Composite products often have different needs in the military and civilian markets. However, a French conglomerate, FABHELI, met the needs of both markets when producing a 2.9-foot CFRP propeller prototype. The goal is to develop and produce a propeller that reduces environmental footprint and energy consumption on an industrial scale.
The group successfully developed a propeller with the potential to reduce ship energy consumption and maintenance costs, improve hydrodynamic efficiency and reduce noise. In addition, the cost of propellers may also attract military and civilian shipbuilders. This technology won the 2018 JEC Asia Composites Innovation Award.
FabHeli comes from "Fabriqué"-the abbreviation for manufacture; and "Heli"-the meaning of French propeller. Loiretech is the designer and manufacturer of composite molds and leads this combined group. Its partners include NAVAL, the designer / builder of naval submarines and surface ships, and Méca, which manufactures innovative composite structures. Subcontractors include Bureau Veritas, a testing, inspection and certification service provider, and AML Shipyard, which provides test capacity for new propellers.
The project started in April 2016 and was digitized and calculated by Mecafrance for the mechanical aspect, and calculated by the Naval Group for the hydrodynamic aspect in order to measure the size of the carbon fiber composite propeller, which is twice the size of its metal propeller. The goal is to meet the demanding specifications-the propellers are subjected to large mechanical loads during use, so it is necessary to develop new design concepts, production technologies and surface treatment methods.
In this project, the United Group demonstrated several advantages of composite propellers over metal propellers. One is to save fuel, which is important for both military customers and the civilian market. For example, 80% of the cost of fishermen going to sea each time is fuel.
To develop an effective design, the consortium uses adaptive profile calculations to calculate, test, and simulate the effects of water flow (hydrodynamic behavior) and blade rotation in various propeller designs.
The blade has three main components. Franck Bourcier, Loiretech's vice president of marketing and innovation, said:
"The leading edge is very important because it has to separate the flow of water. Second is the surface of the blade because you have to keep the blade in contact with the water at all times. If you lose contact at some point, you lose accuracy, which is exactly what What we are after. You must also have a very thin trailing edge to avoid cavitation. The thinner the trailing edge, the better the effect and the higher the efficiency of the blade. Cavitation is the formation of bubbles in the liquid due to the movement of the propeller. "
Of particular interest to the military is that this composite propeller can reduce cavitation and thereby reduce sound emissions, which will make it more difficult for the enemy to use sonar to track ships. The patented coating on the blades also helps reduce noise. Another design goal of the project is to enable individual divers to replace propeller blades. This is currently a difficult two-man task.
Meca Senior Structural Engineer and Manager Samuel Durand said:
"The challenge is to develop an innovative connection between the blade and the root. This connection point can be removed and applied to the resin transfer molding (RTM) process. Joints and connections are often the weakest link in composite materials. For marine propellers, Say, the load on this joint is huge. "
The solution was to add an integrated, overmolded insert at the bottom of each blade, and then secure the blade to the metal hub with eight large screws. This composite thruster is very light and can be operated by one diver.
The team used RTM technology to make the propellers, starting with three dry fiber preforms, one for each side of the propeller blade and the other for connecting the bottom. By calculating the number and orientation of carbon fibers in a specific area, the lay-up operation was optimized to meet the hydrodynamic strength requirements of the blade.
The team developed a custom lightweight composite for the core of the blade and added carbon fiber fabric to the leading and trailing edges to increase impact resistance. These nets do not need to be trimmed, so there is less waste. The research team chose an epoxy resin to provide hardness because it is less susceptible to oil pollution in harbour waters.
Single-shot resin transfer molding (RTM) processes are used to mold thick (up to 30mm) epoxy composite blades, respectively. A three-piece tool with a moving iron core is used to accommodate weakening changes at the bottom of the blade. During the processing, there are three pressure bars at the mixing head, and the resin is cured at 60 ° C. The blades are each attached to a molded metal hub.
"The owner of the shipyard is used to using metal blades, and they are not sure whether the composite blades can meet the mechanical requirements." Bourcier said. The crew tested the propellers in difficult conditions, making sharp turns without reversing at full speed. "It was a huge surprise for them; we were able to prove that the hardness of the concept film was sufficient. They now believe and want to participate in the next project."
The project team is also working to make the product cost effective. According to its calculations, if the number of propellers purchased is less than 30, metal propellers are cheaper than composite materials; but if the number exceeds 50, composite materials have advantages.
The group hopes to build another prototype so that it can replace the two metal propellers on Le Palais with composite propellers and better measure the reduction in cavitation. It also hopes to build a 13-foot-diameter propeller and perhaps design a composite crankshaft to further increase efficiency. Bourcier added that the future development of the propellers will depend on the funding available to the consortium.