A New Start for Marine Propellers

34 REINFORCEDplastics December 2004

In the well-known apocryphal story, alocal person, on being asked fordirections to another part of the

locality, replies: “Well, if I were you, Iwouldn’t start from here!”

A marine engineer, given a cleansheet and asked how he would go aboutdesigning a propeller, might feel thesame way. What ships have now are pro-pellers made from metals – materials thatcorrode in sea water and set up galvanicaction, are easily dented, bent in ground-

ings or strong impacts and are difficult toform into complex shapes. How mucheasier it should be to mould durable pro-pellers in plastic, repeatably and to theexact shape required for peak hydro-dynamic efficiency. Such units could bemade faster and less expensively thanmetal counterparts, without the exten-sive reworking that metal props oftenrequire. Plastic blades would never cor-rode, electrolyse, ‘de-zincify’ or seize tothe drive shaft. If only, our engineer

might wish, plastics could be found thatwere strong enough to serve for thesehighly loaded devices.

Modern reinforced plastics are thefulfilment of that wish, giving designersthe chance for a fresh start in propellerdesign and manufacture. A number ofcompanies have shown the viability ofcomposite propellers, albeit, so far, main-ly those for small craft. The challengenow is to scale these up to larger unitssuitable for ships.

0034-3617/04 ©2004 Elsevier Ltd. All rights reserved.

A new start for marinepropellers?Reinforced plastics offer designers of ship propellers a new approach to design and manufacture. The viability of composite propellers has been demonstrated onsmall craft; the challenge now is to scale-up these designs to produce larger unitssuitable for ships. George Marsh reports.

QinetiQ composite propeller installed on RV Triton. (Picture © QuinetiQ.)

35December 2004 REINFORCEDplastics

A fine example of innovation is thatset by Swedish company ProPulse ABwith a novel concept it patented half adecade ago. The ProPulse® modular pro-peller comprises a metal hub withreplaceable composite blades whosepitch can jointly be adjusted (at bladeinstallation) to one of five settings.Suitable for motors of 20-300 hp, theprops are made from an undisclosed‘high quality composite’ developed forthe application by the Sicomp of Pitea, acompany jointly owned by the Swedishgovernment and the Lulea University ofTechnology. The manufacturer is Formel-produkter of Boden.

Modern reinforced plasticsare giving designers thechance for a fresh start in propeller design andmanufacture.

Tests have shown the blades to bestronger than equivalents in aluminium,the ‘stock’ material used for outboardmotor and sterndrive props. Despiteweighing up to 40% less than their alu-minium counterparts, the blades aretough and resilient, resisting impact,light grounding and damage from cavita-tion. (The latter is a phenomenon inwhich low pressure cavities formed inaerated water behind the blades as thepropeller rotates collapse, sometimeswith considerable force.) Moulding theblades to the precise shape called for bythe 3D CAD design has resulted in highpropulsive efficiency. In case of damage,blades can be removed on the spot andreplaced individually. A trolling fisher-man could, for example, quickly removetwo opposite blades of a damaged four-blade prop, re-adjust the pitch of theremaining two and continue operatingwith those.

US company Pirhana says its pro-pellers have blades which, because theyare made from LNG Engineering Plastics’

Verton long glass fibre reinforcedpolyamide thermoplastic, are 17%stronger than traditional die-cast alu-minium, and have far superior chemicaland corrosion resistance. They also resistabrasion better than metals, and sufferless from leading-edge erosion in watermade abrasive by suspended grit. Theyhave low hydrodynamic friction andhigh propulsive efficiency. The Vertonused is said to retain its strength evenafter prolonged immersion in water. Theblades are injection moulded, a methodthat yields perfectly matched units on arapid fabrication cycle.

As with ProPulse, Pirhana blades canbe replaced individually. During 2004,Pirhana introduced a modular compositeprop-wrench to facilitate prop removal.Reported to be 60% stiffer than a con-ventional metal wrench, the tool has theadditional benefit that it is self-buoyant,which can results in fewer lost wrencheswhen working afloat!

