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52 minutes ago, sosoomii said:

Of course, in reality zero or negative leeway upwind is not possible -the foil arm must be gybed. Is there any info on the arm angle?

Leeway is basically yaw angle of the hull. It can be zero or negative, too depending on how the foil wing and flaps are set up. The foil arm doesn't really do much once the boat is up to speed on foils. 

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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

OK, it sounds like there's some interest in this topic, so here goes.   Any engineering effort starts by defining the requirements.  From this figure, it looks like the average foil area is 1.64

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Hmmm, not sure that’s how I would define leeway. I’d say leeway is the angle of attack of the keel or other primary underwater foil, which is usually is parallel to the boat centreline but not necessarily.  Something must be stopping the boat slipping sideways and it sure ain’t the hull.  If not the foil arms then the rudder?

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2 hours ago, sosoomii said:

Hmmm, not sure that’s how I would define leeway. I’d say leeway is the angle of attack of the keel or other primary underwater foil, which is usually is parallel to the boat centreline but not necessarily.  Something must be stopping the boat slipping sideways and it sure ain’t the hull.  If not the foil arms then the rudder?

Angle of leeway is well defined in many textbooks about sailing theory in a consistent way, and so is angle of attack in aero- or hydrodynamics. Neither is a matter of opinion in most if not all cases.

Definitions of AoA and angle of leeway are mostly not the same thing in sailing applications.

Angle of leeway is defined as an angle on a horizontal plane, never on a canted plane. It's the angle between direction of boat velocity relative to water and projection of centerline of the boat on a horizontal plane. Angle between course over ground and centerline are not the same in current, because drift is not part of leeway angle. Leeway is sometimes used to mean component of velocity vector in sideways direction relative to water rather than an angle of leeway.

Angle of attack is mostly not defined at horizontal plane in sailing applications. Not for AC75, not for any canting keel monohull and not for dalifoils or canting keel of Imoca60. Not even for a monohull with fixed symmetrical keel, if there is any heeling.

First you must define a surface made up by all chord lines, then local AoA is defined as an angle in every plane perpendicular to that surface between flow far upstream and that surface. AoA can, and usually will for non planar wings, vary along span, and also with time as well.

 

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1 hour ago, NotSoFast said:

Angle of leeway is well defined in many textbooks about sailing theory in a consistent way, and so is angle of attack in aero- or hydrodynamics. Neither is a matter of opinion in most if not all cases.

Definitions of AoA and angle of leeway are mostly not the same thing in sailing applications.

Angle of leeway is defined as an angle on a horizontal plane, never on a canted plane. It's the angle between direction of boat velocity relative to water and projection of centerline of the boat on a horizontal plane. Angle between course over ground and centerline are not the same in current, because drift is not part of leeway angle. Leeway is sometimes used to mean component of velocity vector in sideways direction relative to water rather than an angle of leeway.

Angle of attack is mostly not defined at horizontal plane in sailing applications. Not for AC75, not for any canting keel monohull and not for dalifoils or canting keel of Imoca60. Not even for a monohull with fixed symmetrical keel, if there is any heeling.

First you must define a surface made up by all chord lines, then local AoA is defined as an angle in every plane perpendicular to that surface between flow far upstream and that surface. AoA can, and usually will for non planar wings, vary along span, and also with time as well.

 

In your effort to sound superior you have misunderstood me.  :)

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2 hours ago, sosoomii said:

In your effort to sound superior you have misunderstood me.  :)

I think we all talk about the same thing. The foil wing of course always has a positive AOA. However, the angle between the hull's centerline and the direction of movement will be determined by the angle at which the foil wing is attached to the foil arm and by flap deflection. Doesn't matter if we call it leeway or yaw angle, it can be negative, positive or zero depending on how the boat is built and used. In addition, it's not just the yaw angle that changes, but pitch as well, since the foil wing is canted out.

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The job of the centerboard is to match the side force generated by the sails. With symmetrical foil, there has to be some angle of attack (2-3 degrees) to generate the lift.  This is what is usually called leeway.  What you see is that the boat actually does not get as far upwind as it is pointed.  You can read 180 on the compass and 177 on the GPS and both are right.

An asymmetrical foil can generate lift @ 0 degrees AOA, or even at negative AOA.  In which case the boat will make 0 degrees leeway.  As the speed increases, the board can produce more lift than the side force of the sails and actually press the boat to windward or make “ negative leeway” which actually is pushing the boat in a positive direction.  In the normal course of displacement sailing, negative leeway is slow because it increases the induced drag on both the hull and the foil.  

In hydrofoil condition, the course sailed is simply the balance of forces between the side force of the hydrofoil and the side force of the sails.  The hull’s centerline is irrelevant.  If you are making negative leeway, you will achieve a course sailed closer to the wind than you are steering . In the case above, you can be reading 180 on the compass and 183 on the GPS.

On AC75s I doubt the foil arms are contributing anything to the side force.  The leeward foil is generally inclined to windward, like a Moth foil when heeled to windward. The force vector it’s at 90 degrees to the surface of the foil, so it is aimed at about the mid height of the mainsail.  The vertical component has to produce enough force to carry the boat, the horizontal component has to counter the side force of the sails.  

It is possible to have a higher VMG by footing aggressively for additional speed and have the foil carry you to Windward by enough to make up for the lower heading.  How that all works out is design specific. 

SHC

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10 hours ago, sosoomii said:

Hmmm, not sure that’s how I would define leeway. I’d say leeway is the angle of attack of the keel or other primary underwater foil, which is usually is parallel to the boat centreline but not necessarily.  Something must be stopping the boat slipping sideways and it sure ain’t the hull.  If not the foil arms then the rudder?

Yes, I think you need to calculate the leeway resistance from the foil arm as well as the foil itself.  The rudder would also contribute, to a lesser degree.

I tried to balance the lateral forces using just the foil. Fail!

When the foil provided enough lateral resistance, there was way too much vertical lift. So then I tried to use downforce on the rudder foil to balance the excess lift. But doing that screwed up pitch - so I had to increase sail driving force to balance that... which led to increased heeling force... .and round and round I went, with no resolution.

<sigh>

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8 minutes ago, MaxHugen said:

Yes, I think you need to calculate the leeway resistance from the foil arm as well as the foil itself.  The rudder would also contribute, to a lesser degree.

I tried to balance the lateral forces using just the foil. Fail!

When the foil provided enough lateral resistance, there was way too much vertical lift. So then I tried to use downforce on the rudder foil to balance the excess lift. But doing that screwed up pitch - so I had to increase sail driving force to balance that... which led to increased heeling force... .and round and round I went, with no resolution.

<sigh>

Forces balance out fine without any significant lateral force on the foil arm. That really doesn't contribute much - see Steve's post above. The angle of the arm is just wrong and hardly any of it is in the water when the boat flies. During take-off, sure it must be important.

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I roughly traced out, scanned and overlayed in photoshop (to get a clearer view) the foils of the remaining boats and defender from the best most recent photos I could find. Ineos definitely have the most unique foil as ETNZ is a straight carry over from the BWB concepts aerospace companies and NASA come up with all the time along with having a very discrete leading edge extension. LR seem to have copied the wing of an old air racer if you get the analogy. It's also much bigger than I first thought. ETNZ have the most efficient foil and when it comes to induced drag and will accelerate better than all the others IMO but Ineos have more RM and maybe a tad more lift at the expense of more drag. LR seems to be more stable and a happy middle ground. 2063660853_Foilprofiles.png.386f5f6bb988eb2196e6154d43004c60.png

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It is entirely likely that the portion of the foil arm that is resisting the forces driving the boat to windward.

