Kiwing

The new sailing twin skin setup

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12 hours ago, Erwankerauzen said:

Regarding the above-mentionned wing cross sections,

It should be noticed the importance of Reynolds numbers

To make it simple Rn=70x(chord lenght in mm)x(Air Velocity in M/s)

So I would not be surprised the Reynolds number for the twin skin sail would be much higher than for  the glider despite the higher velocity, because the maisail chord is aroung 25  feet (8 meters) while the glider chord is barely 3 feet (1 meter). (Very different Reynolds could mean very different Boundary Layer)

Cheers.

I don't know the Reynolds numbers but I know that the Starfighter which had a very thin, and proportionally wide cord, wing was considered to be have great performances while being the most dangerous fighter.

Lockheed-XF-104-Starfighter-53-7787.jpg

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3 hours ago, Tornado-Cat said:

I don't know the Reynolds numbers but I know that the Starfighter which had a very thin, and proportionally wide cord, wing was considered to be have great performances while being the most dangerous fighter.

Lockheed-XF-104-Starfighter-53-7787.jpg

Widowmaker it came to be known as...

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42 minutes ago, Rasputin22 said:

Widowmaker it came to be known as...

The era of the manned missiles......quick in a straight line, not so good in the bends. The Phantom was a step in the right direction, but it wasn't till the Eagle that they really got their shit together.

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4 hours ago, Tornado-Cat said:

I don't know the Reynolds numbers but I know that the Starfighter which had a very thin, and proportionally wide cord, wing was considered to be have great performances while being the most dangerous fighter.

Lockheed-XF-104-Starfighter-53-7787.jpg

I don't know why people are so preoccupied with aircraft wings.  They are simply irrelevant in the context of high speed foiling vessels which are totally dependant on their righting moment to support the wing/foil.

Once an AC75 gets on it's foils the winning rig will be the one that enables the crew to control the excess power with the minimum drag.  The means they will be able to twist the wing so that the top is nearly no drag or possibly negative power to assist the righting moment and the shape will be different all the way down to the boom. 

There is nothing in the aircraft world that compares with this complexity.

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Maybe not in the aircraft world but there is a natural aeronautical design that has a ballast pod opposing a foiled wing that is much like the righting moment stability that you mention Terry. It is the sycamore seed. A simple but elegant solution in Mother Natures long list of engineering achievements.

 

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On 1/21/2019 at 7:00 PM, Terry Hollis said:

Gliders dot not have to contend with the limitations imposed by the righting moment.

Or windshear.

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13 hours ago, Terry Hollis said:

I don't know why people are so preoccupied with aircraft wings.  They are simply irrelevant in the context of high speed foiling vessels which are totally dependant on their righting moment to support the wing/foil.

Once an AC75 gets on it's foils the winning rig will be the one that enables the crew to control the excess power with the minimum drag.  The means they will be able to twist the wing so that the top is nearly no drag or possibly negative power to assist the righting moment and the shape will be different all the way down to the boom. 

There is nothing in the aircraft world that compares with this complexity.

Different constraints but same objective of minimum drag.

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13 hours ago, Terry Hollis said:

There is nothing in the aircraft world that compares with this complexity.

I would submit that a 737 wing that lands and takes off at 150kt, cruises at 500kts, flies in air of varying densities, optimized for $$$fuel efficiency is  pretty F'ing complex and has had heaps of dev thrown at; to use an example I regularly sit inside.

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2 hours ago, Ex-yachtie said:

Or windshear.

Sailplanes (gliders) are very susceptible to wind shear. One of the most contributing factors to fatal crashes is during the turn onto final when windshear can stall the inner wing in a turn and then you may as well be riding a sycamore seed!

https://youtu.be/zfFGN-3Yglo

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The allegation of 'widow maker' as this term was used in discussions of repeated aircraft failures with loss of the pilot's life are not accounted for fully by talk of the aeronautics of Lockheed's F104 Starfighter.  The planes designers were aware of the aircraft's demand for high takeoff and landing speeds.  They did not fully understand the ease with which elevating the nose of the craft risked creating turbulence across the rear horizontal stabilizer.  Turbulence compromised the effectiveness of the rear stabilizers and pilots' efforts to correct the situation by pushing the nose down met with varying and largely unpredictable results.  The effort was begun by altering the position of the rear stabilizer.

German pilots were first to call the plane 'widow maker.'   Aeronautics played a part in earning this negative appellation; the rest of the story has to include mention of the plane's uncommon sensitivity to anything less than perfect maintenance of the propulsion, hydraulics and aero surfaces.  A tragic series of aircraft failures was remedied after technicians from Lockheed took over responsibility for maintaining the plane from their German counterparts.  Turn-around times suffered from what the Germans routinely achieved but at a terrible cost.  In fairness to the Germans, they had not been advised that anything less than minutiae-level maintenance turned what was otherwise a 'sensitive' aircraft to fly into a plane that turned on its pilot without warning.

