Kiwing

The new sailing twin skin setup

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5 hours ago, Tanton Y_M said:

Voile Epaisse. Thick Sail. 1983 Design.

 

 

Awesome. I love the way you kind of disguised the double surface part by integrating it into the middle of a fairly conventional looking sail. Were the masts composite or alloy?

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^ What an amazing, forward-thinking design. When I was a kid, I idolized Dick Carter - it wasn't until years later that I learned you were part of his team during that magic time period. I still vividly remember Love Machine (not just because of the famous photo). Nice work.

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Richard E. Carter is 90 years old now. Two years ago, I suggested for him to write a book about this "magic time period". Job done .

20180719_165615_001_resized_1.jpg

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Maybe inflatable ribs to change the profile on the fly?

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The rocket scientists behind the Godzilla wing of the AC 34 used to say that the thick wing sail allowed tacks and gybes with continuous air flow and no separation (very draggy) which is not the case for a classic teardrop mast.

I guess the engineers have considered this point seriously as fast tacks and gybes are a significant part of the whole performance in the cup.

In addition, if top main sail negative lift is also a part of efficient trim, in order to achieve the so called Bell shaped lift distribution, it is necessary to have a thick sail in order to fit some "inverting devices" in the middle.

Cheers

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33 minutes ago, Erwankerauzen said:

The rocket scientists behind the Godzilla wing of the AC 34 used to say that the thick wing sail allowed tacks and gybes with continuous air flow and no separation (very draggy) which is not the case for a classic teardrop mast.

I guess the engineers have considered this point seriously as fast tacks and gybes are a significant part of the whole performance in the cup.

In addition, if top main sail negative lift is also a part of efficient trim, in order to achieve the so called Bell shaped lift distribution, it is necessary to have a thick sail in order to fit some "inverting devices" in the middle.

Cheers

Has anyone got any photos of the 'negative lift' twist from AC34 or anywhere else?

I have video of all the races and cannot see the top of the wing inverted.

Thanks in advance.

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I doubt you can observe a -3° AoA of the sail top on video. 

I am not sure (because of  my Alzheimer), but it is possible Glenn Ashby has mentionned it in an interview, a few months ago.

Remember, in addition  to his skipper role, he was also the wing trimmer.

Happy week end Random

 

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14 hours ago, random said:

Has anyone got any photos of the 'negative lift' twist from AC34 or anywhere else?

I have video of all the races and cannot see the top of the wing inverted.

Thanks in advance.

The trimaran didn't have that capability.  The AC72 did.  I'd look for pics of AC72s in SF that show the boat from behind.

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20 hours ago, Erwankerauzen said:

I doubt you can observe a -3° AoA of the sail top on video. 

I am not sure (because of  my Alzheimer), but it is possible Glenn Ashby has mentionned it in an interview, a few months ago.

Remember, in addition  to his skipper role, he was also the wing trimmer.

Happy week end Random

 

So that means that there will be a section of the wing between the positive and negative cambers that is neutral.  That means no lift either way and is pure drag.

If Ashby 'mentions' it then that sounds credible but I have not seen it.  BTW I do think that I would see -3 deg .. if it was there.

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6 hours ago, Basiliscus said:

The trimaran didn't have that capability.  The AC72 did.  I'd look for pics of AC72s in SF that show the boat from behind.

Ok, but no one is talking about the tri, no one is talking about the AC72, we are talking about Bermuda and despite all the imagery I cannot see any evidence for the claim.  Could be like the mythical 'Hummingbird' wing trim that Ken referred to on the coverage, then when the camera finally panned up, the NZ wing was more stable than OR, where Kyle was torturing the sheet in apparently random fashion.  Or it could be like the bullshit 'Beast Mode' that was cover for auto ride height.

Until I can see the wing tip inversion working, I have reason to doubt that it played any real part.

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You'd have to pick the right day in Bermuda.  Inverting the top of the wing is something you only do when the rig is overpowered.  So you're not going to see any inverted flap deflections in the light stuff.  It would have to be one of the days in the Challenger Selection Series when the fairings were blowing off the boats.

