When the AC72s of the 34th America’s Cup first lifted off on their foils they attained great speeds of more than 50 miles per hour, or close to three times faster than the wind, which few of us could ever have imagined. In fast these boats turned out to be much faster than even the designers expected.
However these great speeds also brought great danger, which first came to light when Oracle Team USA capsized their AC72 during a practice session in San Francisco Bay after passing through the so-called “death zone”. But what exactly is the “death zone”? That’s exactly what we’re going to find out in this video.
Modern sailing boats can sail in most directions relative to the wind, except straight upwind, with the fastest point of sail being a beam reach, where the wind is perpendicular to the direction of travel, or sailing across the wind.
Race organizers know that sailing back and forth on a beam reach would just get boring for everybody, so they design courses with upwind and downwind legs instead, because it’s more challenging for the boats and their crew.
Boats can’t sail straight upwind, but they can sail upwind at around 45 degrees to the true wind, so they tack back and forth in a zigzag pattern to get upwind.
But going downwind, they also gybe in a zigzag pattern because it’s faster than sailing dead downwind, as they are still sailing across the wind,
AC72 crews put the boat together on shore with the right size wing for the forecast conditions, to optimize efficiency for upwind and downwind sailing.
But there are two points on the course where the boat has to pass through a beam reach, at the upwind and downwind turning marks. This is the most powerful point of sail, so the boat has a tendency to accelerate rapidly out of control, earning the name “death zone” by popular media.
Many fast multihulls must deal with the “death zone”, but most have soft sails, so they can easily twist off the top of the main sail to depower it, which also moves the power down lower on the sail, and makes it easier to pass through the “death zone”, with the lower part of the sail still driving the boat forward.
But the AC72s have rigid wings so you can’t twist off the top, and the whole wing needs to be eased to depower it.
And if you depower too much you’ll fall of the foils, which is also slow and dangerous.
Oracle Team USA had been practicing on San Francisco Bay in a building breeze, so they were on the verge of being overpowered even before they entered the “death zone” as they attempted to switch from upwind to downwind sailing.
They were hit by a gust in the middle of the Death Zone, the boat accelerated rapidly, they were unable to depower the sails fast enough or turn out of the Death Zone because their rudder came out of the water, so they lost control and capsized.
Thankfully nobody was hurt, however it did set the team back several weeks and cost a few million dollars to repair the boat.
Somehow, they launched the comeback of the century and managed to win the 34th America’s Cup.
This video includes content for educational purposes under fair use:
Oracle Team USA Images: Guilain Grenier
Sail Twist Images: Andrew Madding
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