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 Navy landing craft utility vehicles (Fig. 1). (Lewis, 213)
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Fig. 1. US Navy LCU
1645 Class. |
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.
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Fig. 2.
Accelerating Kort nozzle. |
A number of differing nozzle designs have been developed by MARIN (MArine Research Institute,
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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.
"Landing Craft, Mechanized and Utility - LCM /
LCU." May 1999. Online.
Lewis, Edward V., editor. Principles of Naval Architecture, Volume II:
Resistance, Propulsion, and Vibration. Second revision.
"Nozzle." 2000. The American Heritage
Dictionary of the English Language: Fourth Edition, Online.