Composite materials have a higher specific resistance, stiffness and resistance to corrosion than metallic materials. For this reason, they are used in many industrial sectors such as aeronautics, automobiles and wind turbines. In the nautical sector composites are used primarily for the hulls of boats and yachts, but so far not for big ships.
New great potential for marine propellers
Research has only recently focused on the use of composites for boats’ propellers. The advantages of composite materials include higher elasticity, noise damping and specific resistance with respect to metallic materials. Until now, composite propellers have been used for specific types of vessel, such as warships and submarines.
The advantages gained from using materials that are lighter than traditional metals, such as noise reduction, lower stress transmission in the drive shaft, reduced energy consumption, more adaptable and flexible structural profiles, and higher specific stiffness and resistance to corrosion, all show that composites have great potential for marine propellers.
Tests run on the mechanical properties and vibration characteristics of samples of carbon fibre reinforced composites prove that they can indeed be used as new materials for propellers. Carbon fibre reinforced polymers (CFRPs) are stronger than nickel aluminium bronze (NAB), the material traditionally used for boat propellers. The damping ratio of CFRPs is at least four times higher than that of NAB. The only problem with CFRP propellers is that cavitation can cause surface erosion.
Moreover, the extensive use and progress made in production technologies could reduce the cost of this material in the near future. So we can safely say that composites do indeed have great potential for marine propellers.
First propeller design methods by FabHeli
FabHeli is an innovative French project for the production of composite propellers. The aim of the project, conducted by Loiretech, Méca and Naval Group with the support of the French defence ministry’s Direction Générale de l'Armement, is to design, produce and test composite propellers in real situations. In the naval field, the design, sizing and checking of propellers are governed by guidelines produced by the performance standards and classification offices. The first step in the FabHeli project (conducted in cooperation with Bureau Veritas) was to establish the propeller design methods best suited to the specific qualities of composite materials.
Metallic propellers are traditionally sized using the static load line (maximum constant speed). Pressure loads can derive from the different hydrodynamic calculation methods adopted (analytical, computational fluid dynamics, boundary element method).
The design methodology, and in particular the minimum safety factor, was adapted to account for the properties of composite materials. The mechanical calculation is based on the latest developments in wind turbine blades, which are generally made of composites. The validation method uses both standard tests and specific tests developed during the project.
Ongoing production of blades
The second stage of the FabHeli project is currently ongoing. It involves the fine-tuning of the resin transfer molding process selected to produce the propeller, which means that the blades can be produced using previously optimised composites.
The sea trials were conducted using a one-metre diameter composite propeller that enabled a passenger ship, the Palais — 30 metres long and weighing 84 tons, with two 1100 HP engines — to cover the Lorient-Brest route at a speed of 21 knots.
In spite of the long route and the many manoeuvres performed by the vessel, the day of trials ended on a highly satisfactory note with the propeller fully intact. Loiretech and Méca’s goal now is to progress to full-scale production of composite propellers.