It's no accident the US Air Force wanted little to do with Lockheed's 'missile with a man in it.'    

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3 hours ago, Rasputin22 said:

Sailplanes (gliders) are very susceptible to wind shear. One of the most contributing factors to fatal crashes is during the turn onto final when windshear can stall the inner wing in a turn and then you may as well be riding a sycamore seed!

https://youtu.be/zfFGN-3Yglo

My friend died this way 50 metres above the ground and hit a shear - straight into a tree!

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21 hours ago, Terry Hollis said:

I don't know why people are so preoccupied with aircraft wings.  They are simply irrelevant in the context of high speed foiling vessels which are totally dependant on their righting moment to support the wing/foil.

Once an AC75 gets on it's foils the winning rig will be the one that enables the crew to control the excess power with the minimum drag.  The means they will be able to twist the wing so that the top is nearly no drag or possibly negative power to assist the righting moment and the shape will be different all the way down to the boom. 

There is nothing in the aircraft world that compares with this complexity.

While I think an aircraft that does Mach 3+ at something over 80,000 feet shits all over an AC sail/wing in terms of complexity and technological wizardry (and that was 1950s technology designed and built without computers, more or less), I can agree that your point is valid: totally different design parameters.

 

Sr71_1.jpg

 

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I have it on good authority "one of the designers" of the SR-71 the air intakes for the engines finally doubled the performance of the plane after many different goes it was one of the real learning curves.

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1 hour ago, Rasputin22 said:

That SR-71 looks like it is leaking! 

As I'm sure you know, leaking until air friction brought the skin up to temperature was a feature, not a bug.

Cheers,

Earl

 

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15 hours ago, Tornado-Cat said:

Different constraints but same objective of minimum drag.

Not even close.......

On what planet do military aircraft designers EVER start by profiling their lifting surfaces from a Class/Protocol stipulated one design D-sectioned leading edge spar?

Don't answer that. You might as well take up a hobby redefining Taxonomic rules for all the species of life on the planet and see what reaction get there - because with your reasoning two Mammals - the Platypus and the Blue Whale are hot to trot.....

Your logic reminded me of this.

Logical.png

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This twin skin sail is behind a D section mast, it has to be reversible, its camber must be ajusted along the wing section and the distribution of this camber needs to be adjusted along the span.

All of that, features the best that could be expected from "Adaptative Wing" programs funded by DARPA during decades. 

Unfortunatly, what is possible with a soft membrane of a wingsail is not appropriate for supersonic fighter. In other words wingsail & similar are not at the Jurassic age compared to supersonic fighter wingsection, or gliders. They are just the best trade-off to meet the target within the set of constraints.

One reasonnable assumption in the discussion would be to consider that a bunch of rocket scientists working full time on this subject, with correct salary are likely to achieve a better concept compare to our recreationnal engineering guesses.

Cheers

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The SR-71 is not leaking,

It is probably a kind of condensation, within the leading edge vortex  along the  apex  of this Delta wing.

Ton of workpapers on the subject.

 

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2 hours ago, Erwankerauzen said:

The SR-71 is not leaking,

It is probably a kind of condensation, within the leading edge vortex  along the  apex  of this Delta wing.

Ton of workpapers on the subject.

 

The SR-71 leaked like a sieve until airborne and doing at least Mach 2 or so and the Ti skins expanded from the frictional heat with the air and stopped the leakage. 

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Here is one idea.... Please feel free to shoot me down in flames!

What if the sail was inflatable like a SUP? I was looking at a cross section today and found it pretty interesting; pretty basic but can something be done like this?

Obviously there would have to be panels which can then be inflated at different pressures depending on conditions....

 

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Hey @Purple Headed Warrior I don't see any reason why your idea is not valid.  I reread the rule to see if holes were possible like @barfy's shark edge but they are not when sailing.   I think will be all sorts of tricks to get the right shape for the mode they are sailing in.  But the change of shape will have to happen fast as they adjust from displacement mode to foiling mode and the apparent wind doubles and moves forward?

I will continue to dream about what might be the shapes and how they might be achieved and I hope a few people will enjoy adding their "recreational engineering guesses" (thanks @Erwankerauzen ) to the mix.  However side excursions to SR-71 and the like, are entertaining too.  I personally don't enjoy the negative personal comments but, hey, this is SA.

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6 hours ago, Kiwing said:

Hey @Purple Headed Warrior I don't see any reason why your idea is not valid.  I reread the rule to see if holes were possible like @barfy's shark edge but they are not when sailing.   I think will be all sorts of tricks to get the right shape for the mode they are sailing in.  But the change of shape will have to happen fast as they adjust from displacement mode to foiling mode and the apparent wind doubles and moves forward?

