investigation of perfect duct for naval systems

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Investigation of the Optimum Duct Geometry for A Passenger Ferry

F. elik 1*

A. Dorul 1

Y. Arkan 1Yildiz Technical University,

Dept. of Naval Architecture and Marine Engineering, Istanbul, TURKIYE.

ABSTRACT: The use of a duct around the propeller aims to increase the propulsive efficiency by means of

accelerating the inflow to the propeller (accelerating duct, kort nozzle), to decrease the inflow to the propeller

for reducing the cavitation risk (decelerating duct), or to protect the propeller against damage. In this study

the optimum duct geometry is investigated for a passenger ferry with the aim of protecting the propelleragainst damage and if possible to increase the propulsion efficiency. The effects of various duct sections on

performance of the ducted propellers are analyzed by a ducted propeller analysis method based on lifting

surface theory.

1 INTRODUCTION

Screw propellers are the most common devices in

propulsion of the marine vehicles for the last 170years. The purpose of a propeller is usually production of thrust needed to overcome the shipresistance. This is carried out in reaction to themomentum produced by accelerating the flow as aresult of the energy transferred to the water. The

power delivered to the propeller produces a suddenincrease of pressure at the propeller disk. Thiscauses acceleration of the water in axial directionand produces thrust. While the shaft torque istransferred to the water, it causes induced velocity

losses in rotational, radial or axial directions.Besides the energy losses due to increases in theslipstream, the blade friction losses also occurassociated with the passage of the blades throughthe viscous zone. These energy losses are given inFig. 1 (Glover, 1987). As indicated in this figure,especially axial energy losses are increasingrelated with thrust loading. In order to achievefurther gains in efficiency, additional auxiliary

propulsor devices are required to reduce the axialenergy losses.

Fig.1 Propeller energy losses for a range of thrust loading.

During the last three decades, considerable effort

has been made in order to improve the propulsiveefficiency of screw propellers used on ships. Oneof these propulsors is called duct or known asnozzle found as widespread application.Ducts are generally used to obtain additional gainin efficiency, but also are used to reduce thecavitation risk or to protect the propeller fromdamage. The first type is named as acceleratingduct or kort nozzle and the second type is thedecelerating duct as shown in Fig. 2.In an accelerating duct, the flow velocity isincreased due to the duct. Decelerating duct shapescan cause the flow to be decreased, which keepaway the risk of cavitation but may decreaseefficiency. The decelerating duct is generally

0 10 20 30 40 50 60 70

0 1 2 3 4 5 6 Thrust Loading C oefficient (C T )

E n e r g y

L o s s e

s ( % )

Axial Rotational Drag Total

IX HSMV Naples 25 - 27 May 2011 1


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suitable for navy ships, so it is rarely applied. Thedecelerating duct operates as a pump jet.

(a) (b)

Fig. 2 (a) decelerating duct, (b) accelerating duct.

The duct device was first introduced to marinevessels by Stipa and Kort, and developed byexperimental works. They have showed thatapplication of duct increases propulsive efficiency

for heavily loaded propellers. Interaction of ductand propeller is taken into account by van Manenand Oosterveld (1966) at Maritime ResearchInstitute Netherlands (MARIN). Extensivesystematical experiments on ducted propellershave been carried out using standard nozzles,including the accelerating and decelerating typeswith the Ka Propeller Series. It is seen that the useof an accelerating nozzle produces an increase inefficiency only at a higher thrust loading. For lightloading, Sparenberg (1969) has showed that the

representation of a propeller by an actuator disc ina duct in axisymmetric flow yields the efficiencyof the actuator disc alone regardless of the ductshape.A more comprehensive study has been carried out

by Kerwin et al. (1987) where the flow on the ductwas predicted by using a potential based panelmethod. Ryan and Glover (1972) have presentedan interactive procedure by combiningaxisymmetric surface vorticity analysis for theduct and propeller design based on a lifting linetheory. A similar method has been introduced byGibson and Lewis (1972) where the propeller wasmodeled by an actuator disk. This method isapplicable to propellers of arbitrary radial loading,and it therefore is purely an analysis method. Ryanand Glovers method was then improved to theopen water off-design problem for the

