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The new sailing twin skin setup


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"With most single-luff mainsails, leech tension is the only means of controlling twist." Actually, not so... For one, sideways bend of the mast will induce some "non-linear" twist into the sail - the

Short straw day in the rigging department  https://www.facebook.com/groups/ViF.Velisti/permalink/10159437643030931/  

When I was landsailing, tactics didn't play as big a role as they do in the America's Cup.  Speed is everything, so it didn't pay to mix it up in close quarters.  The yachts tended to do more of their

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2 minutes ago, Kiwing said:

@MaxHugen For the very short time when they are reaching with trying to get up on foils, how is my suggested cross-section of the twin skins relative the TWA?

Mast rotation can only be 'ideal' at one spot up it's height, as AoA changes throughout the sail's height. The teams would know approximately where the CE is - generally higher at low AWS, and lower at high AWS (dependent upon the degree of twist).  The mast rotation would be determined to roughly match where the estimated CE is, for the 'ideal' profile at that point.

Anywhere below that point, and the mast would be "over-rotated", and above that, "under-rotated".

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14 hours ago, ed__miller said:

This is what I am trying to get at. Excuse the crude drawing on a phone!

Top drawing. D section of mast relative to centreline. No mast rotation. Wind direction red arrow. Boltrope of left skin is equidistant to right skin.  And with an eased outhaul you could induce more depth in the left skin. 

 

Bottom drawing. D section of mast relative to centreline. Mast rotated to wind direction. The boltrope of left skin is further forward compared to right skin. Therefore to create the same depth of left skin compared to top drawing would require more outhaul to be eased as the foot length of skin is obviously fixed.  

Maybe @MaxHugen can make a nicer drawing on his CAD assuming the above makes any sense :) 

3B4292C8-47C4-4701-9586-0D94D30B4FCE.png

Unfortunately, I don't have a CAD program... yet.    I use a combination of NACA foil profiles, plus XFoil (via XFLR5, a GUI for XFoil), plus an excellent SVG program, to get the profiles I estimate.

I very much doubt that mast rotation is used to manage the skins, if I interpreted your diagram correctly - although it does influence it a bit.

My understanding is the foot tension of the individual skins is a combination of 2 forces:

  1. General Foot Tension - major  setting - both skins are tensioned overall to decrease/increase sail camber.
  2. Individual Foot Tension - minor setting - skins have an adjustment allowing the windward skin to have proportionately greater tension.

As we've seen, NZ has the additional "CAD" - camber adjustment device - to force a higher degree of camber into the vital lower sections of the sail. My thinking is that they don't want to ease General Foot Tension too much as it affects sail shape higher up. So GFT is set to manage the mid sail sections, and the "CAD" is used to increase camber on the lowest section.

For calcs, I consider every sail in 4 sections, using the "draft stripes" for some (rather rough) preliminary measurements. Each section has a different average AoA and Camber, at different AWS.

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15 hours ago, Erwankerauzen said:

Compared to a windsurf sail or Moth type sails, it is probably less efficient.

I doubt that.  

A solid wing sail is more efficient due to it's rounded leading edge, so the stagnation point moves aft, and more airflow on the leeward side increases velocity and reduces pressure there. They also benefit from the slot between the main foil and the flap.

The twin-skin sail is a compromise between the two. Max Sirena said months ago that they were "nearly" as efficient as the solid wing. Not sure what degree of "nearly" it really is, but I'd guess a fair bit closer to a solid wing, than a single skin sail.

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On 12/27/2020 at 4:05 PM, Kiwing said:

You want the flow to adhere so surely you want the windward skin to start where the mast finishes.  The thicker the more pressure difference and hence more lift. No separation means less drag?

Thickness does not increase the lift.  Camber increases the lift.  Thickness increases the local velocities on both the leeward and windward sides, for no net change to lift, and the flow has to decelerate more toward the leech making it more prone to separation.  If you keep the lee side curvature the same, increasing the thickness reduces the camber and reduces the lift.  

One thing that astounded me early in the 33rd Match was the sail makers did not really speak the language of aerodynamics.  I was expecting conversations like, "We're getting separation in this area of the sail, so we need to reduce the pressure gradient there, and that means we need to change the sail shape forward of there this way."   Instead the conversations were more like "I don't like the way the sail looks, so we should recut it like this."  

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From my suck and see experience you set the D mast  rotation and automatically the windward and leeward mains move to correct? shape. Granted this setup is much smaller scale and far less sophisticated than the AC75s - but it works fine for me.  The two  mains are only connected at mast peak and clew - so the sails can slide on each other.

frgmotions5.jpg

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On 12/28/2020 at 5:09 AM, Erwankerauzen said:

afair, thickness is not the criteria for high lift, instead it is the mean camberline which provides the lift.

So, as your sail has inside mecanisms to control the camber, probably, the windward skin can be cambered much, and only until it touchs the inside mecanism, therefore the max camber is limited.

But the leeward skin has no outside mecanisms which could limit the camber so it seems to be the only solution to get more camber in the whole section at the cost of a thicker wing section.

Compared to a windsurf sail or Moth type sails, it is probably less efficient. With these little single skin sails, if you add 1% camber in the sail it provides 1% increase in mean camberline as they are the same.

But for a twin skin, the windward one being limited by the inside mecanisms,  you must add 2% of camber in the leeward skin to achieve 1% increase in the mean camberline of the whole wing section.

