High-speed sailing

The design principles behind the development of the 2013 America’s Cup racing yachts.

A new breed of lightweight catamarans contested the 34th America’s Cup in San Francisco. The boats were created through a complex process that involved several disciplines including marine hydrodynamics, aerodynamics, structural mechanics and advanced composites. Matthew Sheahan, racing and technical editor of Yachting World, writes about recent developments of these racing multihulls.

Since the inaugural race around the Isle of Wight in 1851, the world’s oldest international sporting trophy has represented the leading edge in yacht racing design and technology. In 2013, the 34th America’s Cup delivered the biggest leap in yacht racing performance of any previous match throughout its 162-year history. Hosted in San Francisco during September 2013, this America’s Cup saw two 72’,

$10 million carbon fibre catamarans rise above the sea’s surface to achieve unprecedented speeds around the course.

Compared to the more familiar 82’ America’s Cup single-hulled, heavyweight yachts with their traditional lead keels, the new breed of lightweight catamarans were less than a third of the weight of the monohulls.

The previous America’s Cup, held in Valencia in 2010, had been a major departure from the norm, an event that set a new agenda for the 2013 Cup. The winning boat was BMW Oracle Racing, a 135’ trimaran owned by American billionaire and founder of the Oracle Corporation, Larry Ellison.

Apart from the vessel’s greater size, the black trimaran was notable for its giant 203’ wingsail mast, taller than the full wingspan of a Boeing 747, in place of a more conventional mast and ‘soft’ sail. The use of a solid wingsail, only the second time in the history of the Cup, was to become one of the key features of the next generation of America’s Cup-class boats. Shortly after winning, Oracle Team USA’s CEO and five-time America’s Cup winner Russell Coutts announced the new vision for the 34th America’s Cup and the radical new boat.

As always, winning the Cup put the victors in a strong position to define the rules for the next event, and as Coutts revealed the details, it was clear that multihulls would be the boats of choice. The move was a controversial one. Only twice before had multihulls been used in the America’s Cup.

At just 72’ long, the new AC72 machines would be significantly shorter than their 2010 predecessors but could sail considerably faster, hitting top speeds of 47knots (54mph). They could also handle a much broader range of wind conditions, originally set at 3-33 knots, but later scaled back to around 6-26 knots.

Righting moments

The previous generation of Cup monohulls weighed around 24 tonnes. A modern AC72 catamaran weighs around 6 tonnes, yet has a similarly sized sail plan. Given such fundamental differences, it is easy to see how big the difference in power-to-weight ratios is. One major downside for the multihull, however, is that beyond a certain angle of heel, the multihull will not recover and a capsize is unavoidable.

But there was another reason why the architects of the new rule wanted to see multihulls rather than monohulls.

Establishing an opponent’s potential performance starts with establishing their boat’s righting moment. The precise shape of the hull is key, and therefore why teams guard such details carefully. A high righting moment generated by a relatively wide hull will be powerful in strong winds but have the disadvantage of greater wetted surface area and hence greater drag in lighter conditions. A narrower hull would have lower drag and therefore perform better in lighter winds, but would have less righting moment and therefore less power in stronger conditions.

Calculating, and indeed limiting, the righting moment for a multihull is much easier, as this is less influenced by the hull shape and more influenced by the basic dimensions, predominantly that of weight times distance. To limit boats’ power, the rules for the AC72s defined a maximum weight and maximum beam. At a stroke, everyone had close to the same potential power output.

DESIGN BASICS

The previous generation of Cup monohulls weighed around 24 tonnes. A modern AC72 catamaran weighs around 6 tonnes, yet has a similarly sized sail plan.

Speed was at the heart of Oracle’s vision for the new look America’s Cup, as were the new sophisticated television images and live audio from crew members, both part of a larger package designed to attract a bigger audience. Upping the pace meant abandoning slow single-hulled boats with their heavy lead keels in favour of lightweight, high speed multihulls. Bringing the racing close to the shore was also part of the new template, again, something that suited shallow-draft multihulls better than deep-keeled monohulls.

At the core of the new design was the issue of stability – righting moment – the ability to resist heeling and the key driving force of any yacht. A monohull generates its righting moment through a combination of the position of the hull’s centre of buoyancy and the righting effect of the keel. In broad terms, the heavier the keel, the higher the righting moment. That said, a heavier keel also means a heavier and potentially slower boat.

By contrast, a multihull generates its righting moment through its wider beam rather than with a keel slung beneath. At the point that one hull is lifted clear of the water, a multihull has a large proportion of its weight acting as a lever to windward. This weight times the horizontal distance from the hull that remains in the water is what generates the high righting moment. More righting moment and a lighter boat means considerably higher performance – see Righting moments.

But while multihulls levelled the playing field as far as righting moment was concerned, there were other drawbacks with this type of boat for America’s Cup racing.

Catamarans may be quick in a straight line but they are slow to manoeuvre. The America’s Cup is a match race, a simple two-boat affair where nimble manoeuvrability is a key part of the game, as crews jostle to outsmart their opponents by pushing their boats through tight turns. Multihulls would be slower and less nimble than their monohull counterparts and therefore weak in this area. At least this was the consensus in the sailing world during the buildup to the event.

