The Kort nozzle is a ducted propeller design that improves thrust efficiency at lower speeds, allowing for smaller propellers and power plants. It was developed from the intubated propeller design of Italian engineer Luigi Stipa and perfected by German inventor Ludwig Kort in 1934. The inner surface of the casing must be contoured like an airfoil to maximize efficiency, and direction control can be provided by rudders or by pivoting the nozzle. However, the design loses efficiency at speeds above 10 knots due to the drag created by the deck.
The Kort nozzle is a low-speed ducted propeller design used on boats and boats consisting of a conventional propeller housed within a specially designed casing. The flow acceleration effect of the shroud and the hydrodynamic internal profile greatly improve the thrust efficiency of the propeller at lower speeds. This allows the use of smaller propellers and power plants on lower speed craft than conventional open propeller designs. Direction control on Kort nozzle equipped vessels can be provided by conventional rudder assemblies positioned in the thrust path or the nozzle itself can be pivoted to supply directional thrust. Unfortunately, the deck adds considerable drag to the design, so the Kort nozzle begins to lose its efficiency edge at speeds above 10 knots (11.5 mp/h-18.5 km/h).
The concepts that form the basis of the Kort mouthpiece design emerged in the early 1900s with the visionary efforts of Italian engineer Luigi Stipa. His designs for an “intubated propeller” powered aircraft were farsighted, and the work he did to establish the optimal parameters for the design helped German inventor Ludwig Kort perfect the marine Kort nozzle drive in 1934. The principle behind the Wrapped propeller concept is quite simple; It is one of the most basic standards of fluid dynamics, that is, that a fluid accelerates when it passes through a restriction in a tube. However, the engineering tweaks that make the design so efficient are a bit more complex.
The physical design of a Kort nozzle unit is similar in all respects to conventional propeller units right down to the center of the propeller. Here the hub and blades are enclosed within a cylindrical casing which is open at both ends. This forms the aforementioned fluid dynamics tube and serves to accelerate the flow developed by the propeller. However, the true magic of the Stipa’s intubated propeller and Kort mouthpiece lies in the profile of the inner surface of the cover.
As Stipa discovered after years of research, the inner surface of the canopy must be contoured in the same way as an airfoil or aircraft wing to maximize design efficiency. This airfoil sees the inner surface of the deck following a gradual curve towards the discharge end and with the top of the airfoil adjacent to the edges of the propeller blade. The speed of rotation of the propeller and the distance between it and the leading edge of the deck are also critical factors in the overall efficiency of the system. All of these variables are subject to the exact final design and purpose of the vessel involved and have led to several different variations, including the Rice pusher nozzle and the small craft Phelix unit.
Direction control in wrapped units can be accomplished in one of two ways. Conventional rudders can be used or the nozzle itself can be turned much like the thrust control on a marine jet drive. Despite all the benefits of propellers wrapped up in the thrust efficiency stakes, the designs have one basic flaw. The deck creates a considerable amount of drag in the water that begins to negate all improvements in thrust efficiency as the speed of the ship increases. This phenomenon reaches equalization at about 10 knots, making the Kort nozzle suitable only for slower vessels that require high thrust efficiency, such as tugboats and harbor tenders.
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