 18.

A new propeller model P1851 was designed by SSPA to improve the cavitationin the transit condition. The circulation distribution used in the designof this propeller was chosen so that the existing duct (see below) will giveoptimal efficiency. The calculations were carried out according to the prin-ciples developed by Dr Dyne [2]. This propeller did not give the required towrope force at bollard pull astern, Fig 13. The blade profiles were then shorten-ed thus reducing the camber. This modification gave som improvement in backingperformance but the required tow rope force was still not achieved.

A third propeller designed by SMM, P1859, had the pitch adapted for the bollardpull ahead conditions. This propeller was better in the transit condition thanthe original propeller P1821 and gave, in spite of reduced backing character-istics, an acceptable force at bollard pull astern.

The geometric shape of the propellers and the pitch distribution at differentloadings are described in Figs 2 and 3.

7

8

Fig 4 Duct model D48, profile

/ \

Transit comdutions

Towing astern G0O.O650

G07O.O876 //

//

/t

t7

Foce (oacing

/

Bak Ioon

I I

03 0]. 05 06 07 08 09 10 11

Radius ratio, x=r/R

Fig 5 Circulation distribution for propeller model P1859

0.3Coa

0.2

0.1

- - - Existing duct D8 (NSMB Na 37)Duct calculated for backing (astern towing 5 knots)

Proposed compromise duct

Fig 6 Duct profiles

110 1.15 1.20 1.5Duct ratioR0/R

Fig 7 Influence of duct ratio on open water efficiency

KY __ = 2.063, J = 0.4

9

-

u-a

'IÇ1h59 in

.015

ti':

016

01.6

:0.45

I..

1144Q.oI-Q-

0.43

042

0.41

0.40

lo

9 jIWb0'=:I «

Fig 8 Test arrangement for ducted propeller at open

water tests

1. DUCT

The duct SSPA D8 which was used in combination with all the above propellermodels was of the same type as NSNB No 37. According to previous experience[3] this duct was found to have relatively good backing properties in com-bination with FP propellers.

To find out the optimal shape of the duct with the CF propeller with bladesturned for backing, the circulation distribution for the latter case wasestimated, Fig 5, and the corresponding duct section was calculated accord-ing to reference [2]. It was found that the diffusor angle for backing(duct ratio at ahead condition) should be much smaller, Fig 6. In order notto deteriorate the ahead performance too much, a mean value of duct ratiois proposed. This duct would be expected to reduce the power demand also inthe transit condition by about 3 % according to the results given in refer-ence [13, Fig 7.

The main dimensions of the duct, D 48 , and its position during the tests

are described in Figs , 8 and 15.

OPEN WATER TESTS

The open water tests were carried out in the SSPA cavitation tunnel No 2 inthe high speed test section [5]. The propeller was nounted on an upstreamshaft, Fig 8. The duct was connected to the duct balance by means of threestruts, which according to previous experience did not influence the flowsignificantly. The water velocity was kept constant and the rates of re-volutions were varied so that a range of advance coefficients was coveredand corresponding points measured. Highest possible RPM and velocity wereapplied to minimize the scale effects.

All the propeller models were tested in backing conditions. The results ofthese tests are given as thrust and torque coefficients against the advancecoefficients for propeller and duct, Fig 9. It is noticeabLe that for almostall conditions the duct thrust is opposite to the propeller thrust. Therelation between the propeller models with regards to propeller efficiencywhen backing is given in Fig 10, indicating that P1821 gives the best per-formance and P1851 the worst from this point of view. The efficiencies forsome FP propellers, Ka 4-7O in the duct NSMB No 37 when backing are alsogiven, indicating the possible limit of the quantity concerned.

Propeller models P1821 and P1859 have been tested also at ahead running.Open water characteristics for different pitch settings are given in Fig 11.As propeller model P1859 is better adapted to the ahead running conditionsit consequently gives the better results of the two, which also can be seenfrom the propeller efficiency curves, Fig 12. In this diagram the optimalcurve for FP propellers with radially constant pitch, Ka 4-7O in duct NSMBNo 37 [3] is also introduced, indicating that the power increase due to theintroduction of a CP propeller (21859) is about 6 % at the transit condi-t ion.