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On 7/23/2018 at 2:22 PM, random said:

Ok, but no one is talking about the tri, no one is talking about the AC72, we are talking about Bermuda and despite all the imagery I cannot see any evidence for the claim.  Could be like the mythical 'Hummingbird' wing trim that Ken referred to on the coverage, then when the camera finally panned up, the NZ wing was more stable than OR, where Kyle was torturing the sheet in apparently random fashion.  Or it could be like the bullshit 'Beast Mode' that was cover for auto ride height.

Until I can see the wing tip inversion working, I have reason to doubt that it played any real part.

The mythical "hummingbird" effect you are talking about is visible, it's nothing as crazy as you are expecting to see.

It's just that Ashby who was playing the twist is like playing the sheet on a main, it makes noticeable kicks in and out at the top of the wing, it's not fluttering about, it's just popping in and out, while the remainder of the wing remains as is, making it look a little more noticeable, like how a soft leech looks in a breeze and you drop some sheet or crank it back in quickly.

The other teams were just playing the wing sheet, which is like setting the mainsail, and then playing the traveller. which... WTF?! If someone did that on a beach cat they would be laughed at.

They were just keeping the majority of the wing set, and de-powering the head, where it matters most, and where it will least effect the drive through the rest of the rig.

The new boats having conventional main sheets will do this as all other boats do, by sheeting on and off.

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13 hours ago, Basiliscus said:

You'd have to pick the right day in Bermuda.  Inverting the top of the wing is something you only do when the rig is overpowered.  So you're not going to see any inverted flap deflections in the light stuff.  It would have to be one of the days in the Challenger Selection Series when the fairings were blowing off the boats.

Uh huh, so you are saying that you haven't seen it ...

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Random,

Try to googlize the  following and you will get the full workpaper, it is too heavy to be downloaded.

You can bet confortably that, if there is some theorical advantage to have a bell-shaped lift distribution (with negative lift at the tip), the AC design team have put it at full use.

These "rocket scientists" are professionnal you know!  

NASA/TP—2016–219072

On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds

Albion H. Bowers, and Oscar J. Murillo Armstrong Flight Research Center, Edwards, California

Robert “Red” Jensen, and Brian Eslinger Jacobs Technology, Inc., Edwards, California

Christian Gelzer Logical Innovations, Inc., Edwards, California.

Good reading

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^ links to earlier discussions on the same subject -  with video, pdf etc

 

 

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Theoretical optimums are all very well but when you are constrained by class rules or physical practicalities then other factors may be of much greater importance than a min Cd or mac Cl/Cd in a particular regime.

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I don't remember the rules but is there anything to prevent teams from adjusting the shape of the twin skin sail? Like different tension in both sail or inflatable elements in between the 2 sheets? How far could they go (spars in between)?

While it won't have the rigidity of a wing, they could probably gain some rigidity and control that way, plus some flexibility in the shape of the sail/wing that they didn't have before (as adjusting the "thickness"). Add the funding and the development that AC will bring and that could be the best thing that this AC will bring to the sailing world.

I don't think this will trickle down to everyone's boat but it could allow for high performance racing with easier logistics than wings but similar performances. I definitely wish for a substantial step forward compare to soft sail and close to wing sail performance. 

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11 hours ago, Lakrass said:

I don't think this will trickle down to everyone's boat but it could allow for high performance racing with easier logistics than wings but similar performances. I definitely wish for a substantial step forward compare to soft sail and close to wing sail performance. 

Let's be clear. The performance of this type of sail will get nowhere near that of the wings we have seen in the last 2 AC's. This is because of the efficiencies of the slotted wing and the ability to accurately control camber and the slot at all levels of the sail.

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17 hours ago, darth reapius said:

The mythical "hummingbird" effect you are talking about is visible, it's nothing as crazy as you are expecting to see.

It's just that Ashby who was playing the twist is like playing the sheet on a main, it makes noticeable kicks in and out at the top of the wing, it's not fluttering about, it's just popping in and out, while the remainder of the wing remains as is, making it look a little more noticeable, like how a soft leech looks in a breeze and you drop some sheet or crank it back in quickly.

The other teams were just playing the wing sheet, which is like setting the mainsail, and then playing the traveller. which... WTF?! If someone did that on a beach cat they would be laughed at.

