Zhao, D.; Guo, C.; Su, Y.; Wang, L., and Wang, C., 2017. Numerical study on the unsteady hydrodynamic performance of a four-propeller propulsion system undergoing oscillatory motions.In this study, the hydrodynamic performance of single- and four-propeller propulsion systems were numerically calculated under open-water and oscillating conditions to elucidate the effect of oscillatory motions on the hydrodynamic performance of propellers of large vessels under wave conditions. The Reynolds-averaged Navier-Stokes method and self-defined motion equations were adopted to describe the coupled rotation/oscillation of propellers in order to simulate their motion with the ship body under actual sea conditions. A numerical simulation of the unsteady flow fields was performed by using an overset-grid approach. The unsteady thrust coefficient, torque coefficient, propeller wake vortex, and blade pressure distribution during the coupled self-rotation and heaving oscillation of the propellers were analyzed at different speeds of advance. The propulsive efficiency and received power of the propellers were calculated and analyzed for various conditions, with a focus on the effect of different oscillation frequencies on the hydrodynamic performance of multipropeller propulsion systems. The results presented in this paper are theoretically meaningful and may be applicable to optimizing the propeller design or arrangement of multiple propellers, predicting the hydrodynamic performance of multipropeller propulsion systems, and rationally determining the power allocation of main engines. This paper provides theoretical support for resolving practical engineering problems.