Originally used for reducing erosion or canal
banks,this device of placing a screw propeller inside
a nozzle or, a duct , popularly
called Kort nozzle, is now an accepted practice for improving the
propulsive performance
particularly in certain types of crafts like tug boats.The advantage lies in
the fact
that the nozzle, which has an airfoil cross-section, also produces thrust in addition to the
thrust produced by the propeller itself.Therefore the total thrust load does not act
wholly through
the propeller shaft but is also distributed over the nozzle attachments to
the hull.Also,the nozzle
round the nozzle attachments to the hull.Also, the nozzle
round the propeller helps in
homogenizing the flow to the propeller thereby reducing
the propeller excited vibration.
For
obtaining high manoeuvrability, the nozzle can be made to swivel about a vertical axis directing
the propeller race in the
desired direction, thus eliminating the need of a rudder.Schottel rudder
propeller is a
compact arrangement incorporating this principle.For interesting details, reference
may be
made to the technical literature published by the Kort Propulsion Company, Schottel
Werft
and other manufacturer such units.
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