We intend to highlight some of the various propulsion systems frequently used on tugs. Today we start with the Kort nozzle.
The term "nozzle" is derived from the Middle English noselle, meaning "nose". Interestingly, "nozzle" also remains in the English language as a slang term for "nose."
The Kort nozzle is a shrouded, ducted propeller assembly for marine propulsion invented by Ludwig Kort. The hydrodynamic design of the shroud, which is shaped like a foil, offers advantages for certain conditions over bare propellers.
Kort nozzles or ducted propellers can be significantly more efficient than unducted propellers at low speeds, producing greater thrust in a smaller package. For the bollard pull it may produce as much as 50% greater thrust per unit power than a propeller without a duct. Tugboats are the most common application for Kort nozzles as highly loaded propellers on slow moving vessels benefit the most.
The additional shrouding adds drag, however, and Kort nozzles lose their advantage over propellers at about ten knots (18,52 km/h).
Kort nozzles may be fixed, with directional control coming from a rudder set in the water flow, or pivoting, where their flow controls the vessel's steering
sources: Wikipedia, Everything 2, US Patent no. 2,139,594