However, by the time the racing got under way, the innovative configurations aboard both challenger Emirates Team New Zealand and defender Oracle Team USA had convinced even the most sceptical that this new breed of multihull had challenged conventional wisdom and that catamarans could provide exciting, closely fought match racing. One of the important factors in this newfound performance was the 40m tall wingsail.

WINGSAILS

On a solid wingsail, leech profile is maintained through physical shape in the wing. Therefore there is no vertical load in the mainsheet, and horizontal load only

Looking more like an aircraft’s wing than even the most advanced soft sail and mast combination, wingsails are rare but they do present several advantages. The first is the ability to maintain an efficient aerofoil shape without inducing the high loads that are required when using control lines to achieve the required shape on a soft sail.

One example is the mainsheet that controls the angle of attack of the wing to the wind. Here the sheet doesn’t need to pull down vertically to maintain the shape of the trailing edge as it does on a conventional sail. Instead, the sheet simply needs to pull the sail in laterally. This makes the mainsheet easier to adjust quickly by the crew as they are not having to haul on such high loads. This in turn means that the boats do not require such highly geared, heavy and slow winches.

The AC72 wingsails were around twice the weight of a conventional mast, but the sheet loads of a soft sail would have been eight to ten times more. With 8-10 tonnes of load on the conventional mainsheet compared to 1 tonne on the wingsail mainsheet, the sail would not only have been slower to operate but physically impossible for the crew to perform as many tacks and tight manoeuvres as they did during the racing.

The wingsail also presents lower drag than a conventional sail plan through manoeuvres and will therefore decelerate less. Conversely, the more abrupt power delivery of a wingsail means that crews can also de-power more easily with a wingsail than with a soft sail and stop the boat more quickly if required. In match racing, decelerating is as important as being able to accelerate and manoeuvre quickly.

The final key advantage is one of lowering the sail plan’s centre of effort. By adjusting the various flap elements that make up the trailing edge of the wing, the height of the centre of effort of the sail plan can be altered. In stronger winds, the centre of effort is lowered to reduce the heeling moment. In lighter wind strengths, it is raised in order to create more heeling force to lift one of the hulls out of the water early, thus halving the total hull drag – see photo labelled Centre of effort.

The wingsail was still only one side of the high performance equation for the new breed of America’s Cup class boats. Lifting the entire boat vertically out of the water on hydrofoils to reduce the hydrodynamic drag of the hulls was the second major step forward.

IMPROVING FOILS

Centre of effort © ACEA/Photo Gilles Martin-Raget

Hydrofoil technology has been around for a long time. Its early development started over 100 years ago when Italian Enrico Forlanini achieved 36.9 knots (42.5 mph) with his 60hp airscrew-driven boat in 1906. Achieving speeds on water to rival those of many of the cars of the day made several engineers sit up and take notice, among them the Wright Brothers and Alexander Graham Bell, both of whom experimented with foil-borne craft. But it wasn’t until 1938 that a sailing boat got up onto foils with Americans Robert Rowe Gilruth and Carl William Price, who managed to achieve the same feat, albeit more slowly, under sail.

Yet despite the breakthrough, sailing hydrofoils have been rare and mainly the domain of speed record attempts rather than mainstream yachting. The French 60’ trimaran L’Hydroptere is one of the best-known modern foiling yachts, having set the outright world sailing speed record in 2009, sustaining a speed of 52.86 knots (60.83 mph) for 500m in 30 knots of wind.

At the opposite end of the size scale, the International Moth Class, a 10’ racing dinghy weighing just 30kg has been leading the way in small sailing hydrofoils. Today the fleet, which has minimal class rules in order to encourage development, is dominated by foiling boats. In addition, the class was the first to race around a course that included both upwind and downwind legs, entirely on their foils.

The lessons learned on the Moths have been valuable in the development of the foiling AC72s. Indeed, many of the design engineers and sailors in America’s Cup teams have been drawn to Moth sailing to learn the tricks and technology firsthand. Yet to do so for the Cup catamarans meant overcoming several fundamental issues.

Hydrofoils work well on powered vessels where the engine can provide sufficient speed for the boat to rise up onto its foils. To get to this point, the vessel needs to push through high levels of drag. Once over the ‘hump speed’, (named after the typical shape of the speed vs drag curve), the hull is raised clear above the water and the overall drag reduces dramatically. On a sailing boat, however, it is harder to guarantee sufficient wind to get the boat to rise. If it can’t then there is a big drag penalty when compared to a non-foiling boat because of the additional resistance caused by the extra hydrofoil surface area.

At speed on foils © ACEA/Photo Gilles Martin-Raget

Sufficient power-to-weight ratio and righting moment are essential to lift the boat clear of the water. In addition, to take full advantage of the big potential speed gains, Cup boats needed to foil on all legs of the course and not just some. In the early stages of design this was thought to be the stumbling block for the AC72s.

