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Kort Nozzles

A nozzle is a hydro dynamically optimized shell encircling the blade tips of a propeller. The combination of propeller and nozzle is called a ducted propeller. A Kort nozzle is a ducted propeller. It was developed first by Luigi Stipa (1931) and later by Ludwig Kort (1934). Simply put, a kort nozzle is the circular steel structure installed around the propeller of a towing vessel. Kort nozzles are installed around the propellers of some vessels to increase the thrust of the propeller. Kaplan propellers are designed to be used with a nozzle in order to give workboats more thrust, by considerably increasing the efficiency of propellers carrying heavy loads while working at low speeds.

Kort nozzles or ducted propellers can be significantly more efficient than unducted propellers at low speeds, producing greater thrust in a smaller package. For tugs and pusher boats, efficiency improvements of around 20% are frequently achievable. Bollard pull can be raised by more than 30%. The efficiencies of a kort nozzle are generally gained under 10 knots of boat speed. When operating at above 10 knots of boat speed, kort nozzles lose their efficiency. Tugboats, towboats and fishing trawlers are the most common application for Kort nozzles as highly loaded propellers on slow moving vessels benefit the most. Kort Nozzles have the additional benefit of reduced propwalk and increased course stability at low speed while towing or pushing heavy loads.

Bernoulli’s Principle states that an increase in the velocity of a fluid creates a decrease in pressure. If you look at the cross section of a Kort Nozzle it has a foil or winglike shape. When moving through the water the wing or foil shape of the kort nozzle causes an increase in the water's velocity and low pressure slightly in front of the nozzle which increases water flow to the propeller. Propeller operation in the nozzle is supplied with a larger water quantity than that available to a free propeller of equal diameter at the same thrust. Water flowing over the nozzle’s shape also creates a slight thrust forward which increases its efficiency.