153

1 INTRODUCTION

Podded propulsion system is a new propulsion

systems have been used for both commercialand

naval ships. Propulsion pods are gondola shaped

devices, hanging below the stern of a ship, which

combine both the propulsive and the steering

functions.

Pod propulsion offers attractive performance

benefitsovermoreconventionalpropulsionsyatems,

especially in the areas of ship noise, hydrodynamic

efficiencyandfueleconomy.Theeliminat

ionoflong

shaftlines, support bearings, stern tubes, and other

underwater protrusions typically with conventional

systemcreatesasmootherlaminarflowoverthehull

andpropeller

The first patent for a podded propulsion system

was in 1826 by William Church and the first

applicat

ion was by John Ericson in 1836. The real

applicationforthispropulsionsysteminthepastwas

appliedtotorpedoes.InJapantherearesomevessels

operating with podded propulsion system and the

results from the application are good especially in

reducingvibra

tionlevelbutrathercomplicateddueto

theconventional propulsionsystem usinglongshaft

locatedbetweeneachother[5].

The podded propulsion system normally uses an

electric motor driven by diesel electric drive. This

propulsion drive has been used in icebreakers and

other special purpose vessels. A pod consists of a

motor locat

ed in a hydro‐dynamically optimizes

housingandstayattachedtothehull.Well designed

pods reduce resistance to motion by 5‐10%. An

optimallydesignedpodshape,positioningandangle

in relation to ship’s hull can increase propulsion

efficiency up to 15% in comparison with an in hull

propulsion system. Pods  also decrease the vessel

vibra

tions and noise levels and provide a more

Experimental Analysis of Podded Propulsor on Naval

Vessel

M.PAbdulGhani,O.Yaakob,N.Ismail,A.S.AKader&A.FAhmadSabki

arineTechnologyCentre,UniversitiTeknologiMalaysia,81310UTMJohorBahru,Malaysia

P.Singaraveloo

ShipClasificationMalaysia,ShahAlam,Selangor

ABSTRACT:Thispaper describes the effect ofpodpropulsor attachmenttotheexistingNaval Vessel hull

formwhichwasdesignedforconventionalpropulsorinaspectsofresistanceandmotioncharacteristics.These

investigationswerecarriedoutona3.0mmodelbyexperimentalworksinthetowi

ngtank120mx4mx

2.5mattheMarineTechnologyCentre(MTC),UniversitiTeknologiMalaysia(UTM).Thebasisshipchosenfor

thisstudyisSealiftclasstypeMPCSS(MultiPurposeCommandSupportShip).Inthisstudy,thedesignfor

thenewpodpropulsorisbasedonaprovendesignandscaleddowntosuitthi

stypeofhullformaccordingly.

This paper describes the resistance comparison between bare and poddedhulls in calm water as well in

waves.Theseakeepingtestforhullwithandwithoutpodinregularwavesatservicespeedof16.8knotswere

carried out atwavelength to model lengt

h ratio, Lw/Lmbetween 0.2 and 1.2. The outcomes from this

experimentalworksonhullwithandwithoutpodwerecompared.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 8

Number 1

March 2014

DOI:10.12716/1001.08.01.17

154

environmentalfriendlyvesseltoshipoperators.Pods

can be dismounted and serviced atsea, making dry

dockingformajorpropulsionrepairsunnecessary.

Severalmodeltestserieshavebeencarriedoutto

define a shape with optimal efficiency. CFD

calculations have been made to investigate the flow

andpressurepatternaroundthePod. Toreachagood

propulsionefficiency,theunderwaterhousingshould

beassmallaspossible.

2 BACKGROUND

Basically, a podded propulsion system consists of a

fixed pitch propeller driven by an electric motor

through a short shaft. The shaft and the motor are

locat

ed located inside a pod shell. The pod unit is

connectedtoship’shullthrougha  strutandslewing

bea

ring assembly. This assembly allows the entire

pod unitto rotate and thus the thrustdeveloped by

the propeller can be directed anywhere over 360



relativetotheship.[4]

Asmallpoddiameterorgondoladiametershould

beusedtogetahightotalefficiencyandtoreducethe

interaction effects between propeller  and pod

housing.

The pod diameter depends on the size of the

electric motor inside the pod. The definition of the

geometric parameters is shown in Fig. 1 and it

s

proposedparticularsasshowninTable1.

