Kiwing

The new sailing twin skin setup

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9 hours ago, Kiwing said:

I believe;

1) more than half of the teams will be able to reverse the top of the softsail and use it to do various things.

2) that this soft sail set up will be at least  70% of the slotted AC34 wing.  This is at odds with some experts but then so were the cycles.  And this is at least twice as good a a single skin sail.

3) The reason Glen latched onto it was because he could see hidden benefits when put together with the secret knowledge they had from their control system for the wing.  It would not surprise me if Glen has the goal of getting close to the performance of the AC34 wing.  But once again some experts will scoff at that dream!

1. If one team works it out, they all will.

2. That goes against all experts and all known aerodynamic theory, but the biggest issue is that I am not sure how you would ever measure this. 

3. Glenn didn't latch on to it because of hidden potential. He latched onto it because of clear and obvious potential. The decision had already been made not to go with a solid wing sail and that they wanted the sails to be capable of being raised and lowered out on the water. He knew this decision had some serious consequences and when he saw the double skinned rig in WA, he would have instantly realised that it solved a number of issues. 

What I cannot understand is the fixation on trying to imply the soft wing will perform better than the experts say. It is a pointless argument and totally irrelevant. It doesn't matter how it performs compared with the solid wings used in the past. The issue is whether it out performs a conventional soft sail rig, which it will do, and whether it has other benefits for a foiling boat over a conventional rig, which it does. 

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2 hours ago, A Class Sailor said:

What I cannot understand is the fixation on trying to imply the soft wing will perform better than the experts say. It is a pointless argument and totally irrelevant. It doesn't matter how it performs compared with the solid wings used in the past. The issue is whether it out performs a conventional soft sail rig, which it will do, and whether it has other benefits for a foiling boat over a conventional rig, which it does. 

Very well put.....

The move for a soft sail (twin skinned or not), is not to force innovation to be more efficient than the wing. But to eliminate the 'need' for the wing; bringing costs down and reducing the logistical problems.

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Move to soft sails was to get sailmakers back into the ACup business - much political pressure from the blue circle conglomerate was brought to bear.  And if you think this is going to reduce costs then dream on!  

The one benefit is in the logistical side for sure but we'll see if that works out in practice soon enough.

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A free standing wing mast that can rotate 360 degrees is MUCH less aero drag than a small section spar held up with wires and spreaders. So would be MUCH better at the dock.

There is a reason that zero production aircraft have been built using single or double skin soft wings in the past century, and only aircraft built for weird flight envelopes (slow and rugged: e.g., Pitts, Cubs, C150) have been built with wires and/or spreaders in the past half century.

So the double skin sail is a silly, silly development. Airplanes tried it, and moved to "free standing wings." It was explored in C class during the 60s by people including Steve Dashew, and was much worse than a wing, and showed no advantage over a rotating spar with fully battened sail. Hence C class cats, who can do anything within a given area, only use wings or single skin sails since that time, and only wings once they got light enough.

Now sure, better materials today, but that improves the single sail more than the double skin sail, and wing much more than either. Especially when one includes the strength and therefore weight of the platform (boat): fundamentally lower on the solid wing, over using very high tensions on a soft sail (single or double).

Someone specified a couple of requirements -- monohull, ballast for self righting, sails that can be dropped. These were simply stipulated requirements, none were specified to achieve ultimate performance or especially bang for the buck or practicality. Sure, more practical than a wing held up by wires, but that is just stupid anyway, a concept for going much slower than 70 knots over the deck. 20 or 30 knots, sure, but not 50 to 80 apparent.

Foiling is different. A solution that looks sorta the same is wrong. As the future will prove.

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What about the double skin wing of modern paragliders and kite boards? Double skin, high aspect ratio, pretty effective.

And remember OR getting backed across the bay with ribs in bda, a control problem as I recall, but I'm sure a pucker moment for all involved in the 20kt breeze. This would really suck to do in Auckland from out towards Tiri in a breeze and swell. Being able to drop a sail is good practice.

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Here is the wing sail of the fastest iceboat. Not two element.  This area has a lot of experimenting to be done.

 

WING SAIL 1.jpg

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Now I agree the main motivations were as above, but now we are here let us not limit our expectations.

My comments are all "believe" and "hope" so meant to stimulate discussion, but they come from not being able to believe these things could have a VMG faster than the wind down wind, or sail up a river against the flow in still air !!!

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15 hours ago, Sailabout said:

do the rules allow adjustable spreaders ( whilst sailing) for this cup?

Spreaders are part of the separate mast drawing package that was scheduled for release on 31 August 2018. As far as I know it's not public.

What is the purpose of adjustable spreaders? If it's to control mast shape, that's forbidden by rule 20.6 which says nothing can control mast shape outside the upper and lower zones, where devices other than spreaders are likely more effective.

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33 minutes ago, RobG said:

Spreaders are part of the separate mast drawing package that was scheduled for release on 31 August 2018. As far as I know it's not public.

What is the purpose of adjustable spreaders? If it's to control mast shape, that's forbidden by rule 20.6 which says nothing can control mast shape outside the upper and lower zones, where devices other than spreaders are likely more effective.

I can't imagine you can change the shape of that enormous D section very easily anyway.  It looks to be an extremely strong 3 dimensional shape to me and hard to flex in any direction.

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4 hours ago, Kiwing said:

Here is the wing sail of the fastest iceboat. Not two element.  This area has a lot of experimenting to be done.

 

WING SAIL 1.jpg

Yet again, you are not comparing like for like, and BTW, that's the land yacht version of the rig. There are significant differences in the requirements for a speed record wing like that and one for a racing yacht. The speed record wing does not need to be able to power the craft on multiple points of sailing, in varying wind strengths, it does not need to be able to tack and gybe efficiently and acceleration is totally irrelevant to the speed record wing. Finally, the speed record wing works in a completely different power to weight range to the wings on the sailing boats, plus the drag from the interface with the support medium (water, ice, land) is completely different.

Stick a wing like this on a sailboat racing round a course and it would be a total dog.

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@A Class Sailor next you will be telling me it will need a spinnaker to go do wind?!  Only joking!

These foilers are virtually into the wind all the time, they could be thought of more like an iceboat than a sailing boat.

I believe you are a little trapped within what has happened and I am trapped in dream land.

I really hope you will be blown away with the end result of this out of left field softsail which I am sure will out perform it's predecessors. 

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2 hours ago, Kiwing said:

@A Class Sailor next you will be telling me it will need a spinnaker to go do wind?!  Only joking!

These foilers are virtually into the wind all the time, they could be thought of more like an iceboat than a sailing boat.

