A propeller (often called a screw if on a ship or an airscrew if on an aircraft) is a type of propulsor with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which exerts linear thrust upon a working fluid such as water or air when rotated. Propellers are used to pump fluid through a pipe or duct, or to create thrust to propel a boat through water or an aircraft through air. The blades are shaped so that their rotational motion through the fluid causes a pressure difference between the two surfaces of the blade by Bernoulli's principle which exerts force on the fluid. Most marine propellers are fixed pitch screw propellers with helical blades fixed to a boss, rotating on a propeller shaft with an approximately horizontal axis. Other types of marine propellers include controllable pitch, variable pitch, self pitching, and vertical axis propellers.
History
Early developments The principle employed in using a screw propeller is derived from stern sculling. In sculling, a single blade is moved through an arc, from side to side taking care to keep presenting the blade to the water at the effective angle. The innovation introduced with the screw propeller was the extension of that arc through more than 360° by attaching the blade to a rotating shaft. Propellers can have a single blade, but in practice there is nearly always more than one so as to balance the forces involved.
The origin of the screw propeller starts with the first records of a water screw, or screw pump, dates back to Ancient Mesopotamia, a cuneiform inscription of Assyrian King Sennacherib (704–681 BC) describes casting water screws in bronze. This is consistent with classical author Strabo, who describes the Hanging Gardens as watered by screws. Later, Archimedes (c. 287 – c. 212 BC) used a screw to lift water for irrigation and bailing boats, so famously that it became known as Archimedes' screw. It was probably an application of spiral movement in space (spirals were a special study of Archimedes) to a hollow segmented water-wheel used for irrigation by Egyptians for centuries. Later, modern propeller designs usually began with truncating a long screw at the tip. Additionally, a flying toy, the bamboo-copter, was enjoyed in China beginning around 320 AD. In 1661, Toogood and Hays proposed using screws for waterjet propulsion, though not as a propeller. Robert Hooke in 1681 designed a horizontal watermill which was remarkably similar to the Kirsten-Boeing vertical axis propeller designed almost two and a half centuries later in 1928; two years later Hooke modified the design to provide motive power for ships through water. In 1693 a Frenchman by the name of Du Quet invented a screw propeller which was tried in 1693 but later abandoned. In 1752, the Academie des Sciences in Paris granted Burnelli a prize for a design of a propeller-wheel. At about the same time, the French mathematician Alexis-Jean-Pierre Paucton suggested a water propulsion system based on the Archimedean screw. In 1771, the steam-engine inventor James Watt in a private letter suggested using "spiral oars" to propel boats, although he did not use them with his steam engines or ever implement the idea. One of the first practical and applied uses of a propeller was on a submarine dubbed Turtle which was designed in New Haven, Connecticut, in 1775 by David Bushnell, a Yale student and inventor, with the help of Isaac Doolittle, a clock maker, engraver, and brass foundryman of New Haven. Bushnell's brother Ezra Bushnell and Phineas Pratt, a ship's carpenter and clock maker, constructed the hull in Saybrook, Connecticut. On the night of September 6, 1776, Sergeant Ezra Lee piloted Turtle in an attack on HMS Eagle in New York Harbor. Turtle also has the distinction of being the first submarine used in battle. Bushnell later described the propeller in an October 1787 letter to Thomas Jefferson: "An oar formed upon the principle of the screw was fixed in the forepart of the vessel its axis entered the vessel and being turned one way rowed the vessel forward but being turned the other way rowed it backward. It was made to be turned by the hand or foot." The brass propeller, like all the brass and moving parts on Turtle, was crafted by Doolittle. In 1785, Joseph Bramah of England proposed a propeller solution of a rod going through the underwater aft of a boat attached to a bladed propeller, though he never built it. In February 1800, Edward Shorter of London proposed using a similar propeller attached to a rod angled down temporarily deployed from the deck above the waterline and thus requiring no water seal, and intended only to assist becalmed sailing vessels. He tested it on the transport ship Doncaster at Gibraltar and Malta, achieving a speed of 1.5 mph (2.4 km/h). In 1802, the American lawyer and inventor John Stevens built a 25-foot (7.6 m) boat with a rotary steam engine coupled to a four-bladed propeller. The craft achieved a speed of 4 mph (6.4 km/h), but Stevens abandoned propellers because of the inherent danger in using the high-pressure steam engines. His subsequent vessels were paddle-wheeled boats. By 1827, the Czech inventor Josef Ressel had invented a screw propeller with multiple blades on a conical base. He tested it in February 1826 on a manually-driven ship and successfully used it on a steamboat in 1829. His 48-ton ship Civetta reached 6 knots. This was the first successful Archimedes screw-propelled ship. His experiments were banned by police after a steam engine accident. Ressel, a forestry inspector, held an Austro-Hungarian patent for his propeller. The screw propeller was an improvement over paddlewheels as it wasn't affected by ship motions or draft changes. John Patch, a mariner in Yarmouth, Nova Scotia, developed a two-bladed, fan-shaped propeller in 1832 and publicly demonstrated it in 1833, propelling a row boat across Yarmouth Harbour and a small coastal schooner at Saint John, New Brunswick, but his patent application in the United States was rejected until 1849 because he was not an American citizen. His efficient design drew praise in American scientific circles but by then he faced multiple competitors.
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