Sailing

Sailing employs the wind—acting on sails, wing sails or kites—to propel a craft on the surface of the water (sailing ship, sailboat, raft, 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. From prehistory until the second half of the 19th century, sailing craft were the primary means of maritime trade and transportation; exploration across the seas and oceans was reliant on sail for anything other than the shortest distances. Naval power in this period relied on sail to varying degrees, depending on the available technology, culminating in the gun-armed sailing warships of the Age of Sail. Sail was slowly replaced by steam as the method of propulsion for ships over the latter part of the 19th century, as steam technology gradually improved through many developmental steps. Steam allowed scheduled services that ran at higher average speeds than sailing vessels. Large improvements in fuel economy allowed steam to progressively outcompete sail, ultimately in all commercial situations, giving ship-owning investors a better return on capital. In the 21st century, most sailing represents a form of recreation or sport. Recreational sailing or yachting can be divided into racing and cruising.…

Sailing employs the wind—acting on sails, wing sails or kites—to propel a craft on the surface of the water (sailing ship, sailboat, raft, 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. From prehistory until the second half of the 19th century, sailing craft were the primary means of maritime trade and transportation; exploration across the seas and oceans was reliant on sail for anything other than the shortest distances. Naval power in this period relied on sail to varying degrees, depending on the available technology, culminating in the gun-armed sailing warships of the Age of Sail. Sail was slowly replaced by steam as the method of propulsion for ships over the latter part of the 19th century, as steam technology gradually improved through many developmental steps. Steam allowed scheduled services that ran at higher average speeds than sailing vessels. Large improvements in fuel economy allowed steam to progressively outcompete sail, ultimately in all commercial situations, giving ship-owning investors a better return on capital. 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. Sailing relies on the physics of sails as they derive power from the wind, generating both lift and drag. On a given course, the sails are set to an angle that optimizes the development of wind power, as determined by the apparent wind, which is the wind as sensed from a moving vessel. 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 which are steering the course. This combination of forces means it is possible to sail both upwind and downwind courses. The course with respect to the true wind direction (as would be indicated by a stationary flag) is called a point of sail. Conventional sailing craft cannot derive wind power on a course with a point of sail that is too close into the wind. ==History== Sailing was probably independently invented in at least two regions: Island Southeast Asia and the Mediterranean (including the Nile and other Middle Eastern rivers). The Caribbean, which meets the criteria for a "nursery" region for maritime experimentation, with warm waters and mutually visible islands, did not develop sailing technology but adopted it after the arrival of European explorers. The geography of the Nile, with a northward-flowing current and the prevailing wind blowing in the opposite direction, was conducive to early use of sail. The same wind-and-current combination on the Guayaquil River in Ecuador, together with the otherwise unexplained origin of the region's sailing craft, may provide a third site of independent invention. Throughout history, sailing was a key means of propulsion that gave greater mobility than overland travel. This greater mobility increased the capacity for exploration, trade, transport, warfare, and fishing, especially compared with overland options. Until the significant improvements in land transportation during the 19th century, if water transport was an option, it was faster, cheaper, and safer than making the same journey by land. This applied equally to sea crossings, coastal voyages, and river and lake travel. Examples of this include the large grain trade in the Mediterranean during the classical period. Cities such as Rome were totally reliant on sailing ships to deliver the large amounts of grain needed. It has been estimated that it cost less for a sailing ship of the Roman Empire to carry grain the length of the Mediterranean than to move the same amount 15 miles by road. Rome consumed about 150,000 tons of Egyptian grain each year over the first three centuries AD. A similar but more recent trade in coal was from mines situated close to the River Tyne to London. Beginning in the 14th century at the latest, it grew as the city's population increased. In 1795, 4,395 cargoes of coal were delivered to London. This would have needed a fleet of about 500 sailing colliers (making 8 or 9 trips a year). This quantity had doubled by 1839. (The first steam-powered collier was not launched until 1852, and sailing colliers continued working into the 20th century.) ==Navigation== ===Point of sail=== 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. The principal points of sail roughly correspond to 45° segments of a circle, starting with 0° directly into the wind. For many sailing craft, the arc spanning 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 "close-hauled". At 90° off the wind, a craft is on a "beam 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". 