As the core power output component of an electric outboard motor, the structural design of the propeller directly determines its propulsion efficiency, operational stability and adaptability to application scenarios. Compared with propellers for traditional fuel-powered outboard motors, electric models feature low speed, high torque, stable power output and no fuel corrosion, which impose higher requirements on the propeller in terms of lightweight design, fatigue resistance and hydrodynamic adaptability. This paper systematically disassembles the structural system of electric outboard motor propellers from the aspects of basic structural composition, core component functions, structural optimization directions, and the collaborative design of materials and structures.

I. Basic Structural Composition: Collaborative Architecture of Three Core Components

The basic structure of an electric outboard motor propeller consists of three core components: blades, hub and shaft. Some high-performance models are equipped with auxiliary connectors such as blade brackets to form a complete power transmission and propulsion system. Each component not only undertakes its own function independently, but also realizes efficient force transmission through structural design, ensuring the accurate conversion of motor mechanical energy into ship thrust.

1. Blades: Core Carrier for Thrust Generation

Blades are the key components that directly interact with water to generate thrust, and their structural parameters (shape, quantity, pitch, cross-sectional form) directly affect propulsion efficiency and hydrodynamic characteristics. Traditional blades mostly adopt a solid structure, while new-type propellers for electric outboard motors have adopted curved tubular blade designs. The open-ended tubular structure allows water flow to enter from one end and exit from the other, and effectively reduces the slip phenomenon—the difference between the actual forward distance of the propeller per revolution and the theoretical pitch—by maintaining the balance of water inflow and outflow, thus significantly improving propulsion efficiency.

The number of blades needs to be adapted to the motor power and application scenarios: 2-3 blades are commonly used for recreational models with 1-5kW power, balancing lightweight design and propulsive force; 4-5 blades can be adopted for high-power working models above 20kW to enhance the stability of power output. Blades are connected to the hub according to the designed pitch, and some models achieve movable connection through blade brackets, which can fine-tune the angle according to water flow conditions and optimize hydrodynamic response performance.

2. Hub: Hub for Component Connection and Force Transmission

Located at the center of the propeller, the hub mainly functions to connect the blades and the propeller shaft, evenly distribute the torque transmitted by the shaft to each blade, and bear the impact force of water flow and the self-weight of components at the same time. Its structural design must balance connection strength and hydrodynamic performance to avoid additional water resistance caused by abrupt shapes.

In terms of structural form, the hub can be integrally molded with the blades (suitable for easy-to-process materials such as plastics and composite materials) or movably connected with the blades through blade brackets. As an auxiliary connector, the blade bracket is composed of rib plates and a concave bonding plate. The end of the rib plate is integrally molded with the hub, and the concave bonding plate is attached to the outer wall of the blade. This structure not only strengthens the connection stability between the blade and the hub, but also reduces fatigue damage at the blade root by dispersing the stress.

3. Propeller Shaft: Core Shaft System for Power Transmission

The propeller shaft is responsible for transmitting the rotational power of the electric outboard motor to the hub and blades, and its structural design directly affects power transmission efficiency and operational stability. Traditional solid propeller shafts tend to generate hub vortex during high-speed rotation—a vortex formed by the pressure difference of water flow before and after the propeller shaft—which increases operational resistance and reduces efficiency. The new hollow propeller shaft design features an open front and rear structure, allowing water flow to enter from the front end and exit from the rear end, balancing the water pressure on both sides of the shaft, effectively suppressing the formation of hub vortex, while reducing the shaft weight and lowering the motor load.

In rim-type electric outboard motors, the propeller shaft structure is further optimized: the propeller is directly fixed to the motor rotor through the blade tips, eliminating the traditional shaft system and gear transmission links, which greatly reduces power transmission loss. Meanwhile, combined with water-lubricated or oil-lubricated bearings, it achieves low-noise operation, making it particularly suitable for noise-sensitive scenarios such as fishing boats.

II. Structural Optimization Directions: Dual Goals of Efficiency Improvement and Scenario Adaptation

With the iteration of electric outboard motor technology, the structural optimization of propellers focuses on the three core goals of maximizing efficiency, minimizing resistance and enhancing durability, forming a diversified direction of structural innovation to adapt to the needs of models with different power levels and application environments.

1. Integrated Structural Design

High-end models adopt a motor-propeller integrated structure, where the propeller blades are directly integrated inside the motor rotor and driven by a rim-type permanent magnet motor, eliminating intermediate transmission components and achieving the maximum improvement of power transmission efficiency. This integrated structure not only simplifies the overall layout and forms a compact unit of the propeller and the motor, but also realizes 360° full rotation driven by the rudder shaft, achieving the function of integrated rudder and propeller and improving ship maneuverability. At the same time, the integrated structure is equipped with a hydraulic tilting mechanism, which can lift the propeller out of the water when the ship is moored, reducing water corrosion and marine organism adhesion and extending the service life.