performance analysis of ducted propeller byCaracostas (1978) based on the surface vorticitydistribution technique. Falcao De Campos (1983)established extensive studies on the calculation ofduct performance in uniform and radialy variableinflow. Glover and Szantyr (1989) presented a

paper to predict the performance of ducted propellers operating in a non-uniform velocity

field. They calculated the pressure distribution andthrust on propeller and duct.In this study, numerical analyses are carried outfor five different duct geometries, and the resultsare compared with each other and the propellerwithout duct. Whether an additional gain inefficiency is investigated for various nozzlegeometries for a high speed passenger ferry.

2 DUCTED PROPELLER ANALYSIS

METHOD

The ducted propeller analyses are carried out usingan analysis method based on the lifting surfacetheory presented by Glover and Szantyr (1989). Inthis method, the hydrodynamic loading on the

propeller blades and on the duct is replaced byappropriate distribution of vorticity while thethickness of the propeller blades and duct ismodeled by the appropriate distribution of sourcesand sinks. These singularities are distributed onthe surfaces built up by the meanlines of the

propeller blade sections and the meanlines of thechordwise duct sections. Kinematic boundaryconditions are utilized to determine the vortexdistributions which represent hydrodynamic loads.The kinematic boundary condition forms origin ofthe lifting surface equation formulation. According

to this condition, the velocity of the flow whichcomes to the lifting surface should be parallel tothe surface. In other words, no flow should passfrom the surface; total of normal velocity should

be zero at all points on the surfaces of the nozzleand the propeller camber line. This condition can

be written as below for propeller and nozzle liftingsurfaces:

1 1 1. . ( ) . . ( )

4

1 1. . ( ) . . ( )

1 1. . ( ) . . ( )

1 1( ) ( ) ( ) ( )

( .

p pv

ps d

dv ds

p d

p pv p pS S

po d p d S S

dv dod d S S

p pc d dc p pS S

n dS n dS r r

n dS n dS r r

n dS n dS r r

q q dS q q dS n r n r

V

). 0 R n

In the equation above, pcq and dcq is written only

if cavitation is determined. Also . R is relevantwith rotation of the propeller and is taken into



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account if the calculation is made only at a pointon the propeller. In case of sheet cavitation on

propeller blades or nozzle, the normal vectorn changes to show the change in the originallifting surface geometry.

3 GEOMETRY OF DUCTED PROPELLERS

In this study, for a passenger ferry, four different

nozzle geometries are investigated if there is any

gain in efficiency or not. While the first two of

these duct sections are conventional (Nozzle 19A,

Nozzle 32), other two sections are newly designed

sections (HR, Rice-Speed).

3.1 Conventional duct sections

Two of the duct profiles which are investigated inthis study are Nozzle 19A (accelerating) and

Nozzle 32 (decelerating) as given in Fig. 3.Some duct designs can cause drag as the speed ofadvance increases. With the Kort nozzle, this drag

becomes more significant at higher speeds and caneventually reduce the overall thrust gain to zero.For Nozzle 19A nozzle, an axial cylindrical part atthe inner side of the nozzle at the location of thescrew, the outside of the nozzle profile is made

straight and the trailing edge of the nozzle isthicker.Detailed information about these nozzle sectionscan be found in Oosterveld (1970).

(a)

(b)

Fig.3 (a) Nozzle 19A, (b) Nozzle 32.

3.2 Rice-Speed and HR nozzles

Conventional duct sections (e.g. Nozzle 19A)cause an additional drag in high speed ships. Forincreasing efficiency in high speed ships, somenozzle sections are developed such as Rice-Speed

nozzle (Rice Propulsion) and HR nozzle (Wrtsila)as given in Fig. 4.

(a)

(b)

Fig.4 Rice-Speed nozzle, (b) HR nozzle.

