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Like many aspects of ship design, designing propellers is a mixture of science and art in the form of experience. There is a lot going on at the stern of a ship that is poorly understood. This is not surprising. Every hull form, engine-propeller combination, interaction with the rudder wake region, and sea condition is different. Wave action introduces a random vertical moment into this equation.
For these reasons, most ship propellers operate at efficiencies around 56%, with the principle design problem being cavitation as rotational speed is increased. In comparison, aircraft propellers operate at 90% efficiency. On a ship a propeller efficiency of less than 50% indicates a poor propeller/hull design.[http://avstop.com/ Aviation Online Magazine] describes basic propeller principles. <ref>http://avstop.com/ac/flighttrainghandbook/basicpropellerprinciples.html</ref>
(Efficiency is measured as a ratio of the propulsive energy produced to the energy consumed in rotating the propeller shaft.)
A double-screw boat is better with two smaller three-bladed propellers turning slightly faster. Propellers for nozzles should be square-tipped Kaplan designs. All propellers should be manufactured to ISO 484 Class 1 standards.
[http://www.vicprop.com Victoria Propeller Ltd.] has an [http://www.vicprop.com/displacement_size_new.php online calculator for propeller size] for displacement and semi-displacement hulls.
== Controllable Pitch Propellers ==
When a blade rotates in water, it creates a high-pressure behind the blades and low pressure in front (towards the hull). This differential provides thrust. However, at the edge of the blades, high pressure leaks to the low-pressure side, resulting in loss of thrust.
[[File:KortNozzle.jpg|Thumbthumb|250px|Leftleft|Kort 19A Nozzle – note the smaller exit side – Photo with permission © Rice Propulsion, http://www.ricepropellers.com/]]
In the 1930s, this caught the attention of Ludwig Kort, an aeronautical engineer from Hanover. He developed nozzles (shrouds or ducts) for propellers. They prevented water from escaping from the blade tips. This improved propulsion efficiency at speeds under 10 knots.