diesel electric propulsion

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Diesel Electric Systems

Revolutionary propulsion

Like most new technologies, marine diesel electric systems were introduced inthe navy long before they were shown to the commercial industry. Their major

advantages are low noise and vibration disturbances, lower energy consumptionand higher flexibility in the ship design. Numerous diesel engines can beconnected in parallel in sound and vibration isolated rooms, almost anywhere onthe lower decks of the ship.

These diesel generators are then responsible for the electric ship propulsion, theheating and any other electrical utilities on board. Running dieselelectric enginesat a stable load allows smoother transients and constant speeds. !hile at anefficient load, marine diesel engines also tend towards optimum fuelconsumption, thereby reducing emissions and the impact on the environment.

Advantages of Diesel Electric Systems

"t should be mentioned that the diesel electric system is extremely valuable forships with low average speed #$% knots&, such as cruise liners. This techni'ue ofcombined diesel electric systems gains importance when the installed powergenerating capacity can be used for various ship functions, and differentsituations such as that needed for passenger services #i.e. electricity, heating&.

The safety aspects of diesel electric systems are commonly regarded as being

related to redundancy in different ways. The number of electric powergeneratingunits is large enough to ensure propulsion capability and steerage wayirrespective of any component failure. "n addition the diesel and electric units canbe located in different compartments to safeguard against loss of power in caseone compartment has been destroyed by fire or flooding. This flexibility alsoallows the optimisation of cargo space volume and arrangement.
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The shipbuilding industry has recently introduced a revolutionary new propulsionsystem known as ()*+ in which the shaft has been replaced by a bulbshapedpropulsion unit driven by an electric motor. The only connection between the bulband the ship is therefore electric cabling. This design significantly reduces wakeeffects, and propulsion efficiency is improved by as much as $%. Two (R)*"+

projects are dedicated to this new system. )(T"()* is looking at the impact of()*+ on ship design and attempting to optimise the shape of the stern. ()*+"N+-R"/-is a support project for )(T"()* and any other future relatedresearch in that it is collecting data from the existing ()* fleet. 0t present1inland has four passenger ships and one icebreaker2tanker inservice usingthis new technology. The 1rench company, /hantiers de l30tlanti'uehas also justdelivered a ()* vessel called 4illennium using two 5%4! ()*+, this vesselrepresents the largest scale use of the technology to date.

Project goal

The final goal of the project is to develop guidelines for the design of poddedships. The intention is to have these guidelines as general as possible, butdevelopment work has been focused on four ship types6 cruise liner, ropax ferry,products tanker, and supply ship. The hydrodynamic development of these shiptypes is the responsibility of those four consortium partners with model basins67+0 for the cruise liner, /T) for the ropax ferry #as outlined in The Naval

Architect's (olish report in 8uly20ugust this year, page $9&, +(0 for the productstanker, and TT for the supply ship. The structure of the project is not, however,based on these ship types but on the following issues, which form eight technicalwork packages6

: 7ydrodynamics: +afety and risk analyses: +tructural safety: "mpact on environment: )perational aspects: -ffects on general arrangement: /ost2benefit evaluation

The last workpackage, which deals with the development of guidelines, willmainly be carried out at the end of the project. The other activities run more orless in parallel, withinput2output from one workpackage to another.
http://dbs.cordis.lu/fep-cgi/srchidadb?ACTION=D&SESSION=112232000-7-27&DOC=2&TBL=EN_PROJ&RCN=EP_DUR:36&CALLER=PROJ_GROWTHhttp://dbs.cordis.lu/fep-cgi/srchidadb?ACTION=D&SESSION=112232000-7-27&DOC=2&TBL=EN_PROJ&RCN=EP_DUR:36&CALLER=PROJ_GROWTHhttp://www.marine.alstom.com/http://dbs.cordis.lu/fep-cgi/srchidadb?ACTION=D&SESSION=112232000-7-27&DOC=2&TBL=EN_PROJ&RCN=EP_DUR:36&CALLER=PROJ_GROWTHhttp://dbs.cordis.lu/fep-cgi/srchidadb?ACTION=D&SESSION=112232000-7-27&DOC=2&TBL=EN_PROJ&RCN=EP_DUR:36&CALLER=PROJ_GROWTHhttp://www.marine.alstom.com/


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0s can be seen from the list above, the goal of the investigation is to have a verybroad base for forming the guidelines. !hen the project was formed in the springof $;;; there were many 'uestions regarding podded ships6 /ould the layout ofthe ship be more effective, ie, could there be space savings when moving from aconventional propellershaft

arrangement with rudders to a podded solution/ :W

alone e(treme good manoeuvring conditions can be reached as the pod can be turned toall wanted directions.

Steerable #ropulsion $nits% &ydrodynamic issues and Design'onsequences

by Tom van Terwisga, 1rans Muadvlieg and 7enk alkhof #40R"N, !ageningen,

TheNetherlands&(aper written on the occasion of the @%th anniversary of +chottel mb7 /o=(resented on $$ 0ugust 5%%$

(ntroduction

The last three decades have shown a strong development in the market forsteerablepropulsion units. This paper addresses several main developments and placesthem in ahistoric perspective. The major objective of the paper is to present a review ofissues relevantto steerable propulsor units. These issues are essentially of a hydrodynamicnature. 0lthoughit is thought that hydrodynamic issues often have a heavy impact on the design,theprofessional background of the authors rather than anything else prompts thechoice for an


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emphasis on hydrodynamic aspects. +tarting from the hydrodynamic aspects, wedrawseveral conclusions towards the design and operations of vessels e'uipped withsteerablepropulsion units.

+teerable propulsion units refer here to those units that are able to activelydeliver a steeringmoment by rotating the thrust vector through the rotation of the thruster. +uchpropulsion unitsmay occur in different concepts. The most renowned example and one of theoldest productsin this range is the steerable thruster unit #1igure $&.Recently, since the early nineties, a distinct concept has made its way into themarine world.This new concept is referred to as podded propulsion #or in short6 pods& and isdistinguished

from the original thruster in that its prime mover is an electric motor, situated inthe hubunderneath the strut, directly driving the propeller #1igure 5&.0igure 1 Steerable t)ruster unit

0igure 2 Podded 'ro'ulsor0part from the steerable thruster and the pod, a number of other steerablepropulsor unitsexist. )ne of the oldest is the oith +chneider /ycloidal propeller #see 1igure ?&.Thispropeller is characterised by a number of foils rotating about a vertical axis, witha blade anglethat depends on the blade position. The blade angle is controlled by amechanical actuatormechanism, which essentially determines the thrust2tor'ue ratio in every position.

0 special type of waterjet that is worth mentioning is the +chottel (umpjet, whichdistinguishes itself by the combination of intake, pump and noEEle in onerotatable unit #see1igure J&. The gain in space and the conse'uent flexibility in the ship design areobvious.(age 5 of $90igure 3 4oit) Sc)neider Pro'eller0igure 5 Sc)ottel Pum' 6etThe paper first aims at providing some historic background to the development ofsteerablepropulsion units. This is followed by a discussion on hydrodynamic issues anddesignconse'uences for perhaps the two most popular steerable propulsors6 thesteerable thruster


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and the podded propulsor.&istoric development

0n early example of a propulsor applying the principle of the vectored thrust isthe ermanVoith Schneider Propeller. The development of the oith +chneider (ropeller

started in $;5Aand the first application powered an inland waterway vessel in $;5;. The first tugwith the+( /ycloidal propeller installed #1igure 9& was launched in $;9%.0igure 7 0irst 4SP Tractor Tug in 1879+chottel has played an essential role in the history of azimuthing thrusters. +ome9% yearsago, +chottel introduced the +chottel Rudder (ropeller +R(. This rudderpropeller could berotated over ?A% deg #vertical axis&, where the full propulsive power could beused for any

angle #1igure A&. These days, aEimuthing thrusters are available up to some A4!, allowingfor a wide range of applicability #1igure >&.(age ? of $90igure : 0irst Sc)ottel Rudder Pro'ellerlaunc)ed in 18790igure ; #argest Sc)ottel RudderPro'ellerThe popularity of steerable thruster units can be explained by the variousapplications of*ynamic (ositioning #*(& or *ynamic Tracking #*T&, both in the offshoreindustry, as well asin other areas of seagoing activities. The conventional thruster unit which iswidely applied in*(2*T applications, makes use of a mechanical power transmission, where theprime mover#mostly a diesel engine or an electric motor& is connected with the propellerthrough one ortwo right angle gears #designated respectively L or G drive&.The increase in popularity of the conventional thruster in the early seventies wascaused byseveral factors, according to Nienhuis O@P. H"n the offshore industry activities wereshiftingtowards increased water depths which in some cases prohibit the use ofconventional passivemooring systems. The flexibility and mobility of *( systems led to its applicationfor theexploitation of marginal oil fields, with the added advantage that assistance ofanchor


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handling vessels is no longer necessary. This latter advantage is also beneficialfor cable orpipe laying vessels, which nowadays may be fitted with dynamic tracking #*T&systems."ndeed there seems to be a trend for oil companies to re'uire the use of actively

controlledships in the vicinity of subsea pipe lines to avoid the risk that these may bedamaged by theuse of anchors.H )ther applications of *( or *T systems can be found indredging vessels#e.g. trenching, stone dumping, beach replenishment& and naval ships #minehunters inhunting or hovering mode, frigates in mine sweeped areas, replenishment at seaoperations&.The number of applications of the rotatable thruster for other ship types alsogrew. This was

a.o. promoted to a large extent by Bussemaker O$P, who proposed tractor tugswith aEimuthingpropellers. "n the mean time the application of rotatable thrusters has grown tomany othership types, such as doubleended ferries, stern drive tugs, inland passengerships, minehunters and offshore workships.The traditional stronghold of the aEimuthing thruster is the application wheregoodmanoeuvrability at low speeds is essential, such as e.g. for *( and *T. !ith thematuring ofthe concept of the aEimuthing thruster and the availability of electric motors witha high powerdensity, the thruster with an electric motor in the pod came within reach. The firstsocalled

podded propulsors, using this design principle came into service in the earlynineties. )ne ofthe main assets of this podded propulsor is probably that it has importantconse'uences forthe general arrangement of the ship as well, because of the different layout of thepropellershaftingengine chain. )ther important aspects refer to the overall propulsive efficiencyandthe manoeuvrability.The idea of placing the electric propulsion motor inside a submerged aEimuthingpropulsorarose in the late $;@%s by Qvaerner 4asaards, together with 0BB "ndustry. 0$.9 4! unitwas first installed in $;;% on the 1innish waterway service vessel +eili O>P.