It isn’t  unusual to have some skeg or equivalent to keep the foil from going to windward  and thus losing angle of attack and lift along with it.

The easy way to tell is to look at the spray. It is always higher on the pressure side.  This is confused by the plume hitting the underside of the foil arm.  

SHC

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On 1/30/2021 at 12:30 AM, erdb said:

I think we all talk about the same thing. The foil wing of course always has a positive AOA. However, the angle between the hull's centerline and the direction of movement will be determined by the angle at which the foil wing is attached to the foil arm and by flap deflection. Doesn't matter if we call it leeway or yaw angle, it can be negative, positive or zero depending on how the boat is built and used. In addition, it's not just the yaw angle that changes, but pitch as well, since the foil wing is canted out.

And the most important one determining that is foil arm cant angle. A variable adjustable by the sailing crew. Cant the arm further out, and leeway angle becomes smaller or negative, as more foil wing lift is oriented to windward for a given vertical lift component, which presumably is still equal in magnitude of weight of boat - rudder vertical lift, assuming the other main foil is airbourne.

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On 1/29/2021 at 9:51 PM, sosoomii said:

In your effort to sound superior you have misunderstood me.  :)

I can only respond to what you post, not what you think. I have no effort to sound superior, as there is no need to. Some concepts just happen to be well defined already, even for a very different kind of sailing boat like AC75.

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On 1/29/2021 at 5:32 PM, Steve Clark said:

The job of the centerboard is to match the side force generated by the sails. With symmetrical foil, there has to be some angle of attack (2-3 degrees) to generate the lift.  This is what is usually called leeway.  What you see is that the boat actually does not get as far upwind as it is pointed.  You can read 180 on the compass and 177 on the GPS and both are right.

An asymmetrical foil can generate lift @ 0 degrees AOA, or even at negative AOA.  In which case the boat will make 0 degrees leeway.  As the speed increases, the board can produce more lift than the side force of the sails and actually press the boat to windward or make “ negative leeway” which actually is pushing the boat in a positive direction.  In the normal course of displacement sailing, negative leeway is slow because it increases the induced drag on both the hull and the foil.  

In hydrofoil condition, the course sailed is simply the balance of forces between the side force of the hydrofoil and the side force of the sails.  The hull’s centerline is irrelevant.  If you are making negative leeway, you will achieve a course sailed closer to the wind than you are steering . In the case above, you can be reading 180 on the compass and 183 on the GPS.

On AC75s I doubt the foil arms are contributing anything to the side force.  The leeward foil is generally inclined to windward, like a Moth foil when heeled to windward. The force vector it’s at 90 degrees to the surface of the foil, so it is aimed at about the mid height of the mainsail.  The vertical component has to produce enough force to carry the boat, the horizontal component has to counter the side force of the sails.  

It is possible to have a higher VMG by footing aggressively for additional speed and have the foil carry you to Windward by enough to make up for the lower heading.  How that all works out is design specific. 

SHC

In your example of an asymmetrical foil with 0 degrees leeway and a course of 177 deg, the heading would also be 177 - not 180.  From the sailor's point of reference, leeway means a loss of ground, but it's not like the heading is fixed and leeway represents a loss.  It's more a case of the course through the water being determined by the aero and hydro lift/drag ratios, and the boat is oriented with respect to the velocity vector according to the angle of attack required to balance the forces.

I don't think it's a matter of the foil carrying the boat to windward.  The hull's centerline is irrelevant, as you say.  It's just like the case of the gybing centerboard.  When the board is rotated, the board still goes through the water at the same angle of attack.  What changes is the bow is rotated off the wind by the amount of board rotation in the case. 

Where an asymmetrical foil does make its gains is in centering its low-drag zone about the actual operating conditions.  With a symmetrical foil, the low-drag zone is necessarily centered about zero lift.

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2 hours ago, Basiliscus said:

I don't think it's a matter of the foil carrying the boat to windward.  The hull's centerline is irrelevant, as you say.  It's just like the case of the gybing centerboard.  When the board is rotated, the board still goes through the water at the same angle of attack.  What changes is the bow is rotated off the wind by the amount of board rotation in the case

This is the pint I was, clumsily, trying to make. If you define leeway angle as the angle between course through water and hull centreline axis it becomes an irrelevant measure in as much as it does it give an indication of how quickly the (underwater) boat is slipping sideways. 

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On 1/29/2021 at 2:08 PM, sosoomii said:

Of course, in reality zero or negative leeway upwind is not possible -the foil arm must be gybed. Is there any info on the arm angle?

I though Foiling Moths were crabbing windward ??

Dont know if it is the same than your negative leeway upwind?

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Apologies if this was previously posted - I get a bit overwhelmed by the volume of posts sometimes - one of the designers, Martin Fischer of LR, states that although the cats and AC75s are about the same downwind, "the cats are not comparable upwind."

 

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1 hour ago, Tornado-Cat said:

I have not been keeping up, do you know how many unseen set of foils the 3 teams have left ?

If I am not mistaken LR and ITUK have used all of them. I don't know ETNZ

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7 hours ago, Mozzy Sails said:

First episode on foils, following the previous discussing the rule.

Just looking at the most visually distinctive element, the frontal profiles.

 

Hey Mozzy, I enjoyed the chat. One thing you guys didn't mention is that the anhedral foils let you change the direction of lift. I've posted this earlier, sorry if if you've seen already, but the point is that for the boat to be in balance, the combined lift vector from the foil wings needs to point towards the center of effort of the sails. In other words, the combined lift vector needs to be in line with the vector sum of lateral sail force, weight and rudder vertical force:

ETNZ.thumb.JPG.9ed31e9ac7a91951d717942413cd3d4e.JPG

(This is because sail lateral = foil lateral and weight + rudder down = foil vertical)

If you have a T foil, the direction of the foil lift vector can only be manipulated with foil cant, which is slow. Therefore, the only tool you have for dynamic roll balance is adjustment of CoE height with sail trim.

If you have a Y foil, you can change the direction of the foil lift vector by deflecting the flaps differently on the foil wing halves. This gives you an additional tool to line up the lift vector with the CoE. 

others2.JPG.9561b29e96a95cfc46dd76e234f306af.JPG

This may be important during maneuvers for example, if you can't balance it all with sail trim.

In addition, it seems to me you could achieve more righting moment with the Y foil if you deflect the flap on the outside wing more than on the inside. However, (without any calculations to prove) I always thought that for minimum drag, the foil pattern should be symmetrical on the two sides of the foil arm. Lots of pros and cons including the ones you discussed in the video.

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7 hours ago, Mozzy Sails said:

First episode on foils, following the previous discussing the rule.

Just looking at the most visually distinctive element, the frontal profiles.

 

When you get to profiles take a look at ETnZs new foil. Smaller area but potentially a thicker foil shape.

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2 hours ago, erdb said:

(This is because sail lateral = foil lateral and weight + rudder down = foil vertical)

If you have a T foil, the direction of the foil lift vector can only be manipulated with foil cant, which is slow. Therefore, the only tool you have for dynamic roll balance is adjustment of CoE height with sail trim.

