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The new sailing twin skin setup


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4 hours ago, nav said:

Watching the stern cam today/yesterday it appeared that there was virtually nothing going on in the lower section of the LR sail. Maybe I don't know what I'm looking for but it might as well have been 'set and forget'. Will this cost them when they encounter the big dogs, or is it the right, simple and elegant way to go?

It seems unfathomable to me that they would not be shaping the lower sections.

Are you sure?

If so, it's hard to see them sticking with the "big dogs".

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On 11/27/2020 at 2:01 PM, Kiwing said:

looking at the mast head control lever, too.

b11.thumb.jpg.85e4fa4fca333eae53e89969e87c32b1.jpg

rotating around that black dot pivot?

Ineos seem to have the capacity to do either reverse, symmetric (either AOA), or as here providing more power

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4 hours ago, Kiwing said:

Ineos seem to have the capacity to do either reverse, symmetric (either AOA), or as here providing more power

That is the point I d love to address,

While this double skin sail is probably less powerfull and a bit more draggy than the 3 elements wing sails, I have the intuition that the intention of the design team, when elaborating the rules for the mainsail;  was to provide an opportunity for competitors to suss out a system which can address more efficiently the management of the pressure distribution along the span, than a classic sail/mast package.

This assumption is supported by the bottom part + upper part of the twin skins , dedicated for control mecanisms.

Other assumption ?

Thank  You Basilicus for the 2 reminders, my post was very candid, and I fell confident ERDB & MAX will put them at full use.

Happy Sunday 

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39 minutes ago, Erwankerauzen said:

While this double skin sail is probably less powerfull and a bit more draggy than the 3 elements wing sails, 

2 element actually (1 slot)

 

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5 hours ago, Erwankerauzen said:

While this double skin sail is probably less powerfull and a bit more draggy than the 3 elements wing sails,

Happy Sunday 

@Erwankerauzen Thank you for saying "probably"

And for all your other thought provoking posts.

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6 hours ago, Erwankerauzen said:

That is the point I d love to address,

While this double skin sail is probably less powerfull and a bit more draggy than the 3 elements wing sails, I have the intuition that the intention of the design team, when elaborating the rules for the mainsail;  was to provide an opportunity for competitors to suss out a system which can address more efficiently the management of the pressure distribution along the span, than a classic sail/mast package.

This assumption is supported by the bottom part + upper part of the twin skins , dedicated for control mecanisms.

Other assumption ?

Thank  You Basilicus for the 2 reminders, my post was very candid, and I fell confident ERDB & MAX will put them at full use.

Happy Sunday 

Some months back, one of the designers from LR (IIRC) said that the twin skin sails were not far behind the solid wing sails.

I'd speculate that may be partially because the twin skin can progressively twist the sail, something the solid wing obviously cannot do.

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6 hours ago, Erwankerauzen said:

...While this double skin sail is probably less powerfull and a bit more draggy than the 3 elements wing sails, I have the intuition that the intention of the design team, when elaborating the rules for the mainsail;  was to provide an opportunity for competitors to suss out a system which can address more efficiently the management of the pressure distribution along the span, than a classic sail/mast package....

You've got me thinking along a different line with regard to the double-luff soft sail.  The multi-element section has a higher maximum lift than a single element section, which is why they are dominant in the C Class catamarans that are under-canvassed downwind.  But in the AC boats, a lack of sail area hasn't been the problem and they only need to operate near maximum lift when sailing downwind in very light air.  So the maximum lift benefit of the slotted flap isn't so important.

Where the double-luff main may have an advantage is not that it is thick, but that it is thinner than a rigid wingsail.  The main element of a rigid wing is necessarily convex on both sides because of symmetry.  But a soft sail is concave on the windward side, reducing the thickness considerably.  When the boundary layer is essentially turbulent over the entire surface, the profile drag is not very sensitive to the section shape, but depends most strongly on the thickness.  And there is a modest drag penalty for having the slot because the higher velocities near the flap leading edge have more skin friction than the corresponding location on a single element section.  So I think the basis for comparison of the double-luff mainsail is not the single-luff main, but the rigid wingsail that is operated in the low to medium lift range.

The other area where the rigid wingsail excels is control of the twist.  The AC72 wingsail flaps could be twisted as much as 40 deg from foot to head, and the twist was controlled at several locations along the span.  This allowed for precise control of twist, and the twist profile could be varied.  For example, one of the changes made in San Francisco to the OTUSA boat was to use more flap deflection in the mid leech (less twist) and let the head twist off more.  It's pretty hard to make that kind of change in a soft sail, where the leech shape is controlled by tension and the twist is only controlled at the foot and head. 

With most single-luff mainsails, leech tension is the only means of controlling twist.  It would be interesting to compare a single-luff sail that had the same kind of twist control at the head with a double-luff sail and with a rigid wingsail.  

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9 minutes ago, nav said:

Sure they can, but you're new here maybe you missed it....

hint: they aren't really 'solid'

I am aware they have sections which they can depower, which is why I emphasised progressively .

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12 minutes ago, MaxHugen said:

...I'd speculate that may be partially because the twin skin can progressively twist the sail, something the solid wing obviously cannot do.

Not so.  The twist of the rigid wingsails is continuously varied along the span.  When Artemis first started sailing their AC50, they had discrete twist with a discontinuity between the flap segments.  They soon found this was draggy, because there is a vortex shed at each discontinuity in the spanwise lift distribution, and they had to revise their design to eliminate the steps.

The OTUSA AC72 wingsail had a control arm at the top of each flap, with a pin that engaged a slot in the neighboring flap's trailing edge.  This ensured the twist was continuous across the junction between the flap segments.  Each flap segment was twisted by the difference in the control arm positions for that segment and the preceding segment.

