Kort Nozzle Thrust

A Kort nozzle is a ducted ring that surrounds a marine propeller, directing the water flow and increasing the static thrust generated by the propeller. By reducing the slipstream’s lateral expansion, the nozzle creates a higher pressure differential across the propeller blades, which is especially beneficial for vessels that operate at low speeds or require high thrust at low advance ratios.

The thrust increase provided by a Kort nozzle can be expressed as a dimension‑less factor, often denoted as k. This factor depends on the nozzle’s geometric characteristics—principally the ratio of the nozzle exit area to the propeller disk area (Aₙ/Aₚ)—and on the propeller’s thrust coefficient (C_T), which captures how efficiently the propeller converts rotational power into thrust.

Designers use the following relationship to estimate the thrust gain: the nozzle amplifies the baseline propeller thrust by a factor that grows with larger area ratios and higher thrust coefficients, but practical limits exist due to increased drag and cavitation risk at higher speeds.