A ducted propeller, also known as a Kort nozzle, is a propeller
fitted with a non-rotating nozzle. It is used to improve the
efficiency of the propeller and is especially used on heavily
loaded propellers or propellers with limited diameter. It was
developed by Luigi Stipa (1931) and Ludwig Kort (1934). The Kort
nozzle is a shrouded 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.
Advantages are increased efficiency at lower speeds (<10 knots),
better course stability and less vulnerability to debris. Downsides
are reduced efficiency at higher speeds (>10 knots), course
stability when sailing astern, and increase of cavitation. Ducted
propellers are also used to replace rudders.
Kort nozzles or ducted propellers can be significantly more
efficient than unducted propellers at low speeds, producing greater
thrust in a smaller package. Tugboats and fishing trawlers are the
most common application for Kort nozzles as highly loaded
propellers on slow moving vessels benefit the most. Nozzles have
the additional benefits of reducing paddlewheel-effect (e.g. the
tendency of a right-hand wheel to back to the left) and reduce
bottom suction while operating in shallow water.
The additional shrouding adds drag, however, and Kort nozzles lose
their advantage over propellers at about ten knots (18.5 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.
Shrouding of this type is also beneficial to navigation in ice
fields since it protects the propeller tips to some extent.
However, ice or any other floating object can become jammed between
the wheel and nozzle, locking up the wheel. Fouled wheels in Kort
nozzles are much more difficult to clear than open wheels.
A research paper by Bexton et 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. This
type of injury has come to be known as a "corkscrew" injury. The
authors also comment that other animals, including harbour
porpoises, have been seen to exhibit similar injuries.
There are two types of ducts; accelerating and decelerating. With
accelerating ducts, the inflow velocity and efficiency of the
propeller is increased. This is the type that is used on heavily
loaded propellers or propellers with limited diameter. As Ludwig
Kort performed extensive research on this type, it is often called
a "Kort nozzle".
With the second type, the inflow velocity is reduced, whereby
pressure is increased, reducing cavitation. This is called a
pump-jet, especially in combination with fixed blades or stators.
MARIN has done extensive research on ducted propellers. Many of the
used profiles are based on the NACA airfoils of which the NACA 4415
has very good characteristics. Most commonly used are nozzle 19A
and 37 of the MARIN series. These have a rounded trailing edge to
ease fabrication and increase efficiency sailing astern. Initially,
the propellers of the Wageningen B-series were used, later the
Kaplan-type with a wider blade tip.
The above material is from Wikipedia,edited by Ijin Marine.