Multi-purpose ducted cp-propellers for a semi-submersible offshore support vessel

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MARINTEKNISKA INSTITUTET SSPA

SWEDISH MARITIME RESEARCH CENTRE SSPA

GÖTEBORG

PUBLICATION NO 92

1982

MULTI-PURPOSE DUCTED CP-PROPELLERS

FOR A SEMI-SUBMERSIBLE OFFSHORE

SUPPORT VESSEL

by

ERIC BJÄRNE and DAN MELITZ

Paper

presented at the International

Conference Offshore Göteborg 81

Göteborg, August 1981

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Distributed by: Liber Distribution S-16289 STOCKHOLM Sweden

ISBN 91-38-06762-5 ISSN 0373-4714

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Fig 1

Ship model 2119-A in SSPA cavitation tunnel No 2

I

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ABSTRACT

A semi-submersible support vessel consisting of a platform connected by columns to two displacing pontoons was designed with two main fixed shaft CP propellers in partially steering ducts. These propulsion units had to fulfil certain requirements at different draughts and propeller loadings, which entailed a difficult compromise design. The main problem vas the reconcilation of the conflicting requirements for almost equal ahead and

astern bollard pull thrust in the operating (deep draught) condition, with cavitation 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 knots at full power. This condition was found to be the most critical one from the cavitation point of view. Thrust reducing and eroding cavitation had to be avoided or at least minimized.

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

the 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 absorbed

by the propellers.

Possibly the design requirements could have been solved more easily by the application of rotational thrusters with angular gears, but due to restricted draught and other space considerations this solution could not be adopted. In this paper three different propeller designs which were investigated are described. The results of the model tests which were carried out in the SSPA cavitation laboratory, Fig 1, and in the towing tank have been analysed and

are discussed.

DESIGN PROBLEMS

Propeller Design

The problems connected with the design of multipurpose CF propellers have been frequently discussed previously, for instance in connection with tug

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6

Bollard pullastern

Half power

Propeller model P1859,SMM design,

=0.79

-'i- P1851,SSRA -- -- =0.82

-- P1821SMM -- -'- :0.78

Bollard pull,ahead

Transit Half power

Full power

r

..

IVÍII

UIIlIV1VA

'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 diadia-meter was in this case restricted to 3 metres. A larger diameter would have been advantageous for all conditions, but

especial-ly

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

The desired rate of revolutions (220 RPM) could be constantly absorbed at the different loadings by adjusting the pitch. Due to the fact that the required pitch is obtained by blade turning, this giving the same change of pitch angle for all radii, the radial pitch and thus, the loading distribution will be

significantly deformed at off-design conditions, Fig 2. This fact disfavours

the CF propellers in relation to FP propellers, but a better utilization of the 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 greatest

extent.

Duct Design

In this case the propeller is placed inside a duct, the properties of which are greatly dependant on the radial loading distribution of the propeller. The shape of the optimal duct is associated with a specific propeller having a given radial circulation distribution. If the circulation distribution is changed 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

a

0.8 0.7 0.6 0.5 0.4 0.3

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Propeller model P1859 SMM design A0 IA0 0.79

Pl85lSSPA --, -'- 0.82

-

n P82lSMM e, - n nO.78

O 100 200

Max. profile thickness S n mm

¿ 10 20 30 40

Camber F ,mm

Fig 3

Propeller main data

3. PROPELLERS

111 the propellers tested had the Kaplan blade shape, Fig 3, ifl order to

obtain acceptable cavitation properties at off-design conditions. The propeller model P1821 had a constant pitch and was designed by Stone Manganese Marine Ltd (SNM) for almost zero loading. This propeller gave

sufficient backing power but unacceptable and even power-reducing cavitation at the transit condition, Figs i6, lT and 18.

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

A third propeller designed by SMM, P1859, had the pitch adapted for the bollard pull ahead conditions. This propeller was better in the transit condition than the 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 different loadings are described in Figs 2 and 3.

