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Sailing faster than the wind is the technique by which vehicles that are powered by sails (such as sailboats, iceboats and sand yachts) advance over the surface on which they travel faster than the wind that powers them. Such devices cannot do this when sailing dead downwind using simple square sails that are set perpendicular to the wind, but they can achieve speeds greater than wind speed by setting sails at an angle to the wind and by using the lateral resistance of the surface on which they sail (for example the water or the ice) to maintain a course at some other angle to the wind.〔A clear explanation, with diagrams, is given at and at 〕〔An explanation with a video is given at 〕〔A more detailed discussion is given at 〕〔A very comprehensive explanation of all aspects of the topic is found in the book: See in particular Chapter 16.〕〔See also the explanation at 〕〔A simple explanation is given at 〕 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.〔(How yachts go faster than the wind Gray, R. ''The Telegraph'' 26 September 2013 )〕 The Extreme 40 catamaran can sail at in winds.〔(【引用サイトリンク】 About eXtreme 40 )〕 The high-performance International C-Class Catamaran can sail at twice the speed of the wind.〔(【引用サイトリンク】 publisher = Sail Magazine )〕 Iceboats can typically sail at five times the speed of the wind.〔See "How fast do these things really go?" in the (【引用サイトリンク】url=http://iceboat.org/faqiceboat.html )〕 By sailing downwind at 135 degrees off the wind, a sand yacht can sail much faster than the wind. The velocity made good downwind is often over twice as fast compared to the same land yacht sailing directly downwind.〔 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.〔(【引用サイトリンク】 AC34 Multihull Class Rule Concept Document )〕〔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 (【引用サイトリンク】 AC34 Monohull Class Rule Concept Document )〕 They proved to be faster, averaging about 1.8 times the speed of the wind with peaks slightly over 2.0. In 2009, the world speed sailing record on water was set by a hydrofoil trimaran sailing at 1.71 times the speed of the wind.〔The 500-meter record was , achieved in winds by Hydroptère, a hydrofoil trimaran, see 〕〔(【引用サイトリンク】 title=Official web site of l'Hydroptère )〕 Also in 2009, the world land speed record for a wind-powered vehicle was set by the sand yacht ''Greenbird'' sailing at about three times the speed of the wind.〔The record was with winds of , see 〕 In late 2012 the Vestas Sailrocket 2 skippered by Paul Larsen achieved a new outright world speed record of 65.45 knots on water, at around 2.5 times the speed of the wind.〔(【引用サイトリンク】title=500 Metre Records )〕 ==Sailing perpendicular to the wind== For example, a boat can sail a course that is perpendicular to the true wind (that is, at 90 degrees with respect to the true wind). As it accelerates, the wind as seen from the boat will increase and the wind will appear to shift forward.〔''Forward'' means making a smaller angle relative to the bow than the angle that the true wind makes relative to the bow. This phenomenon is explained in most sailing manuals, see for example p. 82 of Or see p. 32 and p. 122 of 〕 This is the same effect that causes rain to appear to fall at an angle when seen from a moving car, and is equivalent to the astronomical phenomenon of aberration of light. As the wind increases in speed and shifts forward (because of the acceleration of the boat), the sails have to be trimmed in order to maintain performance. This causes the boat to further accelerate, thus causing a further increase in windspeed and a further forward windshift. Eventually, the sails cannot be trimmed any further and an equilibrium is reached. Although the boat is sailing perpendicular to the true wind, its sails are set for close hauled sailing.〔An explanation of how this applies to iceboats can be found at the bottom of the (【引用サイトリンク】url=http://iceboat.org/faqiceboat.html )〕 The actual speed of the boat in such a situation depends on the wind speed, how close to the wind it can sail, the resistance of the surface (water or ice), and leeway (downwind drift). Normal cruising boats yachts can sail at about 45 degrees off the apparent wind (50 to 60 degrees off the true wind). High performance racing yachts at about 27 degrees (35 degrees off the true wind).〔The formula for the apparent wind is (using the symbols shown in the vector diagrams) tan(alpha)=sin(beta)/ (Boat speed+cos(beta)). The figures shown for the angle of the apparent wind assume a boat speed around 0.3 times windspeed, say for a keelboat in winds. The figures for the angle of the true wind are from the main article on sailing.〕 High-performance multihulls can sail at 20 degrees off the apparent wind. Iceboats can sail even closer to the apparent wind. According to the data provided on p. 406 of the cited book ''High Performance Sailing'', a fast keelboat such as a Soling can sail at 30 degrees off the apparent wind, an 18ft Skiff at 20 degrees, and an iceboat at 7 degrees.〔See also p. 204 of the cited book ''High Performance Sailing''〕 If hull speed is not a limiting factor, and if the strength of the wind is sufficient to overcome the surface resistance, then the speed of the boat as a multiple of the wind speed will depend only on how close it can sail to the wind. For example, assuming that surface resistance is negligible (as for an iceboat), if a boat sails at 90 degrees to the true wind, but at 45 degrees to the apparent wind, then it must be sailing at the same speed as the true wind. That is, if the wind speed is V, then the boat's speed is also V. Elementary trigonometry and elementary vector operations can be used to show that, if a boat sails at 90 degrees to the true wind, but at alpha degrees to the apparent wind, and the wind speed is V, then the boat's speed must be V×cotangent(alpha). The table below shows the values of this function, as a multiple of windspeed. Hull speed is not a limiting factor for an iceboat nor for high-performance multihulls. So a boat capable of sailing at 10 degrees off the apparent wind (which is the case for many iceboats) that sails at 90 degrees to the true wind will be sailing nearly 6 times faster than the wind.〔See page 204 of the cited book ''High Performance Sailing''.〕 It can sail slightly faster, as a multiple of the windspeed, if it sails at a greater angle off the true wind.〔The maximum multiple of windspeed is achieved at an angle of 90+alpha off the true wind. For alpha = 45, the maximum multiple of windspeed is 1.41 at an angle of 135 degrees off the true wind.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sailing faster than the wind」の詳細全文を読む スポンサード リンク
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