It also nearly led to disaster in SF.  Minutes before the last race, the pocket in which one of the pins rode broke.  It was looking like OTUSA might have to retire from the race because they wouldn't be able to properly control the flap.  A shore team crewmember was sent aloft with some fast-setting glue, managed to get it stuck back together and got off the boat in time for them to make the start.  But the whole race, Jimmy Spithill was half expecting it to let go at any time.  That's why, when asked when he knew they had the race won he replied, "When we crossed the finish line!"  They could have lost it at any time.

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10 minutes ago, Basiliscus said:

Not so.  The twist of the rigid wingsails is continuously varied along the span.  When Artemis first started sailing their AC50, they had discrete twist with a discontinuity between the flap segments.  They soon found this was draggy, because there is a vortex shed at each discontinuity in the spanwise lift distribution, and they had to revise their design to eliminate the steps.

The OTUSA AC72 wingsail had a control arm at the top of each flap, with a pin that engaged a slot in the neighboring flap's trailing edge.  This ensured the twist was continuous across the junction between the flap segments.  Each flap segment was twisted by the difference in the control arm positions for that segment and the preceding segment.

It also nearly led to disaster in SF.  Minutes before the last race, the pocket in which one of the pins rode broke.  It was looking like OTUSA might have to retire from the race because they wouldn't be able to properly control the flap.  A shore team crewmember was sent aloft with some fast-setting glue, managed to get it stuck back together and got off the boat in time for them to make the start.  But the whole race, Jimmy Spithill was half expecting it to let go at any time.  That's why, when asked when he knew they had the race won he replied, "When we crossed the finish line!"  They could have lost it at any time.

OK, I didn't know the flaps could be twisted like that.

Why were they in segments, rather than one piece?

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2 hours ago, MaxHugen said:

OK, I didn't know the flaps could be twisted like that.

Why were they in segments, rather than one piece?

The hinge line was curved.  And that was because the spar was straight.

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1 hour ago, Wandering Geo said:

Weren't the Nzers twisting the spar as well in the AC72 (like the C class)

They did for their first wingsail. I'm not sure if their second had a twisting main element or not.

OTUSA tested a twisting main element on their AC45, but it wasn't as fast as the stock wing.  Part of the reason was a twisting main element requires a separate spar with a shell around it.  This isn't as stiff in bending as a D tube structure.  Based on the AC45 tests, they elected not to go with a twisting main element on their AC72.  They were able to get all the lift variation they needed by twisting the flap.

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2 hours ago, Basiliscus said:

They did for their first wingsail. I'm not sure if their second had a twisting main element or not.

OTUSA tested a twisting main element on their AC45, but it wasn't as fast as the stock wing.  Part of the reason was a twisting main element requires a separate spar with a shell around it.  This isn't as stiff in bending as a D tube structure.  Based on the AC45 tests, they elected not to go with a twisting main element on their AC72.  They were able to get all the lift variation they needed by twisting the flap.

That was my recollection, dropped for the 45s and 52s as there flap twist was enough even downwind due to the much higher speeds

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On 1/29/2021 at 10:28 PM, Mikko Brummer said:

Finn inverse.jpg

Thanks to Mikko's masthead Finn pic and explanations, I'm starting to understand how the profile might look in the upper section of the AC75 mainsail.  I tried this in XFoil, and yes! - it produces quite a bit of "negative" lift, and even has a decent L/D ratio.  However, I didn't get too adventurous and force the foil shape to include the actual angle of the mast:

image.png.5c78990ab5f75d37f7e63235f8f4235c.png

Mikko described how they add a lot of cunningham force to do this, I'm wondering if a fair bit of leech tension is also required to get this "S" shape?

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Well, well, well, the "S' shape.

How amazingly interesting these twin sails are, and to think Glen has a "suck it and see" playing field to play on too!

Thank you @Mikko Brummer for sharing some of your magic.  Thank you @MaxHugen for exploring and sharing.

I just believe!!

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13 hours ago, Basiliscus said:

You've got me thinking along a different line with regard to the double-luff soft sail.  The multi-element section has a higher maximum lift than a single element section, which is why they are dominant in the C Class catamarans that are under-canvassed downwind.  But in the AC boats, a lack of sail area hasn't been the problem and they only need to operate near maximum lift when sailing downwind in very light air.  So the maximum lift benefit of the slotted flap isn't so important.

Where the double-luff main may have an advantage is not that it is thick, but that it is thinner than a rigid wingsail.  The main element of a rigid wing is necessarily convex on both sides because of symmetry.  But a soft sail is concave on the windward side, reducing the thickness considerably.  When the boundary layer is essentially turbulent over the entire surface, the profile drag is not very sensitive to the section shape, but depends most strongly on the thickness.  And there is a modest drag penalty for having the slot because the higher velocities near the flap leading edge have more skin friction than the corresponding location on a single element section.  So I think the basis for comparison of the double-luff mainsail is not the single-luff main, but the rigid wingsail that is operated in the low to medium lift range.

The other area where the rigid wingsail excels is control of the twist.  The AC72 wingsail flaps could be twisted as much as 40 deg from foot to head, and the twist was controlled at several locations along the span.  This allowed for precise control of twist, and the twist profile could be varied.  For example, one of the changes made in San Francisco to the OTUSA boat was to use more flap deflection in the mid leech (less twist) and let the head twist off more.  It's pretty hard to make that kind of change in a soft sail, where the leech shape is controlled by tension and the twist is only controlled at the foot and head. 

With most single-luff mainsails, leech tension is the only means of controlling twist.  It would be interesting to compare a single-luff sail that had the same kind of twist control at the head with a double-luff sail and with a rigid wingsail.  

"With most single-luff mainsails, leech tension is the only means of controlling twist." Actually, not so... For one, sideways bend of the mast will induce some "non-linear" twist into the sail - the sail "twists" from the luff as well as the leech. The leech twist is normally close to linear unless there is much roach and "batten poke" with it. In the Finn, the mast is super stiff in lateral up to some 75% of the luff, and then goes very soft in the last 25%, dipping the masthead to leeward in gusts and waves. 