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8

Fig 4

Duct model D48, profile

/ \

Transit comdutions

Towing astern G0O.O650

G07O.O876

/

tt

7

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

0.3 C

o

a

0.2 0.1

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- - - 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

1.5 Duct ratioR0/R

Fig 7

Influence of duct ratio on open water efficiency

K Y __

_{= 2.063, J = 0.4}

-u -a

'IÇ1h59

_in

.015

ti':

016 01.6 : 0.45 I.. 1144 Q. o I- Q-0.43 042 0.41 0.40

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lo

9 jIWb

0'

_=:

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 propeller models 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 blades turned for backing, the circulation distribution for the latter case was estimated, Fig 5, and the corresponding duct section was calculated accord-ing to reference [2]. It was found that the diffusor angle for backaccord-ing

(duct ratio at ahead condition) should be much smaller, Fig

6.

In order not

to deteriorate the ahead performance too much, a mean value of duct ratio is proposed. This duct would be expected to reduce the power demand also in the 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

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OPEN WATER TESTS

The open water tests were carried out in the SSPA cavitation tunnel No 2 in the high speed test section [5]. The propeller was nounted on an upstream

shaft, Fig 8. The duct was connected to the duct balance by means of three

struts, which according to previous experience did not influence the flow significantly. The water velocity was kept constant and the rates of

re-volutions were varied so that a range of advance coefficients was covered

and corresponding points measured. Highest possible RPM and velocity were

applied to minimize the scale effects.

All the propeller models were tested in backing conditions. The results of these tests are given as thrust and torque coefficients against the advance

coefficients for propeller and duct, Fig 9. It is noticeabLe that for almost

all conditions the duct thrust is opposite to the propeller thrust. The relation between the propeller models with regards to propeller efficiency when 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 for some FP propellers, Ka 4-7O in the duct NSMB No 37 when backing are also given, 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 conditions it consequently gives the better results of the two, which also can be seen from the propeller efficiency curves, Fig 12. In this diagram the optimal curve for FP propellers with radially constant pitch, Ka 4-7O in duct NSMB

No 37 [3] is also introduced, indicating that the power increase due to the

introduction 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 different pitch settings (positive and negative) are given in Fig 13. In line with the earlier results, P1859 is found to be the best at ahead running (posi-tive pitch) and P1821 the best at running astern (nega(posi-tive pitch). p1851 was not tested at ahead running. The relation is further conf,rmed in

Fig

l4,

where the Barnaby numbers [6], i e 0.01107 . KT/(Kfl)213, are

compared for the different propellers and pitch settings.

TESTS IN BEHIND CONDITIONS

A complete wooden model of one of the pontoons of the vessel was mounted in the large test section of the cavitation tunnel and fitted in turn with the right-handed mod1 of propellers P1821 and P1859, Fig 1. The position of the propeller and duct is shown in Fig 15. This figure also shows 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 described below, 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 affect the cavitation pattern or pressure pulses during the tests.

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12

Ad

uuuiiui

uiiiu

_UU4IU

Ï!lU

Î-.uii.

_'1

0 0.1 02 03 0.1. 05 OES 0.7 08 0.9 1.0

ii

Advance coeffiaent ,J

-S

X 2 X 2 o -1

00

_X l 2 3 1. S 6 7 X

o

-L 2 X -1 Propeller model P1821 -2- 7

0 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

U

IUU!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 o

Plopetler modeL P1869 Propellermodel P1851

X

o

.5

a

X 3 2 o -2 -3 ¿ -S

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D o D 2 Pro1L model P1859 in duct D48

