Sailing World - - Contents - BY DAVE REED

The faster boat will win. Here’s how.

While many of the Amer­ica’s Cup Class cata­ma­ran’s com­po­nents are one- de­sign, par­tic­u­larly hulls and head­sails, ex­perts have drilled into new ar­eas as never be­fore: foils, wings, aero­dy­nam­ics and con­trol sys­tems. While the boats might look alike, the hid­den dif­fer­ences are crit­i­cal.

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 pre­done 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.

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 end­plate 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. Q

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