Ducted propeller

Downsides are reduced efficiency at higher speeds (>10 knots), course stability when sailing astern, and increase of cavitation.

Tugboats and fishing trawlers 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 (18.5 km/h).

Shrouding of this type is also beneficial to navigation in ice fields since it protects the propeller tips to some extent.

A research paper by Bexton et al. (2012)[6] concluded that ducted propellers were the likely cause of fatal injuries of seals in the northeastern Atlantic.

The authors hypothesized that the seals were drawn through the nozzle and past the rotating propeller blades, incurring curvilinear lacerations to skin and muscle tissue.

The authors also comment that other animals, including harbour porpoises, have been seen to exhibit similar injuries.

The small clearance between the propeller and duct reduces tip vortex, increasing efficiency.

When towing, tugboats sail with low speed and heavily loaded propellers, and are often fitted with ducts.