Wow, we never thought of it like that Ken Read


This quote was given to me by a designer in a current America’s Cup team, describing the type of rapid development still taking place within all the teams now


in Bermuda. One small change to one component and the decision-making reverberations are felt from top to bottom and front to back of the boat. Nothing happens without consequences to many, many other


areas and elements of the boat –many times surprising even the smartest of innovators. Hence the number of times someone stops and realises that ‘wow, I never


thought of it like that’ or ‘wow, this changes everything’. In last month’s article we looked at the underwater appendages


and some of the general trends. This month let’s look at the foils that stick up in the air. The wing, the headsails and the way they interact with the balance and stability of the platform. Let’s take a closer look at the wing elements that are fixed, or


one-design, and those the teams can tweak to their heart’s desire. While the ACC wings are considered ‘one-design’ there are only three one-design pieces to the aerodynamic package: a) the jibs (three sizes) b) the wing plan (size and shape) and specifically c) the front wing element. The jibs are North Sails 3Di Endurance, the most durable version


of 3Di, and, ironically for this market, not the lightest 3Di sails that North Sails make. Each team receives three identical one-design jib sizes and shapes, two of the three ‘end plate’ with the deck, in other words they fully seal themselves to the platform so there is no loss of air pressure. The third is a smaller sail used in TWS 18-20+ really just to help the crew manoeuvre the boat. The jibs are possibly the simplest element of the flying cats but


have still proved themselves vital to the crew’s ability to sail and balance the boat well. One interesting point, in most conditions the apparent wind speed is twice as much upwind as it is downwind. So what looks like nearly identical trim both upwind and down isn’t the case. A typical rough guess at the jib sheet ease to go downwind is 115mm, give or take. And the lead angle can go from about 4° upwind to nearly 12° downhill. The wings are composed of a single front element and a back


element split into three pieces: top, middle and bottom. The teams have built their wings in-house (or chosen a commercial vendor to supply the pieces) but the completed product is then carefully measured for dimensions, centre of gravity (VCG) and overall weight to ensure consistency across the fleet. The front elements can be built anywhere but they must be


one-design not only in weight and centre of gravity but also in bend characteristics and overall stiffness. There are very strict tests performed on each front section before it is deemed legal. The back wing elements have to be identical in plan form but,


unlike the front element, these can diverge in mechanical properties which leads to variations in twist. The beauty of carbon is that you can build it to be rigid, or build


in flex. With flex you have space to manipulate and contort the part with building wind pressure; but I don’t believe that this area of freedom will make enough difference to win or lose the America’s Cup. That said, if you get all the small decisions wrong, it can amount to a boat that just isn’t as fast as it needs to be. One of the things that wasn’t really talked about much in the


last America’s Cup was how Emirates Team New Zealand came out of the blocks with a very different wing set-up and twist profile from Oracle Team USA, not only with a twisting front element but with a radical trim of the sections top to bottom in the back of the wing. One of the biggest changes Oracle made during the last Cup Match was not only to how they sailed the boat but also how they changed their own wing shape and trim as the regatta went on to look much more like ETNZ’s trim. So, as an example, if you radically start tightening the lower leech


and twisting or opening the upper leech you present a completely different profile to the wind and you lower the centre of effort of the wing substantially. A byproduct of a big change away from what the


24 SEAHORSE


initial (neutral) shape looked like is that all of a sudden the structure (ribs inside the wing) experiences increased stresses and strains it was never designed to cope with, which can lead to premature failure. In 2013 the Kiwis went through this test/break/fix process when


testing pre-Cup. Oracle went through this process during the Cup. In fact, with a broken rib in the wing happening literally half an hour before the final historic race, they had a shore crewmember (Jeff Causey) up the mast with more quick-drying glue than you could shake a stick at. Not doing any practice sailing pre-start, giving the broken carbon frame as much cure time as possible, before finally sheeting on to get the boat going over the startline. Imagine what history would say about the 34th America’s Cup


if the hurriedly repaired rib had failed? For this Cup the teams have taken the extra loads of possible


radical trim and twist into account from the outset and therefore the mechanical properties of these parts have been tailored to accommodate this. The current AC wings look identical to each other and have the


same three adjustment elements (the three flaps out at the back) but they can be set up very differently. So aerodynamically let’s just say that each of these wings is made to deliver over 50° of twist. A more practical twist profile to be used at any given time is more like 30° which is approximately 10° per flap. To put that into context, if you were sailing an Etchells, imagine


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