Thickness is definitely one of the criteria. High lift  (and low drag) is about the difference in velocity between the air on the windward side of the airfoil and the leeward side of the airfoil. Depending on airspeed, maximum lift will be at a specific thickness and shape of the foil. A foil that has no thickness will be very far from maximum. And also pretty far from minimum drag. The hard wings on the cats were probably about twice the lift to drag ratio as a normal sail. And that was significantly due to the thickness.

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13 minutes ago, nroose said:

Thickness is definitely one of the criteria. High lift  (and low drag) is about the difference in velocity between the air on the windward side of the airfoil and the leeward side of the airfoil. Depending on airspeed, maximum lift will be at a specific thickness and shape of the foil. A foil that has no thickness will be very far from maximum. And also pretty far from minimum drag. The hard wings on the cats were probably about twice the lift to drag ratio as a normal sail. And that was significantly due to the thickness.

Yep.

There's a good reason why small planes that can take off within 20 feet have an appalling top speed. They pay a massive price in drag for their ultra high lift.

There is current (very nucleate) research into wings that can change form dynamically. Larger camber for take off reducing as it approaches cruise height.

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12 minutes ago, jaysper said:

There is current (very nucleate) research into wings that can change form dynamically. Larger camber for take off reducing as it approaches cruise height.

Most jet airliners do it now, using leading edge extensions for max lift and min drag, and trailing edge extensions for max lift and max drag, or both for even more lift to benefit from the slower speeds the extra drag created.

Watching them during takeoff and landings is my favourite part of the flight....

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13 minutes ago, Sidecar said:

Most jet airliners do it now, using leading edge extensions for max lift and min drag, and trailing edge extensions for max lift and max drag, or both for even more lift to benefit from the slower speeds the extra drag created.

Watching them during takeoff and landings is my favourite part of the flight....

Oh yeah, but do some searching on YouTube for what crazy shit they are trying to achieve now.

It's a whole different level. God knows if it will turn into anything practical any time soon, but it's fascinating to watch.

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8 minutes ago, Sidecar said:

I have seen some of it, and as you say, not likely to turn into anything practical any time soon.

 

I suspect they will need to see significant advances in material science before this sort of shape shifting is practical.

Looking at the internal mechanism, it is probably too fragile for 10K cycles in a commercial environment.

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Love the idea of twin skin sails, a major advantage also being that it allows for a more structurally efficient mast design, which should result in less weight aloft and material cost. Which is sadly more than offset by the cost and complexity of having two mainsails instead of one.

My “mainsail” has no mast in front of it, and I have played around with twin lightweight skins like in a ram air parachute. Two balanced pivoting luff stays up front at an appropriate distance apart to “inflate” and a larger balanced cam at the back, to control the chord depth with some form of webbing/intercostals in between for shape stability to reduce the weight and cost of two sets of battens.

Any paraglide nutters out there who would be interested in developing the idea with this nutter? .....PM me.

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6 hours ago, Sidecar said:

Love the idea of twin skin sails, a major advantage also being that it allows for a more structurally efficient mast design, which should result in less weight aloft and material cost. Which is sadly more than offset by the cost and complexity of having two mainsails instead of one.

My “mainsail” has no mast in front of it, and I have played around with twin lightweight skins like in a ram air parachute. Two balanced pivoting luff stays up front at an appropriate distance apart to “inflate” and a larger balanced cam at the back, to control the chord depth with some form of webbing/intercostals in between for shape stability to reduce the weight and cost of two sets of battens.

Any paraglide nutters out there who would be interested in developing the idea with this nutter? .....PM me.

I was considering a paraglider when jumping out of perfectly good aircraft became too expensive. :P  Built a yacht instead.

Interesting comparison to the twin-skin sail, do you happen to have any diagrams of recent paraglider 'foil' profiles? 

Curiosity has got me.

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6 hours ago, jaysper said:

Oh yeah, but do some searching on YouTube for what crazy shit they are trying to achieve now.

It's a whole different level. God knows if it will turn into anything practical any time soon, but it's fascinating to watch.

Sure is, like morphing wings and who knows what else...

MIT-NASA Flexible Wings

image.png.0e451d91529e4e4d269c2652cf1ac090.png

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7 hours ago, nroose said:

Thickness is definitely one of the criteria. High lift  (and low drag) is about the difference in velocity between the air on the windward side of the airfoil and the leeward side of the airfoil. Depending on airspeed, maximum lift will be at a specific thickness and shape of the foil. A foil that has no thickness will be very far from maximum. And also pretty far from minimum drag. The hard wings on the cats were probably about twice the lift to drag ratio as a normal sail. And that was significantly due to the thickness.

It's camber that creates the low pressure on the leeward side that creates lift, as you can see from an estimated sail profile with 9% camber at 7° AoA.

image.thumb.png.0f60f205a7d0be58d703d05809317077.png

The solid wings are "thick" because they have to present a "camber" on both tacks/gybes. They are designed something like this, from a NACA profile:

image.png.40a141a599e2d81a2d2e6b0c2d71282b.png

 

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1 hour ago, Kiwing said:

Isn't the windward surface of "D" draggy as the flow separates along the surfaces?

No, the flow mainly goes through the gap between the foil and the flap. Quite a while ago @Basiliscus posted a good pic of the airflow of one of these wings, quite interesting. You'd have to do some searching to find it I guess.

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1 hour ago, Kiwing said:

00wing1.thumb.jpg.e4cad6230f6d5d8581b09f6d855861e5.jpg

The max width of this wing is where the right most yellow line is?  Is this relevant to these AC75 twin skins @MaxHugen ?