I will continue to dream about what might be the shapes and how they might be achieved and I hope a few people will enjoy adding their "recreational engineering guesses" (thanks @Erwankerauzen ) to the mix.  However side excursions to SR-71 and the like, are entertaining too.  I personally don't enjoy the negative personal comments but, hey, this is SA.

I'd think weight may cancel this approach out... those SUPs are pretty heavy... but then again.. there was the Goodyear Inflatoplane - I seem to recall a clip of someone wandering out and jumping up and down on the wing without it showing much bending...

 

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On 1/22/2019 at 9:24 PM, RobG said:

While I think an aircraft that does Mach 3+ at something over 80,000 feet shits all over an AC sail/wing in terms of complexity and technological wizardry (and that was 1950s technology designed and built without computers, more or less), I can agree that your point is valid: totally different design parameters.

 

Sr71_1.jpg

 

SR71 or YF12A are the most beautiful and exceptionnal known airplanes (we don't know about the Aurora project), however if you want to chose a military plane you should choose the U2 to compare better with a wing sail.

u2inflight.jpg.e08fa9941d03ca1db7042a410011ff84.jpg

greenbird in Kambalda.jpg

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If AC75s hit 50 knots they will be up there with these things !!

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Ecotricity that beast above did 68 knots.  When you think of that an AC75 doing 50 knots is amazing if it happens!

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18 hours ago, Erwankerauzen said:

The SR-71 is not leaking,

It is probably a kind of condensation, within the leading edge vortex  along the  apex  of this Delta wing.

 

That's what that particular photo looks like to me. Interesting also to learn that SR-71s also leak. What fun to pilot a plane that routinely leaked a flammable liquid.

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3 hours ago, Tornado-Cat said:

…however if you want to chose a military plane you should choose the U2 to compare better with a wing sail.

It was posted in response to a comment that aircraft wings don't have to contend with the issues of sailboats like the AC75s. My point (obviously poorly made) is that they need to deal with very, very different conditions and comparing the two in general is pointless, though there may be some specific features that are comparable. E.g. AC75s don't have to contend with surface temperatures varying from ambient on the ground to 600°C in flight, which is why the SR71 wings are corrugated in places. It's not an aerodynamic enhancement but to allow the surface material (mostly titanium alloy) to expand and contract without breaking or deforming too badly.

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9 hours ago, rh2600 said:

I'd think weight may cancel this approach out... those SUPs are pretty heavy... but then again.. there was the Goodyear Inflatoplane - I seem to recall a clip of someone wandering out and jumping up and down on the wing without it showing much bending...

 

I did consider the weight before I wrote my post... And I agree they are very heavy!

But, they might find a more suitable technical way of producing the same solution without having to consider the commerical price element of the SUPs.

Who knows, just threw it out there.

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Googleize "Inflatable Beam" and you will get a lot of documents on the topic

AFAIR, but cant find it anymore, a Swiss company has patented a concept which use both inflatable beam combined with "walls" in carbon. 

It was something like  "Tensilarity" but cannot find anything, neither in My Documents nor with Google.

Cheers

Erwan

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Wow @Erwankerauzen that looks incredibly interesting.  I think I will be looking deeply at that, looks amazing.  The possibilities are endless not to mention an AC36 softwing.

Thank you for your stimulating contributions.

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@Erwankerauzen That Tensiairity looks very relevant to the softwing of the AC75.  With the amazing membranes they have in sail workshops something will come of it.

How exciting!  Thank you for bringing that amazing concept into this speculatory arena.

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8 hours ago, Kiwing said:

speculatory arena”

Wow. I know they’ve upgraded this site, but I didn’t know they’d upgraded it THAT much!

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On 1/27/2019 at 8:16 AM, Kiwing said:

@Erwankerauzen That Tensiairity looks very relevant to the softwing of the AC75.  With the amazing membranes they have in sail workshops something will come of it.

How exciting!  Thank you for bringing that amazing concept into this speculatory arena.

The main sail must be able to be folded without damage so no additional structure there (the "beam" part), and inflatable battens are prohibited (rule 18.4 d), so where will it be used?

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Tensiarity requires a stiff member, think mast and a membrane that folds up with tension strips in it to give tensile strength which can be blown up to a suitable shape where the compression and tension balance and the buckling forces are controlled by the shape.  Now how they can put that into a softwing will take some very outside the box thinking but it should be possible.

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i wonder if that piece at the top of the sail is flotation and/or a sealed off area keeping water out of the hollow between the skins, in case of capsize.  The double skin sail may perform better when full of air, so i wonder if they will ram air into it.

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13 hours ago, Kiwing said:

Tensiarity requires a stiff member, think mast and a membrane that folds up with tension strips in it to give tensile strength which can be blown up to a suitable shape where the compression and tension balance and the buckling forces are controlled by the shape.  Now how they can put that into a softwing will take some very outside the box thinking but it should be possible.

I love it when you talk dirty. 

