In the well-known apocryphal story, a local person, on being asked for directions to another part of the locality, replies: “Well, if I were you, I wouldn't start from here!”

A marine engineer, given a clean sheet and asked how he would go about designing a propeller, might feel the same way. What ships have now are propellers made from metals - materials that corrode in sea water and set up galvanic action, are easily dented, bent in groundings or strong impacts and are difficult to form into complex shapes. How much easier it should be to mould durable propellers in plastic, repeatably and to the exact shape required for peak hydro-dynamic efficiency. Such units could be made faster and less expensively than metal counterparts, without the extensive reworking that metal props often require. Plastic blades would never corrode, electrolyse, ‘de-zincify’ or seize to the drive shaft. If only, our engineer might wish, plastics could be found that were strong enough to serve for these highly loaded devices.

Modern reinforced plastics are the fulfilment of that wish, giving designers the chance for a fresh start in propeller design and manufacture. A number of companies have shown the viability of composite propellers, albeit, so far, mainly those for small craft. The challenge now is to scale these up to larger units suitable for ships.

A fine example of innovation is that set by Swedish company ProPulse AB with a novel concept it patented half a decade ago. The ProPulse® modular propeller comprises a metal hub with replaceable composite blades whose pitch can jointly be adjusted (at blade installation) to one of five settings. Suitable for motors of 20-300 hp, the props are made from an undisclosed ‘high quality composite’ developed for the application by the Sicomp of Pitea, a company jointly owned by the Swedish government and the Lulea University of Technology. The manufacturer is Formel-produkter of Boden.

Tests have shown the blades to be stronger than equivalents in aluminium, the ‘stock’ material used for outboard motor and sterndrive props. Despite weighing up to 40% less than their aluminium counterparts, the blades are tough and resilient, resisting impact, light grounding and damage from cavitation. (The latter is a phenomenon in which low pressure cavities formed in aerated water behind the blades as the propeller rotates collapse, sometimes with considerable force.) Moulding the blades to the precise shape called for by the 3D CAD design has resulted in high propulsive efficiency. In case of damage, blades can be removed on the spot and replaced individually. A trolling fisherman could, for example, quickly remove two opposite blades of a damaged four-blade prop, re-adjust the pitch of the remaining two and continue operating with those.

US company Piranha says its propellers have blades which, because they are made from LNG Engineering Plastics' Verton long glass fibre reinforced polyamide thermoplastic, are 17% stronger than traditional die-cast aluminium, and have far superior chemical and corrosion resistance. They also resist abrasion better than metals, and suffer less from leading-edge erosion in water made abrasive by suspended grit. They have low hydrodynamic friction and high propulsive efficiency. The Verton used is said to retain its strength even after prolonged immersion in water. The blades are injection moulded, a method that yields perfectly matched units on a rapid fabrication cycle.

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

Echoing ProPulse, US company Composite Marine Propellers considers the materials used in its Comprop series a 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 equipment on Regal, Wellcraft, Glastron and Corona boats, as well as being recommended for spares or replacements on a variety of craft powered by engines up to 225 hp. The 22 inch model weighs just 2.5 lb (1.1 kg), as against 4 lb (1.8 kg) for an equivalent aluminium prop. As well as being more affordable than aluminium units, these props have blades that are designed to flex slightly or break off should they hit an obstruction, so that drive shafts and bearings remain undamaged.

Outboard Marine Corp (OMC), one of the world's leading outboard producers and owner of the Evinrude and Johnson brands as well as OMC stern drives, now offers composite propellers alongside the aluminium and top-of-the-range stainless steel props that have been standard for years. Company experts reckon that stainless props are still the best for stiffness and ultimate performance, but at almost double the price of aluminium equivalents, let alone composite, they are in a premium market. Mercury Outboards offers a line of composite propellers as spares.

Scaling up

Scaling up these small units to larger propeller sizes suitable for ships is something of a challenge. Plastics, even when reinforced, tend to be less stiff than metals and early blades made from them were known to lose propulsive efficiency by flexing. Nevertheless, efforts to overcome the difficulties have continued because of the potential advantages - not least reduced weight and increased durability.

