One could argue that all the fame and glory of the modern-day America’s Cup goes to the sailors, and beyond that, the helmsmen and faces of the respective franchise. But each and every one of these sailors will agree they’re only as good as the platform they’ve been given, and in doing so, they’ll point to their design-team brain trust (and boatbuilders, by extension) that employ complex design and engineering programs to integrate the needs and input of the sailors themselves.
The variables to consider today as the boats take flight are innumerable, says Nick Holroyd, lead designer and technical director with SoftBank Team Japan. Holroyd, a mechanical engineer by trade, has been involved with the Cup for nearly two decades. “The level of complexity and the subtleties are endless,” he says of the new AC50. “In that sense, they’re rewarding of time on the water, because the permutations of how you can set up the foils and the wing are huge.”
First things first: The foils, what do we need to know?
Unlike with the previous Cup, the systems are now very interconnected. When we started down the road with the foiling AC72s, we didn’t really know how to control the boats, so we found a shape solution for the boards that had a lot of inherent stability. With the 72s, the forces were really big, so we had relatively low power, which we still have, but we’re far more sophisticated now with how we use it.
In general, in sailing, whether on a monohull or whatever, anytime you’re generating lift off a keel, you have induced drag. That’s energy lost in the system, and the source of that drag is because you’re generating lift. In the early stages of designing these foils, we were designing where we had this sort of excess of lift and using those to balance out each other. Why I start with that is because that’s where you start feeding into control systems. Now we look at the global stability of the boat, and that stability includes the helmsman’s reaction time, and time for the hydraulics to actually move the board, plus the actual stability of the foil itself. As we work our way down that path, we find we can design boards that are negatively stable because the systems are good enough so the helmsman’s reactions are way better than they were with the AC72.
What do you mean by negative stability?
As in, when you take one of our modern foils and disturb it from equilibrium, the tendency is for it to accelerate away from the equilibrium, without the correction. Whereas before, we had a foil, we disturbed it, and it wanted to pull itself back toward its equilibrium point. Now we have a foil that when you disturb it, it wants to fire the boat out of the water, so without that correction movement in the control system, you have negative stability.
The beauty is we are generating a lot less lift overall, so we are not creating this excess of lift to create the balance point. We are generating just enough to lift the boat up (which weighs roughly 3 tons with all the guys on board) and about a ton of side force from the wing. So now we have boards that are very much L-shaped, and the forces are not fighting each other anymore. As a result, the foils have much less induced drag, and the spans of the foils compared with the 72s are much shorter.
So what’s that done for us? We’ve gone from having boats that only foil downwind — the takeoff speed in the 72s was 20-plus knots — to boats that foil upwind in pretty modest amounts of breeze. Take off is in the midteens. It’s been a drive to efficiency, and the system part of that is coupled into what we’ve been able to achieve.
So how efficient are today’s foils?
The drag you get when creating lift is much more pronounced at low speeds. For example, if I have a certain span, I have water going past that foil at quite low speed. I have to deflect the hell out of it to generate enough momentum into the water to lift the boat. You see that at low speeds — you can see a really defined wave trough behind the foil where it really presses the water surface. At high speed, what I have is tons of water flowing over the foil, and I have to deflect it only a small amount, so what we’ll see evolve is quite specialized foils. This is because the balance where the drag comes from changes significantly through the speed range. Your high-speed boards are skinny, short-span toothpicky-type boards.
What’s been interesting about this is we have to target a boat to win in certain conditions, but this time especially it’s not as predictable. We have to design a much more general-purpose platform. In 6 knots, it might be two hulls in the water and running square downwind, but that’s a design condition for us to consider. With another half-knot of breeze and we can get one hull to fly, and then with another half-knot or so we’re foiling downwind. Then another knot and we’re foiling upwind. So we have to design a foil that will take us from 6 knots on up. The way the physics of sailing the boat across the range changes is remarkable.
And what about at the top end of the wind range, 20-plus knots?
At the high-end, we have water boiling around the foils. The foil geometry becomes dictated by that, but we also need something that performs well in a straight line at high speed. The core need is stability. We’ll see some boats that are really in control and maybe not so fast. There will be others that show flashes of being blindingly quick but can’t be controlled very well, which is why the helmsman’s response times and the amount of power/watts you can put into the hydraulic actuators to move the boards around is big part of it all.
About those control systems, what can you share?