Echoing ProPulse, US companyComposite Marine Propellers considersthe materials used in its Comprop seriesa trade secret, describing them simply as‘fibre-filled resins’. The single-piece four-blade props, up to 22 inch (about 55 cm)in diameter, are offered as original equip-ment on Regal, Wellcraft, Glastron andCorona boats, as well as being recom-mended for spares or replacements on avariety of craft powered by engines up to225 hp. The 22 inch model weighs just2.5 lb (1.1 kg), as against 4 lb (1.8 kg) foran equivalent aluminium prop. As wellas being more affordable than alumini-um units, these props have blades thatare designed to flex slightly or break offshould they hit an obstruction, so that drive shafts and bearings remainundamaged.

Outboard Marine Corp (OMC), oneof the world’s leading outboard produc-ers and owner of the Evinrude andJohnson brands as well as OMC stern

A new start for marine propellers?

Here the instrumentation pod and strain gauges used in trialling the QinetiQ compositepropeller are clearly visible. (Picture © QuinetiQ.)


drives, now offers composite propellersalongside the aluminium and top-of-the-range stainless steel props that have beenstandard for years. Company expertsreckon that stainless props are still thebest for stiffness and ultimate perform-ance, but at almost double the price ofaluminium equivalents, let alone com-posite, they are in a premium market.Mercury Outboards offers a line of com-posite propellers as spares.

Scaling upScaling up these small units to larger pro-peller sizes suitable for ships is some-thing of a challenge. Plastics, even whenreinforced, tend to be less stiff than met-als and early blades made from themwere known to lose propulsive efficiencyby flexing. Nevertheless, efforts to over-come the difficulties have continuedbecause of the potential advantages –not least reduced weight and increaseddurability.

One of the prime movers in the fieldis the German company AIR. Fertigung-Technologie GmbH, founded 12 yearsago by graduates of the University ofRostock, with whom the company stillmaintains close ties. This partnership hasmet stiffness requirements by adoptingcarbon fibre composites for theirContur® range of composite propellersintended for superyachts and ships. Over400 ship sets of Contur advanced com-posite propellers have been sold, rangingin diameter from 50 cm to 5 m. AIR saysits propellers weigh only a third as muchas conventional nickel-aluminiumbronze (NAB) equivalents. Compositeblades can, it says, be thinner at the tipsthan metal, reducing propeller noise typ-ically by 5 dB.

More recently AIR, with RostockUniversity’s academic backing, has comeup with an exciting innovation that pos-itively exploits the flexible qualities ofcomposites. In the latest, ‘smart’ Conturpropellers, carbon, aramid and drawnpolyethylene fibres are disposed withinthe composite in such a way as to pro-vide hydroelasticity. This enables theblades to react to changing load

conditions by altering their pitch, somaintaining optimum propulsive effi-ciency across a range of throttle settings.As a result, fuel consumption can bereduced by up to 15%. In addition, adap-tive self-pitching propellers reach ratedrpm faster than conventional props, sothat the vessel has better acceleration.Cavitation is reduced because, since theblade adapts itself to different loads, theload over a given blade area tends to staywithin the limits at which implosion ofcavities against the blade is induced.Blades are manufactured in closedmoulds by an resin transfer moulding(RTM)-like process, to close tolerances sothat their hydroelastic and other proper-ties are matched.

Contur propellers reduce the cost ofprop maintenance by having separateexchangeable blades, each blade possess-ing a thickened root that slots into thehub. One owner of a 25 kt, 150 ft (45 m)GRP-constructed motor yacht reportedreplacing blades underwater as being‘straightforward’ and quick. Having beenprompted initially to switch from metalprops to composite on experiencingexcessive vibration, he had since run

'countless sea miles' smoothly. In a speedtrial the vessel proved only 0.2 kts slowerthan when she was new with her metalpropellers.

