Another project goal was to come up with a design that would allow a single diver to change propeller blades. That’s currently a difficult, two-person task.
“The challenge was to develop an innovative junction between blades and root, which can be dismounted and adapted to the resin transfer molding (RTM) process,” says Samuel Durand, senior structural engineer and manager at Méca. “Junctions and connections are often the weakest point in composite materials, and for a marine propeller the loads at this junction are huge.”
The solution was to add an integrated, over-molded insert to the foot of each blade and then attach the blades with eight large screws to the metal hub. The composite propeller is light enough for one diver to maneuver.
The consortium manufactured the propeller using RTM, starting with three dry fiber preforms, one for each side of the propeller blade and one at the foot for the connection. Lay-up operations were optimized by calculating the number and orientation of carbon fibers in specific areas to fulfill the hydrodynamic strength requirements for the blade.
The team developed a custom lightweight composite for the core of the blade and added carbon textile at the leading and trailing edge for added impact resistance. These net shape edges don’t need trimming, so there is less waste. The team chose an epoxy resin to provide stiffness and because it is not susceptible to the oils found in harbor waters.
The Bureau Veritas supervised the composite propeller design and manufacture in preparation for future certification that could allow the propeller to be sold on the commercial market.
Last March, at the AML shipyard in Lorient, France, the propeller was installed on Le Palais, a 95-foot, 84-ton ferry ship that can carry 286 passengers. The propeller performed well on its inaugural 10-mile trip to Groix and then on a 200-mile journey to Brest and back.
“The shipyard owner is used to metallic blades, and they weren’t sure that the composite blades could succeed at the mechanical requirements,” Bourcier says. The boat’s crew tested the propeller under difficult conditions, going in reverse at full speed and making sharp turns without any damage.
“That was a big surprise to them; we were able to prove that the concept blade was stiff enough to meet the requirements,” he adds. “They are now believers and want to be involved in the next project.”