Then there's the cunningham - by putting lots of tension, you can add significant compression into the mast and increase its bend in the upper part. At some point, all the luff curve cut into the sail will be all used, and the upper leech will twist off dramatically, while the lower leech will hold on. With the correct combination of mast bend & luff curve, you can tailor exactly when this happens as the wind picks up.

This works with normal, short leech battens (like the Finn), but with full-length battens, the effect is even more dramatic. A good example is the windsurfer sails, that have a 8:1 purchase cunningham and completely bend the mast already ashore by cunningham tension. With the tension on the luff, a normal fold along the luff tries to form, but with the full-length battens, it cannot and instead the battens push very strong on the mast bending it into the curve cut into the luff of the sail. 

This works perfectly for the AC75 as well, luckily they have 2 sails and hydraulic rams to push on the (stiff D-profile) mast to bend it & flatten the sail. You could see the importance & the effect on INEOS, when they jammed their cunningham ram, set the sail in rather depowered-mode for upwind, and could not power up downwind in that race.

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12 minutes ago, Mikko Brummer said:

"With most single-luff mainsails, leech tension is the only means of controlling twist." Actually, not so... For one, sideways bend of the mast will induce some "non-linear" twist into the sail - the sail "twists" from the luff as well as the leech. The leech twist is normally close to linear unless there is much roach and "batten poke" with it. In the Finn, the mast is super stiff in lateral up to some 75% of the luff, and then goes very soft in the last 25%, dipping the masthead to leeward in gusts and waves. 

Then there's the cunningham - by putting lots of tension, you can add significant compression into the mast and increase its bend in the upper part. At some point, all the luff curve cut into the sail will be all used, and the upper leech will twist off dramatically, while the lower leech will hold on. With the correct combination of mast bend & luff curve, you can tailor exactly when this happens as the wind picks up.

This works with normal, short leech battens (like the Finn), but with full-length battens, the effect is even more dramatic. A good example is the windsurfer sails, that have a 8:1 purchase cunningham and completely bend the mast already ashore by cunningham tension. With the tension on the luff, a normal fold along the luff tries to form, but with the full-length battens, it cannot and instead the battens push very strong on the mast bending it into the curve cut into the luff of the sail. 

This works perfectly for the AC75 as well, luckily they have 2 sails and hydraulic rams to push on the (stiff D-profile) mast to bend it & flatten the sail. You could see the importance & the effect on INEOS, when they jammed their cunningham ram, set the sail in rather depowered-mode for upwind, and could not power up downwind in that race.

Great explanation of the various factors affecting twist etc, thanks!

Even I could understand it.  :D

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On 1/29/2021 at 8:39 PM, erdb said:

While I was working on the VPP, one interesing discovery was that as you increase lateral force on the sail, obviously the counteracting lateral force on the foil increases the same way. However, since the foil is canted out at an angle, this lateral force is directly coupled to increasing vertical lift from the foil (the relationship is governed by the cant angle), and to balance out vertical forces, the rudder downforce needs to increase as well. So in a way, you have a lot more righting moment, because the more you push the boat sideways, the more you increase foil lift and rudder downforce. Of course the limit is that pitch moments need to be balanced as well, and you can only increase rudder downforce to a certain level before the stern is sucked down in the water. This I think is the reason why all the boats have their main foils at the aft end of the box allowed by the rules, because if you have more weight forward of the foil, you can increase rudder downforce more and that adds to your righting moment.

Initially, I had quite high rudder downforce values for every point of sail above 10 kts of wind. Then as I started to tune "my boat", trying different foil angles flap angles and weight distribution, it turned out that the best performance at target upwind and downwind angles comes when the rudder vertical force is minimized. However, when the boats bear off into the death zone of reaching angles, there is considerable amount of rudder downforce to provide extra righting moment.

My model spits out these figures for every TWS / TWA combination (double click on the images for full resolution). I'm curious what you think of it. The top panels are the vectors stringed up starting with the blue vector of sail force, and for balance, the vectors should end up at the starting point (0,0). The reference point for all my equations was the cross section of the stern, midline and MWP (see class rule for definition). 

This is for TWS: 20 kts , TWA: 100 deg. 8 kN rudder downforce. The roll moment from the sails is actually close to that 420 kNm, 442 kNm in this case, but the horizontal foil force adds to that (since the CoE of the foil is below my reference point). So the total righting moment foil vertical lift has to provide is ~480 kNm.

1692133995_TWS20TWA100.thumb.png.d3c112270398bbec5004c557a9ccba5d.png

@erdb, truly a great effort, you have practically an AC 75 sailing simulator ready - I reckon only the lateral balance is missing, understandably you can't do it with your simple sail model. Good observation the increased righting moment through the rudder downforce, I would not have come to think about that. The speed & force & moment numbers all make sense and are remarkably close to what you would expect and is now seen in real life. And a very nice presentation here as well - what platform have you been working on, just Excel or something more sophisticated? What's your own background behind that pseudonym erdb (forgive me if I'm naive, but I never understood why people don't post with their own names, is there some risk there?).

I have here now with a better sailtrim at TWS 10 kn, TWA 50°, leeway 0° and BS 28 kn:

Overall   Drive (Fx)  5,70 kN. (corrected this later, main 2,4 kN and jib 3,3 kN)
Overall   Heel (Fz)  39,2 kN
Heeling mom          467  kNm,      of which mainsail 333 kNm and jib 130 kNm. Mainsail now includes mast.

Of the individual parts, the hull drive/drag is ±0, hull sideF 2,5 kN. The rudder & the leeward foil(arm) are drag wise neutral, the windward foil sports a tiny drive of 50 N. With the low resolution, the side forces are probably pretty correct, but the drive underestimated (drag overestimated).

The mainsail twist is now 15° and the Jib 18°. Main sheeting is 2° off CL and jib 7° off CL. I think I did the mistake of twisting the mainsail too much, like a classic sail, when I should have twisted the main less and just adjusted the profile for little lift in the head, thus aligning the profile nicer with the mast. My moment reference is at sea surface right under the tack of the mainsail, so a little lever would need to be added to get the heeling arm correct, to the underwater foil.

