Three of those aboard were injured at 11 pm local time on Tuesday when a huge wave surged over the stern as the vessel surfed down small mountains of water in a 40-knot tailwind.
Two of them were sent flying across the boat and were briefly pinned under water in the stern cockpit.
One of them had been at the helm, and the accident left no one steering the boat.
Under the force of the wave the boat suddenly and violently broached, turning quickly into the wind.
Fighting his way back to the helm, a member of the crew grabbed the wheel and tried to put the 68-ft yacht back on course, but steered the boat too far, causing her to gybe unexpectedly.
The boom swung very fast across the boat and, as it did, two of the crew are thought to have become entangled in the main sheet, the rope used to pull in the sail.
The most seriously injured was Hugh Gregory, who comes from England.