Desafío Español updates on sail development

Desafío Español, the Spanish America's Cup challenger gave last week a very interesting presentation and update on the research it is carrying out together with the CEAMA research institute, in one of the most important parts of an America's Cup campaign, the development and optimization of the sails.

It is an irony that the "motor" of an America's Cup yacht, or any sailing boat, is also the most visible of its parts. Most teams go to great lengths to hide their hull, bulb and keel, but sails are impossible to hide, unless the team decide to train and test far away from indiscreet eyes and cameras. For example, BMW Oracle recently tested in Valencia sheets that are deemed to be revolutionary, only after distancing their boats tens of miles from the Malvarosa beach. Similarly, Alinghi, the America's Cup Defender, will most probably take advantage of their winter move to Dubai to test some new sails (maybe similar to this one that we spotted last July) far away from the other teams.

According to Juan Carlos Sánchez, R&D director, the Spanish challenger will have spent in the 32nd America's Cup a total of 4 million euros (US$ 5 million) for the research, development and manufacture of sails. This sum represents approximately 7% of the team's total budget and is equal to the funds spent on the yacht's appendages (bulb, keel and rudder). Finally, Sánchez estimates that Desafío Español will have developed around 140 sails during this America's Cup campaign. In comparison, it is believed that some of the bigger teams have budgets that can reach 8 million euros (US$ 10 million), dedicated to sails.

This extremely important part of an America's Cup campaign is carried out by the challenger's in-house team of designers and engineers aided by the CEAMA research institute. CEAMA is located in southern Spain and is home to the only wind tunnel in Spain that can provide wind simulations that can approach real-life situations. According to CEAMA, their tunnel is able to produce winds that vary with altitude. In real life, wind speeds are not constant but vary exponentially with altitude, meaning that even for the mere 35 meters of an America's Cup mast, wind speed at sea level is lower than wind speed at the top of the mast. In an extremely competitive environment such as the America's Cup, even a few degrees of further precision could mean the difference between winning and losing.

Gerneral view of the wind tunnel used by Desafío Español

According to the Spanish team, the cooperation between the two entities consists of three important points:

A. Consulting
Both parties will carry out a thorough analysis of the yacht's anemometers in order to reach the highest possible accuracy in a wide range of wind speeds.

B. Study of wind sensors aboard ESP-88
Desafío Español together with CEAMA will carry out the vertical directional calibration of all cup anemometers and zero calibration of all weather vanes aboard ESP-88 as well as the new Desafío Español yacht still under construction.

C. Aerodynamic study of sails
According to the engineers of Desafío Español, under the current version of the rule governing the design of America's Cup Class yachts, the so-called Version 5, it is precisely the propulsion of the yacht the area where the design team has the biggest liberty in order to optimize its performance, mainly through the control of the total sail surface.

Optimizing sail designs under the constraints of a given yacht class is usually done testing real-scale prototypes and sometimes simulating smaller-scale models through numerical methods. Numerical simulations that give good results with low-profile sails upwind can become more complicated and less rellaible in the case of a combination of sails downwind or sails with great curvatures. This is the case of big spinnakers where great curvatures are increasingly frequent. These sails have a large area where the limit layer is separated from the sail surface. This fact together with the turbulent nature of the flow and the twist of apparent wind, seriously compromise the validity and reliability of such simulations.

As a result, one has to seek an alternative or complementary methodology for the evaluation of sail designs through tests in a wind tunnel. This way Desafío Español hopes to develop new prototypes that will allow the evaluation of all design changes in a timely and cost-effective way.

The movement of a sailing yacht is made possible by the aerodynamic push generated by its propulsion sails. This movement forward is counteracted by friction on the surface of the hull, resistance due to waves and heeling as well other aerodynamic resistance forces of minor magnitude. The job of the desing team is to, on the one hand, increase the aerodynamic push from the sails and on the other hand decrease as much as possible resistances.

For a given set of wind conditions, direction and heeling, the response of the sail, that is the propulsion it provides, will depend on its aerodynamic profile as well as the materials it is made of.

The pressure field on a sail will depend on the form of its profile. As a result it is very important to study for a given sail surface what is the optimal tridimensional form that will provide the greatest push, as a function of the apparent wind (direction and speed) and heeling. According to CEAMA and Desafío Español, the best solution is the study in the wind tunnel of a scale model. This is done both directly through observation of the model as well as computer analysis of the digital video recording. This method is very important in the case of very flexible sails, such as spinnakers.

The knowledge of such pressure distribution as well as the tridimensional field of wind speeds around the sail, gives designers an invaluable tool to understand and optimize their designs.

Finally, all information obtained during testing can be used as input for a Velocity Prediction Program (VPP), in order to provide a better approximation of the yacht’s performance in a given race course.

A model sail inside the wind tunnel