They were just keeping the majority of the wing set, and de-powering the head, where it matters most, and where it will least effect the drive through the rest of the rig.

The new boats having conventional main sheets will do this as all other boats do, by sheeting on and off.

And to my eye when it was puffy, like the first few days of training with mihbda footage, it was obvious that GA was trimming the top element to balance the platform as the puffs moved through. Better balance, easier foil trim.

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10 minutes ago, A Class Sailor said:

Let's be clear. The performance of this type of sail will get nowhere near that of the wings we have seen in the last 2 AC's. This is because of the efficiencies of the slotted wing and the ability to accurately control camber and the slot at all levels of the sail.

Oh

It's you again with the home built fluid simulator crossed with a time machine.

Let's be clear,

Please remove the cork

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15 minutes ago, barfy said:

Oh

It's you again with the home built fluid simulator crossed with a time machine.

Let's be clear,

Please remove the cork

Sorry, but I don't understand what you are getting at. Can you please make yourself clear. Are you saying my comments are wrong?

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7 minutes ago, A Class Sailor said:

Sorry, but I don't understand what you are getting at. Can you please make yourself clear. Are you saying my comments are wrong?

It translates to "remove the cork" apparently oneof the main charicters says it to people who are very self inflated- meaning if they would ...

having never seen the control systems for this type of wing I have trouble when someone pontificates efficiency in this manner. Maybe you do this always with regards to subjects that no one has seen developed, but perhaps hold your wind or speak in a speculative manner

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1 minute ago, barfy said:

It translates to "remove the cork" apparently oneof the main charicters says it to people who are very self inflated- meaning if they would ...

having never seen the control systems for this type of wing I have trouble when someone pontificates efficiency in this manner. Maybe you do this always with regards to subjects that no one has seen developed, but perhaps hold your wind or speak in a speculative manner

Sorry, but I am quoting what Glenn Ashby told me, which goes along with what every other wing expert has said. You make out that we are looking at something new and unknown. We are not. The advantages and limitations of both this sort of rig and the wings that were used in the last 2 AC's are actually very well known and while I expect to see improvements over the current generation of double skin sails, there is simply no way around the limitations of an unslotted rig. If you don't know that, you really shouldn't be commenting.

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^

I think in the larger context of wing design...ya don't see many slotted wings on things that fly...

Wait and see grasshopper

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18 hours ago, random said:

Uh huh, so you are saying that you haven't seen it ...

One thing we know is that Basil knows much more about AC wings than you do.

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it's not like etnz has understated their performance metrics before

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4 hours ago, Curious said:
23 hours ago, random said:

Uh huh, so you are saying that you haven't seen it ...

One thing we know is that Basil knows much more about AC wings than you do.

So there has been a lot of chat about something that has not been seen ... mmmmmmm

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5 hours ago, A Class Sailor said:

while I expect to see improvements over the current generation of double skin sails, there is simply no way around the limitations of an unslotted rig.

The slot is required for wings because they are rigid and are not suitable for all wind configuration and to add control. As the flaps are required for an airplane to land to increase lift at lower speed without stalling the wing.

The slot characteristics would actually have a negative effect if they were able to modify the profile of the wing at will to extract maximum efficiency whatever boat speed and wind condition (this is indeed unrealistic hence the slot). That's where twin skin setup has a slight advantage, the profile shape can be modify when on the water (in theory).

Let's be clear, I didn't say it will work, I was just considering the possibility to have structure in between the 2 sails and eventual limitation of this. Then there are plenty of difficulties to get the perfect profile shape on the water with possible changing condition but that's the AC, they might pull something special out of the hat. At least we tend to agree that there should be (significant) improvement over the current twin skins.

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19 hours ago, Erwankerauzen said:

Random,

Try to googlize the  following and you will get the full workpaper, it is too heavy to be downloaded.

You can bet confortably that, if there is some theorical advantage to have a bell-shaped lift distribution (with negative lift at the tip), the AC design team have put it at full use.

These "rocket scientists" are professionnal you know!  

