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Cup Spy - May 11: The Swiss reveal the 'Tubercle' wingfoil - a look at where it works best.

by Richard Gladwell/Sail-World NZ 21 May 17:16 PDT 13 May 2023
Tubercle wingfoil - Alinghi Red Bull Racing - AC40-LEQ12 - Day 26 - May 18, 2023 - Barcelona © Alex Carabi / America's Cup

[This report is backdated to the events of May 11, which occurred while Cup Spy was recovering from appendicitis.]

Only two America's Cup teams came out of the shed today.

The British hoped for some sailing in their test boat T6, however the breeze was not co-operative off Mallorca, and most of the day was spent chasing wind pressure.

In Barcelona, the Swiss Alinghi Red Bull Racing team unveiled one of the more interesting design ideas seen at this stage of the 2024 America's Cup. The "Tubercle" wing borrows concepts seen in nature and also in aircraft design and would seem to have benefits at both the lower end of the speed scale - maintaining flow over the wing, and at the upper end giving some control/delaying cavitation.

What happened in the Cup - May 11, 2023:

  • Alinghi Red Bull Racing did not sail, but did reveal a new wing on their AC40-4, now LEQ12, which has been christened the "Tubercle", due to the presence of a "lumpy" leading edge - akin to the same "lumps" on the leading edge of a humpback whale fin. The idea has been tried on aircraft. The Swiss team did undertake a towing test which lasted 40 minutes.
  • INEOS Britannia were the only team to actually try to sail, this day. While they did get some sailing done much of the day was spent under tow chasing the elusive breeze.
  • American Magic wrapped up their Pensacola based program, yesterday May 10, pending pack-out and transition to their new base at Barcelona, where they will be based until the end of the 2024 America's Cup.
  • Luna Rossa did not sail.
  • Emirates Team New Zealand did not sail due to adverse weather conditions.
  • No further news from the Orient Express Team (formerly K-Challenge).

Alinghi Red Bull Racing - AC40/LEQ12 - Day 25 - May 11, 2023 - Barcelona

The "Tubercle" foil revealed by Alinghi Red Bull Racing is quite a design departure from the contemporary evolution from high drag, high lift, wide thin chord, minimum bulb wings to the low drag, low lift, narrow thicker chord, larger bulb wingfoils.

The key features of the Tubercle foil is the "lumpy" leading edge, which are assumed to be aimed at tidying up flows over the wing, and the minimum sized bulb, meaning that the required weight that has to be built into the wingfoil/bulb has been put into the wing itself.

Tubercles appear on the leading edge of humpback whale fins, and their purpose is to improve the maneuverability of the humpback as it twists and turns at relatively low speed (a humpback whale has a top speed of 15kts, while the AC40/75 are sailing at 30-50kts).

On aircraft the idea of the tubercle was to improve maneuverability and lift as well as delaying stalling. The end game for the Alinghi Red Bull Racing design team would seem to be aimed at using the placement of the tubercles to manipulate the position of cavitation, and get an increase in top end speed, as well as maintaining lift during tacks/gybes - particularly at low speeds. Interestingly the takeoff speed for AC40's/AC75's (15kts) is about the same as the top end speed of a humpback. So he tubercle idea would seem to have applications at both ends of the AC75 performance scale.

While the study of nature is one source of inspiration for the AC wing designers, another interesting area is the development of fighter wings during the early stages of World War 2, where all sides made rapid design in an attempt to gain a speed and maneuverability advantage over their enemy - including the same trade-offs that we now see happening over 80 years later in the America's Cup.

From the AC37 Joint Recon team:

Alinghi Red Bull Racing rolled out their AC40 at 09:40 after being delayed due to morning rain. The team revealed a new port foil configuration, with the one design foil replaced by a swept back foil, with tubercles on the leading edge of the outboard side of the foil.

The foil does not feature any winglets, with flaps visible under the foil and the top surface apparently laminated over. The trailing edge of the foil arm connecting to the foil appears to have a slightly different profile, though we did not have the opportunity to take a side on shot for direct comparison.

GoPro cameras were set up around the boat and the mast to film the performance of the new foil. With a very aggressive chop and no wind, the team only managed a short tow, 3nm North along the coast and back.