One of the prime movers in the field is the German company AIR Fertigung-Technologie GmbH, founded 12 years ago by graduates of the University of Rostock, with whom the company still maintains close ties. This partnership has met stiffness requirements by adopting carbon fibre composites for their Contur® range of composite propellers intended for superyachts and ships. Over 400 ship sets of Contur advanced composite propellers have been sold, ranging in diameter from 50 cm to 5 m. AIR says its propellers weigh only a third as much as conventional nickel-aluminium bronze (NAB) equivalents. Composite blades can, it says, be thinner at the tips than metal, reducing propeller noise typically by 5 dB.

More recently AIR, with Rostock University's academic backing, has come up with an exciting innovation that positively exploits the flexible qualities of composites. In the latest, ‘smart’ Contur propellers, carbon, aramid and drawn polyethylene fibres are disposed within the composite in such a way as to provide hydroelasticity. This enables the blades to react to changing load conditions by altering their pitch, so maintaining optimum propulsive efficiency across a range of throttle settings. As a result, fuel consumption can be reduced by up to 15%. In addition, adaptive self-pitching propellers reach rated rpm faster than conventional props, so that the vessel has better acceleration. Cavitation is reduced because, since the blade adapts itself to different loads, the load over a given blade area tends to stay within the limits at which implosion of cavities against the blade is induced. Blades are manufactured in closed moulds by an resin transfer moulding (RTM)-like process, to close tolerances so that their hydroelastic and other properties are matched.

Contur propellers reduce the cost of prop maintenance by having separate exchangeable blades, each blade possessing a thickened root that slots into the hub. One owner of a 25 kt, 150 ft (45 m) GRP-constructed motor yacht reported replacing blades underwater as being ‘straightforward’ and quick. Having been prompted initially to switch from metal props to composite on experiencing excessive vibration, he had since run ‘countless sea miles’ smoothly. In a speed trial the vessel proved only 0.2 kts slower than when she was new with her metal propellers.

UK importer and distributor Fleetwater Marine reports growing interest in Contur composite propellers from professional charter/leisure craft and workboat operators. Recently the company supplied Associated British Ports (ABP), the UK's leading ports business, with its first set of Contur ‘smart’ self-pitching propellers. Before ordering these, ABP and VT Halmatic (builder of a Nelson 48/50 pilot boat operated by the former), trialled the composite props at sea, against the normal bronze propellers. Results with the Nelson craft showed fuel reductions of almost 10% at full throttle and an impressive 17.5% at mid range. Acceleration was enhanced and noise in the wheelhouse was cut by up to 4%. When several sizeable underwater objects were struck unexpectedly, the composite blades continued to operate smoothly without damage whereas, in the opinion of the on-board crew, metal blades would probably have bent and started to vibrate.

According to Fleetwater Marine's Nicholas Bentley-Buckie, the Contur blade tips are especially resistant to below-water impacts because they are fortified with drawn polyethylene fibres. His graphic explanation of the prop″s self-pitching action is that ‘the blades flatten out with load’ so that optimum pitch is maintained the whole time. This contrasts, he told Reinforced Plastics, with the fixed pitch of conventional bronze blades which is normally set to absorb the full engine power at maximum throttle setting. This means that at the normal cruise settings at which vessels spend most of their time, they are operating away from peak efficiency. The largest prop Fleetwater has supplied to date is a 3 m diameter unit fitted to a mine-sweeper. The company is discussing with a well known local shipbuilder the possibility of fitting Contur props to fast patrol boats.

Research

Composite propellers could bring major benefits to certain shipping sectors, in particular fast ferries and military. Among naval craft, for instance, the non-magnetic nature of plastic props and their quiet running can enhance the stealth of mine countermeasures vessels, while their resilience would help them resist damage from underwater explosions and debris. Research in various countries is directed at achieving the promised benefits by making sizeable composite propellers a reality. Efforts in Germany (the University of Rostock's role has been mentioned) have been paralleled in the USA, UK, Scandinavia, Italy, Greece and elsewhere.

In the late 1990s, the European Community-sponsored Composite Marine Propeller (Comarprop) project brought researchers from four countries together in an effort to define design and manufacturing technologies, confirm the propellers' commercial benefits and establish a basis for their acceptance by classification societies. Investigations ranged from composite materials evaluation to producing full-scale props by resin transfer moulding and trialling them at sea. Work on design methodologies involved finite element modelling and trying to adapt the hydrodynamic design process to allow for the non-isotropic properties of composites. Another focus was how best to incorporate hydroelasticity so as to secure adaptive blade pitching. Project collaborators included the Norwegian Marine Technology Research Institute (Marintek); Dowty Aerospace Propellers and DERA (now QinetiQ), Haslar in the UK; the National Technical University of Athens; and from Italy the Registro Italiano Navale, shipbuilder Fincantieri, CETENA SpA and the Consorzio Armatori per la Ricerca Srl.