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 downwind, sails act substantially like a parachute, with drag predominantly propelling the craft. For craft with little forward resistance, such as 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. ====Effect on apparent wind==== True wind velocity (V T ) combines with the sailing craft's velocity (V B ) to give 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 wind speed on a reach. It diminishes towards zero for a craft sailing dead downwind. 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. Consequently, a sailboat experiences a wider range of apparent wind angles than does an ice boat, whose speed is typically great enough to have the apparent wind coming from a few degrees to one side of its course, necessitating sailing with the sail sheeted in for most points of sail. 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, the point of sail affects the 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 is resisted by the weight of ballast from the crew or the boat itself, and by the boat's shape, especially on a catamaran. As the boat points off the wind, lateral force and the forces required to resist it become less important. On ice boats, lateral forces are countered by the lateral resistance of the blades on the ice and by their spacing, which generally prevents heeling. ===Course under sail=== , showing wind direction at various latitudes in the Northern Hemisphere Wind and currents are important factors to plan for both offshore and inshore sailing. Predicting the wind's availability, strength, and direction is key to harnessing 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 relative to the wind, necessitating a change of tack with the wind on the opposite side of the craft. 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. ====Upwind==== A sailing craft can sail on a course anywhere outside of its no-go zone. If the next waypoint or destination is within the arc defined by the no-go zone from the craft's current position, then it must perform a series of tacking maneuvers to get there on a zigzag route, called beating to windward. The progress along that route is called the course made good; the speed between the starting and ending points of the route is called the speed made good and is calculated by the distance between the two points, divided by the travel time. The limiting line to the waypoint that allows the sailing vessel to leave it to leeward is called the layline. Whereas some Bermuda-rigged sailing yachts can sail as close as 30° to the wind, most 20th-Century square riggers are limited to 60° off the wind. Fore-and-aft rigs are designed to operate with the wind on either side, whereas square rigs and kites are designed to have the wind come from one side of the sail only. Because the lateral wind forces are highest when sailing close-hauled, the resisting water forces around the vessel's keel, centerboard, rudder, and other foils must also be highest to limit sideways motion or leeway. Ice boats and land yachts minimize lateral motion by using the resistance of their blades or wheels. =====Changing tack by tacking===== Tacking or coming about is a maneuver by which a sailing craft turns its bow into and through the wind (referred to as "the eye of the wind" ) so that the apparent wind changes from one side to the other, allowing progress on the opposite tack. The type of sailing rig dictates the procedures and constraints on achieving a tacking maneuver. Fore-and-aft rigs allow their sails to hang limp as they tack; square rigs must present the full frontal area of the sail to the wind, when changing from side to side; and windsurfers have flexibly pivoting and fully rotating masts that get flipped from side to side. ====Downwind==== , flying a sprit-mounted asymmetrical spinnaker on a broad reach A sailing craft can travel directly downwind only at a speed that is less than the wind speed. However, some sailing craft such as iceboats, sand yachts, and some high-performance sailboats can achieve a higher downwind velocity made good by traveling on a series of broad reaches, punctuated by jibes in between. It was explored by sailing vessels starting in 1975 and now extends to high-performance skiffs, catamarans, and foiling sailboats. Navigating a channel or downwind course among obstructions may necessitate changes in direction that require a change of tack, accomplished by jibing. =====Changing tack by jibing===== Jibing or gybing is a sailing maneuver by which a sailing craft turns its stern past the eye of the wind so that the apparent wind changes from one side to the other, allowing progress on the opposite tack. (When this is done when beating to windward, it is usually referred to as wearing.) This maneuver can be done on smaller boats by pulling the tiller towards yourself (the opposite side of the sail). As with tacking, the type of sailing rig dictates the procedures and constraints for jibing. Most fore-and-aft sails with booms, gaffs or sprits are unstable when the free end points into the eye of the wind and must be controlled to avoid a violent change to the other side. Lateen sails are often tacked by wearing, with the sheet being freed so that the sail rotates around the mast (avoiding the "bad tack"). Square rigs, as they present the full area of the sail to the wind from the rear, experience little change of operation when gybing, and may sometimes choose to wear rather than tack to avoid the risk of getting caught in stays. Windsurfers again have flexibly pivoting and fully rotating masts that get flipped from side to side. ==Trimming== dinghy trimmed for a reach with the sail aligned with the apparent wind and the crew providing moveable ballast to promote planing Trimming refers to adjusting the lines that control sails, including the sheets that control the angle of