If you have a Y foil, you can change the direction of the foil lift vector by deflecting the flaps differently on the foil wing halves. This gives you an additional tool to line up the lift vector with the CoE. 

others2.JPG.9561b29e96a95cfc46dd76e234f306af.JPG

This may be important during maneuvers for example, if you can't balance it all with sail trim.

In addition, it seems to me you could achieve more righting moment with the Y foil if you deflect the flap on the outside wing more than on the inside. However, (without any calculations to prove) I always thought that for minimum drag, the foil pattern should be symmetrical on the two sides of the foil arm. Lots of pros and cons including the ones you discussed in the video.

1) Would you please correct that drawing of yours. Extend both red lines as dotted lines downwards to the point where they meet each other. Then relocate the blue line to begin at that point instead from junction of wing halves as it currently and falsely is in your drawing. At the same direction it would point towards a substantially lower position on the mast. The less anhedral there is, the greater the effect, as the starting point of the blue line then relocates much further away from the centerline. Choosing a very low anhedral, the origin for blue resultant can be several nautical miles from the boat (below and leeward) In that case differential flap would change rolling moment much more rapidly than lateral- or vertical force balance. It would be great to show that comparison in the drawing for different anhedrals.

2) Using differential flap deflection like that to make the resultant blue line point lower or higher for rapid roll balance also results mismatch in lateral forces regardless of if anhedral or straight wing is used, if vertical forces remains matched by the magnitude of the blue resultant. Your statements seem to assume otherwise, ie that asymmetrical lift would be independent and free variable for roll balance, when it's not. All the same if there is anhedral or not, but less anhedral means the origin of resultant will be further down, and any increase in side force would necessitate much lower center of effort height than a wing with more anhedral. Of course short term mismatch in lateral forces balance is allowed, it just results lateral acceleration to auto correct it by changing leeway angle.

3) A rudder sideforce is not mentioned, but it does effect all of following: 1 sideforce balance, 2 rolling moment balance, and 3 yaw balance. A long as it is non zero, the foil total force can not point directly toward junction of center of effort of sails and weight vector for roll balance.

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8 minutes ago, NotSoFast said:

1) Would you please correct that drawing of yours. Extend both red lines as dotted lines downwards to the point where they meet each other. Then relocate the blue line to begin at that point instead from junction of wing halves as it currently and falsely is in your drawing. At the same direction it would point towards a substantially lower position on the mast. The less anhedral there is, the greater the effect, as the starting point of the blue line then relocates much further away from the centerline. Choosing a very low anhedral, the origin for blue resultant can be several nautical miles from the boat (below and leeward) In that case differential flap would change rolling moment much more rapidly than lateral- or vertical force balance. It would be great to show that comparison in the drawing for different anhedrals.

2) Using differential flap deflection like that to make the resultant blue line point lower or higher for rapid roll balance also results mismatch in lateral forces regardless of if anhedral or straight wing is used, if vertical forces remains matched by the magnitude of the blue resultant. Your statements seem to assume otherwise, ie that asymmetrical lift would be independent and free variable for roll balance, when it's not. All the same if there is anhedral or not, but less anhedral means the origin of resultant will be further down, and any increase in side force would necessitate much lower center of effort height than a wing with more anhedral. Of course short term mismatch in lateral forces balance is allowed, it just results lateral acceleration to auto correct it by changing leeway angle.

3) A rudder sideforce is not mentioned, but it does effect all of following: 1 sideforce balance, 2 rolling moment balance, and 3 yaw balance. A long as it is non zero, the foil total force can not point directly toward junction of center of effort of sails and weight vector for roll balance.

1) and 2) - I have no idea what you're talking about, but feel free to modify the drawing anyway you want to demonstrate your point. BTW, with those arrows I just wanted to show how the direction of a vector perpendicular to the foil wings' plane can be diverted up or down by differential flap settings. It wasn't meant to show accurate summation of foil wing lift vectors. 

As for your 3rd point, certainly lateral rudder force would come into this as well, but I'm pretty sure it's negligible compared to lateral force provided by the foil. If there was any significant lateral force on those skinny rudders they would visibly bend (which happens sometimes in hard turns, but not while sailing in straight line).

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One of the AC designers, I think in the last AC, described high speed sailing as trying to squeeze the boat with a really narrow angle between the wind power vector and the foil lift vector. The narrower you can make that angle the faster you go, to a point. If the leeway is negative, that angle becomes negative and the whole thing falls apart. Leeway and angle of attack are different on foiling boats. The only way to narrow that angle is through drag reduction (hull out of water, aero drag, etc.) I don't think these boats are crabbing to windward, it defies physics. Just think of slowly increasing the angle on the foil as you're flying along, at some point the boat will stop moving forward, the angle between wind power vector and foil lift vector is too narrow to overcome the inescapable drag.

Now, I do think some of the teams are yawing the hulls (and only the hulls) into the wind to reduce aero drag. I think this was really evident on the kiwi AC50. Need to be able to oversheet the sails to achieve it, but only relative to the hull, not the foils. This might look like crabbing to windward, but it isn't, the sailors are maintaining an otherwise regular heading.

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11 minutes ago, erdb said:

1) and 2) - I have no idea what you're talking about, but feel free to modify the drawing anyway you want to demonstrate your point. BTW, with those arrows I just wanted to show how the direction of a vector perpendicular to the foil wings' plane can be diverted up or down by differential flap settings. It wasn't meant to show accurate summation of foil wing lift vectors. 

As for your 3rd point, certainly lateral rudder force would come into this as well, but I'm pretty sure it's negligible compared to lateral force provided by the foil. If there was any significant lateral force on those skinny rudders they would visibly bend (which happens sometimes in hard turns, but not while sailing in straight line).

Could you not achieve the same thing on a t foil with differential flap settings. I can’t see the difference. Happy to be corrected. 

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44 minutes ago, erdb said:

1) and 2) - I have no idea what you're talking about, but feel free to modify the drawing anyway you want to demonstrate your point. BTW, with those arrows I just wanted to show how the direction of a vector perpendicular to the foil wings' plane can be diverted up or down by differential flap settings. It wasn't meant to show accurate summation of foil wing lift vectors. 

As for your 3rd point, certainly lateral rudder force would come into this as well, but I'm pretty sure it's negligible compared to lateral force provided by the foil. If there was any significant lateral force on those skinny rudders they would visibly bend (which happens sometimes in hard turns, but not while sailing in straight line).

Quick question: are you adding in any vectors for the immersed part of the foil arm?   I haven't really looked into that, but think maybe I should...

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24 minutes ago, Ncik said:

One of the AC designers, I think in the last AC, described high speed sailing as trying to squeeze the boat with a really narrow angle between the wind power vector and the foil lift vector. The narrower you can make that angle the faster you go, to a point. If the leeway is negative, that angle becomes negative and the whole thing falls apart. Leeway and angle of attack are different on foiling boats. The only way to narrow that angle is through drag reduction (hull out of water, aero drag, etc.) I don't think these boats are crabbing to windward, it defies physics. Just think of slowly increasing the angle on the foil as you're flying along, at some point the boat will stop moving forward, the angle between wind power vector and foil lift vector is too narrow to overcome the inescapable drag.