o -.0- P1821 J

Prop Xa 4-70 in duct NSMB 37 (D48) fixed pitch

Fig 10

Open water efficiency at astern thrust

PØØe1Ier model P1859 Piopelter model P1821

uiuiuiu

iuiuiri

iir4

i1U!4I

iii..iu

3 -0 01

02 03 04 05 06 07 08 09

lO Li Advance coefficient J D o 3 6 o

2s

o

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

P071D 1.2 -1025

Mit

-0.7

hiiUUUI

u

UÌLIiU

-.rni;ai

_IidI.UI

-P!uuuI1IU

--J O OEl 02 03

04

05 0.6 0.7 0.8 Advance coefficient J 0.5 > 0f. e

10.3

o 0 0.2 0.1 o o o 2 3 1. s 6 7 8

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14

Prop, model P1859 in duct DI8 P1821

Fig 13

Relation power-thrust

according to open

water tests

o 2 Pitch ratio P0.7 ID ,t. (DL;l prop. K CFPP -7O with C. duct st _{F7/D= 1.1.}

iiiiìi

¡uU

irf4uN

!UI

u._

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

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O

o Prop. model P18591

-e- -i'- --- P1851

in duct D48

-

-r- P1821J

loo a = 1.107 __L213

(Ks)

o

Pitch rotio P07/D

Fig 14

The Barnaby number, B.

Bollard pull conditions

Port Starboard

D C B

Fig 15

Ship model 2119-A, position of propeller and duct

models and pressure transducers

i.-A"

kil

4 3 2

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16 X o o

-7

X o 6 5 1. 3 2 o 15 s o 250

z

1 200 a 150 Propeller modet P18591 Pl82i in ct D48 Non-cavitating propeller Fbwer-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%I 4 6 8 lO t2 4 Daught

iiw-.

P01 -8

_dI

2 1. ₅ ₆ 7 8 Cavitation number a,

Fig 16

Influence of cavitation on propeller thrust and

torque, propeller in behind condition

11 12 13

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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 transit condition, the back cavitation on this propeller was considerable, Fig 18. The back sheet cavitation was very thick, merging into a prominent tip vortex cavitation, which collapsed on the inner duct surface. On propeller model P1859 the back, as well as the tip vortex cavitation, was thinner and

the latter did not collapse on the duct.

The propeller force measurements carried out in connection with the

cavita-tion studies indicated some force reducing cavitacavita-tion at the transit

con-dition on P1821, Fig 16, also confirmed by open water tests at cavitating conditions. A prediction for P1821 based on open water characteristics at cavitating conditions indicates a speed loss of about 0.3 knots due to force reducing cavitation, Fig 17.

P1859 in D48 PrJ 7/01.4 J =0.46

K=0.368

270° P1821 in D48 P07/D =1.274 J =0.41

Ku0.370

2700

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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

«---C

A

- -°- G

Advance coefficient JO,L1

/

_rA

__u-

i

_"o

Iii:

_II

IIUt!UII

I

-

-- __

-01 02 03 Q1 _0.5 0.6 Advance coefficient J

Fig 19

Results of pressure pulse measurements, propeller

model P1821 in duct D48. Duct angle,

=

00,

pitch angle,

p = 30° E

z

o. 10 C o o

o

3 2

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6.2

dr1

3.68

2.27

Fig 20

Cavitation patterns, propeller model P1821 In duct

D48 at transit conditions

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,.- '2 7

0

o2

Q-5

o 20 3 2 o -1 2 3 1. 5

/

w,

_w

I

OT o 2 Pitch 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 pitch settings. In agreement with earlier results, P1859 was found to have a superior performance to P1821 at ahead running loads, Fig 22. P1859 was the only propeller model examined at negative pitch (astern running) in the cavitation tunnel indicating a rather stable face cavitation of limited 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 ahead running and somewhat lower at astern running condition, Fig 21.

