Investigation of the Optimum Duct Geometry for A Passenger Ferry

F. Çelik1*

A. Doğrul1

Y. Arıkan1

Yildiz 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 propeller

against 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 170

years. The purpose of a propeller is usually

production of thrust needed to overcome the ship

resistance. This is carried out in reaction to the

momentum produced by accelerating the flow as a

result of the energy transferred to the water. The

power delivered to the propeller produces a sudden

increase of pressure at the propeller disk. This

causes acceleration of the water in axial direction

and produces thrust. While the shaft torque is

transferred to the water, it causes induced velocity

losses in rotational, radial or axial directions.

Besides the energy losses due to increases in the

slipstream, the blade friction losses also occur

associated with the passage of the blades through

the viscous zone. These energy losses are given in

Fig. 1 (Glover, 1987). As indicated in this figure,

especially axial energy losses are increasing

related with thrust loading. In order to achieve

further gains in efficiency, additional auxiliary

propulsor devices are required to reduce the axial

energy 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 propulsive

efficiency of screw propellers used on ships. One

of these propulsors is called duct or known as

nozzle found as widespread application.

Ducts are generally used to obtain additional gain

in efficiency, but also are used to reduce the

cavitation risk or to protect the propeller from

damage. The first type is named as accelerating

duct or kort nozzle and the second type is the

decelerating duct as shown in Fig. 2.

In an accelerating duct, the flow velocity is

increased due to the duct. Decelerating duct shapes

can cause the flow to be decreased, which keep

away the risk of cavitation but may decrease

efficiency. The decelerating duct is generally

0 10 20 30 40 50 60 70

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

Energ

y L

osses (

%)

Axial Rotational Drag Total

IX HSMV Naples 25 - 27 May 2011 1

suitable for navy ships, so it is rarely applied. The

decelerating duct operates as a pump jet.

(a) (b)

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

The duct device was first introduced to marine

vessels by Stipa and Kort, and developed by

experimental works. They have showed that

application of duct increases propulsive efficiency

for heavily loaded propellers. Interaction of duct

and propeller is taken into account by van Manen

and Oosterveld (1966) at Maritime Research

Institute Netherlands (MARIN). Extensive

systematical experiments on ducted propellers

have been carried out using standard nozzles,

including the accelerating and decelerating types

with the Ka Propeller Series. It is seen that the use

of an accelerating nozzle produces an increase in

efficiency only at a higher thrust loading. For light

loading, Sparenberg (1969) has showed that the

representation of a propeller by an actuator disc in

a duct in axisymmetric flow yields the efficiency

of the actuator disc alone regardless of the duct

shape.

A more comprehensive study has been carried out

by Kerwin et al. (1987) where the flow on the duct

was predicted by using a potential based panel

method. Ryan and Glover (1972) have presented

an interactive procedure by combining

axisymmetric surface vorticity analysis for the

duct and propeller design based on a lifting line

theory. A similar method has been introduced by

Gibson and Lewis (1972) where the propeller was

modeled by an actuator disk. This method is

applicable to propellers of arbitrary radial loading,

and it therefore is purely an analysis method. Ryan

and Glover’s method was then improved to the

open water off-design problem for the

performance analysis of ducted propeller by

Caracostas (1978) based on the surface vorticity

distribution technique. Falcao De Campos (1983)

established extensive studies on the calculation of

duct performance in uniform and radialy variable

inflow. 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 and

thrust on propeller and duct.

In this study, numerical analyses are carried out

for five different duct geometries, and the results

are compared with each other and the propeller

without duct. Whether an additional gain in

efficiency is investigated for various nozzle

geometries for a high speed passenger ferry.

2 DUCTED PROPELLER ANALYSIS

METHOD

The ducted propeller analyses are carried out using

an analysis method based on the lifting surface

theory presented by Glover and Szantyr (1989). In

this method, the hydrodynamic loading on the

propeller blades and on the duct is replaced by

appropriate distribution of vorticity while the

thickness of the propeller blades and duct is

modeled by the appropriate distribution of sources

and sinks. These singularities are distributed on

the surfaces built up by the meanlines of the

propeller blade sections and the meanlines of the

chordwise duct sections. Kinematic boundary

conditions are utilized to determine the vortex

distributions which represent hydrodynamic loads.

The kinematic boundary condition forms origin of

the lifting surface equation formulation. According

to this condition, the velocity of the flow which

comes to the lifting surface should be parallel to

the surface. In other words, no flow should pass

from the surface; total of normal velocity should

be zero at all points on the surfaces of the nozzle

and the propeller camber line. This condition can

be written as below for propeller and nozzle lifting

surfaces:

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 dS S

dv do

d dS S

p pc d dc

p pS S

n dS n dSr r

n dS n dSr r

n dS n dSr r

q q dS q q dSn r n r

V

). 0R n

In the equation above, pcq and dcq is written only

if cavitation is determined. Also .R is relevant

with rotation of the propeller and is taken into

IX HSMV Naples 25 - 27 May 2011 2

account if the calculation is made only at a point

on the propeller. In case of sheet cavitation on

propeller blades or nozzle, the normal vector

n changes to show the change in the original

lifting 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 in

this study are Nozzle 19A (accelerating) and

Nozzle 32 (decelerating) as given in Fig. 3.

