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6 hours ago, Basiliscus said:

I think cavitation on the upper side of the leading edge is unlikely in these foil designs at high speed.  It is more to be expected as sheet cavitation at takeoff speed.  At high speed, I'd expect the foils would be designed so that bubble cavitation occurred more toward the middle of the chord.  

Cavitation on underside of the leading edge could be happening at high speed, but I'd think they'd design their sections so that wouldn't happen, either.  A little trailing edge up flap deflection, compensated for by a little bow-up change in pitch attitude would cure it.  

So a foil that has ventilated won't have any cavitation effects around the air pocket interface? From a armchair view there would seem to be zones of aerated low pressure and high pressure outside the envelope, conditions for cavitation to take place. There seems to be bit much discussion or information about how the boats that breach a tip are dealing with the ventilated wing. Any insight appreciated.

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Just a few interesting bits of the straight-line performances from today: Upwind /Downwind VMGs - race 1: Upwind /Downwind VMGs - race 2: Same story in both races actually.

Thanks to weta27's pics I have created an approximation of NZ's "BFB v2" foil. Main points: Foil area is almost the same, possibly even a smidge larger. Flaps have increased in area as

Data from yesterday's races - no big surprise, but there are some interesting things. I've excluded the first few minutes from Race-1 when INEOS was off the foils. As was obvious during broadcast

Posted Images

9 hours ago, MaxHugen said:

...A question pls: why is the forward half of the lower surface designed so that it produces no lift?

That is driven by the desire to have the onset of cavitation as high as possible, while also trying to get substantial thickness for structure and to house the ballast.  If the forward half produced lift, then that would raise the velocity on the upper surface and lower it on the bottom surface.  The result would be the upper surface reaching the cavitation threshold at a lower speed.  Both the lower and upper surfaces are hard up against the cavitation threshold, due just to thickness.

Each of my posts has a link to the previous post in the series.  If you trace back a post or two, you'll see how that situation evolved.

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8 hours ago, barfy said:

So a foil that has ventilated won't have any cavitation effects around the air pocket interface? From a armchair view there would seem to be zones of aerated low pressure and high pressure outside the envelope, conditions for cavitation to take place. There seems to be bit much discussion or information about how the boats that breach a tip are dealing with the ventilated wing. Any insight appreciated.

Ventilation is a completely different phenomenon from cavitation.  The only thing they have in common is a vapor phase in addition to the water phase near the foil.

Ventilation doesn't require as low a pressure as cavitation - the local pressure only has to be lower than atmospheric pressure for air to be sucked down onto the foil.  But ventilation requires something else that cavitation does not - the flow has to be separated to begin with.  Otherwise, the fast-flowing water will simply sweep the air off the foil.  So ventilation is fundamentally a boundary layer phenomenon.

However, the flow separation can come from some surprising ways.  The C-FLY guys told me about how they had thought they'd designed their surface-piercing main foil so it wouldn't ventilate.  But waves breaking over the foil could trap a pocket of air and that would create a separated zone that would propagate down the foil very quickly.  

Ventilation also requires a path for the air to get to the separated zone.  This can be via a laminar separation bubble at the leading edge that acts like a straw when it is filled with air.  It can even happen by air coming up from behind the boat via the low pressure in the core of a trailing vortex.  A common cure for ventilation is to add fences that block the progress of air coming down the foil.  But fences also add drag, and if the fence comes out of the water, it is not effective.  

The reason cavitation erodes the surface is because it is water vapor that will condense when the local pressure once again becomes higher than the vapor pressure of water.  When bubbles of water vapor condense, they don't shrink symmetrically, but instead form a high speed jet of water that shoots through the bubble as they collapse.  It is the hammering of the surface by these tiny jets that does the pitting.  Air does not condense into liquid as it encounters higher pressure, so there isn't the same kind of erosion cause by ventilation.

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10 hours ago, zillafreak said:

So why is the current class of foiling monohull the fastest yet? Size for size the current the class faster than a multihull of similar size/power. Possibly the fastest boat in the world next to Sailrocket.

There's a lot that has been learned in the last three Cup cycles.  We could be on a Version 3 AC72 by now, and it would be faster than the Version 1 AC72s.  

I think a Version 3 AC72 (or even an F50) could be a very formidable competitor to an AC75 in the prestart.  It could dial up an AC75 until both boats fell off their foils, and then it would be, "Sayonara."

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1 hour ago, Basiliscus said:

Ventilation is a completely different phenomenon from cavitation.  The only thing they have in common is a vapor phase in addition to the water phase near the foil.
 ...

Great explanation and easy to understand. Thanks!  :)

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2 hours ago, Basiliscus said:

...

The reason cavitation erodes the surface is because it is water vapor that will condense when the local pressure once again becomes higher than the vapor pressure of water.  When bubbles of water vapor condense, they don't shrink symmetrically, but instead form a high speed jet of water that shoots through the bubble as they collapse.  It is the hammering of the surface by these tiny jets that does the pitting.  Air does not condense into liquid as it encounters higher pressure, so there isn't the same kind of erosion cause by ventilation.

@barfy, in case you missed a previous post, there's a good YT video on cavitation. It also shows a bubble collapsing which is very cool - video will start at that section:

 

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Another fascinating interview from Mozzy! We have been very spoiled this AC with the gems that just keep popping up!

 

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44 minutes ago, The Advocate said:

I don't think that interview gave us very much at all. The only interesting bit I thought was about the flap hinge.

Very much a puff piece.

I disagree. It didn't give us any LR secrets, but then you can't expect that (plus it looked like somebody from team had final edit).

However it gave lots of insight into the general design process and just more general development culture of the cup. I like these interviews.

What i did find interesting was the comment about how it wasn't only ETNZ that affairs a lot of design areas to support a specific design ideas. It was revealing to hear how much was affected by the runnerless configuration. We now know why they held onto it for so long long, as it affected most other parts of the boat.

 

 

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10 hours ago, Basiliscus said:

That is driven by the desire to have the onset of cavitation as high as possible, while also trying to get substantial thickness for structure and to house the ballast.  If the forward half produced lift, then that would raise the velocity on the upper surface and lower it on the bottom surface.  The result would be the upper surface reaching the cavitation threshold at a lower speed.  Both the lower and upper surfaces are hard up against the cavitation threshold, due just to thickness.

Each of my posts has a link to the previous post in the series.  If you trace back a post or two, you'll see how that situation evolved.

Re-reading your Parts 1-6, with a lot more concentration now.

Need your advice re XFoil... I've been using Analysis "Type 1", as I don't really understand Type 2, which it looks like you use from the polars in the H143 data. But I'm wondering if I should try to get my head around Type 2. 

For a start, the Density default in Type 1 may be an issue? Also don't know how the vals for Chord/Span/Mass affect XFoil?

I'm using an Re calculated using NZ's avg foil chord of 0.32m, at a velocity of 50 knots (Mach 0.075), and the same Ncrit as you do. These are my Type 1 params in XFLR5:

image.png.54e17d8ba330d82e8e2b92c9f6e5c22e.png

Any recommendations would be appreciated. :)

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2 hours ago, MaxHugen said:

Re-reading your Parts 1-6, with a lot more concentration now.

Need your advice re XFoil... I've been using Analysis "Type 1", as I don't really understand Type 2, which it looks like you use from the polars in the H143 data. But I'm wondering if I should try to get my head around Type 2. ...

Any recommendations would be appreciated. :)

The different types of drag polars in Xfoil are due to the fact that the nondimensional lift and drag coefficients are not the same as the actual lift and drag forces.  The performance depends on the actual forces, not the nondimensional coefficients.  The type 2 and type 3 polars are intended to take this into account.

