A Kort nozzle is a precisely engineered duct surrounding a marine propeller, providing greater control of the stream of water passing through it. The term "nozzle" is derived from the Middle English noselle, meaning "nose". Interestingly, "nozzle" also remains in the English language as a slang term for "nose." (Nozzle)
The idea of surrounding a propeller by a nozzle is a very old one. In 1934, however, L. Kort experimentally determined that the use of certain nozzles increased the efficiency (the ratio of power provided by the propeller to power output by the motor) of heavily loaded propellers. At this point, ducted propellers came to be commonly used. Kort nozzles today can be found enhancing such vessels as tugboats, supply vessels, trawlers and Navylanding craft utility vehicles (Fig. 1). (Lewis, 213)
Fig. 1. US Navy LCU 1645 Class.
(Landing Craft, Mechanized and Utility - LCM / LCU.)
The Kort nozzle operates on a similar principle to that of everyday nozzles, such as those found at the ends of garden hoses: a decrease in the area of a pipe translates into an increase in the velocity of the fluid passing through it. Mathematically, this principle is expressed by the equation of continuity, which states that the product of the cross-sectional area and the speed of the fluid is constant anywhere within a pipe. The other physical principle behind the Kort design is Bernoulli's equation, which requires that at every point within a pipe, the sum of the pressure, kinetic energy per unit volume, and gravitational potential energy per unit volume is the same. (Beichner, Jewett, and Serway, 476)
Within the bounds of these requirements (the equation of continuity and Bernoulli's equation) the Kort design emerges. An accelerating Kort nozzle can be seen in cross-section in Fig. 2 and in greater design detail in Fig. 3. The total diameter of the nozzle is approximately twice its length. It is of widest diameter at the intake, thinnest (and hence creating greatest velocity) in the center, where the propeller turns, and thinner at the outlet than at the inlet. This results, according to the two governing equations, in a net velocity and energy gain in the flow of water, pushing the propeller to greater efficiency. When viewing the figures, one should picture the nozzle revolved around a horizontal center line.
Fig. 2. Accelerating Kort nozzle.
(Lewis, 214. Modified by Kathryn Catlin.)
A number of differing nozzle designs have been developed by MARIN (MArine Research Institute, Netherlands) for unique thrust characteristics. For example, Fig. 3 is a MARIN Nozzle No. 37; its rather thick trailing edge is particularly suited for operation on the stern (back) side of a watercraft. MARIN Nozzle No. 19A (not shown) was crafted with a thinner trailing edge for pushing heavy loads. For application on pushboats and tugboats, Nos. 22 and 24 (not shown) were designed with larger length-diameter ratios (.8 and 1, respectively). (Lewis, 215-216) Although all four of these nozzle types present a large improvement over a non-ducted propeller, the differences between them are relatively small and refined. Nozzles No. 19A, 22, and 24 are somewhat more efficient than No. 37; however, No. 37 is much better suited for operation both ahead and astern of a craft, hence its popularity on pushboats that require movement in two directions. (Lewis, 219-220)
Fig. 3. MARIN Nozzle No. 37.
In its seventy-year history, the Kort nozzle has been repeatedly refined and developed for ever more specific uses. It remains the most efficient method of boosting the thrust of a heavily loaded propeller, and its many incarnations continue to grace the sterns of tugboat fleets worldwide.
Beichner, Robert J., John W. Jewett, Jr., and Raymond A. Serway. Physics for Scientists and Engineers with Modern Physics. Fifth edition. Orlando, FL: Harcourt College Publishers, 2000.
"Landing Craft, Mechanized and Utility - LCM / LCU." May 1999. Online. 20 February 2001. < http://mitglied.tripod.de/usnavy/ships/lcm.htm>.
Lewis, Edward V., editor. Principles of Naval Architecture, Volume II: Resistance, Propulsion, and Vibration. Second revision. Jersey City, NJ: The Society of Naval Architects and Marine Engineers, 1988.
"Nozzle." 2000. The American Heritage Dictionary of the English Language: Fourth Edition, Online. 20 February 2001. < http://www.bartleby.com/61/10/N0181000.html>.