Kiwing

The new sailing twin skin setup

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Looks like it has a batten at the bottom like the mule's Twin skin.

Is it Italian?

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3 hours ago, darth reapius said:

Is this what I think it is?

LUNAROSSASOFTWING.png

 :) Where’s that from?

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^ From Luna Rossa team video at 00:54. Didn't they release that they use a cat shorten to comply with surrogate length and were mainly focusing on wing development? (can't find links or source for that)

 

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LR testing the rig. Looks pretty slick already......

There's one heads'l missing though isn't there?

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On 11/24/2018 at 11:43 PM, Erwankerauzen said:

Kiwing,

If your above-mentionned readings are not secret, I am sure many geek loke me would be happy to read then too.

Could you post some reading reference please.

Thanks in advance

Laurent

 

Erwan

@Erwankerauzen,

I have searched and searched but not come up with it yet. I was about distorting wings replacing flaps motivated by radar avoidance and better adhesion for the flow increasing fuel economy for commercial aircraft. I thought I found it on Google scholar from a PhD but maybe it was other research as I can't find it again.

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8 minutes ago, Kiwing said:

I was about distorting wings replacing flaps m

You mean like the Wright Brothers?

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1 hour ago, random said:

You mean like the Wright Brothers?

Way a head of their time obviously.

But not around to help NYYC !!??

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8 hours ago, Kiwing said:

1527090028_doulbleskinatthebottom.jpg.b1135f75d7de2b0bb3758d09f3a7e842.jpg

The triangle on both sides of the mast, at the bottom, is this new?

You mean the gnav (inverted vang)? That’s been around in dinghies and sports boats for a long time to free up space for crew.

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Thanks @dogwatch read all about that.

Now the sail material on each side of the mast pulled back to the boom, almost like a twin skin shaping sail element. Is it to reduce the air disturbance of the gnav or for the actual mainsail performance or a bit of both?

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On 12/13/2018 at 9:45 PM, dogwatch said:

You mean the gnav (inverted vang)? That’s been around in dinghies and sports boats for a long time to free up space for crew.

And to push bend up the mast, making the gnav more effective with less load.

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23 hours ago, Kiwing said:

Thanks @dogwatch read all about that.

Now the sail material on each side of the mast pulled back to the boom, almost like a twin skin shaping sail element. Is it to reduce the air disturbance of the gnav or for the actual mainsail performance or a bit of both?

It's there to provide space for the gnav.  Otherwise the mainsail will set fine on one tack but be distorted by the gnav on the other.

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I am so surprised:-

This is where the innovation will be this round (Ibelieve).  The winning of the cup will be here in the engine room yet no one has made any suggestions about what the Kiwis might do? I think we are in for a surprise but of course the sails are so easy to copy that maybe we wont see it until the second half of the match when the Kiwis need to pull out all the stops to win and not give the challenger time to copy and perfect the new innovation!  May be it will be hidden deep inside so no one can work it out quickly, who knows.

I suggest it will be in the transition from non apparent wing sailing to apparent wind sailing, The ability to change the shape and angle of attack smoothly, not loosing flow, and keeping the acceleration constant up to ultimate  VMG.

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1 hour ago, Kiwing said:

The winning of the cup will be here in the engine room yet no one has made any suggestions about what the Kiwis might do?

A bit hard to speculate when you aren't actively developing and testing a twin skin setup. Also, will North be water tight if NZ come up with something profoundly different? I agree that sails will be critical, along with foil design and boat handling.

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25 minutes ago, Horn Rock said:

A bit hard to speculate when you aren't actively developing and testing a twin skin setup. Also, will North be water tight if NZ come up with something profoundly different? I agree that sails will be critical, along with foil design and boat handling.

My thoughts are;-

foil design 2% of total slow to max VMG performance
boat handling 3% of total slow to max VMG  performance
twinskin 5% of total slow to max VMG performance

For what this 75 year old laser sailor's opinion is worth.  I actually think ETNZ (Ashby, foils, and better control of all elements) had better VMG, that they only used fleetingly last cup and I can't wait to see if they can carry that over?

As far as North is concerned I doubt any thing profoundly different will leek but little tweaks might.  I hope both ways?

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3 hours ago, Horn Rock said:

A bit hard to speculate when you aren't actively developing and testing a twin skin setup. Also, will North be water tight if NZ come up with something profoundly different? I agree that sails will be critical, along with foil design and boat handling.

 

3 hours ago, Horn Rock said:

A bit hard to speculate when you aren't actively developing and testing a twin skin setup. Also, will North be water tight if NZ come up with something profoundly different? I agree that sails will be critical, along with foil design and boat handling.

What NZ eventually comes up with will be done in conjunction with the embedded North Sail Engineers.  Each team of North Engineers are most likely held to confidentiality agreements.

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Regarding the advantage of the twin skins set-up,  AFAIR , for the cup with the big trimaran BMW  ORACLE , the design team, especially Joseph Ozannes, involved a little bit behind the wing project, was mentionning that the airflow remains atttached to the wing during all the tack, creating less drag than a teardrop mast + batten sail which triggers separation during the tack, as a consequence of the "Hinge" of the  teardrop mast + sail.

Separation =probably total loss of driving force + increase in pure drag.

In addition, you have to  wait a few  seconds for the flow to re-attache to the sail when on this other tack, and this time period is prop to the sail chord's  lenght.

I guess  that  one of the main twin skin's competitive advantage is around this issue, which seems consistant for a match racing regata.

Cheers

 

Erwan 

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@Erwankerauzen Thanks, I wonder what the separate NorthSail teams are doing.

Is there any team that is not using NorthSails?

Where is Josepth Ozannes working now? Is he a American NorthSail team?

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Sorry Kiwing,  I have no idea about what the different North-Sails teams are doing.

Joseph Ozannes was involved in CFD within the ORACLE's Design team

His colleague Dimitri Despierres was addressing the FEA/Structural engineering issues.

Both are "rocket-scientists" 

No idea where they are working now. Otherwise I would beg them to help me making a little bit CFD/FEA for my A-Cat morphing wing project.

Cheers

Erwan

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Are there any sail makers who can anonymously help with some insights or is it all too exciting and secret?

I guess there is another option that is is all old and boring and there is nothing much new or exciting!

