Numerical investigation of hull-propeller-rudder interactions for a free-running twin-propeller combatant under straight-line and astern operating condition

This study presents a comparative numerical investigation of the hull–propeller–rudder interactions using Improved Delayed Detached Eddy Simulation (IDDES) for the free-running ONRT model under straight-line and astern operating conditions in calm water. Simulations demonstrate good agreement with experimental data, with a steady-speed error of 2.63%. The results reveal that astern navigation significantly increases total resistance and reduces attainable speed. The astern navigation also leads to a marked degradation of propeller performance, with a significant reduction in the wake fraction. While both conditions exhibit Kelvin-type wave patterns, the astern case shows distinct variations in wave elevation characteristics. The wake dynamics, critical to the hull–propeller–rudder interplay, undergo significant changes: the axial high-speed region and radial suction band shift upward, and tip-vortex instability intensifies with increased small-scale fragmentation. Furthermore, the secondary flow structures induced by these interactions differ substantially, particularly regarding the disappearance of bow-induced flows. This work elucidates how these interaction mechanisms and associated flow details are altered under astern navigation.