NASA/TP—2016–219072

On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds

Albion H. Bowers, and Oscar J. Murillo Armstrong Flight Research Center, Edwards, California

Robert “Red” Jensen, and Brian Eslinger Jacobs Technology, Inc., Edwards, California

Christian Gelzer Logical Innovations, Inc., Edwards, California.

Good reading

I have no doubt they are good reading for insomniacs.  But has anyone here any images or video of inverted wingtips from the last AC?

I have watched them all many times and have not been sharp enough to pick it.  No one?

115282d1483553683t-i-did-fad01b4ea7128ae

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19 minutes ago, Lakrass said:

 I was just considering the possibility to have structure in between the 2 sails and eventual limitation of this.

Check the rules. This is only allowed in the head area up to 4 metres down from the top and the foot area up to 1.5 metres from the bottom which isn't that much on a 26.5 metre rig. For 21 metres of the sail, there can be no structure other than specified battens. The zone at the top is to allow systems to invert the sail and to try to get similar righting moment from the rig as they got from the solid wing, or at least that is what ETNZ have said. They are probably just bullshitting us, because according to random, they weren't doing that in the last AC.

It's also interesting to note that slots in the sails are specifically banned. I wonder why;)

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1 hour ago, random said:

So there has been a lot of chat about something that has not been seen ... mmmmmmm

Basiliscus is a true expert on AC rigs. Not an internet expert, not someone who has just read about them, but a real expert. Why not learn from him?

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On 7/24/2018 at 5:13 AM, Basiliscus said:

You'd have to pick the right day in Bermuda.  Inverting the top of the wing is something you only do when the rig is overpowered.  So you're not going to see any inverted flap deflections in the light stuff.  It would have to be one of the days in the Challenger Selection Series when the fairings were blowing off the boats.

@Basiliscus I thought during a tack or gybe the top of the wing was independently used for stability during the manoeuvre. To lead the whole wing to the other side and to correct poor timing of other elements?

Is depowering the only use?

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30 minutes ago, Curious said:
2 hours ago, random said:

So there has been a lot of chat about something that has not been seen ... mmmmmmm

Basiliscus is a true expert on AC rigs. Not an internet expert, not someone who has just read about them, but a real expert. Why not learn from him?

Yeah I'm all ears and eyes.  Where are the pics?

image.png.07b638e62a2fba670aa71714b98a4710.png

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Demanding pics about AC rigs from him is a bit like demanding pics about Laser technique from Tom Burton. Why not just accept that he knows?

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14 hours ago, random said:

Yeah I'm all ears and eyes.  Where are the pics?

image.png.07b638e62a2fba670aa71714b98a4710.png

Why do you think you are deserving of any real effort to educate you in this matter. If you know who Basiliscus, just accept what he says.  I've heard about righting moment from the top of the wing from a number of different sources some of which are in the public domain and some was direct from AC participants. Either they are all in on some big con which makes no sense or else it was happening. Why do you think so many people with direct knowledge saying it happened should not be trusted and what have they got to gain from making this up?

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20 hours ago, Kiwing said:

@Basiliscus I thought during a tack or gybe the top of the wing was independently used for stability during the manoeuvre. To lead the whole wing to the other side and to correct poor timing of other elements?

Is depowering the only use?

I can't speak for the rig on Aotearoa as Emirates hasn't released the kind of technical information that OTUSA published after the 34th AC.  And I'm not intimately familiar with exactly what the wing trimmer's playbook was during a tack or gybe on USA 17.  But I don't think wing twist was used quite that way on USA 17

I believe the primary controls on USA 17 during a maneuver were the wing sheet/traveler (it often went by either name) and the inverter line.  The inverter was basically a second sheet that was led to the clew of the main element, while the sheet itself was led to the clew of the flap.  The inverter allowed the crew to time exactly when the wing popped through onto the new tack.  The twist was self-tacking, so as the wing popped through, the twist automatically flipped to the mirror image so it would be the same on the new tack as it was on the old one.  

That was in keeping with the different design philosophies of the two rigs.  The Emirates philosophy was evidently to have positive control through the hydraulics, including driving the camber and twist onto the new tack.  The OTUSA philosophy was to allow the wing to passively take up its shape due to the airloads on it, and to determine the shape through limiters and adaptable mechanical ratios in the control system. 