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^ It's definitely new and exciting - at least applied on this scale. The rule is written to allow a head gantry plus other hardware extending down 4 meters from the masthead to not only control twist but also invert the head similar to the AC 50 wings. The only real constraint is that this system must be hoisted with the sail.

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Do we know how much internal structure there will be between the two skins?

Is hydraulics allowed to minutely control the vertical levels of the sail?

Is there more flexibility in these control elements?

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13 hours ago, Kiwing said:

Are there any sail makers who can anonymously help with some insights or is it all too exciting and secret?

I guess there is another option that is is all old and boring and there is nothing much new or exciting!

Kiwing,

Is it a question for me ?

Nothing is very secret.

Cheers

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@Erwankerauzen, no not for you exclusively, I was hoping a few sail makers might come to the party with some insights?

All my reading is theory from Phds etc. Nothing from sail makers except that group Glen is keen on from Western Australia.

Sounds like the setup can do most things the AC35 wings can but my impression is there is a huge variation of what people here think will result.  As I am only a reader and have no practical experience some sail makers (particularly those working on the project) might let out a few whispers?

Are they going to be a similar engine to the AC35 wings?

Less performance? more or roughly equal?

Also are we likely to see some trickle down?

I know it can all be speculation but we might get some hints at least?

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4 hours ago, Kiwing said:

Sounds like the setup can do most things the AC35 wings

Sorry to be blunt, but this is completely incorrect. There are some huge differences. The efficiency of a slotted, multi element rig is way above what can be achieved with the double skin. The AC50 wings could be adjusted in ways that are totally impossible with the double skin rig and which make the wing significantly better. The best way i have had it described is that if you take a scale of 1 to 10, with a conventional mast and sail being a 1 and the AC50 wing being a 10, then the double skin rig would come out at about 3.5 to 4.

A solid wing rig would make the boat faster. Anybody who tells you different is wrong. Even TNZ team members admit that, but make a strong case that the ultimate performance isn't the issue, not least because everybody has the same limitation, but that ease of handling both ashore and on the water are important and make the compromise worthwhile.

As for trickle down, remember this is actually trickle up technology. These sails have been around for a while, on a range of different boats. While i am sure that the rig will benefit from the development of AC teams, the question is whether we will see this type of rig become more popular. My view is that it will not. This rig is automatically very expensive compared with current rigs. You effectively have 2 mainsails, so double the price and the mast is bigger with 2 tracks, so is again more expensive. One estimate I saw suggests for a mast and main combo, the double skin rig costs about 170% of a conventional rig. It's hard to imagine many situations where this would be seen as worthwhile.

 

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@A Class Sailor,  I really respect your opinion, I truly do.

But Kite sailors could develop slotted wings too and they are at the moment with twin skins.  I realise it is very hard to compare.

Aeroplane wings are going toward non slotted bending surfaces, as well, for efficiency I think, save fuel?

So I know I am "throwing the ball out there" but that is what ETNZ does.  Remember the bikes ETNZ encouraged the idea they were too hard caused all sorts of problems etc, etc,. which is what they are doing now but....

I am most likely incredibly wrong but us ETNZ fan boys believe they can run on water with their knees tired together like they did in AC34.  Just let the cat out of the bag too early.

99% you are right and the bikes have been tried many times and don't work????

Just a dreamer - that's me.

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5 hours ago, A Class Sailor said:

Anybody who tells you different is wrong.

^^ visions of grandeur ?

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5 hours ago, A Class Sailor said:

The best way i have had it described is that if you take a scale of 1 to 10, with a conventional mast and sail being a 1 and the AC50 wing being a 10, then the double skin rig would come out at about 3.5 to 4.

I am not an expert in fluid mechanics and I don't think you and your friends are either.

If you use common sense and look out the window next time you are flying and have a look at the wing while cruising at 35,000 feet you will notice that there is no slot.  The only time you will see a slot is when you are landing at a lower speed when more lift is required and the extra drag does not matter.

There may well be some situations with the variable wind that a sail boat encounters where a slot might be beneficial but that will not be every situation.  For the most part they will be more concerned with the correct twist in the rig and maintaining laminar flow.

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9 hours ago, A Class Sailor said:

Sorry to be blunt, but this is completely incorrect. There are some huge differences. The efficiency of a slotted, multi element rig is way above what can be achieved with the double skin. The AC50 wings could be adjusted in ways that are totally impossible with the double skin rig and which make the wing significantly better. The best way i have had it described is that if you take a scale of 1 to 10, with a conventional mast and sail being a 1 and the AC50 wing being a 10, then the double skin rig would come out at about 3.5 to 4.

A solid wing rig would make the boat faster. Anybody who tells you different is wrong. Even TNZ team members admit that, but make a strong case that the ultimate performance isn't the issue, not least because everybody has the same limitation, but that ease of handling both ashore and on the water are important and make the compromise worthwhile.

As for trickle down, remember this is actually trickle up technology. These sails have been around for a while, on a range of different boats. While i am sure that the rig will benefit from the development of AC teams, the question is whether we will see this type of rig become more popular. My view is that it will not. This rig is automatically very expensive compared with current rigs. You effectively have 2 mainsails, so double the price and the mast is bigger with 2 tracks, so is again more expensive. One estimate I saw suggests for a mast and main combo, the double skin rig costs about 170% of a conventional rig. It's hard to imagine many situations where this would be seen as worthwhile.

 

Well said.

The interesting thing about the AWS wing, like the Omer wing, the one on the Swiss lake, the many windsurfer soft wings and all the other twin-skin sails dating back to the 1930s or so is that they all share the same remarkable characteristic - that is, they perform amazingly well when sailing alone and in press releases, but they seem to all have an amazing inability to actually turn up at a regatta and beat comparable boats with conventional rigs.

Decades of experience would indicate that efficiency is not the problem - after all, so many breathless press releases about the unbeatable superiority of such rigs cannot be wrong.  It appears that the only reason is that developing a soft wing or twin-skin sail somehow causes a bizarre allergy to the sound of the winner's hooter. Hopefully they will put up or shut the fuck up after this AC.

 

 

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3 hours ago, Terry Hollis said:

I am not an expert in fluid mechanics and I don't think you and your friends are either.

If you use common sense and look out the window next time you are flying and have a look at the wing while cruising at 35,000 feet you will notice that there is no slot.  The only time you will see a slot is when you are landing at a lower speed when more lift is required and the extra drag does not matter.