UK importer and distributor Fleet-water Marine reports growing interest inContur composite propellers from pro-fessional charter/leisure craft and work-boat operators. Recently the companysupplied Associated British Ports (ABP),the UK’s leading ports business, with itsfirst set of Contur ‘smart’ self-pitchingpropellers. Before ordering these, ABPand VT Halmatic (builder of a Nelson48/50 pilot boat operated by the former),trialled the composite props at sea,against the normal bronze propellers.Results with the Nelson craft showed fuelreductions of almost 10% at full throttleand an impressive 17.5% at mid range.Accel-eration was enhanced and noise inthe wheelhouse was cut by up to 4%.When several sizeable underwaterobjects were struck unexpectedly, thecomposite blades continued to operatesmoothly without damage whereas, inthe opinion of the on-board crew, metalblades would probably have bent andstarted to vibrate.


Over 400 ship sets of Contur Marine composite propellers have been supplied for craftranging from superyachts to minesweepers. UK agent Fleetwater Marine recently supplied aset for an Associated British Ports pilot vessel. The latest hybrid composite Contur props havehydroelastic properties that confer adaptive self-pitching. (Picture courtesy of ConturMarine/Fleetwater Marine.)

37December 2004 REINFORCEDplastics

According to Fleetwater Marine’sNicholas Bentley-Buckie, the Conturblade tips are especially resistant tobelow-water impacts because they arefortified with drawn polyethylene fibres.His graphic explanation of the prop'’sself-pitching action is that ‘the bladesflatten out with load’ so that optimumpitch is maintained the whole time. Thiscontrasts, he told Reinforced Plastics, withthe fixed pitch of conventional bronzeblades which is normally set to absorbthe full engine power at maximum throt-tle setting. This means that at the normalcruise settings at which vessels spendmost of their time, they are operatingaway from peak efficiency. The largestprop Fleetwater has supplied to date is a3 m diameter unit fitted to a mine-sweeper. The company is discussing witha well known local shipbuilder the possi-bility of fitting Contur props to fastpatrol boats.

ResearchComposite propellers could bring majorbenefits to certain shipping sectors, inparticular fast ferries and military.Among naval craft, for instance, thenon-magnetic nature of plastic propsand their quiet running can enhance thestealth of mine countermeasures vessels,while their resilience would help themresist damage from underwater explo-sions and debris. Research in variouscountries is directed at achieving thepromised benefits by making sizeablecomposite propellers a reality. Efforts inGermany (the University of Rostock'srole has been mentioned) have been par-alleled in the USA, UK, Scandinavia,Italy, Greece and elsewhere.

In the late 1990s, the EuropeanCommunity-sponsored Composite Mar-ine Propeller (Comarprop) projectbrought researchers from four countriestogether in an effort to define design andmanufacturing technologies, confirmthe propellers' commercial benefits andestablish a basis for their acceptance byclassification societies. Investigationsranged from composite materials evalua-tion to producing full-scale props by

resin transfer moulding and triallingthem at sea. Work on design methodolo-gies involved finite element modellingand trying to adapt the hydrodynamicdesign process to allow for the non-isotropic properties of composites.Another focus was how best to incorpo-rate hydroelasticity so as to secure adap-tive blade pitching. Project collaboratorsincluded the Norwegian MarineTechnology Research Institute (Marin-tek); Dowty Aerospace Propellers andDERA (now QinetiQ), Haslar in the UK;the National Technical University ofAthens; and from Italy the RegistroItaliano Navale, shipbuilder Fincantieri,CETENA SpA and the ConsorzioArmatori per la Ricerca Srl.

The US Navy, intrigued by the adap-tive blade pitching and other possibili-ties offered by the AIR Fertigung-Technologie composite propellers,recently initiated a three-year evaluationprogramme. The work is being funded bythe US Department of DefenseComparative Testing Office. Since Spring2004, researchers at the Propulsion andFluid Systems Division, West Bethesda,have begun a programme to evaluate

blades designed by the division and builtby the German company in a water tun-nel, in a large cavitation channel and inextensive towing tanks at Carderock.Engineers plan to use data from fibre-optic strain gauges embedded in theblade laminates to validate predictioncodes and estimate propeller service life.During the project large blades, includ-ing those for an 8 m diameter prop, willbe built and fatigue tested at theUniversity of Rostock and the UnitedStates Naval Academy.