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)ver the last five to six years, podded propulsors have become more and moreimportant.(articularly on cruise liners, the units have proven to be of major importance as ameans toreduce cavitation and vibration and hence have lead to a new standard for a high

comfortclass of cruise ships. 0t 40R"N, it all started with the re'uest of Qvaerner 4asaards and(age J of $9/arnival /ruise Lines to compare the results of the twin screw open shaft1antasy class ofships with similar ships provided with pods.Based on encouraging results with pods as main propulsor, /arnival /ruise Linesdecided toselect 0BB 0EipodS propulsion on the last two passenger cruise ships of the1antasy class.

H-lationH, delivered in early $;;@ from Qvaerner 4asa ards3 7elsinki ard wasthus the firstcruise ship fitted with electric aEimuthing propulsion units. Two units wereinstalled withpulling propellers in the front end of the pods. The electric motors feature apower output of$J 4! each and a rotation rate range from %$JA rpm. 0t present, the largestpodded drivesthat are offered by the industry go up to powers of about ?% 4!.The podded propulsor #with the electric motors placed in the pod& have proven tooffer anumber of benefits, Hsuch as a remarkably increased manoeuvrability. The crashstop forinstance was half of the original, and the vessel remains manoeuvrable during acrash stop.)ther benefits are less fuel consumption, reduced engine room siEe and flexiblemachineryarrangement, as well as low noise and vibrations. The need for long shaftlines,conventionalrudders, /(propellers and reduction gears are eliminated, resulting in spaceand weightsavings and reduced need for maintenance.H O>P."n the meantime, all major propulsor manufacturers have developed their ownpoddedpropulsor #1igure @, 1igure ; and 1igure $%&. 0 noteworthy deviation from themainstream poddesign is the +iemens +chottel (ropulsor #++(, see 1igure $$&.0igure < A=i'od from A>>0igure 8 Dol')in from 6o)n Crane,#i's0igure 19 &ermaid from Rolls,Royce


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?ame(a 0igure 11 SSP from Siemens,Sc)ottelThe hydrodynamic design of the ++( is characterised by two propellers, rotatingin the samedirection. By dividing the total thrust over two propellers, a number of potentialadvantages

occur6 The mass flow through the propeller disk is increased when compared to onlyonepropeller. This is caused by the contraction of the streamtube due to the actingpropellerwhen going downstream. The wake of the first propeller at the downstreampropeller has(age 9 of $9therefore a diameter that is smaller than the propeller diameter. /onse'uentlyadditionalmassflow is ingested, leading to a higher efficiency.

The loading over the blades is lower when the thrust is divided over twopropellers,causing improved cavitation characteristics. 0lternatively, at comparablecavitationbehaviour, the blade area ratio can be decreased which decreases the frictionaldragcontribution to the tor'ue.0t a lower loading per blade, there is room to decrease the propeller rotationrate, alsoresulting in smaller frictional drag contributions to the tor'ue. *ecreasing the propeller rotation rate leads to larger rotational losses in the

wake. Theselosses can largely be recovered when a proper stator #such as the stator fins andthe struton the ++(& is placed, downstream or upstream of the propeller.The above tendencies may lead to improved powering performance, which isalmost always atrade off between efficiency and the risk of vibration hindrance and erosion.These potentialadvantages do however not automatically lead to an improved overallperformance of theship. 4uch will depend for example on the constraints with regard to propeller

diameter.0 derivative of the hydrodynamic considerations is that it will be important to havea highpowerdensity electric motor. This motor should be able to operate at low rotation rates,or atthe same rotation rate at a reduced pod diameter. The ++( was the first poddedpropulsorfitted with a (ermanent 4agnet 4otor, allowing for a high power density


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)hrusters% &ydrodynamic issues and design consequencesThis section touches upon some of the more dominant hydrodynamic issues inthe designand operation of thruster units6 thrust effectiveness, maximum thrust density andmanoeuvrability.

Thrust effectiveness(erhaps the most important issue in *(, *T or low speed manoeuvring isknowledge on theeffective forces that the thrusters exert on the ship in the encountered conditions.Theseforces determine the thruster effectiveness for a given input power andconse'uently affectthe selection of the type of thruster, its siEe and the overall thruster layout.Thruster effectiveness does not follow simply from a consideration of a thruster inopen water

conditions. The thruster always operates in the vicinity of the hull and of otherpropulsors inan environment determined by waves and the motions of the ship.Nienhuis O@P acknowledged the following disturbing factors6 H"n the first place,unsteadyconditions are inherent due to the lowfre'uency motions, the variable thrustvectors as wellas the firstorder ship motions. +econdly, the low speeds encountered may leadto inflowdirections, which deviate significantly from the alongship direction. 1urther, it maybeexpected that other propellers operating in the vicinity of the considered thrusteror propellerwill not only alter its effective inflow velocity, and hence its thrust, but will alsoaffect the netforce which this thruster exerts on the ship. Next, the effect of wind and waves,which moreoften than not dominates the current, leads to thrust levels of the propeller whichare not inbalance with the current forces. This is similar to a tug in towing condition. 1inally,restrictedwater #shallow water or the presence of 'uays& is often encountered, changingtheperformance of the propulsion devices.These phenomena all combine to the fact that for a proper design and operationof a vesseloperating at low speeds, it is not sufficient to know the bollard pull of each of thepropellers.+till relatively little knowledge is available for conditions inherent to *(, trackingor low speed


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manoeuvring. These conditions being6 low propeller inflow speeds

drift angle varying from % to ?A% degrees

thrust vectors largely uncorrelated with the current force vector

widely varying propulsion arrangements

restricted water(age A of $9 unsteady dynamic behaviour as a function of waves and low and high fre'uentshipmotions.HThe effectiveness of a whole *(2*T system is often presented in a socalled *(capabilityprediction. +uch a prediction aims at providing the sustainable conditions for aship with agiven thruster configuration. The sustainability is then defined in a simple way bydeterminingthe static balance between excitation forces imposed by the environment andreaction forcesby the thrusters #1igure $5&. 0s explained above, the conditions for a *(operated ship arehighly dynamic and due to amongst others the effect of large inertia and dampingforces andsecond order wave drift forces, this static approach suffers from severelimitations. !ichers etal. O$$P conclude that static analysis #for a monohull& is inade'uate in determiningthe *(capability of a ? axis weathervaning vessel.0igure 12 E*am'le of a DP Ca'ability 'lot s)o(ing t)e reduction inca'ability byt)ruster , )ull and t)ruster @ t)ruster interaction. n t)is case : a=imut)ingt)rusters(ere a''lied eac) of 299 k$ bollard 'ull.

0nother complication in the use of *( capability predictions is the large variety ofcomputational models that are used, each with their own simplifications andneglects. "n manycases for example, no interaction effects with other thrusters or with the hull areused. "n evenmore cases the effects of e.g. bow tunnel thruster degradation in waves are

neglected. Theseare mostly outside the scope of the *( /apability programs, whereas theseeffects can havean important bearing on the capability.To fully incorporate the above effects and other nonlinear effects such as thesecond orderwave drift forces on the hull, one should apply a simulation model that solves thee'uations of


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motion in the time domain #see e.g. !ichers et al. O$$P&.

Maximum thrust densityBecause of structural considerations, the siEe of a thruster is usually heavilyconstrained. This

constraint, together with the desire to keep the number of thrusters as low aspossible, hasposed the issue of the maximum thrust density #thrust per unit propeller diskarea&. 0lthough itis recognised that the thruster efficiency decreases with increasing thrust densityin general,there is nevertheless a drive toward higher thrust densities for *( as a result ofthe overalldesign problem.The minimum dimensions of thrusters are however limited by cavitation inducedthrust

breakdown, cavitationinduced vibrations, erosion and possibly mechanicalconstraintsimposed by the construction. +imple rules of thumb are mainly used in practiceby engineersand propeller manufacturers to determine the minimal propeller siEe in an earlydesign stage.(age > of $9These rules mostly use a propeller tip speed criterion or a power densitycriterion. 4orerefined criteria, deduced from model experiments, such as proposed by 0uf3mQeller O?P and7oltrop O5P, show that parameters as blade area ratio and number of bladesshould also betaken into account. /urrent computational tools such as lifting surface or panelcodes are ableto also show the effect of blade geometry on the maximum thrust density.an Rijsbergen and an Terwisga O$%P review methods to determine the minimalpropellerdiameter originating from fullscale experience, modelscale experiments andtheoretical andcomputational considerations. Their paper focuses on thrust breakdown due tothe presenceof a certain amount of sheet cavitation on the propeller blade. )ther types ofcavitation, suchas (ropeller 7ull ortex #(7& cavitation and erosive bubble cavitation can alsoimpose alimit on the thrust density, but are not yet amenable to computational analysis."t was concluded from this study that the minimum propeller is determined by twocriteria6 0


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nondimensional thrust density criterion QT2n, and a nondimensional tip speed

criterion n.*imensional e'uivalents of these criteria are less reliable because they show toolarge adependency on shaft immersion and efficiency. 1urthermore, the thrust capability

of apropulsor was pointed out to be dependent on wake field, propulsor type #openpropeller,ducted propeller or waterjet& and propeller design. These parameters shouldpreferably beincorporated in the criteria.