While it was clear that these highly powered, lightweight machines were capable of foiling downwind 5-6 knots faster in typical racing conditions than if their hulls were in the water, the question was whether this additional speed would be sufficient to deliver a net gain around the course if the boats didn’t foil on the upwind legs.

Emirates Team New Zealand’s original calculations suggested that the minimum wind speed at which the boat would foil downwind would be 14 knots. As testing and crew training progressed, this limit was to come down to below 10 knots of wind. In addition the team started to learn how to foil upwind. At this point foiling was now definitely part of the game.

Given the range of speeds that the boat would be sailing at, controlling the ride height was the next issue for designers. This was particularly tricky given that the rules prohibited any moving surfaces on the foils such as the flap and elevator type devices that are common aboard the Moths.

To achieve this, teams developed self-levelling systems to control the ride height – see At speed on foils photograph. Riding too high would mean less of the vertical part of the daggerboard would be in the water and therefore less sideways force would be generated. Pushed too hard, the boat would skip sideways. Riding too high also raised the sail plan further above the water’s surface and increased the heeling lever, making the boat less stable in heel and therefore reducing the power available.

The answer was to use hydraulic rams powered by the crew winding handles on grinding pedestals, to angle the foils in towards the centre of the boat, known as canting. By varying the amount of cant, the proportions of vertical to horizontal lift could be changed to suit different points of sail and weather conditions – see Canting.

Learning how to handle these new, highly powered hydrofoiling machines was a big challenge for designers and crew, yet despite the complexity, it was clear to the Kiwis that foiling would be a crucial part of the campaign. Having started with more forgiving but higher-drag foils as the team learned to sail the giant machine, the new designs became more refined in their shape and required more accurate sailing from the crew.

Throughout the early sailing trials, the approach seemed to pay off as the team sailed more confidently in stronger winds and more awkward sea states. Ideally, the team would have built and trialled the widest range of daggerboards across a broad range of conditions, but with each foil taking between 1-3 months to produce and costing $450,000, time and cost limited this development.

Their opponents, Oracle Team USA, started with a more refined looking boat that suggested lower aerodynamic drag and lower-drag foils from the outset. Their first breakdown resulted from a daggerboard failure, but just eight days into their testing programme, the American boat suffered a major setback when the cat tripped up, capsized at speed and suffered serious structural damage. Miraculously, no one was badly hurt, but the accident was a major setback. Three months of testing were lost as the team rebuilt its damaged first boat while reconsidering some of the design issues for the second, more refined boat in its campaign.

In the end, it was this lower drag design that won through as the Oracle crew learned how to handle its sprightly machine. Success came at the eleventh hour for the defenders but provided some of the most dramatic racing that the Cup has ever seen.

But there was also a stark reminder as to the risks involved and the level at which the new technology was being pushed. Seven months after Oracle’s capsize, tragedy struck when British Olympic sailor Andrew Simpson was killed aboard Artemis Racing, one of the other challengers, when the boat capsized and broke up during training.

FOILING AND WINGSAILS

Under America’s Cup rules: - No moving surfaces allowed - Foil cannot extend outside maximum beam, hence the ‘L’ configuration - Boats are not permitted to have both daggerboards lowered for more than 30 seconds

The future for sailing hydrofoils is an exciting one. The fast-paced racing of the America’s Cup not only highlighted the thrill and speed of sailing on foils, but it has also enabled a greater understanding of how such configurations may work without conventional moving control surfaces. There is also a greater understanding of how much more stable a fast-moving boat is when it is riding on its hydrofoils. Given the problems of encountering waves at high speed with conventional, non-foiling boats, hydrofoiling continues to offer the possibility of increased speeds in otherwise challenging sea states.

But foiling is not the future for all. While the America’s Cup has always provided a trickle down for technology, foiling is unlikely to be used on typical cruising boats of the future. The heavy keels of monohulls and even the relatively heavy structure of cruising multihulls, make it extremely difficult to generate sufficient power-to-weight ratios and the necessary righting moments.

However, lessons in control systems and even foil section design could have an influence in other areas of yacht design, be it the methods of draft reducing lifting keels on cruising boats, or more refined high-lift/low-drag foil sections for keels, rudders and even roll stabilisers.

When it comes to the solid wing sails, it is less likely that this technology will trickle down into mainstream yachting. Despite their impressive efficiency and their ability to reduce loads in the control lines, the wings are complex and expensive to build. Stepping (taking down) and removing such wings is also difficult and potentially risky, particularly in windy conditions when the wing still wants to fly whether it is attached to the boat or not. Stepping an AC72 wing takes around 30 people, and like other teams, Emirates Team New Zealand have had several alarming moments when the wing has overpowered them during the stepping process.

For the future of wingsails, practicality is the limiting factor.

Throughout the history of the America’s Cup, technology has always played an important part elsewhere in the sport. From electronic wind instruments developed in the 1930s to the wing keels and Kevlar sails of the 1980s, the America’s Cup has always had a greater influence than has at first been apparent. For all its radical changes, the immediate benefits of this America’s Cup cycle may be difficult to see at present, but the 34th America’s Cup will doubtless provide key developments and technologies for the wider sport in the future, just as it has during the last 162 years.