Propeller boss cap fin (PBCF)

The propeller boss cap fin (PBCF) is an energy-saving boss cap carrying small fins, one per blade, fitted to the propeller hub to break up the hub vortex shed behind the boss. The hub vortex is a swirling low-pressure core that wastes rotational energy and can cavitate and erode the rudder; the fins recover part of that swirl as added thrust and raise propeller efficiency by roughly 1 to 5 percent. Developed in Japan by MOL, the Ship Research Institute, and Mikado Propeller in the 1980s, it is a retrofittable device that needs no shaft or hull change. The gain comes from torque reduction and hub-vortex suppression rather than from changing the main blade loading.