On The Foil­ing Fron­tier

The AC75 Rule for the 36th Amer­ica’s Cup in Auck­land in 2021 presents a unique chal­lenge for de­sign teams to de­liver some­thing never be­fore seen at such a scale.

Sailing World - - Starting Line -

Q The Amer­ica’s Cup is about high-per­for­mance sail­ing and cut­tingedge de­sign and tech­nol­ogy. To­day, it must also em­brace the el­e­vated skill level re­quired to sail such high-per­for­mance craft. Match rac­ing, which is the Cup’s tra­di­tional for­mat, is a dis­ci­pline that adapts to the type of boat be­ing used for the com­pe­ti­tion. Dial-ups, slam dunks and tack­ing du­els have evolved, and the same thing will hap­pen with the AC75. New moves will be coined in the years to come, and cer­tain ma­neu­vers will be equally, if not more, dif­fi­cult to ex­e­cute. Youth and ath­leti­cism are re­quired to sail the lat­est Amer­ica’s Cup ma­chines, but ul­ti­mately, the suc­cess of each ef­fort be­gins with the de­sign and con­struc­tion of the plat­form it­self.

One of the first steps an Amer­ica’s Cup team will take when

drilling into a new de­sign rule is to paint a rough pic­ture of what the rule in­tends, which re­quires ex­ten­sive draw­ings and com­par­isons. The sec­ond step in­volves cre­at­ing a base- line hull, ap­pendage and sail plan for in­put into a pro­pri­etary ve­loc­ity- pre­dic­tion pro­gram. A good VPP can give clear in­di­ca­tions for what vari­a­tions within a de­sign rule are worth ex­plor­ing with re­gard to per­for­mance.

The VPP re­quires a tremen­dous amount of in­put and ac­cu­rate data to sup­port the op­ti­miza­tion process. It will be one of the cen­tral clear­ing­houses for de­sign ideas and what-ifs for the AC75, but it also serves as the back­bone for race mod­els and race sim­u­la­tors that in­tro­duce a heap of ad­di­tional vari­ables. Many ques­tions can be an­swered with a com­bi­na­tion of these tools, but re­al­ity of­ten brings un­ex­pected re­sults, and many lessons can be learned by build­ing some­thing real.

The base- line de­sign will evolve as quickly as teams re­ceive re­sults, and de­sign it­er­a­tions will con­tinue un­til build dead­lines. With any new boat de­sign, and es­pe­cially in a new class, no one area can be ig­nored be­cause there is no sta­tus quo to fall back to. There is a lot of work ahead, for chal­lengers and de­fender alike, but the two pre­vi­ous Amer­ica’s Cup cata­ma­ran classes — the AC72 and the AC50 — are good bench­marks to be­gin com­par­ing the over­all de­sign traits of the AC75.

Weight, Sta­bil­ity and Power

When look­ing at a boat’s per­for­mance po­ten­tial, de­sign­ers and engi­neers must ex­plore the big three: weight, sta­bil­ity and power. When com­par­ing the sail plans (Fig­ure 4), the lower rig height of the AC75 is the first ob­vi­ous dif­fer­ence. The AC75 will weigh slightly more than the AC72, and twice as much as the AC50, which means it will strug­gle in light winds and down-speed ma­neu­ver­ing. With less drag on the sail plan and foils, how­ever, the AC75 should be strong once it’s up and go­ing in strong winds.

Cata­ma­rans have in­her­ent zero-de­gree sta­bil­ity, with low wet­ted sur­face and light dis­place­ment. They can af­ford to have more power in the sail plan in or­der to fly the wind­ward hull, then quickly pop up on the foils. In con­trast, the AC75 mono­hull is wide, which in­tro­duces a lot of wet­ted hull sur­face and a very low right­ing mo­ment in dis­place­ment mode. An In­ter­na­tional Moth uses a nar­row hull to ad­dress the wet­ted-sur­face prob­lem, and the crew’s weight on the tramp con­trib­utes to its right­ing mo­ment. A Moth, how­ever, is dif­fi­cult to sail at low speeds and is prone to cap­size. The AC75, be­ing a much larger boat, re­quires a wider hull for sta­bil­ity and to cre­ate a wide enough plat­form to get the port and star­board foils apart. The power-to-weight ra­tios of the AC75 when it’s not fly­ing ver­sus fly­ing gives an in­di­ca­tion that the tran­si­tion cross­over may be more chal­leng­ing than it was with the cata­ma­rans.