The relation between thrust and power (torque) at zero speed and differentpitch settings (positive and negative) are given in Fig 13. In line withthe earlier results, P1859 is found to be the best at ahead running (posi-tive pitch) and P1821 the best at running astern (negative pitch). p1851was not tested at ahead running. The relation is further conf,rmed inFig l4, where the Barnaby numbers [6], i e 0.01107

. KT/(Kfl)213, arecompared for the different propellers and pitch settings.

TESTS IN BEHIND CONDITIONS

A complete wooden model of one of the pontoons of the vessel was mountedin the large test section of the cavitation tunnel and fitted in turnwith the right-handed mod1 of propellers P1821 and P1859, Fig 1. Theposition of the propeller and duct is shown in Fig 15. This figure alsoshows the position of the pressure transducers for measurement of pro-peller-induced pressure fluctuations on the hull.

It should be pointed out that unlike the self propulsion tests describedbelow, these tests were carried out with one pontoon model only and there-fore the interaction effect between the two pontoons could not be ascertained.However, it is estimated that using one pontoon did not significantly affectthe cavitation pattern or pressure pulses during the tests.

11

12

AduuuiiuiuiiiuUU4IUÏ!lUÎ-.uii.'1

0 0.1 02 03 0.1. 05 OES 0.7 08 0.9 1.0 iiAdvance coeffiaent ,J

-SX

2X

2

o

-1

00Xl

2

3

1.

S

6

7

Xo-L2X

-1

Propeller model P1821

-2- 70 0.1 02 03 OE1. OES 06 0.7 OES 0.9 10 1.1

Advance coefficient J

Fig 9 Open water characteristics, astern thrust,

propeller models in duct D48

IIIIIII!Ul!iU!Ìk1UU

P!1ii

I lU U ur UIUU!idiUl!UlUi

o OEl 02 0.3 0.1. OES 0.6 0.7 0.8 0.9 1.0 1.1

Advance coefficient J

0°X

2

3

L

5

6

oPlopetler modeL P1869 Propeller model P1851

Xo.5a

X

3

2

o

-2

-3

¿

-S

D

o

D

2

Pro1L model P1859in duct D48

o -.0- P1821 JProp Xa 4-70 in duct NSMB 37 (D48) fixed pitch

Fig 10 Open water efficiency at astern thrust

PØØe1Ier model P1859 Piopelter model P1821

uiuiuiuiuiuiriiir4i1U!4I

iii..iu-3 -

0 01 02 03 04 05 06 07 08 09 lO LiAdvance coefficient J

D

o

3

6

o2so

4

3

2

O

2

8 3

0 01 02 03 04 0.5 06 07 0.8 09 1.0 1.1

Advance coefficient J

Fig 11 Open water characteristics, ahead thrust,

propeller models in duct D48

13

P071D 1.2

-1025

Mit-0.7

hiiUUUIuUÌLIiU

-

.rni;aiIidI.UI-

-

P!uuuI1IU-

-JO OEl 02 03 04 05 0.6 0.7 0.8

Advance coefficient J

0.5

>

0f.

e

10.3o0

0.2

0.1

o

o

o

2

3

1.

s

6

7

8

14

Prop, model P1859 in duct DI8P1821

Fig 13 Relation power-thrust

according to open

water tests

o

2Pitch ratio P0.7 ID

,t.(DL;l

prop. KCFPP

-7Owith C.

ductst F7/D= 1.1.iiiiìi

¡uUirf4uN

!UIu._

1\//¡I?

