Will it fly?

The di­vided opin­ions re­gard­ing the new AC75 de­sign for the 36th Amer­ica’s Cup show no signs of con­verg­ing af­ter the re­cent pub­li­ca­tion of the Class Rule.

Boating NZ - - Contents - BY CHRIS­TIAN STIM­SON

Analysing the re­cently-re­leased AC75 De­sign Rule.

Whether the im­ages of the yacht with one foil raised/one low­ered re­mind you of a pee­ing dog or an ag­gres­sive pray­ing man­tis, the fact is there is no ref­er­ence point on which we can lean to un­der­stand the vi­a­bil­ity of the pro­posed de­sign. Be­cause it is so far re­moved from any­thing seen thus far. Even the most ad­vanced of the cur­rent crop of IMOCA 60s only be­come par­tially foil-borne when reach­ing.

With­out doubt the ETNZ and Luna Rossa de­sign teams are among the best and the bright­est, with the most so­phis­ti­cated sim­u­la­tion tools at their dis­posal. If they be­lieve the num­bers to be true, who are we to doubt them? To my mind Schrödinger’s Cat would be ap­pli­ca­ble here.

In his 1935 thought ex­per­i­ment, Er­win Schrödinger presents the sce­nario of a cat in a sealed box. Un­til the box con­tain­ing the cat is opened, and its fate known, the cat may be re­garded as si­mul­ta­ne­ously both alive and dead. I think un­til we ac­tu­ally see an AC75 sail­ing, both pos­si­bil­i­ties ex­ist si­mul­ta­ne­ously – they will ei­ther be an amaz­ing spec­ta­cle or an in­cred­i­ble flop.

I’m bank­ing on the for­mer. But why might they be a flop, with all that brain power di­rected to en­sur­ing they’re not?

Well, fun­da­men­tally, in set­ting out to do for mono­hulls what the AC50S did for mul­ti­hulls – rais­ing the tech­nol­ogy level with foils, wing­sails and con­trol sys­tems – the con­cept of a fully-foil­ing mono­hull has to over­come a num­ber of un­avoid­able en­coun­ters with the laws of physics.

Foils don’t aid sta­bil­ity un­til they are mov­ing forward through the wa­ter – they need flow over them. The force gen­er­ated varies with the speed of flow squared. If you dou­ble the speed you get four times the lift.

The con­verse is also true. If you halve the speed, you get a quar­ter of the lift. So the tran­si­tion from dis­place­ment mode to foil­ing will see huge changes in speed and lift, as will fall­ing off the foils or los­ing lift through ei­ther ven­ti­la­tion or cav­i­ta­tion.

In the event of los­ing lift through ven­ti­la­tion, the right­ing mo­ment will ‘switch off’ but the heel­ing mo­ment will not (or at least not in­stan­ta­neously, no mat­ter how fast the main is eased), so we could see cap­sizes.

To keep the yacht up­right be­fore the foils gen­er­ate enough lift, and to pre­vent the yacht from in­vert­ing if cap­sized,

bal­last is re­quired. Un­like the cata­ma­rans, which have high ini­tial form sta­bil­ity at rest, the mono­hulls need a low cen­tre of grav­ity to keep the rig point­ing sky­wards.

The mass of the bal­last – around 2.3-tonnes shared equally in the two foils – is added mass that needs to be both ac­cel­er­ated and lifted, in­creas­ing the loads on the foils and mak­ing them more sen­si­tive.

Cer­tainly, scale ef­fects and in­er­tia will es­nure things hap­pen slower than on a foil­ing moth, and Dan Ber­nasconi has ex­plained how, in the sim­u­la­tor at least, a par­tial loss of foil area to ven­ti­la­tion is not cat­a­strophic, lead­ing only to loss of ride height, slow­ing a pitch down by the bow.

In ad­di­tion, the top of the main­sail has a ‘gaff’ with a yoke that al­lows twist con­trol with­out high leech ten­sion. In this way heel­ing mo­ment can be con­trolled quite pre­cisely to mit­i­gate against cap­size.

