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An investigation into the effects of ducted tips on a marine propeller tested in an aerodynamic facility Li, Hsing Hui Isabella

Abstract

Earlier ducted tip marine propeller designs had shown reduced tip vortex intensities, which delayed cavitation inception; however, there existed discrepancy between the tip's effects on the efficiency of the propeller. The objective of this project was to confirm the findings in a more controlled environment and gain a direction for optimizing the tip geometry. A dynamometer designed to test full size marine propellers developed in 2004 for the UBC Boundary Layer Wind Tunnel was updated and used for the experiments conducted. Although cavitation cannot be observed in a wind tunnel, changes in the propeller performance will be evident with the use of the dynamometer. The dynamometer is capable of measuring torque and thrust loads up to 20Nm ±0.1% full scale and 196N ±0.03% applied load, respectively. Static and dynamic calibrations indicated that the linearity of the sensors is of a R² value of 1.000 or higher, except for that of the dynamic torque, which had a R value of 0.998. The calibration constants were accurate to the error of the setup: ±1.5%. Two 24"x24" Dyna-Quad propellers were selected for the experiment. One of the propellers was used as a reference propeller; the other was fitted with 0.75" and 1" ducted tips. The propellers were tested at 1000, 1500, 2000, and 2500 rpm. Measurements were sampled at 100 Hz for 15 seconds at advance ratio increments of 0.05. The efficiencies measured decreased with increasing duct size. The peak efficiencies determined for the reference, 0.75" ducted tip, 1" ducted tip, and faired 1" ducted tip propellers were 61.8%, 58.7%, 56.7%, and 58.5%, respectively. Installation of the ducted tips removed lifting area and added wetted area but did not improve the overall propeller performance. Fairing of the 1" ducted tip improved the aerodynamics of the tip, but the performance remains lower than that of the reference. Although the current ducted tips did not show favourable results in the wind tunnel, with the appropriate fairing of the ducts, the design may still be effective in delaying cavitation without the expense of efficiency.

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