I believe you are a little trapped within what has happened and I am trapped in dream land.

I really hope you will be blown away with the end result of this out of left field softsail which I am sure will out perform it's predecessors. 

It is not like the foiling concept is totally new.  The AC36 boats hope to go as fast, or slightly faster than the AC35 cats.  Maybe as fast as the F50's.  So there is really nothing that will blow us away except for the shape of the "hull" that is attached to the foils and the delicate balance to keep it stable.  The principles of foiling and the impact of the sail/wing and high speed apparent wind sailing is getting to be well know.  According to all of the air flow experts, it is impossible for the dual sails to perform as well as the wing.  The designers are just hoping that they can improve the performance over a typical single skin sail.  But if the dual sail is better than a single sail, why don't we already see them in any real sailing competitions?

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OK @Herfy most people here believe the soft sail will be only 10-20% better than a soft sail  and nowhere near the F50 wings.  It follows then that they will not see speeds like the F50 or only in really short bursts when it all come together.  They have a size advantage which in the old days meant much faster but as the AC50s showed over the bigger cats of AC34 foiling means size can be a disadvantage!  I hope/believe you will be blown away by the ability of the soft sails to get up near to the AC50 wings.

The dual skin like the bikes have been around for a while and .....!!??

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48 minutes ago, Herfy said:

It is not like the foiling concept is totally new.  The AC36 boats hope to go as fast, or slightly faster than the AC35 cats.  Maybe as fast as the F50's.  So there is really nothing that will blow us away except for the shape of the "hull" that is attached to the foils and the delicate balance to keep it stable.  The principles of foiling and the impact of the sail/wing and high speed apparent wind sailing is getting to be well know.  According to all of the air flow experts, it is impossible for the dual sails to perform as well as the wing.  The designers are just hoping that they can improve the performance over a typical single skin sail.  But if the dual sail is better than a single sail, why don't we already see them in any real sailing competitions?

We don't see any monohulls with ballasted foils either, but by all accounts the computers think they will be significantly better than any current monohull configurations as well.

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54 minutes ago, Herfy said:

It is not like the foiling concept is totally new.  The AC36 boats hope to go as fast, or slightly faster than the AC35 cats.  Maybe as fast as the F50's.  So there is really nothing that will blow us away except for the shape of the "hull" that is attached to the foils and the delicate balance to keep it stable.  The principles of foiling and the impact of the sail/wing and high speed apparent wind sailing is getting to be well know.  According to all of the air flow experts, it is impossible for the dual sails to perform as well as the wing.  The designers are just hoping that they can improve the performance over a typical single skin sail.  But if the dual sail is better than a single sail, why don't we already see them in any real sailing competitions?

I think we may be in for a revolution with twin skin rigs.

The rotating mast is a red herring and will probably rotate enough to align it's self with the apparent wind and that's it.

The internal structure and mechanism will be as complex as a hard wing so that a full twist will be available.  The need for the sail to be lowered will add to the complexity.

They will only need two internal structures but I can see them using a lot of skins during the course of the series.

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5 hours ago, Herfy said:

  But if the dual sail is better than a single sail, why don't we already see them in any real sailing competitions?

Class rules for the valid purpose of cost control.

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Why the assumption that twin skins are faster, when aerodynamics gurus say that they are not necessarily quicker and that a thin foil, at its optimum trim, can have better characteristics? 

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Nice discussions.

When it comes to compare rig's efficiency, it is important to know in which aerodymaic conditions they will be used.

Of course a slotted 3 elements wingsail would perform better in light winds when it is crucial to extract a maximum power from the wind. The  wingsail concept provides the required Lift coefficient for this purpose.

But as soon as the boat is foiling, the apparent wind increases so much that it is all about drag, and big power is not required anymore. So the main advantage of the wingsail would be its versality (big Hp in low winds andlow drag in high winds)

It has been mentionned by "insiders" and CFD Guru that the double skin set-up has been choosen to avoid the crane/wing issue when the boat is in the harbor. (It is  better to burn money in technology than in logistic) and also this twin set-up project bringsback the sailing industry in the loop, with possibility to implement similar technology for production boats (The fashion argument).

May be the average wind conditions available at the venue for the Cup in 2021/2022 would provide interesting insights regarding the best trade-off for the rig concept. (to much wind for a wing to be necessary)

I feel confident that expected wind conditions for the Cup were the first assumption on the table of the design team.

Double skin main advantage seems to be a little lower drag compared to a teardrop mast/full batten sail  package.

The 10/20% performance improvement as claimed here or there would be meaningfull only if the sailing conditions were disclosed.

Cheers

 

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1 hour ago, Curious said:

Why the assumption that twin skins are faster, when aerodynamics gurus say that they are not necessarily quicker and that a thin foil, at its optimum trim, can have better characteristics? 

Which aerodynamic gurus are saying that?

Some people use single-skin paragliders for hike and fly applications, because they are lighter to carry up a mountain. They are however aerodynamically inferior to the standard double-skin models.

 

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6 hours ago, Terry Hollis said:

I think we may be in for a revolution with twin skin rigs.

The rotating mast is a red herring and will probably rotate enough to align it's self with the apparent wind and that's it.

The internal structure and mechanism will be as complex as a hard wing so that a full twist will be available.  The need for the sail to be lowered will add to the complexity.

They will only need two internal structures but I can see them using a lot of skins during the course of the series.

I get the feeling ETNZ designed their rig, then wrote the rule in a way that provides minimal hints. So there will be internal structures in the top 4m of the sail that are less than 650mm long, which likely slot into something in the back of the mast at the same height that protrudes less than 300mm. It will be hoistable with the sail and won't twist the mast, but it will twist the sail.

They must think that's enough to do the job, and if reverse twist is in the mix, it will do it. But maybe it's not needed given the foil can dial in windward heel at will in a way no other boat has been able to.

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On 1/9/2019 at 7:53 PM, Terry Hollis said:

The basic problem with twisting the twin skin sail is that it's asymmetrical shape is achieved by rotating the mast and this produces an excellent shape for the full height of the sail. Unfortunately twist is what we want and with a rigid mast it is difficult to undo the shape at the top while keeping it lower down.

With the solid wing the starting shape is symmetrical with a hinge in the middle so they are able to twist it easily.

Yes and no. Having a rotating spar does not mean you cannot also fully shape the sail skin mold. So - unlike relatively rigid wing elements - each of the skins has the same twist potential of a normal 2D sail in the sense of smoothly morphing from full in the bottom to twisted and flat in the head.  With the introduction of mechanical control of both camber and twist in the top 4 meters (including the possibility of running negative AoA at the head sections), you have the potential to very smoothly mimic what was being done with the wing in most of the wind range where there is more than enough available lift, and you are focused on shedding heeling/pitching forces and drag. There will of course still be much more overall elasticity in the system compared to a rigid wing, but that might actually be an advantage in the less steady state conditions and rougher waters of the Hauraki gulf.