The Hull Truth

The AC75’S hull shape is “open,” but there are a num­ber of tight con­straints, such as length, which is 20.65 me­ters (67.7 feet), plus or mi­nus 50 mil­lime­ters. The bow’s full­ness is lim­ited by two max­i­mum-beam mea­sure­ments within 4 me­ters of the bow, pre­sum­ably to avoid scow-type hull shapes. There is also a min­i­mum bow vol­ume of 40 cu­bic me­ters, in con­sid­er­a­tion of pitch-pole pre­ven­tion, which the shorter rig will also help with. The cant and axis of ro­ta­tion of the side foils is fixed at 4.1 me­ters apart, which will ul­ti­mately infl uence wa­ter­line beam. Max­i­mum beam is set with a tight tol­er­ance of 4.8 to 5 me­ters, and min­i­mum beam at the tran­som must be 4 me­ters.

De­sign teams will spend a lot of time ex­plor­ing hull shapes within the above lim­its, look­ing for a shape with min­i­mal drag in light-wind dis­place­ment mode while also ad­dress­ing the sta­bil­ity re­quired to gen­er­ate thrust for take­off. Wind­speed lim­its and the ex­pected wind con­di­tions at the venue will be a hot topic be­cause they can in­flu­ence the over­all de­sign pack­age. Crews were com­fort­ably foil- tack­ing the AC50S down into the 8- to 9-knot true wind­speed zone. It will be in­ter­est­ing to see where that zone ends up with the AC75.

The hull has no sig­nif­i­cant lim­its on struc­ture other than in­ter­me­di­ate car­bon­fiber mod­u­lus and a hand­ful of stan­dard AC rule lim­its. Some level of re­cy­cled ma­te­ri­als must be used in the con­struc­tion, which is a re­fresh­ing di­rec­tion for the Amer­ica’s Cup. Teams are al­lowed to build two hulls, with the ear­li­est launch dates of April 2019 for Boat No. 1 and Fe­bru­ary 2020 for Boat No. 2.

The Shape of Foils to Come

The AC75’ s port and star­board foils are com­posed of a foil arm, a foil wing and a flap at­tached to the foil wing. The foil arm will be sup­plied equip­ment (lim­ited to four). The foil wings and wing flaps are open to de­sign but must fit within de­fined pa­ram­e­ters. Teams are re­stricted to build­ing six foil wings and 20 foil flaps. Wings and flaps can also be mod­i­fied up to 20 per­cent of their weight. Each foil assem­bly has a pre­scribed weight of 1,175 kilo­grams that it must hit within 1 per­cent, and a cen­ter of grav­ity that is re­quired to be close to the foil wing for an over­all min­i­mum sta­bil­ity. To avoid ex­cess vol­ume in the foils, steel will likely be the ma­te­rial of choice for the lower por­tion and wing. It’s in­ter­est­ing to note that the two foils con­trib­ute 40 per­cent of the boat’s over­all weight. Re­main­ing weight con­sid­er­a­tions for boat struc­tures, sys­tems and deck equip­ment will be on­go­ing to en­sure the boat stays on weight.

The T-foils will re­quire ex­ten­sive de­sign and engi­neer­ing re­search, much like they did with the AC50S. Ma­neu­ver­ing ca­pa­bil­i­ties will heav­ily in­flu­ence the op­ti­miza­tion process. Foil- wing ar­eas will be gen­er­ally re­duced to the bare min­i­mum for steadys­tate straight- line sail­ing. Ex­pect two to three dif­fer­ent foil wing­spans and then some vari­a­tions in the flaps to suit con­di­tions. Flap lengths are lim­ited to 50 per­cent of the to­tal chord of the wing-flap com­bi­na­tion.

