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==== Apparent-wind-angle limit ====
==== Apparent-wind-angle limit ====
[[File:Speed ratio graph for high-performance sailing craft.png|thumb|Drag angle (''β'' ≈ Apparent wind angle) for high-performance sailing craft as a ratio of V<sub>B</sub> to V<sub>TB</sub> at a course of 135° off the wind.]]
[[File:Speed ratio graph for high-performance sailing craft.png|thumb|Total drag angle (''β'' ≈ Apparent wind angle) for high-performance sailing craft as a ratio of V<sub>B</sub> to V<sub>TB</sub> at a course of 135° off the wind.]]
Given an ideal circumstance of a frictionless surface and an airfoil that can develop power, there is no theoretical limit to how fast a sailing craft can travel off the wind as the apparent wind angle becomes ever smaller. In reality, both sail efficiency and friction provide an upper limit. Speed is determined by the ratio of power developed by the sail over power lost through various forms of drag (e.g. surface drag and aerodynamic drag). Ideally a smaller sail is better, as speeds increase. Unfortunately, a small sail diminishes the ability for a craft—even an iceboat—to accelerate to speeds faster than the wind. The principal limit to speed in high-performance sailing craft is form drag. Efforts to overcome this limit is evident in the streamlined hulls of high-performance iceboats and the improvements in drag reduction on planing dinghies. A fast iceboat, can achieve an apparent wind of 7.5° and a speed of six times the true wind speed on a course that is 135° off the wind. Bethwaite suggests this might be a practical limit for a craft powered by sails.<ref name=":0" />
Given an ideal circumstance of a frictionless surface and an airfoil that can develop power, there is no theoretical limit to how fast a sailing craft can travel off the wind as the apparent wind angle becomes ever smaller. In reality, both sail efficiency and friction provide an upper limit. Speed is determined by the ratio of power developed by the sail over power lost through various forms of drag (e.g. surface drag and aerodynamic drag). Ideally a smaller sail is better, as speeds increase. Unfortunately, a small sail diminishes the ability for a craft—even an iceboat—to accelerate to speeds faster than the wind. The principal limit to speed in high-performance sailing craft is form drag. Efforts to overcome this limit is evident in the streamlined hulls of high-performance iceboats and the improvements in drag reduction on planing dinghies. A fast iceboat, can achieve an apparent wind of 7.5° and a speed of six times the true wind speed on a course that is 135° off the wind. Bethwaite suggests this might be a practical limit for a craft powered by sails.<ref name=":0" />


Revision as of 18:24, 6 August 2020

18ft Skiff in Kiel Harbor

High-performance sailing is achieved with low forward surface resistance—encountered by catamarans, sailing hydrofoils, iceboats or land sailing craft—as the sailing craft obtains motive power with its sails or aerofoils at speeds that are often faster than the wind on both upwind and downwind points of sail. Faster-than-the-wind sailing means that the apparent wind angle experienced on the moving craft is always ahead of the sail.[1] This has generated a new concept of sailing, called "apparent wind sailing", which entails a new skill set for its practitioners, including tacking on downwind points of sail.[2]

History

Bethwaite offers the following chronology of key advances in sailing technology that provided the essential elements of high-performance sailing:[2]

  • 1900s: Moveable ballast and planing hulls were emerging.
  • 1960s: Flexible masts, sail-shaping controls, and knowledge of exploiting wind shifts in racing were developed.
  • 1970s: Powerful rigs, including wingsails, offset by the crew trapezing from racks or wings allowed sailing faster than the wind and downwind tacking.

High-performance sailing craft

Oracle sailing hydrofoil catamaran with wingsail in the 2013 America's Cup

High-performance watercraft that can exceed the velocity of the true wind include sailing catamarans and foiling sailing craft. Ice boats and land-sailing craft are often able to do so. There are also wind-powered vehicles that can travel faster than the wind, such as the rotor-powered Blackbird, which are outside the scope of this article.

Skiffs

Starting ca. 1975, 18ft Skiffs were sailing downwind faster than the speed of the wind. This meant that they had to tack, rather than jibe to change tacks.[3] Other skiffs that can sail faster than the wind include the 29er, and 49er, both designed by Julian Bethwaite.[4]

Multihulls

In 2013, a new class of catamaran was announced for the America's Cup which can achieve well in excess of double the speed of the wind.[5] The catamarans used for the 2013 America's Cup were expected to sail upwind at 1.2 times the speed of the true wind, and downwind at 1.6 times the speed of the true wind.[6][7][8] They proved to be faster, averaging about 1.8 times the speed of the wind with peaks slightly over 2.0.[9]

The Extreme 40 catamaran can sail at 35 knots (65 km/h; 40 mph) in 20–25-knot (37–46 km/h; 23–29 mph) winds.[10] The high-performance International C-Class Catamaran can sail at twice the speed of the wind.[11]

