CFRP propellers are made from Carbon Fiber Reinforced Polymer (CFRP) materials. In CFRP propeller blades, the fibers bear the main hydrodynamic and centrifugal forces. The load-bearing fibers can be laid in different directions along the blade to minimize strain, allowing for optimization of blade performance through the design of fiber arrangement and stacking sequence. As the blade thrust, effective pitch, and twist design are optimized, the performance of CFRP propellers is also enhanced.
Structure of CFRP Propellers
CFRP propellers consist of blades, a boss, and a pressure plate. The blades are CFRP, while the boss is made of a metal material called NAB (Nickel Aluminum Bronze). The boss has an inverted conical groove into which the blades are inserted from the rear, preventing blade detachment.
The pressure plate is fixed at the rear end of the boss with bolts. The inverted conical groove narrows towards the front and radially, preventing loosening once inserted into the specified position. Due to its modular design, individual blades can be replaced if damaged.
Design of CFRP Propellers
While the design of traditional metal propellers can ignore blade deformation during operation, the lower elastic modulus of CFRP necessitates considering elastic deformation in the design.
As with conventional propeller design, the pressure distribution on the wing surface is calculated, and this pressure distribution along with centrifugal force is used as load conditions for structural analysis. This determines the pre-deformation shape of the propeller that will achieve the optimal shape during operation.
When a propeller blade rotates through the area near the ship’s bottom (top position), the wake velocity decreases and the inflow angle increases, resulting in higher loads and potential cavitation. CFRP propellers can suppress cavitation in this high-load top position due to a reduction in pitch angle under load.
Considering the elastic deformation of CFRP propellers, the elastic modulus of model propellers is determined based on the similarity principle of fluid forces related to deflection and bending stiffness, and a polymer with this elastic modulus is selected.
Advantages of CFRP Propellers
Durability
Ships, especially large vessels that navigate in open seas, require propellers to operate continuously for long periods. Large oil tankers typically sail for 1 month at a time, which means that a large propeller with a diameter of 10 meters rotates non-stop for 1 month (720 hours). Considering this premise, propellers naturally need high durability. Because CFRP is a high-strength, lightweight material, it can effectively address this issue.
Lightweight
Generally, in vehicles, weight reduction can improve fuel efficiency. However, propellers are very small components compared to the overall weight of a ship, so even if their weight is reduced, the contribution to overall fuel efficiency is not significant. On the other hand, the lightweight nature of high-speed rotating propellers in water has a good effect on reducing vibration, thereby lowering noise.
| Specifications Of The Propellers And The Shafts | ||
| Original(NAB) | New design(CFRP) | |
| Propeller diameter (mm) | 1,950 | 2,120 |
| Boss diameter (mm) | 363 | 420 |
| Blade area ratio | 0.63 | 0.50 |
| Propeller weight (kg) | 715 | 422(59%) |
| Moment of inertia (kg-m2) | 112 | 36.8(33%) |
Furthermore, during engine rotation, the lightweight propeller allows for smoother engine rotation and reduced engine load, which is particularly important when navigating in stormy weather. CFRP propellers are much lighter than traditional NAB propellers, allowing for increased blade thickness without adding weight, thus improving the propeller’s cavitation performance and reducing vibration and underwater noise.
Precise Shape
Due to the long-term continuous operation of propellers, even slight differences in dimensional accuracy can significantly impact engine performance and durability. In propeller manufacturing, even for large propellers with a diameter of 10 meters, dimensional tolerances must be controlled within the millimeter range, and surface smoothness must also be controlled at the millimeter level.
When manufacturing CFRP propellers, PAM-RTM technology can control polymer impregnation parallel to the blade surface and perpendicular to the surface thickness, producing precise propeller shapes.
Manufacturing Process of CFRP Propellers
CFRP propellers are manufactured using the Vacuum-assisted Resin Transfer Molding (VaRTM) process.
The VaRTM method refers to a process where reinforcing fiber substrates (such as glass fiber or carbon fiber fabrics) are first laid on a mold, then sealed with plastic film and vacuumed. Liquid polymer is then injected, and the negative pressure principle causes it to impregnate all areas of the fiber substrate and cure fully. To facilitate polymer impregnation (diffusion) and achieve a better blade surface, specialized mesh fabric and release fabric are used between the reinforcing fiber substrate and the mold, as well as between the substrate and the plastic film.
While traditional propeller materials like NAB (nickel-aluminum-bronze alloys) have drawbacks such as complex processing, high blade costs, fatigue cracking, and poor acoustic damping, CFRP is becoming an important material for propeller manufacturing. As production costs decrease, the application of CFRP in the propeller field is expected to become more widespread.
Image Source: Fiberdyne1 & Nakashima & Mdpi
