Wärtsilä has reviewed its currentportfolio of controllable pitch propellersbased on recent market requirements,particularly the facts that ships todayare required to have more power,operate at higher speeds anddemonstrate better fuel efficiency withlower noise levels. New hub models incombination with the Efficiency Rudderpromise optimal performance inhigh-strength applications such ascontainer feeders and RoPax vessels.

In shipbuilding, use of the controllable pitchpropeller (CPP) has grown steadily since itsintroduction about 40 years ago. Today thispropeller type is in wide use, especially invessels which benefit from high-output4-stroke engines (Fig. 1 and 2).

Several ship types apply CPPs due to thedirect benefits these offer in operation. Shipoperators normally choose a CPP for one ofthe following reasons:� The use of shaft generators for

operational efficiency� Frequent manoeuvring in confined

waters� The wide range of thrusts and operating

conditions required� The need for fast response to different

thrust requirements� Low operational cost with optimal

engine load characteristics.

Hub development - the historyThe controllable pitch propeller has toprovide two main functions: transfer thethrust and torque, and change the pitch ofthe blade. The first is done by a bladebearing connecting the blade and the hub.The pitch is changed via a mechanical or ahydraulic system that rotates the bladearound its vertical axis. Originally all CPPswere equipped with a mechanicalconnection between a yoke in the hub andan inboard actuator. The modern designstandard is a hydraulic piston positionedinside the hub.

Hub power has grown steadily over theyears, as Figure 1 shows. The following

28 - Wärtsilä 2-2004

The Ship Power Supplier

Historical developm nt of maximum powerapplied per ship ropulsorp

e

0

10000

20000

30000

40000

50000

60000

70000

1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 2020Year

Power [kW]

Fixed pitch propel ers

JetsPodded

Controllable pitch propellers

propulsors

l

Fig. 1 The maximum power applied on a single propulsor has increased dramatically inrecent decades.

Fig. 2 The world’s most powerful controllable pitch propeller, installed on the QueenElizabeth II: power 44 MW at 144 rpm and a maximum ship speed of about 32 knots.The hub diameter is 2.0 m with a five bladed propeller diameter of 6.0 m.

Improved controllable pitchpropeller concept offers bettervessel performance by Teus van Beek

Propulsor TechnologyWärtsilä Propulsion Netherlands BV

table is from an overview of hubdevelopment presented at the launch of theLips C-hub in 1983 with the addition ofrecent advances by Wärtsilä:

1st generation controllable pitch propeller,1955 < 5 MW� split hub grease lubricated� mechanical pitch actuation� mechanical pitch feedback

2nd generation controllable pitchpropeller, 1962 < 11 MW� integral hub, oil lubricated� mechanical pitch actuation� mechanical pitch feedback

3rd generation controllable pitchpropeller, 1972 35 MW� integral hub, oil lubricated� hydraulic pitch actuation� mechanical pitch feedback

4rd generation controllable pitchpropeller, 1983 (C-hub) no power limit� integral hub, oil lubricated� hydraulic pitch actuation� optional electrical pitch feedback� maximum power applied 43 MW,

largest size 2.8 m hub diameter

5th generation controllable pitchpropeller, 1989 (CPS-hub)� integral hub, oil lubricated� wearing plates, hub replacement behind

the ship� hydraulic pitch actuation� mechanical pitch feedback

6th generation controllable pitchpropeller, 1995 (D-hub)� integral hub, with integrated hub cover� hydraulic pitch actuation� mechanical pitch feedback

Hub design - the fundamentalsThe most recent hub designs have far fewerparts than earlier types of controllable pitchpropeller installation, which implies that the

newer designs are more compact, strongerand more reliable. In most modern designs abasic choice has to be made as to theposition of the actuating cylinder. Puttingthe actuating cylinder between the bladesresults in a more compact design and fewerparts whereas an actuating position behindthe hub after the blades allows largeractuating forces or lower actuating pressures.Both solutions are already available.

Torque and thrust are transferred via theblade bearings. The area and diameter ofthe blade bearings and the effective surfacearea determine the torque and thrusttransmitting capacity of a CPP-hub. Theallowed bearing pressure depends on thematerials selection, the occurrence of localpressure peaks and the amount and type oflubrication.

The system must also be able to changethe pitch in all operating conditions. Thestrength of the pitch actuating system isdetermined by the force on the actuatingpin in combination with the allowedpressure on the pin surface. This force hasto overcome both the hydrodynamic forceson the blades and the friction forces in thehub. Together these are called the‘actuating force’.

Ultimately, the actuating force must begenerated by the hydraulic cylinder in thehub. Since this cylinder has a certain area,the force required determines the pressureand the necessary available area. Anoptimum hub design, therefore, must havethe proper balance between the thrust andtorque carrying capacity on the one hand,and the actuating capacity on the other.

The new Wärtsilä hub portfolioSince ship requirement have diversified overtime Wärtsilä decided to develop twodistinct CPP-hub ranges: one for highpower densities and high ship speeds, andthe other for lower power densities andmoderate ship speeds. Recent developmentsin RoPax vessels operating at ship speedsclose to 30 knots result in differentrequirements for the hub forces, notablylarger actuating forces and cavitationproperties (for further details, please refer toReference 2 in the article “Technologyguidelines for efficient design and operationof ship propulsors”, Marine News 1-2004).

