KORT nozzle

The Kort nozzle is an accelerating annular duct surrounding the propeller, patented by Ludwig Kort in the 1930s, that raises thrust at low advance speed by accelerating the inflow and recovering part of the slipstream energy on the duct’s inner aerofoil section. The duct carries a forward-acting thrust of its own and reduces the slipstream contraction, so a ducted propeller delivers more bollard pull per unit power than an open wheel: gains of 20 to 30 percent at zero speed are typical, which is why tugs and trawlers use it. The standard accelerating profile is the MARIN 19A; a decelerating duct instead suppresses cavitation. Benefit reverses at high advance speed, where duct friction outweighs the recovery.