Looking at your foil resistance pie, I'm curious where you got the wave drag from? Spray drag is quite high, but maybe it is so.

LR.jpg

Edited by Mikko Brummer
corrected overall drive
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7 hours ago, Mikko Brummer said:

"With most single-luff mainsails, leech tension is the only means of controlling twist." Actually, not so... For one, sideways bend of the mast will induce some "non-linear" twist into the sail - the sail "twists" from the luff as well as the leech. The leech twist is normally close to linear unless there is much roach and "batten poke" with it. In the Finn, the mast is super stiff in lateral up to some 75% of the luff, and then goes very soft in the last 25%, dipping the masthead to leeward in gusts and waves. 

Then there's the cunningham - by putting lots of tension, you can add significant compression into the mast and increase its bend in the upper part. At some point, all the luff curve cut into the sail will be all used, and the upper leech will twist off dramatically, while the lower leech will hold on. With the correct combination of mast bend & luff curve, you can tailor exactly when this happens as the wind picks up.

This works with normal, short leech battens (like the Finn), but with full-length battens, the effect is even more dramatic. A good example is the windsurfer sails, that have a 8:1 purchase cunningham and completely bend the mast already ashore by cunningham tension. With the tension on the luff, a normal fold along the luff tries to form, but with the full-length battens, it cannot and instead the battens push very strong on the mast bending it into the curve cut into the luff of the sail. 

This works perfectly for the AC75 as well, luckily they have 2 sails and hydraulic rams to push on the (stiff D-profile) mast to bend it & flatten the sail. You could see the importance & the effect on INEOS, when they jammed their cunningham ram, set the sail in rather depowered-mode for upwind, and could not power up downwind in that race.

Further to the Windsurfer sail analogy. As a retired windsurfer I can only agree on the comments made on windsurfer sails. By using ridiculous amounts of Cunningham (downhaul) you can actually make a 12 square meter sail manageable in pretty high winds on a Formula board. A properly set sail will depower properly upwind and still provide enough drive downwind. You will still need to release the outhaul slightly sometimes when going downwind, but there is no way you can adjust the downhaul. The downhaul is set with an 8:1 purchase on the beach while a fit 90 kilo windsurfer puts his foot on the base of the mast and pulls close maximum of what he can manage. Probably the sail could be smaller if this was adjustable. When I was racing the luff pockets were wide (15-20% of chord) but kept stretched out by "camber inducers" which the sail battens fitted into, thereby providing a thin convex/concave profile for the forward part of the sail

The range of adjustment on a 5.5 meter luff on big windsurfer sail is surprisingly small. Maybe within two centimeters from max to min. Too little downhaul and the sail will feel heavy and unmanageable, too much and you will not have enough power and the sail will feel "dead".

Other analogies: The critical point when bearing off reminds a windsurfer very much of the same situation on a board. Commit, and you have a chance. If you don't, chances are that you will end up with an unmanageable change in apparent wind and loose control completely. A windsurfer takes guts to command - can't help but feeling respect when I see these guys bearing off at 50 knots.....

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2 hours ago, Mikko Brummer said:

@erdb, truly a great effort, you have practically an AC 75 sailing simulator ready - I reckon only the lateral balance is missing, understandably you can't do it with your simple sail model. Good observation the increased righting moment through the rudder downforce, I would not have come to think about that. The speed & force & moment numbers all make sense and are remarkably close to what you would expect and is now seen in real life. And a very nice presentation here as well - what platform have you been working on, just Excel or something more sophisticated? What's your own background behind that pseudonym erdb (forgive me if I'm naive, but I never understood why people don't post with their own names, is there some risk there?).

I have here now with a better sailtrim at TWS 10 kn, TWA 50°, leeway 0° and BS 28 kn:

Overall   Drive (Fx)  5,70 kN. (corrected this later, main 2,4 kN and jib 3,3 kN)
Overall   Heel (Fz)  39,2 kN
Heeling mom          467  kNm,      of which mainsail 333 kNm and jib 130 kNm. Mainsail now includes mast.

Of the individual parts, the hull drive/drag is ±0, hull sideF 2,5 kN. The rudder & the leeward foil(arm) are drag wise neutral, the windward foil sports a tiny drive of 50 N. With the low resolution, the side forces are probably pretty correct, but the drive underestimated (drag overestimated).

The mainsail twist is now 15° and the Jib 18°. Main sheeting is 2° off CL and jib 7° off CL. I think I did the mistake of twisting the mainsail too much, like a classic sail, when I should have twisted the main less and just adjusted the profile for little lift in the head, thus aligning the profile nicer with the mast. My moment reference is at sea surface right under the tack of the mainsail, so a little lever would need to be added to get the heeling arm correct, to the underwater foil.

Looking at your foil resistance pie, I'm curious where you got the wave drag from? Spray drag is quite high, but maybe it is so.

LR.jpg

Hi, it's really nice to see some high level analysis on this. My model's aero/hydro part is overly simplistic. My background is biomed science and I'm a sailing addict who always enjoyed understanding the word via physics. :D:D I don't have any experience with CFD and have no sophisticated tools. I've done everything in python, which I decided to learn when we got locked down due to covid. This was a good exercise. 

My model works kind of backwards from what you'd expect. For each TWS/TWA, I start out with thousands of combinations of speed, rudder vertical force and foil arm cant angle. For all these combinations, I calculate straight line forces in 3D, plus pitch and roll moments (not yaw). From these I also get the necessary cl, cdi and CoE height for the sails. Using Tom Speer's vortex worksheet I generated tables of cl, cdi, CoE height for various sail plans and twists and I check which combinations of speed, rudder force etc are possible to achieve with the sails. From those possible combinations, I select the one with the highest speed and that's my polar point for that TWS TWA combo.

For TWS 10kts, TWA 50 I got: 27.2 kts speed, AWS 34.5 kts, total sail drive 6.27 kN, sail heel 37.27 kN.