For sure the new rigs will will have a very hard time generating the same power as the slotted wings downrange (which has been mitigated with the reintroduction of headsails, we shall see how successfully) but up range they are potentially very effective. 

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Head sails will be interesting, as soon as the boat lifts out a code zero will be flapping in the head wind...will be interesting if they will be used. Is there room to develop specific code zero's, ie an innovative quick furl?

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8 hours ago, Curious said:

Why the assumption that twin skins are faster, when aerodynamics gurus say that they are not necessarily quicker and that a thin foil, at its optimum trim, can have better characteristics? 

Probably the same gurus who thought arms could generate just as much power as legs I imagine...

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1 hour ago, Boybland said:

Probably the same gurus who thought arms could generate just as much power as legs I imagine...

Nobody ever thought that arms were stronger than legs.  It was just a matter of if it was worth it, not taking into consideration that it freed up the hands of a cycler to trim the foils.

 

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14 minutes ago, Herfy said:

not taking into consideration that it freed up the hands of a cycler to trim the foils.

 

The ETNZ control system design was so superior to OTUSA's.  I was astonished that, for being so "advanced" in technology Oracle kept inside that mental box.  

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6 hours ago, Purple Headed Warrior said:

Im a bit behind here.... Can someone explain to me this reverse twist? 

Sorry, I mean the ability to twist the head off to leeward to provide lift to windward and therefore righting moment. Maybe someone has a better term.

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It's more like reverse shaping, isn't it? Getting the airfoil shape in the head to pop to windward rather than the normal depth to leeward situation...

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This is a amateur diagram by me to try to show twist at the top of a standard mainsail looking down.

The image on the left is of a standard main and on the right a main with a twisted top in red. The apparent wind has moved forward to reflect the shift of the centre of effort down the sail. This is a depowering strategy?

Twist 1.jpg

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18 hours ago, Kiwing said:

most people here believe the soft sail will be only 10-20% better than a soft sail  and nowhere near the F50 wings.  It follows then that they will not see speeds like the F50 or only in really short bursts when it all come together. 

This is where the thinking totally falls down. It does not follow that because the soft wing is nowhere near as efficient as a solid wing that the AC75's will be slower. They could be quicker. Why? Because the efficiency of the rig is only one factor in the whole equation. If the AC75 has better power to weight ratio, better foils, more righting moment etc, then the less efficient rig won't matter. We need to consider the whole package, not one part of it.

12 hours ago, dogwatch said:

Some people use single-skin paragliders for hike and fly applications, because they are lighter to carry up a mountain. They are however aerodynamically inferior to the standard double-skin models.

I don't think anybody is saying that the double skin rig isn't better than a single skin conventional rig.

14 hours ago, dogwatch said:
19 hours ago, Herfy said:

  But if the dual sail is better than a single sail, why don't we already see them in any real sailing competitions?

Class rules for the valid purpose of cost control.

There are enough development classes around for this to be incorrect. As mentioned before, double skin rigs have been seen in real sailing competitions. To date, their potential hasn't been fully realised. 

People keep reacting as if this is something new, that people know little about it and that there is some huge, untapped potential that nobody has seen. The fact is that this is a well understood, old concept that to date, has never had the money spent on it to realise it's true potential because it would never produce the returns. In the Moths and A's, which have seen these rigs, I would expect it to cost $10,000's to get it right, maybe even into 6 figures. It can be hard enough to make back the cost of development on conventional rigs, never mind complete new concepts. The AC guys will be throwing millions at the problem. For the smaller boats, it simply is uneconomical. 

The other assumption that is well off is that because the AC has chosen to go this way, there must be something everybody else has missed. That is simply untrue. Its like the hard wing rigs. They were very well understood and contrary to what many might think, the AC has probably not advanced their art accept in a very secret area of control systems. The wings used on the top C Class cats are significantly more sophisticated than the AC wings in almost every aspect except those control systems, because the AC rigs were limited by the rule which the C Class is not.

For one last time, let's get it straight. Overall, a double skin rig will be better than a conventional single skin rig and offers the AC boats some significant advantages over the hard wings. The hard wing rig is better again, being able to produce more power when needed and less drag when depowered, but offers some significant disadvantages. While the choice of rig will effect the maximum potential of the AC75, this does not mean it will be slower than the AC50's or the F50's because of many other factors. There will be no way for us to know just how good the soft wings will be because of all the other factors. It will not be possible to say that if the AC75 is faster than the AC50, it has a better rig. 

What I cannot understand is the blind faith shown by mainly ETNZ fanboys which has no basis in anything. It makes no sense

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At normal speeds it would be depowering, but the holy grail not found yet would be for an inverted head to have a windward force benefit against the normal lean of a monohull, and possibly even reductions in drag in the rig and below the water in the foils - flat is fast...

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Sorry I didn't mean to describe all the benefits of the ability to twist the top only a simple idea of what it was after  @Purple Headed Warrior had asked.  I hope he/she got some benefit?

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I also got the apparent wind reason wrong?

I now think it is because the true speed of the wind increases with height above the water?

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13 hours ago, dogwatch said:

Which aerodynamic gurus are saying that?

Some people use single-skin paragliders for hike and fly applications, because they are lighter to carry up a mountain. They are however aerodynamically inferior to the standard double-skin models.

 

Professor Mark Drela of MIT, co-designer of godzilla's wing, designer of the world record holding human powered aircraft and designer/operator of the world human powered watercraft,  says Thin airfoils are capable of the highest CL and CL/CD values, but only within a narrow CL range (or alpha range).....The airfoil has attached flow only in the range alpha = 11-15, or CL = 2.65 - 3.05 , in which the L/D is phenomenal. Above alpha = 15 degrees, the entire surface aft of 45% chord separates suddenly, and below 11 degrees most of the bottom surface separates suddenly, and the drag skyrockets in both cases. So such a thin airfoil is pretty much out of the question on an airplane, even before structural consideration are brought in....But a soft sail allows the possibility of changing the camber of a thin airfoil, which can greatly extend the low-drag range if done appropriately. So a thin airfoil which always has the appropriate camber shape dialed in at any given operating point will in general be superior to a thick airfoil.  Obviously, structural requirements can change the picture entirely. If the airfoil must have a very large bending stiffness, as in the case of a wing or cantilever sail, then the thin airfoil is not an option, and thick airfoils must be used. 

