Considering the complex flow features of a submarine under the maneuvering condition and their close relationship with the maneuverability, a numerical study is performed for the flow around the submarine under the rudder deflection condition. Taking the submarine model DARPA SUBOFF as the study object, the rudder force test with a wide range of rudder angles is simulated based on the open source CFD platform OpenFOAM. Two numerical methods of Unsteady Reynolds-Averaged Navier-Stokes (URANS) method and Delayed Detached Eddy Simulation (DDES) method are employed. A systematic convergence study is performed to clarify the effect of grid spacing and time step on the numerical solution. The computed hydrodynamic forces are compared with the available experimental data, and the effectiveness of the adopted numerical methods is validated. Besides, the obtained rudder forces and rudder parameters are compared and discussed. Based on the obtained flow field details of vortex structures, velocity fields, streamline distribution, turbulence levels, and pressure distribution, the flow mechanisms of the deflected rudders are analyzed. Compared with URANS method, DDES method presents a better capability of capturing the flow field details around the submarine under the tight maneuvering condition, where remarkable flow separation is involved.