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Propeller model P1859 in duct 048 zero speed P07 ID (KTT) -0.85 (-0.290) -0.57 c-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 angles are given in Fig 23. The lateral force obviously increases with duct angle, whilst the axial component of the duct force decreases according-ly. The propeller thrust however increases with duct angle, which gives a

slight reduction of the total axial thrust of the propulsion unit with in-creasing duct angle. The results are given in Fig 21 compared with those for a fully rotational azimuth thruster according to reference

[7]

with the same dimensions. At small duct angles the lateral force of the subject propulsion unit, where only the duct is turned, is somewhat higher than that of the azimuth thruster, where both the duct and the propeller are

turned. At higher angles the order is, as expected, reversed. The azimuth thruster gave at all duct angles higher axial forces, probably due to a

more 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) Duct angle -0° p 300

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22

3300

I-u., z,>-4-, C

200 ee

o

z,

loo

X o IP J X

- P1859

P1821

in duct 048

Fig 23

Duct and propeller forces, zero speed, ahead,

half power and 220 RPM, draught T = 15.25 m

TP

u,

u

o

lo

20 30

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-/

/

X +Ip

-S-7

,-P1859 in duct D48

Azymuth 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

o

10 20 30

Duct angle

3 degrees

5Oo

o->-

I-+ 400

300

200

100

o

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24 7 6 5 4 3 2

0-8 P1859 P1821

}SMM designs in duct 01.8

knots 300

z

200 o w 100 /

I

PpOG9MW

/

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 8 points on the model hull surface, Fig 15. Generally the blade frequency amplitudes 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 range of 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, propeller hull vortex (P1W) cavitation was continuously appearing, Fig 20. The vortex hit the hull just at transducer C, and extremely high pressure pulses were recorded, sometimes around 20 times that of the expected value.

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.7. SELF PROPULSION TESTS

Self propulsion tests were carried out in the SSPA towing tank with a complete ship model (2 pontoons) at light draught (6.5! metres), driven by two ducted propeller models. The full scale power and RPM prediction for 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 P1859

canpared with P1821.

CONCLUSIONS

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

Better propeller efficiency could have been achieved at some loadings with FP 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 improved

the backing properties, perhaps also giving a somewhat improved ahead

per-formanc e.

A larger propeller diameter would have increased the towing forces at zero speed, but again this was not possible in this case due to the restricted

draught.

With the existing duct and the given conditions the propeller model P1859 was found to be a good compromise both with regard to transit speed, tow rope forces at zero speed and cavitation properties when

running ahead in the transit condition. F'urthermore the propeller gave acceptable performance also when running astern.

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

ACKNOWLEDGEMENTS

The present investigations have been carried out at Swedish Maritime Research Centre, SSPA, on behalf of British Petroleum Limited, the British National Oil Corporation and Scott-Lithgow Limited. The authors wish to express their sincere thanks to the heads of these

establishments for their support of the work.

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

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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. Ship

Model Basin Publ. No 345, Wageningen 1970, 130 pp.

R]

DYNE GILBERT. "Systematic Studies of Accelerating Ducted Propellers in Axial 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 SSPA

Large Cavitation Tunnel", Stone Manganese Marine (SIvllvl)/Newcastle

University Conference, Newcastle 1979, pp 7-1 - 7+20 (also SSPA Publ

No 86, Göteborg 1980, 38 pp).

VAN MANEN J D. "Effect of Radial Load Distribution on the Performance of Shrouded Propellers", Trans. Royal Inst. of Naval Architects,

London 1963, vol 105, pp 59-74.

MINSAAS KNUT J and LEHR ERIK. "Hydrodynamical Characteristics of Rotatable Thrusters", The Ship Res. Inst. of Norway (NSFI) Report

69.78, Trondheim 1978.

11. DEFINITIONS

K

B = Barnaby's number = 10.01107 T 2

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 (= Dm T K = Thrust coefficient (= T

_pDn2

K = Torque coefficient (- Q Q _pD5n2

K = Pressure pulse coefficient (

2

P

pD2n2

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n, N = Rate of revolutions (ris and RPMresp.) 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 radius min

S = Max. propeller blade profile thickness

T = Thrust, as index: total or trial

VA Propeller speed of advance

V = Ship speed

V -V

_sA

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