With the implementation of an energy efficiency design index, energy-saving devices (ESDs) have received increasing research interest. In this study, experimental and numerical analyses were performed on a 25-m long ore carrier model to investigate the working mechanism of ESDs comprising a pre-swirl stator and a rudder bulb. The interactions between the hull, propeller, rudder, and ESDs were observed to have a positive effect on the propulsion performance of the large-scale ship model. To elucidate the effects, self-propulsion tests were conducted at sea to determine the thrust and torque coefficients. The sliding mesh method was used to perform numerical simulations of the large-scale ship model for unsteady multiphase flows with different inflow speeds. The wake field at the stern and the pressure distribution on the vessel surface were analysed to investigate the energy-saving mechanism. The results showed that ESDs can effectively ameliorate the propeller slipstream and have a positive interference effect on the large-scale hull. The average gain of the ESDs relative to no-ESDs was up to 3%.