Pretty close numbers to yours I guess considering that a lot depends on how fast the boat is going. I get 7.7 deg twist from the vortex worksheet, but this is jib+main combined.

My hull drag and foil spray/wave drags are basically educated wild guesses :D. I tried to find some simple formulas or published graphs and base my estimation on that, but it's far from perfect. One interesting thing was however that as I played with foil drag numbers, I realized that the boat would only balance if the foil drag is in a certain range. If the drag wasn't right, the forces that come from the geometry and weight of the AC75 would never balance. So in a way I reverse engineered drag from the boats' dimensions. 

It's interesting that hull drive/drag is ±0 for you. How do you get that? Do you look at the sails with and without the hull? For TWS 10, TWA 50 I have 451 N hull drag and 102 N hull side force. Obviously not accurate, but compared to the sail and foil forces these are pretty small.

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14 hours ago, erdb said:

It's interesting that hull drive/drag is ±0 for you. How do you get that? Do you look at the sails with and without the hull? For TWS 10, TWA 50 I have 451 N hull drag and 102 N hull side force. Obviously not accurate, but compared to the sail and foil forces these are pretty small.

Drag in the direction of motion of the boat, in the apparent wind direction there would obviously be drag. This is not unusual, the air drag of a Finn hull is nil, too. Although the fact that the AC is all in the air makes it more draggy. Checked the side force, it's indeed some 2500N, so you are optimistic there. Much of the hull side force could be from the infamous vortex? This is hull only, without the foils.

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On 2/1/2021 at 8:55 AM, Basiliscus said:

.........

It also nearly led to disaster in SF.  Minutes before the last race, the pocket in which one of the pins rode broke.  It was looking like OTUSA might have to retire from the race because they wouldn't be able to properly control the flap.  A shore team crewmember was sent aloft with some fast-setting glue, managed to get it stuck back together and got off the boat in time for them to make the start.  But the whole race, Jimmy Spithill was half expecting it to let go at any time.  That's why, when asked when he knew they had the race won he replied, "When we crossed the finish line!"  They could have lost it at any time.

It is amazing to think a simple pin might have given the Cup to NZ !!!!

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23 hours ago, Kiwing said:

It is amazing to think a simple pin might have given the Cup to NZ !!!!

Yes, sounds like it was very-very close.. 

We have seen a bunch of problems even recently with these still-raw AC75’s, breakages are a real possibility in the remaining races too. 

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I think it is pretty much always like that. The AC boats are on the edge. Disasters don't always happen, but they are never far off!

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6 minutes ago, Mikko Brummer said:

Visualizing flow with massless particles. Note how trailing edge vorticity (the source of induced drag) is shed all along at the leech(es), not just at the "tips". I had the reduce the size & quality of the vid to fit in the max allowed attachment size.

Viewed from aft, would that leech vortex be rotating clockwise?

Seems to be very large for just one leech point?

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At this height, it would appear to be anti-clockwise. At some point (height), it will change direction, there where the vertical loading is at the maximum - the vortices grow from a gradient in the vertical loading. For a single sail, at the max loading height, there is no gradient and the flow would be exiting the leech parallel on both sides of the sail... but this is more complex, with the vorticity from the jib blending with that of the main, and closer to the sea level, vorticity from the hull & windward foil, for instance. In the second pic, vorticity in a plane just behind the boat. You can distinguish vortex cores of the tip vortices from the main & jib tops, and lower down the foils and the bottom keel/skeg.

Nimetön.jpg

Vorticity.png

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2 hours ago, MaxHugen said:

^ What is "vertical loading"? Does that describe the airflow vector towards the head of the sail?

No, I was referring to the "spanwise distribution of loading", thus vertical in case of a sailboat. The spanwise distribution of circulation, as in the case of the "elliptical loading". Look up "lifting line theory" for more.

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7 hours ago, Mikko Brummer said:

Better here at larger vorticity scale

Vorticity2.jpg

What does it look like if you present the signed X component of the vorticity instead of the magnitude?  (Or maybe the component in the apparent wind direction.) That would clarify which way the vortices were rotating.

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On 2/8/2021 at 2:09 AM, MastaVonBlasta said:

FB_IMG_1612703331358.thumb.jpg.ced6223c75e628d338ee6861cbc58277.jpg

Do the right most two represent reversing the top?

Who were Advanced Wing Systems associated with?

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14 hours ago, Kiwing said:

Do the right most two represent reversing the top?

Who were Advanced Wing Systems associated with?

They posted some content with American Magic? 

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I remember Sir Ben saying this twin skin is no better than a single skin main.  Seems that has come home to bite them.

Ability to create RM with the top at low speeds when the foils are not giving them much RM.  And create large camber right at the bottom to give them the most power to get up.  Is what they lack in low wind speeds.

Control of the the top to give them stability for snap tacks/gybes is also costing them dear?

But what would I know?  76yr laser sailor?

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1 hour ago, Mikko Brummer said:

Mostly nonsense, I'm afraid.

I am very interested @Mikko Brummer .  Share some ideas, I would love to explore some research, video and explanations, any chance of you giving a few more clues.  Your stuff above speaks the truth.

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14 hours ago, Kiwing said:

I am very interested @Mikko Brummer .  Share some ideas, I would love to explore some research, video and explanations, any chance of you giving a few more clues.  Your stuff above speaks the truth.

I will, as soon as I find some time to run more accurate simulations. I would like to compare the 2-skin concept against a traditional, small wing mast with a fully battened mainsail. Instinctively, I don't think that there is much in the relatively thick D-mast twin-skin profile, compared to a sleek wing mast & single-ply sail, but it could be I'm wrong. 

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28 minutes ago, Mikko Brummer said:

I will, as soon as I find some time to run more accurate simulations. I would like to compare the 2-skin concept against a traditional, small wing mast with a fully battened mainsail. Instinctively, I don't think that there is much in the relatively thick D-mast twin-skin profile, compared to a sleek wing mast & single-ply sail, but it could be I'm wrong. 