Hydrofoils

There are many varieties of sailing hydrofoils. Monohull examples include the International Moth, Laser, and AC75. America's Cup catamarans have used hydrofoils, since 2013.[12] Other foiling catamarans include, A-Class,[13] C-Class,[14] Nacra 17, Nacra F20,[15] and GC32.[16]

In 2009, hydrofoil trimaran, Hydroptère, set the world speed sailing record on water at 50.17 knots (92.9 km/h), sailing at about 1.7 times the speed of the wind.[17][18] In late 2012, Vestas Sailrocket 2 achieved a new outright world speed record of 65.45 knots (121.2 km/h) on water, at around 2.5 times the speed of the wind.[19]

Iceboats

Iceboats on the Hudson River of New York in the second half of the 19th century were as long as 69 feet (21 m) and sailed as fast as 107 miles per hour (172 km/h), a record exceeding any other conveyance in 1885, set by the Icicle. Iceboats designs dating from the mid 20th century onwards typically consist of a triangular or cross-shaped frame, supported by three skate blades called "runners", with the steering runner in front. Runners are made of iron or steel with sharpened edges, which hold onto the ice, preventing slippage sideways from the lateral force of the wind in the sails, as they develop propulsive lift. Given their low forward resistance, iceboats can typically sail at five to six times the speed of the wind.[3] Classic iceboats and Skeeters have reached speeds of 100–150 miles per hour (160–240 km/h). Record speeds are for a Skeeter: Das Boot, 155.9 miles per hour (250.9 km/h)[20] and for a classic iceboat: Debutaunte, 143 miles per hour (230 km/h).[21][22]

Land-sailing craft

By sailing downwind at 135° off the wind, a land-sailing craft can sail much faster than the wind.[23] The velocity made good downwind is often over twice as fast compared to the same craft sailing directly downwind.[23] In 2009, the world land speed record for a wind-powered vehicle was set by the sailing craft, Greenbird, sailing at about three times the speed of the wind[24] with a recorded top speed of 202.9 kilometres per hour (126.1 mph).[25]

Other high-performance sailing craft

Apparent wind sailing

Whereas iceboats have been able to exceed the speed of the wind, both upwind and downwind for a century, this capability only became routine with the evolution of 18ft Skiffs in the third quarter of the 20th century when their speed tripled from that of the 1950s. Sailing craft that sail faster than the speed of the wind, downwind as well as upwind, are capable of tacking downwind because the apparent wind is always ahead of the mast. This lead to the concept of "apparent wind sailing".[3]

Apparent wind

Main article: Apparent wind
Apparent wind, VA, 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 (C). Because of a small β, the sail is sheeted in for all three points of sail.

Apparent wind is the wind velocity (direction and speed), VA, measured aboard a moving sailing craft; it is the net effect (vector sum) of the boat wind, VB—the air flow over the craft induced by its speed over the earth (equal to in magnitude, but opposite in direction to the craft's speed)—and the true wind, VT. The apparent wind measured aboard a craft under power, traveling in calm conditions, VT = 0 knots, would come from directly ahead and at a speed that is the same as the boat speed over the bottom (VA = VB + 0 = VB). If the craft travels at VB = 10 knots with a tailwind of VT = -5 knots, it experiences an apparent wind of VA = 5 knots directly on the bow (VA = VB + VT = 10 − 5). The apparent wind experienced by a stationary craft is the true wind speed. If a craft proceeds at 90° to a true wind of VT = 10 knots, itself traveling at a speed of VB = 10 knots, then the apparent wind angle would be 45° off the bow and the apparent wind speed would be about 14 knots, calculated as: square root [(VB )2 + (VT )2] = square root [102 + 102] = 14.14. As the craft becomes faster than the true wind, the apparent wind is always ahead of the sail.[26]

When drag angle of the hull is negligible, the formulas for calculating VA and β are:[27]

  • VA = square root {[VT cos (90° – true wind angle)]2 + [VT sin (90° – true wind angle) + VB]2}
  • β = 90° – arctan {[VT sin (90° – true wind angle) + VB] / [VT cos (90° – true wind angle)]}

Sail power

Main article: Forces on sails

A sail generates lift with a forward propulsive component and a sideways component, based on an optimum angle of attack that is constrained by the apparent wind, VA, being forward of and approximately aligned with the sail.[28][29]

  • Decomposition of wind force acting on a sail, generating lift. (FT = Total aerodynamic force, L = Lift D =Drag, α = angle of attack)
    Decomposition of wind force acting on a sail, generating lift.
    (FT = Total aerodynamic force, L = Lift
    D =Drag, α = angle of attack)
  • Conversion of lift into propulsion. (FR = Propulsive force, FLAT = Sideways force)
    Conversion of lift into propulsion.
    (FR = Propulsive force, FLAT = Sideways force)

Beta theorem

β is the apparent wind angle from course over the water.

