When boats become a platform
During the last visit in Barcelona I had the unique opportunity to discuss with the design coordinator, Adolfo Carrau from Alinghi Red Bull Racing Team, about these design miracles of the 37th America’s Cup.
He simply calls them platforms. With about 6 knots of wind, they take off, start flying on the foils, and reach speeds of around 100 km/h. This changes everything: hydrodynamics and aerodynamics become the focus to reduce drag.
The sails create thrust, the foils create lift and drag, and there is plenty to test and optimize. The foil design is highly complex, incorporating all the science aerospace engineering has to offer.
Thank you Adolfo Carrau and Alinghi Red Bull Racing for these insights.
The article is published on the Wave Magazine Nr. 63.
Photos: © see name photographer in each photo
English Translation with ChatGPT
Archimedes Gets Wings
In the first article of our three-part series, we described the history of the America's Cup up to the present day. Now, we take a closer look at the boats and their design. To uncover this, Jürg Kaufmann traveled to Barcelona, where he was able to visit Alinghi Red Bull Racing for two days and speak with designers, sailors, and coaches. Here are the exclusive insights...
We are currently experiencing a new era of the America's Cup. The Cup has always been full of surprises of all kinds, but this one is special. The first surprise: the French AC-Cupper from Team Orient Express is identical to the boat of the defender Emirates Team New Zealand, including the hidden design secrets like the mainsail boom located in the hull.
However, we no longer speak of boats but rather of constructions flying over the water. The designers simply call them "platforms." In Barcelona, I had the opportunity to speak extensively with Adolfo Carrau, the design coordinator, about the design of the current AC75 at the Alinghi Red Bull Racing team base.
The platform must be a solid yet as lightweight as possible structure that can withstand the forces. Additionally, it must accommodate all the technology and fully integrate into the sail design. From back to front, this includes:
READY FOR TAKE-OFF
In the water before take-off, the displacement is optimized to generate maximum lift and minimal water resistance. After take-off, the focus is on reducing air resistance and optimizing airflow into the sails. From about 6 knots of wind, the lift of the foils in the water is sufficient to make the platform fly. The platform, including the underwater hull, is designed to generate enough lift with the choice of sails without delaying the "take-off" moment due to displacement. Once the hull lifts off the water, the lower hull part no longer displaces water but only air.
An AC regatta only starts when wind speeds are between 6.5 and 21 knots. If the wind drops below 6 knots during the regatta, the Cup boats cannot stay on the foils and sail like conventional displacement yachts. This brings up initial questions for the teams: should the design consider the displacement or flight mode more? How much time should the crews invest in training conventional sailing or foiling?
There is also the time and weather component. In August and September, the challengers will sail for the place in the America's Cup match against the defender Emirates Team New Zealand. However, conditions will get tougher from the end of September. Adolfo Carrau compares it to a Formula 1 car driving straight; the winds get stronger, the waves higher – "the road is getting bumpier." The challengers, however, have to get the most out of their design beforehand and are forced to make corresponding compromises. The defender only needs to focus on the conditions from October and additionally benefits from the insightful comparison of how the French sister boat fares against the other four challengers.
By selling the design package, Emirates Team New Zealand got money into their coffers – and the French could catch up a bit in the competition. The Kiwis manage their financial resources sparingly. Instead of air freighting their Cup boat to Barcelona, "Taihoro" came to Spain by cargo ship. The weeks-long training loss will be quickly made up by Grant Dalton's team.
“Boat One. A beautiful machine, a radical idea.”
Adolfo Carrau
KEY POINTS FOR THE DESIGNERS
If the design team has done its homework correctly, the fastest boat comes to the water. If everyone calculated the same way, all boats would be equally fast. Yet, the AC yachts must perform their best in different wind and wave conditions. Who has the best predictions about the capricious Mediterranean waters off Barcelona? Which team has the perfect compromise for the elimination regattas in the summer and the final in November?
It’s not rocket science to make a yacht fast under normal conditions. There is enough experience in sail design, fluid dynamics are known, and even in transforming the ship into a flying object, aerodynamics provides advice. Let's look over the designers' shoulders to see what problems and dilemmas they face.
Once the Cup boat or platform flies, there are still two crucial elements from the designers' perspective: reducing drag and optimizing propulsion. The same physical principles apply as with airplanes. Water has a higher density than air, so the foils or hydrodynamics are crucial. At speeds around 100 km/h, however, aerodynamics play an important role. When looking at the hulls, we see that the design teams have found similar solutions. Only the English boat is more extreme. Whether this pays off will be seen in July when the teams are allowed to train against each other for the first time.
