High-performance sailing


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. The Beta Theorem describes the net effect of the sail and resisting forces in describing the apparent wind angle.

History

Bethwaite offers the following chronology of key advances in sailing technology that provided the essential elements of high-performance sailing:
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 rotor-driven craft, such as the "land yacht", Blackbird, which are outside the scope of this article.

Watercraft

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.
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. 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. They proved to be faster, averaging about 1.8 times the speed of the wind with peaks slightly over 2.0.
The Extreme 40 catamaran can sail at in winds. The high-performance International C-Class Catamaran can sail at twice the speed of the wind.
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. 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.

Iceboats

Iceboats can typically sail at five to six times the speed of the wind.

Land sailing craft

By sailing downwind at 135° 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. 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 with a recorded top speed of.

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 with a tripling of their speed. 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 a new concept of "apparent wind sailing".

Apparent wind

Apparent wind is the wind velocity measured aboard a moving sailing craft; it net effect of the speed of the craft over the bottom and the true wind speed. The apparent wind measured aboard a craft under power, traveling in calm conditions, would come from directly ahead and at a speed that is the same as the boat speed over the bottom. If the craft travels at 10 knots with a tailwind of 5 knots, it experiences an apparent wind of 5 knots directly on the bow. The apparent wind experienced by a stationary craft is the true wind speed. If a craft proceeds at 90° to a true wind of 10 knots, itself traveling at a speed of 10 knots, then the apparent wind angle would be 45° off the bow and the apparent wind speed would be 14 knots. As the craft becomes faster than the true wind, the apparent wind is always ahead of the sail.
The points of sail at which sailing craft can achieve highest speeds span between a beam reach and a broad reach. Normal cruising yachts can sail at about 45° off the apparent wind. High-performance racing monohulls at about 27°. High-performance multihulls can sail at 20° 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° off the apparent wind, an 18ft Skiff at 20°, and an iceboat at 7°.

Points of sail

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. Downwind, 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.

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.
Beam reach
For example, a boat can sail a course that is perpendicular to the true wind. As it accelerates, the wind as seen from the boat increases and the wind appears to shift forward. This is the same effect that causes rain to appear to fall at an angle when seen from a moving car, and is analogous to the astronomical phenomenon of aberration of light.
As the wind increases in speed and shifts forward, 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.
Hull speed is not a limiting factor for an iceboat nor for high-performance multihulls. So a boat capable of sailing at 10° off the apparent wind that sails at 90° to the true wind is sailing nearly 6 times faster than the wind. It can sail slightly faster, as a multiple of the windspeed, if it sails at a greater angle off the true wind.
Broad reach
As stated in the introduction of the book High Performance Sailing, in the section Tacking Downwind, "... any boat which runs 'square' must necessarily sail downwind at some speed less than the wind's speed whereas any boat which tacks downwind has no theoretical limit to its speed. Iceboats, for example, can tack downwind at average speeds many times the wind speed."
The same book states, in section 24.2, "In a True Wind of, the Soling crew will sail the close reach and reaching legs in Apparent Winds little stronger than True Wind.... The 18-foot Skiff crew sails the cross wind legs in much stronger Apparent Winds which approach. Even on the broad reaching legs they must still sail in a strong Apparent Wind which blows from ahead, so they still need to use strong-wind 'going-to-windward' handling techniques even though they are sailing downwind." Figure 24.2 of this book provides vector graphics that show how the Skiff can sail downwind faster than the speed of the wind.
From the detailed data provided for the 2009 record set by a sand yacht, it can be seen that the record was achieved when the craft's course was about 120° off the true wind. That is, the craft was moving faster than the wind although the true wind was behind it. This is possible because the speed of the craft results in a large forward wind shift, so that the craft is close hauled with respect to the apparent wind.
Suppose that a boat is at a standstill, then starts to sail on a course that is 135° off the true wind. The boat accelerates, so the apparent wind is less than the true wind and shifts forward of the true wind. If the boat can reach a speed equal to about 71% of the true windspeed, then the apparent wind is perpendicular to the boat's course and its speed is about 71% of the true windspeed. If that reduced apparent windspeed still generates sufficient force to overcome the resistance of the surface, then the boat continues to accelerate.
That is, the situation is the same as the one explained above, because the boat is still accelerating after having reached a course perpendicular to the apparent wind. In practice, most boats sailing on the water cannot overcome the resistance of the water in order to reach speeds equal to the speed of the wind. However, iceboats can do so, because the resistance of the surface is very small. Thus, an iceboat that starts sailing on a broad reach continues to accelerate until it is close-hauled with respect to the apparent wind.

The Beta Theorem

Garrett describes the Beta Theorem as 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, between the apparent wind and the course over the water. The hull drag angle and sailing rig each have drag angle with respect to the medium flowing past them. The sum of those two drag angles are equal to the apparent wind angle. This theorem applies for every point of sail. A small β denotes high efficiency and a potential for high speed.
Many boats can make good downwind faster by not sailing dead downwind, but instead jibing back and forth. If the boat can accelerate until the apparent wind is at angle α off the bow, then it can be seen from the table above that it can make good downwind faster than the true wind. Such performance is theoretically possible. An easy-to-grasp animation demonstrating the principle of how it can be possible to go faster than the wind can be found at.
However real boats cannot equal the performances shown in the table, although iceboats can come close to them. Indeed, iceboats can make good both upwind and downwind at speeds greater than the wind. And so can sand sailing crafts: during the 2009 land speed record, the land yacht Greenbird was proceeding at about 3 times the speed of the wind on a course about 120° off the true wind. Thus, its speed made good downwind was about 1.5 times the speed of the wind. During a training run the catamaran Alinghi 5, one of the competitors for the 2010 America's Cup, covered to windward and back in 2.5 hours in winds, so its average velocity made good was, about 1.9 times wind speed. This is consistent with the sailing craft being able to sail at about 15° off the apparent wind, see the table above. Indeed, the catamaran sails so fast downwind that the apparent wind it generates is only 5-6° different from that when it is racing upwind; that is, the boat is always sailing upwind with respect to the apparent wind.
During the first race of the 2010 America's Cup, the winning sailing craft USA 17 sailed to windward in 1 hour 29 minutes, in winds of. Thus its velocity made good upwind was about 1.8 times windspeed, consistent with being able to sail about 13° off the apparent wind when sailing upwind. She sailed downwind in 1 hour 3 minutes, so her velocity made good downwind was about 2.5 times windspeed, consistent with being able to sail about 14° off the apparent wind when sailing downwind. During the second race, winds were. USA 17 reached the windward mark in 59 minutes, so her velocity made good was about, about 1.65 times wind speed. The course was a triangle, so the velocity made good downwind was only, about 1.4 times wind speed. USA 17 averaged, about 3.35 times the wind speed, on the faster first triangular leg.
Other sailboats can make good downwind at speeds faster than the wind. Indeed, it can be seen from the polar chart for the Skiff that it can make good about downwind at a windspeed of, by jibing back and forth at about 140° off the true wind. The polar chart in Figure PS1 of the cited book High Performance Sailing shows that boats that were sailing in 1996 were able to make good downwind at about 1.5 times the speed of the wind.