Yes. The mast is 650mm 'long' (x direction), and I used 7.0m as the sail 'foot' length, so Max Width (aft edge of mast) is positioned at about 8.5% of chord length. The mast is 400mm wide (y direction), so it's ~5% of chord length.

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13 hours ago, nroose said:

Thickness is definitely one of the criteria. High lift  (and low drag) is about the difference in velocity between the air on the windward side of the airfoil and the leeward side of the airfoil. Depending on airspeed, maximum lift will be at a specific thickness and shape of the foil. A foil that has no thickness will be very far from maximum. And also pretty far from minimum drag. The hard wings on the cats were probably about twice the lift to drag ratio as a normal sail. And that was significantly due to the thickness.

Sorry NROOSE, but a foil with no thickness like a windsurf rig, seems to deliver significant lift, same remark for Moth or A-Cat rigs.

But I would advise you not to pay too much attention to my above remark regarding thickness & lift, and instead you should read Basilicus's post.

Cheers

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8 hours ago, MaxHugen said:

Interesting comparison to the twin-skin sail, do you happen to have any diagrams of recent paraglider 'foil' profiles? 

 Curiosity has got me.

Offhand, no I don’t.

The difference is that the leading edges, the base (boom) and the top, perhaps by means of a gaff batten(s) can all be held in place without multiple guy wires and strings and would be less collapsible that a paraglider. 

Hence my curiosity.

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4 hours ago, kiwin said:

Lift is a product of the change in direction of the airstream. A bigger mass of air "bent" through a bigger angle is more lift.

41 minutes ago, nav said:

Yep. Simples.

Maximise deflection, minimise drag.

Not really, guys. Try  Bernoulli's Principle.

If you're old enough, you might remember the fly spray pumps... like a bicycle pump, with a thin tube extending from a tank to where air was pushed out at high velocity:

image.png.4af3f20a2961640becc743f29a8596d9.png

No "bending" required!

 

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I have no theory or training in sailing, maths and stats background, computer science life, but I notice and I believe what I see.  Glen and TNZ are making these twin skins sing and dance.  Most sailors know if they really try they notice things that make boats go faster and science learns from them.  This is the case with this twin skin!  In My Humble Opinion.

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2 hours ago, MaxHugen said:

Not really, guys. Try  Bernoulli's Principle.

If you're old enough, you might remember the fly spray pumps... like a bicycle pump, with a thin tube extending from a tank to where air was pushed out at high velocity:

image.png.4af3f20a2961640becc743f29a8596d9.png

No "bending" required!

 

You're not quite right there either max (I think).

Yes Bernoulli's principle applies since the wing creates an area of low pressure on the side where the breeze must travel the longest distance and hence lift.

However, deflecting the direction of the breeze introduces the whole opposite and equal force thing which is how sticking a perfectly flat piece of cardboard out your car window at an angle 100kmh will most definitely induce a force.

No profile but plenty of force.

And yes, certainly Mr Bernoulli gets a say in there as well as the air transitions at the edges, but that's not the whole story.

So if I was fully awake in 200 and 300 engineering classes, that should be a reasonable if half arsed explanation of it.

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6 hours ago, kiwin said:

Lift is a product of the change in direction of the airstream. A bigger mass of air "bent" through a bigger angle is more lift.

I think this is backward. The change in direction of the airstream is a product of the lift. Lift is a product of the difference in speed of the air across the surfaces.

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8 hours ago, Erwankerauzen said:

Sorry NROOSE, but a foil with no thickness like a windsurf rig, seems to deliver significant lift, same remark for Moth or A-Cat rigs.

But I would advise you not to pay too much attention to my above remark regarding thickness & lift, and instead you should read Basilicus's post.

Cheers

I think Basillicus is talking about convex vs concave. So long as the inner surface is concave, it's not going to increase the velocity of that air. The shorter the span on the inside the slower the air will go across it. But you want flow, so you can't just make it flat.

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Bernoulli's theorem accounts for a very small proportion of wing lift. 3-5% max. The explanation whereby the leeward air molecules " have to travel faster to reach the leech at the same time" should trigger all the red flags. Why do you think the leeward air is compelled to  travel faster? Are there rules? Do air molecule police arrive and put slow molecules in the naughty corner?

You can go into all sorts of theories but essentially if you can bend more air through a bigger angle you get more lift. It's a bit like catching a ball and rethrowing it in a different direction. You will experience a reaction force as you eject the ball in the new direction.

This is why more camber generates more lift. It imparts a larger change in direction. And it's why a stalled sail generates less lift, because the flow detaches on the leeward side and doesn't change direction as much.

 

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1 hour ago, kiwin said:

Bernoulli's theorem accounts for a very small proportion of wing lift. 3-5% max. The explanation whereby the leeward air molecules " have to travel faster to reach the leech at the same time" should trigger all the red flags. Why do you think the leeward air is compelled to  travel faster? Are there rules? Do air molecule police arrive and put slow molecules in the naughty corner?

You can go into all sorts of theories but essentially if you can bend more air through a bigger angle you get more lift. It's a bit like catching a ball and rethrowing it in a different direction. You will experience a reaction force as you eject the ball in the new direction.

This is why more camber generates more lift. It imparts a larger change in direction. And it's why a stalled sail generates less lift, because the flow detaches on the leeward side and doesn't change direction as much.