ThebasisshipchosenforthisstudyisSealiftClass

Type MPCSS (Multi PurposeCommand Support

Ship). The ship particulars and its body plan are

showninTable2andFig.2respectively.

Figure1.PodAnatomy

Table1.ProposedPodParticulars

_______________________________________________

ParameterValue

_______________________________________________

Propellerdiameter,Dm 3.887

Podlength,Lp,m 5.995

Poddiameter,Dp,m 2

Podlengthratio,Lp/D 1.542

Poddiameterratio,Dp/D0.514

_______________________________________________

Table2.ShipParticulars

_______________________________________________

ParameterValue

_______________________________________________

Lengthoverall,LOAm 103.000

LengthBetweenPerpendicular,LBPm 97.044

Breadth,m 15.000

Depth,m 11.000

Draught,m 4.409

Displacement,tonnes 4431.57

Speed(Operational),knots 16.8

Speed(Max),knots19.98

_______________________________________________

Figure2.SealiftClassTypeMPCSSofNavalVessel

3 EXPERIMENTALSETUP

Theexperimentshavebeencarriedoutinthetowing

tank of the Marine Technology Centre (MTC),

UnivesitiTeknologiMalaysia(UTM).Thedimensions

ofthistankare:length120meter,width4.0meterand

water depth 2.5 meter. The maximum attainable

speedofthetowingcarriageis5m/swithacceleration

1m/s

.

Table3.ModelTestMatrixforResistanceinCalmWater

_______________________________________________

Run VS(knots) ModelSpeed,Vm(m/s) Fn

_______________________________________________

1 14 1.228 0.226

2 16 1.404 0.259

3 16.8

1.474 0.272

4 18 1.579 0.291

 5 201.7550.324

_______________________________________________

Table4.ModelTestMatrixforResistance&Seakeepingin

RegularWavesatVm=1.474m/s

_______________________________________________

FnWAVECHARACTERISTICS

________________________________________

Lw/Ls Lw Hw Tw ωw Hw/Lw

_______________________________________________

0.272 0.51.50.015 0.980 6.02 1/100

0.272 0.61.80.018 1.074 5.85 1/100

0.272 0.82.40.024 1.240 5.07 1/100

0.272 1.03.00.030 1.386 4.53 1/100

0.272 1.23.60.036 1.518 4.14 1/100

_______________________________________________

155

4 RESULTSANDANALYSIS

Most of the results of the measurements have been

plottedbasedontheFroudenumber.

In general, podded hull has higher resistance

valueduetotheadditionalwettedsurfacearea.Based

on the result obtained, at the design speed (16.8

knots), the total ship resistance value for hull with

pod propulsor is higher tha

n the hull without pod.

Figures 3  and 4shows the resistance for podded

hull is higher than bare hull by differences about

22.5%butthedifferencesbetweenthesetwodecreases

withincreasingofspeed.

Figure3.CtmversusFn

Figure4.CwmversusFn

As shown in Figures 5 and 6, at the maximum

(Lw/Lm=1.2)wavecondition,thetotalshipresistance

forpoddedhullishigherthanforhullwithoutpod.

Thedifferencebetweenthesetwovaluesisabout20%.

Figure5.CtmversusLw/LmatFn=0.272

Figure6.CwmversusLw/LmatFn=0.272

Figure 7 shows the pattern of pitch RAO for the

modelwithandwithoutpodarethesame  butthere

are small deviations in term of magnitude of the

response whilst the values for model with pod are

slightly higher hence the hull with pod produce

higher pitching motion than than the hull without

pod.

gure7.ResponseAmplitudeOperator

5 CONCLUSIONS

From the above the following conclusions can be

drawn:

It was found that the the hull with pod produce

resistance20%higherthanthebarehullandalsoan

incrementabout22%higherinpitchingresponse.

Generally pod technology has already made

significant progress in the commercial shipbuilding

industry. This new technology offers ma

ny unique

advantages not offered by conventional electric

propulsionsystems.Podpropulsionisundoubtedlya

viableoptionforfutureshipbuilding programsforthe

Malaysiannavy.

ACKNOWLEDGEMENTS

Theauthorswishtoexpresstheirsincerethankstoall

staffoftheMarineTechnologyCentre,UTMfortheir

assistanceincarriedoutthi

sexperimentalworks.

156

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