Despite having sails prepared and ready to go at the base, the team did not sail. The waves and chop had no consistent direction, very messy sea state. The AC40 was still able to fly above the chop while on the tow.

When the yacht was brought back to the dock, the mainsail skins were carried on for a quick fit test. It appears adjustments have been made to the mainsail head, specifically about the trailing corners of the skins, possibly with new battens.

Designers were spotted focusing on the diagonal batten and taking pictures of the headboard. The sail was only in view for a short while as sailing was called off and the yacht was promptly craned out. There are no photos of the mainsail, however a video of the crew working on the sail is available for reference.

Session Statistics: Alinghi Red Bull Racing - AC40/LEQ12 - Day 25 - May 11, 2023 - Barcelona

  • Wind Strength: <5kts
  • Wind Direction: 'Xaloc'-South East Wind-Less than 5kn at the time of towing
  • Weather: Sunny 12°C
  • Sea State: Beaufort 4
  • Roll out: 1010hrs Dock Out: 1100hrs
  • Dock In: 1140hrs Crane out: 1230hrs
  • Total Tacks: 0; Fully foiling: 0; Touch & Go: 0; Touch Down: 0
  • Total Gybes: 0; Fully foiling: 0; Touch & Go: 0; Touch Down: 0

INEOS Britannia - LEQ12 - Day 46 - May 11, 2023 - Mallorca

INEOS Britannia were the only team to actually try to sail, this day. While they did get some sailing done much of the day was spent under tow chasing the elusive breeze.

Top sailing journalist Justin Chisholm was on the water and made various observations on the British boat:

If the British team did not have as productive a day as they were hoping for it was not for any lack of trying and persistence on their part.

With a forecast of 10-15 knots of breeze this afternoon the scene was set for another solid day of testing in around Palma Bay. However, the low cloud that persisted since early morning bringing light rain showers, during the rigging and launching of the team's T6 LEQ12 test boat, failed to clear out to the south by early afternoon as forecast and choked off any hope of a decent sea breeze developing.

After delaying by half an hour the team docked out at 1330 and once out of the harbour began to hoist T6's mainsail. But before the sail reached the top of the mast a change of plan was called.

The double skin main dropped, bagged, and transferred to a chase boat before T6 was put on a bow tow for a 40 minute tow testing session along the Palma city front and down as far as S'Arenel.

The mainsail was hoisted at 1500 before the boat was towed at speed to the southwest exit of the bay.

The J2-1 was hoisted at 1520 and after 10 minutes of trying to self-take off in 6-7 knots the crew opted for a fast tow-up that triggered a 25 minute flight that took the yacht west along the Mallorca coast past the offshore rocks at El Torro and beyond the Port Adriano marina.

The flight was assisted by an uncertain and shifty 10-12 knot westerly breeze and hindered by some medium – somewhat confused – chop on top of at times a half metre swell.

With the breeze gradually becoming lighter more unsettled a second prolonged attempt to get airborne came to nothing. The yacht was put on a bow tow at 1650 for the 35 minute tow back into the bay and over to Palma.

Sails were dropped by 1745 with the team docking in at 1750. Another sailing session has been scheduled for tomorrow Friday May 12.

Session Statistics: INEOS Britannia - LEQ12 - Day 46 - May 11, 2023 - Mallorca

  • Weather: Overcast 13° - 21° C
  • Wind Strength 6-12kts Med chop - 0.5metre swell
  • Wind Direction: 260-270°
  • Roll out: 1145hrs Dock Out: 1231hrs
  • Dock In: 1750hrs Crane out: 1820hrs
  • Total Tacks: 12 - Fully foiling: 9; Touch & Go: 1; Touch Down: 2
  • Total Gybes: 3 - Fully foiling: 2; Touch & Go: 0; Touch Down: 1

Crew: Giles Scott, Dylan Fletcher, Leigh McMillan, Luke Parkinson

Additional Images:

This backdated commentary was written and compiled from video, still images and statistical content extracted from the AC37 Joint Recon program and other material available to Sail-World NZ including photo files, and other on the water coverage from the 2010, 2013, 2017 and 2021 America's Cups. Its format is intended to give Sail-World readers a snapshot of all teams' progress on a given day or period.

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