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7 hours ago, david r said:

i wonder if that piece at the top of the sail is flotation and/or a sealed off area keeping water out of the hollow between the skins, in case of capsize.  The double skin sail may perform better when full of air, so i wonder if they will ram air into it.

A masthead flotation device with at least 300kg of buoyancy is required by rule 20.24. The two sail skins are not attached to each other except by the mast and some loose lines between battens near the leech, so it will not retain water any more than a single skin.

If an AC75 capsizes, it will be race over. Raising the boat slowly should allow any water on the sail or in the mast to run out of the leech or head before passing horizontal. 

16 hours ago, Kiwing said:

Tensiarity requires a stiff member, think mast and a membrane that folds up with tension strips in it to give tensile strength which can be blown up to a suitable shape where the compression and tension balance and the buckling forces are controlled by the shape.

An inflatable luff would likely be considered part of a "control system" and is disallowed by:

20.26 With the exception of leech lines of the mainsail, control systems of the mainsail shall only be attached
to or bear upon sail skins of the mainsail in the mast upper zone or mast lower zone.

 

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@RobG I don't agree as battens could also be considered control devices and air pressure can come from the wind and be controlled from the top or bottom of the mast?

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^^ A luff is not a control system, it's a luff, but defender and challenger can interpret what they want through their bodies.

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23 hours ago, Kiwing said:

@RobG I don't agree as battens could also be considered control devices and air pressure can come from the wind and be controlled from the top or bottom of the mast?

Battens are covered by other rules 20. 22 and 20. 23. Rule 20.26 says "control system", not "device".

Tensairity depends on precise control of air pressure between 50mbar and "a few hundred mbar depending on the application". So even if an inflatable luff was allowed, it would be a serious challenge to control the inflation pressure solely using apparent wind during changes to its angle and strength plus varying sail trims and manoeuvres.

22 hours ago, Tornado-Cat said:

^^ A luff is not a control system, it's a luff, but defender and challenger can interpret what they want through their bodies.

A luff is a region of a sail, I don't think anyone's calling it a control system.

It would take some very clever interpretation of the rules to argue that a pressurised bag inside the luff that is intended to control the shape of the sail is not part of a mainsail control system. Though perhaps it might be used inside the mast to stiffen/soften it as required.

 

PS. The editor here converts "20. 22" without the space to "20dogballs". Childish, really.

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1 hour ago, RobG said:

It would take some very clever interpretation of the rules to argue that a pressurised bag inside the luff that is intended to control the shape of the sail is not part of a mainsail control system.

I see it the other way, it takes a good control of the interpretation of the rules to make it a control system.

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3 hours ago, RobG said:

a pressurised bag inside the luff that is intended to control the shape of the sail is not part of a mainsail control system. Though perhaps it might be used inside the mast to stiffen/soften it as required.

Wouldn't this be the interpretation? If you don't change the characteristics of a thingy it isn't controlling anything, thus not a control system?

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The control only happens in the top and the bottom as allowed.

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For all the exciting technology, can anybody please explain how this inflatable idea actually makes the boat go faster. 

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sym.jpg.68d1d07dcc4d837aebd4be3f5a250a64.jpg

See @dorox's post on page one of this thread, a symmetrical design is the least drag, and as we have too much power less drag when foiling is the answer?

What they actually achieve and if they can achieve symmetry is another question.

Hey it is all speculation here right!

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On 1/24/2019 at 12:23 AM, Kiwing said:

Ecotricity that beast above did 68 knots.  When you think of that an AC75 doing 50 knots is amazing if it happens!

Eco hit like 120!!

The 75's will never be close to iceboats nor landsailors. The limiting factor in top speed is aero drag, the 75's have plenty of it.

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@heavym210 What speed in knots did Ecotricity hit? 104 knots sounds better.  I converted from kpm.

I wondered at the time what I had done wrong.

By the way what is your picture of? in the little round it is hard for me to see what it is.

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2 hours ago, heavym210 said:

Eco hit like 120!!

The 75's will never be close to iceboats nor landsailors. The limiting factor in top speed is aero drag, the 75's have plenty of it.

Don't talk about things you clearly don't understand. If it was about aero drag, why is the fastest sailboat only doing 65 knots? The limiting factor is drag, but it is hydrodynamic drag caused by the foils in the water. 

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4 hours ago, Kiwing said:

@heavym210 What speed in knots did Ecotricity hit? 104 knots sounds better.  I converted from kpm.

I wondered at the time what I had done wrong.

By the way what is your picture of? in the little round it is hard for me to see what it is.

It’s my iceboating tipping over. 

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17 hours ago, barfy said:

Wouldn't this be the interpretation? If you don't change the characteristics of a thingy it isn't controlling anything, thus not a control system?

If it's not controlling anything, why is it there?

16 hours ago, Kiwing said:

The control only happens in the top and the bottom as allowed.

