As is ever the case with the Amer­ica’s Cup, the fastest boat won. ETNZ de­signed and de­vel­oped a cat that was faster on all points and in all con­di­tions than any other. How was this achieved on a bud­get a frac­tion the size of those en­joyed by its ri­vals?

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The an­swer lies in an open-minded ap­proach to de­sign, iden­ti­fy­ing the best-per­form­ing op­tion and work­ing to re­move the ob­sta­cles to it rather than com­pro­mis­ing. That, and hav­ing great faith in your de­sign and sim­u­la­tion tools, since the bud­get, ge­og­ra­phy and time­line did not al­low for full-size test­ing along­side the other chal­lengers.

Much has been said of the ‘bru­tal de­brief’ the team went through fol­low­ing the de­feat in San Francisco. Grant Dal­ton re­counted that 20 or so points came out of the meet­ing as the high­est pri­or­i­ties for next time. Top of that list were tech­nol­ogy and ca­pa­ble people, en­cour­ag­ing the ‘new gen­er­a­tion’ of sailors com­ing through the ranks, and giv­ing in­di­vid­u­als re­spon­si­bil­ity with­out con­straints.

Glen Ashby ex­plained the di­rec­tion given to the en­tire team was to ‘throw the ball as far as pos­si­ble, and then run af­ter it hard’. That meant en­vis­ag­ing the best pos­si­ble de­sign, then over­com­ing ob­sta­cles along the way to make it real.

And that’s ex­actly what they did – com­bin­ing ex­cep­tion­ally ef­fi­cient yet sen­si­tive foils de­mand­ing a com­plex con­trol sys­tem, with plen­ti­ful hy­draulic power to al­low pre­cise and fre­quent ad­just­ments to keep them flying.

We’ve heard how cy­cle power was dis­counted early on by other teams be­cause ‘the clips would get tan­gled in the net­ting’ or ‘the mount/dis­mount would be too slow’. With each of these arguments the other teams inched them­selves away from the Cup-win­ning con­fig­u­ra­tion.

Credit has to go to the en­tire ETNZ de­sign team. With­out doubt there were small vic­to­ries along the way that made at­tain­ing the big goal pos­si­ble. That said, the de­sign process was over­seen by Dan Ber­nasconi, who no doubt brought a rigour to the team’s pro­cesses from his ex­pe­ri­ence with For­mula 1, and it is as­sumed he was re­spon­si­ble for the de­liv­ery of – if not the ac­tual ‘go/no go’ de­ci­sion – the de­sign path ul­ti­mately adopted.

Guil­laume Verdier, re­spon­si­ble for de­sign­ing the foils, de­vel­oped a con­cept and ge­om­e­try that drew upon the lessons learned in San Fran and took it to the next level. ETNZ’S foils re­port­edly gen­er­ated the same lift from 20 per­cent less area (and hence less drag) than Or­a­cle USA’S.

They could be con­fig­ured to gen­er­ate more lift for the same drag, or any­where in be­tween. Chris Draper, wing trim­mer for Artemis Rac­ing, ac­knowl­edged the ‘ag­gres­sion in the de­sign choices’ ETNZ had made.


A com­bi­na­tion of many things, each re­in­forc­ing the ef­fec­tive­ness of the others in a vir­tu­ous, up­ward spi­ral rather than a vi­cious, down­ward one. The adop­tion of cy­clors is the most ob­vi­ous, but they are re­ally the so­lu­tion to an­other prob­lem – how to pro­vide suf­fi­cient power to con­trol the foils.

They had the ad­di­tional spin-off of less windage/drag than tra­di­tional arm grinders while af­ford­ing the helms­man a clear view for­ward, and also al­lowed cy­clor Blaire Tuke to have his hands free and fo­cus on the fine con­trol of the foils.

This di­vi­sion of labour – al­low­ing Peter Burl­ing to fo­cus on steer­ing and ob­serv­ing the race track, Glen Ashby to trim the wing and jib and Blair Tuke to con­trol the ride height – ul­ti­mately turned out to be more ef­fec­tive than the tra­di­tional helm-tac-nav tri­umvi­rate which loads the helms­man with both ride height con­trol and wing sheet dur­ing a turn.

