Editing Sailing (section)

==Sail physics==
{{Main|Forces on sails}}
[[File:Points of sail--close-hauled (right) and down wind (left).jpg|thumb|right|Aerodynamic force components for two points of sail. <br />''Left-hand boat'': Down wind with detached airflow like a ''parachute''— predominant ''drag'' component propels the boat with little heeling moment. <br />''Right-hand boat'': Up wind (close-hauled) with attached airflow like a ''wing''—predominant ''lift'' component both propels the boat and contributes to heel.]]
The physics of sailing arises from a balance of forces between the wind powering the sailing craft as it passes over its sails and the resistance by the sailing craft against being blown off course, which is provided in the water by the [[keel]], [[rudder]], [[Foil (fluid mechanics)|underwater foils]] and other elements of the underbody of a sailboat, on ice by the runners of an [[iceboat]], or on land by the wheels of a [[Land sailing|sail-powered land vehicle]].

Forces on sails depend on wind speed and direction and the speed and direction of the craft. The speed of the craft at a given point of sail contributes to the "[[apparent wind]]"—the wind speed and direction as measured on the moving craft. The apparent wind on the sail creates a total aerodynamic force, which may be resolved into [[Drag (physics)|drag]]—the force component in the direction of the apparent wind—and [[Lift (force)|lift]]—the force component [[normal (geometry)|normal]] (90°) to the apparent wind. Depending on the alignment of the sail with the apparent wind (''[[angle of attack]]''), lift or drag may be the predominant propulsive component.  Depending on the angle of attack of a set of sails with respect to the apparent wind, each sail is providing motive force to the sailing craft either from lift-dominant attached flow or drag-dominant separated flow. Additionally, sails may interact with one another to create forces that are different from the sum of the individual contributions of each sail, when used alone.

===Apparent wind velocity===
The term "[[velocity]]" refers both to speed and direction. As applied to wind, ''apparent wind velocity'' ('''V<sub>A</sub>''') is the air velocity acting upon the leading edge of the most forward sail or as experienced by instrumentation or crew on a moving sailing craft. In nautical terminology, wind speeds are normally expressed in [[Knot (unit)|knots]] and wind angles in [[degree (angle)|degree]]s. All sailing craft reach a constant ''forward velocity'' ('''V<sub>B</sub>''') for a given ''true wind velocity'' ('''V<sub>T</sub>''') and ''point of sail''. The craft's point of sail affects its velocity for a given true wind velocity. Conventional sailing craft cannot derive power from the wind in a "no-go" zone that is approximately 40° to 50° away from the true wind, depending on the craft. Likewise, the directly downwind speed of all conventional sailing craft is limited to the true wind speed. As a sailboat sails further from the wind, the apparent wind becomes smaller and the lateral component becomes less; boat speed is highest on the beam reach. To act like an airfoil, the sail on a sailboat is sheeted further out as the course is further off the wind.<ref name=Jobson>{{cite book | last = Jobson | first = Gary | title = Championship Tactics: How Anyone Can Sail Faster, Smarter, and Win Races | publisher = St. Martin's Press | location = New York | year = 1990 | isbn = 978-0-312-04278-3 | pages = [https://archive.org/details/championshiptact00jobs/page/323 323] | url = https://archive.org/details/championshiptact00jobs/page/323 }}</ref> As an iceboat sails further from the wind, the apparent wind increases slightly and the boat speed is highest on the broad reach. In order to act like an airfoil, the sail on an iceboat is sheeted in for all three points of sail.<ref name = Kimball>
{{cite book
 | last = Kimball
 | first = John
 | title = Physics of Sailing
 | publisher = CRC Press
 | date = 2009
 | pages = 296
 | isbn = 978-1466502666 }}</ref>

===Lift and drag on sails===
{{Main|Lift (force)|Lift-induced drag}}
[[File:Sail angles of attack and resulting flow patterns.jpg|thumb|right|Sail angles of attack (α) and resulting (idealized) flow patterns for attached flow, maximum lift, and stalled for a hypothetical sail. The stagnation streamlines (red) delineate air passing to the leeward side (top) from that passing to the windward (bottom) side of the sail.]]