Tom Speer of Boeing, who was also involved in the design of Godzilla's wing, noted on Boat Design Forum that " the notion that because aircraft wings are very efficient and have thick sections, while sails have thin sections and generally lower lift/drag ratios, and therefore a thick sectioned sail will aerodynamically superior to a sail rig with a thin section simply because it is thick, is a mistaken idea. Airplanes have thick sections because they are structurally stronger and because they have to operate efficiently at low lift coefficients in cruise. This is generally not the case for most sailing craft, except for very high-speed craft like landyachts and iceboats" and 'a sail rig can operate at comparatively high lift coefficients even in high winds because it has the luxury of being able to reduce area. This makes the narrower operating range of the thin section acceptable.”

 
Obviously thick sections can work extremely well and may do so in the 75s, but they are not better per se. Even efficiency would have to be defined quite tightly for a real assessment; for example if rigs are unrestricted a taller, lighter single-skin rig may be much faster due to the lower induced drag.
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49 minutes ago, Curious said:

Tom Speer of Boeing, who was also involved in the design of Godzilla's wing, noted on Boat Design Forum that " the notion that because aircraft wings are very efficient and have thick sections, while sails have thin sections and generally lower lift/drag ratios, and therefore a thick sectioned sail will aerodynamically superior to a sail rig with a thin section simply because it is thick, is a mistaken idea. Airplanes have thick sections because they are structurally stronger and because they have to operate efficiently at low lift coefficients in cruise. This is generally not the case for most sailing craft, except for very high-speed craft like landyachts and iceboats" and 'a sail rig can operate at comparatively high lift coefficients even in high winds because it has the luxury of being able to reduce area. This makes the narrower operating range of the thin section acceptable.”

Since when has any AC Cup boat been able to reduce area in high winds?  Everyone knows what the optimum foil and sail shapes are in a given situation but no one knows what the given situation on a racing yacht on foils is.  For that reason having the ability and knowledge to control foils and rigs is everything.

At 7 knots just before foiling the the optimum shape is necessary.  At 35 knots to windward with a 60 knot apparent wind it's all about drag and righting moment and nothing about the best shape.

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"except for very high-speed craft like landyachts and iceboats"

Would seem to apply here

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20 minutes ago, Terry Hollis said:

Since when has any AC Cup boat been able to reduce area in high winds?  Everyone knows what the optimum foil and sail shapes are in a given situation but no one knows what the given situation on a racing yacht on foils is.  For that reason having the ability and knowledge to control foils and rigs is everything.

At 7 knots just before foiling the the optimum shape is necessary.  At 35 knots to windward with a 60 knot apparent wind it's all about drag and righting moment and nothing about the best shape.

AC boats have been able to reduce area since 1851. For many decades they have chosen to change shape instead- but why the insistence that twin skins will do that better?

I don't know how often you have sailed at over 30 knots, but among those of us who have I have yet to meet anyone who claims that it's "nothing to do with the best shape". For a start, having "the best shape" allows you to reduce drag.

 

 

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23 minutes ago, Kiwing said:

"except for very high-speed craft like landyachts and iceboats"

Would seem to apply here

Perhaps. I never said that thick foils could not be better, just that the common assumption that thick foils are inherently better is not accurate.

 

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7 hours ago, Boybland said:

Probably the same gurus who thought arms could generate just as much power as legs I imagine...

No, not the same people at all. 

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1 hour ago, Kiwing said:

"except for very high-speed craft like landyachts and iceboats"

Would seem to apply here

No it does not. There are huge differences between very high speed land yachts and iceboats and the AC boats. Just for starters, they are close to double the speed which makes a significant difference. The other huge difference is the drag they need to overcome. Both weigh a fraction of the weight of an AC boat. The land yacht runs on wheels that have proper bearings, which reduces drag to a negligible point. The ice yachts also have very little drag. 

1 hour ago, Terry Hollis said:

At 7 knots just before foiling the the optimum shape is necessary.  At 35 knots to windward with a 60 knot apparent wind it's all about drag and righting moment and nothing about the best shape.

Sorry, Terry, but this is an ill thought out statement. At all times, optimum shape is everything. If you are after low drag and reduced righting moment, the shape of your sail/rig is how you achieve it. Even if you have the rig bladed out, that is your chosen shape.

 

3 hours ago, Kiwing said:

I now think it is because the true speed of the wind increases with height above the water?

When the wind is above about 5-6 knots, this effect is negligible over the height of a mast. (Read Bethwiate or do a google search on this) What does change is wind angle, producing what sailors refer to as wind shear and which is one reason why we use twist. In apparent wind sailing, the faster you go, the less this is has an effect.

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9 hours ago, Eiar said:

At normal speeds it would be depowering, but the holy grail not found yet would be for an inverted head to have a windward force benefit against the normal lean of a monohull, and possibly even reductions in drag in the rig and below the water in the foils - flat is fast...

Flat is fast - but healed to windward will not only add RM (a real consideration in a class of boat where overall displacement and boat dimensions are so tightly constrained) but also help unload the hydrofoil (allowing it to run less flap and or AOA - thereby operating in a lower Drag setup) - so Healed to windward (upwind) will be Faster.

We have already seen significant numbers of AM sailing with windward heel, to suggest that this will be a standard mode to go to once foiling.

The reasoning for twin skins seems to be that it ticked many if not all the boxes that were sought for. Some of them might include:

  • - It lessened the need for every team to have to dismantle their rigs every day.
  • - It appeased the calls for boats to become boats (as opposed to marinised aircraft) again.
  • - It gave the teams / marine industry a fertile new ground for development that might have commercial potential in a wider sense. 
  • - It is a low drag/high tech solution that fits well with the needs of hydrofoiling, which needs initial rig power for lift off that can be shaped to low drag for high speed - so appeases both Designers & Marketeers.

All these comparisions between and conjucture of this overall solution to existing techology - be that single skin sails or hard wings and everything in between - is Moot.

(Look how much debate went on about the hydrofoils in AC50's - shape, size, control, construction etc - and still how liitle has been revealed - lots of informed commentary - but not a lot of hard data from the original creators - the same will occur here.)

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7 hours ago, A Class Sailor said:

Sorry, Terry, but this is an ill thought out statement. At all times, optimum shape is everything. If you are after low drag and reduced righting moment, the shape of your sail/rig is how you achieve it. Even if you have the rig bladed out, that is your chosen shape.

Perhaps it is ill thought out but the meaning is clear.  The optimum shape at 7 knots is the maximum power that can be extracted in a light breeze this will be a shaped sail.

