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→How Propellers Work
Fixed propellers are the most efficient, strongest and least expensive. But they are at their most efficient only at their designed rpm and hull speed. At other speeds, their efficiency falls off. A fixed propeller is ideal for a boat that cruises long distances at a constant speed. Boats that stop and start a lot or change speed frequently are better with a controllable-pitch propeller.
== Number of Blades ==
The ideal propeller has one large blade, but this would be very unstable. It would set up a lot of vibration in the shaft. Traditionally, trawler propellers were three-bladed. Modern propellers are four, five, six and seven-bladed designs. Six-bladed propellers, at first, were unsuccessful. Prior to the development of thrust systems, submarines were using designs with seven skewed (curved) blades. Generally, trawler blades are an aerofoil shape called B.Troost.<ref>http://traktoria.org/files/personal_submarine/propulsion/propeller/lecture_notes/hydrodynamic_characteristics_of_propellers.pdf</ref>
<ref>Basic Ship Propulsion, Allied Publishers, 2004</ref>
<ref>Marine Propellers and Propulsion, John Carlton, Butterworth-Heinemann, Nov 23, 2012</ref>
With more blades, vibration and noise are reduced, and thrust is more evenly distributed. For example, a boat with 200 shaft horse-power and three blades has 66.6 hp per blade. With five blades, the power distribution is 40 hp per blade. With more blades, a propeller can be smaller. However, larger propellers are better at moving water. Small blades move less water. To compensate, they have to turn faster. But increasing rpm reduces efficiency. Other factors also come into play. For example, higher block coefficients in the hull design reduce propeller efficiency.
Again, like so many boat design issues, the optimum is a compromise between several factors. Generally, the choice is between large slow turning and small fast turning propellers with an increased pitch angle.
Experience shows that the optimum for a single-screw boat is a four-bladed propeller with some skew, to compensate for the disturbed inflow. A four-bladed propeller may, however, increase sympathetic vibration, pushing you back to re-considering three or maybe five blades.
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.
[[Category:PropulsionSystems]]