Manoeuvrability)ne of the most important goals of the aEimuthing thrusters is to have the abilityto direct thethrust in all directions. This allocation offers an excellent freedom in

manoeuvrability and is ofgreat use for the offshore industry, especially for the purpose of dynamicpositioning. 0lso forother ships, it turned out to be a good solution.The large amount of tugs that are presently e'uipped with aEimuthing propellersis a goodexample of ships that are combining onthespot manoeuvrability with there'uired vectoredthrust ability at low and high speeds. The noEEle on the steerable propellercombines thisgood manoeuvrability with a good bollard pull. 0 good example is the ship type

0Eimuthing+tern *rive #0+*& tug. The number of aEimuthing stern drive tugs that aredelivered in therecent years is enormous. The 0Eimuthing +tern *rive tug is hereby developedas thestandard tug type, taking over from the tractor tugs and the conventional tugs.-ven a newtype of tug is developed and e'uipped with +chottel thrusters. This is the RotorStug O9P, ofwhich an impression is given in 1igure $?.

(age @ of $9The concept of three thrusters under the vessel without skegs yielded anenormous freedomin manoeuvrability #1igure $J&, allowing even pure sideways movements of up toA knots#1igure $9&.

0igure 17 Sideste''ing at : knots


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1or ships e'uipped with thrusters and their manoeuvrability, it becomes an issuewhether thehuman helmsman is able to control the ship. This manoeuvring problem is in away related to

the control of the jet fighter 1$A. The 1$A system in itself is course unstable andsomanoeuvrable that one human cannot handle it. (lacing a computer between thecontrols ofthe pilot and the actual steered flaps on the 1$A formed a good solution. 1or theRotor Tug,+chottel developed also such a device, called the 4aster(ilot. 0lso for the shipse'uippedwith *( capabilities, such computer systems are re'uired to allocate the thrustsof thepropellers in such a way that the environmental loads can be withstood, not only

effective, butalso efficient. This means that the *( job has to be done with as little power useas possible.*uring all these manoeuvres, it is important that the thrusters will have as littlemutualinteraction as possible. )ne thruster, blowing in the direction of a secondthruster, reducesthe effectivity of the leeward thruster to a large extent, see 1igure $A fromNienhuis [email protected] 1: &utual interference bet(een t)rusters1or manoeuvring and course keeping purposes, one is interested in thecharacteristics of thepropellers in obli'ue flow at relative high speeds. The side force and thelongitudinal force asfunction of larger forward speeds and obli'ue inflow angles are discussed in OAP.!ithdecreasing skeg siEes and in some cases no skeg at all, the aspect of the coursestabilitybecomes more critical. 1or the above mentioned Rotortug, it was found that forsmall anglesof attack, the side force generated by an operating thruster with noEEle is of thesame order ofmagnitude as a typical skeg. This is illustrated in 1igure $> from O9P. Thesesmaller angels ofattack #say up to $9K& are important for course keeping. 1or an important part, thecoursestabilising effect is due to the noEEle. The following example of a doubleendedferryillustrates that for thrusters without noEEle, the situation is different.(age ; of $9


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Com'arison of lateral force%.-U%%$.-U%95.-U%9?.-U%9

J.-U%99.-U%9A.-U%9% $% 5% ?% J% 9%Angle of attack BdegTransverse force B$+kegThruster0igure 1; Com'aring t)e lateral forces (orking on t)e s)i' due to t)rustersor skeg+ufficient course keeping ability with rotatable propulsors is not trivial. *ouble

ended ferriesare sometimes e'uipped with thrusters without noEEles. "n 1igure $@, reported inOJP anillustration is given of two hull forms. !hile the upper hull form #initial design&suffered from anunacceptable course instability, the lower hull form appeared to show anacceptablebehaviour. "n this case, stability obviously has to come from both the hull formand thepropulsors.0igure 1< Hull form design conseuences for sufficient course kee'ingability#ods% &ydrodynamic issues and design consequences!ith every new development, new uncertainties occur that need to be controlled.7ydrodynamic issues that arose during the development of pods wereuncertainty about scaleeffects in the powerspeed prediction based on model tests, and the loads andstresses thatoccur on the pod during its operational life. 40R"N has recognised theseproblems in an earlystage and has invested in developing and validating an extrapolation method toscale thepowerspeed relation from model to full scale. This is reflected in the pod modelsused forhydrodynamic testing and in a 8oint "ndustry (roject on (ods in +ervice. Theobjectives and adescription of the monitoring campaign are given later.

0 number of design 'uestions arose with the advent of the pod6 !ill pods save money and will they show lower fuel consumption


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!ill they lead to higher passenger and crew comfort, achieved by lowerpropeller inducedhull pressures and excitation forces through better cavitation properties< !hat about the manoeuvrability and the course keeping ability degrees. "t isthereforeexpected that in the near future, more sophisticated wake adapted propellers onpods cangain a few percent in efficiency without sacrificing the excellent vibration levels ofthe ship.(age $$ of $9Manoeuvrability and course "eepingThe introduction of podded propulsors with electric motors in the hub introducedthe vectoredthrust in a new market segment6 the very large powers. This allowed for examplecruise shipsto be e'uipped with pods. The need for this was also obvious. Besides thealready presenttrend to go for an 0ll -lectric +hip, there was a need for better manoeuvrabilitywith cruiseships. /ruise ships are becoming larger and larger while ports stay at similarsiEes andmarine traffic becomes denser. 0 further improvement in controllability of cruiseships shouldtherefore be pursued. The application of the podded propulsors stimulated thisenormously.Besides the almost standard application of pods for cruise vessels, nowadayspods are alsoapplied in other ships. The first application of the ++( was on a chemical tanker,but there are


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other applications possible such as heavy load ships.-'uipping ships with podded instead of conventional propulsion can improve themanoeuvring characteristics of a ship considerably. 7owever, the use of the wordcan shouldbe emphasised here6 worsening is also possible. +everal manoeuvring aspects

are dealt within the following.#o$ speed manoeuvring1or manoeuvrability at low speeds, the pod developments are the necessary leapforward.*ue to the use of new materials and client re'uirements towards all balconyships, thesuperstructure of modern cruise vessels and ferries is becoming very high. Theresulting windloads are enormous. Therefore, more powerful bow and stern thrusters arere'uired.

-specially stern thrusters have insufficient power. Now, the allturnable poddedpropulsorsare overcoming this in the aft ship. The most recently observed trend is that theamount ofbow thruster power is the limiting factor in reaching the vesselsF low speedmanoeuvringtargets.!ourse "eeping ability

0 design conse'uence of the application of pods is that freedom is obtained todesign a veryflat aft ship. This is often favourable from a resistance point of view, and createsa veryhomogeneous flow towards the pod, which is good to avoid cavitation andvibrations.-specially when three or four pods are used, this freedom is also needed from adesign andconstruction point of view. References are the Mueen 4ary "" and the -agle classof cruisevessels. The open aft ship does not have much lateral resistance and hence thecoursekeeping ability will be small. The podded propulsors are furthermore in generalwithoutnoEEle. "t was already stipulated in this paper that nonducted propellers haveinherentlymuch lower course keeping stability than ducted propellers. Together, this makesthat poddedships are in general more course unstable than conventional ships #see 1igure5$&. Therecent trends of applying pods to full ships #such as tankers and LN carriers&can become a


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real challenge from the course keeping point of view.The possible operational conse'uences of an insufficient course stability isserious6 Not fulfilling the "4) resolution 0>9$#$@& towards course keeping ability,

-xcessive steering actions imposed by the autopilot, causing wear and tear of

thebearings and steering engine, increased resistance, loss of propulsive efficiencyandpossible cavitation.0n increased risk of broaching due to the loss of directional and transversestability instern 'uartering waves. "ncreased risk of collisions due to the inability of the ship to counteract turnsade'uately. "ncreased re'uired power because of the additional hull resistance resultingfrom the nonEero

drift angles.-xcessive steering in calm water or waves should be avoided at all times from acavitationpoint of view. The conse'uences are a constantly varying loading of thepropeller, resulting inmany peak loadings. There is an increased risk for adverse effects by cavitationon thepropeller #when the propeller is in obli'ue flow, the cavitation inception speed islower&. +hipsat higher speed may additionally suffer from cavitation on the struts of the pods.(age $5 of $9

0igure 21 Aft s)i' eui''ed (it) conventional 'ro'ulsion arrangement anda 'odarrangementBased on the above, it is the firm belief of 40R"N that the course keeping abilityanddirectional stability should be investigated thoroughly before building the ship.4ore importantthan ever seems here that the behaviour and performance of the vessel is theresult of amarriage of the hull form with the propulsor.%eel angles

