Propeller on a modern mid-sized merchant vessel. The propeller rotates clockwise to propel the ship forward when viewed from astern (right of picture); the person in the picture has his hand on the blade's trailing edge.

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade. Propeller dynamics, like those of aircraft wings, can be modelled by either or both Bernoulli's principle and Newton's third law. A marine propeller of this type is sometimes colloquially known as a screw propeller or screw, however there is a different class of propellers known as cycloidal propellers – they are characterized by the higher propulsive efficiency averaging 0.72 compared to the screw propeller's average of 0.6 and the ability to throw thrust in any direction at any time. Their disadvantages are higher mechanical complexity and higher cost.


Early developments

The principle employed in using a screw propeller is used in sculling. It is part of the skill of propelling a Venetian gondola but was used in a less refined way in other parts of Europe and probably elsewhere. For example, propelling a canoe with a single paddle using a "pitch stroke" or side slipping a canoe with a "scull" involves a similar technique. In China, sculling, called "lu", was also used by the 3rd century AD.

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 are nearly always more than one so as to balance the forces involved.

The origin of the screw propeller starts with Archimedes, who 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. Leonardo da Vinci adopted the principle to drive his theoretical helicopter, sketches of which involved a large canvas screw overhead.

In 1661, Toogood and Hays proposed using screws for waterjet propulsion, though not as a propeller.[1] Robert Hook 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 Hook modified the design to provide motive power for ships through water.[2] 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.[3] In 1771, 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.[4]

However, these were all theoretical ideas. The first use of a propeller on a watercraft was on the Turtle designed in New Haven, Connecticut, in 1775 by Yale student and inventor David Bushnell, with the help of the clock maker, engraver, and brass foundryman Isaac Doolittle, and with Bushnell's brother Ezra Bushnell and ship's carpenter and clock maker Phineas Pratt constructing the hull in Saybrook, Connecticut.[5][6] On the night of September 6, 1776, Sergeant Ezra Lee piloted the Turtle in an attack on the HMS Eagle in New York Harbor.[7][8] The 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."[9] The brass propeller, like all the brass and moving parts on the Turtle, was crafted by the "ingenious mechanic" Issac Doolittle of New Haven.[10]

In 1785, Joseph Bramah in 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.[11] In 1802, Edward Shorter 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 in Gibraltar and at Malta, achieving a speed of 1.5 mph.[12]

The lawyer and inventor John Stevens in the USA, built a 25 foot boat with a rotary stem engine coupled to a four-bladed propeller, achieving a speed of 4 mph, but he abandoned propellers due to the inherent danger in using the high-pressure steam engines, and instead built paddle-wheeled boats.[13]

By 1827, Czech-Austrian inventor Josef Ressel had invented a screw propeller which had multiple blades fastened around a conical base. He had tested his propeller in February 1826 on a small ship that was manually driven. He was successful in using his bronze screw propeller on an adapted steamboat (1829). His ship "Civetta" with 48 gross register tons, reached a speed of about six knots (11 km/h). This was the first ship successfully driven by an Archimedes screw-type propeller. After a new steam engine had an accident (cracked pipe weld) his experiments were banned by the Austro-Hungarian police as dangerous. Josef Ressel was at the time a forestry inspector for the Austrian Empire. But before this he received an Austro-Hungarian patent (license) for his propeller (1827). He died in 1857. This new method of propulsion was an improvement over the paddlewheel as it was not so affected by either ship motions or changes in draft as the vessel burned coal.[14]

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.[15] His efficient design drew praise in American scientific circles[16] but by this time there were multiple competing versions of the marine propeller.

Screw propellers

Although there was much experimentation with screw propulsion until the 1830s, few of these inventions were pursued to the testing stage, and those that were proved unsatisfactory for one reason or another.[17]

Propellers of the RMS Olympic, a sister ship to the RMS Titanic and HMHS Britannic
Smith's original 1836 patent for a screw propeller of two full turns. He would later revise the patent, reducing the length to one turn.