It was kind of like the difference between a jib with two sheets, and a self-tacking jib with one sheet and a jib traveler control.  With twin sheets, you have to do everything during a tack, and if the timing isn't right you can end up with the jib inverted to weather.  A self-tacking jib will take care of itself during the maneuver, but the timing may not be the same as for the twin-sheet jib.

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7 hours ago, A Class Sailor said:

Why do you think so many people with direct knowledge saying it happened should not be trusted and what have they got to gain from making this up?

"Everyone is saying it"

Uh huh.  But the entire event was recorded in great detail, yet not one inverted wing tip can be seen?  It's not that I do not believe it might work in theory, but I want to see it in practice.  So far nada.

image.png

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Has anyone seen any mention of a of the teams developing the twin skin concept?

Is the ETNZ/COR experiment the only public exposure?

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On 7/26/2018 at 3:22 PM, Basiliscus said:

It was kind of like the difference between a jib with two sheets, and a self-tacking jib with one sheet and a jib traveler control.  With twin sheets, you have to do everything during a tack, and if the timing isn't right you can end up with the jib inverted to weather.  A self-tacking jib will take care of itself during the maneuver, but the timing may not be the same as for the twin-sheet jib.

Uh huh.  Nice deflection.

Where are the pics of the inverted wing tip from one of the most photographed events in sporting history?

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For those who don't believe in or haven't been told personally from team members about the inverted top wings in the last America's Cup that were used, what would you need to see?  5 degrees from sheeted in and straight?  10 degrees off?  Or do you need to see the large camber in the bottom of the wing pushed to the very top in the opposite direction and creating large vortices of drag that would slow the boat down and ruin its performance?   If drag is the number one concern for high performance as it always is, the top will never be highly inverted.  No end plate deck seal there.  Too slow.  No one will be able to show a clip of that.

The apparent wind angles were incredibly close.  What might look like twist to a typical sailor, even a high performance skiff or cat sailor, can be wing inversion at the TWA's and speeds they were sailing.  Nothing else compared to date to that level of performance.  What was the top speed we saw in 8 knots of breeze?  Almost 40 on the reach?  There is negligible twist in the apparent wind at those speeds.  Maybe the best analogy is to put your hand out the car window at 60 mph / 100 kmh and notice how much of an angle is required to feel either lift or downforce. Does it take 20 degrees? If you noticed ETNZ flipping their upper leech in and out like a hummingbird sometimes, how can you say it is not inverted sometimes when those on the teams have said it was? 

It's too bad so much of the video is gone from AC35.  The final Cup races that I have seen archived were completed in light air.  Oh well.  Maybe we'll see more in the F50s.

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50 minutes ago, Lost in Translation said:

what would you need to see?  5 degrees from sheeted in and straight?  10 degrees off?

'll take whatever dude.  Something is better than nothing.

Right now we still got nothing.

Edit: but it sounds like the kind of bullshit they would propagate to fuck with the heads of other teams, and laugh about it when they see it appear in the press or in forums like this.  Sorta like OR do about 'Beast Mode'.  Fuck they must have beer flying out their nostrils every time it is mentioned in the pub.

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^ There's apparently a new interview with GA floating around @seahorse int', might shed some light for the unconvinced?

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Apparently ...

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Moving the centre of effort (by even inverting the top) allows:-

1) controlling righting moment by a third factor (1. foil control. 2. crew weight)

2) maximising stability during manoeuvres 

3) controlling either power to get onto foils, or maximise benefit of apparent wind.

NorthSails simulation gives credence to the idea teams might use inversion of the top of the sail for any or all of the reasons.

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44 minutes ago, Kiwing said:

Moving the centre of effort (by even inverting the top) allows:-

1) controlling righting moment by a third factor (1. foil control. 2. crew weight)

2) maximising stability during manoeuvres 

3) controlling either power to get onto foils, or maximise benefit of apparent wind.

NorthSails simulation gives credence to the idea teams might use inversion of the top of the sail for any or all of the reasons.

So?  Where was it used in the last AC?

