Kort nozzles or ‘ducted propeller’ is a shrouded propeller assembly for marine propulsion. They are generally used to improve efficiency and course stability at lower speeds on heavily loaded propellers or propellers with limited diameter, producing greater thrust in a smaller package, great for tugs and fishing vessels.
The downside of use are reduced efficiency at higher speeds, course stability when sailing astern and increase of cavitation.
Cavitation is a form of corrosion, formed as a result of operating pressures dropping below the vapour pressure of the surrounding fluid. This causes tiny gas bubbles to form, which collapse at an increased velocity on the surface of the exposed material.
This constant impact collapse of the bubbles causes fatigue on the substrate resulting in erosion of the surface, which is usually seen as severe pitting or pock marks over the area.
In some Kort nozzle manufacture or remedial fabrication a secondary liner is installed to the propeller shroud. This can be of different metal, which can induce galvanic corrosion in the right circumstances.
Galvanic corrosion or ‘bimetallic corrosion’ is an electrochemical process in which one metal corrodes preferentially when in electrical contact with another, in the presence of an electrolyte. In this instance seawater is the electrolyte.
Corrosion at sea is aggressive and can severly damage expensive equipment, parts or hull in a matter of months rather than years, reducing fabricated steel integrity ten-fold. Design or control is usually implemented as matter of course, to ensure the process is stemmed or reduced as much as possible.
To eleviate the corrosion process ships engineers install sacrificial anodes to the equipment as a sort of corrosion ‘magnet’, but as these corrode and deplete, the efficiency drops and corrosion attacks the least protected area.
Grade 2 IMPACT were called by one of our clients to a tug boat in dry dock which has been operating in the North Sea, which has severe corrosion to the two main Kort nozzles, two bow thrusters and one stern thruster.
We were requested to grit blast each of the areas to remove all evident corrosion.
Fill all pitting using a solvent-free composite metal repair paste, followed by two coats of a solvent-free, chemical resistant ceramic filled protective coating.
The ship had very little dry dock time and repairs had to be addressed promptly to halt further damage whilst in transit.
The ship was back on its way in no time at all.
Propeller blades can suffer from various forms of cavitation corrosion, dependant on the fluctuating pressure fields of its operation.
The various forms of cavitation are usually grouped as;
Some of these forms are relatively benign, but others can be very aggressive on the propeller’s material.
Cavitation is a form of corrosion, formed as a result of operating pressures dropping below the vapour pressure of the surrounding fluid. This causes tiny gas bubbles to form, which collapse at an increased velocity on the surface of the exposed material.
This constant impact collapse of the bubbles causes fatigue on the substrate resulting in erosion of the surface, which is usually seen as severe pitting or pock marks over the area.
Micro-hardness testing of both damaged and undamaged blades shows that, under conditions of cavitation attack, the material in the layers immediately below the surface work hardens and, therefore, becomes brittle and will fail if not addressed.
Grade 2 IMPACT can repair and protect older propellers suffering with cavitation, or prepare and protect against cavitation from new.
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