A Kort nozzle is a hydrodynamically-designed shroud that encircles a ship’s propeller just outside the blade tips. The entire assembly of the propeller and the nozzle is referred to as a ducted propeller. The concept of the ducted propeller was initially invented by the Italian aeronautical engineer Luigi Stipa (1900-1992) for use on aircraft. The German engineer Ludwig Kort (1888-1958) adapted the concept for use on vessels. The pioneering aspect of the ducted nozzle was the foil-shaped shroud, not unlike an airplane wing. The shroud opening is widest in front of the propeller, forcing more than the usual amount of water to pass through the propeller blades. The Kort nozzle improves the overall efficiency of the propeller at low speeds (below ten knots). Above that speed, the inefficiency caused by the drag produced by the shroud exceeds the efficiency of the increased water flow through the propeller. For that reason, the Kort nozzle is used almost exclusively on vessels requiring high thrust at low speed, such as tugs and trawlers. Bollard pull on a tug fitted with a Kort nozzle may be up to 30% higher in some situations than the bollard pull on a similar tug with a traditional open propeller. The Kort nozzle may be fixed, with vessel directional control coming from a traditional rudder, or the nozzle may be designed to pivot, so that the water flow through the nozzle controls the vessel’s direction. The shrouding can provide some protection for the propeller in ice fields. If the ice is particularly thick though, it may get jammed between the propeller blade and the shroud, locking up the propeller. Fouled propellers fitted with Kort nozzles are much more difficult to clear than are open propellers. Several variations of the Kort nozzle and ducted propeller assembly are in production, with selection based on vessel design and operation.