## 1 Introduction  
Propulsion systems play an indispensable role in marine vessels. There are various designs applied to present ship propulsors, such as the open propeller, the ducted propeller, poddded and azimuthing propulsors, etc. Each kind of propulsor has its advantages and disadvantages. Regardless of different types of design, they share the same propulsion technology, i.e., a shaft-driven propulsion, in which the shaft is imperative because the propeller must be connected to it in order to rotate. Furthermore, the other side of the shaft is mounted on the primary engine, which lays within the ship's hull, making the whole structure stretch through the cabin space. In addition, substantial friction power loss and high-level noise and vibrations can occur when they operate. In order to overcome the main drawbacks in conventional shaft-driven propulsion systems as mentioned above, a novel propulsion concept has been brought forward, namely the Rim-Driven Thruster (RDT), also known as the Integrated Motor Propulsor (IMP) [1]. As the name implies, this kind of thruster is rim driven instead of being shaft driven. In an RDT device, the driver motor is integrated into the structure, with the stator embedded in the shroud and the rotor distributed on or in the rim. As the thruster works under water, both the stator and rotor are coated with protection, such as epoxy resin, to prevent water erosion. The propeller blades are directly connected to the rim and rotate with the rotor when the motor works. Figure 1 gives the inner layout of an RDT, in which some key components are shown: the driven motor, the propeller blades, the bearings, and the duct. Several outstanding improvements are realized due to the integrated design of RDTs. The component arrangement of RDTs is much more compact compared to conventional propulsion systems and therefore installation and management are more flexible. The elimination of intermediary equipment, such as a motor cooling system, reduction gear box, and bearing lubricating system, can save a lot of cabin space and reduce ship building costs. The motor efficiency is higher when the RDT is powered by a Permanent Magnet (PM) radial field motor, and power does not have to be applied to the field and thus field power losses are eliminated. In addition, the application of water-lubricated bearings ensures a low risk of water pollution [2]. Moreover, the application of the RDT is not limited to ordinary ships, as underwater Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs) can also be equipped with lightweight and highly compact RDTs to operate with a high performance [3; 4]. Despite many distinguished merits of RDTs, there are still many challenges in their hydrodynamic design and performance analysis compared to traditional propellers. In order to understand the performance of RDTs, [PERSON] et al. [5] designed and manufactured a demonstrator RDT with a 250 mm diameter. Experimental tests were conducted to investigate the influence of the duct and propeller profiles on the overall performance of the thruster. It was found that at bollard conditions the duct fairings could have negative effects on the thrust production due to the resistance of the fairings to the flow. However, at higher advance ratios, the existence of the fairings improved the performance. As for the propellers, symmetric and asymmetric ones were compared under the same working conditions. The asymmetric propeller produced a higher thrust in the forward direction compared to the symmetric propeller. This was expected because the asymmetric profile increased the pitch ratio, resulting in a higher thrust at the same speed. The study also concluded that the smoothness of the surface of the duct and propeller could have an influence on the performance of the thruster. For the hydrodynamic performance evaluation of an RDT, experimental methods are widely employed because of their high reliability and accuracy.