The section of a Rice-Speed nozzle was developedfrom air wing sections displaying highestlift/lowest drag properties (van Manen &Ooosterveld 1966). A comparison of flow around

Nozzle 19A and Rice-Speed nozzles is given Fig.5 (Wrtsila).

Fig.5 Flow around Nozzle 19A and Rice-Speed nozzles.

The nozzles are aerofoil shaped rings placedaround the propeller. Nozzles have found theirapplication in ships for decades with good results.

The main advantage of the nozzle is that itincreases the trust on the propeller. Comparing propellers with and without nozzles shows that thenozzle propeller offers about 25% more total thrust(nozzle and propeller) than an open propeller atzero ship speed (bollard condition). At high shipspeeds this difference becomes less up to the pointwhere the nozzle generates drag instead of thrust.The HR nozzle is very useful for improving the

performance of the propulsion unit of tugs anddredgers. This nozzle generates more thrust at

dredging conditions in combination with morethrust at free-sailing speed, compared withconventional nozzles such as the Nozzle 19Anozzle.



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Fig.6 A comparison of velocity fields between Nozzle 19Aand HR nozzles.

As seen in Fig. 6, according to the velocity fieldsaround the Nozzle 19A and HR nozzles, Nozzle19A shows more drag because of the high velocityfield while HR nozzle produces more lift(Wrtsila).

Nowadays, new nozzle designs are used includingthe HR nozzle. Applications of the newly-designednozzles are available for even high speed shipssuch as RoPax and passenger ferries.

4 ANALYSIS OF DUCTED PROPELLERS

In this chapter, for a fast passenger ferry propulsion efficiency is analysed for speeds between 8 and 20 knots. These analyses are carriedout for the ducted propeller geometries mentionedin previous chapter. For this purpose, propulsionefficiency is calculated for propeller with Nozzle19A, propeller with Nozzle 32, propeller with HRnozzle and propeller with Rice-Speed nozzle. Inall cases, the nozzle length is taken as 0.4D. Alsoanalyses are made for open propeller and propellerwith Nozzle 19A with a nozzle length of 0.5D.Two propeller geometries are designed for open

propeller (Fig. 7) and ducted propellers (Fig. 8),and same propeller section is used in all analysesincluding the open propeller analysis. The

differences in open propeller and ducted propellersare the pitch and nozzle lengths. The designed propellers do not have any rake or skew as the propeller data is given below.

Propeller Data:Ship length, Lbp =59.4 mDelivered power, P D = 2000 kWPropeller diameter, D = 1.46 mPropeller rate of rotation, N = 600 rpm

Number of blades, Z = 4

Ship design speed, V S = 18 KnotsPitch ratio at 0.7R (P/D) = 0.983 (open)Pitch ratio at 0.7R (P/D) = 1.193 (ducted)Blade area ratio, A E/Ao =1.1

Blade Section: NACA66 (a = 0.8) Nominal wake fraction, 1-w = 1.022

The open propeller design is made with lifting linemethod (Celik & Guner 2006), while lifting line isused for the propeller and axisymmetric vortexdistribution (Celik et al. 2010) is used for duct inducted propeller design.

Fig.7 3-D model of the open propeller

Fig.8 3-D model of the ducted propeller with 19A nozzle

The ducted and open propeller performance predictions for a high speed passenger ferry fordifferent nozzle geometries are carried out using alifting surface analysis method developed byGlover and Szantyr (1989).The propulsion efficiencies for open propeller andducted propellers for speeds between 8 and 20knots are given in Fig. 9.



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20

25

30

35

40

45

50

55

60

65

8.00 10.00 12.00 14.00 16.00 18.00 20.00

Ship speed, V (knot)

P r o p u l s

i o n e

f f i c i e n c y

( % )

Open propeller 19-A nozzle+propeller Rice nozzle+propeller HR nozzle+propeller 32 nozzle+propeller 19A DC=0.5D nozzle+propeller

Fig.9 Comparison of propulsion efficiencies of ducted and

open propellers.

As can be seen from Fig. 9, for a fast passenger

ferry in a design speed of 18 knots;

The decelerating nozzle has nearly the same

efficiency in all speeds with respect to the

open propeller. The decelerating nozzle does

not cause additional drag.