The dag­ger-foilil ge­om­e­try is the next ma­jor dif­fer­ence. ence. What fol­lows is sup­po­si­tion based d on ob­ser­va­tion, since the de­tails of the de­signs are still very much un­der wraps.

ETNZ’S foils (in in blue) had a slightly in­board curved ‘shaft’ shaft’ (the part in­side the boat and thatt just reached the wa­ter) com­ing in to the up­per foil at a no­tice­able knuckle. The up­per per foil curves out­board then a sharp tran­si­tion nsi­tion into the hor­i­zon­tal flat and kinked tip. ip.

Many com­mentsnts greeted this kink, but to my mind it’s the he hor­i­zon­tal el­e­ment that holds the se­cret to o their suc­cess – more

on that later. Or­a­cle USA (in red), on the other hand, ex­hib­ited foils with a straighter shaft, a much less pro­nounced knuckle be­tween shaft and up­per foil, a tighter ra­dius into the lower foil and either a gen­tle curve or no de­flec­tion at all in the lower foil to tip.

Less eas­ily de­tected was that ETNZ’S foil tips had a much higher as­pect ra­tio – they were longer in span for ap­prox­i­mately the same area. Broadly speak­ing, foil area de­ter­mines how much lift you can gen­er­ate; the span de­ter­mines how much drag is at­ten­dant with that lift.

The greater span means that ETNZ’S foils gen­er­ate less in­duced drag, or less tip loss. If you can gen­er­ate less drag in to­tal, the thrust from the wing will make the yacht faster in the same wind­speed. If the foils go faster through the wa­ter, they gen­er­ate more lift, in­creas­ing VMG, so you could re­duce the area, which will re­duce the drag fur­ther…

Back to the sig­nif­i­cance of hor­i­zon­tal el­e­ment. Or­a­cle’s foils looked like a slightly more re­fined ver­sion of what

they had in San Francisco. The foil is a slightly curved shaft, so that its lower part changes its an­gle to the hor­i­zon­tal as board ex­ten­sion is in­creased.

The down side of this is, at full ex­ten­sion, a re­duced pro­por­tion of the to­tal force gen­er­ated (ma­genta) is ver­ti­cally up­ward (red), and so re­duces the amount of lift op­pos­ing grav­ity. As a re­sult more area is re­quired, which comes with its at­ten­dant drag penalty (see pix be­low).

ETNZ adopted a hor­i­zon­tal mid-span with all the force act­ing ver­ti­cally – it needed less area to lift the same weight of yacht. As well as adding area, the kinked tip in­creased span to re­duce in­duced drag and pos­si­bly, quite clev­erly, drawn with twist to re­duce the lo­cal an­gle of at­tack and thereby move the cen­tre of pres­sure (ie; lift) out­board which would in­crease right­ing mo­ment (I con­cede this is pure sup­po­si­tion).

Fur­ther­more, the smooth­ness and sta­bil­ity through tacks and gybes was partly due to the hor­i­zon­tal el­e­ment. As the yacht carves through the turn with both boards down, the ‘old’ board on the out­side of the turn is travelling faster than the ‘new’ foil on the in­side of the track. This gen­er­ates more flow over the foil, which pro­duces more lift. In the case of ETNZ that force is ver­ti­cal and helps keep the wind­ward hull up as the boat set­tles on the new tack or gybe.

For Or­a­cle that di­he­dral vee causes the out­side foil to gen­er­ate more force than the new, in­side foil, and cru­cially that force (ma­genta) is in­clined as it’s per­pen­dic­u­lar to the vee. The in­ward com­po­nent (green), be­ing greater on the wind­ward side, will try to push the yacht to lee­ward, in­creas­ing the lo­cal an­gle of at­tack of the new foil un­til the wind­ward foil is re­tracted.

The net re­sult is more drag and un­evenly dis­trib­uted ver­ti­cal lift across the boat. All of which will slow you down and make it harder to bal­ance the boat, and ex­plains ETNZ’S faster speed and greater VMG through tacks and gybes.

