Update
Hotting up… As interest grows in the 5.5 Metres, supply has picked up with new boats proving – contrary to legend – a match for those ‘unbeatable established’ designs. Polish Finn gold medallist Mateusz Kusznierewicz (above) has the perfect CV for the class, multiple Finn titles followed by two world titles in the more technical Star. As in the Star, he and his ex-Finn and Laser friends arrive hiking harder too. The engaging Pole won the Alpen Cup in Garda crewed by Przemyslaw Gacek and another man with multiple Finn Gold Cups, Ed Wright. His Polish Aspire team also just bought a Cape 31; no comment needed there. Nevertheless world champion Peter Morton’s latest Hollom design won the first race in Garda and was leading the second when her rudder fell out. It’s not over yet
NO CONSENSUS – Jack Griffin The AC75s of 2021 delivered exciting and high-quality racing that surprised many people, including your scribe. They mostly stayed right side up. The sailors mastered foiling tacks and gybes and take-offs in all but the lightest airs. The yachts we saw in the 36th America’s Cup diverged greatly
from the November 2018 concept video that introduced the new class. The yachts capsized but the self-righting idea turned out to be a fantasy. We didn’t see the crew hoisting or furling a Code 0. We barely saw the crew at all. We didn’t see them loading the jib sheet on the rule-required winches or casting it off during tacks – both the winches and the crew were tucked into the cockpits to reduce aero drag. The new version of the rule incorporated those lessons. The Code
0 is gone from the rule. The Code 0 bowsprit that extended the 68ft yacht to its nominal 75ft has become a wand to hold wind instru- ments. The winches for the foresail sheets are gone – jibs are self- tacking. Also gone is 1,000kg of weight – the better to enable taking off at the low end of the wind range. And the foil wingspan has increased from 4 to 4.5m. The devilish challenge of designing those wings remains. Accepted wisdom says that the new generation of AC75s will be
closer in performance than the four yachts that raced in 2021. Or will they? The designers learned from what they tried in the first Cup cycle with this class. They learned from seeing what others did. It seems that all the teams will go to the two-helm set-up that Luna Rossa pioneered. We’ve now had two full Cup cycles to absorb the lessons from
Team New Zealand’s slim, high aspect ratio foil wings. It is generally thought that this concept is less stable and harder for the sailors to master, but it delivered the winning combination of lift, manoeu- vrability and low drag on the Kiwi AC50 in Bermuda and again on their AC75 in Auckland. The current state of foil wing design shows that the designers are
far from settling on one concept. Plan forms vary dramatically. Fore and aft offsets of the foil wings relative to the foil arms vary dramat- ically. Foil arm fairings and foil wing bulbs grow and shrink, probably to house actuators for the flaps
The previous rule required that any one hydraulic actuator could
only be mechanically linked to one control surface. Electric actuators were allowed to control multiple control surfaces. The current rule gives more flexibility. The foil arm stocks are still one-design and supplied. They weigh
464kg. Teams can use another 806kg for foil arm fairings, wings, flaps and control systems. Aye, there’s the rub – where to put those 806kg? Put the flap actuators inside fairings attached to the foil wing, like
Ineos’s W-wing? Having that weight outboard increases righting moment but also increases wetted surface. Move the actuators up into the foil arm fairings and connect them to the flaps with pushrods? Slightly less righting moment but also a smaller bulb and less wetted area. Use foil wings with multiple segments or one straight or curved segment with a single flap running across the entire span? Now decide how to deal with laminar and turbulent flow over the wing and around the foil arm and the transition between them. Should you move the main lifting surface of the foil wing aft of the foil arm? To make things even more interesting Alinghi Red Bull Racing
launched a foil wing on their AC40 with tubercles. The August 2011 issue of Seahorse had a strong article speculating on the hydro - dynamic benefit of adopting these forms, seen in nature on the dorsal fins of dolphins and the flippers of humpback whales. Also in 2011 the Journal of Integrative and Comparative Biology,
published by Oxford University, studied the tubercles on humpback whales’ flippers. Humpback whales have exceptional ability to make aquabatic manoeuvres to catch prey, using their wing-like flippers for banking and turning. A 2019 paper published in Cambridge’s Journal of Fluid Mechanics
also explored the effect of tubercles and reported that they may reduce flow separation and suppress the formation of a laminar sep- aration bubble in the tubercle troughs in the transition from laminar to turbulent flow. Tubercles may help avoid stalling the foils during manoeuvres. It will be interesting to see how much they help Alinghi. The images from the joint recon programme show that the
designers are far from settling on the one ‘right’ answer to these questions. The Protocol prohibits sailing an AC40 or AC75 anywhere other than Barcelona from July to October this year. [The Protocol is silent on sailing an own-design LEQ12 test yacht, so Luna Rossa
SEAHORSE 15
ROBERT DEAVES
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