''Lift'' on a sail, acting as an [[airfoil]], occurs in a direction ''perpendicular'' to the incident airstream (the apparent wind velocity for the headsail) and is a result of pressure differences between the windward and leeward surfaces and depends on the angle of attack, sail shape, air density, and speed of the apparent wind. The lift force results from the average pressure on the windward surface of the sail being higher than the average pressure on the leeward side.<ref>{{Citation |first=G.K. |last=Batchelor |author-link=George Batchelor |title=An Introduction to Fluid Dynamics |year=1967 |publisher=Cambridge University Press |isbn=978-0-521-66396-0 |pages=14–15 }}</ref> These pressure differences arise in conjunction with the curved airflow. As air follows a curved path along the windward side of a sail, there is a pressure [[gradient]] perpendicular to the flow direction with higher pressure on the outside of the curve and lower pressure on the inside. To generate lift, a sail must present an "[[angle of attack]]" between the [[Chord (aeronautics)|chord line]] of the sail and the apparent wind velocity. The angle of attack is a function of both the craft's point of sail and how the sail is adjusted with respect to the apparent wind.<ref>Klaus Weltner ''A comparison of explanations of the aerodynamic lifting force'' Am. J. Phys. 55(1), January 1987 pg 52</ref>

As the lift generated by a sail increases, so does [[lift-induced drag]], which together with [[parasitic drag]] constitute total ''drag'', which acts in a direction ''parallel'' to the incident airstream. This occurs as the angle of attack increases with sail trim or change of course and causes the [[lift coefficient]] to increase up to the point of [[aerodynamic stall]] along with the lift-induced [[drag coefficient]]. At the onset of stall, lift is abruptly decreased, as is lift-induced drag. Sails with the apparent wind behind them (especially going downwind) operate in a stalled condition.<ref name = Clancy>
{{Citation
 | last = Clancy
 | first = L.J.
 | title = Aerodynamics
 | place = London
 | publisher = Pitman Publishing Limited
 | year = 1975
 | pages = 638
 | isbn = 978-0-273-01120-0 }}</ref>

Lift and drag are components of the total aerodynamic force on sail, which are resisted by forces in the water (for a boat) or on the traveled surface (for an iceboat or land sailing craft). Sails act in two basic modes; under the ''lift-predominant'' mode, the sail behaves in a manner analogous to a ''wing'' with airflow attached to both surfaces; under the ''drag-predominant'' mode, the sail acts in a manner analogous to a ''parachute'' with airflow in detached flow, eddying around the sail.

====Lift predominance (wing mode)====
Sails allow progress of a sailing craft to windward, thanks to their ability to generate lift (and the craft's ability to resist the lateral forces that result). Each sail configuration has a characteristic coefficient of lift and attendant coefficient of drag, which can be determined experimentally and calculated theoretically. Sailing craft orient their sails with a favorable angle of attack between the entry point of the sail and the apparent wind even as their course changes. The ability to generate lift is limited by sailing too close to the wind when no effective angle of attack is available to generate lift (causing luffing) and sailing sufficiently off the wind that the sail cannot be oriented at a favorable angle of attack to prevent the sail from [[Stall (fluid dynamics)|stalling]] with [[flow separation]].

====Drag predominance (parachute mode)====
When sailing craft are on a course where the angle between the sail and the apparent wind (the angle of attack) exceeds the point of maximum lift, separation of flow occurs.<ref>{{Citation
 | last1 = Collie
 | first1 = S. J.
 | last2 = Jackson
 | first2 = P. S.
 | last3 = Jackson
 | first3 = M.
 | last4 = Gerritsen
 | first5 = J.B.
 | last5 = Fallow
 | title = Two-dimensional CFD-based parametric analysis of down-wind sail designs
 | journal = The University of Auckland
 | date = 2006
 | url = http://syr.stanford.edu/RINA_Steve.pdf
 | access-date = 4 April 2015
 | archive-date = 28 July 2010
 | archive-url = https://web.archive.org/web/20100728123723/http://syr.stanford.edu/RINA_Steve.pdf
 | url-status = dead
 }}</ref> Drag increases and lift decreases with increasing angle of attack as the separation becomes progressively pronounced until the sail is perpendicular to the apparent wind, when lift becomes negligible and drag predominates. In addition to the sails used upwind, [[spinnaker]]s provide area and curvature appropriate for sailing with separated flow on downwind points of sail, analogous to parachutes, which provide both lift and drag.<ref name=Textor>
{{cite book
 | last = Textor
 | first = Ken
 | title = The New Book of Sail Trim
 | publisher = Sheridan House, Inc.
 | date = 1995
 | url = https://books.google.com/books?id=2JIbS0c1XPwC&pg=PA50
 | page = 50
 | isbn = 978-0-924486-81-4 }}
</ref>
{{center|
;Downwind sailing with a spinnaker
}}
<gallery mode="packed" heights="175px">
File:Sailboat on broad reach with spinnaker.jpg|Spinnaker set for a broad reach, generating both lift, with separated flow, and drag.
File:Spinnaker trimmed for broad reach.jpg|Spinnaker cross-section trimmed for a broad reach showing transition from boundary layer to separated flow where vortex shedding commences.
File:Amante Choate 48 photo D Ramey Logan.jpg|Symmetric spinnaker while running downwind, primarily generating drag.
File:Symmetrical spinnaker with following apparent wind.jpg|Symmetric spinnaker cross-section with following apparent wind, showing vortex shedding.
</gallery>

