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{{Short description|Device that transmits rotational power into linear thrust on a fluid}}
{{About||aircraft propellers|Propeller (aeronautics)||Propeller (disambiguation)}}
[[File:Ship-propeller 2000.jpg|thumb|upright=1.35|A 'right-handed' propeller on a merchant vessel, which rotates clockwise to propel the ship forward]]
[[File:Pratt & Whitney Canada PW100 Dash 8 OE-LGD 01.JPG|thumb|upright=1.35|Propeller of [[Pratt & Whitney Canada PW100]] [[turboprop]] mounted on [[Bombardier Q400]]]]

A ''' propeller''' (often called a '''screw''' if on a [[ship]] or an '''airscrew''' if on an [[aircraft]]) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear [[thrust]] upon a working fluid such as water or air.<ref>{{Cite web|url= https://www.britannica.com/technology/propeller|title=Propeller |website=Encyclopedia Britannica |access-date=2019-12-04}}</ref>  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.<ref>{{Cite web|url=  https://www.grc.nasa.gov/WWW/K-12/airplane/propeller.htm | publisher = NASA | title = Propeller Propulsion|date= May 5, 2015}}</ref> Most marine propellers are '''screw propellers''' with helical blades rotating on a [[propeller shaft (ship)|propeller shaft]] with an approximately horizontal axis.{{Efn | On many boats, the prop shaft is not horizontal but dips towards the stern. Although this is often forced upon the designer by hull shape, it gives a small benefit by helping to counter any [[squat effect]].}}

==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 propeller|single blade]], but in practice there is nearly always more than one so as to balance the forces involved.

[[File:Archimedes screw.JPG|thumb|left|[[Archimedes' screw]]]]
The origin of the screw propeller starts at least as early as [[Archimedes]] (c. 287 – c. 212 BC), 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. A flying toy, the [[bamboo-copter]], was enjoyed in China beginning around 320 AD. Later, Leonardo da Vinci adopted the screw 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.<ref>{{Citation | last = Carlton | first = John | title = Marine Propellers and Propulsion | publisher = Butterworth-Heinemann | year = 2012 | page = 363}}.</ref> [[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.{{Sfn | Carlton | 2012 | p = 1}}  In 1693 a Frenchman by the name of Du Quet invented a screw propeller which was tried in 1693 but later abandoned.<ref>{{Cite web|url= https://books.google.com/books?id=LPczAQAAMAAJ |title= A Treatise on the Screw Propeller: With Various Suggestions of Improvement|first= John|last=Bourne|date=April 10, 1855|publisher=Longman, Brown, Green, & Longmans|via=Google Books}}</ref><ref>{{Cite web|url= https://books.google.com/books?id=_3hUAAAAYAAJ&pg=PA18|title= Patents for Inventions: Abridgments of Specifications : Class… |date= April 10, 1857|publisher=Patent Office|via=Google Books}}</ref> 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.{{Sfn | Carlton | 2012 | page = 1}} 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.<ref>Murihead, James Patrick, ''The Life of James Watt, with Selections from His Correspondence… With Portraits and Woodcuts'', London: John Murray, 1858, p. 208</ref>

One of the first practical and applied uses of a propeller was on a submarine dubbed {{ship||Turtle|submersible|2}} which was designed in [[New Haven, Connecticut]], in 1775 by Yale student and inventor [[David Bushnell]], with the help of clock maker, engraver, and brass foundryman [[Isaac Doolittle]]. Bushnell's brother Ezra Bushnell and ship's carpenter and clock maker Phineas Pratt constructed the hull in [[Saybrook, Connecticut]].<ref>Stein, Stephen K., 2017, ''The Sea in World History: Exploration, Travel, and Trade [2 volumes]'', Ed. Stephen K. Stein, ABC-CLIO, Vol. 1, p. 600</ref><ref>Manstan, Roy R.; Frese, Frederic J., ''Turtle: David Bushnell's Revolutionary Vessel'', Yardley, PA: Westholme Publishing. {{ISBN|978-1-59416105-6}}. {{OCLC|369779489}}, 2010, pp. xiii, 52, 53</ref> On the night of September 6, 1776, Sergeant Ezra Lee piloted ''Turtle'' in an attack on {{HMS|Eagle|1774|6}} in [[New York Harbor]].<ref>Tucker, Spencer, ''Almanac of American Military History'', ABC-CLIO, 2013, Volume 1, p. 305</ref><ref>Mansten pp. xiii, xiv.</ref> ''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."<ref>Nicholson, William, ''A Journal of Natural Philosophy, Chemistry and the Arts'', Volume 4, G. G. & J. Robinson, 1801, p. 221</ref> The brass propeller, like all the brass and moving parts on ''Turtle'', was crafted by Issac Doolittle of New Haven.<ref>Manstan, p. 150</ref>

