In the wave of the transformation of marine power systems towards greenization and intelligence, the Rim-driven Thruster (RDT), as a disruptive shaftless propulsion technology, has gradually become a research hotspot and application focus in the field of marine propulsion due to its highly integrated structural design and excellent power performance. Compared with traditional propulsion systems, the Rim-driven Thruster eliminates key transmission components such as the drive shafting and gearbox, realizing the integrated integration of the motor and the propeller, which fundamentally changes the power transmission path. Its power performance is not only reflected in the improvement of propulsion efficiency, but also covers multiple dimensions such as noise control and maneuverability. Starting from the structural characteristics, this paper will deeply analyze the core elements of the power performance of Rim-driven Thrusters, discuss the key factors affecting their performance, and look forward to their optimization directions and application prospects.

I. Structural Characteristics and Power Transmission Mechanism of Rim-driven Thrusters

The core structural advantages of Rim-driven Thrusters lie in "shaftlessness" and "integration". Their basic structure mainly includes components such as rim-mounted motors, propellers, water-lubricated bearings, and fairings. Different from traditional propulsion systems that transmit main engine power through drive shafts, Rim-driven Thrusters embed the rotor of the permanent magnet synchronous motor into the rim at the top of the propeller blades, and the stator is fixed inside the fairing. The rotating magnetic field generated by the stator windings directly drives the rotor to drive the propeller to rotate, realizing direct power transmission. This structural design fundamentally eliminates the energy loss caused by mechanical transmission links such as shafting and gearboxes, and at the same time greatly simplifies the system structure and improves space utilization. According to DARPA (Defense Advanced Research Projects Agency) report data, the use of rim-driven propulsion systems in surface ships can reduce the space occupied by the power system by 60%~70%, providing greater freedom for hull flow field optimization and cabin layout.

In terms of power transmission efficiency, the shaftless design of the Rim-driven Thruster shortens the energy transmission path to the shortest, avoiding energy loss caused by gear meshing and shafting friction in traditional propulsion systems. Data from the China Classification Society (CCS) shows that compared with traditional propeller propulsion systems, the propulsion efficiency of Rim-driven Thrusters can be improved by 15%~20%. In addition, some new-type Rim-driven Thrusters adopt an axial flux motor structure. Through the double-stator axial arrangement of stator-rotor-stator, the radial size and rotor axial length are effectively reduced, the annular flow loss is reduced, and the power transmission efficiency is further improved. The application of water-lubricated bearings replaces the traditional oil-lubricated sealing structure, which not only avoids leakage risks, but also reduces frictional resistance, providing a guarantee for the stable performance of power performance.

II. Core Advantages of Power Performance of Rim-driven Thrusters

(I) High Efficiency and Energy Saving, Significantly Improved Propulsion Efficiency

Propulsion efficiency is the core index to measure the power performance of thrusters. Rim-driven Thrusters achieve high efficiency and energy saving through multiple structural optimizations. On the one hand, the shaftless design eliminates energy loss in the transmission link, maximizing the conversion of mechanical energy output by the motor into propulsive force; on the other hand, the optimized streamlined design of the fairing and the integrated matching with the propeller effectively improve the flow field distribution and reduce energy loss caused by water flow disturbance. Scholars such as Luo Xiaoyuan found through CFD (Computational Fluid Dynamics) numerical simulation that a reasonable fairing structure can significantly improve the thrust and efficiency of the Rim-driven Thruster, and the combined system with a first-order arithmetic difference pre-propeller stator attachment can increase the propulsion efficiency by up to 3%. In practical applications, the "Su Jiaoxun 0001" law enforcement vessel equipped with a 500-kilowatt full-revolving shaftless Rim-driven Thruster achieves a speed of 25 km/h while demonstrating excellent energy consumption economy.

(II) Low Noise and Vibration, Excellent Concealment and Comfort

Vibration and noise are the main drawbacks of traditional propulsion systems, while the structural design of Rim-driven Thrusters fundamentally inhibits the generation of noise and vibration. Shafting rotation and gear meshing of traditional propulsion systems are the main sources of vibration and noise. Rim-driven Thrusters eliminate these mechanical transmission components and only drive the propeller to rotate through electromagnetic force, greatly reducing mechanical noise. At the same time, Rim-driven Thrusters adopt shielded motors and water-lubricated bearings, and the structural design completely submerged in water further weakens the propagation of noise. This low-noise and low-vibration characteristic not only improves the comfort of ship navigation, but also makes Rim-driven Thrusters have unique advantages in fields with high concealment requirements such as military submarines and scientific research ships. The earliest research on rim-driven propulsion systems originated from the U.S. military DARPA's Tango Bravo silent submarine program.

(III) Flexible Maneuverability, Adapting to Diversified Navigation Needs

Rim-driven Thrusters can realize full-revolving control, which can adjust the thrust direction by 360°, greatly improving the maneuverability and mobility of the ship. They are especially suitable for ships that need frequent steering and positioning, such as harbor tugs, law enforcement ships, and scientific research ships. In addition, the modular design of Rim-driven Thrusters allows them to be arranged in multiple unit combinations according to the ship's power needs, and the dynamic distribution of power is realized through an integrated energy management system, further optimizing the power output efficiency. For example, Sun Yat-sen University's "Lanbo No.1" scientific research ship and Kunming Lake Dianchi cyanobacteria cleaning ship all adopt Rim-driven Thrusters, which adapt to complex operating environments by virtue of their flexible maneuverability.