the sails with respect to the wind, the halyards that raise and tighten the sails, and the adjustments to the hull's resistance to heeling, yawing, or progress through the water. ====Reducing sail==== Reducing sail on square-rigged ships could be accomplished by exposing less of each sail, by tying it off higher up with reefing points. Additionally, as winds get stronger, sails can be furled or removed from the spars entirely until the vessel is surviving hurricane-force winds under "bare poles". On fore-and-aft rigged vessels, reducing sail may involve furling the jib and reefing or partially lowering the mainsail, that is, reducing the area of a sail without actually changing it for a smaller sail. This results both in a reduced sail area but also in a lower centre of effort from the sails, reducing the heeling moment and keeping the boat more upright. There are three common methods of reefing the mainsail: Slab reefing, which involves lowering the sail by about one-quarter to one-third of its full length and tightening the lower part of the sail using an outhaul or a pre-loaded reef line through a cringle at the new clew, and hook through a cringle at the new tack. In-boom roller-reefing, with a horizontal foil inside the boom. This method allows for standard- or full-length horizontal battens. In-mast (or on-mast) roller-reefing. This method rolls the sail up around a vertical foil, either inside a slot in the mast or affixed to its outside. It requires a mainsail with either no battens or newly developed vertical battens. ===Hull=== Hull trim has three aspects, each tied to an axis of rotation, they are controlling: Heeling (rotation about the longitudinal axis or leaning to either port or starboard) Helm force (rotation about the vertical axis) Hull drag (rotation about the horizontal axis amidships ) Each is a reaction to forces on sails and is achieved either by weight distribution or by management of the center of force of the underwater foils (keel, daggerboard, etc.), compared with the center of force on the sails. ==Other aspects of seamanship== Seamanship encompasses all aspects of taking a sailing vessel in and out of port, navigating it to its destination, and securing it at anchor or alongside a dock. Important aspects of seamanship include using a common language aboard a sailing craft and managing the lines that control the sails and rigging. ==Sail physics== 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, 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 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—the force component in the direction of the apparent wind—and lift—the force component 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 provides 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 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. 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. As an iceboat sails further from the wind, the apparent wind increases slightly and the boat speed is highest on the broad reach. To act like an airfoil, the sail on an iceboat is sheeted in for all three points of sail. ===Lift and drag on sails=== 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. 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 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 the 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 characteristic coefficients of lift and 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. ====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. 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, 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. ====Wind variation with height and time==== Wind speed increases with height above the surface; at the same time, it may vary over short periods 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. 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 wind at 3 m above the water would be approximately at above the water. In hurricane-force winds with at the surface the speed at would be 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 twist in the shape of the sail to achieve attached flow with height. Gusts may be predicted using the same value that serves as the exponent for wind shear, acting 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, suggests the degree to which a sailing craft must adjust its sail angle to wind gusts on a given course. Drag from its form is described by a prismatic coefficient, C p = displaced volume of the vessel divided by waterline length times maximum displaced section area—the maximum value of C p = 1.0 being for a constant displace cross section area, as would be found on a barge. For modern sailboats, values of 0.53 ≤ C p ≤ 0.6 are likely because of the tapered shape of the submerged hull towards both ends. Reducing interior volume allows creating a finer hull with less drag. Because a keel or other underwater foil produces lift, it also produces drag, which increases as the boat heels. The wetted area of the hull affects the total friction between the water and the hull's surface, creating another component of drag. ==See also== Dinghy racing High-performance sailing Land sailing Racing Rules of Sailing Sailing at the Summer Olympics Single-handed sailing ==Notes== ==References== ==Bibliography== "Transportation and Maps" in Virtual Vault , an online exhibition of Canadian historical art at Library and Archives Canada Rousmaniere, John, The Annapolis Book of Seamanship, Simon & Schuster, 1999 Chapman Book of Piloting (various contributors), Hearst Corporation, 1999 Herreshoff, Halsey (consulting editor), The Sailor's Handbook, Little Brown and Company, 1983 Seidman, David, The Complete Sailor, International Marine, 1995 ==Further reading==

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