0 third important aspect of steering with pods is the occurrence of large heelangles. Thepods are very powerful steering tools. The side force that can be generated is solarge, thatthe steered vessel can suffer from very large heel angles. 0t 40R"N, heel anglesof up to 59Khave been measured with ship models due to regular steering. Qnowing that thepanic limit for


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passengers is at some >K, it is obvious that this is undesired. The designconse'uences arethat the hull form will have to be modified to assure that the heel angles will staywithinacceptable values. "t is important to check this with model tests before the ship is

build.Practical operation of pods and thrusters-xperience from past projects learns that crew training is becoming veryimportant whenships are e'uipped with podded propulsors, which is true for steerable thrusterunits as well.)perating pods is a different way of sailing. The manoeuvring capabilities ofvessels e'uippedwith pods are potentially high, but full use of these capabilities re'uires crewtraining,preferably on a manoeuvring simulator in order to cover also propulsion

emergencies.-xamples of such projects are e.g. the cruise vessels built at 4eyer!erft in(apenburg, whohad to sail through the -ms to reach open sea. ery accurate steering isnecessary and theslightest mistake will cause a risk on the loss of the ship. )ther examples are thetraining oftugmasters, the handling of doubleended ferries such as for the (+* ferries andthe T-+)ferries. 1igure 55 gives an illustration of a training for tugmasters on the handlingof a tugwhile escorting large vessels.(age $? of $90igure 22 &aster training in tug )andling at &AR$Fs simulator centreSafety and structural loadsTo get an appreciation of the structural loads that are met during operations withpodpropelled ships, a large -uropean project was initiated by 40R"N. The reliabilityand safety ofpods under operational conditions had to be monitored on full scale. This 8oint"ndustry(roject was designated H(ods "n +erviceH and has the following objectives6$. 0ssess the reliability and safety of pods under operational conditions5. -valuate the operational performance and benefits for the ship owners?. *evelop design, construction and classification methods."n this 8oint "ndustries (roject, 59 parties are collaborating worldwide. Besides40R"N, theseare the cruise line operators, navies, the pod manufacturers 0BB 0Eipod,Qa4e!a, +iemens+chottel, shipyards and classification societies and TT 1inland #see 1igure 5?&.


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0igure 23 Partici'ants in t)e Pods in Service Project B?vaerner &asa,%ardsmean()ilealso joined t)e 'roject(age $J of $9*uring this project, four vessels will be monitored on fullscale during a period

between A to$5 months of their operational life. The measured ships are the +ummit#4illeniumclass&, theTTlineFs Nils 7olgerson, R//LFs Radiance of the +eas and the 1innish Botnica.*uring themonitoring campaign, there is a focus on structural excitation and response. Tothis end, thefollowing signals are measured continuously6 strains in shaft, gear and podhousing, hullpressure fluctuations, hull accelerations and vibrations and propeller bladestrains.

+imultaneously, the conditions are monitored continuously by registration ofaEimuth shafttor'ue and angle, input power and propeller rpm, ship draft, motions, speed andtrack andwind, waves and current. 1or one of the vessels, the underwaterradiated noisewill bemeasured.1rom the measured 'uantities, important feed back is obtained. This is not onlyhydrodynamicfeed back with respect to the efficiencies and vibrations. 4uch structural feedback ispresented and classification societies are using this to upgrade or determine therules for theclassification of podded vessels. 0 special work group consisting of allclassification societiesis developing and verifying computational and design methods for pod and hullstrength.The first ship, the Botnica, owned by the 1innish 4aritime 0dministration,experienced aextreme severe storm situation #$9 m significant waves& during the monitoringcampaign. Theresults are being analysed during the first months of 5%%$. Then it also willbecome clearerwhat happened during that extreme event. But of course also the otherinformation will be ofimportance to increase the knowledge related to the behaviour of the ship and its()*system during a longer period of time.*inal remar+s


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This paper gives a review of current issues in the design and application ofsteerablethrusters and podded propulsors. )ne can conclude from this review that theconcept ofsteerable thrusters and its design space is relatively well known territory, yet

leaving anumber of pitfalls for the designer. The concept of the podded propulsor isrelatively new, andrelatively little empirical knowledge has yet been accumulated. 7ence, designersandoperators have to rely on model tests, supplemented with /1* calculations thatre'uirerelatively little empiricism. 1or pods, one can state that the necessary empiricalknowledge isgenerated more 'uickly than was the case with the steerable thruster some 9%years ago.

This is achieved through sophisticated model tests supplemented with /1*computations andcomprehensive fullscale measurement campaigns.)n podded propulsors, different applications and more sophisticated designs canbeexpected. 0n extension of the pod applications can be expected toward full blockvessels andcontainer ships. Research programs are already initiated for this. 0 higher degreeofsophistication of the design seems especially possible in an optimisation of thecombined hullform V pod system design #e.g. adaptation of hull lines& and in further reductionsof the poddiameter and the optimisation of the stay #strut arm of the pod&. "n addition, thepropelleroptimisation will lead to a further improvement in efficiency and in cavitation andvibrationreduction. "t is expected that the range of applications will also grow withincreasing insight incourse keeping properties in calm water and waves.

0lthough this paper has dealt especially with hydrodynamic issues, we cannotevade theeverimportant issue of economics. -ven hydrodynamicists can see that areduction of theprice of the pods will definitely be beneficial toward extension of its use.

References%O$P Bussemaker, ). and /orlett, -./.B.= Tractor tug family fitted with rudderpropeller.(roceedings of 5nd "nternational Tug /onvention. London, $;>5


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O5P 7oltrop, 8.= 0 statistical reanalysis of resistance and propulsion data,"nternational+hipbuilding (rogress, ol ?$ No. ?A? #$;@J&O?P Qeller, auf3m 8., H-nige aspecten bij het ontwerpen van scheepsschroeven #indutch&H,

+chip en !erf, *ec. $;AA.(age $9 of $9OJP Qristensen, 7.).7.= The manoeuvrability of double ended ferries. *esignconsiderations,construction and service experience. "nt. /onference on +hip 4otions and4anoeuvrability. R"N0, 1ebruary $;;@, London.O9P Qooren, T., 0albers 0. and Muadvlieg, 1.= Rotor Tugnology= "T+5%%%, The $Ath"nternational Tug and +alvage /onvention, 8ersey, /hannel "slands, 4ay 5%%%.OAP )osterveld, 4.!./. and van )ortmerssen, .= Thruster systems for improvingthemanoeuvrability and position keeping capability of floating objects. )T/ paper

$A59, 4ay$;>5O>P 4arine (ropulsion "nternational= 35%%% ears of (ropulsion 7istory3, +ept.5%%%O@P Nienhuis, C.= 30nalysis of Thruster -ffectivity3, (h* Thesis, )ct. $;;5O;P alkhof, 7enk 7.= H(odded propulsors, it has all just startedH, 4arine(ropulsion 5%%$/onf., The 4otorship, 4arch 555?, , LondonO$%P an Rijsbergen, 4.W. and an Terwisga, T.8./.= 3)n the maximum thrustdensity ofpropellers3, N/T39% "nternational /onference on (ropeller /avitation, Newcastle,?9 0pril5%%%O$$P !ichers, 8, Bultema, +. and 4atten, R.= 37ydrodynamic research on andoptimiEingdynamic positioning system of a deep water drilling vessel3, )ffshore Technology/onference )T/, ol J, no. @@9J, 4ay $;;@


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Diesel Electric Propulsion Systemfor P&O Cruise Liner AuroraElectric Propulsion System or P&O "Aurora" -a Propulsion System with a high Leel o !eunancy

S#$ A#LAS Marine Electronics% system responsiility or'(iesel-electric propulsion system) electrical powergeneration an power istriution system*+ntegrate naigation systemMain ata'Length oerall appro, ./0 m1reath 2..0 m(esign raught appro, /3 m4ross measurement est /5000 tPassenger cains 326

Serice Spee .6 7notsPower plant 850 M9Propulsion ries . , .0 M9

#hruster ries 6 , :8 M9;hiller ries 2 , :28 M9Propulsion an power components supplie y S#$ A#LASMarine Electronics . (iesel-electric main propulsion ries with'- . (oule-wining synchronous motors .0 M9) :60 rpm

- . Synchro-;onerters :.6 M?A 5 (iesel generators6 , :/8 M?A) 55 7?) 50 @) 8:6 rpm. , :.8 M?A) 530 ?) 50 @) :>00 rpm Switchoars 55 7? with :: an :. panels or meiumoltage istriution 6 @armonic =lter an7s reucing the total harmonicistortion o the mainsoltage to ma, 8 B > !ing main sustation units 6 , :8 M?A an 6 , :: M?Aincl meium an


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low oltage switchoars an cast resin transormers or lowoltage supply 8 ;ast resin transormers or engine room an emergencyswitchoar supply

/ A; motors 6 , :8 M9) 55 7?) :.00 rpm or thrustersan 2 , :28 M9)55 7?) 2500 rpm or air-conition compressors2GeneralCor the new Dagship o P&O) uilt atMeyer Shipyar) S#$ A#LAS MarineElectronics was selecte as maincontractor or the complete electricpower generation) main power istriution

an the electric propulsion system1ase on the e,isting e,perienceacuire rom the eliery o more than:60 electrical propulsion ries) S#$A#LAS Marine Electronics has ta7enoer also the system responsiility orthe power generation in cominationwith the propulsion system Mathematicalmoels as well as own sotware

simulation programs allow precisepreictions concerning the ehaiouran the uality o the power networ7