 

I suspect with wings where there is a significant difference in profile from one side to the other,  that Mr Bernoulli will get a bigger say than 2 or 3%.

But yes, my equal and opposite reaction comment was trying to make much the same point. Take some wind and force it to change direction will induce a reactionary force.

If you don't attempt to change the angle of the wind,  you might as well return to the days of square rigging (some hyperbole intended).

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5 hours ago, nroose said:

I think Basillicus is talking about convex vs concave. So long as the inner surface is concave, it's not going to increase the velocity of that air. The shorter the span on the inside the slower the air will go across it. But you want flow, so you can't just make it flat.

The whole "air flows faster on the lee side because it's a longer distance than the windward side, leading to a difference in pressure because of Bernoulli's principle" explanation is complete hooey.  There are so many ways from everyday experience to see this is false.  If it was true, a symmetrical wing section would produce no lift, which would be news to the Red Bull air racers.  A flat wing would produce no lift, which as any kid that's flown a balsa dime-store glider knows, is false.  A jib would produce no lift, because a thin sail has no difference in arc length between the windward and leeward sides.  But as a sailor, you know this to be false.  Angle of attack would have no effect, which is false.  In fact, the air on the lee side actually gets to the trailing edge before the air on the windward side, so there's nothing to the notion that both sides have to get there simultaneously.  And if you calculate how much difference in arc length would be required to get normal levels of lift, the difference would require a far thicker airfoil than any airplane has.

I'm always amazed at the way fluid dynamics has spawned so many "Just So Stories" that are no more true than Kipling's tales. 

 

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24 minutes ago, Basiliscus said:

The whole "air flows faster on the lee side because it's a longer distance than the windward side, leading to a difference in pressure because of Bernoulli's principle" explanation is complete hooey.  There are so many ways from everyday experience to see this is false.  If it was true, a symmetrical wing section would produce no lift, which would be news to the Red Bull air racers.  A flat wing would produce no lift, which as any kid that's flown a balsa dime-store glider knows, is false.  A jib would produce no lift, because a thin sail has no difference in arc length between the windward and leeward sides.  But as a sailor, you know this to be false.  Angle of attack would have no effect, which is false.  In fact, the air on the lee side actually gets to the trailing edge before the air on the windward side, so there's nothing to the notion that both sides have to get there simultaneously.  And if you calculate how much difference in arc length would be required to get normal levels of lift, the difference would require a far thicker airfoil than any airplane has.

I'm always amazed at the way fluid dynamics has spawned so many "Just So Stories" that are no more true than Kipling's tales. 

 

A symmetrical foil produces lift because of angle of attack, which makes the effective length of the leeward side longer than the windward side. The case of a uniform width foil producing lift is about the fact the the inside of a curve is shorter than the outside, and angle of attack produces that affect as well.

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Thank you @Basiliscus
From what I have read Leonardo observed and draw flows that into fluids that we are only just getting to understand and how we get lift is still being debated to completely understand all aspects.  Here, we have a world where we need a lot of traditional sailing knowledge until we get the foils providing RM then we transition to apparent wing which requires a completely different mode.  Fascinating stuff.  Wish I knew half of what is going on!

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I'm trying to figure out how to convert the trimming of these twin skins to controls that most sailors are used to. This is my first stab, let me know where I'm wrong:

  • Shrouds - Rig tension, more in heavy air, less in light air. I don't think this will work any differently in these boats, they probably try and run the rigs very straight without any lateral sag in the middle.
  • Forestay - Rake control, more rake in light air, less in heavy air. Do we think they run a fairly constant rake, or adjust for conditions?
  • Backstays - Function roughly the same as usual, flatten the top section of both mains. Also reduces headstay sag. It's been noted ETNZ runs a good amount of mast bend, I wonder if they are using the backstay as an active trim control to control power and heel angle?
  • Vang - likely non-existent. Leech tension is controlled by the mainsheet, these boats are always sailing tight AWA so don't need a vang.
  • Mainsheet - Leech tension. The boats without a boom must have independent mainsheets for the two sails?
  • Traveller - AoA control. Definitely the most active control on the boat. I assume they change mast rotation in conjunction with this? Or is the mast rotation your coarse control of AoA and then traveller is used mainly as power and angle control as AWS changes?
  • Outhauls - Camber (depth) control, 1 per sail. This is definitely where the most differences between boats exist. How you control the camber down low seems to be a huge factor in low-speed power (marginal foiling).
  • Cunningham - I don't think these sails have/need it. They likely are cut with the draft where you want it, and you wouldn't really need to move it.

I think the sails must also have some kind of camber control at the head as well. It seems the top part of the sail is being treated as a symmetrical foil.

The only traditional way of controlling shape in the mid-section would be batten tension, which they could definitely be actively adjusting, but is that allowed or even worth it?

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2 hours ago, sailer99 said:

..........

I think the sails must also have some kind of camber control at the head as well. It seems the top part of the sail is being treated as a symmetrical foil.

The only traditional way of controlling shape in the mid-section would be batten tension, which they could definitely be actively adjusting, but is that allowed or even worth it?

"...... the top as a symmetrical foil".  Yes you have something there I believe, first time I have thought like that.  Post #1505 looking down on Britannia they do put a slight camber on it but I think TNZ does not seem to.  But using as a symmetrical foil with slight AoA on one side or the other of the wind would give great control.  Thank you @sailer99.
Sorry, as a 75yrold sailing a laser gives me no idea of your other ideas.