2. Hydrodynamically Optimized Structure

In addition to hollow propeller shafts and tubular blades, the overall shape of the propeller adopts a streamlined design, and the transition between the hub and blades is smoothly treated to reduce flow separation and local vortex loss. Some high-performance propellers optimize the blade cross-sectional form through numerical simulation and adopt a variable pitch design—adjusting the pitch along the blade radius to equalize the water flow speed at all parts of the blade, further improving propulsion efficiency and reducing vibration and noise.

3. Strengthened Sealing and Protection Structure

Electric outboard motor propellers need to work in the water environment for a long time, so their structural design must balance sealing and protection performance. Special sealing components are installed at the connection between the propeller shaft and the hub to prevent water from infiltrating into the motor and transmission system; rim-type thrusters wrap the rotor assembly with waterproof sleeves and cooperate with bearing sealing structures to achieve all-working-condition waterproof protection. For propellers used in seawater scenarios, structural optimization is also required to reduce residual gaps and avoid local accumulation of chloride ions that cause corrosion.

III. Collaborative Design of Materials and Structures: Precise Matching of Performance and Scenarios

The structural design of propellers is highly correlated with material selection. The mechanical properties and processing characteristics of different materials determine the feasibility of structural forms, while structural design can maximize the advantages of materials to achieve the collaborative optimization of material-structure-performance. Combined with the power characteristics and application scenarios of electric outboard motors, the adaptation relationship between mainstream materials and structures is as follows:

1. Engineering Plastics: Optimal Choice for Lightweight Basic Structures

Engineering plastics such as Nylon 66 + glass fiber and PP + glass fiber are suitable for recreational models with 1-5kW power, and can adopt an integrally molded blade-hub structure, featuring low processing cost and lightweight design (40% lighter than aluminum alloy of the same size). Tubular blade and hollow propeller shaft structures are easy to realize in plastic materials through injection molding technology. Meanwhile, the impact resistance of plastics can alleviate blade damage caused by small obstacles (such as reefs and fishing nets), but structural reinforcement is required to avoid high-temperature creep and water absorption deformation.

2. Aluminum Alloy: A Balanced Structural Solution for Medium-Power Models

6061 and 7075 aluminum alloys are suitable for medium-power models with 5-20kW power, and can be formed by precision milling. The blades and hub are mostly of an integrated structure, and the blade angle deviation can be controlled within 0.2°, ensuring structural accuracy and propulsion stability. Anodizing treatment is adopted to enhance surface corrosion resistance, and the streamlined hub design balances the requirements of strength and low water resistance. 7075 high-strength aluminum alloy is suitable for models above 20kW, and the connection between the blade root and the hub needs to be thickened to cope with high torque loads.

3. Stainless Steel: Durable Structural Design for Seawater Operation Models

316L stainless steel is suitable for working models above 20kW operating in seawater for a long time due to its extremely strong corrosion resistance, and can adopt a composite structure of stainless steel blades + plastic hub, retaining the corrosion resistance of the blades while reducing the overall weight. The blades are processed by wire cutting and polishing technology, with the surface roughness controlled at Ra ≤ 0.4μm. Combined with the hollow propeller shaft structure, it reduces water resistance and corrosion risks, ensuring long-term stable operation in the marine environment.

4. Composite Materials: Performance Breakthrough Structure for High-End Models

Composite materials such as CFRP (Carbon Fiber Reinforced Plastic/Resin) are suitable for high-end and high-performance models, and can be designed with complex variable cross-section blades and integrated streamlined structures, balancing lightweight design, high strength and low friction coefficient. The high specific strength of composite materials allows the adoption of thinner blade structures, and combined with tubular design and hollow propeller shaft, it achieves a dual breakthrough in propulsion efficiency and maneuverability, making it suitable for high-performance scenarios such as high-speed speedboats and official vessels.

IV. Conclusion

The structural design of electric outboard motor propellers has always centered on the three core goals of high-efficiency propulsion, stable operation and scenario adaptation, evolving from the basic three-element structure of "blades-hub-shaft" to integration, streamlining and compounding step by step. Innovative designs such as tubular blades, hollow propeller shafts and rim-type integrated structures, combined with the collaborative optimization of materials and structures, not only solve the problems of slip and hub vortex of traditional propellers, but also adapt to the power characteristics of electric models, providing precise structural solutions for electric outboard motors of different power levels and application scenarios. In the future, with the development of numerical simulation technology and new materials, propeller structures will be further upgraded towards customization, high efficiency and low noise, becoming a core driver for the performance breakthrough of electric outboard motors.