Team New Zealand bow down and flat out in Bermuda with her trademark teardrop-configured cyclor group and their resident cycling Olympic medallist Simon Van Velthooven (page 20) tucked down at point. The bow-down trim of the AC50s was established early on as the teams pursued maximum downforce and therefore righting moment by using a negative angle of attack on the weather rudder foil. Contrast Pete Burling’s secure helm position with Jimmy Spithill hanging off the back of USA 17 in Valencia in 2010 on page 40
As a schoolboy, working in vacations
for a company that built hydrofoil boats, I learned early on about what the engineers now call ‘stuffs’. At the time they were developing a product called Sea Wings for retrofitting to simple outboard runabouts. In their first iteration the foils were
fibreglass, crude by today’s standards. The lack of precision led to the same sort of crashes, which were a shock to me the first time I witnessed one. Later, riding around on these boats, the cause of the crashes became clear: if an air bubble appeared on the top of the foil, especially at higher speeds when the foils were barely in the water, lift was instantly lost – the same thing that happens with rudders that ‘pull a bubble’ during the onset of a broach. Later, with more precise, machined alu- minium foils, the Sea Wing problem was mostly solved. Nevertheless, if the boats got too high on the foils, bubbles could still result in a crash. With wing-driven sailboats and their
much more complex, suddenly changing loads and angles, not to mention foiling height, foil design is more difficult and avoiding stuffs becomes much harder. By 2017, though, the incidence of such crashes, if not totally eliminated, was greatly reduced. In 2013 foil development rapidly went
from simple T-foils on relatively straight daggerboards to sophisticated L-shaped systems. However, the AC72 rules forbade fore-and-aft angle changes to T-foil equipped rudders that would control the boat’s pitch. Pitch could only be controlled by fore-and-aft angle changes to the dagger boards. I thought at the time it was a pretty unfortunate rule because rudder- mounted elevators only required simple actuation systems that were more efficient; the actual systems had to be exponentially more expensive, not to mention just a whole lot more difficult to use.
44 SEAHORSE In the commercial and military world
hydrofoil systems are controlled largely using fly-by-wire software that combines sensing foil height, craft pitch and roll to keep foil-borne operation steady. Such software was prohibited in the America’s Cup so the boats had to be ‘hand flown’. Hand-flying foil-borne craft, especially
with almost fully submerged foils that are largely unstable in pitch, is extremely diffi- cult, requiring rapid, world-class athlete- type reflexes. The hydraulic control sys- tems, which were powered by human effort, were imprecise. To change pitch angles on the main foils required careful button presses where the longer you held your finger on the button the more the daggerboard angle changed. But hold it too long or not long enough and the result could be unwelcome… The controller only had a vague idea of the angle that would result from all his button pressing. Oracle’s team devised an electro-
mechanical system where a button press created a precise angle change. Press once, you get 0.5°; press twice and you’d get 1°. The system was also adjustable, creating different angle increments depending on the day’s conditions. It was controversial and Team NZ protested saying it was software, but the jury, noting that no software was being used, just human-induced electronic impulses, rejected the protest. This system was a decisive advantage when upwind foil- ing came into play in the second half of the longest ever America’s Cup Match. There was also a major difference in
wing design between Oracle and Team New Zealand. TNZ opted for the C-Class system with a round, load-carrying spar inside a twistable forward element. The TNZ wing was highly adjustable, making it easy to change from deep and very power ful to flat and twisted. The adapt- ability made it ideal for inducing foiling, especially in the lower wind ranges where
the Kiwis were always faster. Oracle, bent on saving weight and reduc-
ing complexity, opted for a forward main element that was monocoque, with no internal structural spar. It was lighter but couldn’t be twisted. In their two-boat testing programme in AC45s they found that the twisting forward element was only superior up to 10kt of true wind speed. Above that Oracle’s tests concluded their choice of a structural wing would be faster in San Fran- cisco’s regular 10kt+ thermal breezes. Carbon rigging continued to evolve
with Carbo-Link supplying all the con- tenders with their streamlined solid rods. Sailing with the foils required new tech-
niques and rapid reactions, especially upwind. Flying high on the foils was fast but too high and the boat suddenly skated sideways. Keeping the foils more deeply immersed limited sideslip but was slower. The biggest change in foil/wing sailing
was the elimination of the big A-sails. On foils the boats were just too fast for the big, relatively full sails, which effectively, in Ashby’s words, ‘applied a handbrake’ at foiling speeds. The jibs evolved to ever flatter, smaller sails doing just enough to power up the lower areas of the wings. Early on it appeared that TNZ had
made the correct wing choice and seemed faster off the wind with the ability to power up more while being able to depower the upper areas of the wing when necessary. As history showed, they domi- nated the early Cup races and almost won convincingly. The turning point during the Match came when Oracle perfected upwind foiling before TNZ. The big improvement apparently started
when Oracle’s Tom Slingsby, after watching TNZ practise, sent the team an email sug- gesting sailing lower upwind to reach foiling speed. The team’s wing designer Joseph Ozanne had maintained up to this point that if the boat were sailed upwind at a ‘tight’
INGRID ABERY
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