The con­trol sys­tems – re­quired to move the foils, rud­ders and wing (known col­lec­tively as con­trol sur­faces) and main­tain the ride height and give ETNZ that elu­sive 100 per­cent fly-time – were equally cun­ning in their de­sign.

We know from the Rule In­ter­pre­ta­tions (which make in­ter­est­ing read­ing in them­selves – http://no­tice­board.­ter­pre­ta­tions ) that one team (we don’t know which) re­quested clar­i­fi­ca­tion un­der In­ter­pre­ta­tion no. 70.

This con­cerned a tablet screen with a sec­ond, elec­tri­cally-iso­lated, trans­par­ent screen, phys­i­cally air-gapped and over-laid. The lower screen shows the tar­get val­ues cal­cu­lated by the on-board com­put­ers, the up­per screen the ac­tual state/po­si­tion of the var­i­ous con­trol sur­faces.

But ‘auto-pi­lots’ are pro­hib­ited as the AC50 class rule 15.4 spec­i­fies that elec­tri­cal sys­tems for ac­tu­a­tion and po­si­tion sens­ing of con­trol sur­faces be elec­tri­cally-iso­lated from each other but also that only man­ual in­puts are al­lowed in terms of al­ter­ing the con­trol sur­faces.

The crew mem­ber moves the con­trol sur­face by either touch­ing the up­per screen at the po­si­tion in­di­cated as the tar­get value or, as in sub­se­quent In­ter­pre­ta­tion no.72, by ad­just­ing some other slider, knob or but­tons to align the ac­tual po­si­tion with the cal­cu­lated, tar­get po­si­tion.

It’s re­ported that the ba­sis of ETNZ’S con­trol sys­tem was

Luna Rossa’s ‘auto-pi­lot’ from their AC72 pro­gramme, which was of­fered once Luna Rossa had with­drawn from AC35. We don’t know which team sub­mit­ted In­ter­pre­ta­tion no.70, but it looks a lot like what Glen Ashby and Blair Tuke were us­ing.

The bal­ance of trans­verse forces on an AC50 is such that the heel force from the rig is op­posed by the lift force from the lee­ward dag­ger-foil and the wind­ward rud­der T-foil. The AC rule al­lows the rud­ders to be raked dif­fer­ently on each side, and ad­justed in each tack or gybe, through a max­i­mum range of 3o. This al­lows the wind­ward T-foil to have a neg­a­tive an­gle of at­tack, pulling the wind­ward cor­ner down and in­creas­ing right­ing mo­ment, while the lee­ward one is neu­tral or close to it.

To in­crease the force gen­er­ated up­wind, when boat speeds are slower, the yacht is sailed in an ex­ag­ger­ated bow-down trim, pitch­ing the stern up and in­clin­ing the T-foil fur­ther to in­crease its an­gle of at­tack. But there is risk as­so­ci­ated with this as it brings the wind­ward T-foil closer to the wa­ter’s sur­face and in­creases the dan­ger of it break­ing the sur­face.

To mit­i­gate this, ETNZ em­ployed two more tech­niques. The first was al­low­ing the plat­form to twist un­der back­stay load. This re­quired clever en­gi­neer­ing to en­sure the plat­form only twisted just enough with­out break­ing, and also meant less lam­i­nate and thus less weight was ex­pended on the struc­ture. ETNZ’S was re­port­edly the light­est of all the AC50S mea­sured in Ber­muda, and that again helps un­load the dag­ger-foils fur­ther.

Sec­ond was the adop­tion of slight wind­ward heel, very much like a wind­surfer. This has three prin­ci­pal ef­fects: one, to bring the weight of the en­tire wing­sail over the cen­tre­line, in­creas­ing right­ing mo­ment; two, to di­rect the force from the wing­sail

above the hor­i­zon­tal which marginally re­duces the heel­ing mo­ment and lifts the lee­ward hull, both of which un­loads the lee­ward dag­ger-foil, mean­ing it pro­duced less drag; and three, to im­merse the wind­ward T-foil more and so mit­i­gate the risk of it be­ing flown so close to the sur­face.