Garrett introduces the beta theorem (or course theorem) as a way to understand how apparent wind angle results from the interplay between the driving force from the wind and the resisting force from the water (or hard surface), the result of the net effect of two counteracting foils, the sail in the air and the keel in the water. When one resolves the ratio of lift to drag for each in its medium, the resulting motion of the sailing craft resolves to an angle, beta (β), between the apparent wind and the course over the water. The hull (below the water) and the sailing rig (above the water) each have drag angle with respect to the medium flowing past them (water or air), they are λ and αm in the accompanying diagram. The sum of those two drag angles are equal to β, the angle between the apparent wind and the course sailed (β = λ + αm). This theorem applies for every point of sail. A small β denotes high efficiency and a potential for high speed.[26] As forward velocity increases, β becomes smaller; on sailing craft with effective underwater foils the drag angle of the hull, λ, becomes smaller with increased speed, it becomes negligible with hydrofoiling craft, and essentially nonexistent for ice boats and land sailing craft.[30]

Apparent-wind-angle limit

Total drag angle (β ≈ Apparent wind angle) for high-performance sailing craft as a ratio of VB to VTB at a course of 135° off the wind.

Given an ideal circumstance of a frictionless surface and an airfoil that can develop power, there is no theoretical limit to how fast a sailing craft can travel off the wind as the apparent wind angle becomes ever smaller. In reality, both sail efficiency and friction provide an upper limit. Speed is determined by the ratio of power developed by the sail over power lost through various forms of drag (e.g. surface drag and aerodynamic drag). Ideally a smaller sail is better, as speeds increase. Unfortunately, a small sail diminishes the ability for a craft—even an iceboat—to accelerate to speeds faster than the wind. The principal limit to speed in high-performance sailing craft is form drag. Efforts to overcome this limit is evident in the streamlined hulls of high-performance iceboats and the improvements in drag reduction on planing dinghies. A fast iceboat, can achieve an apparent wind of 7.5° and a speed of six times the true wind speed on a course that is 135° off the wind. Bethwaite suggests this might be a practical limit for a craft powered by sails.[3]

Points of sail

Main article: Point of sail

The points of sail at which high-performance sailing craft can achieve highest speeds and achieve the best speed made good over a course span between a beam reach (90° to the true wind) and a broad reach (about 135° away from the true wind). According to Bethwaite, having made comparative measurements in a true wind of 15 knots (28 km/h; 17 mph), a displacement Soling can achieve speeds slightly higher than the true wind and sail 30° off the apparent wind, whereas a planing 18-foot Skiff achieves speeds of almost 30 knots (56 km/h; 35 mph) at an apparent wind of 20° and an iceboat can achieve 67 knots (124 km/h; 77 mph) at an apparent wind of 8°.[2]

Under apparent wind sailing, the objective is to keep the apparent wind as far forward, as practical, for the course sailed in order to attain the fastest course made good to the objective. This requires a craft that can exceed the true windspeed, both upwind and downwind; this allows the apparent wind to remain well ahead of the sail on the courses sailed, the fastest of which are reaches. To be avoided is heading too far downwind, where the apparent wind moves behind the sail and the speed drops below the true windspeed as the course trends from a broad reach to running square (dead down wind).[3]

Upwind

Depending on the craft sailed, the course made good into the wind may trend away from its closest point into the wind in order to allow the craft to sail at optimum speed.[3] Bethwaite explains that high-speed sailing demands independent action of both the tiller and the mainsheet, whereby the person at the helm avoids responding to gusts and, instead, eased the mainsheet as needed, thus increasing the boat's velocity made good over the previous technique of pointing the craft more into the wind.[4]

Off the wind

Bethwaite explains that a new reaction to gusts was needed when sailing off the true wind at speeds faster than the wind (with the apparent wind forward of the sail). Whereas a traditional sailor might reflexively steer into the apparent wind in a gust, the correct response is to veer away from the gust, heading more downwind. This has the doubly beneficial effect of relieving the heeling force of the gust and allowing the craft to sail yet faster off the wind.[4]