#he "Aurora" is esigne as a passengeressel with a ery high stanar or allaccommoation areas #his high stanaris conseuently achiee also or themachinery) especially or the propulsionsystem +ntensie saety reuirementso P&O lea to a esign with a high leelo reunancy #his guarantees a highaailaility uner all sailing conitionsSystem Description

#he "Aurora" is euippe with our ieselgenerators proiing all electric energyor the propulsion) all machinery an


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hotel consumers #he meium oltageswitchoar is split into two sections)which ee all essential consumers Aring us system supplies all sustations

ia two short circuit current limitationreactors #wo =,e pitch propellers) eachrien y a synchroconerter system)proie the reuire propulsion power;hiller an thruster motors with irecton line start are connecte to the meiumoltage switchoar Cour =lter circuitsensure a low harmonic istortion actorSynchro-Converter Drive DesinEach propulsion motor is esigne as

si, phase salient pole synchronousmachine with air cooling 1oth o thewining systems are e y separateconerters #his results or the motor ina :.-pulse torue ripple an a smoothoperation Each o this two conerters)which are eeing one motor) is suppliey . parallel transormers with a 20Fshit or a :.-pulse line reaction #his

esign proies an unchange harmonicsignature with a :. pulse line reactionalso in case o operation with only oneo the conerters #he two conertersare conseuently realise with theirown control) e,citation an coolingcircuits

#he conerter power section isesigne as irect water coole systemto reuce the imensionsS!itch"oar# Desin

#he 5)5 7? oltage switchoar isesigne in two separate parts connectey SC 5 tie rea7ers Cor themain consumers li7e generators SC 5rea7ers are use) or the smalleroutgoing panels with a higher switching


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seuence contactors are use A erycompact arrangement was achieewith oule stoc7 compartments

#he sustations operate with separate

SC 5 insulate switchoars eeingthree wining transormersPropulsion $e#un#ancy Concept

#he "Aurora" is euippe with an e,ceptionalhigh leel o reunancy 1esiethe two propulsion motors there are intotal our inepenent conerters Oneconerter is connecte to one winingsystem o the motor +t is controlle anmonitore y it%s own control system

#he same applies or the water coolingcircuit) where the power part o eachconerter is separately coole Eachconerter is een euippe with ane,citation system) which will e automaticallyswitche oer in case o a ailureEach conerter uses separate sensors)so the monitoring is inepenent romeach other As special esign aspect

each conerter is esigne or :60B oit%s nominal power #his uilt-in sparepower will e actiate automatically) ionly one conerter is in operation Atotal conerter power o .>M9 is6thereore installe per shatline 1esiethe lower temperature leel in normaloperation moe this power proies incase o operation with one conerter aery low reuction o spee #estsshow a reuction o only one 7not #hisallows the essel to maintain it%s timescheule;onseuently all power an au,iliaryeeings or one shatline are supplie


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rom iGerent istriution switchoarsAlso the =lter circuits ollow the principleo reunancy +n case o a prolem withone o the =lters) an unlimite operation

is possile without any restrictions#he ene=t o this concept is anaailaility o a partial higher propulsionpower in case o a conerter ailure%ele#ianostic System

#he propulsion system is euippe withintensie iagnostic an ailure ienti=cationsystems A transient recorer orstoring ata in a memory is proie asstanar since many years or all S#$

A#LAS Marine Electronics synchroconerters$ow a teleiagnostic systemis realise) which allows a sericeengineer to perorm an instantaneousealuation o an eent rom his homeofce ia satcom

#he serice engineer can perorm thesame analysis as eing locally at theconerter oar +n many cases the

ailure can e correcte y the shipsstaG engineers y ata transmissionan phone< a, support ia Satcomilter circuits' (uality of the mainsCour =lter circuits in total are proie)each with an incorporate =lter or the8


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improement o the power actor #hisreuces the reactie power anincreases the efciency #he =lters areswitche epening on loa conitions in

orer to aoi a capacitie power actoron the mains)onitorin concept(istriute +


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Second tanker to )ave SSP 'odded 'ro'ulsionS(edis) s)i'o(ners order c)emical 'roducts tanker(it) Siemens SCH!TTE# Pro'ulsor

The +iemens +/7)TT-L (ropu

received an order from +wedish sof *onsX to supply a +iemens +/3podded3 propulsion system for antanker. The order is worth aroundnew vessel is scheduled for delivvessel3s sister ship the H(rosperopropulsion, entered service in No

)wners HRederi 0B *onsXtankH o

in +weden have recently placed abuilders +hanghai -dwards +hipproducts tanker of approximatelymetres overall length. The vesse+iemens +/7)TT-L ++( > (rosystem with an output of 9.$4!.

0t the heart of the ++( propulsiofield electric motor that needs no system. /ompact, hydrodynamictogether with a twinpropeller coneffective propulsion system whichconventional systems, offers bett

substantially less space. These feand cut costs and so bring about in the amount of revenueearningThe fact that the pod swivels alsovessel3s manYuvrability. Thanks construction installing it on boardsimple procedure. 0t the same timkey features such as excellent re


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maintenance costs.

)ne important reason for the newhas been the highly successful pH(rosperoH the new vessel3s sis

service at the end of last year o1or the press only6 *ata of illustrations on re

,etting t-in propellerefficiency from a pod0n innovative podded drive system is designed to give significant improvementsin economy and maneuverability for a broad range of vessels, including cruiseships, chemical tankers, icegoing vessels, offshore units and naval vessels.

0vailable in power outputs from 9 4!to ?% 4! per unit, the ++( #+iemens

+chottel (ropulsor& promises energysavings of better than $%, thanks to acombination of the benefits of the+chottel Twin (ropeller and a newpermanentlyexcited synchronousmotor, developed by +iemens, thatallows the maximum efficiency in thetransmission of electrical energy withina minimum installation space.The unitbuilds on +chottel3s experience in thedevelopment of steerable rightangle

drives #its Rudderpropellers&. Thebenefits of these units in applications upto A 4! are well known and there aremore than 5?,%%% +chottel units of thistype in service.To improve the efficiency of the Rudderpropeller, +chotteldeveloped the +chottel Twin (ropeller #+T(&, where the propeller load isdistributed 9%29% to two propellers, one forward and one aft of a lower housing.


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This housing features two airplane type fins that recover rotational energy fromthe forward propeller.

The +T( achieves efficiencies up to 5% higher than standard Rudderpropellers.7owever, mechanical right angle drives, including the +T(, are limited to around

> 4! per shaft.

The ++( #+iemens +chottel (ropulsor& was developed to make the advantagesof the +T( available for higher powers. This was only possible by incorporatingpowerful electric motors in the lower housing of the aEimuthing drive.

/onventional high power, low speed synchronous motors are so large and heavythat they must be housed within underwater housings with a diameter of as muchas A% that of the propeller diameter, with a dramatic negative influence on theunit3s overall efficiency. +iemens has for some time been developingpermanentlyexcited synchronous motors with a longitudinal electrical flow

design. 0 $,%%% k! propulsion test unit has been in operation on a naval vesselfor several years. This type of unit, available in a power range of about 9 4!?%4! at low speeds, allows a significant reduction in the diameter of the motorand, in turn, in the diameter of the housing of a podded drive. This allowsoptimum hub2propeller diameter ratios to be achieved.

The ++( is a marriage of the +chottel Twin (ropeller and this new type of electricmotor.0 standard Rudderpropeller6 replaces the steering and propulsion systemsof vessels= gives optimum maneuverability without additional stern thrusters=lowers noise and vibration thanks to special supports= occupies less space andre'uires a smaller engine room than a conventional system= can be installed later

in the construction stage than the conventional system, saving on constructiontime and costs.

1urther advantages claimed for the ++( are6 no risk of vibration excitation bygear sets and cooling fans= simple surfacecooled motor= mounting of the lowerhousing is possible without drydocking.

C!&PARS!$

0 propulsion analysis was performed comparing the ++( with the propulsionsystem of the >%,%%% gt cruise vessel !enturybuilt by 4eyer !erft in $;;9. Tank

tests were performed by +0, (otsdam, ermany, taking into consideration theoriginal tank tests of the vessel by ++(0, othenburg, +weden.

The [email protected] m L)0 !entury displaces ?9,5%% tons on a design draft of >.9 m.*esign speed is 55 knots. "ts diesel mechanical propulsion system includes twoshaft lines, each with a 9.@ m diameter propeller absorbing $J 4! at $5% rpm.

+0 (otsdam carried out tank test and cavitation tunnel analyses of both6


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a standard podded drive system using two units each with a 9.5 mpropeller diameter and propeller speeds of $A% rpm #hub diameter A% ofpropeller diameter&=

an ++( system with two units each with propeller diameters of 9.J m andpropeller speeds of $9% rpm.

The study indicated that the ++( system would reduce propulsion plant powerconsumption by $%, translating into either an %.9 knot speed increase or a $%fuel savings. Though the study considered the advantages of the standardpodded drive3s lower resistance #due to the absence of stern thrusters and shaftbrackets&, its speed was found to be no better than with the conventionaldrive.The ++( gives considerable space savings in applications such as cruiseships. "n addition, a weight comparison of the $J 4! ++( installation and aconventional diesel electric installation indicates a total weight of 9$% tons for the++( units, and associated cabling and structures. This compares with a totalweight of >A% tons for the conventional system.

E#ECTRCA# S%STE&

The hydrodynamic re'uirement that the lower housing diameter not exceed ?%J% that of the propeller, ruled out conventional synchronous motors. Theconcept selected was, instead, the permanentlyexcited synchronous motor.+iemens has many years3 experience in designing this type of motor for navalsubmarines.