There may well be some situations with the variable wind that a sail boat encounters where a slot might be beneficial but that will not be every situation.  For the most part they will be more concerned with the correct twist in the rig and maintaining laminar flow.

You could also try reading what the AC wing and airline wing designer on SA says. He is very definitely an expert in fluid dynamics.

When I look out the window when flying I see a wing moving at steady speed and fairly similar angle of attack. I don't see a wing stalling before a start, accelerating at the gun, dealing with major airspeed fluctuations, and changing its angle of attack dramatically.

You could also ask yourself whether C Class and AC wing designers are so stupid as to go to all the hassle of fitting slots if they are not needed. Or you could consider that since you are not a fluid dynamics expert, you could try to learn from the people who are instead of assuming they are wrong.

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Who is the AC wing designer? and who is the aeroplane wing designer? so I can read what he says.

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Well, in this debate regarding the respective advantage of wings vs sails..., it is important to consider the sailing conditions.

2/3 elements wingsails can deliver a lift coefficient around 2.4 while a good teardropmast + battens sails is around 1.7 max. (2D figures)

But it is not necessary to have a big power in the rig  without the required righting moment.

The A-Cat serie will probably never go to a wing sail like the C-Cat, just because they don't need it.

The extra power from the wing would be useful only under 6 knts TWS.

One can also notice there is already a slot in this new AC rig:  Between the jib and the main sail.

I am not sure the wing sail was put at full use in the last cup, most of the time the angle between the main element and the flap was very tiny as the boat was sailing fast with 40 to 50 knts apparent wind.

Overall I think like Terry Hollis,  it is all about drag & lift distribution along the span to minimize induced drag in all conditions.

The twin skin is likely to minimize separation you would see on a classic teadrop mast rig, and the mecanisms we are likely to discover inside the top part of the sail will be here to achieve the appropriate lift distribution at the top (elliptical  or bell-shaped ).

Fair Winds for 2019

Erwan

 

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3 hours ago, Kiwing said:

Who is the AC wing designer? and who is the aeroplane wing designer? so I can read what he says.

They are the same person, but forum rules won't allow me to reveal who it is. There are other posts, reactions to the guru's posts, that may allow you to work it out. 

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@Xlot Thank you.  You wont hear me for a few days while I digest all that, and thank you @Curious.

Are their any sailmakers who can weigh in with some practical thoughts and experience?

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Wow there is a lot in your post @Erwankerauzen I really enjoy reading your interesting and informative contributions.

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Regarding airplane wings, most are not designed to fly inverted. 

A wing sail has to be inverted so it can sail on either tack.

 

wing.jpg.b81dceef166e3f0211164d17f6824475.jpg

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13 hours ago, Terry Hollis said:

If you use common sense and look out the window next time you are flying and have a look at the wing while cruising at 35,000 feet you will notice that there is no slot. 

This is the mistake so many make. When our sailboats are doing 500+ knots, then the comparison might be relevant. Curious makes a good point when he said

10 hours ago, Curious said:

You could also ask yourself whether C Class and AC wing designers are so stupid as to go to all the hassle of fitting slots if they are not needed.

It wasn't just the Ac wing designers. It was the rule makers as well.

 

 Why would they create a rule that made the wings so much more complex and expensive if they didn't have to.

9 hours ago, Erwankerauzen said:

The A-Cat serie will probably never go to a wing sail like the C-Cat, just because they don't need it.

We would build wing sails if we could find a way of getting them to survive big crashes/capsizes at speed of 25 knots +. It's pointless having a rig that would mean a capsize would finish your regatta. Ben Hall built a wing for his A about 8-9 years ago, before the days of foiling, and while he had limited success, he sold it to BMW Oracle who used it as a test bed for their wing on the DoG tri. They found a problem that once fixed, turned it into a serious weapon that was significantly faster than a standard A. That problem was that the wing wasn't stiff enough, particularly the second element and the slot closed under load.

11 hours ago, Curious said:

The interesting thing about the AWS wing, like the Omer wing, the one on the Swiss lake, the many windsurfer soft wings and all the other twin-skin sails dating back to the 1930s or so is that they all share the same remarkable characteristic - that is, they perform amazingly well when sailing alone and in press releases, but they seem to all have an amazing inability to actually turn up at a regatta and beat comparable boats with conventional rigs.

Decades of experience would indicate that efficiency is not the problem - after all, so many breathless press releases about the unbeatable superiority of such rigs cannot be wrong.  It appears that the only reason is that developing a soft wing or twin-skin sail somehow causes a bizarre allergy to the sound of the winner's hooter. Hopefully they will put up or shut the fuck up after this AC.

Based on results to date in classes i know, I should totally agree with you on this. Double skin rigs built for Moths and A Class have failed to prove they are quicker around a course. I think there are a number of reasons for this, not least because the standard rigs of those boats are very highly developed and double skin sails haven't had the same level of development. For this reason, i am actually optimistic that the AC boys will end up with a double skin set up which out performs the best single skin rigs, but there is no way that they will come anywhere near close to a proper slotted wing sail, something the TNZ guys readily admit.

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Semi off topic but wasn't one reason for soft sails for AC36 so they did not have to crane wings on and off every day? 

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9 hours ago, Erwankerauzen said:

< snip >

The twin skin is likely to minimize separation you would see on a classic teadrop mast rig, and the mecanisms we are likely to discover inside the top part of the sail will be here to achieve the appropriate lift distribution at the top (elliptical  or bell-shaped ).

<snip>

 

This (bolded) is the key component that requires double surface skins.The permitted gantry (twist control) mechanism and proactive control of the spanwise lift distribution allows the head portion of the sail to be flown at very low (and even negative) AoAs. So the twin skin not only 'hides' the mechanism (reducing parasitic drag), but is actually critical to having this mechanism function properly since both cases require  relatively symmetric sections to be aerodynamically efficient.

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2 hours ago, Nigel Texas said:

Regarding airplane wings, most are not designed to fly inverted. 

A wing sail has to be inverted so it can sail on either tack.

 

wing.jpg.b81dceef166e3f0211164d17f6824475.jpg

Well above my pay grade. No doubt I'm projecting my own misunderstandings but, too, I think there may be some confusion on differences between wings on boats and airplanes. So, FWIW, here are some of my thoughts on the very basics:

A symmetric wing sail can be set to work on either tack.