Yes, that would be fine. But it's only 4 m at the top and 1.5 m at the bottom (which includes the boom) of a 26.5 m tall mast. While the concept of tensairity has some interesting features, no–one has proposed how it can be applied to an AC75 mainsail within the rules in a way that is better (by some objective criteria) than alternatives (that are unknown to the great unwashed) that are likely based on existing, proven mechanisms.

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Aero drag can be a big part of total drag. 

ie. Let 's compute the aero drag of the crew trapezing its A-Cat,  windward @ 20 knts boat speed with 14 knts TWS.

Just the crew DRAG

assuming its aera is: 1 sq meter

 Its drag coefficient is 0.25

The apparent wind velocity 15 m/s

The crew drag is = 0.61 x 1 x 15 x 15 x 0.25= 34 Newtons

The total driving force is probably between 175 N and 250 N

So just the crew drag is a significant part of the total drag.

Just like mentionned by Heavym210.

Happy week end

Erwan

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At some point Fallow speculates that the twin skin has the potential to be 20% better than a standard rig.

That brings me back to earth a bit and while I like the idea they are exploring the idea of improving sailing for more people I wonder if 20% is enough to see one on an A class cat?

probably not a hobbie,  Maybe the bigger more expensive boats where first across the line is important?

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I understabd improvements are mainly in the high lift area, so not sure  this development will be really worth the extra weight for an A, to gain a bit more power dowwind in the light

we'll see

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3 hours ago, 17mika said:

I understand improvements are mainly in the high lift area, so not sure  this development will be really worth the extra weight for an A, to gain a bit more power dowwind in the light

we'll see

It was the reduction of drag which would give an edge I would have thought.  But Glen Ashby will know and watch his "A" power plant for the next worlds or the one after next?.

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An interesting conundrum for them;

- One of the reasons we went in this direction is the possibility of 'trickle-down'

- One of the main advantages of the 'TwinSkin' ;) over conventional sails is reduced drag at foiling speeds

- What % of the WWF-leet sails at 3x windspeed? (And what % of those that do, have not already tried it?)

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On 2/24/2019 at 2:23 AM, Kiwing said:

At some point Fallow speculates that the twin skin has the potential to be 20% better than a standard rig.

That brings me back to earth a bit and while I like the idea they are exploring the idea of improving sailing for more people I wonder if 20% is enough to see one on an A class cat?

probably not a hobbie,  Maybe the bigger more expensive boats where first across the line is important?

Not just the 20% more powerful - which is the grunt they need to get foling - but as or more important, is the ability to operate at lower drag than single skin sails - which is highly desirable when foiling. 

This 20% statement falls in step with Botin who has been quoted as saying 10-20% over single skin rigs. 

The weight argument Fallows describes is compelling compared to the solid wing rigs of before - but as no one knows what hardware will or wont be used by the various teams, then it all becomes speculative.

Also, if teams run windward heel then added rig mass gets offset by the additional RM that is created in what is otherwise a tightly controlled global weight limit - that is, of course, until the dynamics get bent all out of shape and throws the whole show on its ear. 

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Actually @Boink I still believe Glen Ashby has his eyes on a much bigger percentage.  And I agree (while you sound much more (knowledgeable) that Drag is the main benefit.

So; on another tack, so to speak.

No boom, just a traveller on the flat smooth deck with just heads above (like the Mule) and not a lot of wing movement (compared to a single skin immersed boat) and steering the boat to the appropriate angle of attack until foiling (ignoring VMG until foiling), probably healed to windward too am I in crazy land?

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2 hours ago, Kiwing said:

Actually @Boink I still believe Glen Ashby has his eyes on a much bigger percentage.  

Here we go again! Your blind and irrational optimism is crazy. You do realise that Fallow is better qualified to talk about the rig than Ashby? Why do you keep thinking that Glenn is the designer and technical guru? Sure, he is a very smart sailor who has been a successful sailmaker of small boat sails, but he is one cog in a big design wheel and he certainly isn't the best person on the team when it comes to rig design. He might even come up with some of the ideas, but in terms of the technical development, it's not Glenn.

Why do you keep coming up with the idea that a 2 skin rig can perform better than both the independent experts and the ETNZ experts say is possible? Why would ETNZ play down thw performance potential of the rig, when they have played up the overall performance of the boat? 

I suspect that part of the issue is that you equate the rig to the potential of the AC75. I doubt very much that the rig is going to be the limiting factor in the overall performance of the boat, again, like with the 50's, the biggest issue is likely to be cavitation on the foils, just like it was thye biggest challenge for Paul Larsen with Sailrocket. He could have developed a more efficient rig (he has said so) but the big problem wasn't there.

 

On 2/24/2019 at 9:29 AM, Kiwing said:

But Glen Ashby will know and watch his "A" power plant for the next worlds or the one after next?.

I'll watch, but having chatted to him about it, I would bet a lot against him using a double skin rig. 