Type 1 polars are what you'd get from a wind tunnel test.  The Reynolds number is constant throughout the whole run as the angle of attack is varied. It is what the wing would experience in flight if a plane were to do a wind-up turn at a constant speed, rolling into the turn from level flight and steadily increasing the g's until the wing stalled.

The problem with type 1 polars is more often than not one is interested in the performance under steady state conditions.  When a plane or hydrofoil is flying at different speeds, the Reynolds number is changing with speed and the angle of attack is changing with speed.  So you need to run a whole series of type 1 polars and then interpolate between them in order to get the profile drag vs speed at the correct Reynolds numbers.

You can shortcut this process with a type 2 polar.  The type 2 polar assumes the lift (in N) is constant.  This is not a bad approximation for the lift because the lift for a wing generally has to equal the weight.  The equilibrium lift coefficient will vary  inversely with the square of the speed, with low speed corresponding to high lift coefficients and high speed corresponding to low lift coefficients.  Since Reynolds number varies with speed, this means the Reynolds number will be low at low speed and high lift coefficient, and Reynolds number will be high at high speed and low lift coefficient.  The type 2 polar behaves this way, varying the Reynolds number by the square root of the lift coefficient.  So you can make one run and have the appropriate Reynolds number for each steady-state lift coefficient without having to interpolate between a number of type 1 polars.

FWIW, Xfoil's type 3 drag polar also varies the Reynolds number with lift coefficient, but this is oriented toward sizing the wing area at the design stage.  Induced drag depends on the span, but doesn't depend on the planform area.  Profile drag, on the other hand, does depend on the area.  If you keep the span constant and vary the area by changing the chord, the Reynolds number will also change.  Keeping the lift (in N) constant again but this time also keeping the speed constant, a larger chord will result in a lower lift coefficient and a higher Reynolds number.  This will increase the drag due to extra wetted area, but not quite as much as you'd expect because of the increased Reynolds number and the variation of drag coefficient with lift coefficient.  The type 3 drag polar varies the Reynolds number in this manner.

The ideal planform area is such that the wing operates at the maximum profile lift/drag ratio.  You can size the wing so the lift coefficient corresponds to the maximum lift/drag ratio of the type 3 polar.  If the wing is larger than this size, the lift coefficient will be lower, the Reynolds number higher, and the profile drag coefficient will be lower as well.  But the increased wetted area will overcome the savings in drag from the reduced drag coefficient.  If you make the wing smaller than the ideal size, there is a savings in wetted area but the increase in the drag coefficient outweighs the savings due to wetted area.  Two different section shapes will vary their drag coefficients differently with Reynolds number and lift coefficient and so will have different ideal wing sizes.  Using the type 3 drag polars, you can compare different sections and the one that has the highest maximum 2D lift/drag ratio will be the most efficient section.  This will be true for that section at its ideal size compared to the other sections at their ideal sizes.

That's why I used type 2 drag polars when designing the sections.  The type 2 drag polars corresponded to operating along the red loading line in the cavitation diagrams.

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With regard to the settings in XFLR5, you can use the actual water density and kinematic viscosity.  You don't need to use the default values for air.

If you are using a type 2 polar, I think the easiest way to determine the Reynolds number is to consider what the speed would be at a lift coefficient of 1.0.  You can look that up on my cavitation diagrams by following your design loading line.  The speed at Cl=1.0 along with the average chord will give you the Reynolds number to put into Xfoil/XFLR5.

Of course, when you're working the nitty gritty of an actual design there will be different sections designed for different spanwise stations along the wing, and for those you'd use the local chord instead of the average chord.  But section characteristics don't change very rapidly with Reynolds number, especially for the kinds of speeds we're interested in, so the average chord is fine for preliminary design.

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12 hours ago, MaxHugen said:

@barfy, in case you missed a previous post, there's a good YT video on cavitation. It also shows a bubble collapsing which is very cool - video will start at that section:

 

I saw that, great video. I still think the elephant in the room is the ventilated outer wing, with no fences, and what that does to the efficiency of 50% of the wing. I've seen heaps of explanations and examples of vortex and tip ventilating, but other that a lot of boats crashing to surface I haven't heard any of the wise men speaking to the loss of lift. There were early comments here when the first videos showed tip breach and full ventilation without falling to earth, but not much commentary for Awhile.

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10 hours ago, The Advocate said:

I don't think that interview gave us very much at all. The only interesting bit I thought was about the flap hinge.

Very much a puff piece.

Exponentially more interesting than you

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On 3/7/2021 at 12:02 AM, erdb said:

Presumably, the FCS reports the cant angle the same way for all boats regardless of the length of the foil arm. We just don't know exactly what their reference point it.

There is a big bolt that goes through the end of the foil arm that the teams attach to. 

I bet it's that. 

But isnt it quite easy to back calculate from Max's diagram?

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On 3/6/2021 at 7:13 PM, erdb said:

OK, here is my take on the anhedral foil righting moment debate:

First, we have to clarify whether we're talking about steady-state balance or some dynamic situation. For simplicity, I'll talk about the steady state balance for now.

For this comparison, let's assume that we have two boats that are equally balanced, sailing with the same speed, TWA with the same sail plan. The only difference is that one has a T-foil, the other an anhedral. If we start from here, it's important to realize that the total vertical and total horizontal foil forces have to be the same for both boats. From here, it follows that you can't freely change the force distribution between the wing halves. That distribution will depend on the foil arm cant angle.

It turned out to be quite an interesting geometrical problem. I used Max's measurements for anhedral angle and coordinates of center of effort for the wing halves, and used my VPP for vertical and horizontal foil forces required. The effect of sticking out the outside tip of the foil was omitted.

Here is what happens at TWS=20kts upwind - need for max righting moment (double click to see it better):

1574351370_uw20.thumb.png.02533bfcd9a0d3e6b155c964247a7c5d.png

There's a lot to digest here, I've been staring at these for half an hour now:))) Horizontal axis is foil arm cant angle - as we go right, the foil is canted out more and more. Changes in all the forces and arm lengthes were included in these calculations.

For a given sail setup, the T foil can only have one specific cant angle to balance forces. This is indicated by the dotted vertical line. The horizontal dashed line shows total forces and righting moment generated by the T foil. For the horizontal and vertical forces, this is the same as the sum of forces on the anhedral wing halves. Red and green lines represent the inside and outside wing halves of the anhedral foil. RM is referenced to the foil arm rotation axis on the hull for simplicity. We're only interested in the differences anyway.

On the left graph, what you see is that with low cant angle, the outside half contributes minimally to horizontal forces, and the inside half has to carry almost all the force. As you cant the foil out, this balance changes with a cross-over around 64 deg cant angle. 

The second graph shows vertical force, and it's quite confusing at first. It shows the same thing that at low cant angle, the outside half contributes minimally, and almost all the vertical lift is carried by the inside. You might think this is wrong (I did first), since the angles should work the opposite way compared to horizontal forces, the outside half is much closer to horizontal - shouldn't it generate the vertical lift? The way it works out is that at those low cant angles, the total lift generated by the outside wing half is minimal.

This can be seen on the third graph. Basically, you can think of it as you need a certain wing angle to counteract the sail forces. At low arm cant angle, the outside wing's angle is just really not aligned well with these forces, so it can't carry much. The two halves become equally loaded at around 62 deg foil cant, which is well below the cant angle that a T foil would use. The load on each wing half is a tiny bit higher than the wing half of the T foil, because of the anhedral angle.

Finally, righting moment is shown on the right side. The blue line indicates the combined righting moment of the two wing halves. As you can see, righting moment of the T foil (dashed line) is reached at a much lower cant angle with the anhedral, and you can generate much more RM with the anhedral foil if you further increase cant angle. If you are limited by how much of the outside tip can stick out of the water, the anhedral foil generates a lot more righting moment.