A hinged wing sail need not have any slots. eg.

image272_0.jpg

Slots, slats, flaps and other high lift devices make sense when you need more force. Sailboats that have an optimal upwind rig may be under-powered when attempting to get max downwind VMG. Drag in the downwind condition is an asset for slow boats and even for very fast boats the cost is discounted relative to upwind. Downwind a boat may be able to balance a greater force from the sails. Downwind there is less wind and hence less sail force to work with. The ratio of lift to drag is not a universal goodness score.  This is not a battle between Good and Evil. Lift and drag are decompositions that help with the math. I think easier to visualize the problem in terms of total forces. Typically the problem is to maximize velocity made good or minimize time around the course. These problems can not be optimized for all points of sail and wind speeds with a fixed sail geometry.  Kinda like with airplane wings, it's useful to have a relatively clean mode for high apparent winds when excess force is available and a high force mode for low apparent winds. 

Apologies if that's all stooopidly obvious or incorrect or both. 

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How great!  thank you all for adding your ideas.

Had not thought about the inversion aspect which might give twin skins some advantages???

I have many hours of consideration and investigation here!!  Thank you so much.

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6 hours ago, A Class Sailor said:

This is the mistake so many make. When our sailboats are doing 500+ knots, then the comparison might be relevant.

It must of escaped your attention then that all aircraft from 50 knot microlights to supersonic fighters use the same concept .. no slot unless landing.

The main feature of the AC75 is the fact that it uses a rotating mast which allows the inner and outer skins to be tensioned differently so that they can produce an asymmetrical shape just like an aircraft wing, you would be struggling to emulate that feature in a practical manner on a Moth.

As has been pointed out the crew will be occupied with de-powering the rig most of the time rather than trying to extract more power so I stand by what I said .. they need good controls to provide twist and shape to allow laminar flow so that drag is minimised in the high apparent winds that are expected. 

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4 hours ago, weightless said:

...The ratio of lift to drag is not a universal goodness score.  This is not a battle between Good and Evil. Lift and drag are decompositions that help with the math. I think easier to visualize the problem in terms of total forces. ...

Actually, there is a fundamental relationship between lift/drag ratio and sailing performance.  The basic sailing performance equation is:
Vb = boat speed
Vt = true wind speed
gamma = course to true wind (0 = straight upwind)
beta = apparent wind angle, measured between apparent wind and course through the water
Vb = Vt*sin(gamma - beta)/sin(beta)
Vmg = Vt*sin(gamma - beta)*cos(gamma)/sin(beta)

This comes directly from applying the law of sines to the wind triangle, and is exact for all sailing craft when the terms are defined as above.

It turns out that the apparent wind angle, beta, is the sum of the aerodynamic and hydrodynamic drag angles:
lift = force component at right angles to the apparent wind or course through the water
drag = force component parallel to the apparent wind or course through the water
ea = arctan(aero_drag/aero_lift)
eh = arctan(hydro_drag/hydro_lift)
beta = ea + eh

At any point of sail (gamma), you want to minimize the apparent wind angle (beta), and this leads to maximizing the aero and hydro lift/drag ratios.  Maximizing the force comes into it when it improves the hydro lift/drag ratio.  For example, a boat that sails dead downwind has a hydrodynamic lift/drag ratio of zero.  So heading up to get more side force improves the hydro lift/drag ratio and speeds the boat up.  Likewise, operating the rig at angles of attack past maximum aero lift/drag ratio can improve performance when the additional hydrodynamic lift results in a better hydro lift/drag ratio.  

Very high performance craft (such as landyachts, iceboats, and foilers) approach the ideal of the "constant beta boat", in which the apparent wind angle is constant for all points of sail, and the lift/drag ratios are also constant.  If you assume a constant apparent wind angle, you'll find best Vmg occurs near 45 deg and 135 deg to the true wind, and maximum speed near a beam reach.  So there are good reasons why we tend to sail at approximately those angles.  And it's all about lift/drag ratios.

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@Basiliscus First thank you for this thought provoking clear sighted concept.

If the lift-drag ratio for a slotted wing similar to the f50 cats was 100 and the l-d ratio for the best "normal" sail boat jib and main were say 10.

What range would you speculate the new twin skin setup of the proposed AC75 will be ?

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24 minutes ago, Basiliscus said:

  Likewise, operating the rig at angles of attack past maximum aero lift/drag ratio can improve performance when the additional hydrodynamic lift results in a better hydro lift/drag ratio.  

Thank you for all of that. I very much appreciate the professional analysis.  Your formulation is elegant. I have seen the problem laid out as you do by Marchaj. Obviously, I can't argue. However, as I was telling the story from the point of view of the sails only I think it is fair to say that optimizing lift to drag of the sails alone is not the name of the game.

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8 hours ago, NeedAClew said:

Semi off topic but wasn't one reason for soft sails for AC36 so they did not have to crane wings on and off every day? 

I'd say this is right on topic.

Yes, this is a powerful reason to explore an old concept, but  ........

*  Easier handling and transportation

* No need for tower cranes

* Simplified

construction

* Easier repair

* More durability

 ..... are all attractive features.

Of course, ya gotta make twin sails develop enough power for foiling!

 

 

 

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4 hours ago, Terry Hollis said:

you would be struggling to emulate that feature in a practical manner on a Moth.

Why? It's been done. It's been done on an A Class and it has been done on small yachts. Where do you think the idea came from? Glenn Ashby saw it when he went "walk about" in West Australia after the last cup.

 

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11 minutes ago, A Class Sailor said:

Why? It's been done. It's been done on an A Class and it has been done on small yachts. Where do you think the idea came from? Glenn Ashby saw it when he went "walk about" in West Australia after the last cup.

 

I did not say it could not be done, I said it would not be practical on a Moth.

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2 hours ago, weightless said:

...However, as I was telling the story from the point of view of the sails only I think it is fair to say that optimizing lift to drag of the sails alone is not the name of the game.

I discovered the same thing when I was playing with a simple landyacht VPP.  I had arranged it so the angle of attack for maximum aerodynamic L/D was below maximum lift.  But I discovered the best performance was with an angle of attack higher than best aero L/D.  It took me a while to realize that because the side force on the chassis was dependent on the side force applied from the rig, that the L/D of the rig and the L/D of the chassis were not independent.  By loading up the chassis more, it improved the L/D of the chassis and gained performance even at the expense of the aerodynamic L/D.  