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I'm understand you @A Class Sailor but You are a realist and I am a dreamer, one in a thousand I am right and you are wrong. Does not stop me dreaming.

So how about the boomless rig, on a deck like the mule? What are your more informed thoughts (than mine) on that?

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14 hours ago, Kiwing said:

So how about the boomless rig, on a deck like the mule? What are your more informed thoughts (than mine) on that?

This is the really funny one. In Australia, there are 2 main A Class sailmakers, Ashby and Brewin. Brewin uses a boomless sail while Ashby has a boom. Glenn says you need a boom for control. I am not really sure what to make of the AC rig being boomless, unless the control devises at the bottom of the sail provide a similar function. 

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The twin-skin application for smaller boat has a big problem:

It would be important to get a good idea of the actual drag of today's A-Cat rig for instance

and honestly, it is a bit difficult as the sail's surface roughness is not like 

standard surface roughness used in wind tunnel to study boundary layers,

so analogies are challenging, and no wind tunnel datas for such rigs,  so...

Ironnically, it is probably easier to compute the drag of a twin skins package than

the drag of an A-Cat rig.

Cheers

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I am speculating we will see the wing "joined" to the deck, hence no boom and no sheet, no traveller, no attachment to the deck at all, controlled from within.

After watching that NYYC video my head is buzzing with the potential for some amazing outcomes !

Kiwi number 8 wire thinking verse amazingly refined, skillfully put together, very high Tech design.  Perhaps simulator verse simulator.

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On 2/26/2019 at 2:46 AM, Kiwing said:

Actually @Boink I still believe Glen Ashby has his eyes on a much bigger percentage.  And I agree (while you sound much more (knowledgeable) that Drag is the main benefit.

So; on another tack, so to speak.

No boom, just a traveller on the flat smooth deck with just heads above (like the Mule) and not a lot of wing movement (compared to a single skin immersed boat) and steering the boat to the appropriate angle of attack until foiling (ignoring VMG until foiling), probably healed to windward too am I in crazy land?

Well we are in there together - crazy land that is. Remember power is only needed to get foiling - then it is all about power delivery at least or lowest drag - which is where the twin skin will shine over conventional rigs. 

The closest analogy I can think is that contemporary yacht rigs (mast, spreaders, shrouds and single skin sails) are the Bleriot plane wings of c.1909, whereas the D mast and twin skin setup are the classic Fabric wings on plywood ribs type affair. More developed, better solutions, alllowing more speed and control - but not the analogies given before about trans and supersonic aircraft - because the speed ranges (and wing shapes) do not translate.

Agreed about flat smooth deck - also reckon that a scow like hull with minimal freeboard (think massive SUP or scaled up quant 23) type shapes are a distinct possibility .

I think a boom will remain just to enable macro scale control of the lower half of the sail - but yes deck sealed for whatever span the rule and handling mechanics will permit. I think batten control at the top of the rig with either internal or external actuators will be critical. Whatever solution proves fast will be that which has control at lowest drag.

The new NYYC AM video about Airbus, shows more of the heeled to weather than not healed to weather shots; though experience tells us  that they only show what they want us to see and I am tring to remain open to how it will all shake out. I remain persuaded that where the box rule limits foil span, boat weight and rig dimensions - the gain in RM from heeled to weather is too great a free ride to not chase.

Also in the video I saw the trim control seemed all about fine but quick leech adjustment adjusting twist (therfeore power) to keep it in the groove - it will be all about the pre starts, engagements and crosses that sort the men from the boys.....

Much to look forward to as the campaigns develop.

Bleriot.jpg

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On 2/27/2019 at 4:28 AM, Kiwing said:

I am speculating we will see the wing "joined" to the deck, hence no boom and no sheet, no traveller, no attachment to the deck at all, controlled from within.

You need to explain how that is possible within the rules. There can be nothing controlling the sail in the region above 1.5m above the bottom of the sail until 4m from the top, a 21m span. All that is allowed is the sail's connection to the mast and some battens (tightly specified). You can't have a GNAV or similar (at least I don't think it's practical if kept within the 1.5m lower region), and you need to be able to control sail rotation somehow (again, only through something attached in the upper and lower regions). 

To me, a boomless AC75 would need to work the same way it works on multis: mainsheet on the clew and traveller on the deck. But I'm very open to whatever ideas you have of how it might work as long as they are plausible within the rule.

 

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@RobG

It seems to me that a boom, like the mule, distorts the bottom of the wing.

Picture a thinnish sheet of Carbonfibre taking the place of a boom inside the bottom of the wing.  Able to bend to the shape of the lee side skin while providing a compression element holding the bottom of the skins (wing) down to the deck.  With a pneumatic element to stop bubbling

You need some assumptions: -

1)      The hull is pointed in the optimum direction for the most power regardless of VMG

2)      The wing is shaped for the most power to lift onto the foils.

