Alinghi Challenge AC37
- Thread starter strider470
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SCARECROW
Super Anarchist
Priscilla
Super Anarchist
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Sailbydate
Super Anarchist
San Francisco Bay, 1:15pm, September 25, 2013.
Priscilla
Super Anarchist
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dg_sailingfan
Super Anarchist
If anyone wants to watch the Full Alinghi Red Bull Racing Team Announcement here it is. The Société Nautique de Genève has been kind enough to upload their Full Presser onto the Alinghi YT Channel 
floater
Super Duper Anarchist
September 20, 2013.
"With Team New Zealand well ahead in light winds near the end of the fourth leg, the first race on September 20 was abandoned by the race committee due to the race exceeding the 40-minute time limit."
Sea Breeze 74
Anarchist
September 18, 2013
Race 2. Having taken the series to match point in the previous race, ETNZ made a perfect start to streak away from Oracle towards mark one. Sadly the race committee signaled a wind delay as they crossed the start line. Racing for the day was ultimately postponed.
The Tractor was built for a decent breeze but those cheating mother fuckers changed the upper wind limits on us. We should have walked away with the cup that day.
Race 2. Having taken the series to match point in the previous race, ETNZ made a perfect start to streak away from Oracle towards mark one. Sadly the race committee signaled a wind delay as they crossed the start line. Racing for the day was ultimately postponed.
The Tractor was built for a decent breeze but those cheating mother fuckers changed the upper wind limits on us. We should have walked away with the cup that day.
Audio, EB interview, available only in Italy
America's Cup, the new challenge of Alinghi - Altri Sport - Rai Sport
America's Cup, the new challenge of Alinghi - Altri Sport - Rai Sport
Sea Breeze 74
Anarchist
floater
Super Duper Anarchist
I don't recall that second race, on the 18th, off the top of my head. But I was sitting there front and center, on the sea-wall, for the light air drifter on the 20th (imo, the last best chance NZ had to take the series). ETNZ just plain out-sailed Oracle - left them for dead. The difference was that the Kiwis grasped that a negative VMG was okay. And that's what they did, they powered up into a very slight breeze although it meant sailing away from the mark (both boats were in exactly the same wind here btw) just to get some apparent flow across their sails. From there, they were able to get enough speed to jibe away and on down the course (right in front of the spectators btw - it was fucking one for the ages). For whatever reason, Oracle refused to do this and stubbornly held on to a miniscule positive VMG and remained glued to the water.
And this, understanding that a negative VMG can be a good tactic, was present even in the most recent series. I would not accuse Burling or Tuke of this error. But it seems to me that both Spithill and Ainsley (same brain trust aboard oracle on the 20th) may have made the same mistake in AC 37. Anybody recall either of their two boats choosing VMG over boat speed - and by doing so, losing the race?
And this, understanding that a negative VMG can be a good tactic, was present even in the most recent series. I would not accuse Burling or Tuke of this error. But it seems to me that both Spithill and Ainsley (same brain trust aboard oracle on the 20th) may have made the same mistake in AC 37. Anybody recall either of their two boats choosing VMG over boat speed - and by doing so, losing the race?
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pusslicker
Super Anarchist
Basiliscus
Anarchist
Sorry to be so late to this discussion. I've been fighting a battle in a different domain lately.
I think cavitation is reasonably well understood. For most engineering purposes it is enough to predict the limits within which cavitation cannot occur and to stay within them. High-speed cavitation doesn't become evident until a few knots above the point of incipient cavitation so it's a conservative point to start.
Predicting the performance after the onset of cavitation requires very sophisticated CFD. You have two fluid phases to consider - water and water vapor. It takes time for water to evaporate into water vapor, and it takes time to condense back into water. All the while the fluid phases are being swept through zones with different pressures, so conditions are changing rapidly. On top of that, you have the boundary layer effects, including flow separation, to consider. All of this translates into time, cost, and expertise needed to calculate just a few points.
I think the hard part is not understanding cavitation, but balancing the conflicting demands of trying to go to the limits of what is possible within the Design Rule. You want the foils to be small to minimize the wetted surface area. You want all the span the Rule allows to minimize the lift-induced drag. These pressures drive the foil to be thin. But the foil needs thickness for structural stiffness and strength, plus (on the AC75) volume to hold ballast and flap mechanisms. Small foils need to operate at comparatively high lift coefficients and that leads to leading edge cavitation at takeoff.
So there are a lot of conflicting demands that need to be reconciled with the team's assessment of risk. Is it better to have a boat that is more forgiving when trying to get up if it means sacrificing some performance once it is up? How probable are different wind conditions during the match?
I posted a 6-part series in which I tried to convey how one might go about whittling away at the design of a hydrofoil section in order to satisfy these constraints.
What really is poorly understood is ventilation. Ventilation adds another fluid phase, air. Surface and boundary layer effects are unavoidable. And there's not a straightforward engineering criterion for predicting the onset of ventilation the way there is for cavitation.
Thank you for chiming in, Basiliscus!
Some of the points you have made here were made also by Brit on Friday, it's so damn nice to get informed perspectives.
Brit brought up this exact same tradeoff point too. He also commented on how many maneuvers were performed in AC36, due to the boundary lines (which will be a touch wider in AC37).
Do you still have that nice Tri down by Hood Canal? Am fantasizing about retiring to the same or a similar island's west-facing coast.
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enigmatically2
Super Anarchist
Just because you can doesn't mean you should. The problem with setting limits in any rule is that those limits become the target. This does not always produce the best result.
The IACC rule, for example, had a max beam limit of 5.000 m or thereabouts. NZL 32 had a beam of about 3.5 m.
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