The Kort nozzle is a low speed, ducted propeller design used on ships and boats consisting of a conventional propeller housed within a specially designed shroud. The shroud's flow accelerating effect and hydrodynamic inner profile greatly improve the thrust efficiency of the propeller at lesser speeds. This allows for smaller propellers and power plants to be used on lower speed vessels than those for conventional, open propeller designs. Steering control on Kort nozzle-equipped vessels may be supplied by conventional rudder assemblies placed in the thrust path or the nozzle itself may pivot to supply directional thrust. Unfortunately the shroud does add considerable drag to the design which is why the Kort nozzle begins to lose its efficiency edge at speeds above 10 knots (11.5 mp/h-18.5 km/h).
The concepts forming the basis of the Kort nozzle design emerged in the early 1900s with the visionary efforts of Italian engineer Luigi Stipa. His designs for an “intubed propeller” driven aircraft were farsighted, and the work he did on establishing the optimum parameters for the design helped German inventor Ludwig Kort perfect the Kort nozzle marine drive in 1934. The principle which underpins the shrouded propeller concept is fairly simple; it is one of fluid dynamics' most basic standards, i.e., that a fluid accelerates when passed through a restriction in a tube. The engineering tweaks which make the design so efficient are a little more complex, however.
The physical layout of a Kort nozzle drive is similar in all respects to conventional propeller drives up to the propeller hub itself. Here the hub and blades are enclosed within a cylindrical shroud which is open on both ends. This forms the fluid dynamics tube mentioned previously and serves to accelerate the flow developed by the propeller. The real magic of the both Stipa's intubed propeller and the Kort nozzle, however, lies in the profile of the inside surface of the shroud.
As Stipa discovered after years of research, the inner surface of the shroud should be contoured in the same fashion as an airfoil or aircraft wing to maximize the efficiency of the design. This airfoil profile sees the shroud's inner surface follow a gradual curve toward the discharge end and with the highest part of the profile adjacent to the propeller's blade edges. The rotational speed of the propeller and the distance between it and the leading edge of the shroud are also critical factors in the overall efficiency of the system. These variables are all subject to the exact end design and purpose of the vessel involved and have given rise to several different variations including the Rice thrust nozzle and the small craft Phelix drive.
Steering control on shrouded drives may be achieved in one of two ways. Conventional rudders may be used or the nozzle itself can be turned much like the thrust control on a marine jet drive. Notwithstanding all the benefits of shrouded propellers in the thrust efficiency stakes, the designs do have one basic flaw. The shroud creates a considerable amount of drag in the water which begins to negate all improvements in thrust efficiency as the vessel's speed increases. This phenomenon reaches equalization at approximately 10 knots, thereby making the Kort nozzle suitable only for slower craft requiring high thrust efficiency such as tugs and harbor tenders.