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Sailing employs the wind �acting on sails , wingsails or kites �to propel a craft on the surface of the water sailing ship , sailboat , windsurfer , or kitesurfer , on ice iceboat or on land land yacht over a chosen course , which is often part of a larger plan of navigation. A course defined with respect to the true wind direction is called a point of sail. Conventional sailing craft cannot derive power from sails on a point of sail that is too close to the wind.
On a given point of sail, the sailor adjusts the alignment of each sail with respect to the apparent wind direction as perceived on the craft to mobilize the power of the wind. The forces transmitted via the sails are resisted by forces from the hull , keel , and rudder of a sailing craft, by forces from skate runners of an iceboat, or by forces from wheels of a land sailing craft to allow steering the course.
In the 21st century , most sailing represents a form of recreation or sport. Recreational sailing or yachting can be divided into racing and cruising. Cruising can include extended offshore and ocean-crossing trips, coastal sailing within sight of land, and daysailing.
Until the mid of the 19th century, sailing ships were the primary means for marine commerce; this period is known as the Age of Sail. Throughout history sailing has been instrumental in the development of civilization, affording humanity greater mobility than travel overland, whether for trade, transport or warfare, and the capacity for fishing. The earliest representation of a ship under sail appears on a painted disc found in Kuwait dating between and BC.
They would go selling and teaching other civilizations how to build, sail, and navigate the ships. There were improvements in sails, masts and rigging ; improvements in marine navigation, including the cross tree and charts of both the sea and constellations, allowed more certainty in sea travel. From the 15th century onwards, European ships went further north, stayed longer on the Grand Banks and in the Gulf of St.
Lawrence , and eventually began to explore the Pacific Northwest and the Western Arctic. According to Jett, the Egyptians used a bipod mast to support a sail that allowed a reed craft to travel upriver with a following wind, as late as BC.
Such sails evolved into the square-sail rig that persisted up to the 19th century. 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 What Is Sailing Boat Used For 40 , rudder , underwater foils and other elements of the underbody of a sailboat, on ice by the runners of an ice boat , or on land by the wheels of a 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. 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. The term " velocity " refers both to speed and direction.
As applied to wind, apparent wind velocity V A 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 knots and wind angles in degrees. All sailing craft reach a constant forward velocity V B for a given true wind velocity V T and point of sail.
The craft's point of sail affects its velocity for a given true wind velocity. 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.
In order to act like an airfoil, the sail on a sailboat is sheeted further out as the course is further off the wind. In order to act like an airfoil, the sail on an iceboat is sheeted in for all three points of 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. 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 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. 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. 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.
Sails allow the 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 stalling with flow separation.
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. In addition to the sails used upwind, spinnakers provide area and curvature appropriate for sailing with separated flow on downwind points of sail, analogous to parachutes, which provide both lift and drag. Spinnaker cross-section trimmed for a broad reach showing transition from the boundary layer to separated flow where vortex shedding commences.
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. Wind shear occurs because of friction above a water surface slowing the flow of air.
Additionally, apparent wind direction moves aft with height above water, which may necessitate a corresponding twist in the shape of the sail to achieve attached flow with height. 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.
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. A sailing craft's ability to derive power from the wind depends on the point of sail it is on�the direction of travel under sail in relation to the true wind direction over the surface.
In points of sail that range from close-hauled to a broad reach, sails act substantially like a wing, with lift predominantly propelling the craft. In points of sail from a broad reach to down wind, sails act substantially like a parachute, with drag predominantly propelling the craft. For craft with little forward resistance ice boats and land yachts , this transition occurs further off the wind than for sailboats and sailing ships.
Wind direction for points of sail always refers to the true wind �the wind felt by a stationary observer. The apparent wind �the wind felt by an observer on a moving sailing craft�determines the motive power for sailing craft. The waves give an indication of the true wind direction. The pennant Canadian flag gives an indication of apparent wind direction. Reaching : the pennant is streaming slightly to the side as the sails are sheeted to align with the apparent wind.
Running : the wind is coming from behind the vessel; the sails are "wing and wing" to be at right angles to the apparent wind. True wind velocity V T combines with the sailing craft's velocity V B to be the apparent wind velocity V A , the air velocity experienced by instrumentation or crew on a moving sailing craft. Apparent wind velocity provides the motive power for the sails on any given point of sail. It varies from being the true wind velocity of a stopped craft in irons in the no-go zone to being faster than the true wind speed as the sailing craft's velocity adds to the true windspeed on a reach, to diminishing towards zero, as a sailing craft sails dead downwind.
Sailing craft A is close-hauled. Sailing craft B is on a beam reach. Sailing craft C is on a broad reach. Boat velocity in black generates an equal and opposite apparent wind component not shown , which adds to the true wind to become apparent wind. Apparent wind and forces on a sailboat. As the boat sails further from the wind, the apparent wind becomes smaller and the lateral component becomes less; boat speed is highest on the beam reach.
Apparent wind on an iceboat. As the iceboat sails further from the wind, the apparent wind increases slightly and the boat speed is highest on the broad reach. The sail is sheeted in for all three points of sail. The speed of sailboats through the water is limited by the resistance that results from hull drag in the water. Ice boats typically have the least resistance to forward motion of any sailing craft. On conventional sailboats, the sails are set to create lift for those points of sail where it's possible to align the leading edge of the sail with the apparent wind.
For a sailboat, point of sail affects lateral force significantly. The higher the boat points to the wind under sail, the stronger the lateral force, which requires resistance from a keel or other underwater foils, including daggerboard, centerboard, skeg, and rudder.
Lateral force also induces heeling in a sailboat, which requires resistance by weight of ballast from the crew or the boat itself and by the shape of the boat, especially with a catamaran. As the boat points off the wind, lateral force and the forces required to resist it become less important.
Wind and currents are important factors to plan on for both offshore and inshore sailing. Predicting the availability, strength and direction of the wind is key to using its power along the desired course. Ocean currents, tides and river currents may deflect a sailing vessel from its desired course.
If the desired course is within the no-go zone, then the sailing craft must follow a zig-zag route into the wind to reach its waypoint or destination. Downwind, certain high-performance sailing craft can reach the destination more quickly by following a zig-zag route on a series of broad reaches.
Negotiating obstructions or a channel may also require a change of direction with respect to the wind, necessitating changing of tack with the wind on the opposite side of the craft, from before. Changing tack is called tacking when the wind crosses over the bow of the craft as it turns and jibing or gybing if the wind passes over the stern.
Winds and oceanic currents are both the result of the sun powering their respective fluid media. Wind powers the sailing craft and the ocean bears the craft on its course, as currents may alter the course of a sailing vessel on the ocean or a river.
A sailing craft can sail on a course anywhere outside of its no-go zone. Because the lateral wind forces are highest on a sailing vessel, close-hauled and beating to windward, the resisting water forces around the vessel's keel, centerboard, rudder and other foils is also highest to mitigate leeway �the vessel sliding to leeward of its course.
Ice boats and land yachts minimize lateral motion with sidewise resistance from their blades or wheels. Tacking from starboard tack to port tack.
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