So while you're right that you can't purely optimize the L/D of the rig independently from the boat, it's still the place to start.  Drag reduction always yields a performance gain.  Increasing the lift may improve performance, but not necessarily.  It depends on how much the drag increases, both aerodynamically and hydrodynamically, as the lift is increased.  OTUSA sailed their AC72 with a daggerboard lifting pole that acted like a slat, which allowed the windward daggerboard shaft to have attached flow.  The aerodynamic thrust from the daggerboard offset most of the drag from the entire windward hull.  However, the increased side force increased the drag on the leeward daggerboard, and the slat-shaped lifting pole on the leeward hull had more windage than a round pole.  The drag from daggerboard and leeward pole negated the gains from the windward pole and daggerboard, and the aerodynamic lifting pole was scrapped.  When adding lift, you really can't say much about the effect on performance unless you have a VPP that considers all the interactions.

Dave Hubbard did an interesting analysis when Oracle Racing was sizing the soft sail rig for the trimaran, USA 17.  There were no limits on the rig design, so it all came down to the fundamentals.  Hubbard showed that there was an optimum rig size for each wind strength.  If the rig were too high or the sail area too large, the lift had to be reduced to stay within the righting moment available, and the parasite drag was higher than need be.  If the rig were too short, then the induced drag was increased and performance suffered.  So there was an optimum area and height for each wind strength.  It turned out that the best aspect ratio was remarkably similar across the wind range.  It looked like the entire rig was just scaled up and down for different wind strengths.  Just what the optimum aspect ratio was depended on the righting moment, windage, and the resistance of the hull.

If the double-luff sail is to have an aerodynamic advantage over a rigid wingsail, I believe it's going to be the ability to vary the size according to the wind strength.  It's not going to have an edge on maximum lift or control of camber and twist.  Of course, the rationale for the double-luff sail may not be aerodynamic at all.  It could be all about logistics.  Or fashion.

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21 minutes ago, Basiliscus said:

Drag reduction always yields a performance gain. *

Thanks for that. I really hope you're working on a book. Seriously.

* It would be sad if it were to apply in this case but In the wider context of sailing the apparent wind isn't always ahead of the beam.

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Nice readings , thanks to everybody for feeding this interesting discussion.

AFAIR, there is an old book

Title: High Speed Sailing

Author:Norwood

Which provides the vectors graphs and equations to understand the fundementals of high speed sailing.

Unfortunatly, I  have only photocopies and cannot find them in my library.

Cheers

Erwan 

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I should have add:

AFAIR In the history of the Little America Cup (LAC) and the  C-Cat serie:

 it seems that the huge load of the leech on the rear beam of the catamaran structure was a significant structural challenge.

The first wing concept (probably  developped by Dave Hubbard) addressed this issue.

Cheers

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59 minutes ago, Erwankerauzen said:

The first wing concept (probably  developped by Dave Hubbard) addressed this issue.

 

Miss Nylex (and some odd contraptions) pre-dates the Patient Lady series - which had the first competitive wings

 

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10 hours ago, Basiliscus said:

I discovered the same thing when I was playing with a simple landyacht VPP.  I had arranged it so the angle of attack for maximum aerodynamic L/D was below maximum lift.  But I discovered the best performance was with an angle of attack higher than best aero L/D.  It took me a while to realize that because the side force on the chassis was dependent on the side force applied from the rig, that the L/D of the rig and the L/D of the chassis were not independent.  By loading up the chassis more, it improved the L/D of the chassis and gained performance even at the expense of the aerodynamic L/D.  

So while you're right that you can't purely optimize the L/D of the rig independently from the boat, it's still the place to start.  Drag reduction always yields a performance gain.  Increasing the lift may improve performance, but not necessarily.  It depends on how much the drag increases, both aerodynamically and hydrodynamically, as the lift is increased.  OTUSA sailed their AC72 with a daggerboard lifting pole that acted like a slat, which allowed the windward daggerboard shaft to have attached flow.  The aerodynamic thrust from the daggerboard offset most of the drag from the entire windward hull.  However, the increased side force increased the drag on the leeward daggerboard, and the slat-shaped lifting pole on the leeward hull had more windage than a round pole.  The drag from daggerboard and leeward pole negated the gains from the windward pole and daggerboard, and the aerodynamic lifting pole was scrapped.  When adding lift, you really can't say much about the effect on performance unless you have a VPP that considers all the interactions.

Dave Hubbard did an interesting analysis when Oracle Racing was sizing the soft sail rig for the trimaran, USA 17.  There were no limits on the rig design, so it all came down to the fundamentals.  Hubbard showed that there was an optimum rig size for each wind strength.  If the rig were too high or the sail area too large, the lift had to be reduced to stay within the righting moment available, and the parasite drag was higher than need be.  If the rig were too short, then the induced drag was increased and performance suffered.  So there was an optimum area and height for each wind strength.  It turned out that the best aspect ratio was remarkably similar across the wind range.  It looked like the entire rig was just scaled up and down for different wind strengths.  Just what the optimum aspect ratio was depended on the righting moment, windage, and the resistance of the hull.

If the double-luff sail is to have an aerodynamic advantage over a rigid wingsail, I believe it's going to be the ability to vary the size according to the wind strength.  It's not going to have an edge on maximum lift or control of camber and twist.  Of course, the rationale for the double-luff sail may not be aerodynamic at all.  It could be all about logistics.  Or fashion.

I used that argument (~ :)) for a Gunter rig on the L 7- rotating wing stump, but round top mast, no sleeve, although a sleeve there wouldn’t be too big a pain in the ass, but heavier, so full battens would’ve been on the chopping block.  Easier to me$$ with gust response too....

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6 hours ago, Erwankerauzen said:

Nice readings , thanks to everybody for feeding this interesting discussion.

AFAIR, there is an old book

Title: High Speed Sailing

Author:Norwood

Which provides the vectors graphs and equations to understand the fundementals of high speed sailing.

Unfortunatly, I  have only photocopies and cannot find them in my library.

Cheers

Erwan 

I lost mine.  Maybe it was too small a book?