Now, I do think some of the teams are yawing the hulls (and only the hulls) into the wind to reduce aero drag. I think this was really evident on the kiwi AC50. Need to be able to oversheet the sails to achieve it, but only relative to the hull, not the foils. This might look like crabbing to windward, but it isn't, the sailors are maintaining an otherwise regular heading.

Leeway is really not a good term for foiling boats, but that's how a lot of people think about it coming from displacement boats. As you said, efficiency is all about lift drag ratios of the sail plan and the foil(s). The hull is just the structure that connects the sails and the foils.

Yaw angle is a much better term and negative leeway just means that the hull is rotated with the bow off the wind. The yaw angle of the hull depends on how the foil is attached to the boat. If you change the foil's angle, the boat will still travel in the same direction (foil AOA is still the same), but now the hull's yaw angle is different. The yaw angle doesn't fundamentally change anything, but it affects hull aero, lateral distance between the rudder and the foil and the relationship of the jib and the main. 

The first generation boats sometimes sailed with a lot of pitch (bow down) and yaw (bow off the wind - or negative leeway), especially downwind, when speed was high and the AOA of the foil decreased (just think about how reducing the AOA on the canted foil changes pitch and yaw angles). The 2nd generation boats with more pronounced keels have to sail with fairly constant pitch and yaw angles to make the hull aero effective. They probably achieve this with much better foil flap control, which in a way works the same way as changing the AOA of the foil.

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3 minutes ago, MaxHugen said:

Quick question: are you adding in any vectors for the immersed part of the foil arm?   I haven't really looked into that, but think maybe I should...

Nothing lateral or vertical, only some drag, but even that is negligible. 

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37 minutes ago, amc said:

Could you not achieve the same thing on a t foil with differential flap settings. I can’t see the difference. Happy to be corrected. 

It can affect the balance, but much less. Setting the flaps differently would just shift the lift vector sideways a bit, but wouldn't change the direction of the vector. The lift has to be perpendicular to the lifting surface. Also, from rule interpretations it seems ETNZ is using / trying to use a system that actuates the flaps together.

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Some thoughts from Mozzy's video (most interesting yet btw, partly because its the area I am weakest on)

Are the boats reaching cavitation speeds? Well some of the effects we have seen on AM, and to a lesser extent LR on the bear away do look a lot like cavitation resulting in a loss of lift. Though LR looked better in the semis than RRs. I wonder whether it is coincidence that of the challengers, GB looked most steady and fastest on that bear away, and was also fastest downwind? Have they done most work on avoiding cavitation? Will NZ have more of a problem with their t-foils?

NZ's foils may provide a greater RM, and allow the end of the foil to come out of the water, but it can't do both simultaneously, because the the tip comes out the 'beam' decreases So I assume that they would use max RM upwind, and drag reduction downwind. But from the previous point, would they then hit cavitation downwind?

If they have better RM,  then they both can take more power from sails (obvs), but also need more power to lift-out and stay on foils. Which matches the deeper camber of the main they seem to have versus others. I would also expect them to have the main a touch higher up the traveller to correct for the AOA of the main whilst still retaining the lateral force, but we haven't seen the stern camera shots to show this.

Tip ventilation, max RM and anti-cavitation make no difference in the lightest stuff, so which will be quicker there i wonder?

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2 minutes ago, enigmatically2 said:

Are the boats reaching cavitation speeds? Well some of the effects we have seen on AM, and to a lesser extent LR on the bear away do look a lot like cavitation resulting in a loss of lift. Though LR looked better in the semis than RRs. I wonder whether it is coincidence that of the challengers, GB looked most steady and fastest on that bear away, and was also fastest downwind? Have they done most work on avoiding cavitation? Will NZ have more of a problem with their t-foils?

Cavitation is a function of both form and speed.  There's a cool video on YT of a schoolboy demonstrating cavitation with a flat disk perpendicular to water flow - at just ~10 knots.  Unfortunately I can't find it now.

So for the rudder to lose lift, a lot would depend on AoA as well as speed, and could result in either cavitation or ventilation I think.

Avoiding overly sharp turns at high speed may help avoid this.

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6 hours ago, erdb said:

Hey Mozzy, I enjoyed the chat. One thing you guys didn't mention is that the anhedral foils let you change the direction of lift. I've posted this earlier, sorry if if you've seen already, but the point is that for the boat to be in balance, the combined lift vector from the foil wings needs to point towards the center of effort of the sails. In other words, the combined lift vector needs to be in line with the vector sum of lateral sail force, weight and rudder vertical force:

ETNZ.thumb.JPG.9ed31e9ac7a91951d717942413cd3d4e.JPG

(This is because sail lateral = foil lateral and weight + rudder down = foil vertical)

If you have a T foil, the direction of the foil lift vector can only be manipulated with foil cant, which is slow. Therefore, the only tool you have for dynamic roll balance is adjustment of CoE height with sail trim.

If you have a Y foil, you can change the direction of the foil lift vector by deflecting the flaps differently on the foil wing halves. This gives you an additional tool to line up the lift vector with the CoE. 

others2.JPG.9561b29e96a95cfc46dd76e234f306af.JPG

This may be important during maneuvers for example, if you can't balance it all with sail trim.

In addition, it seems to me you could achieve more righting moment with the Y foil if you deflect the flap on the outside wing more than on the inside. However, (without any calculations to prove) I always thought that for minimum drag, the foil pattern should be symmetrical on the two sides of the foil arm. Lots of pros and cons including the ones you discussed in the video.

Thanks @erdb. We actually talk about flaps and separate actuators in the a later video, recorded at the same time. It's an important point and is hard to separate out from frontal shapes.

I think a video just on achieving flight stability (roll and pitch) is needed. I shall try and tackle that soon (probably using some of your drawings if you could you send me a blank scale drawing?). 

The differential in flap actuation is a difficult point. Many people (myself included) discuss the bulb  / intersection as if it were a fixed pivot. In reality, it's just a point where forces must be balanced with the opposing forces created by the rig (like you show in the top diagram).  

With the two flaps independent you could create a rolling (or torque) about the bulb, with the effect of moving the lift outboard and increasing righting moment. So the wing is always trying to roll the boat to windward, and the power in the sails is trying to roll to leeward. This is true even for a straight wing (although, would require two independent flaps which ETNZ have not got). This where we a moving away from the balanced forces at the end of the foil arm and discussing where that force is created.

I guess where I struggle, and I am happy to admit this, is when considering the direction of the lift acting on either side of the foil changes things compared to to altering the magnitude of lift. 

 

 

 

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1 minute ago, MaxHugen said:

Cavitation is a function of both form and speed.  There's a cool video on YT of a schoolboy demonstrating cavitation with a flat disk perpendicular to water flow - at just ~10 knots.  Unfortunately I can't find it now.

So for the rudder to lose lift, a lot would depend on AoA as well as speed, and could result in either cavitation or ventilation I think.

Avoiding overly sharp turns at high speed may help avoid this.

I realise there are other factors, hence why mostly questions. But avoiding sharp turns isn't great tactically

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1 minute ago, enigmatically2 said:

I realise there are other factors, hence why mostly questions. But avoiding sharp turns isn't great tactically

I think they round marks with a greater radius in strong winds, although I don't know if that's specifically to avoid rudder issues.

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7 minutes ago, MaxHugen said:

I think they round marks with a greater radius in strong winds, although I don't know if that's specifically to avoid rudder issues.

