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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 ==