The Making of A One-Design Hard Sail
Rig Tech interview from our Jan/Feb 2011 issue. The one-design wing for the AC45 is a scaled-down version of the mammoth sail used to win the 33rd America's Cup, but designing it is by no means simple.
When BMW Oracle Racing revealed its 223-foot wingsail for USA-17, racing sailors and casual fans of the America’s Cup were once again schooled in the incredible efficiency of racing with a “wingsail.” After two lopsided races, the power and inherent advantages of this massive foil were obvious, so it comes as no surprise that the Cup defender will employ the same for the 34th America’s Cup as well. Hard sails will power both the AC72 and the AC45, the latter a one-design pre-Cup boat for syndicates in training, so we’ll be seeing plenty of wings in 2011. To better understand how the engineers are tackling this new task, we sought out mast designer Scott Ferguson (the 2010 Laser Master World champ), who was involved in the development and deployment of USA-17’s wing. He’s one of an army of engineers inside the Oracle Design Group, and the team’s immediate task is to create the wing for the AC45, which will be the sport’s first-ever, one-design wing package.** **
**Is it easier to design a hard wing than it would be to design the perfect rig-and-soft-sail package? **
I wouldn’t say easier, just different. The main reason we have soft sails is because they are much, much easier to handle, get up and down, reef, and store, but it is a considerable compromise from the pure foil shape, like a rudder, keel, or airplane wing. The efficiency is as much as 80 percent better, but you’re still faced with determining the right amount of area. In theory, you need less area for a specific wind speed with a hard wing, compared to a conventional setup. But on the other hand, you want to have more area in light air, and for downwind sailing, which means you’re overpowered when the wind increases. So, you still have the same old problem of controlling the shapes. With soft sails, this comes with mast bend and sail shape, which is a bit more complicated to model properly than the twisting flap elements on a hard wing.
What are the fundamental parts of a hard wing, what do they do, and why are they called elements?
I’m not sure where the term elements came from—probably from Dave Hubbard. When we were working on USA-17 we went on to start acronyms for the “Flap Elements” and “Main Elements” A middle frame in a flap, for example, was called a “FEIF (Flap Element Intermediate Frame). The USA-17 wing is a two-element wing. The C Class Catamaran wings are three-element wings, the third element being a short tab at the trailing edge of the main or leading-edge element. Imagine an infinite number of elements that could create any shape you desire—it’s great from a pure aerodynamic standpoint, but as always, there’s a compromise between weight and structure and performance, and we are looking for the right balance.
In developing the sail for the AC45, where does the design team start—with the spar, and then on to the leading edges and trailing edges?
The main focus, initially, is the structural spar and how it is supported with rigging. Headsails create a fundamental difference compared to the C Class wing, which only needs to support the side forces from the hard wing. Headsails require significant headstay tension in order to create reasonable sail shapes. This means we need a running backstay and hence very high mast compressions. USA-17’s wing had compressions in the range of 60 tons (132,000 pounds). With the AC45 we will see about 10 times less than that, but this is still a significant amount of compression. The span is another big factor: the longer the span, the more efficient the wing area, but at the expense of structural buckling, length, weight, and likelihood of pitchpoling. We’re using the same exact principles we used with USA-17’s wing in an effort to try and simplify the design process.** **
How intricate must the control lines be to adjust all the elements?**
The control system will be a simplified version of what was used for USA-17, which is not much different than the 1988 wing that was on the 60-foot cat Stars & Stripes. Each flap has a line attached to a control arm. It’s similar to a rudder quadrant that leads to a steering wheel, except in this case there are multiple flaps leading to one wheel that controls the twist of the flaps. The upper flaps generally have less load than the lower flaps. We interact with the aero guys to come up with both flap angle targets and loads.
How do you make sure the wing is manageable across a wind range, assuming the class will race in everything from 5 to 30 knots of wind?
It will be very interesting to see these boats sailing in 30 knots of wind. I’m not sure if I could keep my Laser upright around a whole course in 30. In 5 knots, we have headsails to increase the area enough to fly a hull. The righting moment is fixed, so it really doesn't matter how windy it is—the loads come primarily from the boat’s stability. As the wind increases, we reduce sail area by dropping the headsails and increasing twist in the wing.** **
How aggressive is the airfoil shape in terms of being a more forgiving profile than that of USA-17?
It’s a little more forgiving because of the wider range of conditions the boats might face and smaller yachts will pitch more, which is disruptive.** **
Is it designed to survive a high-speed capsize?
It’s like asking if a boat will survive hitting a rock—is it rock No. 2 or rock No. 7? There will certainly be significant damage to the film and light frames, especially if any water gets inside. We don’t know what the capsizing loads are, but the structural spar of the wing is designed to handle side load of about half the boat weight in normal sailing, and many times that in compression.