Kort Nozzle Propellers: Unsuitable For Aviation Use?

Even though some vertical takeoff and landing aircraft and dirigibles use them, are Kort Nozzle propellers largely unsuitable for general aviation use?

By: Ringo Bones

This sort of propulsion system is more well-known n the maritime, rather than the aeronautical engineering field, although there are experimental vertical takeoff and landing  / VTOL aircraft designs from the 1960s, and my first fascination into one is from an early 1960s aviation magazine published in India that I first saw in our public library back in the 1970s. Mostly featured in futuristic looking VTOL aircraft designs from the early 1960s - like the tilting ducted fans of the X-22 - and in more contemporary dirigible designs, it seems that the Kort Nozzle propeller is largely unknown in the field of general aviation.

Originally developed for maritime propulsion, the Kort Nozzle propeller is an assembly of a conventional screw propeller and a short nozzle named after the device’s inventor, L. Kort, a German engineer. The nozzle is rigidly attached to, or forms part of, the structure of the ship, and the propeller of the ship revolves inside the nozzle.  

The basic principle of a screw propeller working inside of a cylindrical tube or a tunnel was proposed soon after the development of practical screw propellers. Robert Griffith patented a ship stern in 1845 that provided for a screw propeller working in a short tunnel. Other variations of such hull construction were patented by C.A. Parsons in 1877, J.I. Thornycroft in 1879 and by many others. Kort revived the idea around 1925 but generally improved it by making the tube into a short nozzle, wider at the mouth than at the exit and with airfoil-shaped cross sections. By practical tests, Kort demonstrated that by this arrangement, a generally increased thrust was obtained for the same power input, as compared with the conventional screw propeller.

Even though highly efficient propellers – in terms of converting engine power output to forward thrust – were already invented and used in maritime applications from the 1850s onwards – early aviation pioneers more often than not ignored the science and engineering behind this designs and thus failed to achieve true heavier than air powered flight in the 19^th Century.  Many historians cite the Wright Brothers developing their own working propeller – albeit independently from existing maritime propeller design advancements made since the mid 19^th Century – that was largely responsible for the success of their epoch making December 17, 1903 first flight.

From a maritime propulsion engineering perspective, the practical gain of the Kort Nozzle propeller over the conventional open propeller for equal propeller diameters is well substantiated. The gain is greatest when the ship speed is low and the propeller is heavily loaded, than is, when the slip velocity is large. For this reason, Kort Nozzle propellers are used principally on towboats, fishing trawlers, and similar vessels which pull heavy loads at low speeds. For such vessels, the addition of a Kort Nozzle may increase the towrope pull per shaft horsepower by as much as 30 to 40-percent. On the other hand, for fast sea-going ships, the Kort Nozzle offers no advantages because the small gain in efficiency obtained by it is usually offset by increased appendage resistance of the hull. For the same reason, Kort Nozzles, or similar devices, have not been found advantageous for use in aircraft and most general aviation applications.