'\ ' I '

"I

t'li,\

T15kN /

- ;=,-flPkN ---- P1821 SMM ä

T-26kN

01 0.2 03 0./. 05 06 0.7 08 09 1.0

Advance coefficient J

Fig 12 Open water efficiency at ahead thrust

C,

>

Q

0.3

0.2

0.1

o

o

4

3

2

2

3

4

5

O

o Prop. model P18591-e- -i'- --- P1851 in duct D48- -r- P1821J

loo a = 1.107 __L213(Ks)

oPitch rotio P07/D

Fig 14 The Barnaby number, B. Bollard pull conditions

Port StarboardD C B

Fig 15 Ship model 2119-A, position of propeller and duct

models and pressure transducers

15

i.-

A"

kil

4

3

2

16

Xoo-7

Xo6

5

1.

3

2

o

15

s

o

250

z

1

200a

150

Propeller modet P18591Pl82i in ct D48

Non-cavitating propellerFbwer-reducìng cavitation

Fig 17 Estimated influence of power-reducing cavitation,

propeller model P1821 in duct D48. Pitch angle,

w = 32°, draught, T = 6.54

J-o

--I, -

/ OEa1

';2- -1- i.o

O7

T in al 220 P%I4 6 8 lO t2 4 Daught

iiw-.P01 -8

_dI

2 1. 5 6 7 8Cavitation number a,

Fig 16 Influence of cavitation on propeller thrust and

torque, propeller in behind condition

11 12 13

Ship speed V5 knots

00

180°00

180°

Fig 18 Cavitation patterns, propeller models at

transit conditions, draught T = 6.54 m

Cavitation Studies

Due to the very high blade tip loading (pitch) of P1821 in the transitcondition, the back cavitation on this propeller was considerable, Fig 18.The back sheet cavitation was very thick, merging into a prominent tipvortex cavitation, which collapsed on the inner duct surface. On propellermodel P1859 the back, as well as the tip vortex cavitation, was thinner andthe latter did not collapse on the duct.

The propeller force measurements carried out in connection with the cavita-tion studies indicated some force reducing cavitation at the transit con-dition on P1821, Fig 16, also confirmed by open water tests at cavitatingconditions. A prediction for P1821 based on open water characteristics atcavitating conditions indicates a speed loss of about 0.3 knots due toforce reducing cavitation, Fig 17.

P1859 in D48PrJ 7/01.4J =0.46K=0.368

270°

P1821 in D48P07/D =1.274J =0.41

Ku0.370

2700

17

18

5

0

7

o

8 9 10 1 12 13

Ship speed ,V knots

. PHV cavitation from propeller to transducer C

s

Transøucer E«---CA- -°- G

Advance coefficient JO,L1

/

_rA__u-i

_"o

Iii: IIIIUt!UIII

- -- __-

01 02 03 Q1 0.5 0.6Advance coefficient J

Fig 19 Results of pressure pulse measurements, propeller

model P1821 in duct D48. Duct angle, = 00,

pitch angle, p = 30°

E

zo.

10

Co

oo

3

2

6.2

dr1 3.68

2.27

Fig 20 Cavitation patterns, propeller model P1821 In duct

D48 at transit conditions

19

J = 0.16 J = 039 J 0.47

,.- '2 70

o2Q-5o

20

3

2

o

-1

2

3

1.

5

///

w,wI

OT

o 2Pitch ratio, P07/D

L

3

2

o;o 2

Pitch ratio P07/D

Open water tests- - - Tests with propeller

behind ship model

Fig 21 Power and thrust measurements, propeller models in

behind conditions, zero speed

The cavitationwas also studied at zero speed loadings at different pitchsettings. In agreement with earlier results, P1859 was found to have asuperior performance to P1821 at ahead running loads, Fig 22. P1859 wasthe only propeller model examined at negative pitch (astern running) inthe cavitation tunnel indicating a rather stable face cavitation oflimited extent at the loadings of current interest, Fig 22.

Bollard Pull Tests

The propeller thrust was also measured in the behind condition at zero speed.Compared with the open water tests, the thrust was somewhat higher at aheadrunning and somewhat lower at astern running condition, Fig 21.

