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America's Cup - Re-interpreting the Interpretation + Video

by Richard Gladwell, Sail-World on 17 Jun 2017
Oracle Team USA with her lower sidestay flying slack and without any tension Richard Gladwell www.photosport.co.nz
In a plot that could have come straight out of 'Yes Minister', more has emerged on the curious Measurement Interpretation 93 which was re-interpreted to mean the opposite just four days after first being published.

The issue was first raised by Sail-World earlier in the America's Cup regatta, at the daily media conference given each morning by Race Director, Iain Murray, who as well as his race management role is also a previous America's Cup skipper, and designer among his many credentials.

The interpretation was first published on May 20, after one of the America's Cup teams noticed that Oracle Team USA and Softbank Team Japan were sailing with slack lower side stays.

The complaint centred around America's Cup Class Rule 25.11 which requires rigging to be tensioned.

The Measurement Committee produced an Interpretation on six days before the start of the regatta on May 20, which clarified the meaning of the rule - saying that all rigging had to be under tension (i.e., not slack) and stipulated that the minimum tension should be hand tension.

They answered several questions regarding the relevant class rule 25.11

The first question answered was 'Does Rule 25.11 require all elements of wing rigging to be tensioned (not left un-tensioned
on slack strops) when top and bottom strop lengths are measured?'

The simple answer given was 'Yes.'

In its original interpretation, the Measurement Committee went further and defined 'tensioned' to mean at least hand tension.

Four days later the Measurement Committee produced a new Interpretation of the same rule which said now that 'it is permitted for some rigging elements to be left slack and the amount of slack is not regulated.'

There had been no change in the America's Cup Class Rule in the four days.

The other related Interpretations, on similar points, were mentioned in the first Interpretation.

No rules had changed to justify the new Interpretation. The original Interpretation was logical and made perfect sense.

There was no obvious reason why it should have been changed less than a week before the start of the Qualifiers.

The gain from running slack lowers is that all the forestay tension (around 3.4tonnes) is shifted onto the after shrouds or backstays.

The idea is that load transmitted through the backstays will bend the platform slightly, pulling the rudder aileron to have a more acute angle of attack in the water, beyond the allowed three degrees. This should enable an AC50 to be pushed harder and faster.


No oversight
At the start of the 35th America's Cup Regatta, it was announced that World Sailing had authorised the Measurement Committee to make decisions without being subject to review by the International Jury, which is usual in all racing and measurement questions.

Many believe that once an interpretation has been issued that is the end of the matter - coming from the perspective that the Measurement Committee has looked at an issue, considered it and then made ruling which right or wrong will prevail for the regatta.

The reason for accepting the ruling without further question is simple - that teams can work with the ruling and design their systems accordingly. They don't expect to be in a situation where they act on one interpretation only to find that another team has managed to bend the ear of the Measurement Committee and persuade them that black is really white and get a new interpretation substituted.

The situation was further compounded by the introduction of a new rule - this time in the Racing Rules that govern the America's Cup.

Two days after the America's Cup Qualifiers a new rule was signed off restricting teams to lodging just one 'Claim of Non-Compliance' (CNC) in each of the Qualifiers, each stage of the Play-offs and in the Match.

If the complaint is upheld the complaining team gets an additional CNC 'opportunity'.

Of course, the flaw in this limitation is that the teams are racing with multiple measurement certificates - and yet only one challenge is allowed. A fundamental of sailing is that it is self-policing, and multiple challenges must be allowed to prevent rule fiddling - with oversight from both competitors and measurement scrutineers being essential to ensure rule compliance.

Majority Rules
The change to the Racing Rules was, as has happened so often in this Regatta, signed off by the majority of competitors. The same five who are also signatories to the so-called Framework - a secret agreement signed between all the teams except Emirates Team New Zealand, which covers the conduct of the 36th and 37th America's Cups.

All but one the teams that signed that rule change have now been eliminated from the America's Cup, and one that didn't sign is stuck with it.