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Is it this one?

https://www.amazon.com/High-Speed-Sailing-Design-Factors/dp/0229115950

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I think so, as there is no much book written by Norwood

but I have not the photocopy of the cover, so I cannot recognize it

It is cheap, I will buy to put between Marcaj and F Bethwaithe

Cheers

Erwan

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On 1/6/2019 at 5:31 PM, Basiliscus said:

If the double-luff sail is to have an aerodynamic advantage over a rigid wingsail, I believe it's going to be the ability to vary the size according to the wind strength.  It's not going to have an edge on maximum lift or control of camber and twist.  

The size variation allowed in the rules makes it hard for me to believe there will be much benefit form playing with area. The difference between biggest and smallest mainsail is 6.8% and that is with the mast height staying the same. It seems you could only reef 1 metre but with the D section of the mast, any reefing would result in huge drag. Because the main is measured on 1/4 height girths, you might get a gain due to aspect ratio changes but I find it hard to believe that the effectiveness of a wing in "blading out" or even getting reverse camber to work, or the in inherent advantage of lift/drag and control of camber and twist  is going to be beaten by being able to make a soft sail slightly larger or smaller with a better aspect ratio soft sail. 

On 1/6/2019 at 5:31 PM, Basiliscus said:

Of course, the rationale for the double-luff sail may not be aerodynamic at all.  It could be all about logistics. 

I think this is where you are getting closer. Logistics is certainly something ETNZ have stated was a reason. This is what they said

Quote

We want something where teams can take the mainsail down and leave the rig in at the dock as well as potentially make mainsail changes on the water, but have something that aerodynamically is superior to a conventional mainsail without being heavier

ETNZ hjave never made any claims about the soft wing being more efficient or "better" than a solid wing. Their comments indicate they are trying to improve on a standard rig, not a solid wing. It seems that it is only people on here who try to make out that the soft wing might or will be better than a solid wing like we saw on the 72's and 50's. The only thing I see as questionable in what ETNZ have said is the question of weight. Assuming they can make the mast the same weight as a conventional mast, which i think is suspect because of the size, they need to make each of the 2 mainsail skins half the weight of a conventional main, despite it having 2 sets of battens and the internal control structures allowed. i think it is a big ask to make they proposed rig as light as a conventional sail.

The biggest benefit I see with the soft wing over the solid wing is the ability not to trash it is there is a big wipe out and I think there will be wipe outs.

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1 minute ago, A Class Sailor said:
On 1/5/2019 at 8:31 PM, Basiliscus said:

Of course, the rationale for the double-luff sail may not be aerodynamic at all.  It could be all about logistics. 

I think this is where you are getting closer. Logistics is certainly something ETNZ have stated was a reason. This is what they said

I wonder how much durability and repair costs play into this. I suppose the differences in those things depend on how fancy they internal bits of the new rigs end up.  I haven't been following closely. Are the rig details available yet?

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54 minutes ago, Boink said:

http://gre69.wixsite.com/advancedwingsystems/untitled-c13zx

AWS are US Magic partner - and West Australian based (re: Ashby post AC Desert Travels)  - are you able to connect the dots?

So are you saying that American Magic has a partnership with AWS and thus an inside track on the technology?  The gap in the AWS sails reminds me of the discussions we had in the NYYC thread about the dual sail and where you could see the second sail through the window on the Mule.

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6 hours ago, A Class Sailor said:

The size variation allowed in the rules makes it hard for me to believe there will be much benefit form playing with area. The difference between biggest and smallest mainsail is 6.8% and that is with the mast height staying the same. It seems you could only reef 1 metre but with the D section of the mast, any reefing would result in huge drag. Because the main is measured on 1/4 height girths, you might get a gain due to aspect ratio changes but I find it hard to believe that the effectiveness of a wing in "blading out" or even getting reverse camber to work, or the in inherent advantage of lift/drag and control of camber and twist  is going to be beaten by being able to make a soft sail slightly larger or smaller with a better aspect ratio soft sail. 

I think this is where you are getting closer. Logistics is certainly something ETNZ have stated was a reason. This is what they said

ETNZ hjave never made any claims about the soft wing being more efficient or "better" than a solid wing. Their comments indicate they are trying to improve on a standard rig, not a solid wing. It seems that it is only people on here who try to make out that the soft wing might or will be better than a solid wing like we saw on the 72's and 50's. The only thing I see as questionable in what ETNZ have said is the question of weight. Assuming they can make the mast the same weight as a conventional mast, which i think is suspect because of the size, they need to make each of the 2 mainsail skins half the weight of a conventional main, despite it having 2 sets of battens and the internal control structures allowed. i think it is a big ask to make they proposed rig as light as a conventional sail.

The biggest benefit I see with the soft wing over the solid wing is the ability not to trash it is there is a big wipe out and I think there will be wipe outs.

3 topics are touched on here:

1. They will never sail with a reef. The added drag from the unfaired D section mast would outweigh any gain from running a smaller area. I believe that all teams will be chasing hard to have very precise control over the two skins, possibly leading to the ability to turn off the camber and drag to neutral or even attempt to reverse it as was hinted at in the last cycle. This would be dependent upon the properties of 3Di type membranes to accomodate such manipulation without going slack between the extreme load cases - and causing additional skin/friction drag. The development of these 3Di type of membranes will happily allow the teams to hide the batten control systems that they will surely develop if these aims are to be achieved. Unfortunately for us and this forum, it will not be revealed for a very long time - look how little has been revealed about the wing control systems from the last cycle, and back there they could run clear film membrane - but often chose not to for both secrecy and placement of sponsor logo reasons (conveniently). I would suggest that hints of what are going on internally are a combiation of Bermuda wing control and something like what Softwing.ch has shown in their material. Lots of batten control from both cable and soild structures.

I cannot even recall what the wind limits are for the next cycle, to call for a reef. 

2. Logistics was a genuine reason why this seed developed into what we see today - and I believe that was both altruistic and also because of the desire to run much larger boats in this cycle. The Kiwis are still a remarkably pragmatic bunch.