This time around we are allowed specifically for appendage rake, so we can use more-modern control systems where we can measure a position. The helmsman sets a command that he wants the board at “X,” and we have a displacement transducer or whatever that measures the board position and a little computer that says the board is currently at X, plus or minus whatever. We can get very fast time-to-targets, and then we can have the computer drive the board to that target. In San Francisco, we were pressing a button that said open the valve for .03 of a second and see what happens. The problem is that if you do that at 20 knots, it’s a different result than at 40 knots, so the boats were hard to sail because of that. We are in the modern world of control systems, but we’re a still a long way away. It’s been a fun science project. But one of the things that is making our lives easier is that we now have hydraulic accumulators: nitrogen-charged gas chambers that we can pump up to the relative pressure of a dive tank. When you see the guys winding the handles, a lot of what they’re doing is recharging the accumulators. We are using hydraulic power from those accumulators to control rake, boards up and down, and many other things.
The other advantage of working through an accumulator is that it can deliver about 400 watts over a sustained period of time. If you have a guy winding the handles, you can have oil delivered at 400 watts, which is many cubic centimeters per minute at any given pressure. If he’s pre-done all that work and put it into the accumulator, you can open a valve and instantaneously have much higher pressure coming out, so you can move boards much faster than operating them directly from the pump.
The efficiency of these systems, however, is in the many components, right?
Yes. We’re trying to achieve very fine levels of control with minimum power. There have been advances right down to the very fine mechanical engineering of valves and stuff; the custom in this industry is huge. I’d guess there are maybe 30 or 50 valves spread across the platforms, and that’s not including check valves and all the pressure-relief valves.
ORACLE Team USA reveal their ACC boat at the team base in Bermuda.
Given how much the aerospace industry already knows about foil shapes, how is it that the foil packages that are so closely guarded by the teams could be so different at the end day?
There are a couple of reasons. One is the dependency on the control systems: How much stability can I pull out of the foil, and how much do I rely on the control systems? That’s the number-one driver and dictates the different way the boats are being sailed. In terms of how people end up with different things, your choices in how you think the game will be played out at a tactical level. The last Cup started where we were foiling downwind, and that’s where the game was going to be won and lost. Then in the end, it was all won upwind. So where on the course are we trying to get our performance? Maneuvers, for example, how we are prepared to trade away straight-line speed for a board that is more forgiving through a lot of maneuvering.
We now have foils that are really quite unstable, so they have to be actively managed throughout every maneuver, and you have all these challenges: You first have to get the foil down, and when you first drop it into the water, you want no lift on it, so you actually drive it down to full extension to get it into the water. At some point, as the boat changes from one jibe or tack to the other, you essentially have the roll moment on the rig, which is going to change one side to the other. The weight will come off one foil and onto the other, so you’re going to have to manipulate both foils, because if you leave the one where the weight is coming off it and you don’t reduce the angle of attack on it, the foil will just keep producing the same amount of lift until it fires that side of the boat up into the air. So the decisions around the relative importance of maneuvering versus straight-line sailing is all that can drive different-looking foils.
There’s a lot happening through the turns than meets the eye, isn’t there?
Oh, yes. The lift you generate on the foil is proportional to the speed, so the first thing that is going to happen is you lose drive force at some point through the turn. You might be going into a jibe close to 40 knots, and your bottom speed might be 28, and you have to come out and accelerate. So to deal with that lift on the foil, you’re going to need to alter the angle of attack on it. As the rig loses its drive force in the middle of the maneuver — the drive force in the rig basically pushes the bow down, so the back of the boat is effectively heavier — you have to do something with the rudders to try to keep the pitch of the boat reasonably level. That means, in general, increasing rake on the boards to keep the boat up so it’s level. Then you’re having to increase rake on the rudders to keep the front of the boat up.
The third piece is the rig roll moment being in one direction; there’s suddenly lift on one side of the boat. Until the rig pops through, the roll moment is in the opposite direction, so you have to transfer all the lift from one daggerboard to the other. At that point, you start accelerating again, and then have to change the rudder rake again. You now have excess drive force, and the bows are being pushed down really hard. Typically, the helmsman will manage one board and the rudders, and one of the crew will manage the new board until the helmsman gets across and takes flight control back.
