Ducted fan is recommended as a small WIG (wing in ground-effect) craft propulsion system instead of a propeller
because it not only provides enough startup thrust but also keeps the water from disturbing the propeller tips.The vortex distribution model of the ducted fan was established
and the numerical results were used to substitute the Prandtl Loss.Through the extension of Betz conditions to the ducted fan
a complete method of ducted fan design was established.The performance of a ducted fan model tested in the wind tunnel shows a satisfactory result.