Hi Mikko, would you have an image of a wing mast profile? Preferably together with the sail, to estimate it's percentage of chord. I'd like to try running an estimation of such a profile in XFoil, and compare it to my interpretation of the twin skin profile.

PS: the AC75 mast is approx 9% of chord. I also tried narrower profiles, but they seemed more restricted in AoA range. Might just be the profile I tried though.

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2 hours ago, MaxHugen said:

Hi Mikko, would you have an image of a wing mast profile? Preferably together with the sail, to estimate it's percentage of chord. I'd like to try running an estimation of such a profile in XFoil, and compare it to my interpretation of the twin skin profile.

PS: the AC75 mast is approx 9% of chord. I also tried narrower profiles, but they seemed more restricted in AoA range. Might just be the profile I tried though.

The design is in 3D, and there's perspective, but maybe you can get something from this. The mast profile is actually by @Basiliscus, originally for a Moth. 

wing mast2.png

wing mast.jpg

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15 minutes ago, Mikko Brummer said:

The design is in 3D, and there's perspective, but maybe you can get something from this. The mast profile is actually by @Basiliscus, originally for a Moth. 

wing mast2.png

wing mast.jpg

Thanks Mikko, should be able to derive a profile from this.

Will start with a NACA profile that roughly matches the mast etc, then generate a similar profile to the AC75 mast/mainsail profile I use.

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6 hours ago, Mikko Brummer said:

I will, as soon as I find some time to run more accurate simulations. I would like to compare the 2-skin concept against a traditional, small wing mast with a fully battened mainsail. Instinctively, I don't think that there is much in the relatively thick D-mast twin-skin profile, compared to a sleek wing mast & single-ply sail, but it could be I'm wrong. 

FWIW...

The AC75 mast dimensions are 600mm (x) by 450mm (y). For a sail of 7m at the foot, the mast has a thickness of  5.9% (of chord) at 7.9% from LE.

From your pics, the wing mast length (x) is 10.7% of mast/sail profile. With the 30% of mast thickness, that's about 3.2% of mast/sail chord, at ~4% from LE.

I tried to create a dat file of the wing mast + sail to use in XFoil, but without a way to convert an svg file to a dat file, my manual attempts were almost useless. So I adapted an Eppler 376 profile, changing thickness to ~3.2% at ~4% from LE, and tried that to compare with my estimate of the AC75 twin skin profile.

The comparison was done at 30 knots AWS, 6% camber, and 11° AoA.

Despite the inaccuracies, it suggests the twin skin has a slight advantage over the wing mast with single skin. However, flattening the windward skin of the twin skins may produce better results at lower wind ranges, but I haven't tried this.

image.thumb.png.194b2b97795f4a3f8b792f5d0e3749d7.png

image.png.aa2f3438274f7c22907e25dbc5b2e4ac.png

Just in case the dat files are of any use to you, I've attached both profiles plus the wing mast.

WingMast (modified NACA 0030).dat
EPPLER 376 (6403).dat

AC75 6406.dat

WingMast (modified NACA 0030).dat

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This might amuse (warning! old school!) Gottingen & ISA sections l/d paper. The double membrane approach is persnickety as far as angle of attack and cambers. I tried some double membrane windsurfer sails back in the 80’s, felt fast/slow fast/slow, slick/draggy within a small timeframe and a lot of face plants, since the center of effort moved around a lot.  Hard on the aft hand. Harnesses not recommended.  Almost impossible to keep in the groove, and fast aoa changes kind of worked- like what we’re seeing in the AC.  Kind of wonder if they need jibs and twist just to make them work at all.  As heavy and rigid as RAFs are, there’s a reason for them, although the aero for them can be kind of streaky too.
 

https://apps.dtic.mil/dtic/tr/fulltext/u2/a280301.pdf

 

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8 hours ago, Mikko Brummer said:

I will, as soon as I find some time to run more accurate simulations. I would like to compare the 2-skin concept against a traditional, small wing mast with a fully battened mainsail. Instinctively, I don't think that there is much in the relatively thick D-mast twin-skin profile, compared to a sleek wing mast & single-ply sail, but it could be I'm wrong. 

How about comparing to a large wingmast with single mainsail?  They are about equal in speed to the rigid wingsail rigs in landyachts, which sail at similar apparent wind angles.  The wingmast could have a similar sized D tube as the twin mainsail mast.  That would make for a good comparison that minimized the differences in the rig and concentrated on the differences in the sails.

FWIW, landyacht sails are made flat, with no broadseaming.  They get enough camber just from the air loads and mast rotation.

One thing I've always wondered, should the trailing edge planform of the mast be convex or concave?  Most of the older C-class masts seemed to have convex planforms, but I'd think that would feed draft into the sail when the mast rotation was flattened - just the opposite of what one would want to do.  Concave should pull draft out when flattened.  A hooked planform for the mast would affect the twist at the head when rotated, but I don't know if that would be a good thing or a bad thing.

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2 hours ago, MaxHugen said:

FWIW...

The AC75 mast dimensions are 600mm (x) by 450mm (y). For a sail of 7m at the foot, the mast has a thickness of  5.9% (of chord) at 7.9% from LE.

From your pics, the wing mast length (x) is 10.7% of mast/sail profile. With the 30% of mast thickness, that's about 3.2% of mast/sail chord, at ~4% from LE.

I tried to create a dat file of the wing mast + sail to use in XFoil, but without a way to convert an svg file to a dat file, my manual attempts were almost useless. So I adapted an Eppler 376 profile, changing thickness to ~3.2% at ~4% from LE, and tried that to compare with my estimate of the AC75 twin skin profile.

The comparison was done at 30 knots AWS, 6% camber, and 11° AoA.

Despite the inaccuracies, it suggests the twin skin has a slight advantage over the wing mast with single skin. However, flattening the windward skin of the twin skins may produce better results at lower wind ranges, but I haven't tried this.

image.thumb.png.194b2b97795f4a3f8b792f5d0e3749d7.png

image.png.aa2f3438274f7c22907e25dbc5b2e4ac.png

Just in case the dat files are of any use to you, I've attached both profiles plus the wing mast.

