A new start for marine propellers

18th March 2005 By: Karel Smrcka

Reinforced plastics offer designers of ships’ pro- pellers a new approach to design and manu- facture. The viability of composite pro- pellers has been demonstrated on small craft; the challenge now is to scale up these designs to produce larger units suitable for ships.

What ships have now are propellers made from metals –materials that corrode in seawater and set up galvanic action, are easily dented, bend 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 hydrodynamic efficiency. Such units could be made faster and less expensively than metal ones, without the extensive reworking that metal props often require. Plastic blades would not corrode, electrolyse, ‘dezincify’ or seize to the driveshaft. If only, many engineers 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. Several companies have shown the viability of composite propellers, although, so far, mainly those for small craft. The challenge now is to scale these up to larger units suitable for ships.

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

Tests have shown the blades to be stronger than equivalent 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, readjust the pitch of the remaining two and continue operating with those.

US approach
US company Pirhana says its propellers have blades which, because they are made from LNG Engineering Plastics’ Verton long glassfibre-reinforced poly- amide thermoplastic, are 17% stronger than traditional diecast 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 in 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 result 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 it simply as ‘fibre-filled resins’. The single-piece four-blade props, up to about 550 mm in diameter, are offered as original equipment on Regal, Wellcraft, Glastron and Corona boats and are recommended for spares or replacements on a variety of craft powered by engines up to 225 hp. The 550 mm model weighs just 1,1 kg, compared with 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.

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 German company AIR Fertigung Technologie. It has adopted carbon-fibre composites for its 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 500 mm to 5 m. Contur propellers weigh only a third as much as conventional nickel-aluminium bronze equivalents. Composite blades can be thinner at the tips than metal, reducing propeller noise typically by 5 dB Contur propellers reduce the cost of prop maintenance by having separate exchangeable blades, each possessing a thickened root that slots into the hub.