A Study of Propeller Operation Near a Free Surface

This paper presents a study of the effects of the interaction of the free surface with a marine propeller operating close to it, including the effects of depth and cross flow. Extensive simulations using the naval hydrodynamics computational fluid dynamics solver REX were conducted to study the open water characteristics, transient blade loads, and flow behavior of propeller 4661, with comparison to fully submerged experimental data at 0 degrees and 30 degrees shaft inclination with respect to the incoming flow. Deeply submerged computations and experiments show that the cross flow results in an increase in thrust and torque and lower efficiency relative to uniform inflow. Near the surface, computations show that effects on thrust and torque increase more dramatically as the propeller load increases. Furthermore, the presence of the free surface breaks the symmetry resulting in highest blade force losses when the blade is near top dead center. As the propeller approaches the surface, the amplitude of the higher harmonics of blade force increases. A demonstration calculation for a self-propelled submarine sailing deep and near the surface shows that operating closer to the surface results in higher propeller loads and lower efficiency.