They may, but in at least one of the races, LR lost loads to GB because they did.

May partially explain the dual board roundings at the leeward mark though

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8 minutes ago, Mozzy Sails said:

Thanks @erdb. We actually talk about flaps and separate actuators in the a later video, recorded at the same time. It's an important point and is hard to separate out from frontal shapes.

I think a video just on achieving flight stability (roll and pitch) is needed. I shall try and tackle that soon (probably using some of your drawings if you could you send me a blank scale drawing?). 

The differential in flap actuation is a difficult point. Many people (myself included) discuss the bulb  / intersection as if it were a fixed pivot. In reality, it's just a point where forces must be balanced with the opposing forces created by the rig (like you show in the top diagram).  

With the two flaps independent you could create a rolling (or torque) about the bulb, with the effect of moving the lift outboard and increasing righting moment. So the wing is always trying to roll the boat to windward, and the power in the sails is trying to roll to leeward. This is true even for a straight wing (although, would require two independent flaps which ETNZ have not got). This where we a moving away from the balanced forces at the end of the foil arm and discussing where that force is created.

I guess where I struggle, and I am happy to admit this, is when considering the direction of the lift acting on either side of the foil changes things compared to to altering the magnitude of lift.

Mozzie, I did the following diagram quite a while ago to convince myself that the net vertical and horizontal forces were the same for "equivalent" T and Y foils - they are of course.  I'm not sure about the torque you mention, but from the bottom right diagram, the Y foil appears to have net horizontal force a bit further outboard than the T foil, no flap differential used though.

image.png.b855cf783a3a41d5f175040c0025d09f.png

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Thanks @Mozzy Sails, really enjoyed the latest vid.  I couple of thoughts popped into my head whilst watching it:

  1. It seems to me that the more the leeward tip comes out of the water the more the lateral centre of effort of the lifting surface moves to weather, which shortens the lever arm & therefore reduces righting moment?  If you take anhedral to the extreme (which I appreciate you can't actually do within the box) then the leeward most half of the foil is generating all of the lift so you have significantly lengthened the lever arm.   Does the box limit anhedral too much for this to be a serious consideration?
  2. Is there not a significant drag penalty having the foil breaking the surface?

Thanks & keep up the good work.

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9 minutes ago, MikeBz said:

Thanks @Mozzy Sails, really enjoyed the latest vid.  I couple of thoughts popped into my head whilst watching it:

  1. It seems to me that the more the leeward tip comes out of the water the more the lateral centre of effort of the lifting surface moves to weather, which shortens the lever arm & therefore reduces righting moment?  If you take anhedral to the extreme (which I appreciate you can't actually do within the box) then the leeward most half of the foil is generating all of the lift so you have significantly lengthened the lever arm.   Does the box limit anhedral too much for this to be a serious consideration?
  2. Is there not a significant drag penalty having the foil breaking the surface?

Thanks & keep up the good work.

Leverage is an interesting one. 

Just thinking about the windward foil, the more arm and foil you can get to the bottom of the box, the more leverage you have. 

But, the more complex side, and I guess is a function of what we are discussing above, is where the fulcrum is on the submerged leeward foil arm? A longer arm for the T will push that surface further to leeward... however, they then fly with foil tip out, which moves it back to windward. 

But, the other aspect, is that with a T foil, you can fly with slightly more cant, without as much foil wing coming out (for a given ride height to MWL). And more foil cant = more leverage. 

And we see ETNZ doing that.

1959303851_uwcant.png.03e371b9ecad6f3587d1887d075a14c8.png
 

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^ You're probably already up on this, but if - as we assume - NZ are using a simplified mechanism that does not use differential flap settings, they have only 2 flap tips to create vortices when flaps are deflected, versus 4.

However, LR look to have flat sections on their bulb at the flap base, which may provide end plating to avoid the vortices there?

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31 minutes ago, Mozzy Sails said:

But, the other aspect, is that with a T foil, you can fly with slightly more cant, without as much foil wing coming out (for a given ride height to MWL). And more foil cant = more leverage.

Assuming that 2 boats (one with T foils, the other with Y foils) both fly with just the tip surfacing, the amount of maximum cant would be governed by:

1. Flight height - distance from foil cant axis to water surface

2. Heel angle - positive heel (ie to leeward) would allow a higher cant

I had originally calculated max foil at 25°, but this was optimistic. Once I had actual data from the ACWS, it averaged to about 21° upwind. It's a couple of degrees less downwind. Note that although I used a Y foil in the diagram, the T foil would be at the same angle for a surfacing tip.

image.png.e4289877733ee909ff775c3e1de3aba9.png

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Just now, MaxHugen said:

Assuming that 2 boats both fly with just the tip surfacing, the amount of maximum cant would be governed by:

1. Flight height - distance from foil cant axis to water surface

2. Heel angle - positive heel (ie to leeward) would allow a higher cant

I had originally calculated max foil at 25°, but this was optimistic. Once I had actual data from the ACWS, it averaged to about 21° upwind. It's a couple of degrees less downwind. Note that although I used a Y foil in the diagram, the T foil would be at the same angle for a surfacing tip.

Yes, Y or T makes no difference to the cant angle where the tip of the foil just starts to pierce (assuming same ride height and heel). This is fairly obvious, as both foil tips are in the same corner of the rule box. 

What's different with a T foil, is that as you increase cant angle from the position described above you lose less foil span per degree of extra cant compared to a Y foil. As the surface  / intersect angle is greater. 

Essentially, T foil is more forgiving for riding a little high and more forgiving of canting out a little further, in terms of the foil area you lose. 

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3 minutes ago, Mozzy Sails said:

Yes, Y or T makes no difference to the cant angle where the tip of the foil just starts to pierce (assuming same ride height and heel). This is fairly obvious, as both foil tips are in the same corner of the rule box. 

What's different with a T foil, is that as you increase cant angle from the position described above you lose less foil span per degree of extra cant compared to a Y foil. As the surface  / intersect angle is greater. 

Essentially, T foil is more forgiving for riding a little high and more forgiving of canting out a little further, in terms of the foil area you lose. 

Agree... maybe that's part of the reason NZ went with the T,  let's them cant to the max, but with less risk?

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Just been thinking about this. General consensus is that GB foils have the largest area.

If we assume that at a given point all other factors are equal, those foils will be producing as much lift as others. But because they are larger, even those the pressure distribution is not even, that lift will be spread over a wider area, thus implying the pressure difference between top and bottom on average will be lower. 

Which means in turn that the pressure above the foils is not likely to be as low, thus they are less likely to suffer from cavitation than all the others. Which may explain why GB was faster than everyone else downwind, and looked more stable and able to turn quickly on the bear-aways.

By the same token, NZ, with the smallest area foils are likely to suffer cavitation earlier than everyone else.

Make sense?

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On 1/31/2021 at 1:34 PM, Mozzy Sails said:

First episode on foils, following the previous discussing the rule.

Just looking at the most visually distinctive element, the frontal profiles.

 

Excellent video, thanks.

So, to sum up, the T provides more power, deeper, for less profile drag and the possibility to get out of the water more fluently.

However teams with test boats did try it and did not keep it, we can assume because not stable enough.

So if TNZ has T ones now, is it because :

1) they did not test it before ?

2) they do still have a light anhedral which is enough for stability ?

3) they have other sets of foils, anhedral,  that we did not see yet ?

