Editing Point of sail (section)

{{Short description|Direction of travel under sail relative to true wind direction over surface}}
[[File:Points of Sail.svg|300px|thumb|Points of sail and approximate [[apparent wind]] for a conventional sailboat on starboard [[Tack (sailing)|tack]]]]

A '''point of sail''' is a [[sailing]] craft's [[Course (navigation)|direction of travel]] under sail in relation to the true [[wind direction]] over the surface.

The principal points of sail roughly correspond to 45° segments of a circle, starting with 0° directly into the wind. For many sailing craft 45° on either side of the wind is a ''no-go'' zone, where a sail is unable to mobilize power from the wind. Sailing on a course as close to the wind as possible—approximately 45°—is termed ''beating'', a point of sail when the sails are ''close-hauled''. At 90° off the wind, a craft is on a ''beam reach''.  The point of sail between beating and a beam reach is called a ''close reach''. At 135° off the wind, a craft is on a ''broad reach''. At 180° off the wind (sailing in the same direction as the wind), a craft is ''running downwind''.<ref name=":2">{{Cite book|last=Rousmaniere, John|title=The Annapolis book of seamanship|others=Smith, Mark (Mark E.)|date=7 January 2014|isbn=978-1-4516-5019-8|edition=Fourth|location=New York|pages=47–9|oclc=862092350}}</ref>

A given point of sail (beating, close reach, beam reach, broad reach, and running downwind) is defined in reference to the [[#True wind versus apparent wind|true wind]]—the wind felt by a stationary observer. The [[Forces on sails|motive power]], and thus appropriate position of the sails, is determined by the [[apparent wind]]: the wind relative to an observer on the sailing craft.<ref name=":2" /><ref name="Kimball" /> The apparent wind is the combined effect of the [[velocity|velocities]] of the true wind and of the sailing craft.<ref name=":2" />

A sail with the airflow parallel to its surface, while angled into the apparent wind, [[Sailing#Lift predominance (wing mode)|acts substantially like a wing]] with lift as a force acting perpendicular to its surface. A sail with the apparent wind perpendicular to its surface, [[Sailing#Drag predominance (parachute mode)|acts substantially like a parachute]] with the drag on the sail as the dominant force. As a sailing craft transitions from close-hauled to running downwind, the lifting force decreases and the drag force increases. At the same time, the resistance to sidewards motion needed to keep the craft on course also decreases, along with the sideways tipping force.<ref name=":2" />

There is a zone of approximately 45° on either side of the true wind, where a sail cannot generate lift, called the "no-go zone". The angle encompassed by the no-go zone depends on the [[airfoil]] efficiency of the craft's sails and the craft's lateral resistance on the surface (from [[hydrofoil]]s, [[outrigger]]s, or a [[keel]] in the water, [[iceboating|runners on ice]], or [[landsailing|wheels on land]]). A craft remaining in its no-go zone will slow to a stop—it will be "in irons".<ref name="Kimball">
{{cite book
 | last = Kimball
 | first = John
 | title = Physics of Sailing
 | publisher = CRC Press
 | date = 2009
 | pages = 296
 | url = https://books.google.com/books?id=Xe_i23UL4sAC&q=Kimball+%22physics+of+sailing%22
 | isbn = 978-1466502666 }}</ref>