A report into the destruction of the IMOCA 60 BT’s coachroof and near sinking in last year’s Transat Jacques Vabre race has concluded that a combination of the material’s stiffness and an unusually large, or rogue wave – “a hydrodynamic event of significance” – caused the failure.

BT was being raced two-handed last November by Sebastien Josse and Jean-François Cuzon and was around 210 miles north-west of the Azores in 30-50 knots of wind when the boat was struck by a huge wave.

It stove in the coachroof, as shown in the photo below, allowing thousands of litres of water to enter the boat and almost sinking her.

In an emergency operation, the two crew were airlifted off the boat. Fortunately, the boat survived the storm and the BT shore team was able to arrange for a salvage of the boat in the aftermath of the gale and it was towed to the Azores by a tug.

The boat was surveyed and the designers, Farr Yacht Design (FYD), checked the engineering calculations and worked on a solution from the repair.

The report concludes that the coachroof panels, while compliant with class scantling rules, could have been stronger but also that a contributory factor may have been the stiffness of the coachroof panels. ‘There is more than reasonable evidence to suggest that the coachroof failure may have had as much to do with the stiffness of the panels as their strength,’ it says.

What convinced FYD that the destruction was the result of an exceptionally large wave, a so-called rogue wave, was that damage was also seen in other areas that had undergone repeated significant loading during the Vendée Globe and other races. There was damage to one of the yacht’s forward topside panels, though not to the internal structure of the hull shell.

They say: ‘Based on what can be deduced about the loading event one can say in all seriousness that if the coachroof had been twice as strong we would likely still have seen damage, which gives some idea of how far outside the normal bounds this event was.’

As a result of the findings, FYD and the boat’s owners, OC Group, decided to repair the coachroof by replacing the smaller, stiff panels with slightly larger panels that are both stronger and more flexible.

The power of huge waves to damage even the toughest of yachts has been well documented and I’ll blog a bit more about that in the next few days. I’ll also look at the myth and reality of rogue waves, so do check back and in the meantime do add any comments.