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.{{Sfn | Carlton | 2012 | pp = 1–2}}

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 {{ship||Doncaster|1792 ship|2}} at Gibraltar and Malta, achieving a speed of {{convert|1.5|mph|kph|abbr=on}}.<ref name="Carlton, p.2">Carlton, p. 2</ref>

In 1802, American lawyer and inventor [[John Stevens (inventor, born 1749)|John Stevens]] built a {{convert|25|ft|m|adj=on}} boat with a rotary steam engine coupled to a four-bladed propeller. The craft achieved a speed of {{convert|4|mph|kph|abbr=on}}, but Stevens abandoned propellers due to the inherent danger in using the high-pressure steam engines. His subsequent vessels were paddle-wheeled boats.<ref name="Carlton, p.2"/>

By 1827, 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.<ref>Paul Augustin Normand, ''La Genèse de l'Hélice Propulsive [The Genesis of the Screw Propulsor]''. Paris: Académie de Marine, 1962, pp. 31–50.</ref>

[[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.<ref>Mario Theriault, ''Great Maritime Inventions'' Goose Lane Publishing (2001) pp. 58–59</ref> His efficient design drew praise in American scientific circles<ref>{{Citation|url= http://www.cogulus.com/cgi-bin/viewer.cgi?type=writings&file=1848_10_033|archive-url=https://web.archive.org/web/20110708174319/http://www.cogulus.com/cgi-bin/viewer.cgi?type=writings&file=1848_10_033 |url-status=dead|title= Patch's Propeller | work = Scientific America | volume = 4 | number = 5  | date = October 10, 1848 | page = 33 | via = The Archimedes Screw | access-date = 31 January 2010|archive-date= July 8, 2011}}</ref> but by then he faced multiple competitors.

===Screw propellers===
Despite experimentation with screw propulsion before the 1830s, few of these inventions were pursued to the testing stage, and those that were proved unsatisfactory for one reason or another.<ref>{{cite book|author=Smith, Edgar C.|year=1905|title=A Short history of Naval and Marine Engineering|url= https://books.google.com/books?id=TSg7AAAAIAAJ|publisher=University Press | place = Cambridge|pages=66–67}}</ref>[[File:F. P. Smith's original 1836 screw propeller patent.jpg|thumb|right|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 [[Sweden|Swedish]] engineer then working in Britain, filed his patent six weeks later.<ref name=bourne84>Bourne, p. 84.</ref> 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 {{convert|30|ft|m|adj=on}}, {{convert |6|hp|kW|adj=on|lk=in}} canal boat of six [[tons burthen]] called ''Francis Smith'', which was fitted with his wooden propeller 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.<ref name=bourne84/> Smith would subsequently file a revised patent in keeping with this accidental discovery.
[[File:EricssonCounterRotatingScrews.png|thumb|Ericsson's original patent for a contra-rotating screw propulsion.]]
In the meantime, Ericsson built a {{convert|45|ft|m|adj=on}} 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 steamer]]s, 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.{{Efn | name=ogden_note | In the case of ''Francis B. Ogden'', Symonds was correct. Ericsson had made the mistake of placing the rudder forward of the propellers, which made the rudder ineffective. Symonds believed that Ericsson tried to disguise the problem by towing a barge during the test.}}  Following this rejection, Ericsson built a second, larger screw-propelled boat, ''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|1843|6}}.<ref name=bourne87-89>Bourne, pp. 87–89.</ref>

[[File:Illustrirte Zeitung (1843) 21 335 1 Archimedische Schraube des Dampfschiffes Archimedes.PNG|thumb|left|Screw propeller of {{SS|Archimedes}}]]
Apparently aware of the Royal 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. This revived Admiralty's interest and Smith was encouraged to build a full size ship to more conclusively demonstrate the technology.<ref name=bourne85>Bourne, p. 85.</ref>