So what's the downside? Unless you are at the cant angle when the loads on the wing halves are equal (where the lines cross on the third graph), one of your wing halves will carry more lift than the other, so basically you waste some wing area on the other wing half. Plus, if the flap is deflected from its optimum angle, drag will increase further. However, you can see that LR's foil is designed pretty well. The wing halves are loaded equally almost at the same cant angle, when the RM reaches the necessary amount (horizontal dahsed line), around 62 - 63 deg cant.

For contrast, here is what happens at TWS=10kts downwind, when you need less RM:

1021911571_dw10.thumb.png.484d6f28d2264c2c67323ab0fae61b14.png

Note that the cant angle is around 63 deg when the total righting moment is at the required value (dashed line). If you look at the lift on the wing halves, you'll see that the inside half is carrying lot more than the outside at this cant angle, so the foil is not balanced out as nicely as upwind. It's also interesting to see how the cant angle barely changes for the anhedral, whereas the T-foil's cant angle went from 67.5 deg (20kts upwind) to ~58 deg (10kts downwind). This explains why in my previous post, cant angles for ETNZ and AM were the highest upwind, and the lowest downwind.

To sum up, the anhedral foil certainly offers more flexibility, but it's top speed performance may be hindered in some conditions. 

Finally some of my previous histograms of cant angles supporting the above analysis. This is from the first ACWS race, in about 15-16kts of wind, upwind and downwind cant angles:

510053014_uwfcant.png.1ac3da66281e263a98fd5a7393d194d3.png  796790026_dwfcant.png.8d07be5c2f64704ed52fd487c53373ed.png

Note how narrow the histograms are for ETNZ, they have to set the cant at a certain angle for balance, whereas LR can play with it depending on how they want to distribute loads between the wing halves. You can also see, how ETNZ's cant angle is much higher upwind, and much lower downwind than LR's

The more I read this, the more intuitive it feels.

Why would force on each side of the arm be equal? 

Even without flap differential the inboard wing would load up more due to leeway increasing its AoA more than the outboard wing. But with flap differential you could produce the same loading difference but with less leeway. 

Result is thay LR can ay around with different CoE heights without canting or can move foil can without having to change CoE height? 

The thing that still has me confused is thay the upwind cant angles seem very high. I'm not sure they cant be achieved without the hull touching down or the foil surfacing. Os thay becuase the VPP was just left to keep getting faster VMG without limit on maximum cant?

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1 hour ago, barfy said:

I saw that, great video. I still think the elephant in the room is the ventilated outer wing, with no fences, and what that does to the efficiency of 50% of the wing. I've seen heaps of explanations and examples of vortex and tip ventilating, but other that a lot of boats crashing to surface I haven't heard any of the wise men speaking to the loss of lift. There were early comments here when the first videos showed tip breach and full ventilation without falling to earth, but not much commentary for Awhile.

Ventilation drastically reduces Lift, because air is ~1000 times less dense than water.

Calculating just when ventilation will occur gets a bit complex, as any wave action is also a factor. Looking at the following diagram, it shows how ambient air is pushing it's way into the low pressure area on the top side of the foil.

The Y foil has a shallower angle to the water surface, and with less water above the foil, air is able to force it's way into and along the length of the foil easier. As ventilation occurs, the foil will produce less lift, and sink down until ventilation ceases.

image.png.3e52e6e910916f648bb50cc3c0f8bcb0.png

Does this help any?

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22 minutes ago, Mozzy Sails said:

The more I read this, the more intuitive it feels.

Why would force on each side of the arm be equal? 

Even without flap differential the inboard wing would load up more due to leeway increasing its AoA more than the outboard wing. But with flap differential you could produce the same loading difference but with less leeway. 

Result is thay LR can ay around with different CoE heights without canting or can move foil can without having to change CoE height? 

The thing that still has me confused is thay the upwind cant angles seem very high. I'm not sure they cant be achieved without the hull touching down or the foil surfacing. Os thay becuase the VPP was just left to keep getting faster VMG without limit on maximum cant?

I don't quite follow erdb's diagrams. In the second from left, the point at which Vertical force should be equal for both wings of a Y foil is when the foil is at 0° cant, ignoring leeway for the moment. That means the foil arm would be at a cant of 42°, not at 60°. (foil arm cant - 42 = foil cant)

As you mention, leeway will increase the AoA of the windward wing more than the leeward wing. An example of leeway calcs (thanks to @enigmatically2's equation) :

  • Foil cant = 22°
  • Foil AoA = 0°
  • Foil anhedral = 16°
  • Windward wing cant = 38°
  • Leeward wing cant = 6°
     
  • Leeway = 2°
  • Windward wing AoA = 1.23°
  • Leeward wing AoA = 0.21°
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19 minutes ago, MaxHugen said:

I don't quite follow erdb's diagrams. In the second from left, the point at which Vertical force should be equal for both wings of a Y foil is when the foil is at 0° cant, ignoring leeway for the moment. That means the foil arm would be at a cant of 42°, not at 60°. (foil arm cant - 42 = foil cant)

As you mention, leeway will increase the AoA of the windward wing more than the leeward wing. An example of leeway calcs (thanks to @enigmatically2's equation) :

  • Foil cant = 22°
  • Foil AoA = 0°
  • Foil anhedral = 16°
  • Windward wing cant = 38°
  • Leeward wing cant = 6°
     
  • Leeway = 2°
  • Windward wing AoA = 1.23°
  • Leeward wing AoA = 0.21°

0 degrees foil cant is foil symmetry line vertical (in line with big grey gravity arrow)?

Wouldn't the inboard wing be producing more lift in the vertical then? Becuase of leeway, so greater AoA?

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4 minutes ago, Mozzy Sails said:

0 degrees foil cant is foil symmetry line vertical (in line with big grey gravity arrow)?

Wouldn't the inboard wing be producing more lift in the vertical then? Becuase of leeway, so greater AoA?

Yes, that's why I added the caveat " ignoring leeway for the moment ".

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8 minutes ago, MaxHugen said:

Yes, that's why I added the caveat " ignoring leeway for the moment ".

Yeah, got that. But is ERDB ignoring leeway? He's just saying how much load and lift each wing would have to produce to balance forces?

At 0 degrees cant the outboard foil would be producing very little lift. And AoA may be negative (depending on pitch). 

Terrible drawing alert. Below is 0 degree foil cant, blue is flow, yellow lift. 

20210308_104436.png

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On 3/6/2021 at 11:06 PM, Mozzy Sails said:

I don't think you can really separate out vertical and horizontal in this way. 

I few days later, I realise I am wrong.

Ironically I did argue the CoL in  vertical shifted outboard to my friends but was was persuaded differently. Should have stuck with my gut for the video.

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Just now, Mozzy Sails said:

I few days later, I realise I am wrong.

Ironically I did argue the CoL in  vertical shifted outboard to my friends but was was persuaded differently. Should have stuck with my gut for the video.

It's okay to be wrong Mozzy and to make the occasional mistake, it gives the rest of us hope :D

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48 minutes ago, Mozzy Sails said:

Yeah, got that. But is ERDB ignoring leeway? He's just saying how much load and lift each wing would have to produce to balance forces?

At 0 degrees cant the outboard foil would be producing very little lift. And AoA may be negative (depending on pitch).

I don't know if erdb is using a Leeway value to calc the relevant forces for each wing of an anhedral foil.

From this previous post we can see that without any leeway correction, the leeward wing is producing 88% more vertical force than the windward wing, at 20° cant.  That's a lot to equalise using the differential leeway AoA effect.

