Luna Rossa Prada Pirelli ready to launch the AC 75, here’s what the new boat might look like

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In green and yellow are the possible new hull shapes of the ac 75. The green solution is more conservative, recreating the air tunnel but not going to recreate particular flow accelerations that will be greater in the yellow version, but with increased downforce risks or generally imposing a more complex design of the “match” between hull, foil and deck.

If the America’s Cup were a poker game, the moment we are about to experience could be called a “show-down,” which is when the cards are finally revealed to play it all out. And April will be just that, the month of the first showdown, where the unions will show the fruit of the research and design work of the past two years, namely the new AC 75s.

Luna Rossa Prada Pirelli will launch its new hull on April 13, and as we told you, this year it will literally be forbidden to make mistakes: in fact, only one AC 75 can be built, not two as in the last edition.

So what will these third-generation AC 75s look like? Probably up to 3 percent faster than seen in the 2020/2021 America’s Cup in Auckland and with some design innovations for hull and foil shapes. Often in evaluations of these boats there is a lot of focus on the foils, rightly so too, but it is forgotten that for the appendages to work best in the water you also need a hull with certain aerodynamic characteristics that “cooperates” with the appendages.

This was the winning secret of Team New Zealand, which with Te Rehutai could afford to use very small, narrow, and fast foils because it had a hull that helped the lift of the appendages due to the cushion of air created under the hull. How then will the hulls of future AC 75s evolve?

In an extensive feature in the April issue of the Journal of Sailing, which has just arrived on newsstands and is also available in a digital version, we tell you about it with analysis by mauro Giuffrè and Federico Albano. The following is a small preview about the hulls.

AC 75 – The shapes of the new hulls.

With full foiling boats such as the AC75s, the function of the hull actually changes, as seen in the last edition, the debut of the new class. Beyond some specific shapes to facilitate the “take-off” of the craft, the designers work on the hull shapes primarily to minimize its aerodynamic drag while at the same time trying to take advantage of its design to ensure an optimal and stable flight attitude.

The V-shaped sections of the old Luna Rossa will most likely not be revived in the new generation of AC 75

Starting with the “pseudo” living work of the hull, the Luna Rossa-style V-shaped versions at the last Cup seem for these reasons destined to disappear. Te Rehutai already presented a beginning of solutions that are likely to be taken much further to the extreme in the next bowl.

The outer part of Luna Rossa Te Rehutai’s hull was already virtually flat, while the inner part had a depression that actually created a tunnel in which air could pass from the bow to the stern.

This is where the first fork in the road opens up for designers, and it will be interesting to find out which path will be chosen since both concepts, which we think will be followed and which we were already exposing to you in the opening pages of the report, have positive and negative sides.

A hint of what the preferred choices are we had with AC40, who have already picked up some of the concepts from the past edition and made them their own. In these boats, the hull is almost completely flat, going to try to reduce the frontal section of the boat by focusing mainly on minimizing drag.

AC 75 – The importance of the “tunnel”

In red is the air tunnel under the hull that designers tried to recreate on the new AC 75s.

The other viable route is the de facto creation of two long tunnels under the boat that would go to channel air flows due to motion and discharge them aft. This presents an initial analogy with today’s Formula 1, where aerodynamic downforce in modern single-seaters in the top motorsport series is generated by two Venturi channels located under the car that converge in the rear diffuser.

The main difference, of course, is that the Venturi channels of a Formula 1 car generate downforce, whereas a Cup hull must actually generate some lift to simplify conduction and stabilize flight.

This means that the hull must work in full harmony with the deck profile, turning the hull into a true wing. This process could be particularly complex to fine-tune with the risk that the tunnels could accelerate the aerodynamic flow to such an extent that they would in turn go on to generate downforce and thus tend to pull the boat back toward the surface of the water.

Conversely, should they function properly they could lead to accelerated flows around the deck, which would increase the pressure on the sails and decrease the wind height gradient. However, all of this remains an extremely difficult process to calibrate, especially for variable conditions, so the two design paths remain very open in this regard.

The ultimate goal remains to generate a volume of high-pressure air (a kind of “cushion”) immediately below the boat under sail, to which the boat can virtually “lean” thereby achieving stable flight and precisely minimizing drag. Both paths can lead to this result; we will see how they will be implemented.

By Mauro Giuffrè and Federico Albano

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