3. Weight minimisation will still be a major goal - not because the teams will run windward heel (veal heel) - both increasing RM and unloading the demands upon the foil, but also because of the gains in acceleration and earlier take off. It will also help when things get a bit hectic (dymanic) in waves and gusts - which are much more likely at this venue. Nor do I believe that they will ever be as light as a teardrop mast and one skin sail. The efficiency gains are also not ever going to be revealed in their fullest sense - but I think this is the real fundamental cause of why the rule makes wrote what they did. Call it trickledown or any other name - you have a team with top shelf competition winning solutions from Bermuda - based in a country that is already world leading in marine industry capablities - why not germinate new fertile areas for development? The opportunity to showcase technology that more directly attributual to everyday sailing was always high on Prada's wishlist. If a sailmaker can sell in future 2 mainsails and associated hardware - rather than one? if it is associated with the highest levels of performance then it will be utilised eleswhere. You might see more line control systems and hydraulic winches on fast cruising boats - it breaks down the age long held belief that you need to grind everything into place. 

Performance is one thing - and marketing completely the other - look how the profileration of spoilers all over vehicles in motorsport has led to styling cues for everyday road cars now also covered in crappy plastic wannabes - and consider the outcomes - Increased fuel burn from both the additional weight and drag and a negligable improvement in vehicle handling in the vast cases of use.

I was recently reminded of the connection between efficiency gains and actual performance. Hollom wrote a fantastic article about development of new Moth Foils for the Thinnair moth. It was a comprehensive look at seeking gains wherever they could be found, he discussed, among other things, the foil solution that ETNZ utilised and what it led to and then talked about moth foils with flaps, and rudder foils with flat vs. anhedral profile -  the end result has a section design that is 20% lower viscous drag than the established class leading foil.

The gains are there to be had - the gains that they see........

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.4 knot speed gain........ So don't fluff your start, any tack, gybe or make rounding.......

A great example of how AC teams need to manage the conundrum: how explore all avenues of development - but for the sake of preserving both time & budget, to explore other potential fields of discovery - stop at chasing perfection when excellence will do.

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1 hour ago, Herfy said:

So are you saying that American Magic has a partnership with AWS and thus an inside track on the technology?  The gap in the AWS sails reminds me of the discussions we had in the NYYC thread about the dual sail and where you could see the second sail through the window on the Mule.

They are listed as an official supplier to the team.

See screenshot from their website.

AMAC - AWS Supplier.jpg

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32 minutes ago, Boink said:

The development of these 3Di type of membranes will happily allow the teams to hide the batten control systems that they will surely develop if these aims are to be achieved. Unfortunately for us and this forum, it will not be revealed for a very long time - look how little has been revealed about the wing control systems from the last cycle, and back there they could run clear film membrane - but often chose not to for both secrecy and placement of sponsor logo reasons (conveniently). I would suggest that hints of what are going on internally are a combiation of Bermuda wing control and something like what Softwing.ch has shown in their material. Lots of batten control from both cable and soild structures.

This isn't allowed in the rules. Control systems can only work in the top 4 metres and bottom 1.5 metres of the sail. There are other rules controlling what you can't do with battens which basically mean you only have control over 5.5 out of 26.5 metres of the sail.

 

43 minutes ago, Boink said:

Weight minimisation will still be a major goal - not because the teams will run windward heel (veal heel) - both increasing RM and unloading the demands upon the foil, but also because of the gains in acceleration and earlier take off.

The weight of the b oats is very closely controlled with only 30kgs between minimum and maximum 7570-7600kgs. 

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37 minutes ago, A Class Sailor said:

This isn't allowed in the rules. Control systems can only work in the top 4 metres and bottom 1.5 metres of the sail. There are other rules controlling what you can't do with battens which basically mean you only have control over 5.5 out of 26.5 metres of the sail.

 

The weight of the b oats is very closely controlled with only 30kgs between minimum and maximum 7570-7600kgs. 

So are you suggesting that this control of battens (and by implication - sail shape) is not important or worth chasing?

Furthermore - are you suggesting that by making the rig as light as possible - thereby lowering VCG - is not beneficial? 

Any weight saved aloft can be put back into structure low down, possibly adding global rigidity and or improving stability.

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18 minutes ago, Boink said:

So are you suggesting that this control of battens (and by implication - sail shape) is not important or worth chasing?

Where did you get that from? I quoted the rule. You said that there would be systems to control the battens (Lots of batten control from both cable and solid structures). I was pointing out the limitations under the rules, which means that for most of the sail, you cannot control the battens.

23 minutes ago, Boink said:

Furthermore - are you suggesting that by making the rig as light as possible - thereby lowering VCG - is not beneficial? 

Any weight saved aloft can be put back into structure low down, possibly adding global rigidity and or improving stability.

I am suggesting that as there are strict limits on weight, both minimum and maximum, there isn't exactly any room to play with this.

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How did the AWS Moth wing go in competition?

Can anyone find results for the AWS sportsboat? If they didn't join in with the sportsboat class on the river, why not?

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Yep, but how long do they need? Their "revolutionary" rig has been around for years now. The site for the AWS 8 metre sportsboat is dead. The designer's site includes pics of one boat but that seems to have been in 2016.The same year they said the boat would be doing sportsboat races on the Swan. It's been entered in the RFBYC sportsboat class for two years but doesn't seem to have started once.

I can find only one result, a midweek race where the 8m wingsail boat finished almost five minutes behind a Thompson 870, a few seconds ahead of a Melges 24, and about 3 1/2 minutes ahead of a good BW8. Not terrible, but not revolutionary. So once again we see big claims about double skin sails, but no big results.

 

 

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2 hours ago, A Class Sailor said:

This isn't allowed in the rules. Control systems can only work in the top 4 metres and bottom 1.5 metres of the sail. There are other rules controlling what you can't do with battens which basically mean you only have control over 5.5 out of 26.5 metres of the sail.

 

I both like and dislike this rule. While it prevents to be the focused area of the boat and to make huge gap between boats (compared to a situation where every team could develop their twin skin unconstrained), it also prevents to have different ideas/concepts and the best possible progress.

The boats which run twin skins setup until now are relatively passive with not much adjustment of sail on the water. Real advantage over soft sail and to catch wing performance would be if they have more control over the "sail profile", being able to adjust, profile thickness, camber, twist, etc. (and knowing when and how to adjust each) on the water. The America Cup would be the perfect place for such development.

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2 hours ago, A Class Sailor said:

The weight of the boats is very closely controlled with only 30kgs between minimum and maximum 7570-7600kgs. 