In strong winds a bladed sailed is not it's optimal shape .. it's the best shape for the circumstances.

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Thanks A-Class Sailor to mention F Bethwaite wind profiles

If you compute AWA & AWS for an  A-Cat sailing at 18 knts windward & 28 knts Downwing at respectively 45° windward and 135° Downwind you will observe:(figures from memory, just as an illustration).

Let's consider following assumptions, just like if all A-Cat Anarchist would have feed an EXCEL database of there respective polar measurements, and with a little quantitative analysis one would have posted a reliable A-Cat Polar (I know it is a little bit masturbation, but we can dream)

True Wind Velocity : 14 knts = 7 m/s   Velocities taken @ 20 feets heigh

Windward @ 45° TWA- Boat speed 18knts=9m/s - AWS 30knts=15m/s - AWA=19.50°

Downwind@135° TWA-Boat Speed 28knts=14m/s -AWS 23knts=11.5m/s- AWA=28.5°

So windward you have +70% power available than downwind

Almost not shear (Twist) windward < 4° But some wind gradient: ie Lower AWS at the trampoline than at the head (probably 70% of AWS at the head)

Downwind you have more shear around 10° and no gradient= same AWS all along the span.

As the AWS is much higher Windward than Downwind for the same TWS, the implication of these observations can provide an explanation for the new elliptical outline of A-Cat sails optimized for foiling compared to fat (squared) head of non-foiling A-Cat.

Windward: AWS is well above than required for an elliptical lift distribution to be optimum, so the lift distribution will be lower loaded up and more loaded down,

Check theory on minimizing induced drag under bending moment constraint.

It is likely to be a Bell-Shaped lift distribution with a CoE around 33% height instead of 42% for an elliptical one.

Then when it come to Downwind, the apparent wind is much lower but you have more shear (twist)and in this case (at least at the designed wind velocity) the same AWS all along the mast put you on the way to get the elliptical lift distribution, as long as you can provide the correct twist in your sail, and therefore achieved the minimum induced drag.

Regarding the ongoing debate Single vs Double skin  or Thin vs Thick, I would like to remind that CFD gurus have explained many times, because above 6 knts true wind, the wind boundary layer above the water is already fully turbulent (See F. Betwaite's Book)

So in this turbulent environment, the double skin solution (the thick one) would just

1-reduce power because the thicker is your asymetric section, the lower is the mean camberline of the wing section, and until separation, the higher the camberline, the higher the power.

2- Increase section drag (which is proportional to thickness, in fully turbulent conditions)

But as I remember what an A-Cat  Australian champion explained to me, at the Euro-Cat in Carnac more than 10 years ago when he used to win F 18 World championships with his buddy.

It was about A-Cat rig's drag, and AFAIR the leeward drag of the less than perfect hinge between mast and sail was considered as increasing dissipation, and dissipation is proportional to wind velocity ^3 (Please remember my assumption : AFAIR )

So the double skin is likely to address this issue, but I am not able to run any calculations and demonstrate it, while it is college level to demonstrate the higher power and lower drag of deck sweeping sails vs gaping sails.

So you can see the design option is a complex trade-off, involving so much parameters that its main virtue, is to prompt us to put our brain at full use, maintaining a correct level on the forum.

Happy Sunday

Erwan

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18 hours ago, NeedAClew said:

The ETNZ control system design was so superior to OTUSA's.  I was astonished that, for being so "advanced" in technology Oracle kept inside that mental box.  

Why are you so astonished? Otusa had a non foiling 72 and barely caught up (and wouldn’t have if etnz didn’t fly their way home in nz one day.   They then thought that their shared design with their challengers would be enough for them to win on the merits of their spend, and etnz with little budget and time focused on hydro and control systems and spanked them.

otusa made one breakthrough in their entire time in the cup.  They picked the wing on 17 after breaking their big ass mast and wing boxed into a corner.

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4 hours ago, Erwankerauzen said:

Thanks A-Class Sailor to mention F Bethwaite wind profiles

If you compute AWA & AWS for an  A-Cat sailing at 18 knts windward & 28 knts Downwing at respectively 45° windward and 135° Downwind you will observe:(figures from memory, just as an illustration).

Let's consider following assumptions, just like if all A-Cat Anarchist would have feed an EXCEL database of there respective polar measurements, and with a little quantitative analysis one would have posted a reliable A-Cat Polar (I know it is a little bit masturbation, but we can dream)

..........

So you can see the design option is a complex trade-off, involving so much parameters that its main virtue, is to prompt us to put our brain at full use, maintaining a correct level on the forum.

Happy Sunday

Erwan

Thanks, but that was slightly over my head....

 

 

jet2.gif

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9 hours ago, Boink said:

Flat is fast - but healed to windward will not only add RM (a real consideration in a class of boat where overall displacement and boat dimensions are so tightly constrained) but also help unload the hydrofoil (allowing it to run less flap and or AOA - thereby operating in a lower Drag setup) - so Healed to windward (upwind) will be Faster.

We have already seen significant numbers of AM sailing with windward heel, to suggest that this will be a standard mode to go to once foiling.

The reasoning for twin skins seems to be that it ticked many if not all the boxes that were sought for. Some of them might include:

  • - It lessened the need for every team to have to dismantle their rigs every day.
  • - It appeased the calls for boats to become boats (as opposed to marinised aircraft) again.
  • - It gave the teams / marine industry a fertile new ground for development that might have commercial potential in a wider sense. 
  • - It is a low drag/high tech solution that fits well with the needs of hydrofoiling, which needs initial rig power for lift off that can be shaped to low drag for high speed - so appeases both Designers & Marketeers.

All these comparisions between and conjucture of this overall solution to existing techology - be that single skin sails or hard wings and everything in between - is Moot.

(Look how much debate went on about the hydrofoils in AC50's - shape, size, control, construction etc - and still how liitle has been revealed - lots of informed commentary - but not a lot of hard data from the original creators - the same will occur here.)

I would add to this that there also may be an effective high wind fast mode that uses normal heel to produce downforce from the rig, which also effectively increases RM. Such a mode would also add insane amounts of load on the rotating arm, but that is an inherent risk of this design, since the entire platform/rig can can be rotated at will around the X-axis relative to any given foil position.

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7 hours ago, Loose Cannon said:

Why are you so astonished? Otusa had a non foiling 72 and barely caught up (and wouldn’t have if etnz didn’t fly their way home in nz one day.   They then thought that their shared design with their challengers would be enough for them to win on the merits of their spend, and etnz with little budget and time focused on hydro and control systems and spanked them.

otusa made one breakthrough in their entire time in the cup.  They picked the wing on 17 after breaking their big ass mast and wing boxed into a corner.

