Parametric Study on Hub Vortex Reducing Effects of Propeller Boss Cap Fins by Force and Wake Field Measurements in Open Water and Self-Propulsion Conditions

Propeller boss cap fins (PBCF) is one of the most popular ESDs in the industry. The present study aims to investigate effects of design variations of PBCFs on the propulsive efficiency and propeller wake field, with special attention on hub vortex dynamics. The wake fields and force on the whole propulsive system were measured by a towed underwater stereoscopic particle image velocimetry (SPIV) system and a propeller open water (POW) test dynamometer, respectively. Design parameters of PBCFs, i.e., the fin surface area and the angle of attack onto the fins, were varied to control fin loading on the PBCF. In the wake field, root vortices generated from the propeller blades were separated by PBCF and did not form a strong hub vortex, which caused pressure drop on the propeller boss cap. The hub vortex reduction practically increased total thrust, as evidenced in the global force measurement results. In PBCF design variations, Total efficiency increased linearly as the pitch angle and fin chord length decreased. The global force measurement results implied that PBCF in light loading separated root vortices efficiently. Hub vortex reduction by PBCF in light loading was also confirmed by the wake field measurement. In the case of low fin height, however, root vortices were not blocked and actually merged to form a hub vortex. Therefore, the primary function of PBCF, i.e., reducing hub vortex, was not effective anymore and the total efficiency decreased. In heavy loading conditions, axial velocity near the center retarded further, causing greater drag and diminishing the total efficiency. The model tests were also conducted in self propulsion condition, to reveal that the new PBCF with reduced loading also improves the energy saving performance when it works in the wake of the ship.