[[File:Great Britain propeller and rudder wideshot.jpg|thumb|A replica of {{SS|Great Britain}}'s first propeller. A four-bladed model replaced the original in 1845. The ship was originally designed to have paddles, but plans changed after screw propellers were shown to be much more efficient.]]
{{SS|Archimedes}} was built in 1838 by [[Henry Wimshurst]] of London, as the world's first steamship{{Efn | The emphasis here is on ''ship''. There were a number of successful propeller-driven vessels prior to ''Archimedes'', including Smith's own ''Francis Smith'' and Ericsson's ''Francis B. Ogden'' and ''Robert F. Stockton''. However, these vessels were ''boats'' – designed for service on inland waterways – as opposed to ''ships'', built for seagoing service.}} to be driven by a [[#History of ship and submarine screw propellers|screw propeller]].<ref>"The type of screw propeller that now propels the vast majority of boats and ships was patented in 1836, first by the British engineer Francis Pettit Smith, then by the Swedish engineer John Ericsson. Smith used the design in the first successful screw-driven steamship, ''Archimedes'', which was launched in 1839."  Marshall Cavendish, p. 1335.</ref><ref>"The propeller was invented in 1836 by Francis Pettit Smith in Britain and John Ericsson in the United States. It first powered a seagoing ship, appropriately called ''Archimedes'', in 1839." Macauley and Ardley, p. 378.</ref><ref>"In 1839, the Messrs. Rennie constructed the engines, machinery and propeller, for the celebrated ''Archimedes'', from which may be said to date the introduction of the screw system of propulsion…" ''Mechanics Magazine'', p. 220.</ref><ref>"It was not until 1839 that the principle of propelling steamships by a screw blade was fairly brought before the world, and for this we are indebted, as almost every adult will remember, to Mr. F. P. Smith of London. He was the man who first made the screw propeller practically useful. Aided by spirited capitalists, he built a large steamer named the "Archimedes", and the results obtained from her at once arrested public attention." MacFarlane, p. 109.</ref>

The ''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 ''Archimedes'' led to a [[tug-of-war]] competition in 1845 between {{HMS|Rattler|1843|6}} and {{HMS|Alecto|1839|6}} with the screw-driven ''Rattler'' pulling the paddle steamer ''Alecto'' backward at {{convert |2.5|kn|kph}}.<ref>[https://bowcreektoanatahan.wordpress.com/propeller-versus-paddle-the-tug-of-war-between-hms-rattler-and-the-alecto/ Propeller versus Paddle: The Tug of War between HMS Rattler and the Alecto], Bow Creek to Anatahan.</ref>

The ''Archimedes'' also influenced the design of [[Isambard Kingdom Brunel]]'s {{SS|Great Britain}} in 1843, then the world's largest ship and the first screw-propelled steamship to cross the [[Atlantic Ocean]] in August 1845.

{{HMS|Terror|1813|6}} and {{HMS|Erebus|1826|6}} were both heavily modified to become the first Royal Navy ships to have steam-powered engines and screw propellers. Both participated in [[Franklin's lost expedition]], last seen in July 1845 near [[Baffin Bay]].

Screw propeller design stabilized in the 1880s.

=== Aircraft ===
[[File:Precision air ATR72 5423a.gif|thumb|[[ATR 72]] propeller in flight]]
{{main|Propeller (aeronautics)}}

The [[Wright brothers]] pioneered the twisted [[aerofoil]] shape of modern aircraft propellers. They realized an air propeller was similar to a wing. They verified this using [[wind tunnel]] experiments. They introduced a twist in their blades to keep the angle of attack constant. Their blades were only 5% less efficient than those used 100 years later.<ref>Ash, Robert L., Colin P. Britcher and Kenneth W. Hyde. [https://web.archive.org/web/20110604093014/http://www.memagazine.org/supparch/flight03/propwr/propwr.html "Wrights: How two brothers from Dayton added a new twist to airplane propulsion."] ''Mechanical Engineering: 100 years of Flight'', 3 July 2007.</ref> Understanding of low-speed propeller [[aerodynamics]] was complete by the 1920s, although increased power and smaller diameters added design constraints.<ref>{{cite book |url=http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/ |title=Pilot's Handbook of Aeronautical Knowledge |publisher=U.S. Federal Aviation Administration |year=2008 |location=Oklahoma City |pages=2–7 |id=FAA-8083-25A}}</ref> 

[[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 [[Santos-Dumont 14-bis|14 bis biplane]]. Some of his designs used a bent aluminium sheet for blades, thus creating an airfoil shape. They were heavily [[Camber (aerodynamics)|undercambered]], and this plus the absence of lengthwise twist made them less efficient than the Wright propellers. Even so, this may have been the first use of aluminium in the construction of an airscrew.

==Theory==
[[File:RMS Olympic's propellers.jpg|thumbnail|left|Propellers of {{RMS|Olympic}}. The outer two are counter-rotating.]]
{{Main|Propeller theory}}
In the nineteenth century, several theories concerning propellers were proposed. The [[momentum theory]] or disk actuator theory – a theory describing a [[mathematical model]] of an ideal propeller – was developed by [[William John Macquorn Rankine|W.J.M. Rankine]] (1865), [[Alfred George Greenhill|A.G. Greenhill]] (1888) and [[R.E. Froude]] (1889). The propeller is modelled as an infinitely thin disc, inducing a constant velocity along the axis of rotation and creating a flow around the propeller.