So through a jibe, for example, the helmsman steers down and starts to feel the roll moment. The power comes out of the rig as the apparent wind goes forward. The bit that makes it quite difficult is that when the new board drops into the water, the physics are unpredictable; depending on how hard it hits the surface. For example, if you’re high flying, the board hits the water with a lot of momentum, and quite often with a big splash. Once that happens, you’ve blown all the water away, and as the board goes deeper, the water sort of folds in over the top, and you get this bubble attached to the foil that gets carried down. The board then can’t generate any lift until that bubble washes away. It’s a fraction of second, and while sometimes it will go in nice and clean and hook up right away, there’s this moment of trying to figure out what it’s going to do.
The wing basically needs camber to be generation power, so we drive the camber through from one side to the other. Again, you want to be able to control the roll moment as it’s going from one side to the other, so the other thing that happens — and it sounds weird, and it’s more pronounced in the tacks than the jibes — is that one side of the boat is going about 3 knots faster than the other. You have one foil generating twice as much vertical force as the other one. When you see them do it well, it just happens, but when you try to step back and consider the physics and all the variables, it’s really impressive to see them do it.
When you watch them on the water, you can see whether it’s going to be a good jibe just by how well they keep the stability going into it. If they get a little bit off-balance, it really compounds. The boat gets a little unstable, too much heel on or whatever, and with that you get a little bow-down pitch. Suddenly, the [pre-set] angle of attack is off, and the foil wants to suck down really hard. You have to get it down and get a quick rake correction before the turn.
OK, enough on the foils. How about the wing?
The complexity in the wing design is that we have a system of cables and hydraulics to control the twist of the three flaps going up the wing. We have four control stations — one at the bottom and three above that — so we can sort of bend the wing into the shape we want. The complexity is when you twist a flap, a certain amount of the aerodynamic force is carried by the structure inside the flap. It takes a force to twist it, so you can choose to build that structure torsional rigidity and the flap will take a lot of the load. There’s a wide range of shapes, so what gets hard is to build control systems that give you control over all those shapes and lets you do that with relatively low power across the full range, from light air where we have the whole thing fully wicked up all the way up the wing, to heavy air where in up-range we can actually invert the lift on the top of the sail where we can keep twisting it until the head of the rig is lifting to windward.
With a multihull, the point at which the windward hull lifts off is the point at which you have maximum righting moment. We can do that in 6 knots of wind. As the breeze gets stronger, I have to move my center of effort closer and closer down to the platform. With a conventional soft sail rig we can move the center of effort down to about 15 percent of the rig height, twisting it until I generate a roll moment in the opposite direction at the head. On these boats, we can get it down to about 10 percent of the rig height, which is essentially using only the bottom panel. We still have the same roll moment, but we’ve halved the moment arm that wants to tip the boat, and still have four times the drive force. That’s why the boats keep going faster and faster down the course. In terms of wing design, the complexity is how to get a control system that is efficient and low-powered, but the loads of the control system are dictated by the structural choices I make in the wing. It’s a complicated piece of engineering. I’d rather build a board that’s 10 percent less stable, and use that power to control the board position more accurately and faster.
What about canting the boards inboard and outboard? At what point does this come into play?
There’s a trade-off with that. The board can’t go outside the beam, by rule. The more you move them outboard toward the max beam, the more righting moment you get. The force you’re using to lift the boat out of the water is getting farther out toward max beam, farther away from your center of gravity, so you have more roll moment. The price you pay is generally the foils become less and less stable as you go outboard. In flat-water moderate conditions, we can sail the boat at maximum power, so board canting is a way of dialing in the limit of stability. There might be times when the boat is flat out of oil, so you might choose to sail the boat a little bit slower but make my life easier while you generate some oil pressure, and you can do that with slightly less cant; you can be a knot slow, but you can reduce the amount of power that’s going to the wing sheet to make the boat easier to sail and catch up on some oil.
The third area of exploitation is the aerodynamics of the boat. Very subtle stuff here, it seems.
Some teams have chosen to put more area in the front beam versus the back beam. When you look down on top of the boat, you’re allowed 33 square meters of projected area, so your design optimization is how to best spend that and put it various places. In terms of the distribution, it comes back to how you set up the wing, to some degree. The front beam acts like the wing tip you’d see on an aircraft rudder; it has an end-plate effect where it helps smooth out the induced drag vortex you get off the bottom of the wing. Higher up the wind range, where the bottom of the wing is loaded really hard, you tend to get a strong vortex off the bottom of the wing, so if you’re designing into that area of the wind range, you’d distribute your area into the front beam. For lighter air, where lift is distributed across the length of the wing, you’d probably start to spread that area around a little more and look to minimize the drag off the crossbeams and so on.