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On 1/1/2021 at 3:42 AM, sailer99 said:

I think the sails must also have some kind of camber control at the head as well. It seems the top part of the sail is being treated as a symmetrical foil.

The only traditional way of controlling shape in the mid-section would be batten tension, which they could definitely be actively adjusting, but is that allowed or even worth it?

They are permitted to actively control the top 4m and lowest 1m of the sail.

As for the mid section, wouldn't they also use mast bend to reduce camber?

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On 12/31/2020 at 4:42 PM, sailer99 said:

 

  • Cunningham - I don't think these sails have/need it. They likely are cut with the draft where you want it, and you wouldn't really need to move it.

 

Why? Don't the same factors apply as traditional sails? For a larger 'groove'  such as needed in less stable wind conditions you put leech tension on (using cunningham or halyard), for higher pointing in more stable conditions you ease that tension?

Or can you achieve that by using the mast rotation to control the luff incident angle?

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On 12/31/2020 at 4:42 PM, sailer99 said:

I'm trying to figure out how to convert the trimming of these twin skins to controls that most sailors are used to. This is my first stab, let me know where I'm wrong:

  • Shrouds - Rig tension, more in heavy air, less in light air. I don't think this will work any differently in these boats, they probably try and run the rigs very straight without any lateral sag in the middle.
  • Forestay - Rake control, more rake in light air, less in heavy air. Do we think they run a fairly constant rake, or adjust for conditions?
  • Backstays - Function roughly the same as usual, flatten the top section of both mains. Also reduces headstay sag. It's been noted ETNZ runs a good amount of mast bend, I wonder if they are using the backstay as an active trim control to control power and heel angle?
  • Vang - likely non-existent. Leech tension is controlled by the mainsheet, these boats are always sailing tight AWA so don't need a vang.
  • Mainsheet - Leech tension. The boats without a boom must have independent mainsheets for the two sails?
  • Traveller - AoA control. Definitely the most active control on the boat. I assume they change mast rotation in conjunction with this? Or is the mast rotation your coarse control of AoA and then traveller is used mainly as power and angle control as AWS changes?
  • Outhauls - Camber (depth) control, 1 per sail. This is definitely where the most differences between boats exist. How you control the camber down low seems to be a huge factor in low-speed power (marginal foiling).
  • Cunningham - I don't think these sails have/need it. They likely are cut with the draft where you want it, and you wouldn't really need to move it.

I think the sails must also have some kind of camber control at the head as well. It seems the top part of the sail is being treated as a symmetrical foil.

The only traditional way of controlling shape in the mid-section would be batten tension, which they could definitely be actively adjusting, but is that allowed or even worth it?

Rake I would expect to go back in more wind. However, it would also change jib sheet angle and they don't seem to have lots of adjustment to cope with that, so I doubt they change rake much.

They also have mast rotation which is independent of AoA (traveller).  It will have an affect on camber the relative entry angle for a give AoA. 

Also looks like the new INEOS main has a significant scope of inhaul. Something for all the Finn boys onboard to talk about!

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8 hours ago, MaxHugen said:

They are permitted to actively control the top 4m and lowest 1m of the sail.

As for the mid section, wouldn't they also use mast bend to reduce camber?

You're right, they would use mast bend for the mid-section of the sail. I didn't know about the active control limits, but they make a lot of sense. I guess mast-bend would be used for mid-section camber, and then a separate control for upper camber.

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8 hours ago, enigmatically2 said:

Why? Don't the same factors apply as traditional sails? For a larger 'groove'  such as needed in less stable wind conditions you put leech tension on (using cunningham or halyard), for higher pointing in more stable conditions you ease that tension?

Or can you achieve that by using the mast rotation to control the luff incident angle?

I think this is less of a twin-skinned thing and more of a symptom of these molded sails. From my experience with the composite sails, the draft position is set and a Cunningham isn't very effective for trying to move it. I could definitely be wrong about that though.

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1 hour ago, sailer99 said:

You're right, they would use mast bend for the mid-section of the sail. I didn't know about the active control limits, but they make a lot of sense. I guess mast-bend would be used for mid-section camber, and then a separate control for upper camber.

Yes. Importantly the question is how that affects the leach at that point and the amount of luff curve used. The leach overall, but particularly the middle of the leach helps gust response, ie allows the boat to avoid drag when the gust hits but importantly helps accelerate the boat through the gust. A boat will try to "stand up" if this is not happening correctly. Adding mast bend and twist to the equation allows luff shaping and added tuning capabilities. A good comparison is the Tornado which has a semi rotating deck-stepped aerofoil section with diamonds. 

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2 hours ago, danstanford said:

Ken Read's thoughts on these boats and particularly the sail controls. 

He definitely becomes far more interested and animated when the conversation turns to the technical aspects of sails doesn't he!

He should start talking technical like that when he's commentating, stop trying to dumb it down bring new fans to sailing in general and start trying to bring in new America's Cup fans that already understand sailing.

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In that video Ken recalls a conversation with BA earlier in the cycle about the twin skin main; BA apparently said "it may have well as been single skin"... this suggests that, at least early on, INEOS were not putting enough emphasis on the sail/rig package. I remember the early mainsails on B1 looked a lot rougher than other teams, particularly the deck sweeper sections. Could it be that INEOS are still playing catch-up?

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9 hours ago, Boybland said:

He definitely becomes far more interested and animated when the conversation turns to the technical aspects of sails doesn't he!

He should start talking technical like that when he's commentating, stop trying to dumb it down bring new fans to sailing in general and start trying to bring in new America's Cup fans that already understand sailing.