Propeller model P1859 in duct D1.8 Propeller model P1821 in duct 0/.8

Propeller model P1859 in duct 048 zero speed

P07 ID

(KTT)

-0.85(-0.290)

-0.57c-0.190)

o(-0.015)

0.95(0.404)

1.30(0.678)

Propeller model P1821 in duct 048 zero speed

Fig 22 Cavitation patterns, towing at zero speed,

behind conditions

The results of the measurements of duct forces at different duct anglesare given in Fig 23. The lateral force obviously increases with ductangle, whilst the axial component of the duct force decreases according-ly. The propeller thrust however increases with duct angle, which gives aslight reduction of the total axial thrust of the propulsion unit with in-creasing duct angle. The results are given in Fig 21 compared with thosefor a fully rotational azimuth thruster according to reference [7] with thesame dimensions. At small duct angles the lateral force of the subjectpropulsion unit, where only the duct is turned, is somewhat higher thanthat of the azimuth thruster, where both the duct and the propeller areturned. At higher angles the order is, as expected, reversed. The azimuththruster gave at all duct angles higher axial forces, probably due to amore efficient propeller (conventional blades arid constant radial pitch).

When backing the lateral forces were at all duct angles of almost negli-gable magnitude.

P07/D

(KTT)

0.800(0.294)

1.270(0.669)

21

Duct angle -0° p 300

22

3300I-

u.,z,>-

4-, C

200

eeoz,

loo

X

o

IP J X

- P1859P1821

in duct 048

Fig 23 Duct and propeller forces, zero speed, ahead,

half power and 220 RPM, draught T = 15.25 m

TP

u, uo lo 20 30

Duct arLe , pdegrees

-

///////

X +Ip

-S-

7,-

P1859 in duct D48Azymuth thruster occ. to ref. [7].Tractor propeller P927 P/D= 1.0

Fig 24 Comparison of propulsion units, half power and

220 RPM, D = 3.0 m

23

o 10 20 30Duct angle 3 degrees

5Oo

o->-I-+

400

300

200

100

o

24

7

6

5

4

3

2

0-8

P1859P1821

}SMM designs in duct 01.8

knots

300

z

200

ow

100

/

I

IIPpOG9MW

//

I

/A!II¿aLI,p -/A

NT

g 10 11 12 13

Ship speed.V5knots

Fig 25 Full scale prediction according to the SSPA

standard procedure

Pressure Pulse Measurements

The pressure pulses caused by propeller model P1821 were measured at 8points on the model hull surface, Fig 15. Generally the blade frequencyamplitudes were fairly low and the pressure pulse coefficients not signi-ficantly influenced by the extent of cavitation, Fig 19.

At transducer C the pressure pulses were measured over an extensive rangeof advance coefficients and cavitation numbers. The results of these measure-ments form the basis for the level curves for similar pressure pulse co-efficients given in Fig 19. At one advance coefficient, J = 0.16, propellerhull vortex (P1W) cavitation was continuously appearing, Fig 20. The vortexhit the hull just at transducer C, and extremely high pressure pulses wererecorded, sometimes around 20 times that of the expected value.

25

.7. SELF PROPULSION TESTS

Self propulsion tests were carried out in the SSPA towing tank with acomplete ship model (2 pontoons) at light draught (6.5! metres), drivenby two ducted propeller models. The full scale power and RPM predictionfor propellers P1821 and P1859 are given in Fig 25. This diagram indi-cates a speed gain of 0.2 knots at constant power with propeller P1859canpared with P1821.

CONCLUSIONS

As already mentioned better positioning properties could gave been obtainedwith fully rotational azimuth thrusters, which in this case could not beadopted due to draught and space restrictions.

Better propeller efficiency could have been achieved at some loadings withFP propellers, which could not be fitted in this case since the main pro-pulsion was by means of electric, fined revolutions motors.

An alternative duct with a somewhat reduced duct ratio slight have improvedthe backing properties, perhaps also giving a somewhat improved ahead per-formanc e.

