The MOD70 Virbac-Paprec capsized on Thursday afternoon during a training session ahead of the Transat Jacques Vabre, off Belle Ile.
Her crew Jean-Pierre Dick and Roland Jourdain were sailing 15-20 knots of wind.
They were surprised by a strong gust and could not stop their multihull overturning.
Their mast broke into three pieces.
Roland Jourdain was able to protect themselves under the hull, while Jean-Pierre Dick was violently ejected into the water.
Initially, both crew were declared safe and sound, with some back pain for Jean-Pierre.
Eventually Dick was airlifted to hospital in Lorient.
A compression of a vertebrae was diagnosed, it remains under observation for two days.
The boat was towed to Lorient with the The Cross, SNSM (lifeboat) in attendance long with the Virbac-Paprec technical team.
Jean-Pierre Dick later commented:
'I'm still in shock.Everything went very quickly. I saw a strong gust get behind us. I released the mainsail carriage but it was obviously not not enough.
'Everything changed quickly, I fell from a great height, and I hit something and before falling into the water. It was violent.
'Fortunately, I was able to reach the hull very quickly. I could soon feel that I had back pain. We waited for rescue in sadness.'
Roland Jourdain commented: 'We were sailing in 15 knots of wind but with irregular established gusting to 18-20 knots. The conditions were manageable. Suddenly, there was a stronger gust than the others. The boat is rose and rose. It hovered for a few endless seconds. I released the Solent jib. We thought it was going to come back upright ... but the boat capsized.
'I was very afraid for Jean-Pierre. This is the first capsize I have experienced, I am very shaken. '