In 1835, two inventors in Britain, John Ericsson and Francis Pettit Smith, began working separately on the problem. Smith was first to take out a screw propeller patent on 31 May, while Ericsson, a gifted Swedish engineer then working in Britain, filed his patent six weeks later.[18] Smith quickly built a small model boat to test his invention, which was demonstrated first on a pond at his Hendon farm, and later at the Royal Adelaide Gallery of Practical Science in London, where it was seen by the Secretary of the Navy, Sir William Barrow. Having secured the patronage of a London banker named Wright, Smith then built a 30-foot, 6-horsepower canal boat of six tons burthen called the Francis Smith, which was fitted with a wooden propeller of his own design and demonstrated on the Paddington Canal from November 1836 to September 1837. By a fortuitous accident, the wooden propeller of two turns was damaged during a voyage in February 1837, and to Smith's surprise the broken propeller, which now consisted of only a single turn, doubled the boat's previous speed, from about four miles an hour to eight.[18] Smith would subsequently file a revised patent in keeping with this accidental discovery.

In the meantime, Ericsson built a 45-foot screw propelled steamboat, Francis B. Ogden in 1837, and demonstrated his boat on the River Thames to senior members of the British Admiralty, including Surveyor of the Navy Sir William Symonds. In spite of the boat achieving a speed of 10 miles an hour, comparable with that of existing paddle steamers, Symonds and his entourage were unimpressed. The Admiralty maintained the view that screw propulsion would be ineffective in ocean-going service, while Symonds himself believed that screw propelled ships could not be steered efficiently.[19] Following this rejection, Ericsson built a second, larger screw-propelled boat, the Robert F. Stockton, and had her sailed in 1839 to the United States, where he was soon to gain fame as the designer of the U.S. Navy's first screw-propelled warship, USS Princeton.[20]

Screw propeller of SS Archimedes

Apparently aware of the Navy's view that screw propellers would prove unsuitable for seagoing service, Smith determined to prove this assumption wrong. In September 1837, he took his small vessel (now fitted with an iron propeller of a single turn) to sea, steaming from Blackwall, London to Hythe, Kent, with stops at Ramsgate, Dover and Folkestone. On the way back to London on the 25th, Smith's craft was observed making headway in stormy seas by officers of the Royal Navy. The Admiralty's interest in the technology was revived, and Smith was encouraged to build a full size ship to more conclusively demonstrate the technology's effectiveness.[21]

A replica of SS Great Britain's first propeller was created for this museum ship. The real propeller was replaced with a four-bladed model in 1845. The SS Great Britain was initially designed to have paddles but the design was modified after screw propellers were proven to be more effective and efficient.

SS Archimedes was built in 1838 by Henry Wimshurst of London, as the world's first steamship[22] to be driven by a screw propeller[23][24][25][26]

Archimedes had considerable influence on ship development, encouraging the adoption of screw propulsion by the Royal Navy, in addition to her influence on commercial vessels. Trials with Smith's SS Archimedes led to the famous tug-of-war competition in 1845 between the screw-driven HMS Rattler and the paddle steamer ; the former pulling the latter backward at 2.5 knots (4.6 km/h).

She also had a direct influence on the design of another innovative vessel, Isambard Kingdom Brunel's SS Great Britain, then the world's largest ship and the first screw-propelled steamship to cross the Atlantic Ocean in 1845. Propeller design stabilized in the 1880s.

Aircraft propellers

ATR 72 propeller in flight

The twisted aerofoil shape of modern aircraft propellers was pioneered by the Wright brothers. While some earlier engineers had attempted to model air propellers on marine propellers, the Wrights realized that a propeller is essentially the same as a wing, and were able to use data from their earlier wind tunnel experiments on wings. They also introduced a twist along the length of the blades. This was necessary to ensure the angle of attack of the blades was kept relatively constant along their length.[27] Their original propeller blades were only about 5% less efficient than the modern equivalent, some 100 years later.[28] The understanding of low speed propeller aerodynamics was fairly complete by the 1920s, but later requirements to handle more power in smaller diameter have made the problem more complex.

Alberto Santos Dumont, another early pioneer, applied the knowledge he gained from experiences with airships to make a propeller with a steel shaft and aluminium blades for his 14 bis biplane. Some of his designs used a bent aluminium sheet for blades, thus creating an airfoil shape. They were heavily undercambered, and this plus the absence of lengthwise twist made them less efficient than the Wright propellers. Even so, this was perhaps the first use of aluminium in the construction of an airscrew.

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