If someone with a VPP doesn't respond, I might give the calcs a go tomorrow - there's a fair bit to it.

 

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10 minutes ago, MaxHugen said:

I don't know if erdb is using a Leeway value to calc the relevant forces for each wing of an anhedral foil.

From this previous post we can see that without any leeway correction, the leeward wing is producing 88% more vertical force than the windward wing, at 20° cant.  That's a lot to equalise using the differential leeway AoA effect.

If someone with a VPP doesn't respond, I might give the calcs a go tomorrow - there's a fair bit to it.

 

Guess we should wait for erdb to answer. 

The way I read it is it's about 20 degree foil cant when both wing halves are producing equal vertical lift and this differs from your zero leeway calculation at 20 degrees because erdb suggest inboard foil is producing more total lift at that cant. 

Not sure if that is leeway prediction or just a result of force balancing for a given CoE. 

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9 hours ago, MaxHugen said:

Ventilation drastically reduces Lift...

image.png.3e52e6e910916f648bb50cc3c0f8bcb0.png

Does this help any?

What’s the direction of travel of this foil - out of the page or right to left? If it’s out of the page I don’t understand the water jet or why it appears to have a stagnation point? If right to left that’s more representative of planing than foiling. 

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7 hours ago, MaxHugen said:

I don't know if erdb is using a Leeway value to calc the relevant forces for each wing of an anhedral foil.

From this previous post we can see that without any leeway correction, the leeward wing is producing 88% more vertical force than the windward wing, at 20° cant.  That's a lot to equalise using the differential leeway AoA effect.

If someone with a VPP doesn't respond, I might give the calcs a go tomorrow - there's a fair bit to it.

 

 

6 hours ago, Mozzy Sails said:

Guess we should wait for erdb to answer. 

The way I read it is it's about 20 degree foil cant when both wing halves are producing equal vertical lift and this differs from your zero leeway calculation at 20 degrees because erdb suggest inboard foil is producing more total lift at that cant. 

Not sure if that is leeway prediction or just a result of force balancing for a given CoE. 

Sorry guys took some time off :D. There are obviously several ways to look at it, and as with all my other posts, there's a good chance I'm wrong.

But... I think what we agree on with Mozzy is that the cant angle of the foil is only one thing. The other is: what forces you need to generate with the foil. Indirectly that's the same as including leeway into the calculations.  When the boat is towed with no sails on, foiling with both foils down such that the end of the foil arm is vertical, the forces are symmetrical between the wing halves of the anhedral foil. Both halves would produce equal lift. However, if you have sails up, you have a lateral force component that you need to counter. If you tried to sail with the foil in the same position (foil arm end vertical), the inside wing would have to produce much more lift than the outside, since it's the only wing half that can counter the lateral force. The outside one is angled the wrong way (there may be some lateral horizontal force combinations, when it would even need to generate negative lift - theoretically). To achieve this force distribution, you would probably angle the flap more on the inside, but the AOA of the two wing halves are different, too, since you have leeway (or yaw angle depending on how you look at it). That's why I said in a previous post, that just by looking at these 2D diagrams showing the foils from behind, you can't solve the problem, because you need to know what the counter forces are.

Let's imagine for example that you have a 90 deg anhedral foil, canted out 45 deg, so one wing is vertical, the other is horizontal. The vertical wing counters the lateral sail forces, the horizontal carries the weight of the boat. As you sheet  in the sails and increase lateral forces, the vertical wing would need to produce more and more lift to counter the lateral forces. The foil is still in the same position, your 2D diagram looks the same, but the force vectors are different, therefore, righting moment is different, too. Whether you increase the lateral force on the vertical wing by increasing flap angle or yawing the boat (or increasing leeway) is a secondary question. It depends on how you want to sail your boat.

Now one limitation of how I compared the two foil setups is that I assumed the sail forces are identical. However, they are obviously different between LR and ETNZ, so the loading of the foils will be different as well, and that will change the optimal cant angle and the righting moment, too. This is why they didn't converge on the same design, because within their own packages, each team considers their solution the best.

 

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1 hour ago, sosoomii said:

What’s the direction of travel of this foil - out of the page or right to left? If it’s out of the page I don’t understand the water jet or why it appears to have a stagnation point? If right to left that’s more representative of planing than foiling. 

That's a stock hydrofoil pic, looking at it from front or rear.

The top side has a low pressure area which produces lift. If the pressure is low enough, you could think of it as "sucking" air down - that's ventilation.

On the bottom side it's high pressure, thus it's "forcing" water up.

I don't know what you mean by a "stagnation point"?

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1 hour ago, erdb said:

Sorry guys took some time off :D. There are obviously several ways to look at it, and as with all my other posts, there's a good chance I'm wrong.

How are you calculating the leeway angle to get the AoA of each wing of the foil, and then the forces for each?

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53 minutes ago, MaxHugen said:

How are you calculating the leeway angle to get the AoA of each wing of the foil, and then the forces for each?

I didn't calculate it here. I just solved the geometrical questions of how with a given cant angle, you can produce the same vertical and lateral forces as a T foil. Whether and how these forces can be generated is a completely different question depending on speed, yaw angle, pitch, speed, flap angle etc.

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15 hours ago, MaxHugen said:

Ventilation drastically reduces Lift, because air is ~1000 times less dense than water.

Calculating just when ventilation will occur gets a bit complex, as any wave action is also a factor. Looking at the following diagram, it shows how ambient air is pushing it's way into the low pressure area on the top side of the foil.

The Y foil has a shallower angle to the water surface, and with less water above the foil, air is able to force it's way into and along the length of the foil easier. As ventilation occurs, the foil will produce less lift, and sink down until ventilation ceases.

image.png.3e52e6e910916f648bb50cc3c0f8bcb0.png

Does this help any?

Well it does, but, the foil isn't sinking. And the ventilation doesn't stop. And ventilation of rudder in this class and the last has been catastrophic.

So, without fences, what's to stop the entire outboard wing to loose most lift? The overheads show air along the entire wing.

What does this do to the lift vector diagrams you fellows are debating?

Or mozzys RM calculations?

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Ventilation is a big problem for rudders the stern flies out of the water when it happens, ETNZ had such an experience in Burmuda. 

The foils that support the boat are sort of self-correcting ventilation leads to a drop-down into more water, not a catastrophe. 

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1 hour ago, coercivity said:

Ventilation is a big problem for rudders the stern flies out of the water when it happens, ETNZ had such an experience in Burmuda. 

The foils that support the boat are sort of self-correcting ventilation leads to a drop-down into more water, not a catastrophe. 

Or the stern sinks in this class and the boat flies out of the water.

But certainly you would sort of notice when your boat loses 50% of lift as the tip breaches.

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22 hours ago, Mozzy Sails said:

The thing that still has me confused is thay the upwind cant angles seem very high. I'm not sure they cant be achieved without the hull touching down or the foil surfacing. Os thay becuase the VPP was just left to keep getting faster VMG without limit on maximum cant?

I originally calculated the maximum practical foil cant, with just the tip out, to be 25°. But that was from the Rules diagram, as there was nothing better to use.

Since then, a great stern shot of NZ indicated it was more likely to be 22°, and indeed, that is generally the cant NZ use upwind from the data.  (ie  64° foil arm cant)

image.png.e788ab0640a03e32f13173699a1d5875.png

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35 minutes ago, MaxHugen said:

I originally calculated the maximum practical foil cant, with just the tip out, to be 25°. But that was from the Rules diagram, as there was nothing better to use.

