© Guillaume Gatefait / MerConcept

Aerodynamics, a growing challenge in the design of the new generation of racing machines5 min de lecture

Interviews, surveys, key events and news from Eurolarge Innovation members… Every quarter, the Bretagne Sailing Valley® News covers the economic and technological news of the Breton competitive sailing industry. Discover below the survey dedicated to aerodynamics.

The 36th America’s Cup set the bar: while in recent years architects and designers have focused mostly on how best to generate lift and make boats fly, the priority for many now has changed towards limiting their aerodynamic drag. And the key is in new methods and tools for more performance. 

The launch of François Gabart’s Ultim SVR Lazartigue trimaran on July 22nd made a lasting impression. There is not doubt that in the pursuit of aerodynamic performance we have never moved on so much. No more coachroof, but a cockpit integrated into the central hull with two small Rafale-style bubbles; a boom that almost touches the back crossbeam; very neat beam fairings. “Aerodynamics was clearly the area on which we could make the most gains,” said Antoine Gautier, director of research at MerConcept. “From the moment you decide to remove the coachroof a whole new architecture must be invented.”. In particular the transmission system between the small internal steering wheels and the helm system which controls the three rudders.

On these new Ultims which lift off and fly in 13 knots of wind, the fluid drag is less and less of a concern. But in 20 knots of true wind an Ultim is sailing at over 30 knots upwind. So there is nearly 100 km / h of apparent wind passing over the deck! 

“Under these conditions, the proportion of aero drag exceeds that of hydro drag by 10% analyzes Gautier Sergent, Managing Director of North Sails France which  supplies a good part of the current generation of offshore flying boats. The Vannes loft is at the forefront of these advances.

“Since Oracle’s America’s Cup victory in San Francisco in 2013, we have seen the growing significance of aero. Before you were expressing potential gains as a percentage reduction in drag, it didn’t necessarily resonate as a real gain. Today, with the evolution of simulation tools, we can detect measurable speed gains, and that speaks volumes to the teams,” continues Sergent.

With a design office of six engineers (out of around forty employees), three of whom only work on simulation, North Sails is increasingly involved on projects further upstream than before. “Numerical simulation tools require elements of drag to be quantified. So the designers need us from the outset. This is an exciting new level  of collaboration.

New collaborations

This new working method is also integrated at VPLP Design, as explained by Xavier Guilbaud, a designer who notably worked on SVR Lazartigue: “On the latest Ultims, we carried out the full aerodynamic assessment of the complete platform to quantify the drag of each element. Of course, the beams are the most disturbing element, but the trampolines are also a real topic. We would like to be able to remove them as we did on USA 17 (the trimaran that won the America’s Cup in 2010). The helm crawled over the rear beam when tacking! This is not realistic on an ocean racing boat, so textiles are used, especially behind the beams and on the sides of the central hull where the friction is most noticeable.”

Vital on the Ultims, the topic is also taken into important but at more relative levels in the Imoca or Ocean Fifty (ex Multi50). On these boats, it is above all the interaction between the rig and the platform that is studied: “We integrated the topic from the second year of sailing on Charal 1,” says Pierre-François Dargnies, technical manager of BeYou Racing. “We tested new configurations this winter, far away from Lorient to remain discreet as we considered the new boat (a Manuard plan under construction at CDK Technologies, in Lorient, NDLR).

In Imoca, the main job is to maximize the end plate effect between the sails and the deck, that is to say to minimise the passage of air from one side of the sail to the other at bottom and thus creates vortexes. On Charal 1, the innovation consisted of a textile panel under the boom to fill the empty space with the deck. “The important thing,” Pierre-François Dargnies continues, “is to quantify the gain of each system in relation to the weight or the complexity it generates. Numerical simulation gains are sought from North Sails for each wind angle and strength in order to make the decision.”

However, the increase in pure performance should not come at the expense of the safety of the sailor or the weight gain of the boat. This is why the use of aero covers is limited on the Ocean Fifty, which is more sensitive than the Ultims to the risk of capsizing. This is also the reason why the idea is not on the agenda over in the Imoca, as Pierre-François Dargnies explains: “You have to remember that the Vendée Globe 2020 was run at very low speeds. Our first concern on Charal 2 is nevertheless to bring the average single-handed speed closer to the pure performances obtained in a full racing crew. So you have to deal with the sea first before you start looking at the gains in the rig!”

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