Update
The 1989/90 Vendée Globe and a young Loïck Peyron (inset) is requested by race HQ to assist Philippe Poupon whose sleek Philippe Briand-designed ketch is capsized deep in the South Atlantic, failing to right itself. In a remarkable piece of seamanship, as Poupon set about cutting away his mizzen to reduce weight aloft Peyron rehearsed manoeuvring around the stricken yacht under sail, then successfully getting a line across before towing Poupon head to wind again under sail – at which point the now-sloop came upright. Peyron finished the race as runner-up to race winner Titouan Lamazou… but that’s not why the first Vendée Globe made him famous
POETRY IN MOTION – Jack Griffin At this writing, New York Yacht Club American Magic’s second boat, Patriot, has sailed. Ineos Team UK have christenedBritannia II and will shortly be sailing. Luna Rossa Prada Pirelli are fitting out their second boat in Auckland. Same for Emirates Team New Zealand. All four teams will be training in their raceboats from late October
through November and early December. Racing begins (finally!) on 17-20 December with all four teams in the America’s Cup World Series and Christmas Race. Both American Magic and Ineos Team UK have departed from the scow design of their first boats, and added a bustle. Ineos Team UK’s bustle looks the most extreme – longer, wider and more rectangular than anything we’ve seen so far. By the time you read this we will know how the Italians and the Kiwis have evolved their hull designs. With all four teams sailing their second boats in Auckland, the
audience will expand from the AC geeks like your writer, and the wider sailing and general sports audiences will try to understand how these boats fly. Lift. It has been said that anyone can explain how a rocket works – but to understand lift you need to be a rocket scientist. Unfortunately, many explanations of the physics of lift are either
incorrect or incomplete. One of the most widespread explanations of lift is plain wrong: the fluid flowing over the upper surface of a foil (aero or hydro) has a longer distance to travel and therefore has to be faster to rejoin the fluid flowing past the lower surface. The Bernoulli effect creates a lower pressure on the upper surface result- ing in lift. Wrong. There is no reason why the fluid molecules moving past the upper and lower surfaces have to rejoin at the trailing edge of the foil. Lift is much more complex than that. Bernoulli is definitely involved, but don’t forget Newton. An accurate but mostly unhelpful explanation is that lift occurs
because fluid flow obeys the Navier-Stokes equations with a no- slip condition on solid surfaces. Doug McLean, former chief engineer at Boeing, admirably explains the ‘real physics’ of lift without the
14 SEAHORSE
mathematics in his excellent and dense book, Understanding Aero- dynamics. As McLean says, ‘Explaining lift in physical terms is more difficult than most people realise.’ McLean devotes 38 pages to exploring the misconceptions and explaining the real physics. At risk of oversimplification, the concepts can be boiled down
to four factors that affect lift: angle of attack (AoA), foil shape, fluid density and flow speed. AoA is easy to understand and demonstrate: just put your hand out of the window of your car when moving at around the speed of an AC75 – about 40kt. Angle your palm up or down and you’ll understand the Newtonian physics of AoA. Likewise, most people understand the effect of flow speed. When
the pilot pushes the throttle forward an aeroplane rolls down the runway until it reaches take-off speed – about 54kt for a Cessna and 140kt for a commercial airliner. An AC75 in displacement mode needs to build enough speed to get into foiling mode, carefully sheeting on with very little righting moment, until it is up on the foils and massive righting moment kicks in. The importance of fluid density is also clear. A Cessna 172 weighs about 760kg; an AC75 ten times more – 7,600kg. Water is about 700 times more dense than air. Travelling through water, an AC75 needs much smaller foils and much less speed than our Cessna to get airborne. The fourth factor in generating lift is the one the America’s Cup
designers get paid for: foil shape. A flat surface with no camber is perfectly capable of generating lift, as our hand-out-of-the-car-window experiment demonstrates. But the AC75 foil wings are far, far more complex. The class rules for the AC72s in 2013 and the AC50s in 2017 forbade flaps. AoA was the main control – achieved by raking the entire daggerboard. Foil shape was crucial, but without movable flaps the design task was very different from that posed by the AC75 Class Rule. The wings on the AC75 foil arms have movable flaps. In fact, the
foil arms may not be raked, only raised and lowered by the one- design foil cant system (FCS) supplied by the defender. AC geeks will compare all the teams’ foil wing designs – span, chord, thick- ness, bulbs, winglets and more. We won’t be able to see deformation
ALEA
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