WingMast (modified NACA 0030).dat
EPPLER 376 (6403).dat

AC75 6406.dat

WingMast (modified NACA 0030).dat

Might you post the links separately?  Can’t get them to download.  Thanks

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I really like that video, but I don't see the foot tension running into the mainsheet at all, how would compression (pulling fwd) translate into a load on the car or sheet when the liner is linearly pulling straight down?

Also I would posit that perhaps the jerk traveler is caused by the load of the mainsheets running fwd and binding the car on the track

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2 hours ago, Basiliscus said:

How about comparing to a large wingmast with single mainsail?  They are about equal in speed to the rigid wingsail rigs in landyachts, which sail at similar apparent wind angles.  The wingmast could have a similar sized D tube as the twin mainsail mast.  That would make for a good comparison that minimized the differences in the rig and concentrated on the differences in the sails.

FWIW, landyacht sails are made flat, with no broadseaming.  They get enough camber just from the air loads and mast rotation.

One thing I've always wondered, should the trailing edge planform of the mast be convex or concave?  Most of the older C-class masts seemed to have convex planforms, but I'd think that would feed draft into the sail when the mast rotation was flattened - just the opposite of what one would want to do.  Concave should pull draft out when flattened.  A hooked planform for the mast would affect the twist at the head when rotated, but I don't know if that would be a good thing or a bad thing.

Like this?

6165685F-87D3-44EE-B911-42CEFECCF187.png

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6 hours ago, MaxHugen said:

FWIW...

The AC75 mast dimensions are 600mm (x) by 450mm (y). For a sail of 7m at the foot, the mast has a thickness of  5.9% (of chord) at 7.9% from LE.

From your pics, the wing mast length (x) is 10.7% of mast/sail profile. With the 30% of mast thickness, that's about 3.2% of mast/sail chord, at ~4% from LE.

I tried to create a dat file of the wing mast + sail to use in XFoil, but without a way to convert an svg file to a dat file, my manual attempts were almost useless. So I adapted an Eppler 376 profile, changing thickness to ~3.2% at ~4% from LE, and tried that to compare with my estimate of the AC75 twin skin profile.

The comparison was done at 30 knots AWS, 6% camber, and 11° AoA.

Despite the inaccuracies, it suggests the twin skin has a slight advantage over the wing mast with single skin. However, flattening the windward skin of the twin skins may produce better results at lower wind ranges, but I haven't tried this.

image.thumb.png.194b2b97795f4a3f8b792f5d0e3749d7.png

image.png.aa2f3438274f7c22907e25dbc5b2e4ac.png

Just in case the dat files are of any use to you, I've attached both profiles plus the wing mast.

WingMast (modified NACA 0030).dat
EPPLER 376 (6403).dat

AC75 6406.dat

WingMast (modified NACA 0030).dat

I think it would be interesting to take the actual AC75 D tube shape and extend it aft to form a teardrop shape.  The mast chord would be maybe 20% of the total chord.   Given the low apparent wind angles for these boats, I think a large wingmast would be more appropriate.  The main drawback I see to that is it would be very stiff in chordwise bending.

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4 hours ago, Basiliscus said:

I think it would be interesting to take the actual AC75 D tube shape and extend it aft to form a teardrop shape.  The mast chord would be maybe 20% of the total chord.   Given the low apparent wind angles for these boats, I think a large wingmast would be more appropriate.  The main drawback I see to that is it would be very stiff in chordwise bending.

Are the teams allowed to glue the 2 skins together at 20% chord?  (With a foaming glue?)

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7 hours ago, Amati said:

Might you post the links separately?  Can’t get them to download.  Thanks

Resent to you via PM.

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there was a great interview of Max Sirena by LiveSailDie. He said on of the kiwi strengths was to test concepts 'quick and dirty'. To get ahead of the competition on proving ideas over refining them. At the moment, this doesn't look the most refined solution, but it looks the best concept. And they've still got plenty of time to refine. 

 

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I certainly think the two teams with the worst sails are out first. Luna Rossa seem to be able to change their sail mode quickly, from super deep to fully bladed out.

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38 minutes ago, kiwin said:

I certainly think the two teams with the worst sails are out first. Luna Rossa seem to be able to change their sail mode quickly, from super deep to fully bladed out.

LR and NZ also have an extra few square metres at deck level that are actually working at producing power low down.

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On 2/18/2021 at 10:06 AM, Basiliscus said:

I think it would be interesting to take the actual AC75 D tube shape and extend it aft to form a teardrop shape.  The mast chord would be maybe 20% of the total chord.   Given the low apparent wind angles for these boats, I think a large wingmast would be more appropriate.  The main drawback I see to that is it would be very stiff in chordwise bending.

And weight......

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The convex trailing edge was driven by the crap sailcloth available at the time. At the time there was a huge problem getting rid of twist because all the sailcloth was too stretchy. One solution, by non other than Austin Farrar was to have the trailing edge of the wing mast more or less match the inevitable sag in the leech.

SHC

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16 hours ago, MaxHugen said:

LR and NZ also have an extra few square metres at deck level that are actually working at producing power low down.

As I understand it, LR have their boom below deck level which is the same level as the bottom of the mast, then they stretch a piece of canvas at deck level between the boom and the foot of the mainsail, this enables them to have a nice shape down to deck level.

ETNZ have their deck much lower than than the foot of the mast and comply with the rules by mounting the mast at the required height on a little stub,  the effect of the arrangement is to give them nearly an additional metre of sail area below the bottom of the mast, not nicely shaped like LR but gives them the extra power they need to get on their small foils in light air.

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5 hours ago, Steve Clark said:

The convex trailing edge was driven by the crap sailcloth available at the time. At the time there was a huge problem getting rid of twist because all the sailcloth was too stretchy. One solution, by non other than Austin Farrar was to have the trailing edge of the wing mast more or less match the inevitable sag in the leech.