Well, ultimately I was astonished because I had been drinking the red Kool-Aide.  Eg, 

 https://www.forbes.com/sites/oracle/2017/02/16/new-oracle-team-usa-boat-an-engineering-marvel-data-machine/amp/

With that terabyte of data and Exadata data machines and such, they stick with a camo wheel (see YouTube consultant video) ...complacency.  To paraphrase, if what you have is a shitload of money, throwing money at it looks like the answer to every problem. 

New Zealand's control system with separation of controllers was just superior. Hats off to them.

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15 hours ago, Terry Hollis said:

In strong winds a bladed sailed is not it's optimal shape .. it's the best shape for the circumstances.

I think that's something of a tautology. :-)

In regard to the rotating mast and sail twist, you can also use the apparent wind angle as a reference and hold the mast always facing the apparent wind (i.e. 0° rotation). Then consider the boom to be inducing twist to windward (positive rotation), which produces an aerodynamic wing shape and positive AoA. The dual skin just means a more aircraft–like shape that can be controlled (at the foot at least) with outhaul. Other controls and battens help to carry the shape up the sail.

A similar mechanism can be used in the upper zone using battens instead of a boom. The number of battens in the upper zone is unlimited and there can be controls that extend to about 900mm from the mast, so additional battens can act like as a series of booms to tightly control the head shape, including twisting to leeward (call it negative twist where 0° twist is aligned with the apparent). So generating lift to windward isn't hard to imagine. The consequences for drag need to be considered, and there is a region of transition between +ve and -ve twist.

It seems to me that given the control available from flapped foils, the boats can use a small amount of windward heel to gain equivalent RM and have the top of the sail generating power, or at least bladed, and not being used for RM. To me that's the biggest advantage the monos have over the AC50s and 72s and why they have the potential to be faster.

Is anyone considering the type of vang/gnav these boats will have? That seems to be a pretty critical control for sail shape.

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Here's how I understand getting RM from the head. The lines represent the AOA of the sail, longer at the bottom and shorter at the top, black "+ve" and red "–ve". 

Twisting off to leeward beyond the apparent wind gives lift to windward. At least that's how I understand the theory. Without suitable control devices, the head will just flap. But with some internal structure, it can likely be quite effective. The top of the sail doesn't need much shape, it might be roughly symmetric.

1181530284_Sailtwist.thumb.png.5bb4a91f31ee3278807233fd84ff4810.png

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Thank you everyone for your great contributions to this interesting thread.

Smoke is coming out my ears as I try to process all.

How about this as try at twist?

Twist 2.jpg

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20 hours ago, Herfy said:

Thanks, but that was slightly over my head....

Sorry Herfy, I hope it did not spoil your week-end

To make it simple: For fast & Foiling boats: I mean It is all about drag

with (V=apparent wind speed)

1-The Section &Friction Drag is proportional to V square:  FD=function(V^2)

2-The Induced Drag is inversly proportional to  V square   ID=function(1/V^2)

3-The Drag from Dissipation is proportional to V cubed     DD=function(V^3)

As Far As I understood.

Just for the rig, I guess the optimization process is far from trivial.

Cheers

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924919434_Twist4.jpg.5436ef1bdb63aa0d05cbf2cbe38e2314.jpg With the mast pointed at the apparent wind the top can be reversed.

Can the soft sail be made with a higher aspect ratio like the AC 50 wing?

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If you dig into how AWS get their SRW get it to work then, (apologies for acronyms - but if you have read the Sail World stuff and or are familiar with AWS stuff from beforehand then you will have no issues)  the biggest query comes from the Mast rotation issue.

Counterintuitively, the AWS employes a system whereby the mast is rotated to leward from centreline to induce camber. Read that again - they rotate the mast to leeward of whatever tack they are on. This adds camber to leeward and reduces camber on the windward skin. 

The AWS mast profile is somewhat square faced and this means that the stall point on its face shifts to the weather corner that faces the apparent wind.

Then by allowing leech slip the amount of camber can be fine tuned to allow for overall rig twist or power requirements. 

The issue with respect to the AC design is that the mast profile is a strict OD prescribed D section (which can then be modified by each team but only in specific areas - see attached diagram)

So taking the shape of the mast profile as per the AC rule, not only does the mast section not allow this rotation to leeward whilst keeping the stall point working favourably, but all the photos of AMAC in operation show a rotation of the mast to the windward axis - to keep the stall point on the correct loctation of the curved D surface. If the AC mast was rotated to leeward the leeward surface would show ugly kinks in its surface at both aft corners of the mast where it becomes sail - that would be far from optimal.

The AMAC mast does rotate - but to windward on each tack and the spreader attachments do allow this - and has been discussed at length here (earlier in this forum) and elsewhere. So what elements of AWS are AMAC utilising? 

The amount of photos are not comprehensive - nor super detailed - to be expected at this stage in the cycle.  But the photos do not suggest that the luffs of each skin are being moved forward and back to induce or remove camber in the manner that the complete AWS solution would require. So is it their Batten slip solutions that are being utilised?

Have they developed some form of thin batten connection (read low profile or whatever terminology you choose) - that preserves a fine trailing edge to the two sail skins, allowing the two sets of battens to slide back and forth over themselves, and also be positively connected to each other?

We do know that on the twin skin setup that each batten location is mirrored exactly on both skin surfaces. If the battens are postively connected then why?

The apparent wind is always from ahead on these boats - so there is little chance of the wind blowing the two elements apart. Locking the two elements together may have flow on benefits in transfering shape or leech tension aross both skins. Certainly insuring the 2 skins cannot blow open would be handy at removing likelyhood of accidental damge or revealing any batten control technology secrets held within the two skins. But most simply and likely, by locking the two batten ends together in a sliding mechanism, allows one to be pulled against the other. 

I have alluded to the possibility of batten control from the outset - I think that on the balance of probablity - those that can finely control both surfaces using tension or compression, using a combination of slides, levers and string control - will outperform those that can't.

These teams have the might of Airbus and the like, to implement thinking, engineering and carbonology at levels that would leave us slack jawed and impressed. 

slwfa18_stl_technology-ferguson-3.jpg

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32 minutes ago, Boink said:

I have alluded to the possibility of batten control from the outset - I think that on the balance of probablity - those that can finely control both surfaces using tension or compression, using a combination of slides, levers and string control - will outperform those that can't.

Everybody knows that the key will be the allowed controls at the top and bottom of the mainsail, because that is the only area where you are allowed to control the sail. The rules specifically ban the control of the 12 battens (6 full and 6 part length) that are allowed outside of the mast upper zone (4000mm from the top) and the mast lower zone (bottom 1200mm from bottom). In other words, the battens in 80% of the sail cannot be controlled. Or have I missed something.