Since then, a great stern shot of NZ indicated it was more likely to be 22°, and indeed, that is generally the cant NZ use upwind from the data.  (ie  64° foil arm cant)

image.png.e788ab0640a03e32f13173699a1d5875.png

From looking at the footage, upwind they fly with more foil out, but windward heel. Is 500mm their average flight height? Looks like less by eye. I guess we have data on this. I guess wing flex will bring more tip out as well. 

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8 hours ago, Stingray~ said:

About ride height

 

My only take-away from this was...

No mention at all about an "end plating" effect of hull to water to prevent pressure migration at the sail, as was extensively debated here months ago.

Encouraging more airflow to the sails is one (good) thing, but I have yet to buy into hull/water "end plating".

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1 hour ago, Mozzy Sails said:

From looking at the footage, upwind they fly with more foil out, but windward heel. Is 500mm their average flight height? Looks like less by eye. I guess we have data on this. I guess wing flex will bring more tip out as well. 

Many variables as always. Given a probable 5% margin of error in the diagram, it could be between 475-525 mm. :lol:

Pitch will bring the bow area closer to the water of course, they don't always have the tip out, they don't always have the cant at 22°, and as you say, heel and/or foil flex changes foil/water height... etc.

We don't have access to ride height data.

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So another thought (whilst I'm desperately looking for reasons to support my strong belief LR will win), in the pressure diagrams from Vittorio's video, we see that the boats generate a large zone of high pressure to windward of both the hull and the sails.   I'm thinking that high pressure is going to be pushing on the hull just as much as the sail, as the pressure difference is the same and the area is the same.  So the fact that ETNZ has lowered the deck and created a bit more sail area below the reference level of the base of the mast is not that significant in the situation that the sails are sealed to the hull and the hull is close to the water.   If fact the high pressure for NZ generated by that extra sail area is going to push on the inside of the crew cubby just as much as it pushes on the sail.   Sure the high pressure also pushes on the outside of the crew cubby, but that's the same as the high pressure pushing on LR's hull.

 

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30 minutes ago, sfigone said:

If fact the high pressure for NZ generated by that extra sail area is going to push on the inside of the crew cubby just as much as it pushes on the sail. 

This is what I mean:

662454278_Screenshotfrom2021-03-0913-11-47.png.de28e30d723d90d34869ae4be5485990.png

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8 hours ago, MaxHugen said:

My only take-away from this was...

No mention at all about an "end plating" effect of hull to water to prevent pressure migration at the sail, as was extensively debated here months ago.

Encouraging more airflow to the sails is one (good) thing, but I have yet to buy into hull/water "end plating".

I can't listen to them for more than a few seconds, but as they say the air not passing under the hull is a positive... That's end-plating. It's important and it's happening. You can explain its positive effects in many different ways (keeping the windward side pressure higher or reducing vortices), but ultimately it increases the lift/drag ratio of the hull+sails and makes the boat faster.

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On 3/8/2021 at 4:24 AM, Mozzy Sails said:

The thing that still has me confused is thay the upwind cant angles seem very high. I'm not sure they cant be achieved without the hull touching down or the foil surfacing. Os thay becuase the VPP was just left to keep getting faster VMG without limit on maximum cant?

9 hours ago, Mozzy Sails said:

From looking at the footage, upwind they fly with more foil out, but windward heel. Is 500mm their average flight height? Looks like less by eye. I guess we have data on this. I guess wing flex will bring more tip out as well. 

 

8 hours ago, MaxHugen said:

Many variables as always. Given a probable 5% margin of error in the diagram, it could be between 475-525 mm. :lol:

Pitch will bring the bow area closer to the water of course, they don't always have the tip out, they don't always have the cant at 22°, and as you say, heel and/or foil flex changes foil/water height... etc.

We don't have access to ride height data.

In my race stats, I also calculate "real cant angle", which is heel + cant:

1902572560_cantheel.thumb.png.51119bc1b590517b95f308b9bacf0c13.png

AM did get up to ~66 degrees in the higher wind range (upwind) by heeling to windward way more than the others:

heel.thumb.png.721fbb0dbe5de07d3d149bad613412e6.png

Maybe this is why they didn't have a fat keel on their boat, so they could heel windward more. It all comes back to what they can do with their sails.

BTW, we do have ride height data as well, but I don't know what the reference point, and if we can compare the numbers between boats. I guess the sensor is on the prod, and I'm not sure the prod's shape is identical on all boats. Pitch angle will of course also affect it. Anyway here it is  - unfortunately no data from the ACWS, but some interesting differences from LR. The 2m+ heights are all from the RR, and in all their Prada semi and final races they were between 1 - 1.5m. They changed their foils in between, right?

height.thumb.png.420bf7ccf462be81886801bb53675f1d.png

 

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@ERDB so, the 67.5 or so degree cant for upwind given in the forces on an anhedral post. 

Where did that come from/ Is that just what the VPP says they should cant to for 20knots TWS upwind?

Considering they can't get that canted? What does it mean, just that they would be easing sails before this point?

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9 hours ago, MaxHugen said:

My only take-away from this was...

No mention at all about an "end plating" effect of hull to water to prevent pressure migration at the sail, as was extensively debated here months ago.

Encouraging more airflow to the sails is one (good) thing, but I have yet to buy into hull/water "end plating".

Yes I am yet to buy in to that as well.

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1 hour ago, Mozzy Sails said:

@ERDB so, the 67.5 or so degree cant for upwind given in the forces on an anhedral post. 

Where did that come from/ Is that just what the VPP says they should cant to for 20knots TWS upwind?

Considering they can't get that canted? What does it mean, just that they would be easing sails before this point?

Yeah I took the vertical and horizontal foil forces from my VPP at 20kts TWS, 43 deg TWA, which was the predicted target angle for best upwind VMG. The forces should be in the ballpark, but obviously not perfect. I noticed that the boats tend to sail lower and faster than my VPP suggested, but the VMG prediction was spot on. Maybe the cant angle limit is the reason they can't point that high?

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Just a few interesting bits of the straight-line performances from today:

Upwind /Downwind VMGs - race 1:

1283233917_uwvmg.png.aa6ea5dbd3dbddd48e005d4f9e58a252.png158401086_dwvmg.png.fccc61e45d401d49d4f275c6f67e9c9c.png

Upwind /Downwind VMGs - race 2:

1329043279_uwvmg.png.debba939b95b25c69285bce7defbbcdb.png1068476415_dwvmg.png.bc3e05f35c0541726c4c3729e5dbcd08.png

Same story in both races actually. Upwind - nothing in it, downwind - ETNZ is fast! The problem for ETNZ is that the downwind legs are over too quickly, and it's hard to make a pass even with this extra vmg. Actually the speeds are similar, but ETNZ points much lower:

 1025980950_dwsbs.png.b6cc3e990e1a522137a0ada1dbbaaf06.png265073594_dwtwa.png.ef7cf399b97fc56efccacd325d2c6cb5.png

In both races, LR sailed the shifts much better (race1 and 2):

favtack.png.d08908bd538602820c85c40c4da0243a.pngfavtack.png.f16e646ce799e527a2f0e10bf23da4a6.png

Some interesting changes in how the boats are sailed. Look at the cant angles - race 1 upwind, downwind:

1323067766_uwcant.png.ef6a079b8ab078b71d4122df46c3b02a.png831504200_dwcant.png.528b0eb189b1ee71d5db82a9861f1bb5.png

Race 2 upwind, downwind:

264341255_uwcant.png.dcb719fe780ec23f4883542c2306db00.png1647682490_dwcant.png.9bcf2c498a8554d3eb4ee57756867f32.png

ETNZ has a completely different technique compared to the ACWS. They're shifting through a wide range of cant angles all the time. All the way up to 68 degrees (!), and it wasn't even that windy today (13-14 kts).  In contrast, LR changed their cant angle much less than they used to, and while upwind ETNZ still cants their foil out more, the average cant angles downwind are similar now between the two teams. 

