Ship propellers vary in size and purpose. They all provide propulsion by creating a pressure change in the water on either side of the blades, similar to how a water or wind turbine works. The propeller is mounted on a shaft which is spun by an engine and runs through the hull, where it pushes water against a rudder to allow the ship to steer.
Number of Blades
The number of blades a propeller has determines how well the vessel will accelerate and how great its top speed will be. The fewer blades a propeller has, the lighter it will be, and less force will be required to accelerate the propeller from a stop. Larger ship propellers typically have more blades, as the greater the surface area a blade has, the more force is exerted on it; this can lead to blades snapping off.
The pitch of a propeller is the amount of water that a blade displaces in the course of one revolution. The greater the angle the blade makes, the more water gets displaced. Some ship propellers have a fixed pitch, meaning the blades are cast as part of the propeller, and cannot change in orientation. Variable pitch propellers, also called "controlled pitch propellers," can have the blade pitch controlled to give them more responsive handling and increased efficiency.
The downside to controlled pitch propellers is that they incorporate more moving parts which decreases the overall reliability of the propeller; and they can leak hydraulic fluid into the sea.
Propellers must be made of very rigid materials to withstand the stresses of pushing against water without warping; however, if a propeller hits a shoal or submerged object, it will damage the shaft system --- and possibly the engine --- if the blades themselves do not give way first. Copper alloy is often used for large propellers, compared to bronze or steel propellers used in smaller boats. Copper will also resist the corrosive force of salt water.
Some ships have a second propeller called a "contra-rotating propeller." It rotates in the opposite direction of the primary propeller. This results in lower propeller noise. A contra-rotating propeller system requires a smaller --- and less expensive --- diameter propeller, or a reduced engine speed with less fuel use for peak performance.
- Marine Insight: Propeller, Types of Propellers and Construction of Propellers
- Manbw: Basic Principles of Ship Propulsion
- "Basic Ship Theory"; K. J. Rawson, E. C. Tupper; 2001
- Photo Credit General Photographic Agency/Valueline/Getty Images
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