# A Review of Progress and Hydrodynamic Design of Integrated Motor Pump-Jet Propulsion
## 1 Introduction
Its downside is that an electric-driven motor is mounted in the pod, causing the diameter of the pod to be extremely large, reducing the propulsion efficiency even further [2]. The pump-jet propeller (Figure 1c) is usually composed of a pipe, a writing element (guide vane), and a rotor (impeller). Its working flow field is more complex, as it is an intricate multiconnected area compared to the internal flow field of the pump-jet blade and the open flow of the propeller, due to the existence of the pipe. The interaction between various components, the thruster, and the propulsion carrier affects the overall performance of the thruster. The rapid development of today's power electronics technology has subsequently promoted the birth of a new type of underwater propulsion device, the integrated motor  
Figure 1: Different marine propulsion systems: (**a**) conventional propulsion system; (**b**) podded propulsor; (**c**) pump-jet propulsion; and (**d**) shaftless IMP [3]. propulsor (IMP), also known as the shaftless rim-driven propulsor/thruster (RDP/RDT) (Figure 1d). [PERSON] et al. (2017) explained that this type of propeller is driven by the rim instead of by the shaft ([PERSON] and [PERSON], 2014). In the IMP propeller equipment, the drive motor is integrated into the structure, while the motor stator is embedded in the shield (pipe). The rotor is distributed on the rim or in the rim, and the propeller blades are directly connected to the rim. When the motor is operating, the blades follow the rotor, and they rotate together. The motor rotor and stator need to be equipped with sealing devices to prevent the entrance of water and various marine debris, because the propeller works underwater. Figure 2 shows three integrated motor thrusters with different structures. In Figure 2a,b, the structure is the same, the only difference being whether there is a hub between the two blades, which is evident in Figure 2b. As shown in Figure 2c, [PERSON] and [PERSON] (2014) explained that, in addition to the hub, there are more guide vanes (rear spiral) behind the rotor ([PERSON] and [PERSON], 2014). The rotor blades generate thrust, while the guide vanes are used to eliminate the rotational movement of the liquid flow and play a role of diversion. A deceleration-type duct is used. Most of the ducts in the IMP thruster are of the decelerating type, which can achieve the effect of deceleration and pressurization to delay the generation of cavitation and achieve the purpose of noise reduction ([PERSON] and [PERSON], 2014). According to [PERSON] et al. (2020), the IMP has several advantages compared to the conventional shaft-driven thruster, pod propulsor, and pump-jet propulsor ([PERSON] and [PERSON], 2014):  
1. The IMP thruster has a high degree of integration, and the cabin reduces the complicated propulsion drive shaft system, auxiliary components, propulsion motors, and other equipment into a compact structure with a light weight. 2. According to previous research, it has a low vibration and noise, since the motor and the propeller are integrated; the motor eliminates the noise in the split propulsion device when driving the propeller, and there is no longer any deceleration between the prime mover and the propeller transmission gear reduction gearbox, which is the main source of underwater radiation noise ([PERSON] and [PERSON], 2014). The back spiral stator eliminates the rotational movement of the fluid, reduces the flow velocity into the rotor blade, and increases the static pressure around the blade. The back spiral stator also delays the onset of cavitation at the tip of the blade and reduces the vortex cavitation noise, thus improving the invisibility. 3. It has a high propulsion efficiency, eliminating friction loss at the output shaft and improving the system propulsion efficiency. The back spiral stator can recover the rotational energy of the liquid flow by effectively increasing the propulsion energy.