To main­tain this tricky bal­ance of wind­ward heel, bow-down trim, hooked in on a twist­ing plat­form at 30 to 40 knots on foils on the verge of let­ting go in a straight line is go­ing to be hard enough. Add in tack­ing and gy­bing while the other team tries to stop you and it’s clear the reliance on the con­trol sys­tems func­tion­ing and the hy­draulic oil pres­sure (to al­low mi­cro-trim­ming of the con­trol sur­faces) is crit­i­cal to the suc­cess of the con­cept.

A large part of main­tain­ing that bal­ance is the wing. Glen Ashby’s screen could pos­si­bly have worked in a sim­i­lar way to the board con­trol sys­tem with an air-gapped over­laid LCD, and what was no­tice­able early on was the very dif­fer­ent trim­ming tech­nique he was us­ing.

Most ob­vi­ous was the ab­sence of a winch, opt­ing for a ram in­side the wing base in­stead. Many wing trim­mers have re­marked that, un­like a soft sail, you can’t ‘see’ the ef­fects of trim change, so it is best (eas­i­est?) to rely on the tar­get num­bers for twist and play the sheet. While no doubt Glen Ashby had tar­get num­bers to re­fer to, he spent a lot more time watch­ing the wing sail than any other wing trim­mer.

He also had a dif­fer­ent ap­proach, favour­ing play­ing the twist on the (smaller) up­per el­e­ments and min­imised sheet ad­just­ments, as op­posed to a pre-set twist and play­ing the sheet on the (larger) lower span.

It caused BAR crew­man Fred­die Carr to re­mark that the ETNZ wing was ‘flap­ping like a hum­ming bird’s wing’. This would have had a huge ef­fect on the heel­ing mo­ment. Hav­ing the long­est lever arm it gen­er­ated more mo­ment for less hy­draulic pres­sure lost and gave less vari­ance in the side force on the dag­ger-foils than play­ing the sheet would.

Or­a­cle took full ad­van­tage of the five-day break to re­con­fig­ure their yacht. They re­port­edly shed al­most 100kg, they com­bined their heavy-airs dag­ger-foil (lower drag) with light airs tips (high span, high lift), and opted to run the boards at less than full ex­ten­sion so the tips were closer to hor­i­zon­tal (but risk­ing a touch-down).

The rud­der T-foils were en­larged (for more right­ing mo­ment), but the rud­ders them­selves were skin­nier to com­pen­sate for the added drag. They also changed their wing trim style to be closer to Glen Ashby’s, at least as far as they could given the deck lay­out and hy­draulic pres­sures.

The end re­sult was they in­creased the yacht’s speed but at the ex­pense of con­trol, both in the sense of sen­si­tiv­ity and ac­tu­a­tion, as they lacked the in­put de­vices and the hy­draulic oil to man­age the now skit­tish yacht.

The one race they did take off ETNZ, in the strong­est winds of the match it­self, could most likely be at­trib­uted to ETNZ nurs­ing their crit­i­cally-dam­aged foils – cracked as a re­sult of be­ing taken out of range in a heavy airs race with Artemis, but kept se­cret so as not to ex­pose the vul­ner­a­bil­ity.

As the breeze died, so did Or­a­cle’s chances, and ul­ti­mately ETNZ righted the wrongs from San Fran. By adopt­ing ef­fi­cient foils, so­phis­ti­cated con­trols, plen­ti­ful power gen­er­ated by cy­clors, who, with their hands free, were able to share the work­load and with their heads down gen­er­ated less aero drag, ETNZ had a boat that was in­trin­si­cally faster, and that kept get­ting faster through­out the tour­na­ment through con­stant re­fine­ment.


8-1 was ETNZ’S match point score in 2013 8-1 was ETNZ’S race win/loss score in 2017 1-7 was Or­a­cle’s score­board in 2017 17 is the name of Or­a­cle’s boat The writ­ing was on the wall.


As the breeze died, so did Or­a­cle’s chances, and ul­ti­mately ETNZ righted the wrongs from San Fran.



ABOVE RIGHT ETNZ show­ing the way. It all added up to faster, smoother tacks and gybes.

ABOVE The cy­clors’ su­pe­rior mus­cle power pumped the hy­draulic fluid around the ves­sel more ef­fi­ciently.

In­no­va­tion, team­work, ded­i­ca­tion and sheer bril­liance. What a team!

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