Leeway still has a double peak sometimes and for both boats. Both had negative leeway on port and positive leeway on starboard:

1776923072_uwleew.png.b2552e8801fefb9468a266d892eea16e.png67873128_dwleew.png.ef507825997b82c1fddc24877dc71d88.png

I'll analyze tacks and jibes later, but it was obvious that LR's acceleration out of the tacks was much better. It will be an interesting regatta for sure. Amazing to see that despite all the differences in design, the boats are so close in performance. On one hand, if my favored tack analysis is any good, ETNZ left a little bit in the bag today by constantly sailing on the wrong tack. On the other, if I had to chose between better tacking vs better VMG downwind, I'd take better tacking. Overall, it makes a much bigger difference I think, because it opens up your options - tactical against the other boat or picking shifts.

 

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1 hour ago, erdb said:

ETNZ left a little bit in the bag today by constantly sailing on the wrong tack.

In the post-race interview for Race1 PB was pretty casual about being off-phase in the 2nd beat.

'Never really thought it was worth doing the extra maneuvre, probably a wrong move in hind-sight'

Race 2 they were forced to be off-shift by LR/tacking on them.

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1 hour ago, Mozzy Sails said:

ETNZ data on the America's cup virtual eye site seems to be out of sync with foils cant Vs everything else. But it looks likes they are canting out before tacks

Heading:

image.thumb.png.8c6bd2bea331b6aed56143e27ae757fe.png

Foil Pos
image.thumb.png.9a0de4c0334cab4022e88e44d7184c53.png

NZ's foil cant seemed excessive to me, especially in R2, so I watched some of the stern camera footage to check on heel.

Sure enough, they are are using 1.0-2.5° heel to increase cant upwind. Eg:

image.thumb.png.0f326fa93461096cc95a2c978d2c8d2f.png

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45 minutes ago, erdb said:

Hmm, maybe that's ETNZ's cant angles were all over the place for me, too. I have to check. I just ran my stuff on the files without looking at it in detail.

They look correct, but just out of sync. So I think your histograms are a correct reflection. I went through quite a bit of the virtual eye stuff. Also onboard communications they are talking about cant angles being 'good'. Whereas at the ACWS they never mentioned them. 

The sync just makes it hard to correlate with other metrics.. like speed gain, TWA, VMG, heel, pitch etc. 

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Hello girls and boys, AC fans. 

I am joining party here and throwing myself to teoretical lions of this forum.

Want to share my wiew on AC75 hydro-aero forces and their balance. I hope It might clear things for some people.

 

@Mozzy and @MaxHugen, You are both somewhat wrong. you complicate simple things and then you succed to pull wrong assumptions, aka "TNZ has bigger RM becouse they have bigger leever by x mm." Give up urge to break forces to componets where you dont have to.

Here is how I see forces on AC75; under few assupmtions: Yact is steady foiling with no heel AND! no LEEWAY relative to foil arm! Ruder forces disregarded. Foil arm enters water at angle 68 deg to WP. Flaps at equal angle.

Now, boat and crew have weight. Lets say it is 8ooo kg. Lets say gravity g is 10 m/s2 Force FG=8000*g=80kN. We know this is somewhere at vertical center line. (exact position along vertical CL not important now) To lift boat up, force equal to FG is needed, in SAME DIRECTION! This is Fz. Fz is parallel to FG, (we don't know direction of Fz, only that it is upward and paralel to FG)

Sails produce side force; FS. (Lets asume boat is upright and FS is directed parallel to waterplane) FS is high in the air.

FS force must be balanced by some other force, in this case hydrodinamic force comming from foils, arms, rudders etc; Let this be Fy. We dont know where this Fy is, only that it is parallalel to the waterplane.

 

Now, Lets draw point which lay at intersection of FG and Fs dirrections. It is somewhere on vertical centerline. This is POINT AT WHICH Combined FG and FS resultant force is acting. (FULCRUM?) Lets call this force FRGS.

Lets dive in the water and see forces there.

There is only ONE force there, Foil lift! Lets call it FF. (remember, no leeway...) I dont know how big FF is, but I know its dirrection on exactly! This dirrection lays on foil wing symmetry plane (as defined in class rules)at same angle at which foil arm enters water, and becouse no leeway and simetrical wings, FF is Shooting straight trough foil arm CL, at same 68 degrees from water, and... and shooting dirtectly at FRGs fulcrum! AHA!! yes, at exact same point high up...

WE do know that FRGs is of same magnitude as FF. They are on same direction, which means no moment about any x axis is needed to balance yacht, Yacht is sailing in balance.

WE know: FG=FFz=Fz, FS=FFy

From this we can calculate all forces of interest, like hydro FFz and FFy, FS... Or let Autodesk ForceEffect app do it.

Screenshot_20210310_222344~2.jpg

Screenshot_20210310_221200_com.autodesk.fbd.activities.jpg

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1 minute ago, gigi said:

Hello girls and boys, AC fans. 

I am joining party here and throwing myself to teoretical lions of this forum.

Want to share my wiew on AC75 hydro-aero forces and their balance. I hope It might clear things for some people.

 

@Mozzy and @MaxHugen, You are both somewhat wrong. you complicate simple things and then you succed to pull wrong assumptions, aka "TNZ has bigger RM becouse they have bigger leever by x mm." Give up urge to break forces to componets where you dont have to.

Here is how I see forces on AC75; under few assupmtions: Yact is steady foiling with no heel AND! no LEEWAY relative to foil arm! Ruder forces disregarded. Foil arm enters water at angle 68 deg to WP. Flaps at equal angle.

Now, boat and crew have weight. Lets say it is 8ooo kg. Lets say gravity g is 10 m/s2 Force FG=8000*g=80kN. We know this is somewhere at vertical center line. (exact position along vertical CL not important now) To lift boat up, force equal to FG is needed, in SAME DIRECTION! This is Fz. Fz is parallel to FG, (we don't know direction of Fz, only that it is upward and paralel to FG)

Sails produce side force; FS. (Lets asume boat is upright and FS is directed parallel to waterplane) FS is high in the air.

FS force must be balanced by some other force, in this case hydrodinamic force comming from foils, arms, rudders etc; Let this be Fy. We dont know where this Fy is, only that it is parallalel to the waterplane.

 

Now, Lets draw point which lay at intersection of FG and Fs dirrections. It is somewhere on vertical centerline. This is POINT AT WHICH Combined FG and FS resultant force is acting. (FULCRUM?) Lets call this force FRGS.

Lets dive in the water and see forces there.

There is only ONE force there, Foil lift! Lets call it FF. (remember, no leeway...) I dont know how big FF is, but I know its dirrection on exactly! This dirrection lays on foil wing symmetry plane (as defined in class rules)at same angle at which foil arm enters water, and becouse no leeway and simetrical wings, FF is Shooting straight trough foil arm CL, at same 68 degrees from water, and... and shooting dirtectly at FRGs fulcrum! AHA!! yes, at exact same point high up...

WE do know that FRGs is of same magnitude as FF. They are on same direction, which means no moment about any x axis is needed to balance yacht, Yacht is sailing in balance.

WE know: FG=FFz=Fz, FS=FFy

From this we can calculate all forces of interest, like hydro FFz and FFy, FS... Or let Autodesk ForceEffect app do it.

Screenshot_20210310_222344~2.jpg

Screenshot_20210310_221200_com.autodesk.fbd.activities.jpg

Wow, that is a splashy entrance!
Welcome to fray, Gigi.