The increase in length of Nozzle 19A causes a

decrease in efficiency. A decrease of % 10 in

nozzle length gives an increse of % 3 of

efficiency in design speed.

In speeds under 16 knots, the efficiencies of all

ducted propellers are nearly same. In high

speeds, the efficiencies differ for ducted

propellers.

In design speed, the highest efficiency is in HR

nozzle. This is % 6 more than Nozzle 19A and

% 8 more than open propeller. The difference

in efficiency between Nozzle 19A and HR

nozzle increases with the increase in speed.

5 CONCLUSIONS

A gain in propulsion efficiency can be

obtained for even high speeds by using

nozzles.

It is possible to increase propulsion efficiency

with modern nozzle designs.

In this study, the decelerating nozzle (Nozzle

32) has nearly no effect on the propeller.

The open propeller is more advantageous in

speeds of over the design speed.

In design speed, Rice-Speed and HR nozzles

can provide up to % 10 more gain compared to

open propeller and Nozzle 19A. This result is

similar with the claims of Rice-Speed and HR

nozzle designers.



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References

van Manen, J.D. and Oosterveld, M.W.C. (1966) Analysisof Ducted Propeller Design. Trans. SNAME, Vol. 74, 522-561, 1966.

Sparenberg, J.A. (1969) On Optimum Propellers with A Duct of Finite Length. Journal of Ship Research, 35(2): 115-61, 1969.

Kerwin, J.E., Kinnas, S.A., et. al. (1987) A Surface Panel Method for the Hydrodynamic Analysis of Ducted Propellers. Trans. SNAME, Vol. 95, 93-122, 1987.

Ryan, P.J. and Glover, E.J. (1972) A Ducted Propeller Design Method: A New Approach Using Surface Vorticity Distribution Techniques and Lifting Line Theory. Trans. RINA, Vol. 144, 545-563, 1972.

Gibson, I. S. and Lewis, M. A. (1972) Ducted Propeller Analysis by Surface Vorticity and Actuator Disc Theory. Proc. Symposium on Ducted Propellers, pp.1-10, RINA,1972. London.

Caracostas, N. (1978) Off-Design Performance Analysis of Ducted Propellers. Proc. Propellers/Shafting78 Symposium ,SNAME, pp. 3.1-3.18, 1978. Virginia, USA.

Falcao de Campos, J. A. C. (1983) On the Calculation of Ducted Propeller Performance in Axisymmetric Flows.Technical Report 696, Netherlands Ship Model Basin,Wageningen, 1983. The Netherlands.

Glover, E.J. and Szantyr, J. (1989) The Analysis ofUnsteady Cavitation and Hull Surface Pressures for Ducted

Propellers. Trans. RINA, Vol. 132, 65-78, 1989.

Oosterveld, M.W.C. (1970) Wake Adapted Ducted Propellers. Publication No. 345, Netherlands Ship Model Basin, Wageningen, 1970. The Netherlands.

Glover, E.J. (1987) Propulsive Devices for Improved Propulsive Efficiency. Trans. Institute of Marine Engineers,Vol. 99, The Institute of Marine Engineers, London.

Celik, F. and Guner, M. (2006) An Improved Lifting Line Model for the Design of Marine Propellers. MarineTechnology, SNAME, Vol. 43, No: 2, 100-113, 2006.

Celik, F., Guner, M., Ekinci, S., (2010) An Approach to the Design of Ducted Propellers. Transaction B: Mechanical Engineering, Scientia Iranica, Vol. 17, No: 5, 406-417, 2010.

www.uhfg.se/pdf/fuelsavings.pdf , W rtsila.www.ricepropulsion.com , Rice Propulsion.

http://www.uhfg.se/pdf/fuelsavings.pdfhttp://www.uhfg.se/pdf/fuelsavings.pdfhttp://www.ricepropulsion.com/http://www.ricepropulsion.com/http://www.ricepropulsion.com/http://www.uhfg.se/pdf/fuelsavings.pdf

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