 SHC

Same reason the DN pop their mast in the negative lateral bend?

40306348124_fd9f616822_b.jpg

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@Mikko Brummer thank you for that ice boat info.

The TNZ loop hole, lowering the deck and getting more sail area down low, has enabled them to lower the COE and use more of the top for control?  Which means they can use smaller foils? and when foiling reduce them further by hanging some of them out of the water, or using the lift to windward to crab and soak up increasing their VMG.  VMG is the name of the game as speed brings cavitation into the picture, a big hill to climb?

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On 2/22/2021 at 6:33 PM, Terry Hollis said:

ETNZ have their deck much lower than than the foot of the mast and comply with the rules by mounting the mast at the required height on a little stub,  the effect of the arrangement is to give them nearly an additional metre of sail area below the bottom of the mast, not nicely shaped like LR but gives them the extra power they need to get on their small foils in light air.

I think the extra sail area and lowering of CE is a minor side benefit. For me the mast stump was about lowering the deck to minimise the frontal area facing 40 to 50 knots AWS. Plus channeling the airflow, which in itself means that the (additional) sail area low down is even more effective.

I will go one further..... air (and water) pushed away sideways is wasted energy. Best is to push it under. TR’s heavily flared bows best pushes air under the flat sections to help create an air cushion when close to the water, assisting lift off. Once it is up and away, the effect is minimised if not lost, but the job has been done, which means you need smaller foils for lift off, which pays back at higher speeds. TR, relative to LR, pushes less air overall out of the way, and more of it under than sideways.

DF78887E-B74D-44B0-A6FC-1D8B989EF344.jpeg

Edited by Sidecar
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On 2/22/2021 at 8:33 PM, Terry Hollis said:

As I understand it, LR have their boom below deck level which is the same level as the bottom of the mast, then they stretch a piece of canvas at deck level between the boom and the foot of the mainsail, this enables them to have a nice shape down to deck level.

ETNZ have their deck much lower than than the foot of the mast and comply with the rules by mounting the mast at the required height on a little stub,  the effect of the arrangement is to give them nearly an additional metre of sail area below the bottom of the mast, not nicely shaped like LR but gives them the extra power they need to get on their small foils in light air.

 

2 hours ago, Sidecar said:

I think the extra sail area and lowering of CE is a minor side benefit. For me the mast stump was about lowering the deck to minimise the frontal area facing 40 to 50 knots AWS. Plus channeling the airflow, which in itself means that the (additional) sail area low down is even more effective.

I will go one further..... air (and water) pushed away sideways is wasted energy. Best is to push it under. TR’s heavily flared bows best pushes air under the flat sections to help create an air cushion when close to the water, assisting lift off. Once it is up and away, the effect is minimised if not lost, but the job has been done, which means you need smaller foils for lift off, which pays back at higher speeds. TR, relative to LR, pushes less air overall out of the way, and more of it under than sideways.

DF78887E-B74D-44B0-A6FC-1D8B989EF344.jpeg

Yes you are right about the hull advantages, my comment on the sails is determined by the topic we are discussing.

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  • 2 weeks later...

@Basiliscus I am wondering about wing wash on the trailing boat.

Is wing wash similar on landyachts?  and if so how big is and does it vary greatly with wind speed as some suggest at low true wind speeds it is huge, not so much at high true wind speeds?

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@Mozzy Sails what evidence is there of bad air in these races?

Does it effect a boat 200 metres behind, 500 metres behind?

Does it effect a boat on the hip ? and how wide of the hip can they stay there?

By the way, you (and your pals) give great informational videos and I also love the opinion pieces as well.

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4 hours ago, Kiwing said:

@Basiliscus I am wondering about wing wash on the trailing boat.

Is wing wash similar on landyachts?  and if so how big is and does it vary greatly with wind speed as some suggest at low true wind speeds it is huge, not so much at high true wind speeds?

 

43 minutes ago, Kiwing said:

@Mozzy Sails what evidence is there of bad air in these races?

Does it effect a boat 200 metres behind, 500 metres behind?

Does it effect a boat on the hip ? and how wide of the hip can they stay there?

By the way, you (and your pals) give great informational videos and I also love the opinion pieces as well.

There's no good way to get the detail, but the building consensus is that these very powerful boats leave a huge amount of bad air behind them and to leeward. It's evident in the lack of any lead changes, as well as the difficulty getting power in the starting area after a jibe. And it just makes sense. Powerful rigs are going to disturb a bunch of air.

Land yachts are generally much more easily driven, and much less powerful, AFAIK, so I would think these boats have more "wing wash".

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18 hours ago, Kiwing said:

@Basiliscus I am wondering about wing wash on the trailing boat.

Is wing wash similar on landyachts?  and if so how big is and does it vary greatly with wind speed as some suggest at low true wind speeds it is huge, not so much at high true wind speeds?

When I was landsailing, tactics didn't play as big a role as they do in the America's Cup.  Speed is everything, so it didn't pay to mix it up in close quarters.  The yachts tended to do more of their own thing, although the racecourse was somewhat constrained by the size of the lakebed, so it wasn't purely a matter of banging the corners.  

What was cool was watching the dust entrained in the lower trailing vortex.  The yachts would lay down a vortex trail that persisted for a considerable time.  You could see it drifting diagonally across the lakebed as the yacht laid down fresh trail.  It was a cool demonstration of the fact that the wake trails at the apparent wind angle, because although the vortex was drifting with the true wind, the yacht was laying down fresh vortex at the speed of the boat.  The result was a vortex trail that sort of crabbed sideways as it extended back from the boat at the apparent wind angle.

This photo doesn't show the trails very well, but you can see there is a yacht to windward that is out of the picture whose trail is extending well behind the two yachts.

Desert-Kruk9-Copy.jpg

When the yacht is constrained by stability, the lift on the rig is approximately constant as the speed changes.  So the lateral wake wash velocity decreases as the speed increases.  That would make the wash stronger in light winds.

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