4agnetic flux is generated by high performance magnetic elements. enerallyarranged on the motor3s rotor, these substitute for the conventional excitation

winding and such auxiliaries as sliprings, rectifier, cooling air ducts and coolingfans. Besides significant volume and weight savings, this gives a considerablegain in efficiency by avoiding core losses and heat losses due to the excitationcurrent.

The flux distribution selected for the ++( application was longitudinal. Thisavoided the need for a disctype rotor, giving flexibility in selecting therelationship between axial and radial dimensions of the motor3s activecomponents. The resulting design is very similar to that of a conventionalsynchronous motor and has similar electrical characteristics, avoiding problemswith electrical supply via conventional converters.To optimiEe the drive

configuration, a selfcommutated converter is re'uired.

*epending on load re'uirements, the ++( will be offered with a cyclo or (!4converter.

*epending on propulsion system demand, the motor will be designed with eitherone winding system or two independent winding systems #in the latter case,emergency operation with half the motor is possible&.


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0 motor designed for a $J4! drive will have thefollowing characteristics6

)utput power $J 4!

+upply voltage ?.? kRated current 5.; k0

-fficiency %.;@

(ower factor cos phi %.@9

4ax speed $9% rpm

Number of poles $@

The motor will be suppliedby a directly watercooled,fuseless and short circuitproofed cycloconverter.To reduce total harmonicdistortion in the ship3s

network, each individual drive system is designed with $5 pulse configuration.The converter will be selected to guarantee a near sinusoidal shape, resulting ina low level of structureborne noise.+0"N+!hile power savings of $% havebeen indicated by studies of a >%,%%% gt cruise ship, +chottel and +iemensbelieve that for other types of ships, power savings may be even higher. 4L

P!DS 0!R R!PAG+iemens+chottel propulsor systems will be powering two new Ro(ax vesselsordered by the TTLine shipping company and to be built by ++! 1[hr und+peEialschiffbau mb7 of Bremerhaven. The ships will be operating in the ferryservice between Travem\nde and Trelleborg. The +iemens 4arine -ngineering+ubdivision in 7amburg will be supplying and installing all the electricalmachinery and systems as a turnkey contract worth around ]J% million.

-ach of the $;% m ships will be propelled by two +iemens+chottel ++( $%(ropulsors. The power output of each ++( $% will be $$ 4!. The turnkeyproject also includes all the automation e'uipment employing proven ^+"4)+"40/ 99 e'uipment and the communications systems.

The ++( utiliEes a compact permanent magnet electric motor that allows the podto have a lower profile, permitting more efficient water flow into the propellers.


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The ++( is particularly suitable for passenger vessels such as TTLineFs two newRo(ax ferries because its twin screws produce much lower noise and vibrationlevels so passengers can enjoy higher standards of comfort.

+ince the (ropulsor comes in siEes from 9,%%% k! to ?%,%%% k! it is suitable for

the whole range of outputs needed by seagoing vessels.1urther development of electric propulsion systems will undoubtedly beexpanding the range of application of podtype propulsion systems in the future."n the permanentmagnet electric motor and the twin propeller concept, the+iemens+chottel (ropulsor is employing two basic technologies that, since theyare both innovative and trendsetting, provide a solid foundation for successfulpenetration of the market. 4L

P!D PR!P"#S!$

by!. :ewis

"nterest in pod propulsion has been stimulated by the successful sea trials ofseveral ice breakers and cruise liners during the last few years. 0t presentelectrically driven aEimuth drives of up to 5% 4! per unit are available.

1rom the hydrodynamic standpoint the primary advantages of this type of driveunit are

lower power re'uirement lower level of propellerinduced pressure pulses improvement of the manoeuvrability

Po(ering Performance

4uch research and development work has been done by the various podproducers for the purpose of improving the pod drive performance.

By optimiEing the pod propellers as well as the configuration of the pod housings,the efficiency of the units can be improved dramatically. The use of modern /1*
mailto:[email protected]:[email protected]:[email protected]


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methods for optimiEation has led to a better understanding of the flow around thepod housings, also in connection with the working propellers. The figures belowshow the pressure distribution on a pod housing. The distribution is different onthe starboard and port side due to the influence of the working propeller on theflow.

+tarboard +ide (ort +ide

#0bb. part$.ps und part5.ps&

4odel tests in the towing tanks with pod units include both propulsion tests andopen water tests.

7+0 has developed new model pod testing e'uipment which enables accuratemeasurements of the propeller tor'ue and thrust at both ends of the drive. Thepod unit thrust is also measured. The photograph shows a +iemens+chottel(ropulser #++(& with two Jbladed propellers installed on 7+03s pod openwater device.

++( model installed on 7+0 pod open water device


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(od units are also well suited as booster drives for increasing the speed ofexisting vessels. 7+0 has carried out extensive model tests for a commercialconversion project6 a cruise vessel with an additional aEimuthing pod on thecenterline. !ithin the scope of the work, the powering performance of fourdifferent pod systems was investigated.

H/osta /lassicaH conversion with additional ++(

Diesel @ Electric Po(er Plant*iesel V electric power plants and propulsion solutions have clear merits whichmay vary from one ship type to another. The safety aspects of diesel V electricpropulsion are commonly regarded as being related to redundancy in differentways. The number of powergenerating units is large enough to ensurepropulsion capability and steerage way irrespective of any component failure. "naddition the units can be located in different compartments to safeguard against

loss of power in case one compartment has been destroyed by fire or flooding.

0 summary of advantages6

1lexibility, the installed power generating capacity can be used forvarious ship functions and different situations=

(ropeller tor'ue capability, full tor'ue at any propeller speeds=

(ermits running diesel engines at a stable load with smoothertransients=

(ermits running diesel engines at a constant speed=

(ermits running diesels engines at a more efficient load at optimumspecific fuel consumption, hereby reducing emissions and impacton the environment=

Cniform machinery= simple spare parts logistics, maintenance, crewtraining, etc=


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1lexibility in location of main engines allows to optimise cargospace volume and arrangement=

Redundancy, both in the sense of safety and freedom inmaintenance routines.

#ropulsion for #erformance

/ommercial marine propulsion has been seeing a good deal of innovation

over the past few years, most of it in the cruise ship sector where poddedpropulsion drive and gas turbines have been brought into play. ery largediesel engines, the largest ever built for commercial application, are beingdeveloped for megacontainer vessels capable of transporting nearly$%,%%% T-Cs. "n the workboat sector, there has been a near fullscale shift toaEimuthing drive for shipassist tugs, and tug owners have been choosingfrom a wide variety of drive combinations. 1ast ferry operators have beendemanding cuttingedge propulsion technology, and new hull forms arebeing developed to use advanced mediumspeed diesels as well ascompact gas turbines. )vershadowing all these developments are environmental regulations,many emanating from /alifornia, that will influence marine propulsiondesign over the next decade. !hile propulsion e'uipment manufactures have come up with a numberof new concepts to reduce pollution and enhance ship operation, not allinstallations have gone completely as planned. "n the cruise sector, /arnival /orporation has settled out of court with


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1inland3s 0BB "ndustry )y for problems the cruise line has experiencedwith 0BBdeveloped 0Eipod podded propulsion drives fitted on two/arnival cruise liners, the -lation and (aradise. Repairs to the (aradise,said to have involved pod bearings and seals, were carried out byirginia3s Newport News +hipbuilding while the -lation was handled at

/alifornia3s +an 1rancisco *rydock. /arnival demanded compensationfrom 0BB because of canceled cruises and lost income due to the twodrydockings. Newport News has also handled /elebrity /ruises, newcruise ship Millennium, which experienced problems with its 4ermaidpodded drives, resulting in a forced speed reduction of about four knots.4ost recently, the new -uropean car ferry Nils %olgerssonexperiencedbearing failure in a +iemens+chottel podded propulsion unit during seatrails in the Baltic +ea, which re'uired drydocking of the vessel forcomplete removal of the pod. Nevertheless, the benefits of poddedpropulsion are many and manufacturers feel that Honce the bugs are

worked outH they will become a standard propulsion device. (odded drive,in fact, has already been specified for the /oast uard3s new icebreakerbeing planned as a replacement for the $;JJbuilt &S!! Mac"ina$onthe reat Lakes. 0lthough podded propulsion has yet to be fitted to a largecontainership, it is already finding application in the tanker field. +wedishoperator Rederi *onsotank has taken delivery of a $A,@%% dwt producttanker, the Prospero, from /hinese shipbuilders that makes use of a single+iemens +chottel propulsion pod. The unit has two propellers, one at eachend, and is powered by four ;L5% !artsila diesel generating plants with a

combined output of A,J@% k!. This gives the J>@foot vessel a loadedspeed of $J.9 knots and the capacity to accommodate about $,%%% cubicmeters more cargo than it would if conventional dieselscrew propulsionwas used. (odded propulsion has also been chosen for a series of larger$%A,%%% dwt tankers being built for 1inland3s 1ortum )il as by 8apan3s+umitomo 7eavy "ndustries. 1ortum is a pioneer 0Eipod user, having hadpods fitted to two of its smaller ships, the &i""u and #unni, almost adecade ago. The new +umitomobuilt vessels will be H*ouble 0ctingTankersH in that they will move forward in open water but will use the

0Eipods to move sternfirst in ice conditions. (odded propulsion is also making its way into the ferry sector and 0BBhas developed a compact version of the 0Eipod that is being installed on adoubleended ferry being completed in -urope for service on the Baltic+ea. The smaller version of podded propulsion unit uses a permanentmagnet motor with direct cooling by the surrounding sea water. This in turnallows a smaller diameter propeller to be used which, according to 0BB,gives more dynamic efficiency. 0 Eipod has been designed to have a reversible propeller for working


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&illennium Class S'ecifications

The 4illennium class represents the latest in advanced cruise ship technology and drivesystems. 4illennium class are ;$,%%% tons, ;A9 feet long and can cruise at up to 5J knots. Theycarry $,;9% passengers and $,%%% crew. The "nfinity has the world3s first conservatory at sea

#including six 4agnolia trees&, a cyber cafe, 59,%%% s'uare foot spa, "nternet access in everystateroom and numerous other features. They are built to (anamax standards and so arecertified to pass through the (anama and +ueE canals. 0ll 4illennium class will be built by the/hantiers de L30tlanti'ue shipyard at +t. NaEaire, 1rance. -ach ship costs over C+* ]?9%million.