A screw turning through a solid will have zero "slip"; but as a propeller screw operates in a fluid (either air or water), there will be some losses. The most efficient propellers are large-diameter, slow-turning screws, such as on large ships; the least efficient are small-diameter and fast-turning (such as on an outboard motor). Using [[Sir Isaac Newton|Newton's]] laws of motion, one may usefully think of a propeller's forward thrust as being a reaction proportionate to the mass of fluid sent backward per time and the speed the propeller adds to that mass, and in practice there is more loss associated with producing a fast jet than with creating a heavier, slower jet. (The same applies in aircraft, in which larger-diameter [[turbofan]] engines tend to be more efficient than earlier, smaller-diameter turbofans, and even smaller [[turbojet]]s, which eject less mass at greater speeds.)<ref>How propellers work - https://www.deepblueyachtsupply.com/boat-propeller-theory</ref>

===Propeller geometry===
The geometry of a marine screw propeller is based on a [[helicoid]]al surface. This may form the face of the blade, or the faces of the blades may be described by offsets from this surface. The back of the blade is described by offsets from the helicoid surface in the same way that an [[aerofoil]] may be described by offsets from the chord line. The pitch surface may be a true helicoid or one having a warp to provide a better match of angle of attack to the wake velocity over the blades. A warped helicoid is described by specifying the shape of the radial reference line and the pitch angle in terms of radial distance. The traditional propeller drawing includes four parts: a side elevation, which defines the rake, the variation of blade thickness from root to tip, a longitudinal section through the hub, and a projected outline of a blade onto a longitudinal centreline plane. The expanded blade view shows the section shapes at their various radii, with their pitch faces drawn parallel to the base line, and thickness parallel to the axis. The outline indicated by a line connecting the leading and trailing tips of the sections depicts the expanded blade outline. The pitch diagram shows variation of pitch with radius from root to tip. The transverse view shows the transverse projection of a blade and the developed outline of the blade.<ref name="PNA ChVII" />

The ''blades'' are the foil section plates that develop thrust when the propeller is rotated
The ''hub'' is the central part of the propeller, which connects the blades together and fixes the propeller to the shaft. This is called the ''boss'' in the UK.
''Rake'' is the angle of the blade to a radius perpendicular to the shaft.
''Skew'' is the tangential offset of the line of maximum thickness to a radius

The propeller characteristics are commonly expressed as dimensionless ratios:<ref name="PNA ChVII" />
* Pitch ratio ''PR'' = propeller pitch/propeller diameter, or P/D
* Disk area A<sub>0</sub> = πD<sup>2</sup>/4
* Expanded area ratio = A<sub>E</sub>/A<sub>0</sub>,  where expanded area A<sub>E</sub> = Expanded area of all blades outside of the hub.
* Developed area ratio = A<sub>D</sub>/A<sub>0</sub>,  where developed area A<sub>D</sub> = Developed area of all blades outside of the hub
* Projected area ratio = A<sub>P</sub>/A<sub>0</sub>,  where projected area A<sub>P</sub> = Projected area of all blades outside of the hub
* Mean width ratio = (Area of one blade outside the hub/length of the blade outside the hub)/Diameter
* Blade width ratio = Maximum width of a blade/Diameter
* Blade thickness fraction = Thickness of a blade produced to shaft axis/Diameter

===Cavitation===
{{main|Cavitation}}
[[File:Cavitating-prop.jpg|thumb|left|Cavitating propeller in [[water tunnel (hydrodynamic)|water tunnel]] experiment]]
[[Image:Cavitation Propeller Damage.JPG|right|thumb|[[Cavitation]] damage evident on the propeller of a personal watercraft]]
[[File:Propeller & anti-cavitation plate & Schilling rudder.jpg|thumb|Bronze propeller & anti-cavitation plate, & [[Schilling rudder]] (on a river barge)]]

[[Cavitation]] is the formation of vapor bubbles in water near a moving propeller blade in regions of very low pressure. It can occur if an attempt is made to transmit too much power through the screw, or if the propeller is operating at a very high speed. Cavitation can waste power, create vibration and wear, and cause damage to the propeller. It can occur in many ways on a propeller. The two most common types of propeller cavitation are suction side surface cavitation and tip vortex cavitation.

