The Kort nozzle is a shrouded, ducted propeller assembly for marine propulsion. The hydrodynamic design of the shroud, which is shaped like a foil, offers advantages for certain conditions over bare propellers.
Kort nozzles can be significantly more efficient than propellers at low speeds, producing greater thrust in a smaller package. For the Bollard Pull it produces about 50% greater thrust per unit power than a propeller. Tugboats are the most common application for Kort nozzles.
The additional shrouding adds drag, however, and Kort nozzles lose their advantage over propellers at about ten knots (19 km/h).
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