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^ Did you have your eyes and ears closed?  The only time I "saw it" was early in the gulf with the 72 but then it went under wraps! And while I don't really care about your persistent quest. I wish I had somehow got and kept that video.

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On 8/23/2018 at 4:25 PM, nav said:

^ There's apparently a new interview with GA floating around @seahorse int', might shed some light for the unconvinced?

If it's so convincing, why isn't he doing it on his A Class? It's class legal and somebody has already done it

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19 minutes ago, Kiwing said:

^ Did you have your eyes and ears closed?  The only time I "saw it" was early in the gulf with the 72 but then it went under wraps! And while I don't really care about your persistent quest. I wish I had somehow got and kept that video.

No, I have been watching.  I have studied all races in the last AC and have not seen any negative camber in the wings.  If you can find the 72 video that would be pretty cool but I am interested in the 50s, the last cup.  If the technique works they would have used it.

So far all I have is the sound of crickets.

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random

Negative camber is different from inverting the top.

GroupamaC which has some common designers with ETNZ has  "little streamlined struts" at the top of her wing to control........ top inversion.

But wether it was actually used is another story, no too much hands available on this 2 crews boat.

Do you aim to replace Doug Lord??

 

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Who?

Did he want to see something to prove it wasn't bullshit as well?

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I've never seen you so you must be bull shit too.

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Yeti?  Bigfoot even?

Apparently there is a Yowie near where I live.

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The issue is that the reverse camber is really small and hard to spot. You cannot see it from behind or the side. You need to look from above to see it. Luckily there is one good photo that does show it

yandy170209.jpg

It's very clear from this photo that the top of the wing is cambered in the opposite direction to the bottom of the wing.

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@A Class Sailor Thank you great spotting.  I never doubted it was used more than I saw as these very skilled and able sailors made the most of what was available to them.

Be interesting how the teams this time will develop and use this ability with a much more difficult environment.

And I believe there will be some trickle down to the top level sailors.

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4 hours ago, A Class Sailor said:

It's very clear from this photo that the top of the wing is cambered in the opposite direction to the bottom of the wing.

There is a significant caveat though that the sail is under enormous stress at the time.

Determination of reverse camber requires calculation of some datum of zero camber (with respect to the apparent wind? lift vector analysis?…?), which is likely not aligned with the boom. Also, 3D calculations or interpretations from a single, off–axis 2D image are likely to be imprecise, and being taken from a long lens quite some distance away can lead to misleading artefacts. So I'd be cautious in using the image as conclusive proof of reverse camber, it might just be the effect of the massive deceleration of the rig and change in apparent wind on a typically twisted sail—the energy has to go somewhere, so the head is going to fall off to one side or the other, and probably favour going to leeward.

But it certainly looks like reverse camber. ;-)

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5 hours ago, A Class Sailor said:

The issue is that the reverse camber is really small and hard to spot. You cannot see it from behind or the side. You need to look from above to see it. Luckily there is one good photo that does show it

 

It's very clear from this photo that the top of the wing is cambered in the opposite direction to the bottom of the wing.

image.png.751e1b4aef23713ce9b3f37643a60037.png

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8 hours ago, A Class Sailor said:

The issue is that the reverse camber is really small and hard to spot. You cannot see it from behind or the side. You need to look from above to see it. Luckily there is one good photo that does show it

yandy170209.jpg

It's very clear from this photo that the top of the wing is cambered in the opposite direction to the bottom of the wing.

"Airbrakes to full, she'll pull-up no worries"

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Even sailing at 90deg to the surface, she was the fastest boat out there....  8)

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On 8/27/2018 at 9:28 AM, A Class Sailor said:

The issue is that the reverse camber is really small and hard to spot. You cannot see it from behind or the side. You need to look from above to see it. Luckily there is one good photo that does show it

yandy170209.jpg

It's very clear from this photo that the top of the wing is cambered in the opposite direction to the bottom of the wing.

Whenever i've stuffed the bows in on my cat and managed to have a glance up, the top always looks like shit !

I'd have to think that the loadings involved on something that goes considerable faster, then stops would make the top of their rig look like crap as well !

