Figure 12. Ordinary propeller with hub cavitation (top) and the Propeller equipped with PBCF that cancels hub cavitation (bottom).

From

Investigation of Different Methods of Noise Reduction for Submerged Marine Propellers and Their Classification

Mohammad A. Feizi Chekab, Parviz Ghadimi, Seyed Reza Djeddi, Mehdi Soroushan

American Journal of Mechanical Engineering. 2013, 1(2), 34-42 doi:10.12691/ajme-1-2-3