====Wind variation with height and time====
{{Further|Wind gradient#Sailing}}
Wind speed increases with height above the surface; at the same time, wind speed may vary over short periods of time as gusts.

[[Wind shear]] affects sailing craft in motion by presenting a different wind speed and direction at different heights along the [[mast (sailing)|mast]]. Wind shear occurs because of friction above a water surface slowing the flow of air.<ref>
{{Citation
 | last1 = Deacon
 | first1 = E. L.
 | last2 = Sheppard
 | first2 = P. A.
 | last3 = Webb
 | first3 = E. K.
 | title = Wind Profiles over the Sea and the Drag at the Sea Surface
 | journal = Australian Journal of Physics
 | volume = 9
 | issue = 4
 | pages = 511
 | date = December 1956
 | doi = 10.1071/PH560511 | bibcode = 1956AuJPh...9..511D
 | doi-access = free
 }}
</ref> The ratio of wind at the surface to wind at a height above the surface varies by a power law with an exponent of 0.11-0.13 over the ocean. This means that a {{cvt|5|m/s|kn|adj=on}} wind at 3 m above the water would be approximately {{cvt|6|m/s|kn}} at {{cvt|15|m|ft|sigfig=1}} above the water. In hurricane-force winds with {{cvt|40|m/s|kn}} at the surface the speed at {{cvt|15|m|ft|sigfig=1}} would be {{cvt|49|m/s|kn}}<ref>
{{cite web
 |last        = Hsu
 |first       = S. A.
 |title       = Measurements of Overwater Gust Factor From NDBC Buoys During Hurricanes
 |publisher   = Louisiana State University
 |date        = January 2006
 |url         = http://www.nwas.org/ej/pdf/2006-EJ2.pdf
 |access-date  = 19 March 2015
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20160304190331/http://www.nwas.org/ej/pdf/2006-EJ2.pdf
 |archive-date = 4 March 2016
}}
</ref> This suggests that sails that reach higher above the surface can be subject to stronger wind forces that move the centre of effort on them higher above the surface and increase the heeling moment. Additionally, apparent wind direction moves aft with height above water, which may necessitate a corresponding [[Sail twist|twist in the shape of the sail]] to achieve attached flow with height.<ref>
{{Citation
 | first1 = A.
 | last1 = Zasso
 | first2 = F.
 | last2 = Fossati
 | first3 = I.
 | last3 = Viola
 | title = Twisted flow wind tunnel design for yacht aerodynamic studies
 | series = 4th European and African Conference on Wind Engineering
 | year = 2005
 | pages = 350–351
 | place = Prague
 | url = http://www.research.ed.ac.uk/portal/files/17535106/Zasso_etal_EACWE4.pdf }}</ref>

Gusts may be predicted by the same value that serves as an exponent for wind shear, serving as a gust factor. So, one can expect gusts to be about 1.5 times stronger than the prevailing wind speed (a 10-knot wind might gust up to 15 knots). This, combined with changes in wind direction suggest the degree to which a sailing craft must adjust sail angle to wind gusts on a given course.<ref name=Hsu>
{{cite web
 | last = Hsu
 | first = S. A.
 | title = An Overwater Relationship Between the Gust Factor and the Exponent of Power-Law Wind Profile
 | work = Mariners Weather Log
 | volume = 52
 | issue = 1
 | publisher = National Oceanic and Atmospheric Administration
 | date = April 2008
 | url = http://www.vos.noaa.gov/MWL/apr_08/overwater.shtml
 | access-date = 19 March 2015 }}
</ref>