Focus on foils and superventilation • Live Sail Die
Since the beginning of the SP80 adventure, optimization and research on superventilating hydrofoils have been at the heart of the project and have mobilized numerous resources. SP80 really started when Xavier, Mayeul and Benoit, our three co-founders, mounted ventilated fins on Benoit’s kitesurf board and realized the enormous potential of this new technology.
“I had a real feeling of stability at high speed,” says Benoit. We hit over 40 knots without struggling to accelerate or stay on the board. We knew then that there was something to dig on that side.
When the project was officially launched in October 2019, it was only natural to ask Prof Mohamed Farhat of the Laboratory of Hydraulic Machines at EPFL for his expertise, a recognized specialist in cavitation. Its laboratory has a unique infrastructure: a cavitation tunnel capable of propelling water at more than 50m / s (180km / h). It is in this tunnel that the foils of the mythical Hydroptère, the boat that made us all dream when we were kids, have been optimized.
WHAT DO YOU KNOW ABOUT SUPERVENTILATION?
The first tests on ventilation fins were carried out in the tunnel in autumn 2019 by Aurore Kerr as part of her master’s thesis at EPFL. Reproducing superventilation in the cavitation tunnel was not an easy task as Aurore explains:
“We were the first to work on superventilation in this tunnel. It involved a lot of challenges. We had to test a lot of things before we found the right solution.
Thanks to this work, the team was able to understand a lot of things about overventilation and the conditions in which it occurs (speed, angle of attack of the foil, shape of the profile, etc.) and precisely measure the forces involved. on the foil. The values obtained confirmed those obtained thanks to the numerical simulations developed by the team.
On a subject which has so much to discover, the experimental approach appears necessary and complementary to numerical simulation. You can’t trust simulations if you don’t have the experimental data with which to compare them, ”explains Charles de Sarnez, head of the fluid dynamics team.
The only problem of Prof. Farhat’s cavitation tunnel is that the foil is enclosed between 4 walls and it is impossible to model the surface of the water. It is an essential step in the understanding of ventilating hydrofoils because the interaction between the foil and the water surface is a very complex phenomenon and difficult to reproduce in a numerical simulation. The team was able to count on the support of METAS (Swiss Federal Institute of Metrology) which granted us access to its 142m long trailer.
AND WHAT IS A CAVITATION TUNNEL?
A cavitation tunnel is an experimental installation for testing the behavior of bodies submerged in water (a hydrofoil for example). A powerful pump sets the water in motion in a closed loop circuit and the narrowing of the tunnel section at the test area accelerates the water thereby recreating high speed sailing conditions. The pressure in the tunnel is also precisely controlled to facilitate the onset of cavitation or ventilation.
Anaëlle Manon, a master’s student in mechanical engineering at EPFL, followed these tests with the help of Flavio Noca, professor at HEPIA and EPFL. In one day at the Berne towing basin, the team was able to do more than 40 different runs, testing the same fins that Benoit had used on his kiteboard in 2018 and the foil that had been mounted on the prototype during the summer tests a few months. before.
“These tests were very informative and allowed us to put precise values on the sensations Benoit had during his kite tests or when he piloted the prototype,” explains Anaëlle.
Through all of these test campaigns, the SP80 hydrodynamics team has gained a unique understanding of superventilation, a subject that has so far been little studied in the world.
This research has allowed us to develop in parallel very precise numerical simulation models which allow us to test thousands of different designs in a few hours and thus to explore forms of hydrofoil that we would never have thought of.
These designs could well be found on our boat during our 80 knot acceleration world record attempt, stay tuned!