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Just now, Tornado-Cat said:

Excellent video, thanks.

So, to sum up, the T provides more power, deeper for less profile drag and the possibility to get out of the water more fluently.

However teams with test boats did try it and did not keep it, we can assume because not stable enough.

So if TNZ has T ones now, is it because :

1) they did not test it before ?

2) they do still have a light anhedral which is enough for stability ?

3) they have other sets of foils, anhedral,  that we did not see yet ?

I would suggest it is more likely that they can warrant the T Foils due to their more sophisticated sail control systems..

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1 minute ago, Tornado-Cat said:

Excellent video, thanks.

So, to sum up, the T provides more power, deeper for less profile drag and the possibility to get out of the water more fluently.

However teams with test boats did try it and did not keep it, we can assume because not stable enough.

So if TNZ has T ones now, is it because :

1) they did not test it before ?

2) they do still have a light anhedral which is enough for stability ?

3) they have other sets of foils, anhedral,  that we did not see yet ?

I think other teams tested it, but couldn't suffer the ventilation that came with running a tip out. And if you don't run the tip out, then there is a profile drag penalty for T as you have a longer foil arm, plus maybe some losses with a tighter angles at the intersection), and maybe teams wanted to play around with flaps in different planes. 

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4 hours ago, Mozzy Sails said:

I think other teams tested it, but couldn't suffer the ventilation that came with running a tip out. And if you don't run the tip out, then there is a profile drag penalty for T as you have a longer foil arm, plus maybe some losses with a tighter angles at the intersection), and maybe teams wanted to play around with flaps in different planes. 

It makes ETNZ's tiny, skinny 'wafer-thin' foil-arm-to-foil mount even more important :-)

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25 minutes ago, rh3000 said:

It makes ETNZ's tiny, skinny 'wafer-thin' foil-arm-to-foil mount even more important :-)

Yeah and having the foils far aft of the arm and bulb will help reduce the pressure around the intersections as it is not all meeting in one area..

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what is the load requirement on the vertical arm to produce a balances leeway force

with ETNZ having a longer arm to the Tee foil bulb, is there an advantage to distribute the leeway force over the larger area?

i imagine that teams with smaller leeway area will be higher loaded therefore prone to cavitation. potentially why ETNZ can get away with a skinnier stock arm down to the foil

 

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6 hours ago, enigmatically2 said:

Just been thinking about this. General consensus is that GB foils have the largest area.

If we assume that at a given point all other factors are equal, those foils will be producing as much lift as others. But because they are larger, even those the pressure distribution is not even, that lift will be spread over a wider area, thus implying the pressure difference between top and bottom on average will be lower. 

Which means in turn that the pressure above the foils is not likely to be as low, thus they are less likely to suffer from cavitation than all the others. Which may explain why GB was faster than everyone else downwind, and looked more stable and able to turn quickly on the bear-aways.

By the same token, NZ, with the smallest area foils are likely to suffer cavitation earlier than everyone else.

Make sense?

If this make sense then people would put on the biggest prop possible on their boat ?

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5 hours ago, Mozzy Sails said:

I think other teams tested it, but couldn't suffer the ventilation that came with running a tip out. And if you don't run the tip out, then there is a profile drag penalty for T as you have a longer foil arm, plus maybe some losses with a tighter angles at the intersection), and maybe teams wanted to play around with flaps in different planes. 

You may be right, I think that a flat T foil is very unstable which is why other teams left it, but that a light anhedral like those for windsurfers can help.

The very thin profile of the end of TNZ arms are made necessary because  T foils require longer arms. The aft position of the foil may prevent the loss at the intersections.

Have we seen all 6 set of foil for each team ? I will be interesting to see what they use for the CSS final.

Any way, wait for the next Mozzy video, the most interesting in this forum IMO.

 

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42 minutes ago, mako23 said:

If this make sense then people would put on the biggest prop possible on their boat ?

No, because that expands the radius which then increases the speed at the tips for the same engine speed. It does suggest they should have prop blades that are not sparse, i.e. they blades together fill a lot of the circumference. Which they do

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8 hours ago, enigmatically2 said:

Just been thinking about this. General consensus is that GB foils have the largest area.

If we assume that at a given point all other factors are equal, those foils will be producing as much lift as others. But because they are larger, even those the pressure distribution is not even, that lift will be spread over a wider area, thus implying the pressure difference between top and bottom on average will be lower. 

Which means in turn that the pressure above the foils is not likely to be as low, thus they are less likely to suffer from cavitation than all the others. Which may explain why GB was faster than everyone else downwind, and looked more stable and able to turn quickly on the bear-aways.

By the same token, NZ, with the smallest area foils are likely to suffer cavitation earlier than everyone else.

Make sense?

When you say faster than everyone else downwind. I assume you are referencing the challengers (if in fact they are), as the only real race with ETNZ showed them to be slower upwind and downwind. So I am not sure where that leaves your theory. As we are not seeing cavitation issues with ETNZ...Yes lift is spread over a larger area which could translate to a more forgiving setup. But you are carrying round more surface area and therefore more drag. So if you are hoping the ETNZ will have cavitation issues...That is not a very wise strategy 

573864E8-03A8-4592-A943-30D18964904D.gif

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1 hour ago, Tornado-Cat said:

You may be right, I think that a flat T foil is very unstable which is why other teams left it, but that a light anhedral like those for windsurfers can help.

How is a T foil "unstable", and how does anhedral reduce that?

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On 2/1/2021 at 12:01 PM, erdb said:

..the combined lift vector from the foil wings needs to point towards the center of effort of the sails.

This doesn't sound right. The force vectors and the moments all need to sum to 0, and the centre of effort of the sails affects the moments, but the lift vector doesn't need to point to the CoE.

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2 hours ago, Ncik said:

This doesn't sound right. The force vectors and the moments all need to sum to 0, and the centre of effort of the sails affects the moments, but the lift vector doesn't need to point to the CoE.

It kind of works out that way, and it's because the unique arrangement with the canted out foil. At least as long the foil doesn't have much anhedral. If we assume that the wing halves exert about the same amount of lift, the combined lift vector will be perpendicular to the foil's plane (in line with the end of the foil arm). Therefore the ratio of vertical and lateral foil force components = tangent(alpha) - see below. This means that the ratio of vertical and lateral force depends only on cant angle.

f_coe.JPG.ff7cadaa4c2783364fb3df597b6c1c56.JPG

Now you may think that all that matters is the heeling moment from the sails, and the CoE can go higher if the lateral force is reduced at the same time (or vice versa). However, the balance breaks, because if you reduce the lateral sail force, the lateral foil force decreases as well (since they have to be equal for balance), and if the lateral foil force decreases, the vertical lift decreases as well, because their ratio is set by cant angle. So now the boat is losing altitude, and you have to do something to balance it out again - change cant angle, boat pitch or heel, sail trim etc. 

The AC50 catamarans were different, because the foils were mostly L shaped, and the vertical and horizontal forces were mostly decoupled. In the same way, the more anhedral the foil on the AC75, the more the lateral and vertical forces are decoupled, and you can manipulate them separately with the flaps. This is why I think the anhedral foils are easier to sail compared to ETNZ's straight foils.

