Effects of Propeller Pitch and Number of Blades on Energy Saving of an ECO-Cap

Propeller hub cap with fins (HCWF) is an energy-saving device, which is easily installed and replaced on ships in comparison with the other energy saving devices. Nakashima Propeller has developed a HCWF named ECO-Cap. This study discusses relations between energy-saving effects, propeller geometries, and flow fields behind the propellers based on propeller open water tests and underwater stereoscopic particle image velocimetry measurements in a towing tank using models of an ECO-Cap and multiple propellers. The test results showed that the ECO-Cap can suppress hub vortices and improve propulsion efficiency up to 4.7%. The propeller which has less number of blades or root-loaded pitch distribution strengthened hub vortices during a normal cap without fins. The intensity of the hub vortices was different during the normal cap between the propellers but almost the same level regardless of the propellers during the ECO-Cap. The results suggest that potential energy-saving amount by the ECO-Cap largely depends on the intensity of the hub vortices during the normal cap. Including the energy-saving effects of the ECO-Cap into the propeller open water efficiency, ranking of the propulsion efficiency changed from the original propeller open water efficiency without the ECO-Cap. The results showed importance of designing propellers and HCWFs considering interaction between components.