A propeller fan is a rotating mechanism that uses specially angled blades to generate thrust. Commonly known as a prop or a screw, it is a tool used primarily as a way to provide locomotion, mostly in airplanes and boats. The difference between a propeller fan and a fan used to circulate air is an emphasis on creating high pressure as opposed to high volume.
The propeller fan, in essence, dates back to the Greek scientist Archimedes, whose eponymous screw design worked to lift water up from a lower source. This screw concept would not be translated to propulsion for roughly 1,500 years, when, in the mid-18th century, two scientists, J. P. Paucton and James Watt, separately proposed using a propeller fan in both air and water-based vehicles, respectively. Though aircraft would take more than another century to quite literally get off the ground, the first screw-driven ships were introduced in the 1800s and, along with the steam engine, revolutionized travel.
Since the Wright Brothers' successful flight at Kitty Hawk, North Carolina in 1903, the propeller has been the simplest and most reliable source of propulsion in aircraft. In both sea and flying machines, a propeller relies on Newton's third law of motion, which states, "To every action there is always an equal and opposite reaction." A prop pushes air or water behind the craft and causes reactionary propulsion forward. The angle of the prop blades, the speed of their rotation, and various other factors all affect how much velocity is imparted by the process.
Both air and water props can have different numbers of blades. Aircraft propellers typically have anywhere from two to eight blades, while marine screws generally have three, four, or five. Complex mathematical equations are employed to determine the correct size, angle, and thickness of each blade for optimum performance. Incorrect specifications can result in a loss of power, responsiveness, and sometimes outright failure of the propeller.
Unique to marine propellers, given their use within a fluid setting, a condition known as cavitation can occur if a screw begins to spin too quickly, or too much power is pushed through it. One or more bubbles form around the screw and quickly collapse, generating a shockwave that can seriously damage the propeller, its blades, or surrounding parts. In addition to the damage that can occur as a result of a single, large cavitation, trauma can occur over time as the result of many small cavitations that occur in the same place. These are usually due to a minor defect or flaw in a screw. This too can cause damage and premature failure.