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14 minutes ago, gigi said:

FG=8000*g=80kN. We know this is somewhere at vertical center line. (exact position along vertical CL not important now)

so you say they are wrong by assuming the teams have different leavers/weight combos ... then you go on to make the simplification that both teams have the exact same leaver ... ... ... they don't

 

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3 minutes ago, Lickindip said:

so you say they are wrong by assuming the teams have different leavers/weight combos ... then you go on to make the simplification that both teams have the exact same leaver ... ... ... they don't

 

My point is that forces align for no leeway case. ( and other asumptions stated)numbers are for ilustrative purpose.

 

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5 minutes ago, gigi said:

My point is that forces align for no leeway case. ( and other asumptions stated)numbers are for ilustrative purpose.

 

then do one for each boat, the RC32(x), RB32(x) position is different for each boat

each boat will have a slightly different CG point not aligning with the CL of the boat depending on the cant they are running on each foil and the weight distribution in each foil, position of crew etc etc etc

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11 minutes ago, Lickindip said:

then do one for each boat, the RC32(x), RB32(x) position is different for each boat

each boat will have a slightly different CG point not aligning with the CL of the boat depending on the cant they are running on each foil and the weight distribution in each foil, position of crew etc etc etc

Yea, but RC is always aligned with my FRGs. There is case for every cant angle... Biger cant angle means bigr side force, thus total force.

Mozzys video prior cup match got me thinking. What he was wrong in video was not to include negative RM moment from Fy force componet, taking into account only Fz. From there all assumptions about bigger RM arm  was off. It is biger but not way that was drown in video.

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one more thing to think: to cant futher out means to lower CE from sails. (again for no leeway case)

Now lower sail CE gives more FS side force. But by canting more also you have more FFY automaticaly. so again all is in bllance.

So more cant = more twist in sails? So les upwwind angle, more speed? sound ETNZ to me.

What is fascinating now is that boats are locked in configs. Now is up to the sailors tu use best whaeet they have, and they have very differently tuned boats. Great times.

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Welcome Gigi

The forces balance, even with leeway though. Unless the boat is accelerating the forces will balance. 

So the question in more who's forces balance with the greatest FS. 

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Ok PB will get better and responds well to pressure in the starts specifically.

The sail choice has an element of luck in it as was seen in the second race ETNZ pulled back a lot of distance on the final leg and it was wind change in my opinion. 

ETNZ on average looked untroubled in front and LR but held ETNZ out by 7 seconds. 

The wind tomorrow may demand a CZ or alternatively no racing. Saturday is also light. there will be more wins and loses to each team, but 13 races in yesterdays conditions would see ETNZ win more. 

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28 minutes ago, coercivity said:

Ok PB will get better and responds well to pressure in the starts specifically.

The sail choice has an element of luck in it as was seen in the second race ETNZ pulled back a lot of distance on the final leg and it was wind change in my opinion. 

ETNZ on average looked untroubled in front and LR but held ETNZ out by 7 seconds. 

The wind tomorrow may demand a CZ or alternatively no racing. Saturday is also light. there will be more wins and loses to each team, but 13 races in yesterdays conditions would see ETNZ win more. 

Sail choice seems to be the most undercommented  topic in this race, in and out of this forum. Somewhat amusing in a sailing competition. 

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2 hours ago, gigi said:

Hello girls and boys, AC fans. 

I am joining party here and throwing myself to teoretical lions of this forum.

Want to share my wiew on AC75 hydro-aero forces and their balance. I hope It might clear things for some people.

 

@Mozzy and @MaxHugen, You are both somewhat wrong. you complicate simple things and then you succed to pull wrong assumptions, aka "TNZ has bigger RM becouse they have bigger leever by x mm." Give up urge to break forces to componets where you dont have to.

Here is how I see forces on AC75; under few assupmtions: Yact is steady foiling with no heel AND! no LEEWAY relative to foil arm! Ruder forces disregarded. Foil arm enters water at angle 68 deg to WP. Flaps at equal angle.

Now, boat and crew have weight. Lets say it is 8ooo kg. Lets say gravity g is 10 m/s2 Force FG=8000*g=80kN. We know this is somewhere at vertical center line. (exact position along vertical CL not important now) To lift boat up, force equal to FG is needed, in SAME DIRECTION! This is Fz. Fz is parallel to FG, (we don't know direction of Fz, only that it is upward and paralel to FG)

Sails produce side force; FS. (Lets asume boat is upright and FS is directed parallel to waterplane) FS is high in the air.

FS force must be balanced by some other force, in this case hydrodinamic force comming from foils, arms, rudders etc; Let this be Fy. We dont know where this Fy is, only that it is parallalel to the waterplane.

 

Now, Lets draw point which lay at intersection of FG and Fs dirrections. It is somewhere on vertical centerline. This is POINT AT WHICH Combined FG and FS resultant force is acting. (FULCRUM?) Lets call this force FRGS.

Lets dive in the water and see forces there.

There is only ONE force there, Foil lift! Lets call it FF. (remember, no leeway...) I dont know how big FF is, but I know its dirrection on exactly! This dirrection lays on foil wing symmetry plane (as defined in class rules)at same angle at which foil arm enters water, and becouse no leeway and simetrical wings, FF is Shooting straight trough foil arm CL, at same 68 degrees from water, and... and shooting dirtectly at FRGs fulcrum! AHA!! yes, at exact same point high up...

WE do know that FRGs is of same magnitude as FF. They are on same direction, which means no moment about any x axis is needed to balance yacht, Yacht is sailing in balance.

WE know: FG=FFz=Fz, FS=FFy

From this we can calculate all forces of interest, like hydro FFz and FFy, FS... Or let Autodesk ForceEffect app do it.

Screenshot_20210310_222344~2.jpg

Screenshot_20210310_221200_com.autodesk.fbd.activities.jpg

Hey gigi, I agree with you, and this is how I built my VPP. I've posted this diagram several times to explain how the foil cant angle is determined by the CoE of the sails. f_coe.JPG.057b03e4a40d8d6570b4bb82963fb443.JPG

However, this is only true if you assume (as you said) that the combined foil force vector lines up with the symmetry plane of the foil. Where it gets more complicated, and this was the subject of recent posts, is when an anhedral foil has its flaps deflected differentially. The overall balance is still the same, the resultant lift vector needs to point towards the sails' CoE, however, it doesn't line up with the symmetry plane of the foil. So you can play a little by changing cant angle, and deflecting the foils differentially, and the question was how that would affect righting moment.

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4 hours ago, Mozzy Sails said:

They look correct, but just out of sync. So I think your histograms are a correct reflection. I went through quite a bit of the virtual eye stuff. Also onboard communications they are talking about cant angles being 'good'. Whereas at the ACWS they never mentioned them. 

The sync just makes it hard to correlate with other metrics.. like speed gain, TWA, VMG, heel, pitch etc. 

Yeah, you're right, the timing is only off by a little.

I enjoyed the video, and was happy to see the histograms featured. Let me know if you want to look at other stuff or present things in a different way.

I wonder if what changed in the way they sail is that instead of adjusting the sail twist and CoE to the foil cant angle, now they keep the sail at a more efficient shape and adjust the cant instead to match changing in forces. I still don't know how they achieve those crazy cant angles. I guess they poke the tip of the foil out more than we thought.

 

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14 hours ago, erdb said:

Leeway still has a double peak sometimes and for both boats. Both had negative leeway on port and positive leeway on starboard:

1776923072_uwleew.png.b2552e8801fefb9468a266d892eea16e.png67873128_dwleew.png.ef507825997b82c1fddc24877dc71d88.png

 

Why would both boats have negative leeway on port and positive leeway on starboard?