3)      Once foiling the wing takes a symmetrical shape (least drag) using only angle of attack for lift

4)      The boat direction, once foiling, is tuned to best VMG

This allows:-

1)      minimal movement of the wing relative to the deck.

2)      Most aerodynamic shape for the hull when foiling (least drag)

3)      Good control of the middle of the wing because the bottom is the right shape.

I have not decided whether this requires the leach to be attached to the deck for strength or it can be done like the bottom of ETNZ’s AC50 wing?

For a 75 year old Laser sailor that is my jumbled thoughts.

Maybe with the help of some other clever people we can develop a more robust guess.

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Thank you very much VARAN for posting these interesting interviews.

We can find many answer to the different questions regarding the features of this twin skin rig.

I will keep the 20% extra HP and the drag minimization target as a benchmark for other applications.

Cheers

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I must be on Iggy by most people.  I expected at least a little reaction from my no boom post.

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I go back to my earlier suggestion about using one sail that splits into a Y near the mast so that it attaches to both sides of the D mast like a twin sail.  It would make the sail more aerodynamic at the mast (less drag) but would be lighter and easier to handle than a twin sail.  You can tapper back where the Y attaches into one sail to optimize drag/lift.  The Y could also be "inflated" or adjusted as needed during the race to optimize performance...

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Can't see how you would stop the windward 'skin' from wrinkling horribly with that ^ set up - both sides have the same length and terminate at the same point (i.e. where they become a single skin). What am I missing?

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The boomless discussion by some of the press is so bad that they highlight an A-class catamaran sailed by Glenn Ashby and call it boomless.  One picture clearly shows a boom on his boat (as he always done at the worlds level), and says it is boomless.

Boomless has been done for many years.  As one example, Randy Smyth has done many rigs without a boom including trimarans, cats, etc.  As another, some sailors have had great racing success with boomless.  Some have not.

When I sailed boomless, it took more force to sheet the main.  That boom is doing quite a bit of work and allows a lower purchase on the mainsheet.  

On the plus side, boomless allows for some unique shapes (like a jib rather than a traditional main) with varying sheet tension.  It is also cleaner aerodynamically.  

Will these boats want that kind of twist from boomless or will they want moth-like sail controls where angle of attack is the most important adjustment?  TNZ focused on twist control and even twist inversion with the last AC with their hydraulic wing controls.  Those boats stayed at very high speed all of the time and had high righting moments all the time.  Not so with the AC75 in at least some of those dimensions such as very limited righting moment before take off.

Unless I am misunderstanding something, the boat has almost no righting moment from its keel at low speed when it moves the two keels out from under it.  One is to windward and is balanced almost entirely by another to leeward.  Maybe the leeward one stays under the boat at the beginning.  Definitely a new level of reliance on keel movement coordination and perhaps form stability to some degree though that is slow.

Not many have sailed rotating mast, two skin mains with control systems at the top several meters of mast to better mimic the controls of the last AC's wing.  It seems a little hard to predict the use of a boom or not given the unique factors of this rig and boat and a small part of the total solution at this point.  

The sails need to have enough ump down low in the sail plan to get the boat to foiling speed with very little righting moment.  Then they need the power to build speed and and then they need to reduce drag and reduce power as they go faster and faster.  Not an easy design brief for sure.

I would focus more on the top of the rig than the bottom to manage those transitions.  I think that is where the action and innovation will be.

 

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9 hours ago, nav said:

Can't see how you would stop the windward 'skin' from wrinkling horribly with that ^ set up - both sides have the same length and terminate at the same point (i.e. where they become a single skin). What am I missing?

I think you are right, hadn't thought of that.  

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@Lost in Translation imagine a sheet of carbon fibre (or similar material) part of the bottom of the two skins, able to keep the skins tight.  You control the shape of the lee skin with the sheet pulling back to the deck.  Similarly the windward skin.  The carbon fibre acts like a boom a bit?

One more step of speculation is to have a "Mule" deck with the softwing sealed to it.  This requires a loss of movement of the traditional mainsail. This could be compensated by the direction of the hull which, ignoring VMG, is pointed in the direction of maximum power to lift the boat onto the foils.  When on foils the coarse is optimised for VMG.

RM Can the foil flaps provide righting moment?

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There is no problem to sheet a sail by a traveler or mainsheet system that runs under the deck for aero purposes.  The Australians in the A-Class have been doing it for years.  Makes for a very clean deck.  

Leech tension will be very important and not something that can be ignored or hoped for.  Either with massive sheet tension or a boom and reduced sheet tension.

If the bottom of the sail is rigid enough to be a boom it is no longer a soft sail and in fact stronger than a typical wing which does not need to serve as a fulcrum for the rest of the leech tension.

The problem with any boom to date for a deck sealed main is the windage.  I would guess they will figure out something clever there.  Maybe a tacking boom that bends to the shape of the foot and could be internal to the twin skins.