Final note, to be perfectly accurate, the foil force vector has to point towards the intersection of CoE height and the axis of combined vertical weight+rudder forces. That axis isn't necessarily in the middle, but it's very close to midline on an AC75, because the crew is almost equally divided between the two sides and the rudder is also in the middle. However, moving crew to windward shifts this axis to windward, and the CoE can go higher as expected with the crew hiking out more.

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To all the contributors to this thread - thank you.

This thread and the sheer knowledge of some contributors or indeed those like me who are not engineering of physics orientated but simply curious contribute to all SA participants knowledge.

I have a query. Does the deck layout of TNZ tunnel air straight through the deck and provide some automatic yaw control (makes the bow point directly to the wind?)

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13 hours ago, Mozzy Sails said:

... And if you don't run the tip out, then there is a profile drag penalty for T as you have a longer foil arm...

Is there not a (significant) drag penalty for running the tip out (surface piercing)?

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7 hours ago, uflux said:

When you say faster than everyone else downwind. I assume you are referencing the challengers (if in fact they are), as the only real race with ETNZ showed them to be slower upwind and downwind. So I am not sure where that leaves your theory. As we are not seeing cavitation issues with ETNZ...Yes lift is spread over a larger area which could translate to a more forgiving setup. But you are carrying round more surface area and therefore more drag. So if you are hoping the ETNZ will have cavitation issues...That is not a very wise strategy 

 

Obvs I was comparing with the challengers. We have no info on NZ essentially The AWCS is obviously irrelevant as a performance comparison (especially when considering GB).

It isn't a question of hoping that NZ will have cavitation issues (though yes, that would be nice), but all the teams have designers who know shit more than us. So if they have different solutions, it isn't because one is obviously better than the other, but that they have made different compromises. I know to some any theory that suggests the NZ solution may not be the best in all respects is flawed, but they will all have advantages and disadvantages. This thread is surely about trying to work out what those are.

My theory may be bollocks, but not because it suggests NZ may have made a compromise

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36 minutes ago, MikeBz said:

Is there not a (significant) drag penalty for running the tip out (surface piercing)?

Yes there is, but it seems at about 30 knots it pays to sail with the tip out (Tom Partingtons calcs not mine), at least using the equation he has for a surface piercing energy loss (waves and spray). The graph below was flashed up on screen in my last video (a crappy photo of a monitor, rather than a screenshot, so hard to see). 

The overall gain isn't huge, but I would say noticeable. However, I wonder if they have any ventilation and how far down their foil it reaches, and what drag this adds, at least from a reduction of span. Very hard to calculate.

But... even if surface piecing can be 'drag neutral', but it allows you to run 1 degree more cant, then it would be a good feature. You don't really see ETNZ sail tips out downwind where they sail with much less cant (their cant settings seem very modal). And you would think if it were all about drag reduction at high speed they would sail tips out downwind. But they don't... so my conclusion is ETNZ have designed foils that tolerate their tips running free to increase upwind RM. 


image.png.2bd8dfe536c2a6fb8a12ae907a29591e.png


And, if I am really speculating, the other teams are using variable flap settings to ever so slightly move the centre of lift outboard along the wings to increase their RM. 
Here's a video of Grant Simmer from before INEOS "in rough terms the righting moment is from the centre of lift of that foil, which would be about at the T junction, to the CoG of the boat which is creating that righting moment". 

 

 

 

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18 minutes ago, Mozzy Sails said:

You don't really see ETNZ sail tips out downwind where they sail with much less cant (their cant settings seem very modal). And you would think if it were all about drag reduction at high speed they would sail tips out downwind. But they don't... so my conclusion is ETNZ have designed foils that tolerate their tips running free to increase upwind RM. 

Until we see ETNZ race for real in the AC match, we really won't know how much or how often ETNZ will run with the tip out. ETNZ is also running flat foils which are much more progressive in breaching than anhedral foils, where the outer wing seems to breach fairly frequently in choppy conditions.

Moths have flat main foils and some reasonably frequently have the tip exposed when going to windward. It doesn't seem to cause ventilation or other performance issues (Moth ventilation seems to come from the strut, not the foil tip).

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4 hours ago, erdb said:

It kind of works out that way, and it's because the unique arrangement with the canted out foil. At least as long the foil doesn't have much anhedral. If we assume that the wing halves exert about the same amount of lift, the combined lift vector will be perpendicular to the foil's plane (in line with the end of the foil arm). Therefore the ratio of vertical and lateral foil force components = tangent(alpha) - see below. This means that the ratio of vertical and lateral force depends only on cant angle.

f_coe.JPG.ff7cadaa4c2783364fb3df597b6c1c56.JPG

Now you may think that all that matters is the heeling moment from the sails, and the CoE can go higher if the lateral force is reduced at the same time (or vice versa). However, the balance breaks, because if you reduce the lateral sail force, the lateral foil force decreases as well (since they have to be equal for balance), and if the lateral foil force decreases, the vertical lift decreases as well, because their ratio is set by cant angle. So now the boat is losing altitude, and you have to do something to balance it out again - change cant angle, boat pitch or heel, sail trim etc. 

The AC50 catamarans were different, because the foils were mostly L shaped, and the vertical and horizontal forces were mostly decoupled. In the same way, the more anhedral the foil on the AC75, the more the lateral and vertical forces are decoupled, and you can manipulate them separately with the flaps. This is why I think the anhedral foils are easier to sail compared to ETNZ's straight foils.

Final note, to be perfectly accurate, the foil force vector has to point towards the intersection of CoE height and the axis of combined vertical weight+rudder forces. That axis isn't necessarily in the middle, but it's very close to midline on an AC75, because the crew is almost equally divided between the two sides and the rudder is also in the middle. However, moving crew to windward shifts this axis to windward, and the CoE can go higher as expected with the crew hiking out more.

This is a profound and fundamental insight into the AC75 dynamics. Wonder if the designers were aware of it when creating the rule?

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40 minutes ago, Mikko Brummer said:

This is a profound and fundamental insight into the AC75 dynamics. Wonder if the designers were aware of it when creating the rule?

I think they will have. Because, it's identical to how all sailing boats work. The only difference is vertical lift comes from a foil, rather than buoyancy / hull lift. 

If I am at maximum righting moment (full hike or dropped to trapeze knots), but I want more power I cannot simply sheet in, as I will heel. So all I can do from that point forward is move the centre of effort down in the sail to give me greater leverage. 

My skiff is fully powered upwind at 8 knots TWS (flat wiring), yet, we keep going faster right up to 15 knots TWS. How could this be unless we were able to further increase our righting moment by reducing the CoE in our sails (by pulling on Cunningham)? 

What happens in a normal dinghy is if we want to increase the horizontal yellow line (more power), we move it down the mast (lower CoE). In return, the side force on the foil must increase to keep the forces equal and opposite (as lift and CoG cannot change). 

IMG-20190629-WA0027 (2)_LI.jpg

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14 hours ago, MaxHugen said:

How is a T foil "unstable", and how does anhedral reduce that?

My guess is that a completely flat T foil is "slippery", which would explain why the other teams abandonned it and why TNZ has a very small anhedral to channel the water, like foiling windsurfers.

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5 hours ago, Mikko Brummer said:

This is a profound and fundamental insight into the AC75 dynamics. Wonder if the designers were aware of it when creating the rule?

