DUCTED PROPELLERS

A ducted propeller is a propulsion device consisting of a conventional open propeller surrounded by a duct or nozzle. Ducted propellers find extensive applications in both marine and aeronautical fields. The duct surrounding the propeller may be used either to accelerate or to decelerate the flow in the propeller plane. In marine applications the flow accelerating duct is known as the "Kort Nozzle" and the flow decelerating device is generally referred as the "pump-jet"

The concept of surrounding the propeller by a shroud is very old. Kort Nozzle ducted propellers have been widely used in tugboats and trawelers where the propeller is small and heavily loaded. The advantage of using a ducted propeller for these vessels is that the flow is accelerated and thus an improvement in propulsive efficiency is obtained In addition the duct shares a fair proportion of the thrust and thereby reduces the propeller loading. The flow accelerating pump-jets find considerable applications for the propulsion of submarines and torpedoes since the lower velocity in the propeller plane helps to delay the cavitation induced noise. The duct itself however will produce a negative thrust.

Table 1

Comparison of measured and predicted duct thrusts

Aerodynamic studies of a Kort Nozzle Ducted Propeller

DUCTED PROPELLER

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