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I finally got time to look at some old MIHBDA vids that I remembered showed quite a lot of twist. Of course it's hard to say for sure with apparent wind, but at @00:30 the boat is hard on the wind just about to tack so I reckon they are sheeted in all the way on the lower sail. The top is quite twisted off.

Link to MIHBDA vid

Also worth watching other vids from the same day is it's quite puffy and fresh and the angle camera to wind gives quite a few head on shots.

 

Sailors_or_Pilots_Emirates_Team_New_Zealand_Set_Sail__Part_4_-_YouTube.jpg

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Wow that is amazing shot !  Might take Glen Ashby to tell us what is going on there.

But you are right @barfy there is plenty of twist there and where the Apparent wind (bottom and top) is is anyone's guess, let alone what they are doing(about to do).

Glen, please or some one close ask him?

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So the shot shows a main with more twist at the top than the bottom ... ah what's the amazing part?

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Tell me where the apparent wind is at the top and at the bottom, where the centre of effort of the wing is and what ETNZ is about to do?

Most pictures of sailing boats you can tell at a glace, maybe not what they are about to do in the long term but in the next 30 seconds yes.

Try to guess then watch the video and be amazed, I was and still am.

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15 minutes ago, Kiwing said:

Tell me where the apparent wind is at the top and at the bottom, where the centre of effort of the wing is and what ETNZ is about to do?

Most pictures of sailing boats you can tell at a glace, maybe not what they are about to do in the long term but in the next 30 seconds yes.

Try to guess then watch the video and be amazed, I was and still am.

I can tell you exactly where the apparent wind is at the top and the bottom.  They have adjusted the shape of the main to accordingly.

They are on starboard, the top of the main is twisted off because of the increased wind velocity at the top, and being adjusted as they go ... then they tack.   So?

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46 minutes ago, random said:

I can tell you exactly where the apparent wind is at the top and the bottom.  They have adjusted the shape of the main to accordingly.

They are on starboard, the top of the main is twisted off because of the increased wind velocity at the top, and being adjusted as they go ... then they tack.   So?

If you compare the top element to jib angle seems like the top element is way off to leeward. As I said, you really need to form your own opinion as there are no hard and fast indicators of apparent wind.

I don't understand how you can tell EXACTLY what the apparent wind angle is.

But, the boat does tack soon after. I would think they are sheeted on with the bottom element.I can then assume the top element is negative.

Imho

Last post on this, as no real video evidence that can prove one way or da other.

Unlike foil movement .

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I am afraid it would never be possible to observe a negative AoA at the top of thse wings, for a simple reason:

Assumption: The crew reponsible for the trimming has a minimum understanding of wing section Lift/Drag issues)

As it is about symetrics wing sections with a significant thickness for the main one (Element 1)

The lift/drag sweet spot for this kind of wing section is probably within -4° to +4°

In other words, even if inverted, as it is controlled by a smart crew, I doubt negative AoA to be more negative than -4° ( on a cruise mode of course).

So you see random, the actual issue is how to detect on a video this tiny 4° maximum difference between the wing section and the apparent wind ?

To see this inversion you would need to be on the top of the wing, checking the alignment of the wind vane with the center line of the wing section.

For a 2 feet long wind vane,; A 4° angle means a 4.15 cm gap or  around 1.60 inches.

Cheers 

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Also, America's Cup Event Limited is ACE?  Wouldn't it be ACEL?  ;)

WetHog  :ph34r:

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From my recollections, where on MIHBDA videos there was definite flap inversion was in the bear-away at the mark

Not sure I have time to have to do some research tonight, but I am pretty confident that some videos showed the top of the main inverting in the big breeze bear-away

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4 hours ago, WetHog said:

 

Also, America's Cup Event Limited is ACE?  Wouldn't it be ACEL?  ;)

WetHog  :ph34r:

Nope not in NZ - "Limited" describes a limited liability legal entity and is typically ignored when trading.

But good to see you focusing on the big issues ;-)

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Interesting, thanks. Wish we’d had this available when designing the wing sail for our Miss Lancia C-Cat in 1976, since it confirms there’s no way in hell one can match a slotted wing’s Clmax with a one-element profile. And this was the overriding factor until foiling brought apparent wind so much forward that efficiency became predominant instead.