Suction side surface cavitation forms when the propeller is operating at high rotational speeds or under heavy load (high blade [[lift coefficient]]). The pressure on the upstream surface of the blade (the "suction side") can drop below the [[vapor pressure]] of the water, resulting in the formation of a vapor pocket. Under such conditions, the change in pressure between the downstream surface of the blade (the "pressure side") and the suction side is limited, and eventually reduced as the extent of cavitation is increased. When most of the blade surface is covered by cavitation, the pressure difference between the pressure side and suction side of the blade drops considerably, as does the thrust produced by the propeller. This condition is called "thrust breakdown".  Operating the propeller under these conditions wastes energy, generates considerable noise, and as the vapor bubbles collapse it rapidly erodes the screw's surface due to localized [[shock wave]]s against the blade surface. 

Tip vortex cavitation is caused by the extremely low pressures formed at the core of the tip vortex. The tip vortex is caused by fluid wrapping around the tip of the propeller; from the pressure side to the suction side. This [https://www.youtube.com/watch?v=GpklBS3s7iU&feature=PlayList&p=218220F6C5BD650E&playnext_from=PL&index=18 video] demonstrates tip vortex cavitation. Tip vortex cavitation typically occurs before suction side surface cavitation and is less damaging to the blade, since this type of cavitation doesn't collapse on the blade, but some distance downstream.

==Types of propellers==

===Variable-pitch propeller===
[[Image:Controllable pitch propeller schematic.svg|thumb|right|A controllable-pitch propeller]]
[[File:Rudder and propeller Beached fishing vessel Nørre Vorupør Denmark 2017-04-14.jpg|thumb|right|A variable-pitch propeller on a fishing vessel]]
Variable-pitch propellers may be either controllable ([[Variable-pitch propeller (marine)|controllable-pitch propeller]]s) or automatically feathering ([[folding propeller]]s). Variable-pitch propellers have significant advantages over the fixed-pitch variety, namely:
* the ability to select the most effective blade angle for any given speed;
* when [[motorsailer|motorsailing]], the ability to coarsen the blade angle to attain the optimum drive from wind and engines;
* the ability to move astern (in reverse) much more efficiently (fixed props perform very poorly in astern);
* the ability to "feather" the blades to give the least resistance when not in use (for example,  when sailing). For large airplanes, if the engine is uncontrollable, the ability to feather the propeller is necessary to prevent the propeller from spinning so fast it breaks apart.

===Skewback propeller===
An advanced type of propeller used on the American [[Los Angeles-class submarine]] as well as the German [[Type 212 submarine]] is called a '''skewback propeller'''. As in the [[scimitar propeller|scimitar blades]] used on some aircraft, the blade tips of a skewback propeller are swept back against the direction of rotation. In addition, the blades are tilted rearward along the longitudinal axis, giving the propeller an overall cup-shaped appearance. This design preserves thrust efficiency while reducing cavitation, and thus makes for a quiet, [[Stealth technology|stealthy]] design.<ref name="JMCWebCreation and Co, 2009">{{cite web |title=Silent propellers|url= http://www.francehelices.fr/silent-propellers.htm|website=France helices |publisher=JMC Web Creation & Co|access-date=July 21, 2017 |date=2009|archive-url=https://web.archive.org/web/20070926223647/http://www.francehelices.fr/silent-propellers.htm|archive-date=September 26, 2007|url-status=dead}}</ref><ref>{{Citation | title = About Propellers | publisher = GSI Tek props | place = UK | url = http://www.gsitek-props.co.uk/about_propellers.htm}}</ref>

A small number of ships use propellers with [[winglet]]s similar to those on some airplane wings, reducing tip vortices and improving efficiency.<ref>Godske, Bjørn. "[http://ing.dk/artikel/dansk-udviklet-energibesparende-propeller-solgt-til-man-dieselturbo-128613 Energy saving propeller]" (in [[Danish language|Danish]]) ''[[Ingeniøren]]'', 23 April 2012. Accessed: 15 March 2014. [https://translate.google.com/translate?hl=da&sl=da&tl=en&u=http%3A%2F%2Fing.dk%2Fartikel%2Fdansk-udviklet-energibesparende-propeller-solgt-til-man-dieselturbo-128613 English translation]</ref><ref>Godske, Bjørn. "[http://ing.dk/artikel/kappel-propellere-baner-vej-succes-hos-man-167005 Kappel-propellers pave the way for success at MAN]" (in [[Danish language|Danish]]) ''[[Ingeniøren]]'', 15 March 2014. Accessed: 15 March 2014. [https://translate.google.com/translate?hl=da&sl=da&tl=en&u=ing.dk%2Fartikel%2Fkappel-propellere-baner-vej-succes-hos-man-167005 English translation]</ref><ref>"[http://www.mandieselturbo.com/1019133/Press/Press-Releases/Trade-Press-Releases/Marine-Power/Propeller-Equipment/Kappel-Agreement-Secures-Access-to-Major-Market.html Kappel agreement secures access to major market]", Man diesel turbo, 30 August 2013.</ref><ref>"[http://www.support-project.eu/supportknowledge/defaultinfo.aspx?topicid=145&index=6 Kapriccio Project] {{Webarchive |url= https://web.archive.org/web/20140315174525/http://www.support-project.eu/supportknowledge/defaultinfo.aspx?topicid=145&index=6 |date=2014-03-15}}" [[European Union]]. Accessed: 15 March 2014.</ref><ref>"[http://www.marinelink.com/news/article/industry-pays-tribute-to-innovation-awards/307142.aspx Industry Pays Tribute to Innovation Awards Winners]" ''Marine link'', 3 October 2002. Accessed: 15 March 2014. Quote: "Winner: the energy-saving Kappel propeller concept from the European Commission-funded Kapriccio propulsion research project. Blades curved towards the tips on the suction side reduce energy losses, fuel consumption, noise and vibration"</ref><!--may not be skewback-->

