That piece of New Zealand’s foils disturbing the dreams of Luna Rossa’s designers

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One of the most distinguished guests of Il Giornale della Coppa, Luca Bassani, had told us something in one of the episodes of our program that had us a bit suspicious. He had reported this to us, “I talked to Mario Caponnetto (the ‘wizard’ of fluid dynamics on Luna Rossa ed.) and he told me about a component in the foil foot of Team New Zealand that seems to be very interesting because they were able to make a very thin profile.”

THE ANSWER IN THE PHOTOS

We were intrigued by this, so we went and made some enlargements of the photos we have available to try to figure out what this mystery component was. We don’t know if our research pinpointed the correct part, but in fact carefully examining the photos of Team New Zealand and Luna Rossa something is discernible, and we are not talking about the shape of the foils.

The appendages consist of the arm, which has that vaguely S-shape, and by the end piece, in a different color, which may be T-shaped or Y-shaped. The secret of kiwis, apart from their T-shaped foils which we will discuss shortly, may be at the top of the end piece of the appendage, just above the two wings. Let’s go through what it is in detail with a sequence of photos.

THE DETAILS.

The piece in question is the black-colored part of the appendage that lies between the arm (in yellow) and the horizontal wings. In the following images the detail and comparison.
On the kiwi boat the correspondent is also black here, between the gray of the arm and the end fins.
You can see how Team New Zealand’s foil has a fairly noticeable tapering just before the bulb and wing attachment, you can see the profile tapering unmistakably, starting with a certain section and ending with a narrower one where the bulb and wings go to attach.
The same thing cannot be observed on Luna Rossa, where the profile of the same portion of the appendage is distinctly more stubby and does not taper; it appears to be constant.

This is no small difference, considering that this is an important immersed part of the appendix. Obviously having made a tapered profile means that less hydrodynamic drag is generated in the water, which combined with wings that are quite a bit narrower and leaner than Luna Rossa’s explains why rumors from Auckland describe Team New Zealand as so fast. How much faster and under what conditions? Quantifying the difference without knowing the details of the measurements is impossible, we can assume that these shapes become very effective in strong winds when the boat can fly very steadily, and at that point the less friction there is the faster it will be. Conversely in low winds, very skinny shapes can mean instability, with the appendages likely to make the Kiwis’ boat a bit more wobbly. The fuller shapes of Luna Rossa’s appendages below 12-13 knots, on the other hand, provide the precise control and clean foiling that the Italian boat showed at length in the final against Ineos. Luna Rossa, however, is expected to take to the water these days with some new components; in fact, there is talk of a new boat setup designed especially for the final.

Which of the two solutions will prove more successful?

Mauro Giuffrè


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