Some duct designs can cause drag as the speed of

advance increases. With the Kort nozzle, this drag

becomes more significant at higher speeds and can

eventually reduce the overall thrust gain to zero.

For Nozzle 19A nozzle, an axial cylindrical part at

the inner side of the nozzle at the location of the

screw, the outside of the nozzle profile is made

straight and the trailing edge of the nozzle is

thicker.

Detailed information about these nozzle sections

can 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. For

increasing efficiency in high speed ships, some

nozzle sections are developed such as Rice-Speed

nozzle (Rice Propulsion) and HR nozzle (Wärtsila)

as given in Fig. 4.

(a)

(b)

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

The section of a Rice-Speed nozzle was developed

from air wing sections displaying highest

lift/lowest drag properties (van Manen &

Ooosterveld 1966). A comparison of flow around

Nozzle 19A and Rice-Speed nozzles is given Fig.

5 (Wärtsila).

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

The nozzles are aerofoil shaped rings placed

around the propeller. Nozzles have found their

application in ships for decades with good results.

The main advantage of the nozzle is that it

increases the trust on the propeller. Comparing

propellers with and without nozzles shows that the

nozzle propeller offers about 25% more total thrust

(nozzle and propeller) than an open propeller at

zero ship speed (bollard condition). At high ship

speeds this difference becomes less up to the point

where the nozzle generates drag instead of thrust.

The HR nozzle is very useful for improving the

performance of the propulsion unit of tugs and

dredgers. This nozzle generates more thrust at

dredging conditions in combination with more

thrust at free-sailing speed, compared with

conventional nozzles such as the Nozzle 19A

nozzle.

IX HSMV Naples 25 - 27 May 2011 3

Fig.6 A comparison of velocity fields between Nozzle 19A

and HR nozzles.

As seen in Fig. 6, according to the velocity fields

around the Nozzle 19A and HR nozzles, Nozzle

19A shows more drag because of the high velocity

field while HR nozzle produces more lift

(Wärtsila).

Nowadays, new nozzle designs are used including

the HR nozzle. Applications of the newly-designed

nozzles are available for even high speed ships

such 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 carried

out for the ducted propeller geometries mentioned

in previous chapter. For this purpose, propulsion

efficiency is calculated for propeller with Nozzle

19A, propeller with Nozzle 32, propeller with HR

nozzle and propeller with Rice-Speed nozzle. In

all cases, the nozzle length is taken as 0.4D. Also

analyses are made for open propeller and propeller

with 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 analyses

including the open propeller analysis. The

differences in open propeller and ducted propellers

are 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 m

Delivered power, PD = 2000 kW

Propeller diameter, D = 1.46 m

Propeller rate of rotation, N = 600 rpm

Number of blades, Z = 4

Ship design speed, VS = 18 Knots

Pitch ratio at 0.7R (P/D) = 0.983 (open)

Pitch ratio at 0.7R (P/D) = 1.193 (ducted)

Blade area ratio, AE/Ao =1.1

Blade Section: NACA66 (a = 0.8)

Nominal wake fraction, 1-w = 1.022

The open propeller design is made with lifting line

method (Celik & Guner 2006), while lifting line is

used for the propeller and axisymmetric vortex

distribution (Celik et al. 2010) is used for duct in

ducted 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 for

different nozzle geometries are carried out using a

lifting surface analysis method developed by

Glover and Szantyr (1989).

The propulsion efficiencies for open propeller and

ducted propellers for speeds between 8 and 20

knots are given in Fig. 9.

IX HSMV Naples 25 - 27 May 2011 4

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)

Pro

pu

lsio

n e

ffic

ien

cy

(%

)

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.

IX HSMV Naples 25 - 27 May 2011 5

References

van Manen, J.D. and Oosterveld, M.W.C. (1966) Analysis

of 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

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SNAME, pp. 3.1-3.18, 1978. Virginia, USA.

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Glover, E.J. and Szantyr, J. (1989) The Analysis of

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Glover, E.J. (1987) Propulsive Devices for Improved

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Celik, F., Guner, M., Ekinci, S., (2010) An Approach to the

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Engineering, Scientia Iranica, Vol. 17, No: 5, 406-417, 2010.

www.uhfg.se/pdf/fuelsavings.pdf, Wärtsila.

www.ricepropulsion.com, Rice Propulsion.

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