America's Cup- Oracle and Team NZ designers reveal AC72 secrets
by Richard Gladwell on 23 May 2014
For the past few weeks two designers from Oracle Team USA and Emirates Team New Zealand have been giving an almost two hour presentation in various venues around New Zealand, as part of the Institution of Professional Engineers NZ centenary celebrations.
Emirates Team New Zealand NZL5 practice before day eight of America’’s Cup 34. Chris Cameron/ETNZ© http://www.chriscameron.co.nz
Sponsored by leading New Zealand engineering consultancy, Beca, the free-entry presentation has travelled to six centres in the North and South Islands and has attracted packed houses.
The two design engineers Andy Kensington from Team NZ and Neil Wilkinson from Oracle, both graduates from New Zealand universities have been involved in America's Cup teams for 13 and 23 years respectively.
Kensington opened running through a few of the differences between AC72 technology and the earlier monohulls used in the America's Cup and looked at the canting keelboats. Then he turned to why Emirates Team NZ settled on the foiling concept from the outset.
The first reason was obvious - that it would be faster, and surprisingly given later events, foiling was also safer, noting that calculations showed that a conventional multihull of the AC72 type would be difficult in winds above 18kts, and even more so in a bear-away situation.
Emirates Team NZ took a multifaceted approach, building a 'waka' a non-sailing foiler which was towed on two North Island lakes, and then the more widely known SL33's on the Waitemata, before going full size in the AC72.
The full progression was shown in a series of video clips, including the SL33's being used to test basic concepts before switching to foilers, and then wingsailed foilers.
In a series of graphics Kensington demonstrated the component forces in a large foiling multihull constrained by the AC72 rule. He also touched on the effect of cavitation on hydrofoils, which influenced the section shape. Then they moved onto various permutations of foil trying to find the optimum between lift, drag and speed.
With the AC72 outed as a foiler, he compared Team NZ's first AC72 with the second, noting that the platform alone took 45,000 man hours and the wingsail a further 20,000 hours to build.
Downwind speed between the first and second boat moved from 35+kts in Boat#1 to 45+kts in Boat #2. Upwind the top speed moved from 18+kts to 28+kts, with massive speed gains being made between the Louis Vuitton Final and the America's Cup Match.
Kensington put Oracle's comeback down to luck coupled with a dramatic increase in upwind VMG as well as tacking speeds and good starts. The difference between the two boats being revealed in one telling upwind image, where both boats are heading towards the camera. Oracle is clear of the water and foiling beautifully, but still getting maximum lateral resistance from her foils, while Emirates Team NZ is struggling to lift her leeward hull clear of the water.
For Oracle Team USA's part, Neil Wilkinson acknowledged that the team had gone into the 34th America's Cup with a head start in the technology stakes as a result of what had been learned in the 2010 America's Cup from the 120ft wingsailed trimaran they sailed to win in two races.
He elaborated on the engineering aspects of foils and the different way they worked upwind and down and also in the transition while gybing and tacking - plus how the foils are canted for up and downwind sailing
Wilkinson claimed that Oracle Team USA had been designed as a foiler from the outset, but broke a foil on the first day of sailing. After that incident, he built a test bed for foils, which consisted of a heavy steel flatbed onto which the foil was strapped before the expected load forces were applied of up to seven tonnes. The deflection of the carbon foil under load was staggering.
He then turned to the construction of the foil cases and how these worked in the boat, with up and downhaul lines. OTUSA and ETNZ used different systems. They also differed int he layout of the boats, with Oracle Team USA opting for a more aerodynamically refined package with the crews being carried more in the hulls rather than the ETNZ approach that was to carry the crew more on the platform.
In another series of load diagrams, Wilkinson elaborated on the forces operating on the AC72s, particularly around the foil cases with a lift of about six tonnes, a lateral force of seven tonnes. At the boat end of the foil there is a staggering 22 tonnes at the pivot point of the foil and 16tonnes on the control ram at the top of the foil - all supported within the AC72 hull structure.