^^This

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10 hours ago, Boybland said:

He definitely becomes far more interested and animated when the conversation turns to the technical aspects of sails doesn't he!

He should start talking technical like that when he's commentating, stop trying to dumb it down bring new fans to sailing in general and start trying to bring in new America's Cup fans that already understand sailing.

I think Ken drops a nice compromise, asking others to explain some generalities, bumping in with some gems..Ken, Nathan, Shirley team is great..loose the other bumbler...wasted space

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The way they are using Shirley is demeaning. She has so much to offer and has been limited to saying “its all terribly exciting”. 


they should dumb it down far less in my view. Nathan is good when he gets a chance but any idiot can see the boats are going fast. They should be explaining the technical side far more. That’s what makes it interesting. Anyway, bit off-topic. 

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36 minutes ago, KingMonkey said:

The way they are using Shirley is demeaning. She has so much to offer and has been limited to saying “its all terribly exciting”. 


they should dumb it down far less in my view. Nathan is good when he gets a chance but any idiot can see the boats are going fast. They should be explaining the technical side far more. That’s what makes it interesting. Anyway, bit off-topic. 

Never off topic here..I agree about the calls to Shirley for "what do you see I the water". A link, ( Python speak), is needed, but something other than "how do conditions look". Maybe, "how is the battle shaping up at water level", or "what do you think of xxxx performance on the last leg".

 

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On 1/4/2021 at 4:02 AM, Mozzy Sails said:

Rake I would expect to go back in more wind. However, it would also change jib sheet angle and they don't seem to have lots of adjustment to cope with that, so I doubt they change rake much.

They also have mast rotation which is independent of AoA (traveller).  It will have an affect on camber the relative entry angle for a give AoA. 

Also looks like the new INEOS main has a significant scope of inhaul. Something for all the Finn boys onboard to talk about!

Tack height?

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13 hours ago, barfy said:

I think Ken drops a nice compromise, asking others to explain some generalities, bumping in with some gems..Ken, Nathan, Shirley team is great..loose the other bumbler...wasted space

I was impressed at just how much more interesting he was when he was talking about sails.

What he was saying in that interview is literally exactly the kind of stuff I persoanlly want to hear while watching yacht racing. It's really the same for any sport for me, the better the insights from the commentator the better it is to watch.

I firmly believe that sailing has so much down time between races and during delays etc...  there are hours and hours when the dumbed down explanations can happen.

During racing just give us the good stuff ken!

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13 hours ago, KingMonkey said:

The way they are using Shirley is demeaning. She has so much to offer and has been limited to saying “its all terribly exciting”. 


they should dumb it down far less in my view. Nathan is good when he gets a chance but any idiot can see the boats are going fast. They should be explaining the technical side far more. That’s what makes it interesting. Anyway, bit off-topic. 

Not sure I agree, Monkey.

Example: My car uses turbo stratified injection (like a bloody big leaf blower. ;-)). I don't really understand the intricacies of how it works, but I can certainly appreciate that this technology makes it's quick. I think that level of understanding is good enough for a lot of folk watching AC75 yacht racing too.

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30 minutes ago, Sailbydate said:

Not sure I agree, Monkey.

Example: My car uses turbo stratified injection (like a bloody big leaf blower. ;-)). I don't really understand the intricacies of how it works, but I can certainly appreciate that this technology makes it's quick. I think that level of understanding is good enough for a lot of folk watching AC75 yacht racing too.

Especially for the NZ audience (which i would wager is the majority) cos despite the vast majority of NZers watching the racing, very few of them have a clew what a halyard is.

For them, the coverage is about spot on.

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1 hour ago, Sailbydate said:

Not sure I agree, Monkey.

Example: My car uses turbo stratified injection (like a bloody big leaf blower. ;-)). I don't really understand the intricacies of how it works, but I can certainly appreciate that this technology makes it's quick. I think that level of understanding is good enough for a lot of folk watching AC75 yacht racing too.

I'm not sure it's as complicated as the name suggests. The turbo part is that air is delivered compressed, via a turbocharger. Stratified injection means the fuel is injected into the cylinder at high pressure, often in multiple very rapid bursts, to end up with a fine and more even distribution of fuel particles that burn more completely. 

There are different ways to achieve it, but that's all that's going on. 

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2 minutes ago, buckdouger said:

I'm not sure it's as complicated as the name suggests. The turbo part is that air is delivered compressed, via a turbocharger. Stratified injection means the fuel is injected into the cylinder at high pressure, often in multiple very rapid bursts, to end up with a fine and more even distribution of fuel particles that burn more completely. 

There are different ways to achieve it, but that's all that's going on. 

I read the brochure and got that much thanks, Buck. But it does illustrate my point rather well.

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15 hours ago, KingMonkey said:

The way they are using Shirley is demeaning. She has so much to offer and has been limited to saying “its all terribly exciting”. 


they should dumb it down far less in my view. Nathan is good when he gets a chance but any idiot can see the boats are going fast. They should be explaining the technical side far more. That’s what makes it interesting. Anyway, bit off-topic. 

I'm not sure I agree either... she currently has the closest view of the action, in the thick of it, and (based her comments in her podcast) loves it!

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55 minutes ago, Sailbydate said:

I read the brochure and got that much thanks, Buck. But it does illustrate my point rather well.

Fair enough, wasn't sure based on the leaf blower comment what 'intricacies' meant to you.