The main foils can only ro­tate on one for­ward/aft cant axis. In­stead of pulling a foil up and down through a bear­ing case when tack­ing and jib­ing, as it was with the cats, the foils on the AC75 will ro­tate up and out of the wa­ter. In many ways, this is a much sim­pler sys­tem than those used on the AC50S and AC72S. Ad­di­tion­ally, the AC50 and AC72 ro­tated the en­tire foil to change the lift of the main foil, whereas the AC75 has only the flap ro­tat­ing, like the flap on a Moth. The Moth uses a wand mech­a­nism to pro­vide au­to­matic con­trol of the flap, but with the AC75, the crew will need to man­u­ally op­er­ate the flap.

The L-shaped foils of the last two Cups are gone, re­placed by the AC75’S T-style foils. Much time and money were spent in the last Cup an­a­lyz­ing dy­namic foil sta­bil­ity ver­sus per­for­mance. The V- shaped foil ori­en­ta­tion used on foil­ing cata­ma­rans like the GC32 pro­vides sta­ble flight with min­i­mal ad­just­ment, but at the ex­pense of hy­dro­dy­namic drag. The T-foils of the AC75 class can be ori­ented to pro­vide both lift and side force to min­i­mize the in­duced drag (drag due to lift), which al­lows more speed po­ten­tial, pro­vided the crew and con­trol sys­tems can han­dle the loss in dy­namic sta­bil­ity.

Only a sin­gle rud­der and rud­der wing is al­lowed. When foil­ing, the rud­der and the main foil work to­gether to lift the boat out of the wa­ter. Hav­ing the rud­der on cen­ter­line does rob some right­ing mo­ment from the boat be­cause it is off­set to wind­ward of the main foil by roughly 5 me­ters. Ideally, one would po­si­tion the foils on the boat so the rud­der takes very lit­tle lift. How­ever, the rule im­poses lim­its on the

cen­ter-of-grav­ity lo­ca­tion and lim­its on the pri­mary foil lo­ca­tion. As a re­sult, teams will have a ten­dency to: a) place the rud­der as far back in the boat as pos­si­ble; b) push the boat’s cen­ter of grav­ity to the rule’s for­ward-most limit with deck sys­tems and com­po­nents; c) push the main foil against the aft limit. All of this is in­tended to limit lift on the rud­der wing, or in other words, try to cen­tral­ize the boat weight onto the main foil, which is much far­ther to lee­ward.

Rud­der lift will be ad­justed by ro­tat­ing the en­tire rud­der assem­bly about the lower bear­ing, sim­i­lar to the AC50S, and will be ac­tively con­trolled dur­ing ma­neu­vers to keep the boat level and maybe also make ac­tive ad­just­ments while straight-line sail­ing.

Power in the Sail Plan

Sails re­turn af­ter tak­ing a back seat to the rigid multi-ele­ment wings of the past seven years. The AC75 Rule pro­vides a frame­work for a dou­ble- sur­face main­sail and a per­mit­ted con­trol sys­tem, and con­trol ar­eas at the top and bot­tom of the mast (Fig­ure 3) that per­mit greater shape con­trol than a nor­mal main­sail. The main­sail area is lim­ited to a min­i­mum and max­i­mum, within 7 per­cent. The mast is a large one-de­sign “D sec­tion,” sim­i­lar to the AC50 wing, and it will serve as the lead­ing edge of the main­sail and struc­tural tube. The mast will pivot on a ball that also trans­mits com­pres­sion load from all of the sup­port rig­ging, all of which will be sup­plied to the teams. The hatched area in Fig­ure 3 shows the only area al­lowed for mod­i­fi­ca­tion on the mast where the main­sail track or boltrope groove will be in­cor­po­rated into the struc­ture.

Head­sails are fairly stan­dard, with their profi les gov­erned by leech per­pen­dic­u­lars and girth lim­its. Small, medium and large code-zero-style head­sails are al­lowed within these lim­its and will be cho­sen based on the day’s sail­ing con­di­tions, up­wind and downwind. If foil­ing con­di­tions ex­ist, I sus­pect there will be one head­sail choice for the race. Fig­ure 1 shows the re­stricted po­si­tion­ing of the mast and tack po­si­tions of all of the head­sails, ef­fec­tively plac­ing the rig and sails in the boat at a spe­cific lo­ca­tion. Self-tack­ing jibs, in­ter­est­ingly enough, are not al­lowed, pre­sum­ably un­der the guise of not al­low­ing an au­to­matic sys­tem, but it will also give the crew some­thing to do.

