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Type 095 - Rim-driven Tunnel Thruster

On the 30 May 2017, Chinese state broadcaster CCTV showed a new episode of its documentary program Focus Report. Chinese Navy Rear Admiral Ma Weiming made various remarks about developing a number of new systems over the years. Popular Science, on its Eastern Arsenal blog, wrote: " In the interview, he seemed to state that the PLAN is fitting its newest nuclear attack submarines with a “shaftless” rim-driven pumpjet, a revolutionary and silent propulsion system." Such a propulsor is different from the MHD caterpillar popularized in 'Hunt for Red October'.

Indeed, TAN Weizhong et al of the School of Energy and Power Engineering, Wuhan University of Technology wrote in 2015 that "Shaftless rim-driven propulsion system is a new type of ship propulsion system developed in recent years,which is a successful attempt of innovative mode of thinking on ship propulsion system. The shaftless propulsion system can effectively reduce the cabin space occupied by propulsion system and increase ship propulsive efficiency significantly. It can also meet the concealment requirement of warship by reducing vibration and noise. Therefore, it has wide application prospect and great application value in both military and civilian areas."

The rim-driven thruster concept offers the low noise emissions and compact dimensions, water-lubricated hydrodynamic bearings and a hubless propeller design without any obstacles in the flow path.

Such systems have become a very important piece of equipment, allowing movements of floating vehicles to be facilitated, above all, but not exclusively, in the marine field. By the installation of one or more of these tunnel thrusters in the quickwork of a vessel, it becomes possible not only to increase maneuvering and evolution ability of the vehicle upon which they are mounted, but it is also possible to help in the implementation of their dynamic positioning system. Such systems are generally arranged transverse to the fore-and-aft axis of the marine unit in the quickwork of the hull and the tubular duct comes out at the sides of the hull where apertures are provided coinciding with the ends of said tunnel. Still generally, the axis of rotation of the propeller inside the duct is arranged transverse to the fore-and-aft axis of the marine vehicle.

A rim driven thruster [RDT] comprises an annular housing, a propulsor assembly, a magnetic rotor assembly and a stator assembly. The annular housing defines a flow path extending along an axis. The propulsor assembly is supported within the housing and comprises propeller blades extending radially from the axis of the flow path. The propeller blades are configured to rotate about the axis. The magnetic rotor assembly is mounted to radially outer ends of the propeller blades. The stator assembly comprises spaced propeller drive modules mounted to an inner circumferential surface of the annular housing. The propeller drive modules are configured to provide electromagnetic torque to the magnetic rotor assembly.

RDTs are advantageous for submerged operation because the electro-magnetic motor is removed from the center of the propulsor. In such a configuration, electrically active components of the stator assembly are positioned within the housing so as to be easily insulated. Moreover, the motor is positioned so as to minimize hydraulic drag. Specifically, the stator assembly is positioned within the annular housing and the rotor assembly is positioned in close proximity to the housing at the outer diameter of the blades. The stator and rotor assemblies are, however, still exposed to hydraulic drag when submerged. Thus, it becomes desirable to reduce the thickness of the rotor and stator assemblies to further minimize hydrodynamic losses.

Typical RDTs utilize conventional slotted stator cores in the stator assembly. In these designs, however, it is difficult to accommodate multiple windings in the narrow and shallow slots that are needed to achieve favorable thickness dimensions. Another proposal for reducing stator core thickness has included the use of a slot-less stator winding and spiral wound stator core laminations. This stator assembly design is expensive, difficult to manufacture and suitable only for small motors. There is, therefore, a need for a permanent magnet motor configuration having favorable hydraulic drag properties in an easily and inexpensively manufactured configuration.

Early development for this technology performed in Rostock, Germany at the company AIR. The project was then taken over by Voith, and the development of the Voith Inline Thruster and Voith Inline Propulsor was then continued. After the Voith factory was shut, the development team decided to offer its experience in rim-driven thrusters on its own through the newly established brand, Silentdynamics.

Schottel, a German company, offers a rim thruster that provides reduced noise and vibration, high efficiency as well as savings in space and weight are important thruster requirements. The Schottel Rim Thruster [SRT] is a quiet, space- and weight-saving drive. It converts electric power directly into propulsion - without transmission losses or noise caused by a gearbox. The stator of the electric motor is integrated into the tunnel and the thruster blades are fastened to the inside of the rotor. An optimized hydrodynamic design results in considerable reduction of cavitation. Thanks to its robust design, the SRT guarantees an effectively reduced maintenance effort. The electric motor is permanently cooled by the surrounding water, preventing it from overheating. Wear parts can be easily exchanged by a diver. In addition, the water-lubricated slide bearings are easily accessible for maintenance.

The purpose of Schottel's SRT lateral thrust control unit is to apply a lateral force onto the ship through the thrust-producing propeller, which is provided in the tunnels which are arranged transversely to the longitudinal axis of the watercraft, to thus increase the maneuverability of the watercraft.

In 2015 Norwegian Cruise Line selected a Rolls-Royce permanent magnet tunnel thruster to upgrade the propulsion package onboard cruise vessel Norwegian Epic. According to Rolls-Royce, the permanent magnet tunnel thruster offers numerous advantages over traditional tunnel thrusters including reduction in noise and vibration, an increase in power output of around 25% from the same size propeller, and is removable underwater eliminating the need for dry docking.

The permanent magnet tunnel thruster design concept comprises a permanent magnet motor in a rim, which drives the propeller in the centre. The permanent magnet motor consists of two main parts – a stator that carries a number of electrical coil windings, and a rotor fitted with a number of very strong permanent magnets. A rotating magnetic field is created by the stator which interacts with the fields of the permanent magnets on the rotor, which generates force to drag the rotor around, providing the mechanical power.

Tunnel thruster systems, however, suffer from a number of disadvantages and limitations that are inherent in the flow of water through a cylindrical passage, that is, the tunnel of a tunnel thruster system, and the interaction between a propeller and the water flowing in the tunnel. For example, the thruster tunnel inherently restricts the volume of the water flowing through the propellers region of influence, thereby correspondingly restricting the thrust than can be generated by the propeller, and the interaction between the water and the tunnel boundaries presents a significantly higher flow resistance compared to a propeller acting in an open flow region, both of which result in significantly reduced efficiency compared to a propeller acting in an open flow region.

The effects of the tunnel on water flow characteristics also often result in the generation of high levels of noise due to propeller cavitation. The turbulence in the water flow around the propellers creates irregular and unpredictable velocity variations along the propeller blade surfaces, thereby making it difficult to optimize the propeller design in order to reduce noise and increase efficiency and performance.

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