After reviewing its current hub portfolio,Wärtsilä decided to add a new hub to theseries in the higher power range, the‘E-hub’. Based on the current hub design,the E-hub incorporates the newest feedbackfrom service experience, the requirements of

2-2004 Marine News - 29

Fig. 3 Basic hub mechanism functions (left, transfer of thrust and torque: right, actuating).

Power range [kW]

0 2500 5000 7500 10000 12500 15000 17500 20000 22500 25000

D hub

C hub

CPS hub

E hub

1995

1983

2004

1989

Fig. 4 The range of controllable pitch propellers available from Wärtsilä.

RoPax vessels and ice strengthening, andoptimum cavitation performance. With theD-hub range now complete, this provides acomprehensive programme of CPPs for anyapplication and requirement, Figure 4.

We examine each of the hub types inmore detail below.

The C-hubThe C-hub, in operation since 1983, is thehub with the largest number ofapplications. The actuating cylinder ispositioned in the blade foot area. Specialhub contours have been developed for highspeeds. The hub concept is well suited tomedium power density applications and foruse with a nozzle.

The CPS-hubThe CPS-hub design, in operation since1989, is very service friendly. For example,the hub can be easily removed from theshaft in dry dock.

The hub is always built of stainless steeland very strong, which makes it well suitedto RoPax applications and ice-strengthenedvessels.

The D-hubThe D-hub was developed in 1995 tocombine low cost and high strength. It isapplied to bow thrusters and to steerablethrusters, especially in the smaller powerrange (< 3 MW). The steps in the series are

therefore fairly small and this has led tosuccessful application in both smaller andlarger vessels.

The D-hub’s success lies mainly in itslow number of parts. Given the position ofthe actuating cylinder, the aft cover isintegrated in the hub body. A separate aftcover is therefore unnecessary, and so noseparate bolt connection and machiningsurface are required, which increasesreliability and strength. The result is ahighly cost-effective solution combinedwith high reliability.

The E-hubThe E-hub has been specially developed forapplications requiring high strength such as

30 - Wärtsilä 2-2004

The Ship Power Supplier

Fig.5 Layout of the C-hub. Fig.6 The CPS concept.

Fig.7 D-hub layout. Fig. 8 Layout of the new E-hub.

RoPax vessels and ice-strengthenedpropellers. The hub contour is flush and soparticularly suitable for high-ship speeds.

The E-hub series will be set up inparallel to the D-hub. The first E-hubs areexpected to enter operation at thebeginning of 2005. The concept utilizes thestrong points of the D, C and CPS concept.The blade bearing, the blade seal and thesolutions used are based on proven designtechnology. Optimized construction andthe minimum number of parts givesreliability and reduced cost.

E-hub featuresCurrent design experience, especially inRoPax vessels at speeds beyond 25 knots,

has shown the importance of the hubcontour to achieving the best cavitationproperties. The E-hub concept does this invarious ways:� The blade bolts have been tilted so that

the blade root can be easily faired on theblade foot

� No flow disturbances are caused by theblade bolt holes

� The blade bolts themselves are level withthe surface of the blade foot.

A special feature of the new hub concept isthe blade foot sealing, Figure 10. This sealwas developed for the CPS-hub for theheaviest wear conditions (sandy waters) andhas shown excellent operating performance.

The basic feature of this seal is double

sealing capacity at the blade foot for thesame number of parts. The outer lip keepsout dirt while the inner lip has optimumlubrication conditions to guarantee longlife. The seal is also very flexible, whichallows it to follow the surface of the bladefoot with minimum variation in surfacepressures. This will avoid local wear of theblade foot and minimize repair cost.

An important aspect of the new hub isits strength. Optimum strength has beenobtained in various ways including:� Reduction of local pressure peaks in the

blade bearing arrangement� Selection of the best material

combinations for wear and strength� Optimum combination of the thrust and

torque force carrying capacity versus theactuating capacity for a given missionprofile.

The new design makes the dimensions ofthe new E-hub smaller than for previoustypes, leading to better hydrodynamicperformance.

Combination withthe Efficiency RudderThe outstanding features of the Wärtsilä’sEfficiency Rudder (described in MarineNews 1-2004) can be extended whencombined with the new CPP range;applying the Efficiency Rudder incombination with the D- and E-hubs willgive optimum operational performance. Toillustrate how this works in practice wedescribe the Efficiency Rudder’s applicationin two ship types: container feeders andRoPax vessels.

The highest efficiency gain with theEfficiency Rudder is achieved using theoptimum conventional aft bodies with

2-2004 Marine News - 31

Fig.9 The flush hub contour guarantees the best cavitation performance at high speed even beyond 30 knots.

Fig. 10 Blade foot sealing.

bulbous sterns. The efficiency gain increasesfor vessels with:� Increased wake fraction, i.e. increased

fullness of aft body (for single screwvessels especially)

� Increased size of propeller hub as a resultof increased power density and/or icestrengthening.