{{See also|astern propulsion}}

===Modular propeller===
A [[modular propeller]] provides more control over the boat's performance. There is no need to change an entire propeller when there is an opportunity to only change the pitch or the damaged blades. Being able to adjust pitch will allow for boaters to have better performance while in different altitudes, water sports, or cruising.<ref name="Smrcka, 2017">{{cite web|last1=Smrcka|first1= Karel|title=A new start for marine propellers|url= http://www.engineeringnews.co.za/article/a-new-start-for-marine-propellers-2005-03-18/rep_id:4136|website=Engineering News|access-date=July 21, 2017|date=March 18, 2005}}</ref>

===Voith Schneider propeller===
[[Voith Schneider propeller]]s use four untwisted straight blades turning around a vertical axis instead of helical blades and can provide thrust in any direction at any time, at the cost of higher mechanical complexity.

=== Shaftless ===
{{Main|Rim-driven thruster}}
A rim-driven thruster integrates an electric motor into a ducted propeller. The cylindrical duct acts as the stator, while the tips of the blades act as the rotor. They typically provide high torque and operate at low RPMs, producing less noise. The system does not require a shaft, reducing weight. Units can be placed at various locations around the hull and operated independently, e.g., to aid in maneuvering. The absence of a shaft allows alternative rear hull designs.<ref>{{Cite web |date=July 2017 |title=Are rim-driven propulsors the future? |url=https://www.rina.org.uk/Are_rim-driven_propulsors_the_future.html |access-date=2023-01-29 |website=www.rina.org.uk |archive-date=2022-05-24 |archive-url=https://web.archive.org/web/20220524153614/https://www.rina.org.uk/Are_rim-driven_propulsors_the_future.html |url-status=dead }}</ref>

=== Toroidal ===
{{main|Toroidal propeller}}
Twisted-[[toroid]] (ring-shaped) propellers, first invented over 120 years ago,{{cn|date=April 2023}} replace the blades with a-circular rings. They are significantly quieter (particularly at audible frequencies) and more efficient than traditional propellers for both air and water applications. The design distributes vortices generated by the propeller across the entire shape, causing them to dissipate faster in the atmosphere.<ref>{{Cite web |last=Blain |first=Loz |date=2023-01-27 |title=Toroidal propellers: A noise-killing game changer in air and water |url=https://newatlas.com/aircraft/toroidal-quiet-propellers/ |access-date=2023-01-29 |website=New Atlas |language=en-US}}</ref><ref> {{cite patent
| country       =US
| number        =US10,836,466B2
| title         =TOROIDALPROPELLER
| pubdate       =2020
|inventor1-last=Sebastian
|inventor1-first=Thomas
| url =https://patentimages.storage.googleapis.com/d6/fe/53/bea4417ed89176/US10836466.pdf
}} </ref>

==Damage protection==
===Shaft protection===
[[File:Propeller rubber bush failed.jpg|A failed rubber bushing in an [[Outboard motor|outboard]]'s propeller|right|thumb]]
For smaller engines, such as outboards, where the propeller is exposed to the risk of collision with heavy objects, the propeller often includes a device that is designed to fail when overloaded; the device or the whole propeller is sacrificed so that the more expensive transmission and engine are not damaged.