The US Navy, intrigued by the adaptive blade pitching and other possibilities offered by the AIR Fertigung-Technologie composite propellers, recently initiated a three-year evaluation programme. The work is being funded by the US Department of Defense Comparative Testing Office. Since Spring 2004, researchers at the Propulsion and Fluid Systems Division, West Bethesda, have begun a programme to evaluate blades designed by the division and built by the German company in a water tunnel, in a large cavitation channel and in extensive towing tanks at Carderock. Engineers plan to use data from fibre-optic strain gauges embedded in the blade laminates to validate prediction codes and estimate propeller service life. During the project large blades, including those for an 8 m diameter prop, will be built and fatigue tested at the University of Rostock and the United States Naval Academy.

Through this programme, the Navy hopes to gain an appreciation of design, manufacturing and performance issues associated with such blades. In particular, engineers seek insight into locating and orientating fibres to create the direction-dependant stiffness that is responsible for the blades' adaptive behaviour. If, overall, the technology proves able to deliver propellers that run more quietly and with less vibration, greater hydrodynamic efficiency and lower life cycle cost than present metal types, the US Navy could became an influential adopter of advanced composite propellers and blades.

In the UK, research organisation QinetiQ headed a team that has designed, built and carried out sea trials on a 2.9 m diameter, five-blade composite propeller. Dowty Propellers, a Smiths Aerospace company, manufactured the blades to a QinetiQ design, while Wartsila Propulsion in the Netherlands produced the bronze hub and assembled the propeller.

Designed and built to warship standards, the hybrid glass/carbon composite propeller was specifically intended for fitting to QinetiQ's trimaran warship demonstrator, the RV Triton, in place of the latter's normal fixed-pitch bronze unit. It was fitted during a routine docking period at Falmouth before under-going extensive sea trials in Falmouth Bay during 2003. During the trials, a data logging system built into the propeller tail cone collected outputs from strain gauges fitted to the blades.

The QinetiQ design trades some weight reduction for increased blade thickness.

“The use of lighter composite materials means that blades can be made thicker without significantly adding to the weight of the propeller,” project manager Colin Podmore explains. “Thicker blades offer the potential for improved cavitation performance, so reducing vibration and underwater signatures.”

Even so, weight savings of some 20% were achieved compared with normal bronze blades, a figure that could be raised to 30-40% if the hub was also made of composite.

According to QinetiQ, the design team applied lessons learned from the Comarprop programme. In particular, the Qine-tiQ design varied markedly from that of Comarprop ensuring that certain problems (believed to have included blade separation issues) could not be reproduced. Classification society Det Norske Veritas was involved in approving the propeller, although the unit was not fully classed since its ability to meet the society's standard 25-year life requirement could not be demonstrated in the time available.

Composite propellers have much potential for applications where weight is critical.

The Falmouth trials provided valuable load data that can now be used in refining hydrodynamic and structural design models. Knowledge was also gained about the acoustic performance of a rotating composite structure and its impact on the galvanic environment at the aft end of a vessel. Overall the tests were regarded as successful, the propeller demonstrating a smooth take up of power and reduced vibration.

QinetiQ personnel who worked on the project believe that composite propellers have much potential for applications where weight is critical, such as with the podded propulsors used by certain cruise ships and other vessels.

Capability leader Cathy Kane adds that future shipowners are also likely to be attracted by the longevity of reinforced plastic propellers and the associated savings in through-life costs.

Kane says that more work is required to provide clients like the UK Ministry of Defence with an idea of what savings can be achieved when, for example, cost-effective manufacturing technologies applicable to production quantities ranging from one to dozens are incorporated. She tells Reinforced Plastics, however, that Ministry of Defence interest is high and officials will be keen to maintain the investigation impetus.

So far, polymer composites have given only a foretaste of what they could do for ship propellers, despite having established themselves on small craft. Slowly, however, the evidence is growing - test results are accumulating, design codes are emerging, and classification societies are becoming involved. Once a few more designers and constructors have become confident enough to re-think their choice of materials, fibre reinforced plastics could begin to deliver a fresh start in the way that ships are propelled.