A ducted propeller, also known as a Kort nozzle, is a marine propeller fitted with a non-rotating nozzle. It is used to improve the efficiency of the propeller and is especially used on heavily loaded propellers or propellers with limited diameter. It was developed first by Luigi Stipa (1931) and later by Ludwig Kort (1934). The Kort nozzle is a shrouded propeller assembly for marine propulsion. The cross-section of the shroud has the form of a foil, and the shroud can offer hydrodynamic advantages over bare propellers, under certain conditions. Advantages are increased efficiency at lower speeds (<10 knots), better course stability and less vulnerability to debris. Downsides are reduced efficiency at higher speeds (>10 knots), course stability when sailing astern, and increase of cavitation. Ducted propellers are also used to replace rudders. History Luigi Stipa in 1931 and later Ludwig Kort (1934) demonstrated that an increase in propulsive efficiency could be achieved by surrounding the propeller with a foil-shaped shroud in the case of heavily loaded propellers. A "Kort Nozzle" is referred to as an accelerating nozzle and is generally a MARIN 19A profile or a MARIN 37 profile. Advantages and disadvantages Kort nozzles or ducted propellers can be significantly more efficient than unducted propellers at low speeds, producing greater thrust in a smaller package. Tugboats and fishing trawlers are the most common application for Kort nozzles as highly loaded propellers on slow moving vessels benefit the most. Nozzles have the additional benefits of reducing paddlewheel-effect (e.g. the tendency of a right-hand wheel to back to the left) and reduce bottom suction while operating in shallow water. The additional shrouding adds drag, however, and Kort nozzles lose their advantage over propellers at about ten knots (18.5 km/h). Kort nozzles may be fixed, with directional control coming from a rudder set in the water flow, or pivoting, where their flow controls the vessel's steering. Shrouding of this type is also beneficial to navigation in ice fields since it protects the propeller tips to some extent. However, ice or any other floating object can become jammed between the wheel and nozzle, locking up the wheel. Fouled wheels in Kort nozzles are much more difficult to clear than open wheels. A research paper by Bexton et al. (2012) concluded that ducted propellers were the likely cause of fatal injuries of seals in the northeastern Atlantic. The authors hypothesized that the seals were