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Boats and foils comparison


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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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2 hours ago, Blitzkrieg9 said:

Too many different answers and I'm more confused than when I posited my claim.  ...

"The righting moment available limits the sail trim".  This. This is exactly what I posited. I believe that possibly LR has a greater righting moment available at the highest wind range so their sail trim is less limited, so they can increase power and go faster compared to ETNZ. 

Yes, it it confusing, because you need to consider the balance of all 6 Degrees of Freedom simultaneously. These are:

Directional Forces

  • Forward/backward (surge) - x axis
  • Left/right (sway) - y axis
  • Up/down (heave) - z axis

Rotational Moments (force x distance)

  • Pitch (transverse axis) - x plane
  • Roll (longitudinal axis) - y plane
  • Yaw (normal axis) - z plane

RM is a function of a rotational moment in the x plane. Heel from the sail force is countered by the mass of the boat, ± the force exerted by the rudder foil.

There are two ways to increase the amount of sail power that can be balanced using Moments in the y plane (roll).

First, you can reduce the Heeling moment by lowering the Centre of Effort of the sails by:

  • Using twist to depower the top of the sail.
  • Having some sail area lower as is the case with NZ.
  • Reducing sail area at the top, such as the "batwing".

Second, you can increase the Righting moments by

  • Moving the foil ballast as far outboard as possible, such as in a T foil.
  • Moving crew location as far outboard as possible - NZ crew are ~20cm further out than on LR.
  • Moving the foil's Vertical Lift centre of effort as far outboard as possible, which can be done using differential flap settings, by up to ~20cm.

Perhaps if you explain why you believe that LR has a greater RM at higher wind ranges, we can look at it further?

image.png.b96c36b1f5f1e41bf3405f9e9e48ac43.png
https://en.wikipedia.org/wiki/Six_degrees_of_freedom

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39 minutes ago, MaxHugen said:

Yes, it it confusing, because you need to consider the balance of all 6 Degrees of Freedom simultaneously. These are:

Directional Forces

  • Forward/backward (surge) - x axis
  • Left/right (sway) - y axis
  • Up/down (heave) - z axis

Rotational Moments (force x distance)

  • Pitch (transverse axis) - x plane
  • Roll (longitudinal axis) - y plane
  • Yaw (normal axis) - z plane

RM is a function of a rotational moment in the x plane. Heel from the sail force is countered by the mass of the boat, ± the force exerted by the rudder foil.

There are two ways to increase the amount of sail power that can be balanced using Moments in the y plane (roll).

First, you can reduce the Heeling moment by lowering the Centre of Effort of the sails by:

  • Using twist to depower the top of the sail.
  • Having some sail area lower as is the case with NZ.
  • Reducing sail area at the top, such as the "batwing".

Second, you can increase the Righting moments by

  • Moving the foil ballast as far outboard as possible, such as in a T foil.
  • Moving crew location as far outboard as possible - NZ crew are ~20cm further out than on LR.
  • Moving the foil's Vertical Lift centre of effort as far outboard as possible, which can be done using differential flap settings, by up to ~20cm.

Perhaps if you explain why you believe that LR has a greater RM at higher wind ranges, we can look at it further?

image.png.b96c36b1f5f1e41bf3405f9e9e48ac43.png
https://en.wikipedia.org/wiki/Six_degrees_of_freedom

Thanks for the great analysis. I will do my best to explain my reasoning.  It has something to do with wing loading and generating lift.  From Wikipedia on wing loading:  "Wings generate lift owing to the motion of air around the wing. Larger wings move more air..."  since I'm discussing the foils, just substitute water for air.  So it appears that LR foils, all things being equal, generate more lift since they are 30% larger.    

BUT, the faster the rate of travel the less foil you need for the same amount of lift: "The faster an aircraft flies, the more lift can be produced by each unit of wing area, so a smaller wing can carry the same mass in level flight".  

Agreed?   

So, now let's look at this theoretical.   Take off the mast and the sails and put 4 350HP engines on the stern. Perfectly calm day with no wind or currents. As you ramp up your speed, the LESS foil you need to keep the boat nice and level. All the "power" is being generated directly in line with the boat. There is no heel, you don't need more lift...  you need the same lift at all speeds, so if you had a magic foil, it would get smaller the faster you travel.  

Agreed?

Now, let's put the sails back up, remove the engines, and get back to reality.  The problem with using wind as your source of power is that your power isn't directly in line with your boat, right?  For every extra bit of power you capture, you get a little bit heeling movement with it!  Damn!