Both these factors apply to the vessels wedescribe below. The optimum increase inefficiency in obtained by:� Integrated rudder torpedo� Propeller blades designed to take more

inwards loading, to further reducepropeller induced vibration

� Slim rudder profile� Extended horn structure� Unique flap mechanism (the Efficiency

Rudder can also be supplied withoutflaps).

The Efficiency Rudder is optimized to fitwith the controllable pitch propeller

installation. This ensures perfect alignmentof the mechanical systems, overall designintegration of both systems, andresponsibility for the entire package withone supplier. The rudder profile isoptimized for each project. Variationsinclude the type and presence of flapsspecial fishtails and special rudder profiles.

Container feederThe main characteristic of a containerfeeder is the position of the machineryinstallation, which is as far aft as possible toensure maximum loading capacity in thehold. Consequently, a single 4-stroke dieselis selected in combination with a gearboxand a single propeller. The superstructureof the ship is normally aft just above thepropeller and the engine.

The operational profiles of these vesselsshow that they operate close to themaximum power they have available and

call in at a lot of ports, which emphasizesthe importance of manoeuvring. AnEfficiency Rudder in combination with acontrollable pitch propeller reduces boththe direct fuel cost and the pressure pulsesby 20%, which will improve the operationof the vessel. Normally the EfficiencyRudder is equipped with a flap to furtherenhance its good manoeuvring properties.

Figure 12 shows how, in the case of asingle-screw vessel, the propeller shaft canbe withdrawn simply by disassembling theEfficiency Rudder.

The operational benefits for a givensingle-screw vessel are shown in Figure 13,which shows the pressure pulses obtained atmodel and full-scale levels. Initial pressurepulses were as high as 5.4 kPa. Furtheroptimization of the propeller design reducedthe blade rate pressure amplitude to 2.9 kPa.Adding the torpedo effect of the EfficiencyRudder resulted in a further decrease to

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The Ship Power Supplier

Efficiency Rudder Conventional rudder

1. Rudder horn

2. Rudder blade

3. Flap

4. Flap mechanism

5. Rudder stock

6. Torpedo

7. Fore torpedo

8. Fairing of hub

Fig.11 Efficiency Rudder layout compared to a conventional rudder installation and disassembly of the main components.

1.7 kPa. This was confirmed by the full-scalemeasurements, which indicated 1.9 kPa.

Engine power predictions for the samecase indicated an overall powerimprovement of 16%; 12% of this was dueto the effect of the Efficiency Rudder(10% from the torpedo effect, 2% from theslender rudder profile and headbox), and4% to the optimized blade design. Theresulting fuel savings led to a very shortpay-back time for this investment.

RoPax vesselDevelopments in RoPax vessels in recentyears have clearly demonstrated the need forlarge power capacity and increased shipspeed, which naturally makes all aspects ofpropeller design more critical than ever.The manoeuvring requirements for thesevessels remain high as before, and for thisreason many of these vessels are equippedwith flapped rudders.

Here too, the Efficiency Rudder offersmultiple benefits. The propeller design givesa significant increase in efficiency. Althoughthe percentage of gain for twin-screw vesselsis normally smaller than for single-screwapplications, the large power demand andfuel consumption of these vessels areexpected to justify the investment inEfficiency Rudders in many cases.

The cavitation performance of the vesselis enhanced, likewise, because theinteraction between hub and rudder iscompletely avoided, the pressure pulses arereduced and the slender design of therudder in the head box area prevents anynegative interaction of the tip vortex flowsbetween the propeller and the rudder.

The Efficiency Rudder is expected toattract a lot of interest especially forhigh-powered RoPax applications.

Experience with a number of projects fortwin-screw vessels demonstrates a power

reduction of 4-6%, and these results areconfirmed by independent model institutes.Moreover the effect of the torpedo part ofthe Efficiency Rudder further reducespressure pulses. Model tests indicated areduction of 10-20%, which is verybeneficial for crew and passenger comforton board. �

References:

/1/ G.H.M. Beek and E.P.H.M. de Mulder,"A New Generation Controllable Pitch Propeller",Lips Propeller Symposium, Drunen, 1983.

/2/ T. van Beek," Technology guidelines forefficient design and operation of shippropulsors", Marine News 1-2004, Wärtsilä.

2-2004 Marine News - 33

6

5

4

3

2

1

0

5.4

2.85

1.76 1.9

2.7 2.78

1.85

0.3

1.1 1.20.89

0.3

1st order of PBF 2nd order of PBF 3rd order of PBF

Fig.13 Model and full-scale test results for a single-screwcontrollable pitch propeller equipped with an Efficiency Rudder.

Fig.14 Efficiency Rudder layout of twin-screw vessel.

Secure rudderDismount rudderstock crimp connection

Dismount fore torpedoRemove tiller if necessary

Loosen horn/torpedo boltconnection and lower blade/flap/torpedoShaft can be withdrawn

Before dismounting

Fig. 12 Shaft withdrawal for a controllable pitch propeller installation with an Efficiency Rudder for a single-screw vessel.