I'm sure they were aware. I wasn't initially. First I thought there's a bug in my program, because the CoE ended up always at the same height regardless of speed, TWS or TWA. It only changed if I changed the cant angle. That made me realize what's happening. It's actually pretty cool, because I can calculate the CoE both based on geometry as shown above and by finding the balance in all forces and moments, and the CoE height comes out the same with usually only a few cm difference due to rounding errors. 

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4 hours ago, Mozzy Sails said:

I think they will have. Because, it's identical to how all sailing boats work. The only difference is vertical lift comes from a foil, rather than buoyancy / hull lift. 

To me that difference is what makes this situation unique. On your skiff, the lateral and vertical forces are not linked. If you're hit with a puff, you have two options: you can either let the sail out and reduce side force or keep the side force the same, but twist the sail more so the CoE moves lower. The boat will remain afloat, regardless how you react. On the AC75, it's more complicated, because to stay on foils, you need to maintain the vertical balance as well, and the lateral and vertical forces are coupled due to the fact that the same foil provides both.

BTW I'm happy to be proven wrong, I'm here to learn. I'm relatively confident that this theory is true for straight T foils and when the boat is in static balance. I'm sure strange and very complex things happen when the boat is out of balance and there are dynamic changes of flow and loads especially with anhedral foils. I wouldn't be able to model it, but I have a feeling that the anhedral foils may provide some sort of automatic balance, kind of like the acute angle foils did on the catamarans.

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7 hours ago, Mozzy Sails said:

Yes there is, but it seems at about 30 knots it pays to sail with the tip out (Tom Partingtons calcs not mine), at least using the equation he has for a surface piercing energy loss (waves and spray). The graph below was flashed up on screen in my last video (a crappy photo of a monitor, rather than a screenshot, so hard to see). 

The overall gain isn't huge, but I would say noticeable. However, I wonder if they have any ventilation and how far down their foil it reaches, and what drag this adds, at least from a reduction of span. Very hard to calculate.

But... even if surface piecing can be 'drag neutral', but it allows you to run 1 degree more cant, then it would be a good feature. You don't really see ETNZ sail tips out downwind where they sail with much less cant (their cant settings seem very modal). And you would think if it were all about drag reduction at high speed they would sail tips out downwind. But they don't... so my conclusion is ETNZ have designed foils that tolerate their tips running free to increase upwind RM. 


image.png.2bd8dfe536c2a6fb8a12ae907a29591e.png


And, if I am really speculating, the other teams are using variable flap settings to ever so slightly move the centre of lift outboard along the wings to increase their RM. 
Here's a video of Grant Simmer from before INEOS "in rough terms the righting moment is from the centre of lift of that foil, which would be about at the T junction, to the CoG of the boat which is creating that righting moment". 

 

 

 

Just a thought...Due to the foil having a swept leading edge wouldn't any ventilation run spanwise root to tip with the flow, rather than tip to root? Therefore the ETNZ running a tip out of the water be pretty impervious. I just haven't seen ventilation run against the flow? 

ED595A55-07C0-499D-B6DF-726C58217065.jpeg

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18 minutes ago, Tornado-Cat said:

My guess is that a completely flat T foil is "slippery", which would explain why the other teams abandonned it and why TNZ has a very small anhedral to channel the water, like foiling windsurfers.

Hmmm, I don't actually understand what "slippery" means?

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3 minutes ago, erdb said:

To me that difference is what makes this situation unique. On your skiff, the lateral and vertical forces are not linked. If you're hit with a puff, you have two options: you can either let the sail out and reduce side force or keep the side force the same, but twist the sail more so the CoE moves lower. The boat will remain afloat, regardless how you react. On the AC75, it's more complicated, because to stay on foils, you need to maintain the vertical balance as well, and the lateral and vertical forces are coupled due to the fact that the same foil provides both.

BTW I'm happy to be proven wrong, I'm here to learn. I'm relatively confident that this theory is true for straight T foils and when the boat is in static balance. I'm sure strange and very complex things happen when the boat is out of balance and there are dynamic changes of flow and loads especially with anhedral foils. I wouldn't be able to model it, but I have a feeling that the anhedral foils may provide some sort of automatic balance, kind of like the acute angle foils did on the catamarans.

No I agree... no one is trimming buoyancy upwind in a dinghy (although position of lift does move around with heel)! And similarly, we don't have to do anything to our foils to increase their lateral resistance when we lower the CoE in the sails... it just happens. 

But you can't have more vertical lift than the boat weighs so whilst they can manipulate where the centre of lift is on the foil with two flaps (giving more or less RM) surely once powered up you would always want max righting moment and then just trim sails as per a dinghy from then on? 

Going round corners is another thing. When you spin in a dinghy the weight goes to the outside the CoB shift catches it as you heel. In these boats the flight controllers have to 'catch' that shift as they turn. 

 

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Since AM is out of the AC.  

It seems like they would be free to conduct wind tunnel or water modeling test on various foils, hulls and rudder configurations until they sign on to the next AC.  Testing that was not within the rules of a team that was an active participant of the AC.  There seems to be some "holes" in their computer modeling that could be improved by more simulated flow testing at various speeds.

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30 minutes ago, Mozzy Sails said:

No I agree... no one is trimming buoyancy upwind in a dinghy (although position of lift does move around with heel)! And similarly, we don't have to do anything to our foils to increase their lateral resistance when we lower the CoE in the sails... it just happens. 

But you can't have more vertical lift than the boat weighs so whilst they can manipulate where the centre of lift is on the foil with two flaps (giving more or less RM) surely once powered up you would always want max righting moment and then just trim sails as per a dinghy from then on? 

Going round corners is another thing. When you spin in a dinghy the weight goes to the outside the CoB shift catches it as you heel. In these boats the flight controllers have to 'catch' that shift as they turn. 

 

Another issue for the AC75 is that heel causes them to pitch up, as foil lift is off-centre.

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Another video... bulbs... big fat ugly bulbs.... I guess when Airbus design your wings you have to have fuselage to. 
Do you mind me posting these videos here? I feel like I may be spamming. 

 

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If these guys had actually designed a wing to push out the onset of cavitation, they'd have a better appreciation of thickness ratio, especially at the root.

And, contrary to what they said in the video, induced drag has nothing to do with aspect ratio.  I'll say that again, because it's a common misconception: induced drag does not depend on aspect ratio.  It depends on span and how the wake wash is distributed across the span, and the span is fixed by the Design Rule.  So varying the area by scaling the chord does not change the induced drag.  Changing the area does strongly affect the parasite drag, however.

It would be interesting to compare the foil designs on the basis of wetted aspect ratio, which is the square of the span divided by the total wetted area, including the wings, bulb, and immersed strut.  In his book, Aircraft Design: A Conceptual Approach, Dan Raymer cites the examples of the Boeing B-47 and Avro Vulcan.  Two bombers of similar vintage, designed for similar missions, but having wing planforms of strikingly different aspect ratio.  However, when compared on the basis of wetted aspect ratio, they are actually quite similar, as was their lit/drag ratios.  The reason was all the extra wetted area of the B-47's fuselage, pylons and nacelles, and empennage.

Boeing_B-47E_Stratojet_51-2394.jpg?fit=1XH558_(G-VLCN)_Avro_Vulcan_-_Last_Flight

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Another good video @Mozzy Sails I hadn't realised the little tabs on the back of ETNZ's arms could be housing weight (or control systems?) as part of the "foil" (but not etc).

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