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The majority of Kort nozzles, either fixed or steerable units, are fitted on tugs. The high bollard pulls being demanded by salvage, coastal towage and harbors tugs, makes the fitting of Kort nozzles essential. To achieve the same bollard pull with an open propeller requires over 45% more power for the same propeller.
It is commonly thought the maximum bollard pull is only reached on trials, but the experience has shown when an emergency arises it is only the highest possible pull, combined with impeccable maneuverability that gets tug and tow out of danger. Only the Kort nozzle provides a combination of both.
Various combinations of nozzles are in service, including twin steering nozzles - with and without independent steering controls - triple screw steering nozzles and a number of units fitted with patented Hannan slots.
The Hannan slot is a device for increasing the astern bollard pull without detriment to the performance ahead and is particularly effective with controllable pitch propellers.
M.T."Stanechakker" built by Hall Russell of Aberdeen for the Shetland Towage Co. and fitted with twin Kort steering nozzles.
This diagram shows the flow through the Kort Nozzle which results in the increased low speed thrust. The cross section of the nozzle is, in effect, the same shape as an aeroplane wing but instead of 'lift', a forward thrust is produced. There is no increased thrust on the shafting, the extra thrust is passed through into the ship's hull via the nozzle.
12' 6" diameter Kort steering nozzle for tug-barge unit building in U.S.A.