$e( Drive and Pro'ulsion Tec)nologies ntroduced

The 4illennium class employs two new drive system technologies.

1irst, they are the world3s first gas turbine powered cruise ships. (ower is generated bytwo - L459%%U aeroderivative gas turbine engines from - 4arine -ngines division, -

0ircraft -ngines. The L459%%U is a combined gas turbine and steam turbine integrated electricdrive systems #/)-+&. -ach 55foot, $$,%%%pound engine produces J%,9%% horsepower at

?A%% R(4. The exhaust gas temperature is ;A9 degrees 1ahrenheit. The gas turbines are thecleanest burning powerplants for any cruise ship in operation today.

+econd, the ship is powered throughthe water by two Qamewa #RollsRoyce 0B& 2

0lstom pod propulsion systems called4ermaid_. -ach 4ermaid pod propulsionsystem consists of a $;.9 4! electric motorturning an $@foot fixed pitch propeller. Theelectric motor is contained within the pod,completely submersed, and has infinitelyvariable speed control. 4ost importantly, the

two pods can be rotated through ?A%degrees, providing thrust in any direction.The propellers normally point forward, buttheir infinite speed adjustment and infinitedirectional adjustment allow the ship to besteered in any direction at any speed up to 5J knots. The propulsion pods not only allow therudder to be eliminated, but putting the power unit in the pod frees up substantial space onboard.

)ther advantages to the pod propulsion system are that the ship can easily dockanywhere without tugboat assistance, and that by pointing the propellers into the oncoming water,pressure pulses are reduced or eliminated. 0 propeller3s pressure pulses create intrusivevibrations within the ship= reduction of propulsion system noise and vibration has long been a keydesign criteria for cruise ships.

Production

Royal /aribbean ordered four 4illennium class ships to be delivered by the end of 5%%5.The first, 4illennium, was delivered in mid5%%%. The "nfinity was delivered on 1ebruary 5A, 5%%$,over a month late, and was not inaugurated until 0pril 5;, 5%%$.

/hantiers de L30tlanti'ue is 1rance3s biggest shipyard and has built most of the world3slargest and most advanced oil tankers, over $5% advanced technology warships and producesJ% of the world3s cruise ships. The shipyard employs over @,9%% people.


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Teet)ing Problems Plague >ot) S)i's

-ven though the shipyard3s advanced technology capabilities are well established, this hasnot prevented the 4illennium class from experiencing a series of expensive and debilitatingteething problems.

The first ship, the 4illennium, had to be taken out of service only a few months afterinauguration due to unacceptably high vibration levels amidships, traced to the gas turbines. Theship was put in drydock at Newport News where it was fitted with a ducktail and additional buffersection in the stern. +everal cruises had to be cancelled in that case.

The "nfinity was in its in final stages of completion just as the 4illennium3s unwantedvibration problems surfaced. "nfinity3s launch was delayed by over a month as a solution wasengineered and incorporated into the ship. +everal of "nfinity3s early cruises were cancelled.

"n 8anuary 5%%$, the 4illennium was once again out of service for two weeks due to anHunderperformingH electric motor in one of the 4ermaid propulsion units. The weak motor limitedthe ship3s top speed to 5%.9 knots instead of 5J knots, making it impossible to stay on schedule.Two cruises were cancelled during the repair.

The "nfinity3s portside pod drive bearing failure sidelined it for two weeks and forced thecancellation of two more cruises. 7owever, it was not immediately clear why the "nfinity had toreturn to drydock for repairs. 0ccording to RollsRoyce, the entire 4ermaid propulsion system canbe serviced or replaced in the water.

Pod Drive Tec)nology uestions Persist

Bearing failures and other problems are not uni'ue to /elebrity, lending credence toseveral leading experts3 opinions that pod drive systems are not yet a mature product ortechnology.

"n *ecember 5%%%, 0BB "ndustry reached a financial settlement with /arnival /ruise Lines

over a propeller bearing failure in one of the $J 4! 0Eipods which power the company3s ship(aradise. The (aradise3s 0Eipod bearing failure was blamed on lubrication problems, although ananalysis pointed to a HseriesH of unspecified problems.

The latest generation of 4od 4ropulsion ystems, the :ermaid, are developed by


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%/*9++9/+

The Commercial im-!riven Permanent "agnet

"otor Propulsor Po#$ill %an $larcom, &uha 'anhinen, and Frie#rich "ewis %4atent 4ending

ABSTRACT

Podded propulsion is gaining more widespread use in the marine industry and is prevalent in

newer cruise ships in particular. This propulsion system can provide many advantages to the shipowner, including improved propulsion efficiency, arrangement flexibility, payload and harbor

maneuverability. A new unique podded propulsor concept is being developed that allows

optimization of each element of the system. The concept integrates a ducted, multiple blade row

propulsor with a permanent magnet, radial flux motor rotor mounted on the tips of the propulsorrotor blades and the motor stator mounted within the duct of the propulsor. This concept,

designated a Commercial im!"rive Propulsor Pod #C"P$, when compared to a conventional

hub!drive pod, offers improved performance and attributes in a number of areas, including%

smaller weight and size, and equal or improved efficiency and efficiency bandwidth, cavitationand hull unsteady pressures. The combination of these C"P attributes and performance


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parameters could allow the ship designer greater flexibility to provide improved ship performance

at reduced cost, as compared to that of a hub!drive pod. The advantages extend across the entire

operating range, from sea trial to off design conditions. The advantages when compared to a hubdrive

pod could allow a C"P to achieve higher ship speeds, or to be applied to a wider range ofplatforms, or to extend the operating envelope of those platforms. The present paper discusses the

C"P&s advantages for both the ship designer and operator, compared to currently available

hub!drive pods."ill Ian "larcom is with eneral $ynamics ;lectric "oat 6;"7, roton, 3onnecticutJuha 1anhinen is with $eltamarin -td, 1elsinki, !inland!riedrich :ewis, is with the 1amburg hip :odel "asin 61I#7, 1amburg, ermany

+/*9++9/+T*!+CT*

eneral $ynamics ;lectric "oat 6;"7 hasdeveloped a commercial rim2driven propulsor pod63


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ma(imie efficiency, and reduce maneuver resistance ofthe pod. The duct diameter is driven by the rotor andstator blade design, while the duct thickness is driven bythe motor design.Scale "o#el ',#ro#,namic Test esults Summar,

n# Conclusions

# complete series of model scale hydrodynamicperformance tests were conducted on a small2scalemodel of the 3


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angle of incidence. The small amount and types ofcavitation, e(hibited above incidence speed by the3


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propulsion machinery designers, builders, testers andowners. Other efforts included gathering reports andpapers, technical research, attending presentations andsymposia, etc., covering numerous types of platformsand propulsion systems. !rom that research and ane(panding knowledge base of the physical andperformance features of ;"0s 3


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"esides the hydrodynamic efficiency advantagedemonstrated by model testing, the 3


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/*9++9/+drive efficiency, have been used in other commercialmarine papers and appear reasonable by comparison toother more comple( estimating methods evaluated.Pro;ecte# Efficienc, /ains re Conservative

ome factors not accounted for in this savingsprojection that bias the results in favor of the hub2drivepod, and thus bring some degree of conservatism to thisprojection areN4rojections #re "ased on ea Trial 3onditions,=)m9sec headwind 6"eaufort +, M)kts77.These are not representative of even average conditionsover the life of these ships, which include operating atthe following conditionsN $eepwater conditionsN Trim, wind, current, waves

and hull fouling are factors having significant

impact on ship resistance. # V%) loading factor isconsidered a normal adjustment from sea trial toaverage deep2water conditions. 5n heavy weatherthe overload condition can easily be )/. hallow water conditionsN Water depth also has

e(tremely strong influence on resistance. 5n onereport it was noted that that for panama( sie cruiseships 6M*m draft7 strong depth impact starts around>/m water depth and in %)m deep water thesevessels can typically only reach )/ of top speed. -ow speed operationN #t lower speeds in particular,

sea trial conditions are the most unrepresentative,since lower speed ranges are likely in shallow

depth, high harbor maneuvering conditions wherepropeller loading would be considerably increased.#nd in those maneuvering conditions the pods areusually turned into a crabbingX orientation, inwhich they are typically oriented between >/ to &/degrees to each other to allow rapid thrust vectoring6e.g., see !igure )7. The to %+ knot poweringportion of the !igure powering comparison isbased on both pods powering from the /o angle ofincidence positionR the crabbing position changesthis. Thus higher blade loads will be e(periencedduring low speed operation than has been analyed,and those operations will be at inflow angles of

incidence to the pod, both factors increasing theadvantage of the 3


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Figure < 4otential 3


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significant advantage.arrower Ship $eams? n# +ni5ue ConfigurationsNThe pod sie itself offers an obvious benefit fornarrow beam ships, evident by the length comparison ofTable % and also depicted in !igures F and *. "utbeyond the conventional twin screw ships as tested the3L is astatement that the four2pod configuration wasselected after a three2pod configuration wasevaluated 6+=.):W each7R the three2podconfiguration was abandoned due to e(cess per podweight, in e(cess of >// tonnes. ince the 3


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allowing for a more rationale distribution of themachinery and load items, and it might thussupport a three2pod configuration where the hubdrivecould not.