Con­trol and Foil-cant­ing Sys­tems

As with the pre­vi­ous two Cup cy­cles, the speed and ac­cu­racy re­quired to ad­just sub­merged foils will have a huge im­pact on over­all boat per­for­mance. The abil­ity to sail the boat con­sis­tently at the apex of con­trol­lable flight could be one of the dif­fer­ence mak­ers. The hard­ware and soft­ware re­quired for such pre­cise flight, there­fore, will be one of the fo­cal points of the de­vel­op­ment teams. The AC75 Class Rule specifi cally al­lows battery power for the ad­just­ment of all un­der­wa­ter ap­pendages (ex­cept steer­ing), com­pletely re­liev­ing the crew from pro­duc­ing the power to achieve move­ment, but still re­quir­ing man­ual in­put from the crew to con­trol the an­gle of the foils. The sys­tem to con­trol and change the cant of the main foils to put the foils in and out of the wa­ter will be sup­plied equip­ment that can­not be mod­i­fied, so the speed and power to move these will be the same for all of the teams.

All sails, mast con­trols, hal­yards, etc., can only be ad­justed us­ing di­rect crew en­ergy. Un­like the rigid- wing sails of Cups past, the main­sail will re­quire sig­nifi cant leech ten­sion to hold shape. This is a big dif­fer­ence com­pared to the AC50 and AC72 wings, which could be ad­justed both in twist and over­all an­gle with much less load and en­ergy. Fast ad­just­ment with wings is much eas­ier, giv­ing more heel con­trol with less en­ergy re­quired. Trim­ming through twist alone might not be as easy as it was with the AC50S. There­fore, we could see vang sheet­ing or highly effi - cient low-fric­tion traveler cars for fast heel con­trol as a fo­cus for de­vel­op­ment, along with top main­sail twist con­trol, which has a very open de­sign space in the top 4 me­ters of the mast and sail.

Power to the Crew

Un­til the pre­vi­ous Cup and 2010’s Deed of Gift match be­tween Or­a­cle Team USA and Alinghi, stored en­ergy, or en­ergy on de­mand, wasn’t al­lowed. In the IACC Class (1990 to 2007), more than half the 17-per­son crew were ded­i­cated strictly to power pro­duc­tion (grinders) to fa­cil­i­tate ef­fi­cient tacks, jibes, sail hoist­ing and re­triev­ing. In 2013, we saw the race­courses short­ened and the crew re­duced to 11. Once foil­ing was in­tro­duced, ex­treme fit­ness was re­quired be­cause eight of 11 sailors were grind­ing al­most all the way around the race track to sup­ply power de­mands and keep the boat op­er­at­ing at peak per­for­mance. In the 2017 Cup in Ber­muda, the AC50 fur­ther re­duced the crew to six sailors and short­ened the course yet again. Ac­cu­mu­la­tor tanks were al­lowed to buf­fer en­ergy pro­duced by the lim­ited crew so it could be used when­ever re­quired. The AC75 Rule has an 11-crew limit, but again, a sig­nif­i­cant por­tion of the crew will be ded­i­cated to en­ergy pro­duc­tion that three or four pri­mary sailors will pro­duce.

Tra­di­tion­ally, large boats re­quire large crew num­bers to han­dle and change a num­ber of sails to get the boat around the race track. These foil­ing boats, how­ever, are dif­fer­ent an­i­mals in that ap­par­ent-wind an­gles and ap­par­ent- wind­speeds hardly change be­tween sail­ing up­wind and downwind, which is why chang­ing sails is less com­mon for this high-per­for­mance breed. Tra­di­tional spin­nakers and gen­nakers that re­quire tech­nique and many hands have no place on a foil­ing sail­boat. Q

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