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They can be connected to controlled battens top and bottom (vertically), right?

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1 hour ago, A Class Sailor said:

Everybody knows that the key will be the allowed controls at the top and bottom of the mainsail, because that is the only area where you are allowed to control the sail. The rules specifically ban the control of the 12 battens (6 full and 6 part length) that are allowed outside of the mast upper zone (4000mm from the top) and the mast lower zone (bottom 1200mm from bottom). In other words, the battens in 80% of the sail cannot be controlled. Or have I missed something.

I have already answered you about this issue (in my posting #284). But in case you have forgotten:

  On 1/8/2019 at 5:05 PM, 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.....

 

So, Yes; I think you have missed something.

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7 hours ago, Varan said:

As I remember it, the Moth sail was effectively a single skin wrapped around a round mast and funky boom (not pictured anywhere) to create the shape. I think a few people tried it (not me, I was much lower than mid–fleet…) but they found it over powered and difficult to control. The consensus was that the theory had potential, but needed more development and practice. That means getting someone toward the pointy end of the fleet to commit to learning how to use it and assist in refinement.

I'm with Curious on the sports boat claims: if it's such a weapon, why isn't anyone using them (AWS dual skins)?

It may well be that the key to making it work better than a pocket luff or wing mast + single skin is the head control, which means it might trickle down to racing yachts and sports boats but not cruising boats or dinghies.

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On 1/11/2019 at 9:21 PM, RobG said:

Sorry, I mean the ability to twist the head off to leeward to provide lift to windward and therefore righting moment. Maybe someone has a better term.

Thanks Rob.... That makes sense and impressive

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5 hours ago, Boink said:

Have they developed some form of thin batten connection (read low profile or whatever terminology you choose) - that preserves a fine trailing edge to the two sail skins, allowing the two sets of battens to slide back and forth over themselves, and also be positively connected to each other?

We do know that on the twin skin setup that each batten location is mirrored exactly on both skin surfaces. If the battens are postively connected then why?

The apparent wind is always from ahead on these boats - so there is little chance of the wind blowing the two elements apart. Locking the two elements together may have flow on benefits in transfering shape or leech tension aross both skins. Certainly insuring the 2 skins cannot blow open would be handy at removing likelyhood of accidental damge or revealing any batten control technology secrets held within the two skins. But most simply and likely, by locking the two batten ends together in a sliding mechanism, allows one to be pulled against the other. 

I have alluded to the possibility of batten control from the outset - I think that on the balance of probablity - those that can finely control both surfaces using tension or compression, using a combination of slides, levers and string control - will outperform those that can't.

My understanding is that batten connections are more or less unrestricted in the upper and lower zones, so interlocking sliders and other devices are possible to control the leeches and sail shape.

Between the zones, battens can't be directly controlled or adjusted (while sailing) at all. Further, they can only connected by tethers within 0.4m of the luff or leech that can't transmit compressive force (so effectively strings or straps) and can't be adjusted (presumably while sailing). These tethers seem to be only to help with handling the sail.

20.23 A batten on one sail skin may be connected to a batten on another sail skin provided the connection is within 0.400 m of the leech or luff of a sail skin, within the mast lower zone, or within the mast upper zone. In addition to these connections, tethered connections between battens, or batten pockets, are permitted anywhere provided such connections are no longer than 600 mm and cannot take compressive forces. Except for battens that are entirely within the mast lower zone or mast upper zone, connections between battens shall not be adjusted.

It's ambiguous whether the first sentence means connections in the lower zone must also be within the 400mm limit, but "or" means connections in the upper zone aren't and are therefore unrestricted.

 

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Sail Shape control Devices inspired from Biomimetic

A Professor at Berlin University has been working for decades on biomimetics.

Just try to googleize the following key words.

Gripper inspired from fin ray

Could be a starting point for wing section control

Cheers

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Thanks Erwan, looks very interesting and we fish alot so I am interested in exploring this natural effect.

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4 hours ago, Kiwing said:

1540151634_softsailcrosssectioon.jpg.978ab2a7a34245f59a784edd59e57bcf.jpgIs this a valid comparison ?

The wing will be shaped something like that while they are not foiling, as soon as they start foiling there will be a huge increase in the apparent wind speed so they will need to work within the limits imposed by the righting moment which means they start feathering from the top down.

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11 hours ago, Terry Hollis said:

The wing will be shaped something like that while they are not foiling, as soon as they start foiling there will be a huge increase in the apparent wind speed so they will need to work within the limits imposed by the righting moment which means they start feathering from the top down.

I'll have to investigate if at slower speeds the "mule" sail above is better.

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1111.jpg.0d7a8e5c1bffe35bec9dadc25f648658.jpg

From fish fins apparently. When you apply a force to "structure A" fixed at the bottom it distorts unusually as in "structure B".  @Erwankerauzen showed the hint to the research above.

I wonder if it can and will be used in the new soft wing?

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18 hours ago, Kiwing said:

1540151634_softsailcrosssectioon.jpg.978ab2a7a34245f59a784edd59e57bcf.jpgIs this a valid comparison ?

No! A sailboat wing has to do so much more, and changes shape to generate more or less lift as needed and moves the position of the centre of effort of the wing along the wing's length as needed. They operate at different speeds with the glider's lower end being the top end for a sailboat while the top end is significantly higher. What you have done is to grab a single moment in time for one part of the sailboat wing and equated it to a glider. You need to consider the other operating conditions. Also, because what is required from each wing is so different, you cannot say that because one particular wing is best for a particular application (glider), it is the best wing for another (sailboat).

I still cannot get over the blind faith that the soft wing must have nearly as good if not better performance than the solid wings used in the previous AC's. There seems to be a belief that it wouldn't have been chosen if that wasn't so. Why the fixation? Why the need for it to be true? Isn't it enough that they have chosen a rig that will out perform a conventional soft sail rig without the problems of the solid wing. Why does it need to be close to the performance of a solid wing? 

 

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The optimum speed for  glider is 40 to 50 knots for the slowest sink rate.

11111.jpg.c51957d20a7187a683e338c7bd9e76cd.jpg

That is why I have chosen a glider wing.

I am sure you require a different cross section for the getting onto foils and maybe the mule sail shape will be better for that short period (20%? of race time).  If so I think they will change the shape during the "take-off" time.