 

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Yes a boom that bends to the shape of the foot of the sail, and is internal is where I think they may be going.

Sealed to the flush deck is that possible too? Would that produce power down real low so as not to require as much RM?

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On 3/8/2019 at 7:33 AM, Kiwing said:

Yes a boom that bends to the shape of the foot of the sail, and is internal is where I think they may be going.

Sealed to the flush deck is that possible too? Would that produce power down real low so as not to require as much RM?

This should get them onto foils early so they can target VMG early.

Long tail sperm shape is my guess for prefoiling.

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The article linked on post#470 shows extra forms that may be connected to the mast, that are in between the 2 skins and give shape to the front 1/3 of the sail.  I wonder why they say this new rig will be half the weight of a wing? Sail mylar is much heavier than wing mylar.  There are battens and internal fairing ribs.  The top of the rig has extra stuff.  If a boom is used it will be beefy, plus the beef needed for a vang system.  Double outhauls and downhauls.  Possibly 4 tracks mounted to the D mast, 1 for each sail and 2 for the internal fairing pieces.  Damn those wings must be heavy to be twice as heavy as all of those items.  Or the article is wrong like they  are wrong about that Acat picture showing a boom, but saying it's boomless.

They could possibly be thinking the large jib will keep them foiling like the gc 32 foilers do, which still use the genoa when the main is reefed.  I don't see how they claim a 50 knot potential with the drag of a large jib sail in the mix.

It will be curious to see how they crack this nut.  Rather than using a giant A cat rig or a giant 18foot skiff rig, they forge ahead into new ground.....a halyard hoist-able wing.

 

 

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39 minutes ago, david r said:

The article linked on post#470 shows extra forms that may be connected to the mast, that are in between the 2 skins and give shape to the front 1/3 of the sail.  I wonder why they say this new rig will be half the weight of a wing? Sail mylar is much heavier than wing mylar.  There are battens and internal fairing ribs.  The top of the rig has extra stuff.  If a boom is used it will be beefy, plus the beef needed for a vang system.  Double outhauls and downhauls.  Possibly 4 tracks mounted to the D mast, 1 for each sail and 2 for the internal fairing pieces.  Damn those wings must be heavy to be twice as heavy as all of those items.  Or the article is wrong like they  are wrong about that Acat picture showing a boom, but saying it's boomless.

They could possibly be thinking the large jib will keep them foiling like the gc 32 foilers do, which still use the genoa when the main is reefed.  I don't see how they claim a 50 knot potential with the drag of a large jib sail in the mix.

It will be curious to see how they crack this nut.  Rather than using a giant A cat rig or a giant 18foot skiff rig, they forge ahead into new ground.....a halyard hoist-able wing.

Mmmmm I like the last line "It will be curious to see how they crack this nut"    Yes indeed, this is the thread to put your speculation, which is "this rig will be much heavier than the AC35 wings. - We will see (or perhaps never find out?)

I remember many well informed people JS etc, suggesting that bikes were the wrong idea. When in fact I believe they were the big difference, most of which we did not see.  The wing control ideas will be carried over to this twin skin system and "crack this nut" with air as their internal fairing ribs and other ideas speculated here!

But because of the secrecy that has pulled it's vale of most of the design development in AC, we might never find out?

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3 hours ago, david r said:

The article linked on post#470 shows extra forms that may be connected to the mast, that are in between the 2 skins and give shape to the front 1/3 of the sail.

Where is that? I can only see the following image, which doesn't show any internal structure:

yysw239256.jpg

Between the upper and lower zones, no internal structure can extend more than 100mm from the back of the mast (rule 20.10) so there are no internal fairings and ribs (outside of the zones), just six full–length battens and six 1m leech battens (rule 20dogballs). The mast itself is virtually one–design.

Quote

I wonder why they say this new rig will be half the weight of a wing?

50% is hard to believe but significantly lighter (maybe 75% of a wing) isn't. They have some pretty amazing CAD that accurately models both and they've built and sailed a (scaled) AC75 dual–skin sail so should have a pretty good idea what a full–sized rig will weigh.

According to the article, the skins of a dual skin sail can be lighter as the load is shared between them, not 50/50 but each skin can be 65% of the strength of a single skin of the same size. There are no internal frames or ribs, and no hinge or flap mechanisms.

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black CF would be invisible in that image.

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Hi Everybody,

Regarding the boom/boomless debate, some time ago, in the A-Cat serie, the boom

was useful to undirectly "control" the twist of the sail.

If you wanted your mainsail to twist under cunningham tension, everything else equal, you would achieved more twist with footsail tension along the boom, than without.

(For a journalist boomless is different from wishboneless)

Cheers

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https://www.sail-world.com/photo/192795

it was post 457 that had the drawing of internal rib things that look like they would hold shape on the leeward side of the sail on each tack after the mast rotates.

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