The new Rule appears to be yet another example where the America's Cup teams negotiate a position between themselves on new rules or rule changes, and then that gets 'codified.' In other words turned into a process where facts are applied to the formula, and the outcomes reached. It all sounds very fair and reasonable, but can lead to horse-trading over other rules and issues, and is not the way any other sailing event is run.

The concerns of the minority, of course don't carry much weight, and there is no protection from a cartel.


No difference in slack or tensioned?

Despite wondering and suspicion over the reasons for the re-interpretation of the original Interpretation 93, one leading multihull sailor has the view that there was nothing in it.

Steve Clark, whose experience with wingsails dates back to 1968 when his father Van Alan Clark built one. Both father and son have had a long association with the 26ft long Int C-Class catamaran, which many America's Cup teams checked out very carefully before embarking on their wingsailed multihull designs.

Steve Clark along with Duncan MacLane developed the wingsail used on Cogito to win the Little America's Cup in 1996.

'The one design ACC rule required wings to have two sets of shrouds, ' Clark explains. 'The rule also specifies the rake of the wing and the forestay tension. The original FEA (Finite Element Analysis) studies of the wing and platform were based on both shrouds and lowers carrying some of the load.

'The lowers have the effect of reducing the span of unsupported column below the hounds and making the wing less likely to buckle in compression. They also share some of the shroud load and distribute this between two sets of chain plates.

'In the event, teams found that the wings withstood the compression loads without loading the lowers and that it was also unnecessary to distribute the shroud load over two sets of chain plates. So the question of the minimum tension arose, which started everyone thinking about what the advantages or disadvantages were being gamed.


'Speculation, based on the first Interpretation 93 was that by controlling the lower tension, you could effect the way the platform wracks.

'Platform wracking, which is uncoupling the fore and aft trim of the two hulls, allowing one to take a bow up trim and the other a bow down trim, has generally been seen as a bad thing.

'Oracle's first AC72 (which broke up spectacularly under the Golden Gate in 2012) was roundly criticised by multihull experts because of this. However, Oracle's design teams goals were misunderstood.

'Subsequent design research shows that there is quite a bit of righting moment available by having the windward rudder T foil develop down force.

'The AC 72 rule did not permit the rudder foils to be adjusted during racing, whereas the AC50 rule permits + 3 deg of differential between the leeward and windward rudder.

'In 2013, Oracle attempted to accomplish this by allowing the entire platform to wrack. This is not necessary in 2017. Or is it?

'Would another degree of differential, caused by allowing the platform to wrack more make a measurable difference in performance. Would loosening the lowers allow this to happen?

'My opinion is that the lower shrouds would have no impact on how stiff the platform was.


'In catamarans with single forestays, the wracking resistance is driven by the “S” bending of the stern beam and the torsion stiffness of the hulls in between the main and stern beam chocks.

'In effect, the weather inboard stern chock has to move down, and the leeward inboard stern beam chock has to move up. The stern beam itself has to distort into a sort of “S” shape accommodate this movement.

'It is possible that design teams spent the time modelling the permutations of stern beam laminates and hull laminates within the one design specification to achieve this additional few degrees of rudder differential. This is the kind of thing you spend design resources on when you are prevented from doing more creative work.

'In short. This is not a performance issue, ' he concludes.

Others in the America's Cup teams did not entirely agree with Steve Clark's analysis choosing to believe that Oracle Team USA and Softbank Team Japan were using the differential shroud tension to somehow bend the AC50 platform to achieve a better rudder angle and pick up more speed as well.

Now it would seem that Emirates Team New Zealand, as the remaining Challenger, and earlier the other eliminated Challengers have looked again at the possibilities under the re-interpreted rule, and determined if indeed there are any gains to be had.

Indeed for the first day of the 35th America's Cup Match, Emirates Team NZ were seen sailing with slack lowers, the same as Oracle Team USA.

Webasto AUS 2020 FOOTER 1Highfield Boats - Sailing - FOOTERX-Yachts AUS X4 - FOOTER - 3

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