The ring propulsor concept employs a permanent magnet motor located in a nozzle, and a ring fastened to the propeller tips. Blades are driven by the rotating ring which plays role of the electric motor rotor. This kind of shaftless propulsor, sometimes called Ring Propeller or Integrated Motor-Propulsor, can be used in submersibles and off-shore underwater equipment. Several designs were developed and tested by AEG-Telefunken/Jastram Forschung (Germany), Harbor Branch Oceanographic Institution (USA), and others. In this work, theoretical models of the ring propulsor are developed The lifting line model is hosed on the classic moderately loaded propeller lifting line model. This model is modified in order to take into account new boundary conditions for the radial circulation distribution and nozzle surface effect on the induction factors. The lifting surface model also includes new features, such as radial, axial, and angular non-uniformity of the flow induced by a nozzle. Both models employ corrections on fluid viscosity in lift and drag force components. In performance prediction, special attention should be paid to the losses in a gap between rotating parts and the stationary nozzle. These losses are estimated based on the Taylor's formula. The axisymmetric propulsive nozzle is simulated as a thick lifting body. The nozzle-propeller interaction is taken into account by the iteration technique. Numerical simulations are performed to demonstrate typical circulation distributions, as well as induced velocities.

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