Through this programme, the Navyhopes to gain an appreciation of design,manufacturing and performance issuesassociated with such blades. In particu-lar, engineers seek insight into locatingand orientating fibres to create the direc-tion-dependant stiffness that is responsi-ble for the blades’ adaptive behaviour. If, overall, the technology proves able to deliver propellers that run more quiet-ly and with less vibration, greater hydrodynamic efficiency and lower lifecycle cost than present metal types, theUS Navy could became an influentialadopter of advanced composite pro-pellers and blades.


Computerised rendition of Contur blades. (Image courtesy AIR. Fertigung-Technologie GmbH.)


In the UK, research organisationQinetiQ headed a team that hasdesigned, built and carried out sea trialson a 2.9 m diameter, five-blade compos-ite propeller. Dowty Propellers, a SmithsAerospace company, manufactured theblades to a QinetiQ design, whileWartsila Propulsion in the Netherlandsproduced the bronze hub and assembledthe propeller.

Designed and built to warship stan-dards, the hybrid glass/carbon compositepropeller was specifically intended forfitting to QinetiQ’s trimaran warshipdemonstrator, the RV Triton, in place ofthe latter’s normal fixed-pitch bronzeunit. It was fitted during a routine dock-ing period at Falmouth before under-going extensive sea trials in FalmouthBay during 2003. During the trials, a datalogging system built into the propellertail cone collected outputs from straingauges fitted to the blades.

The QinetiQ design trades some weightreduction for increased blade thickness.

“The use of lighter composite materi-als means that blades can be made thick-er without significantly adding to theweight of the propeller,” project manag-er Colin Podmore explains. “Thickerblades offer the potential for improvedcavitation performance, so reducingvibration and underwater signatures.”

Even so, weight savings of some 20%were achieved compared with normalbronze blades, a figure that could be

raised to 30-40% if the hub was also madeof composite.

According to QinetiQ, the design teamapplied lessons learned from the Comar-prop programme. In particular, the Qine-tiQ design varied markedly from that ofComarprop ensuring that certain prob-lems (believed to have included blade sep-aration issues) could not be reproduced.Classification society Det Norske Veritaswas involved in approving the propeller,although the unit was not fully classedsince its ability to meet the society’s stan-dard 25-year life requirement could notbe demonstrated in the time available.

Composite propellers have much potential forapplications where weight is critical.

The Falmouth trials provided valuableload data that can now be used in refin-ing hydrodynamic and structural designmodels. Knowledge was also gainedabout the acoustic performance of arotating composite structure and itsimpact on the galvanic environment atthe aft end of a vessel. Overall the testswere regarded as successful, the propellerdemonstrating a smooth take up ofpower and reduced vibration.

QinetiQ personnel who worked onthe project believe that composite propellers have much potential for appli-cations where weight is critical, such aswith the podded propulsors used by cer-tain cruise ships and other vessels.

Capability leader Cathy Kane addsthat future shipowners are also likely tobe attracted by the longevity of rein-forced plastic propellers and the associat-ed savings in through-life costs.

Kane says that more work is requiredto provide clients like the UK Ministry ofDefence with an idea of what savings canbe achieved when, for example, cost-effective manufacturing technologiesapplicable to production quantities rang-ing from one to dozens are incorporated.She tells Reinforced Plastics, however, thatMinistry of Defence interest is high andofficials will be keen to maintain theinvestigation impetus.

So far, polymer composites havegiven only a foretaste of what they coulddo for ship propellers, despite havingestablished themselves on small craft.Slowly, however, the evidence is growing– test results are accumulating, designcodes are emerging, and classificationsocieties are becoming involved. Once afew more designers and constructorshave become confident enough to re-think their choice of materials, fibre rein-forced plastics could begin to deliver a fresh start in the way that ships are propelled. ■


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A New Start for Marine Propellers