I am for more detail from the commentators, it's a unique opportunity to hear from experts about something we are interested in. For the casual viewer there are the visuals. If someone is going to become a fan, I doubt it's the commentating being lay-oriented enough that's going to make the difference. 

If they are going to remain casual observers and make up the numbers, I'm not sure I care, and I'm not sure their viewership changes much for the sport. 

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2 minutes ago, buckdouger said:

Fair enough, wasn't sure based on the leaf blower comment what 'intricacies' meant to you.

I am for more detail from the commentators, it's a unique opportunity to hear from experts about something we are interested in. For the casual viewer there are the visuals. If someone is going to become a fan, I doubt it's the commentating being lay-oriented enough that's going to make the difference. 

If they are going to remain casual observers and make up the numbers, I'm not sure I care, and I'm not sure their viewership changes much for the sport. 

Except....New Zealand. Sorry but its just true.

In no country in the world, it seems, does the general populous give a flying fuck about the America's Cup....except....New Zealand.

Now I have no particular issue with you not giving a shit about the general populace of New Zealand. However, ETNZ has an enormous vested interest in ensuring they (and the Government as a consequence) want them to keep going.

However, what was Ernie's and Lazza's excuse?

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1 minute ago, buckdouger said:

Fair enough, wasn't sure based on the leaf blower comment what 'intricacies' meant to you.

I am for more detail from the commentators, it's a unique opportunity to hear from experts about something we are interested in. For the casual viewer there are the visuals. If someone is going to become a fan, I doubt it's the commentating being lay-oriented enough that's going to make the difference. 

If they are going to remain casual observers and make up the numbers, I'm not sure I care, and I'm not sure their viewership changes much for the sport. 

All good, except it's not really the AC franchise's job to invigorate/expand the sport of sailing, IMO.

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I enjoyed the article and the table setting it did so we can better understand what they are struggling with. However this sentence is hard for me to understand, is it possible for someone to explain the reasons? I certainly have seen lots of depth created for power to get them up on their foils and then the sail getting flattened to reduce drag and all the makes sense to me. This however is stretching my brain:

If you are going to have two skins, they’d better be more efficient than a single one, and that means finding a way to make the windward surface flatter than the leeward surface. 

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^ Simplified, a rigid wing, a la AC50 gets it's 'shape' by controlling the angle between the elements, but the elements themselves remain symmetrical on both tacks.

A single skin sail is also 'symmetrical'

With the twin-skin set up if you controlled both clews together just like a single skin you would be missing out on a lot of power and adding drag.

The advantage this rig has over the 2 above is the ability to make the sail (element) shape asymmetric from tack to tack.....and that means finding a way to make the windward surface flatter than the leeward surface. 

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28 minutes ago, danstanford said:

I enjoyed the article and the table setting it did so we can better understand what they are struggling with. However this sentence is hard for me to understand, is it possible for someone to explain the reasons? I certainly have seen lots of depth created for power to get them up on their foils and then the sail getting flattened to reduce drag and all the makes sense to me. This however is stretching my brain:

If you are going to have two skins, they’d better be more efficient than a single one, and that means finding a way to make the windward surface flatter than the leeward surface. 

I think that it is primarily to maintain laminar flow, thus reducing drag. This would particularly apply in lower wind speeds with a deep camber.

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1 hour ago, danstanford said:

I enjoyed the article and the table setting it did so we can better understand what they are struggling with. However this sentence is hard for me to understand, is it possible for someone to explain the reasons? I certainly have seen lots of depth created for power to get them up on their foils and then the sail getting flattened to reduce drag and all the makes sense to me. This however is stretching my brain:

If you are going to have two skins, they’d better be more efficient than a single one, and that means finding a way to make the windward surface flatter than the leeward surface. 

The ideal wing is shaped like an aircraft wing which is an asymmetrical shape, flat on the bottom and curved on the top. A twin skin main can attempt to copy that shape with the windward side flat and the leeward side curved, if the skins are adjusted separately with the rotating mast.

A solid wing cannot copy the shape because it is symmetrical and can only be bent in the middle.

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Sorry Guys, I could understand this if the sides were reversed. Everything I have ever read about foils says they get their lift predominately from the fact that the air has a longer path around the lifted side. Therefore the windward side should be less flat than the leeward, shouldn't it?

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19 minutes ago, danstanford said:

Sorry Guys, I could understand this if the sides were reversed. Everything I have ever read about foils says they get their lift predominately from the fact that the air has a longer path around the lifted side. Therefore the windward side should be less flat than the leeward, shouldn't it?

The lifting side is the leeward side.

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Dan

The longer path is only a very small proportion of the lift.

The asymmetric twin skin with the short solid leading edge enables attached flow across a wide range of awa and chord depth. Without the flattened windward skin, the drag of the deep chord (like when they just complete a tack, or at the low end of the foiling range) would be too high. They look like effective chord depths (for the mains) up near 20% at max. Try that with a single skin main and see exactly how fast you do not go...!

Upwind, single skin main depths cannot be much deeper than about 12%

Lift is almost linearly proportional to both aoa and chord depth, provided there is flow attachment... Lift can only exist perpendicular to the local deflecting flow.

So the windward side cannot generate material amounts of lift.

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4 hours ago, danstanford said:

Sorry Guys, I could understand this if the sides were reversed. Everything I have ever read about foils says they get their lift predominately from the fact that the air has a longer path around the lifted side. Therefore the windward side should be less flat than the leeward, shouldn't it?

