Is there a mainsail problem behind Alinghi Red Bull Racing’s dismasting?

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37th America’s Cup
Alinghi Red Bull Racing
ARBR AC75 B2
©Ivo Rovira / America’s Cup

In Barcelona on the next America’s Cup race course there was a significant setback for Alinghi Red Bull Racing, which dismasted during a training session in a stiff breeze on the new AC 75. The mast of the Helvetic boat literally collapsed, breaking into two pieces, as the AC 75 was making a foothold from upwind to downwind, with wind strength estimated at around 20 knots.

Fortunately, no crew problems were reported, and the hull appears to have emerged from the accident without significant damage. The reason for the incident remains to be understood, but there are already fairly realistic hypotheses in this regard.

Alinghi Red Bull Racing – The reasons for the dismasting

One can clearly see how, just above the red line on the mainsail, the detachment of the mainsail from the mast begins, an event that perhaps triggered the subsequent dismasting.

As pointed out in an interesting analysis by Vittorio D’Albertas on the Sail Ring Video Blog, there is a frame from the video of the incident that shows a detail that could be crucial in explaining why the mast collapsed. It would appear that the mainsail (which on AC 75s is double with two overlapping “skins”), in the area where the mast then breaks, detaches from the channel where it is inferred. This causes the load that must support the shaft to no longer be evenly distributed along the entire structure, which would cause the shaft to collapse. This would have happened if the mainsail detachment had occurred before the mast failure and not as an outcome of it. However, the video posted in this regard does not unequivocally clarify the timing of the mainsail detachment.

The mainsail has not simply slipped off the cheeks of the mast channel, but in the next frame it can be seen to be rather torn. Indeed, it can be seen that the space created between the sail and the mast is uneven.

At the same time, the detachment may have been caused by the breaking of the mast at that point, which in turn triggers the breaking and detachment of the sail. A structural failure of the pole, which inevitably involves the mainsail as well.

In this second frame you can see how the sail’s detachment is uneven; it did not simply detach from the canallet but tore. Is this the cause of the tree collapse or a consequence of it? The dynamics are not 100% clear.

If the sail had instead exited the mast before it broke, two hypotheses can be made: the first, the most trivial one, is a construction defect that caused its failure; the second hypothesis, on the other hand, is a bit more complex and concerns more the design of the sail and its “match” with the mast. In fact, in strong wind conditions it is likely to think that Alinghi was sailing with significant mast pre-flexion to thin the sail profile.

When this happens all the fat of the mainsail is drawn forward from the mast, the “turn” of the sail, i.e., the curvature of the luff, is absorbed by the mast, and clearly the forces involved and the pulls on the membrane structure increase. If the sail curvature is too little in relation to how much the mast flexes, critical conditions can be encountered, which in extreme cases can lead to sail breakage. Of course, we do not know whether this was the case; it is only a hypothesis.

When will we see Alinghi back on the water? In such cases, the most important thing for the union is not just to get back in the water as soon as possible, but rather to do so with very clear ideas with respect to what happened. In fact, there is no point in returning to the water if you are not certain of the reasons for the accident. Verisimilar to think that the Swiss team will be in the pits for a few weeks, a resting joke certainly annoying, but one that comes at a time when all things considered there is still plenty of time to remedy the problem and get going again.

Mauro Giuffrè

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