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2 hours ago, gigi said:

Hello girls and boys, AC fans. 

I am joining party here and throwing myself to teoretical lions of this forum.

Want to share my wiew on AC75 hydro-aero forces and their balance. I hope It might clear things for some people.

 

@Mozzy and @MaxHugen, You are both somewhat wrong. you complicate simple things and then you succed to pull wrong assumptions, aka "TNZ has bigger RM becouse they have bigger leever by x mm." Give up urge to break forces to componets where you dont have to.

Here is how I see forces on AC75; under few assupmtions: Yact is steady foiling with no heel AND! no LEEWAY relative to foil arm! Ruder forces disregarded. Foil arm enters water at angle 68 deg to WP. Flaps at equal angle.

Now, boat and crew have weight. Lets say it is 8ooo kg. Lets say gravity g is 10 m/s2 Force FG=8000*g=80kN. We know this is somewhere at vertical center line. (exact position along vertical CL not important now) To lift boat up, force equal to FG is needed, in SAME DIRECTION! This is Fz. Fz is parallel to FG, (we don't know direction of Fz, only that it is upward and paralel to FG)

Sails produce side force; FS. (Lets asume boat is upright and FS is directed parallel to waterplane) FS is high in the air.

FS force must be balanced by some other force, in this case hydrodinamic force comming from foils, arms, rudders etc; Let this be Fy. We dont know where this Fy is, only that it is parallalel to the waterplane.

 

Now, Lets draw point which lay at intersection of FG and Fs dirrections. It is somewhere on vertical centerline. This is POINT AT WHICH Combined FG and FS resultant force is acting. (FULCRUM?) Lets call this force FRGS.

Lets dive in the water and see forces there.

There is only ONE force there, Foil lift! Lets call it FF. (remember, no leeway...) I dont know how big FF is, but I know its dirrection on exactly! This dirrection lays on foil wing symmetry plane (as defined in class rules)at same angle at which foil arm enters water, and becouse no leeway and simetrical wings, FF is Shooting straight trough foil arm CL, at same 68 degrees from water, and... and shooting dirtectly at FRGs fulcrum! AHA!! yes, at exact same point high up...

WE do know that FRGs is of same magnitude as FF. They are on same direction, which means no moment about any x axis is needed to balance yacht, Yacht is sailing in balance.

WE know: FG=FFz=Fz, FS=FFy

From this we can calculate all forces of interest, like hydro FFz and FFy, FS... Or let Autodesk ForceEffect app do it.

Screenshot_20210310_222344~2.jpg

Screenshot_20210310_221200_com.autodesk.fbd.activities.jpg

Awesome! But there is the small matter of showing us yours (or your wife's) tits!

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Just now, MaxHugen said:

Why would both boats have negative leeway on port and positive leeway on starboard?

Beats me. Leeway is calculated as the difference in compass heading and GPS course. If the compass is off by a few degrees, so that its reference line is not completely parallel with the centerline of the boat, that could cause an error like this. It's weird that both boats have the same kind of error, but I guess it's possible. Maybe there is something metal/magnetic on both boats that messes with the compass? Or some magnetic disturbance in the Haruaki bay:D! If it was current, it should be reversed downwind vs upwind, but it's not.

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22 minutes ago, erdb said:

Yeah, you're right, the timing is only off by a little.

I enjoyed the video, and was happy to see the histograms featured. Let me know if you want to look at other stuff or present things in a different way.

I wonder if what changed in the way they sail is that instead of adjusting the sail twist and CoE to the foil cant angle, now they keep the sail at a more efficient shape and adjust the cant instead to match changing in forces. I still don't know how they achieve those crazy cant angles. I guess they poke the tip of the foil out more than we thought.

 

Well, thanks for your plots. 

I saw the cant data in the virtual eye, but your histograms show it so much better. 

I think max is on to something with it being linked to heel. 

One idea is more cant and leeward heel going in to the tack. Ride height would stay same but effective cant angle would decrease. Moving CoE to leeward helps heading up. 

Other idea is they are using leeward helm to balance the helm in a straight line. 

But... becuase the cant data is.out of sync it's hard to tally these two. 

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Can anyone please offer any info on the wind conditions for each race and how they differed - either the prestart measurement or notable changes during each race?

I have been quite surprised how poor the info packages are here compared to Bermuda where the wind speed was often overlayed on the graphics and changes in TWD were often at the top of the screen.

Here they seem more obsesses with the on-boat footage graphics which have tons of numbers and look fancy but which are practically pointless to most viewers

Any thoughts on the wind changes which may offer some insight into the different performances in each race?

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47 minutes ago, erdb said:

Yeah, you're right, the timing is only off by a little.

I enjoyed the video, and was happy to see the histograms featured. Let me know if you want to look at other stuff or present things in a different way.

I wonder if what changed in the way they sail is that instead of adjusting the sail twist and CoE to the foil cant angle, now they keep the sail at a more efficient shape and adjust the cant instead to match changing in forces. I still don't know how they achieve those crazy cant angles. I guess they poke the tip of the foil out more than we thought.

 

This is quite important in terms of work done to trim the boat as moving the sails in any way requires grinding. However moving the foils to achieve trim uses battery power. So simply moving the foils means less grinding. 

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15 minutes ago, Mozzy Sails said:

Well, thanks for your plots. 

I saw the cant data in the virtual eye, but your histograms show it so much better. 

I think max is on to something with it being linked to heel. 

One idea is more cant and leeward heel going in to the tack. Ride height would stay same but effective cant angle would decrease. Moving CoE to leeward helps heading up. 

Other idea is they are using leeward helm to balance the helm in a straight line. 

But... becuase the cant data is.out of sync it's hard to tally these two. 

I have histograms for heel and effective cant (heel-corrected cant angle) as well. Trends are still the same (ETNZ vs LR)

Race 1:

947203873_uwheel.png.7f2586fcbdd0ca92cd28027b15233c5f.png570529890_uweffcant.png.bfd572a652b1ca0eb5620e91b785387c.png

 

1432170252_dwheel.png.3e279dd5b638ce3da68f6a0598b855df.png1617778304_dweffcant.png.8945e865c4acdf79390c4eb85d98388c.png

Race 2:

549548145_uwheel.png.0d08f481db69fc6bf2c1c8d7150dfd80.png183226283_uweffcant.png.5ead2ec5d7d147b43931d630c8108f04.png

867379508_dwheel.png.bc8d9be285a7e687c5ad3a0cbf72dfdf.png1770804796_dweffcant.png.35c94f2d896e44ba5c7829b7d3da1065.png

It's weird that ETNZ only heels windward downwind, whereas upwind they are close to level with a slight heel to lee (on average). Any idea why they'd do that?

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21 minutes ago, Mozzy Sails said:

Well, thanks for your plots. 

I saw the cant data in the virtual eye, but your histograms show it so much better. 

I think max is on to something with it being linked to heel. 

One idea is more cant and leeward heel going in to the tack. Ride height would stay same but effective cant angle would decrease. Moving CoE to leeward helps heading up. 

Other idea is they are using leeward helm to balance the helm in a straight line. 

But... becuase the cant data is.out of sync it's hard to tally these two. 

I've had a look at @dorox's stats app , and something odd is going on... in every upwind beat in each race, both foils start at moderate cant and then gradually increase throughout the tack.

Wondering if the hydraulics are playing up... slow loss of pressure?

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3 minutes ago, amc said:

This is quite important in terms of work done to trim the boat as moving the sails in any way requires grinding. However moving the foils to achieve trim uses battery power. So simply moving the foils means less grinding. 

That's a good point. Reliability of the FCS may be a factor as well. Now they're more confident messing with it all the time than before.

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