(IV) High Reliability and Reduced Maintenance Costs

Rim-driven Thrusters eliminate vulnerable components such as shafting and gearboxes, simplify the system structure, and reduce failure points. The water-lubricated bearings used are suitable for complex water environments such as sediment, without dynamic seals and additional auxiliary cooling systems, further improving the reliability and durability of the system. At the same time, the modular installation method makes maintenance operations more convenient, which can greatly reduce the operation and maintenance costs of the ship. Practice by Guangzhou Marine Engineering Equipment Co., Ltd. has shown that the application of Rim-driven Thrusters in civil ships can significantly reduce long-term maintenance investment in operation.

III. Key Factors Affecting the Power Performance of Rim-driven Thrusters

(I) Structural Design Parameters

Structural design is the foundation for determining the power performance of Rim-driven Thrusters. The core parameters include propeller blade design, fairing structure, motor type, and bearing characteristics. Parameters such as the number of propeller blades, airfoil, and pitch directly affect the thrust output and energy conversion efficiency; the shape and size of the fairing and the matching degree with the propeller determine the flow field quality. An unreasonable fairing structure will lead to flow separation and increase resistance loss. The selection of motor type is also crucial. Traditional radial flux motors have problems such as large radial size and high rotational resistance, while axial flux motors can effectively reduce volume, reduce losses, and improve propulsion efficiency by optimizing the magnetic field direction. In addition, the material performance and structural design of water-lubricated bearings affect frictional resistance and service life. The use of sliding bearings made of polymer materials with a reasonable chute angle can reduce flow loss and improve system stability.

(II) Operating Condition Parameters and Operating Environment

The power performance of Rim-driven Thrusters is significantly affected by navigation conditions and the external environment. In terms of operating condition parameters, the advance speed and rotational speed directly determine the thrust and torque output. There is an optimal value of propulsion efficiency at different speeds, and the optimal operating condition range can be determined through CFD numerical calculation. In terms of the operating environment, factors such as water flow speed, flow direction, water temperature, and sediment content all affect power performance. For example, the sediment environment will aggravate bearing wear and affect rotational accuracy, while the sewage discharge hole design on the surface of the Rim-driven Thruster housing can effectively discharge sediment entering the gap, ensuring stable performance.

(III) Coupling Effect Between Hull and Thruster

The power performance of Rim-driven Thrusters does not exist in isolation, and its flow field coupling effect with the hull significantly affects the overall propulsion efficiency. The quality of the stern flow field directly affects the inflow conditions of the thruster, while the installation position and number of Rim-driven Thrusters will change the stern flow field distribution. Therefore, in the design process, it is necessary to comprehensively analyze the interaction between the hull and the thruster through fluid-structure interaction calculation methods, realize integrated optimization design, and avoid power loss caused by coupling effects.

IV. Optimization Directions of Power Performance of Rim-driven Thrusters

(I) Refined Optimization of Structural Design

In the future, it is necessary to further promote the refined structural design of Rim-driven Thrusters, and comprehensively optimize parameters such as propeller airfoil, fairing shape, and motor magnetic field distribution through multi-objective optimization algorithms. For example, bionic design is used to optimize the blade structure to improve cavitation resistance and propulsion efficiency; numerical simulation is used to optimize the stator-rotor gap to reduce electromagnetic loss and flow loss. At the same time, the development of new and efficient axial flux motors will further reduce the volume and improve power density, providing core support for the improvement of power performance.

(II) Integrated Application of Intelligent Control Strategies

Combined with the development trend of ship intelligence, the integration of intelligent control strategies with Rim-driven Thrusters can realize dynamic optimization of power performance. By equipping sensors to real-time monitor navigation conditions and thruster operating status, machine learning algorithms are used to predict optimal operating parameters, and frequency conversion technology is used to accurately adjust motor speed and torque, so that the thruster is always in an efficient operating range. In addition, the development of an advanced integrated energy management system realizes the coordinated control of multiple thruster units, improving the efficiency and reliability of the overall power system.

(III) Deepening of Test and Simulation Technology

Test and simulation technology are important means to optimize the power performance of Rim-driven Thrusters. It is necessary to further improve the CFD numerical simulation method, improve the accuracy of fluid-structure interaction calculation, and realize accurate prediction of the hydrodynamic performance and strength performance of the thruster. At the same time, build large-scale professional test facilities, carry out sea trials and tank tests, verify the accuracy of simulation results, and provide data support for structural optimization and performance improvement. Facilities such as the thruster loaded test tank and variable Froude number circulating test flume built by Guangzhou Marine Engineering Equipment Co., Ltd. provide important guarantees for the performance testing of Rim-driven Thrusters.

V. Conclusion

Relying on its shaftless and integrated structural advantages, Rim-driven Thrusters show significant advantages in power performance dimensions such as propulsion efficiency, noise control, and maneuverability, which are in line with the development trend of green ships and intelligent ships. They have broad application prospects in fields such as civil ships, military ships, and underwater equipment. Structural design parameters, operating conditions, and hull-propeller coupling effects are the key factors affecting their power performance. The power performance can be further improved through refined structural optimization, intelligent control integration, and deepening of test and simulation technology. With the continuous breakthrough and maturity of technology, Rim-driven Thrusters will surely promote a revolutionary transformation of marine power systems, providing core power support for the green and sustainable development of marine transportation, marine development and other fields.