Wilkinson freely admitted that most of the initial design developments in the AC72 came from Team New Zealand while Oracle watched, copied and modified.
An example of this was the development of foils - where Oracle wanted to emulate the Team NZ shape and improve on it, but could not get the daggerboard close enough to the bottom of the boat when retracted, as required by the AC72 rule. So an ingenious 'sliding door' was fitted around the daggerboard slot in the AC72 hull to allow the daggerboard to move closer to the bottom the hull when retracted thus complying with the rule.
Wilkinson then moved onto the foil controls - handled through buttons on the steering wheel, with foil lift buttons on one site of the wheel and foil pitch on the opposite side of the steering wheel. Further controls were on four touch panel points in the opposite cockpit, plus a further set in the floor of one of the cockpits operated by one of the grinders.
While Oracle Team USA undoubtedly made many mistakes in the 2011-2013 campaign, the key issue was how the team understood what happened, learned from these and used the experience to improve the boat's performance. The team did have a very good, what he called 'forensic program' which allowed design engineers to analyse multiple data streams in a line graph format, to look for unexpected issues and be able to accurately draw conclusions as to cause and effect, reducing the chances of repeat errors, and creating a data platform for speed and boat handling development.
A couple of moments of truth for the US team were the nosediving incidents, the first of which wrecked the wingsail and almost completely destroyed the boat. The second had less dramatic results but was a significant learning experience. One of the tools used to analyse what happened in these incidents was a multiple moving plot of the data gathered from various key readout points, which would enable the forensic team to look at how the various elements interplayed, together with similarly time stamped video.
In the example shown a nosedive incident was triggered when the afterguard tried to adjust the foil pitch control using hydraulics, which at the critical moment had lost pressure. The readout showed that a wingsail twist adjustment was being done at the same time draining the hydraulic pressure to the point where the foil control failed to work - triggering some re-design work to avoid a re-occurrence of the issue. In fact four changes to the hydraulic system were made to avoid a re-occurrence of this situation
In the video, Wilkinson explains how the read-out analyse process worked and the information streams that can be correlated and understood.
The pitch control on the Oracle daggerfoil was one of the many controversies of the 34th America's Cup, being the subject of three Jury Hearings just a few weeks before the start of the Match itself. Wilkinson explained the adjustment system with a series of graphics and video.
He went on to highlight the key differences between Oracle and Team NZ development campaigns being:
- Time on the water: Team NZ were launched and sailing before Oracle and stayed that way right through the campaign (Team NZ has 85 days on the water and performed 2000 tacks and 2000 gybes in this build up)
- Disasters: Oracle broke her foils on the first day, losing 5 weeks, and capsized on Day 8, losing 15 weeks
- Wings: Oracle had prior experience with the wingsail development from the 2010 America's Cup. ETNZ's main element wingsail twist was more complex than OTUSA's. However Oracle had tried the same setup and did not believe that it offered significant advantages.
- Foils: Oracle Team USA made a gain on ETNZ having slightly thinner boards with a thicker radius and more highly stressed.
- Daggerfoil Control system: Oracle used an electro-hydraulic system operable from either side of the boat. ETNZ used a mix of electro-hydraulics and manual systems.
Key differences which determined the Match
- Wing tune: Oracle improved upwind drive through increased lower aft loading. (interestingly this significantly increased the wingsail trim sheet loads, and the subsequent hydraulic demands required the crew to grind hard to maintain power from the race start to the finish)
- Crew work: Oracle mastered foiling tacks, gybes and upwind foiling. (While ETNZ led the way in the first two elements, the NZers admitted they never mastered upwind foiling to the extent achieved by Oracle)
-Board control: Oracle had improved positive positioning of the board pitch through electro/hydraulic systems development
- Rudder/Elevator refinement: Oracle added small anti-cavitation mods to the intersection of the elevators and rudder