@A Class Sailor I have no blind faith that the soft wing will be as good as the AC35 wing.  But I do believe that it will be a lot better than a soft single skin sail, maybe half way between.  I know most of the reason ETNZ chose this design was to get sail makers more into the game and to increase the trickle down benefits from the AC but I also believe there is some secret knowledge from AC35 which will enable the Teams to produce a better sail design.

I also think the history of this design is far from complete development of this idea.

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6 hours ago, Kiwing said:

I have no blind faith that the soft wing will be as good as the AC35 wing.  But I do believe that it will be a lot better than a soft single skin sail, maybe half way between.

Well, at least we are making progress and you are reducing your expectations.

On 1/10/2019 at 8:43 AM, Kiwing said:

I believe;

this soft sail set up will be at least  70% of the slotted AC34 wing.  This is at odds with some experts but then so were the cycles.  And this is at least twice as good a a single skin sail.

  It would not surprise me if Glen has the goal of getting close to the performance of the AC34 wing.  But once again some experts will scoff at that dream!

You started with 70%+ and are now at 50%. I will stick with my 35-40% but at least I can now just call you optimistic rather than delusional ;):D

 

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When looking closely at the wing sections presented above, one can seee that the maximum thickness of the glider's wing is well aft and remains almost constant from the mast to 60% of the chord.

It looks like a "Laminar" wing section which seek to maintain the leeward side's Boundary layer laminar as long as possible.

But it can happen only because the glider is flying high in the sky ,in an unperturbated air flow.

Unfortunatly the wingsail is working in the boundary layer of the air above the water and it will not achieve any laminar rooftop on the leeward side.

The same wing section would have different Lift & Drag coef in these 2 different environments.

CFD Gurus suggest, using XFOIL,  to use:

-NCrit=9 and free transition parameters for the glider

-NCrit=1 and/or Forced Transition @ 0.05 (5% chord) for the wingsail.

Happy Week-end 

 

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So ignorant question I will admit to: for a given foil configuration, does wing vs soft sail change foiling liftoff speed?  Second, what speed might that be for AC75? 

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@NeedAClew My guess is that the softwing will have better Power down low speed and hence achieve and earlier lift off.  However changing shape to match the quickly changing apparent wind will be the challenge.

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4 hours ago, NeedAClew said:

So ignorant question I will admit to: for a given foil configuration, does wing vs soft sail change foiling liftoff speed?  Second, what speed might that be for AC75? 

 

3 hours ago, Kiwing said:

@NeedAClew My guess is that the softwing will have better Power down low speed and hence achieve and earlier lift off.  However changing shape to match the quickly changing apparent wind will be the challenge.

If only it were that simple. If you had a soft wing and a hard wing of the same area, the hard wing produces a lot more power in light winds and would get the boat foiling significantly earlier. But that is only a small part of the story. Most "experts" are saying the 75's will start foiling in more breeze than the 50's but I am not so sure because while a hard wing might be more efficient and develop more power in lighter winds, there is still no substitute for rig size and righting moment. I am going to guess at how much wind is needed to achieve take off. 8 knots downwind and 11-12 knots upwind.

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5 hours ago, NeedAClew said:

So ignorant question I will admit to: for a given foil configuration, does wing vs soft sail change foiling liftoff speed?  Second, what speed might that be for AC75? 

I will have no effect on the lift–off boat speed other than might result from a difference in weight. I have no idea what that is, but I suspect a dual skin rig will be heavier than a comparable wing (height and sail area), but maybe not enough to matter.

It might affect the windspeed required, but that would also be dependant on the windspeed the rig is designed to operate in.

Comparison of the performance of rigid wing and dual skin sails at this point is speculation that will disappear down a rabbit hole of ifs, buts and maybes because there are zero real–world examples of this specific dual skin sail. And it will continue to be an unknown until the two face off in comparable boats and conditions (unless someone who knows what they're talking about produces some comprehensive modelling results to show the differences).

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1 hour ago, A Class Sailor said:

 

I am going to guess at how much wind is needed to achieve take off. 8 knots downwind and 11-12 knots upwind.

 

IIRC the gist of the initial AM simulation, refined by Basiliscus, was that an AC75 will take off (distinct from staying on the foils) at around 16 kts boat speed - provided the load is equalized between leeward and windward foils. This displacement boat speed should be achieved with 13 kts TWS on a beam reach, and never going upwind

With weight concentrated on the leeward foil, take off boat speed goes up in excess of 20 kts. 

I haven’t seen different, more recent simulations

 

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2 hours ago, Xlot said:

 

 AC75 will take off (distinct from staying on the foils) at around 16 kts boat speed - provided the load is equalized between leeward and windward foils. This displacement boat speed should be achieved with 13 kts TWS on a beam reach, and never going upwind

With weight concentrated on the leeward foil, take off boat speed goes up in excess of 20 kts.

Do you think or have you read somewhere that in displacement mode these boat can reach 16kts with 13 kts TWS ?

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TC remember AC34 those boats were not going to foil just foil assist!

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On 7/3/2018 at 6:00 PM, Kiwing said:

I am thinking this new engine is not nearly as powerful or controllable as the AC35 wing.

Being over 70 and the highest performance boat I have sailed is a paper tiger cat my opinion is hardly up there.

However I am in software and am aware of how good simulation software is these days so I am speculating partly on the figures put out by ETNZ from their simulations.

Does any one want to discuss this (preferably without character assassination!)

I remember Dennis Conner experimenting with a twin membrane genoa in the 1987 AC. We also experimented with twin skin windsurfer sails, and someone actually went in to production with one in the 90s. I built such a mainsail for my J-22 in the early 90s. It showed real promise in light winds, but was ultimately unwieldy and too complex to manage in heavy air. Of course, my resources were limited at the time, and the demands of running a sailmaker and canvas shop didn't leave much time for further development. Maybe they can make it work.

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3 hours ago, Remodel said:

I remember Dennis Conner experimenting with a twin membrane genoa in the 1987 AC. We also experimented with twin skin windsurfer sails, and someone actually went in to production with one in the 90s. I built such a mainsail for my J-22 in the early 90s. It showed real promise in light winds, but was ultimately unwieldy and too complex to manage in heavy air. Of course, my resources were limited at the time, and the demands of running a sailmaker and canvas shop didn't leave much time for further development. Maybe they can make it work.

@Remodel with some sort of internal structure, a hydraulic control system which can produce changeable winglike shapes, and an reverseable top is 70% of the AC50 wing possible?  The new sail cloths and membranes they have these days will help tremendously wont they?  Am I "delusional" as @A Class Sailor suggests?
Looking at the "Mule" cross section and the "Glider"  cross section I posted earlier can you help with interpreting @Erwankerauzen's XFoil post? which is too technical for me, please.

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