SHIPS鈥� propellers could be about to undergo their biggest transformation since Brunel鈥檚 SS Great Britain, the first large iron-hulled ship with a screw propeller, was launched more than 150 years ago. Sea trials have shown that replacing a conventional metal propeller with one made of carbon composite can help reduce corrosion and maintenance, and make warships stealthier.
Propellers are normally made from an alloy of nickel, aluminium and bronze (copper and tin), known to marine architects as NAB, while the hull is usually steel. Putting the two in seawater causes a current to flow 鈥� just as in the school experiment in which children create a battery by sticking different metals into a potato. This current eats away the iron in the hull near the propeller.
To counter this, naval architects fix platinum-coated titanium plates to the ship鈥檚 bottom and pass an opposing current through them. But ships that use this system, known as impressed current cathodic protection (ICCP), have to go into dry dock at regular intervals for the titanium plates to be replaced.
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There are other problems, too. 鈥淚f you get it wrong, the back end of a ship can corrode away relatively quickly,鈥� says Colin Podmore of the Centre for Marine Technology in Gosport, Hampshire, which is run by the UK鈥檚 defence research company Qinetiq. Worse still, as far as navies are concerned, ICCP produces a characteristic electromagnetic signal that can be picked up by enemies.
So Qinetiq commissioned Dowty Propellers of Cheltenham in the UK and Wartsila Propulsion of Drunen in the Netherlands to design and make a 2.9-metre propeller from layers of carbon composite coated with fibreglass and mounted on a NAB hub. Some small boats have composite propellers, but they have never been built on this scale before.
In February, the new propeller (pictured left) was mounted on the lab鈥檚 98-metre trimaran test vessel, the RV Triton. Sea trials showed immediate benefits, such as reduced vibration in the ship, says Podmore. 鈥淭he use of the lighter composite material meant that the blades could be thicker without significantly adding to the weight of the propeller.鈥�
In addition, the hull protection current could be cut by up to 60 per cent, significantly reducing the telltale signal it emits. That means the titanium plates will last longer between replacements, says Robin Oakley of Qinetiq, which could lead to significant savings. 鈥淐ruise ships don鈥檛 earn money if they are in dry dock,鈥� he points out. It may be possible to reduce the current even further by making the whole prop from composite materials, though the ICCP system would still be needed to prevent corrosion caused by small variations in different parts of the hull.
Composite propellers would also be cheaper to make, as they are ready to use when they come out of the mould, rather than requiring days of finishing like metal propellers. This would make it more feasible to design propellers for specific ships, as Qinetiq did for the Triton, which reduced the noise of the propeller.
But the sea trials were not an unqualified success. One of the five supposedly identical composite propeller blades 鈥渟ang鈥� as it drove the ship, resonating at a particular frequency. Qinetiq is still trying to find out why. And tougher tests remain: even metal propellers can be damaged by big pieces of driftwood, and no one knows how a composite propeller will stand up to such impacts. 鈥淲e鈥檝e yet to see how it will handle a railway sleeper striking it at sea,鈥� says Podmore.