Typically in smaller (less than {{convert|10|hp|kW|abbr=on|disp=or}}) and older engines, a narrow [[shear pin]] through the drive shaft and propeller hub transmits the power of the engine at normal loads. The pin is designed to [[Shear stress|shear]] when the propeller is put under a load that could damage the engine. After the pin is sheared the engine is unable to provide propulsive power to the boat until a new shear pin is fitted.<ref>{{Citation | last= Getchell | first= David | title= The Outboard Boater's Handbook | url= https://books.google.com/books?id=YpMTd7-Mb3sC | isbn= 978-0-07023053-8 | year= 1994 | publisher= McGraw Hill Professional }}</ref>

In larger and more modern engines, a rubber [[bushing (isolator)|bushing]] transmits the [[torque]] of the drive shaft to the propeller's hub. Under a damaging load the [[friction]] of the bushing in the hub is overcome and the rotating propeller slips on the shaft, preventing overloading of the engine's components.<ref>{{Citation | title= Admiralty Manual of Seamanship | url= https://books.google.com/books?id=jUdZlpHWShkC| isbn= 978-0-11772696-3 | publisher = Ministry of Defence (Navy)| place = Great Britain| year= 1995}}</ref> After such an event the rubber bushing may be damaged. If so, it may continue to transmit reduced power at low revolutions, but may provide no power, due to reduced friction, at high revolutions. Also, the rubber bushing may perish over time leading to its failure under loads below its designed failure load.

Whether a rubber bushing can be replaced or repaired depends upon the propeller; some cannot. Some can, but need special equipment to insert the oversized bushing for an [[interference fit]]. Others can be replaced easily. The "special equipment" usually consists of a funnel, a press and rubber lubricant (soap). If one does not have access to a lathe, an improvised funnel can be made from steel tube and car body filler; as the filler is only subject to compressive forces it is able to do a good job. Often, the bushing can be drawn into place with nothing more complex than a couple of nuts, washers and a threaded rod. A more serious problem with this type of propeller is a "frozen-on" spline bushing, which makes propeller removal impossible. In such cases the propeller must be heated in order to deliberately destroy the rubber insert. Once the propeller is removed, the splined tube can be cut away with a grinder and a new spline bushing is then required. To prevent a recurrence of the problem, the splines can be coated with anti-seize anti-corrosion compound.

In some modern propellers, a hard polymer insert called a ''drive sleeve'' replaces the rubber bushing. The [[rotating spline|splined]] or other non-circular cross section of the sleeve inserted between the shaft and propeller hub transmits the engine torque to the propeller, rather than friction. The polymer is weaker than the components of the propeller and engine so it fails before they do when the propeller is overloaded.<ref>{{cite patent | inventor1-last = Karls | inventor1-first = Michael | inventor2-last = Lindgren | inventor2-first = Daniel | pubdate = 1994-03-08 | issue-date = January 16, 1996 | title = Torsionally twisting propeller drive sleeve and adapter | country = US | number = 5484264 }}</ref> This fails completely under excessive load, but can easily be replaced.

===Weed hatches and rope cutters===
[[File:Right-handed 3-blade bronze propeller & stainless steel rope cutter.jpg|thumb|Bronze propeller & stainless steel rope cutter]]
Whereas the propeller on a large ship will be immersed in deep water and free of obstacles and [[flotsam]], [[yacht]]s, [[barge]]s and [[Launch (boat)|river boats]] often suffer propeller fouling by debris such as weed, ropes, cables, nets and plastics. British [[narrowboat]]s invariably have a '''weed hatch''' over the propeller, and once the narrowboat is stationary, the hatch may be opened to give access to the propeller, enabling debris to be cleared. Yachts and river boats rarely have weed hatches; instead they may fit a '''rope cutter''' that fits around the prop shaft and rotates with the propeller. These cutters clear the debris and obviate the need for divers to attend manually to the fouling. Several forms of rope cutters are available:<ref>{{Citation | title = Yachting World rope cutter test | date = 14 April 2015 | publisher = Yachting monthly | url = https://www.yachtingmonthly.com/gear/propeller-rope-cutter-test-30012}}</ref>
#A simple sharp edged disc that cuts like a razor;<ref>{{Citation | title = Simple disc cutters | publisher = ASAP Supplies | url = https://www.asap-supplies.com/propeller-drivetrain/rope-cutters}}</ref>
#A rotor with two or more projecting blades that slice against a fixed blade, cutting with a scissor action;<ref>{{Citation | title = Spurs scissor-action rope cutter | publisher = Spurs marine | url =  https://www.spursmarine.com/shaft-cutters/}}</ref><ref>{{Citation | title = Stripper scissor-action rope cutter | work = Rope stripper | url  = http://ropestripper.com/strippers.php}}</ref><ref>{{Citation | title = Gator cissor-action rope cutter | work = Prop protect | url = http://www.propprotect.com/}}</ref>
#A serrated rotor with a complex cutting edge made up of sharp edges and projections.<ref>{{Citation | title = Images of rope cutters | work = Bing | publisher = Microsoft | type = search | url = https://www.bing.com/images/search?q=yacht+rope+cutter&id=9A2642834983B967EF5261F4A95842DA499E0528&form=IQFRBA&first=1&scenario=ImageBasicHover}}</ref>