See also

References

  1. ^ Jobson, Gary (1990). Championship Tactics: How Anyone Can Sail Faster, Smarter, and Win Races. New York: St. Martin's Press. pp. 323. ISBN 0-312-04278-7.
  2. ^ a b c Bethwaite, Frank (2007). High Performance Sailing. Adlard Coles Nautical. ISBN 978-0-7136-6704-2.
  3. ^ a b c d e f Bethwaite, Frank (2008). Higher performance sailing. London: Adlard Coles Nautical. ISBN 978-1-4729-0131-6. OCLC 854680844.
  4. ^ a b c Bethwaite, Frank (2013-05-12). Fast Handling Technique. New York: A&C Black. pp. 5–6. ISBN 978-1-4081-7860-7.
  5. ^ How yachts go faster than the wind Gray, R. The Telegraph 26 September 2013
  6. ^ "AC34 Multihull Class Rule Concept Document" (PDF). 34th America's Cup. Retrieved 2010-09-14.
  7. ^ "New high performance yachts for 34th America's Cup" (PDF). 34th America's Cup. 2 July 2010. Retrieved 2010-09-14.
  8. ^ The monohull concept for the 34th America's Cup called for a design that would achieve 1.0 times true wind speed upwind and 1.4 times downwind, see "AC34 Monohull Class Rule Concept Document" (PDF). 34th America's Cup. Retrieved 2010-09-14.
  9. ^ "Emirates Team New Zealand gets leg up on ORACLE TEAM USA". 2012-13 America's Cup Event Authority. 7 September 2013. Archived from the original on 21 September 2013. Retrieved 8 September 2013.
  10. ^ "About eXtreme 40". eXtreme40. Archived from the original on 2010-08-12. Retrieved 2010-08-25.
  11. ^ "The Winged World of C Cats". Sail Magazine. Archived from the original on March 14, 2010. Retrieved 2010-08-25.
  12. ^ Clarey, Christopher (2016-06-09). "Sailing Into America's Cup History in Chicago". The New York Times. ISSN 0362-4331. Retrieved 2020-08-03.
  13. ^ Griffits, Bob (February 11, 2014). "Worlds @Takapuna: Day 1, Report by Bob Griffits | International A-Division Catamaran Association". www.a-cat.org. Retrieved 2020-08-02.{{cite web}}: CS1 maint: url-status (link)
  14. ^ Block, Alan (22 September 2013). "Foiling 'Little Cup' Cats set for prestigious C-Class Championship Trophy". www.yachtsandyachting.com. Retrieved 2020-08-02.{{cite web}}: CS1 maint: url-status (link)
  15. ^ McArthur, Bruce (2020). "Nacra 20 sailboat". sailboatdata.com. Archived from the original on 27 July 2020. Retrieved 27 July 2020.
  16. ^ "GC32s to replace Extreme 40s". www.extremesailingseries.com. Retrieved 2020-08-02.
  17. ^ The 500-meter record was 51.36 knots (95.12 km/h; 59.10 mph), achieved in 30-knot (56 km/h; 35 mph) winds by Hydroptère, a hydrofoil trimaran, see "Hydroptère World Records". World Sailing Speed Record Council. September 23, 2009. Retrieved 2010-08-25.
  18. ^ "Official web site of l'Hydroptère". Retrieved 2010-08-25.
  19. ^ "500 Metre Records". World Sailing Speed Record Council.
  20. ^ Spectre, Peter H. (2006). A mariner's book of days, 2007. Dobbs Ferry, NY: Sheridan House. ISBN 1-57409-226-X. OCLC 173009383.
  21. ^ Dill, Bob (March 2003), "Sailing Yacht Design for Maximum Speed" (PDF), The 16th Chesapeake Sailing Yacht Symposium, Anapolis: SNAME
  22. ^ Smith, Doug (January–February 2004). Sailing on slivers of steel. Boy Scouts of America, Inc. pp. 18–21. {{cite book}}: |work= ignored (help)CS1 maint: date format (link)
  23. ^ a b Bob Dill (July 13, 2003). "Frequently Asked Questions". North American Land Sailing Association. Retrieved 2010-08-25.
  24. ^ The record was 126 mph (109 kn; 203 km/h) with winds of 30–50 mph (48–80 km/h), see Bob Dill (April 5, 2009). "Measurement report for Speed Record Attempt Made by Richard Jenkins in the Yacht Greenbird on March 26, 2008". North American Land Sailing Association. Retrieved 2010-08-25.
  25. ^ Editors (March 27, 2009). "Wind-powered car breaks record". BBC New, UK. Retrieved 2017-01-28. {{cite web}}: |last= has generic name (help)
  26. ^ a b Garrett, Ross (1996). The Symmetry of Sailing: The Physics of Sailing for Yachtsmen. Sheridan House, Inc. p. 268. ISBN 9781574090000.
  27. ^ McEwen, Thomas (2006). Boater's Pocket Reference: Your Comprehensive Resource for Boats and Boating. Anchor Cove Publishing, Inc. p. 182. ISBN 978-0-9774052-0-6.
  28. ^ Batchelor, G.K. (1967), An Introduction to Fluid Dynamics, Cambridge University Press, pp. 14–15, ISBN 978-0-521-66396-0
  29. ^ Klaus Weltner A comparison of explanations of the aerodynamic lifting force Am. J. Phys. 55(1), January 1987 pg 52
  30. ^ Kimball, John (2009-12-22). Physics of Sailing. CRC Press. ISBN 978-1-4200-7377-5.
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