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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.
== Controllable Pitch Propellers ==
The pitch of a propeller can be fixed-true, fixed-variable, flexible or controlled. Think of pitch as the number of threads per inch on a screw.
Fixed propellers have a fixed (true) or variable pitch from the tip of the blades to the hub. A fixed-pitch propeller has a constant pitch at any radius from the hub to the tip. Think of this as a screw with a constant number of threads per inch.
A variable-pitch propeller has a pitch that increases from the hub to the tip, but is still fixed in the metal. Think of this as a variable number of threads per inch, i.e., the threads are getting further and further apart. Variable pitch propellers reduce cavitation and are used on high-speed boats, or on very large boats to compensate for varying inflow velocities due to the draft of the hull.
Flexible-pitch (pitch-adapting) propellers are made of a metal hub with carbon-fibre composite blades inserted. The blade pitch angle adapts as the load on the blade changes, reducing cavitation and increasing hydrodynamic efficiency by around 15%.<ref>A.I.R. Fertigung-Technologie GmbH, http://www.air-composite.com/englisch/html/unter/frameset_unternehmen.html</ref> Designed initially for high-end yachts, they are being tested starting in 2004 in larger sizes for the US Navy.
A controllable-pitch propeller has blades whose pitch angle can be adjusted mechanically while the propeller is turning. This requires a special hollow propeller shaft for the mechanical bits. This means the engine can run at its optimum rpm all the time. To speed up, or slow down, you change the pitch of the propeller. To go from forward to reverse, you simply reverse the pitch of the blades.
Controllable-pitch propellers yield the best overall fuel economy when vessel speeds vary. They are also an advantage in areas like Europe where fuel costs are higher. When vessel speed is constant most of the time, a fixed-pitch one is better because of the reduced complexity and cost.
== Kort/Rice Nozzles ==
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.
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.
Decreasing the exit side of the shroud, thus forcing the water to accelerate as it exited the propeller, made further improvements to the Kort nozzle. Eventually, several types of Kort nozzle were developed, with the Type 19 most suited to a trawler. Today, many companies make Kort Nozzles.
Later, the Mexican company [http://www.ricepropulsion.com/ Rice Propulsion] improved on the Kort nozzle by using aerodynamic duct shapes derived from wing sections displaying the highest lift and lowest drag.
== References ==
[[Category:PropulsionSystems]]