A larger propeller diameter would have increased the towing forces at zerospeed, but again this was not possible in this case due to the restricteddraught.

With the existing duct and the given conditions the propeller modelP1859 was found to be a good compromise both with regard to transitspeed, tow rope forces at zero speed and cavitation properties whenrunning ahead in the transit condition. F'urthermore the propeller gaveacceptable performance also when running astern.

It is the authors' hope that this paper will be of some guidance forfuture designs of multi-purpose units.

ACKNOWLEDGEMENTS

The present investigations have been carried out at Swedish MaritimeResearch Centre, SSPA, on behalf of British Petroleum Limited, theBritish National Oil Corporation and Scott-Lithgow Limited. Theauthors wish to express their sincere thanks to the heads of theseestablishments for their support of the work.

Thanks are also extended to the staff of SSPA who took part in theevaluation and analysis of the test results as well as to SMM Pro-pellers Limited who designed two of the models tested.

10. REFERENCES

LANCASTER GERALD H and HAINES JOHN R. "Towing Vessel Screw Propulsion",6th Intern. Tug Convention, Hamburg 1979, pp 211-218.

DYNE GILBERT. " A Method for the Design of Ducted Propellers in a Uni-forni Flow", SSPA Publ. No 62, Göteborg 1967, 50 pp.

OOSTERVELD M W C. " Wake Adapted Ducted Propellers", Netherl. ShipModel Basin Publ. No 345, Wageningen 1970, 130 pp.

R] DYNE GILBERT. "Systematic Studies of Accelerating Ducted Propellers inAxial and Inclined Flows", Symp. on Ducted Propellers, The Royal Inst.of Naval Architects, London 1973, pp ll4-l214.

LINDGREN HANS and BJÄENE ERIC. " Ten Years of Research in the SSPALarge Cavitation Tunnel", Stone Manganese Marine (SIvllvl)/NewcastleUniversity Conference, Newcastle 1979, pp 7-1 - 7+20 (also SSPA PublNo 86, Göteborg 1980, 38 pp).

VAN MANEN J D. "Effect of Radial Load Distribution on the Performanceof Shrouded Propellers", Trans. Royal Inst. of Naval Architects,London 1963, vol 105, pp 59-74.

MINSAAS KNUT J and LEHR ERIK. "Hydrodynamical Characteristics ofRotatable Thrusters", The Ship Res. Inst. of Norway (NSFI) Report69.78, Trondheim 1978.

11. DEFINITIONS

KB = Barnaby's number = 10.01107

T2

L (KQ)

C = Propeller blade width

D = Propeller diameter; as index: duct

F = Propeller profile mean line camber (max)r

G = Circulation coefficient (=

g = Acceleration due to gravity force

H = Draught of propeller shaft below w.s.V

J Advance coefficient (=DmT

K = Thrust coefficient (=T pDn2

K = Torque coefficient (- Q

Q pD5n2

K = Pressure pulse coefficient (

2

PpD2n2

26

n, N = Rate of revolutions (ris and RPM resp.)

P = Propeller pitch, as index: propeller

P01 = Propeller pitch at O.7R

PD = Delivered shaft power

p0 = Static pressure at propeller shaft [=101 300 + yH(Pascal)]

Vapour pressure of water

2dp = Propeller-induced pressure pulses, peak to peak value

Q = Propeller torque

R = Propeller radius (D/2)

r = radius

= Duct radius at entrance

R = Minimum duct radiusmin

S = Max. propeller blade profile thickness

T = Thrust, as index: total or trial

VA Propeller speed of advance

V = Ship speedV -VsA

w Wake fraction (-

X = Force in the direction of ship centre line

x Radius ratio (r/R)

Y Force lateral to ship centre line

= Duct angle in relation to shaft centre line

F = Circulation

y = Weight by volume (kg/rn3)

= Propeller efficiency (=TVA/2iînQ)

P = Density of water (=

= Cavitation number based on rates of revolution

= Pitch angle (arctg ------irxD

pu -pv

/0 ( 2

27