THE RIGGING
Thrust through sails means that each sail must be optimally trimmed individually even under enormous loads. There are three sails, not just two. The mainsail consists of two independent sails that form a wing together. None of the six AC boats have a backstay, which increases the load on the mainsail trimming system and drastically changes the load distribution compared to a normal yacht. A traveler like on many yachts or a mainsail boom hidden in the hull – we already saw these approaches on Luna Rossa Pirelli in the last Cup and now also on Emirates Team New Zealand and accordingly the French. The same applies to the jib, the headsail – there also seem to be different design solutions: with a traveler or with its own boom. The tack points and loads of all three sails are constantly adjusted hydraulically. The mast is a one-design by Southern Spars, so replacement masts are available. But the detailed work of team-specific changes should not be underestimated. Bravo Alinghi Red Bull Racing.
AERODYNAMICS
The hull construction not only affects water resistance but also ensures that the wind flows more effectively through the sails. A well-thought-out hull can reduce wind resistance and at the same time increase sail efficiency, resulting in a higher overall speed. In this Cup, we only see the heads of the helmsmen and trimmers; all elements of the platform are aerodynamically arranged and "packed." The Italians go a step further: the helmsman sits in a cockpit like in an airplane. Increasing propulsion and reducing drag, is a simple goal but a complex task.
In the week after the visit to Barcelona, the Swiss mast broke. The exact cause was not published, but one theory confirms the enormous loads in the mast and rigging. If the pressure from the mainsail trimming system decreases, the pressure on the Cunningham increases. Some experts believe this led to the mast breakage.
For me, the positive aspect of this accident is the professionalism that the Swiss team demonstrates. Two days later, they were already sailing again. Good to know:
We are already in the middle of aerodynamics and hydrodynamics. The designers differentiate between two types of resistance that influence the overall resistance of the yacht.
The diagram on page 63 shows how these different types of resistance behave with changes in sailing speed. Similar to an airplane wing, parasitic resistance is more significant at lower speeds. As speed increases, induced resistance becomes more dominant.
THE FOILS
The big topic and infinitely complex: the basics of physics like resistance, lift, angle of attack, cavitation, flow separation, and reduction of spray play a crucial role here. The foils are controlled by two large arms: the front large foils also have adjustable flaps that are hydraulically controlled by hand. This allows not only the lift to be regulated but also the height above the wind at which the platform can sail. Some teams work with two flaps, allowing finer adjustments at high speeds but also creating more turbulence. Every advantage has its downside. The Italians currently work with one large flap per foil. It remains to be seen what will be used in the Cup.
The wingtips are bent up like on jets; these so-called "winglets" help to avoid vortices at the foil tips in various forms and reduce local cavitation. Vortex generators are installed to control flow separation and maintain a more even water flow over the surface. The foil on the rudder is controlled by the angle of attack. The associated mechanics and hydraulics have been placed at the stern. Different design solutions are also evident here.
HYDRAULICS
The hydraulic system on the platforms consists of three different systems, each controlling its own area. Two of them are one-design; the pressure is built up with energy from batteries and is the same for all teams. This is to save costs and also achieve a certain standard of safety. The hydraulic system for trimming the sails gets its pressure power from the "cyclists" of the crew and was developed by the team.
FLIGHT CONTROL SYSTEM
This controls the settings or flaps of the foils. The current question is whether one large flap over the entire foil or two smaller flaps should be used, as all teams sail with T-foils. Additionally, this system also controls the angle of attack of the rudder system with the foil.
FOIL CANTING SYSTEM
This moves the two large arms with the foils. The forces are so great that the teams decided in the last America's Cup to build a common solution to ensure safety and keep costs under control. The foil arms must withstand about 35 tons of load.
SAIL CONTROL SYSTEM
The third hydraulic system on the platform is responsible for sail trimming. In the images, we see the technology for trimming the mainsail, consisting of two sails. A regatta means about 30 maneuvers. The task of the "cyclists" is far more complex than one might think. No human, no matter how well trained, can deliver 30 high performances in about an hour.
Nils Theuninck, the Swiss Finn sailor, is one of the "energy generators" and said he is very
happy to be a real sailor and thus better able to estimate when to deliver the most power. Every cyclist can only deliver a limited number of top peaks, with recovery sometimes taking only a few seconds.
WHO WILL WIN THE CUP?
The best boat and the best crew. Some experts believe that in the end, the speeds will be similar. If that is true, it means a return to real match racing – and that with platforms flying over the water at 100 km/h. It remains exciting...
THE AUTHOR
Jürg Kaufmann has been at home in the international regatta scene for decades, first as a sailor, then for 20 years as an international match race umpire.
The co-initiator of Wave Magazine is one of the top photographers in yachting, has been present at three Olympic Games, and various Volvo Ocean Races, and is currently covering his fourth America's Cup event. www.juergkaufmann.com
