When operated under large water immersion, surface piercing propellers are prone to be in heavy load conditions. To improve the hydrodynamic performance of the surface piercing propellers, engineers usually artificially ventilate the blades by equipping a vent pipe in front of the propeller disc. In this paper, the influence of artificial ventilation on the hydrodynamic performance of surface piercing propellers under full immersion conditions was investigated using the Computational Fluid Dynamics (CFD) method. The numerical results suggest that the effect of artificial ventilation on the pressure distribution on the blades decreases along the radial direction. And at low advancing speed, the thrust, torque as well as the efficiency of the propeller are smaller than those without ventilation. However, with the increase of the advancing speed, the efficiency of the propeller rapidly increases and can be greater than the without-ventilation case. The numerical results demonstrates the effectiveness of the artificial ventilation approach for improving the hydrodynamic performance of the surface piercing propellers for high speed planning crafts.

The high-speed and efficient swimming characteristics of tuna are valuable for designing bio-inspired underwater vehicles. Tuna use their highly deformable caudal fins as propulsors during swimming. Caudal fin deformation is categorized into skeletal-controlled active deformation and fluid-induced flexible passive deformation. To investigate how flexible passive deformation affects propulsion performance, simulations of four caudal fins with varying flexibilities under two $St$ numbers in a uniform flow are conducted using the finite volume method. This study finds that the medium-flexibility caudal fin achieves a higher time-averaged thrust coefficient without sacrificing efficiency under both high and low $St$ numbers. At a high $St$ number, the medium-flexibility caudal fin enhances thrust by reducing detrimental secondary flows. At a low $St$ number, the medium-flexibility caudal fin increases thrust by strengthening vortex rings, which induces a stronger backward jet.