#lso consider a booster pod arrangement, such as

the 3osta 3lassica e(tension project. The smaller

3 to >.)k4a, containerships with installed propulsion power in the range

of +/ to >/:W would typically allow > to )k4aand tankers would typically allow ) to =k4a.3learance for a cruise ship has typically been +)to >) of the propeller tip diameter to achieve itsvibration tolerance levelR other ship types would ofcourse have different typical clearances. The tip 6orhull7 clearance both provides distance to dissipateenergy from the source 6propeller cavitation7 aswell as placing the propeller in a more benign wakeand thus limiting cavitation.


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5n the 3


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propulsion in some


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3


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Figure 22 3 to %9+ thestrut is e(posed to the strong propeller discharge,and there have been reports that this e(posed areais highly susceptible to cavitation erosion as wellas vibration e(citation. The 3


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uses oil2lubricated roller bearings for both the shaftradial and thrust bearings, which have become amaintenance problem for many ship operators,re8uiring e(pensive dry2dock periods todisassemble the pod6s7 and replace roller bearings.The current bearing problems appear to beaggravated by early seal failures, in some reportedinstances at least, that introduce seawater into thebearing cavity and lead to rapid bearing failure.#lso, the failed seals and flooded bearing cavitiescan allow oil to escape the pods and becomepenaliing environmental spills. 3ooling ystemN -ess cooling system e8uipment is

re8uired since the 3


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realistic to consider this operating mode as being ofsome interest to a commercial ship owner9operator. #salready noted the 3


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:arine ;lectronics 4odded 4ropulsion ystem,$ %./>=./%9+///K)L The 4 4ropulsor, #n 5ngenious 4odded $riveystem,X %)&@)>* /+&*%*"EC6T+E

6 The 4ennsylvania tate @niversity #pplied


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The following limited list of individuals wereconsidered the greatest supporters, and most gratefullyacknowledgedN!rom ;lectric "oatN #l !ranco, cott !orney, 3has.t. ermain, $an Hane, 3harles Hnight, :ichelle -ea,$onald Thompson, pyro 4appas, tu 4eil, :arkWarburton. !rom 1I#N John STRACT(odded propulsion is prevalent in the marine industry. (oddedpropulsion systems provide many advantages to the ship owner,including increased propulsion efficiency and reduced construction


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cost. To evaluate the potential of a new pod configuration, aprototype machine was constructed and tested. This prototypemachine was mainly constructed of composite parts. The propeller,housings, structural blading, motor canning and fairings wereconstructed of composite materials. /omposite materials were

chosen as a cost saving, schedule reduction, performanceenhancement and as a technology demonstration. This paper willreview the unit construction, and test results, focusing on thelessons learned for the composite part manufacture.Q- !)R*+6 (ods, /omposites

$TR!D"CT!$(odded propulsion has become a shipbuilding standard for commercial ships.This propulsion alternative offers the ship owner many advantages. Theadvantages include reduced ship ac'uisition costs, improved propulsionefficiency and improved ship arrangements. (odded propulsion has taken theform of a driver contained in a hull, turning the propeller by a shaft. The driver

hull #or pod& includes a shaft sealing system to prevent water from entering. Thepod is connected to the ship hull by a strut. The strut can be connected directlyto ship structure or to an aEmuthing system. The aEimuthing system offers theship owner improved maneuvering over standard rudder systems. 1igure $ is anartistFs depiction of one of the most popular podded propulsion systems,produced by 0sea Brown Boveri #0BB&.1igure $6 0BB 0Eipod (odded (ropulsion `$

0n alternate podded propulsion system has been invented, incorporating thedriver or motor on the rim of the propeller. The motor rotor is attached to the rimand driven by the motor stator, which is located outside the rotor. Thispropulsion system has been shown analytically and empirically to offer improved

efficiency and reduced weight when compared to current podded propulsion.The configuration of a rim driven pod #R*(& is represented by 1igure 5. The podconfiguration and many details are (atent (ending.1igure 56 Rim *riven (od /oncept

0 prototype R*( was designed and constructed to provide an empiricaldatabase to validate the analysis. This hardware was designed to be testedstatically, and so reproduced the flow path and driver hardware properly. Theexternal shape was not faired due to the static test conditions. The bearingsused for the initial demonstration were angular contact ball bearings. Thissystem offered a low risk, low cost alternative to hydrodynamic bearings as areexpected to be used in many future applications. The unit was manufactured of

composite materials to minimiEe costs and schedule. /ost and scheduleadvantages stemmed from the nonproduction nature of the part and novelcomposite manufacture techni'ues employed. The composite material alsooffered some definable performance advantages. 1igure ? is a picture of theassembled prototype R*(.1igure ?6 (rototype Rim *riven (ropulsion (od

DES-$The prototype R*( was designed for static operation, the thrust was absorbed in


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a moored barge. The integration of the various components to create the R*(assembly was fully evaluated in the design and manufacturing stages to precludeproblems in future units. )ther features of the prototype R*( included6 Rolling -lement #ball& bearings enclosed in a pressure compensated housing.

+tatic lip seals to prevent bearing oil from leaking into the water.

0 permanent magnet #(4& motor design.0 two stage propeller blade set, the first rotating, the second stationary,canceling swirl from the first to maximiEe efficiency. Two composite stator can concepts, a solid can and a hollow pressurecompensated can.0 solid composite canning for the motor rotor.

/omposite blading manufactured with a patent pending concept, whichmanufactures each blade independently and then joins them into a monolithicstructure. -nd bells and cones manufactured of composite materials.

1igure J is a cross sectional drawing of the prototype R*( with a pressurecompensated hollow stator can. This drawing represents the option that wasmanufactured, assembled and tested.1igure J6 (rototype R*( *rawingThe design attributes of the prototype R*( are6

^ $5% 7orsepower^ 9%% R(4^ 5J (oles

^ $JJ +tator +lots^ A (hase 4otor^ %.J9 "nch -lectrical ap&A$"0ACT"REThis section will review the manufacture of the key parts of the prototype R*(,the methods used and the lessons learned. These key parts include the rotor,the stator can, and the stationary blading. The manufacturing methods chosenreflected the re'uired part configuration. The two primary manufacturing J5@ olts

>97E ;@.$@@ 4otor -fficiency

methods used were a vacuum assisted resin transfer molding #0RT4& andfilament winding. The 0RT4 methods used both soft and hard molds. Theresin system for the parts was a *)! *-R-Q0N- inyl -ster with - V lassreinforcement. 0 metallic structure was included as a permanent part of someparts as reinforcement, load carrying, wear surfaces or thread pads. (artshrinkage rates and cure were calculated based on empirical 0RT4 data. 0


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5 average shrink rate was used for the thick parts. This rate was proven to beappropriate for the reinforcement, resin and fillers used.RotorThe rotor was manufactured #using a patent pending process& by first moldingimpeller sections that include the entire blade span and portions of the hub and

shroud, see 1igure 9$. These sections were then assembled with adhesive toform a complete set of blades. The set of blades was then placed in a mold witha glass wrap on the outside of the blade assembly, see 1igure 95. Thisassembly was then injected to form the shroud. The metallic hub was insertedwith glass fiber and injected with resin, see 1igure 9?. The part was thendemolded and the rotor finishmachined to accept the motor rotor. The motorrotor is then installed, see 1igure 9J. lass was packed around the rotor, andthe rotor 0RT4 injected to form a solidly canned part, see 1igure 99.1igure 9$. /omposite Rotor +egments1igure 9?. Rotor with +hroud and 7ub1igure 95. Rotor +egments in Tooling

1igure 9J. Rotor with 4otor Rotor1igure 996 /ompleted Rotor with 4otor Rotor /annedStatorThe stator vanes were manufactured #using a patent pending process& in thesame manner as the rotor, by first molding sections that include the entire bladespan and portions of the hub and shroud, see 1igure A$. These sections werethen assembled with adhesive to form a complete set of blades, see 1igure A5.The set of blades was then placed in a mold with a glass wrap on the outside ofthe blade assembly. This assembly was then injected to form the shroud. Theassembly was then installed in a 0RT4 cylinder on which aluminum stiffenersand thread plates were attached with resin, see 1igure A?. The part was thendemolded, see 1igure AJ. The part finish machining completed the effort, see1igure A91igure A$6 +tator +egments 1igure A56 0ssembled +tator +egments1igure AJ6 0RT4 +tator 0ssembly 1igure A?6 +tator 0ssembly1igure A96 1inished +tatorStator CanThe stator can was manufactured using two different methods. The first methodused was a solid encapsulation. 0 hollow, pressure compensated canningmethod was also used. 1igure >$ is the motor stator that was canned.1igure >$6 4otor +tatorThe solid canning was manufactured by inserting the motor stator in a hard toolwith locating features and dry glass

diesel electric propulsion

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