A ucted fan is a propulsion arrangement whereby a propeller is mounted within a cylindrical shroud or duct. The duct prevents losses in thrust from the tips of the propeller and, if the duct has an airfoil cross-section, it can provide additional thrust of its own. Ducted fan propulsion is used in aircraft, airboats and hovercraft. A kind of ducted fan, known as a fantail or by the trademark name Fenestron, is also used to replace tail rotors on helicopters.