But back on AC75 subject,  a double membrane sail is not going to help very much at the critical moment of take-off

 

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The Liebeck wing sections can achieve similar high Cl, but their stall behavior not easy to manage and the big Cl can be achieved with a laminar rooftop, not a turbulent one like for our sails or wings.

Coming back to the twin skin set_up, it seems we are talking about the main only, but:

If you consider the relative size of the jib, the height of the hound,

I think we should consider it as a slotted rig :

With the jib being the first element and the main sail the second element.

Cheers

 

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I can't wait to see the first twin skin sail up.

I think there is potential for an improvement in apparent wind sailing for boat with greatly reduced friction.

The Moths might be the first to benefit?

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1679914526_TwinskinDmastUK.jpg.0f2d6fcbce724b3af6ef9530118b1400.jpgWell Spotted

UK is using a twin skin D mast !

Are they using a twin skin sail ?

I don't do FB so can't get a good close up.

Is this as interesting as the foils?

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Thank you @BLAK for bringing this to my attention.

I believe the winning of the cup is in the development of the engine.

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On 9/25/2018 at 6:48 AM, Kiwing said:

I can't wait to see the first twin skin sail up.

I think there is potential for an improvement in apparent wind sailing for boat with greatly reduced friction.

The Moths might be the first to benefit?

It's been done on Moths and A Class and there has already been a fair amount of development done. To date, it hasn't been quicker in those classes than their standard rig, probably because the standard rigs are so highly developed.

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@A Class Sailor Kite are the highest performing and they are twin skinned. 

I realise I know nothing about kite sailing only watched it from a distance but looks very fast and very controllable.  These boats should have even more control over the wing/sail shape so should make some improvements.

These improvements may be the difference with these very slippery boats.

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Any pictures available for the Twin skin Moths or A-Cat experiences?

May be these experiences were conducted long time ago, when the drivers of performance were different: (pierce wave, mast stifness...)

A double skin sail is not likely to improve power directly, everything else equal, a single skin would achieve a bit more camber (and therefore a little more power) than a double skin sail.

If surface roughness is an issue, of course, a clean surface double skin sail, will improve not only the drag and will resist better to TE separation, and can achieve a little more power at large AoA.

Regarding this issue, I have no idea if the experience on RAF 30 wing section in a wind tunnel as reported by Marchaj (Aero-Hydrodynamics of Sailing, page 340 Fig:2.82) is relevant to appraise the effect of surface roughness on sail's lift.

Happy Sunday

Erwan

MARCHAJ 2.82.jpg

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I haven't got any pictures of either. The Moth rig was some time ago, but the A Class rig was this year. 2 were built by Advanced Wing Systems, the company i am pretty sure that gave ETNZ the idea of the soft wing.They have made soft wings for some time and even now with the AC teams looking at the problem they still probably have more knowledge on this than anybody else.

The issue of developing new rigs for classes like the Moth and A Class is that the current rigs are very highly developed and efficient. They particularly suit the type of boat they are used on. It makes gains from new rigs hard.

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there was a twin sking moth rig developed in Italy few years ago. It was had glimples of good speed, but was not really well tuneable how standard rigs are. Also, it was of course quite heavy.

It would need a lot of time to develop properly, so the project was aborted. I may have some old photos somewhere

 

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On 11/11/2018 at 9:33 PM, Kiwing said:

What about ice yachts, most have wings now, but are there any twin skins?

The large ones (such as Skeeters) have wing MASTS. Sails are "normal" fully battened. See for good info: https://www.iceboat.org

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@Alan Crawford Do you know, or have you seen a twin skin sail on an Ice Yacht?

I have not found much evidence of a twin skin sail actually proving really superior to a normal sail when added to an existing boat.  Moths are a case in point, but there the added weight might negate the small perceived benefit.  This might apply to Ice yachts too.

With an AC75 maybe the reduction in drag claimed by the SRW initiative and the movement of the resulting lift force forward in the direction of movement is enough to bring major benefits when the boat is designed to lever these benefits?

The design of sailing boats to make the most of apparent wind is a very new science!

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