Think of a hose, with nothing on the end of it. When you turn on the tap moderately, water comes out in a stream approximately the same diameter as the hose. Now put a nozzle of the hose, and the same volume of water comes out in a thinner stream but it has been accelerated. Although perhaps not intuitive, pressure has decreased.

With a wing, or a sail, the direction of the airflow is changed, effectively forcing more air through a narrower area - just like the nozzle on the hose. This accelerates the air, and consequently the pressure reduces. Thus it is not that the air has further to travel, but it is travelling faster, that produces the low pressure which is part of the reason for the lift.

This is not the full explanation, as Newton's laws are also at play, but hopefully this helps explain it a bit for now.

image.png.f6748085925dbee11ff6eec234b559b6.png

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Max, the drawing really helped. I guess I was stuck in the land of single skin sails. When I think of a deeper draft I really only was seeing the windward side of the sail in my mind's eye perhaps that is from setting sails and helming looking at the inside of the foil for the most part. 

Sorry to be so thick Everybody and thanks for the answers. 

Dan 

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9 hours ago, MaxHugen said:

Think of a hose, with nothing on the end of it. When you turn on the tap moderately, water comes out in a stream approximately the same diameter as the hose. Now put a nozzle of the hose, and the same volume of water comes out in a thinner stream but it has been accelerated. Although perhaps not intuitive, pressure has decreased.

Pressurised fluids in a pipe are entirely different to free flowing air over a sail.

All air passing over or around a sail is travelling slower than the surrounding air, none of it travels faster. The air on the windward side travels slower than the air over the leeward side (ignoring separation bubbles and stalled air), but all of it is slower than air further away from the sail on either side.

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13 minutes ago, RobG said:

Pressurised fluids in a pipe are entirely different to free flowing air over a sail.

All air passing over or around a sail is travelling slower than the surrounding air, none of it travels faster. The air on the windward side travels slower than the air over the leeward side (ignoring separation bubbles and stalled air), but all of it is slower than air further away from the sail on either side.

Incorrect. Google "why does air flow faster over a wing".

This is a screenshot I had saved which shows bursts of coloured smoke being released milliseconds apart, to show that effect. Top flow accelerates, bottom flow decelerates.

image.png.a85d946405f85e64b72e3c64a48b2950.png

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Rob

The most deflected air, sailing upwind, is at a slightly higher velocity than the free stream apparent....Just for a small area on the leftward side near the jib bluff in a conventional setup.

This flow is enhanced by the mainsail.

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I found a gif at Wikipedia that is an interesting illustration of airflow around a foil:

Karman_trefftz.gif.0ef54a8e5b3d4759f84abaaaa8356cf1.gif

At left is the free stream, and you can see that it is affected well in advance of, and particularly above, the foil. The air stream above the foil has advanced almost twice that below the foil, although the degree of advancement is dependent on airflow velocity.

It also demonstrates the advantage a yacht might gain when attempting to overtake, if they sail to leeward in the faster air stream rather that to windward - providing they stay out of the turbulent airflow zone shown at centre right and below.

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1 hour ago, MaxHugen said:

I found a gif at Wikipedia that is an interesting illustration of airflow around a foil:

Karman_trefftz.gif.0ef54a8e5b3d4759f84abaaaa8356cf1.gif

At left is the free stream, and you can see that it is affected well in advance of, and particularly above, the foil. The air stream above the foil has advanced almost twice that below the foil, although the degree of advancement is dependent on airflow velocity.

It also demonstrates the advantage a yacht might gain when attempting to overtake, if they sail to leeward in the faster air stream rather that to windward - providing they stay out of the turbulent airflow zone shown at centre right and below.

Sorry Max, but the gif is obviously wrong.  It has the air moving at the very top of the diagram (away from any wing effect) moving much faster than the air at the bottom of the diagram (away from any wing effect).  The wind is at different speeds way before it even touches the wing.

See Basiliscus #1735

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1 hour ago, MaxHugen said:

I found a gif at Wikipedia that is an interesting illustration of airflow around a foil:

Karman_trefftz.gif.0ef54a8e5b3d4759f84abaaaa8356cf1.gif

At left is the free stream, and you can see that it is affected well in advance of, and particularly above, the foil. The air stream above the foil has advanced almost twice that below the foil, although the degree of advancement is dependent on airflow velocity.

It also demonstrates the advantage a yacht might gain when attempting to overtake, if they sail to leeward in the faster air stream rather that to windward - providing they stay out of the turbulent airflow zone shown at centre right and below.

Also, what you are describing is the very first example in the Generation of Lift - Old Theories  that is listed in the article that YOU posted on Modern Sail Theories back in post #1500.  

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57 minutes ago, The_Alchemist said:

Sorry Max, but the gif is obviously wrong.  It has the air moving at the very top of the diagram (away from any wing effect) moving much faster than the air at the bottom of the diagram (away from any wing effect).  The wind is at different speeds way before it even touches the wing.

See Basiliscus #1735 

If you look at the black dots you will see that they form a straight line at the left. This is the free stream. That the air stream is affected before reaching the foil - and above - is well understood.

In the post you quoted, Basiliscus discussed a misconception: " The whole 'air flows faster on the lee side because it's a longer distance than the windward side, leading to a difference in pressure because of Bernoulli's principle' explanation is complete hooey. "

Note that this does not dispute that the airflow is faster on the leeward side, but the reason for why it does.

[edit] From what info are you stating that the airflow is "away from any wing effect" ?

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