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Sea trials of a ducted tip propeller designed for improved cavitation performance

University of British Columbia

Tip vortices produced by marine propellers can be suppressed by attaching tip devices to the propeller blades, which will obstruct the roll-up process and reduce the strength of the vortices. This will decrease the induced drag on the blades and delay onset of the cavitation associated with the low pressure near the core of the vortices. However, most of the previously tested tip devices have suffered large drag penalties due to the added parasite drag of the device, which have resulted in a net loss of propeller efficiency. In this research s e a trials of a ducted tip propeller has shown that flowthrough ducts installed at the blade tips will suppress the tip vortex roll-up and result in a substantial delay of the onset of tip vortex cavitation, without reducing the efficiency of the propeller. These a trials consisted of efficiency measurements and cavitation observations of a conventional propeller that was tested and subsequently modified by replacing a radial fraction of the blade tips by flowthrough ducts that are aligned with the blades a n d bent to follow the outer radius of the propeller. Previous research on the ducted tips suggested that an efficiency improvement could be expected when the propeller operates at low advance ratios. These a trials indicate, however, that the ducted tips caused an increase in the propeller efficiency when it operates at high advance ratios, i.e. close to its peak performance. The success of the ducted tip propeller can mainly be attributed to the effectiveness of the ducts as tip vortex suppressing devices; first of all by obstructing the path of the vortex, and secondly, by diffusing the vortex through mixing of the external and internal flow at the exit of the ducts. However, the low parasite drag of the ducts also play an important role by keeping the drag penalty at a minimum. The ducted tip propeller was originally thought to be useful for boats with heavily loaded propellers, such as tug boats and trawlers. The current research has shown that the ducted tips may have a potential for application on propellers for a much wider aspect of ships, possibly only limited by the tip speed of the propellers.

Thesis/Dissertation

application/pdf

2009-02-14

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http://hdl.handle.net/2429/4591

Mechanical Engineering

University of British Columbia

UBCV

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