Again, as ex­plained by ETNZ, the added mass (ap­prox three times that of an AC50) ac­tu­ally makes the yacht more pow­er­ful at higher speeds, even though the lighter AC50 is quicker on the foils in lighter winds ini­tially.

So it could be ar­gued that adopt­ing the mono­hull is solv­ing a problem that didn’t need solv­ing: a cata­ma­ran is al­ready known to be a good op­tion for a foil­ing race boat. Be­ing light, stiff and car­ry­ing no par­a­sitic bal­last weight, with wet­ted area and drag that re­duces by half with a small in­crease in heel an­gle, the mul­ti­hull op­tion is hard to beat.

Why not just stick the new, dou­ble- skin main­sails on the AC50S and solve the one big neg­a­tive as­pect of op­er­at­ing those cats?

The counter-ar­gu­ment goes along the Kennedy-es­que lines of ‘we do this – and the other things – not be­cause they are easy, but be­cause they are hard’. Why dis­miss what the fu­ture of mono­hull sail­ing might be­come?

At some point in the fu­ture dis­place­ment sail­ing will be seen in the same light as square-rig­gers and old gaffers – a quaint, sepia-toned nostal­gic in­dul­gence. The real busi­ness of sail­ing ef­fi­ciently (be­cause burn­ing fos­sil fuel will be out­dated too) will be foil­ing, no mat­ter what the con­fig­u­ra­tion of ves­sel.

Now, I’m not say­ing the cruis­ing yacht of the fu­ture will look like an AC75, but just as the Wright Broth­ers’ Kit­ty­hawk bears lit­tle re­sem­blance to a Gulf­stream cor­po­rate jet, or the Model T Ford to a For­mula 1 car, both can trace their lin­eage back to those points in time. We can iden­tify the steps on the way that were nec­es­sary to reach the cur­rent state of de­sign.

We’re only at the be­gin­ning of this par­tic­u­lar evo­lu­tion­ary jour­ney.


It re­turns to a box rule as we had be­fore the AC50 cat, in which a box of di­men­sions sets the en­ve­lope, and teams op­ti­mise their de­sign within that en­ve­lope. This en­sures close­ness of rac­ing, by lim­it­ing how far a team can de­part from the rest of the fleet while leav­ing enough room for cre­ativ­ity (and cun­ning) to play a role.

Some parts are spec­i­fied in their ge­om­e­try (such as the rig) and some are sup­plied (such as the foil arms) to limit the de­gree of spend­ing – both in terms of money and de­sign hours – al­low­ing teams to fo­cus on the ar­eas that do make the big­gest dif­fer­ence.

The hull length is set at 20.7m (68ft) plus a 2.16m bowsprit, taking the length to 22.86m (75ft). Beam is set at be­tween 4.8m and 5.0m. The weight (ex- crew) is set at 6,195kg, while the D- sec­tion mast is 26.5m long.

The pivot points for the foils are set at 4.10m apart and 0.52m above the mea­sure­ment wa­ter plane (MWP) within a zone 10m to 12m from the tran­som ref­er­ence plane (TRP).

Foils may only cant through a range that does not cross the cen­tre line, but there is no out­board limit other than that set by the al­low­able range of ar­tic­u­la­tion of the Foil Cant Sys­tem (FCS).

Un­like the AC50S, foil rake is not ad­justable. Foil forces are var­ied by chang­ing the global an­gle of the yacht (by rak­ing a sin­gle 3m-wide rud­der T-foil) and ad­just­ing a trail­ing-edge foil flap to in­duce cam­ber.

The foil arms are sup­plied, as are the bat­ter­ies and rams in the FCS, but foil wings and foil tabs are open in terms of de­sign and must fall within a boxed area that lim­its span to 4m and chord to 1.1m.

Foil wing/flap as­sem­blies must be sym­met­ri­cal about the foil arm and weigh around 1,150kg each. These form a bal­last keel to hold the

RIGHT Ar­tic­u­lated foils en­sure the AC75 will be sel­f­right­ing in the event of a cap­size.

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