==Propeller variations==

A cleaver is a type of propeller design especially used for boat racing. Its leading edge is formed round, while the [[trailing edge]] is cut straight. It provides little bow lift, so that it can be used on boats that do not need much bow lift, for instance [[Hydroplane (boat)|hydroplanes]], that naturally have enough hydrodynamic bow lift. To compensate for the lack of bow lift, a [[hydrofoil]] may be installed on the lower unit.{{clarify|what lower unit|date=April 2024}}{{cn|date=April 2024}} Hydrofoils reduce bow lift and help to get a boat out of the hole and onto plane.{{clarify|what hole|date=April 2024}}{{cn|date=April 2024}}

==See also==
* {{annotated link|Screw-propelled vehicle}}

===Propeller characteristics===
* {{annotated link|Advance ratio}}
* {{annotated link|Axial fan design}}

===Propeller phenomena===
* {{annotated link|Propeller walk}}
* {{annotated link|Cavitation}}

===Other===
* {{annotated link|Azimuth thruster}}
** {{annotated link|Azipod}}
* {{annotated link|Bow thruster}}
* {{annotated link|Ducted propeller}}
** {{annotated link|Pump-jet}}
* {{annotated link|Folding propeller}}
* {{annotated link|Helix}}
* {{annotated link|Impeller}}
* {{annotated link|Kitchen rudder}}
* {{annotated link|Maneuvering thruster}}
* {{annotated link|Modular propeller}}
* {{annotated link|Paddle steamer}}
* {{annotated link|Pleuger rudder}}
* {{annotated link|Propulsor}}
* {{annotated link|Stern thruster}}
* {{annotated link|Supercavitating propeller}}
* {{annotated link|Variable-pitch propeller (marine)|Variable-pitch propeller}}
* {{annotated link|Cyclorotor|Voith-Schneider propeller}}
* {{annotated link|Wake-equalising duct}}

===Materials and manufacture===
{{ external media
|float=right
| video1 =[https://www.youtube.com/watch?v=l9UbnJlhrHA Construction of Wooden Propellers 1] [https://www.youtube.com/watch?v=OR3e8waXuWk 2] [https://www.youtube.com/watch?v=WS-XqOsxBHY 3], NASA Langley
}}
* {{annotated link|Balancing machine}}
* {{annotated link|Composite material}}

==Notes==
{{Notelist}}

==Citations==
<!--This article uses the Cite.php citation mechanism. If you would like more information on how to add references to this article, please see http://meta.wikimedia.org/wiki/Cite/Cite.php -->
{{Reflist|refs=

<ref name="PNA ChVII" >{{cite book|title=Principles of Naval Architecture |editor1-last=Comstock |editor1-first=John P. |publisher= Society of Naval Architects and Marine Engineers |date= 1967 |edition=Revised |last1=Todd |first1=F.H. |chapter=VII: Resistance and Propulsion |pages=397–462}}</ref>

}}

==External links==
{{Commons category|Propellers}}
* [http://www.titanic-titanic.com/titanic_propellers.shtml Titanic's Propellers]
* [http://www.heliciel.com/en/helice/calcul-helice-aile/theorie%20helice%20ailes.htm Theory calculation propellers and wings]: detailed article with blade element theory software application
* [https://books.google.com/books?id=wCcDAAAAMBAJ&pg=PA122 "What You Should Know About Propellers For Our Fighting Planes", November 1943, ''Popular Science''] extremely detailed article with numerous drawings and cutaway illustrations
* [https://web.archive.org/web/20110708180137/http://www.cogulus.com/archimedes/index.html Archimedes Screw History]: The story of marine propulsion
* [http://www.heliciel.com/en/Histoire-helice.htm propellers history]: The story of propellers
* [http://www.wartsila.com/products/marine-oil-gas/propulsors-gears Propulsors and gears]: Wartsila Marine Propellers
* [https://marinerspoint.in/propeller-drop/2020/02/ Propeller Drop] {{Webarchive|url=https://web.archive.org/web/20210420010629/https://marinerspoint.in/propeller-drop/2020/02/ |date=2021-04-20 }}: Measured by [[feeler gauge]]
* ''[[Scientific American]]'', [https://books.google.com/books?id=YIE9AQAAIAAJ&pg=PA232 "History of the Screw Propeller"], 1881, pp. 232

{{Authority control}}

[[Category:Propellers| ]]
[[Category:Watercraft components]]
[[Category:Swedish inventions]]