Kort Nozzle
The accelerating propeller duct: MARIN 19A geometry, static thrust gain of 20 to 40 percent, the advance ratio crossover, and where the duct becomes drag.
A Kort nozzle is an accelerating duct fitted around a propeller so that the duct’s own aerofoil section generates a forward directed lift component while accelerating the inflow at the disc. The standard tug profile is the MARIN 19A series. Published static gains over an open propeller of the same diameter and power span 20 to 40 percent, which is why almost every tug propeller runs in a nozzle.
The gain is a function of loading rather than a fixed bonus. As advance ratio rises the duct’s thrust contribution falls and eventually becomes drag, with the break even against an open propeller commonly quoted near 10 knots. A full scale case from the era when the comparison was still being made gives the shape: a 93 foot, 250 hp single screw towboat gained 37 percent at the bollard and 25 percent at its normal towing speed of about 4.5 knots.
The full article will cover duct geometry and the 19A section, accelerating against decelerating ducts, steerable and fixed installations, the effect on the static merit coefficient, cavitation and erosion at the duct inner surface, and the trade against free running speed.