That's consumed by the 30kg range for crew (1120—1150kg), the only unrestricted items are mast and mainsail. Everything else must weigh what it the rule says it should within ±1% (§3.11), with ballast added for anything that's under weight. That allows approximately ±76kg for the whole boat harvested from individual components, including supplied equipment (which likely will be as close to the specified weight as practical). Supplied equipment is 15% of the mass, so that leaves about 65kg (or about 0.85% of the mass without mast and mainsail) to play with.

The centre of mass of various components and location of ballast is also specified, restricting how excess weight can be distributed.

But it's open slather on the mass of the mast and mainsail and its distribution—within the fairly extensive design parameters of rule 20..

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3 hours ago, RobG said:

But it's open slather on the mass of the mast

I don't believe that it is. The rule effectively controls minimum weight through a minimum required laminate and one design mast fittings. Unless the rule writers have come up with a mast spec that is not strong enough, I expect all the competitive boats to have very similar mast weights.

I can see that there might be some minor differences in mainsail weight but I have seen nothing to suggest that this could be a source of real competitive advantage.

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You may have quoted me before I added "…within the fairly extensive design parameters of rule 20." where "extensive" should have been "restrictive". :-)

A bit tongue–in–cheek, you're absolutely correct that the mast design is very tightly specified as to be almost one design. But with tolerances expressed in mm and nothing in kg, and express permission to add material within the allowed dimensions, there's more wriggle–room that with any other aspect of the design in regard to weight at least. Though adding unnecessary weight to the rig likely has no performance benefit.

The biggest weight saving aloft will be to have a single skin mainsail, which is allowed but would look a bit weird as it must be attached to one side of the mast or the other, it's not allowed to be in the middle of the rear face. I think that's a pity, as it would have been good to see a wing mast + soft sail vs double skin mainsail.

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Being an absolute fanboy, I "believe" Etnz will be able to invert/reverse the top of the soft wing and move the centre of effort considerably.

Reading here there seems to be a variation in "our"opinions as to whether this will be possible, and if so how many teams will be able to do it.

By "invert/reverse" I mean get some lift at the top of the sail in the opposite direct to the main lift of the sail hence helping with depowering the soft sail. Helping with balance in critical manoeuvres by quickly changing RM as well.

Anyone going to express an opinion on that subject?

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18 hours ago, Curious said:

Yep, but how long do they need? Their "revolutionary" rig has been around for years now. The site for the AWS 8 metre sportsboat is dead. The designer's site includes pics of one boat but that seems to have been in 2016.The same year they said the boat would be doing sportsboat races on the Swan. It's been entered in the RFBYC sportsboat class for two years but doesn't seem to have started once.

I can find only one result, a midweek race where the 8m wingsail boat finished almost five minutes behind a Thompson 870, a few seconds ahead of a Melges 24, and about 3 1/2 minutes ahead of a good BW8. Not terrible, but not revolutionary. So once again we see big claims about double skin sails, but no big results.

I missed this. I have no details of any race results but i do know that they built 2 rigs for A Class foilers. The report was that on downwind it was quick but upwind it was very poor (or it could have been the other way around!). It was meant to appear at the last worlds but it wasn't quick enough so the owner stuck with a standard rig.

None of this makes me think that the double skin rig is a bust. I am convinced it will be quicker than a standard rig. The biggest issue to date has been not enough development time. Take the A Class and Moth rigs. They have basically been the first iteration and it is hardly surprising that they cannot out perform what are highly developed and very specialist "standard" rigs. With the full resources of AC teams being used, i am sure we will see benefits, particularly in keeping flow over the rig in tacks and gybes. I still believe that it will get nowhere near the performance of the solid wing sails we saw in the last 2 cups, but so what. This wasn't just a performance decision. There are other factors as well and therefore the rig must not be only judged on performance.

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36 minutes ago, Kiwing said:

Being an absolute fanboy, I "believe" Etnz will be able to invert/reverse the top of the soft wing and move the centre of effort considerably.

Reading here there seems to be a variation in "our"opinions as to whether this will be possible, and if so how many teams will be able to do it.

By "invert/reverse" I mean get some lift at the top of the sail in the opposite direct to the main lift of the sail hence helping with depowering the soft sail. Helping with balance in critical manoeuvres by quickly changing RM as well.

Anyone going to express an opinion on that subject?

Seeing as how a soft wing is soft and flexible, I don't see how one part can generate a force in one direction while another part generates the opposite force. 

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20 hours ago, A Class Sailor said:

Where did you get that from? I quoted the rule. You said that there would be systems to control the battens (Lots of batten control from both cable and solid structures). I was pointing out the limitations under the rules, which means that for most of the sail, you cannot control the battens.

Yes, there are limitations - but the fact that that they can induce batten shape at the both the head and foot is still an avenue that they will chase. The beauty of TPT is building a membrane that translates whatever properties and stresses they choose to propagate over a far greater area than the batten location represents.

Take for example the effect of the humble cunningham - a control applied at one point over a large area. By applying your reasoning;  this control could be done away as it is too localised.

Me, I'll keep it. And I'll take all the batten control that is permitted too..... 

20 hours ago, A Class Sailor said:

I am suggesting that as there are strict limits on weight, both minimum and maximum, there isn't exactly any room to play with this.

Not in the rig though, as also supported by Rob G. They have to base their mast upon a given Mandrel as dimensioned in the rules and add structure as they desire. This was also written about by Eric Hall.

So lots of opportunity for diversity here.

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1 hour ago, Kiwing said:

Being an absolute fanboy, I "believe" Etnz will be able to invert/reverse the top of the soft wing and move the centre of effort considerably.

Reading here there seems to be a variation in "our"opinions as to whether this will be possible, and if so how many teams will be able to do it.

By "invert/reverse" I mean get some lift at the top of the sail in the opposite direct to the main lift of the sail hence helping with depowering the soft sail. Helping with balance in critical manoeuvres by quickly changing RM as well.

Anyone going to express an opinion on that subject?

By hanging that big gantry at the top of the mast they made it plain it was going to be used to control the twist in the twin skin sail.

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34 minutes ago, dogwatch said:

Seeing as how a soft wing is soft and flexible, I don't see how one part can generate a force in one direction while another part generates the opposite force. 

The gantry on top allows the twist and there is nothing to stop them have an internal structure to control the shape at the top.

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