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A novel permanent-magnet (PM) motor used in an integrated motor propeller (IMP) is designed. The motor has no bearings and employs a Halbach array in its rotor structure. Optimization is conducted in such aspects as PM structure, magnetizing angle of PM, and the best ratio of PM length to poles. The designed motor has a thinner rotor and a larger air gap, and a 3-D coupled-field finite element method (FEM) is used to analyze the temperature distribution of this water-cooled thruster. Losses in windings and stator core are calculated in 3-D eddy current field, and the loss is used as the input to calculate the thermal field. Furthermore, a multicomponent fluid method is proposed to deal with the influence from rotating rotor upon the water convection in the air gap. An experiment of this IMP is done under water and temperatures in it are obtained. The temperature results of experiment and calculation are in good agreement, verifying the effectiveness of the analyzing method proposed in this paper.