So now, when we increase speed we need LESS foil because of the extra lift generated by the foils, BUT we also need MORE foil because the faster we go the more the wind tries to heel us over!!!  So instead of requiring constant lift as on a foiling motorboat, we need to increase lift on a sailboat the faster we go. 

So...  isn't it possible that at lower speeds these boats have "excess lifting capacity"...  they CAN generate more lift than they need.  But at higher speeds they might run out of lifting capacity?  Even with more wind available, they can't use it because the heeling effect is greater than the foils can counter?  Because on these sail boats, the faster you go, the more foil lift movement  you need. (I think its the yF vector or something).  

Thoughts?

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21 minutes ago, Blitzkrieg9 said:

Thanks for the great analysis. I will do my best to explain my reasoning.  It has something to do with wing loading and generating lift.  From Wikipedia on wing loading:  "Wings generate lift owing to the motion of air around the wing. Larger wings move more air..."  since I'm discussing the foils, just substitute water for air.  So it appears that LR foils, all things being equal, generate more lift since they are 30% larger.    

BUT, the faster the rate of travel the less foil you need for the same amount of lift: "The faster an aircraft flies, the more lift can be produced by each unit of wing area, so a smaller wing can carry the same mass in level flight".  

Agreed?   

So, now let's look at this theoretical.   Take off the mast and the sails and put 4 350HP engines on the stern. Perfectly calm day with no wind or currents. As you ramp up your speed, the LESS foil you need to keep the boat nice and level. All the "power" is being generated directly in line with the boat. There is no heel, you don't need more lift...  you need the same lift at all speeds, so if you had a magic foil, it would get smaller the faster you travel.  

Agreed?

Now, let's put the sails back up, remove the engines, and get back to reality.  The problem with using wind as your source of power is that your power isn't directly in line with your boat, right?  For every extra bit of power you capture, you get a little bit heeling movement with it!  Damn!

So now, when we increase speed we need LESS foil because of the extra lift generated by the foils, BUT we also need MORE foil because the faster we go the more the wind tries to heel us over!!!  So instead of requiring constant lift as on a foiling motorboat, we need to increase lift on a sailboat the faster we go. 

So...  isn't it possible that at lower speeds these boats have "excess lifting capacity"...  they CAN generate more lift than they need.  But at higher speeds they might run out of lifting capacity?  Even with more wind available, they can't use it because the heeling effect is greater than the foils can counter?  Because on these sail boats, the faster you go, the more foil lift movement  you need. (I think its the yF vector or something).  

Thoughts?

The Vertical force (Fz) required doesn't change with speed, as boat mass is constant, so Fz - boat mass = 0 when these forces are balanced.

The Fz from the foil has to vary a small amount, as there is either an additional Fz provided by the rudder  foil, or even a negative force at higher boat speeds.  But lets just say it's a constant for now.

As boat speed increases, the foil has to maintain this constant Fz, and this is done by altering the foil AoA, and/or the flap angle. If this Fz is not maintained the boat will either rise or fall.

At low speeds, the narrow foils of NZ need more foil AoA and flap angle than LR to produce the same amount of Fz, and get up on the foil.

By about 30 knots, both foils are probably at 0° AoA, and the required Fz is achieved though the flaps alone. Due to LR's larger foils, the flap angle required is only ~0.4°, whereas NZ would need ~3.0°.

This calc also includes the effect of 0.5° of leeway, as this effectively increases the apparent AoA of the foil to the water flow.

"BUT we also need MORE foil because the faster we go the more the wind tries to heel us over!"  Not really.  The primary function of the foil is to provide Vertical force to support the boat, and Lateral force to limit leeway. These are "directional forces", not "rotational moments" which are balanced between Heel and RM.

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5 minutes ago, MaxHugen said:

 

"BUT we also need MORE foil because the faster we go the more the wind tries to heel us over!"  Not really.  The primary function of the foil is to provide Vertical force to support the boat, and Lateral force to limit leeway. These are "directional forces", not "rotational moments" which are balanced between Heel and RM.

Gotcha!  This (the heel vs. RM) is my confusion. I don't understand it quite yet, but now I know where my error lies and what I need to go educate myself on. Thanks!

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To put Max’s eloquent technical explanation into layman’s terms:

Once the leeward foils are lifting the entire weight of the boat, any extra lift does not provide extra righting moment, because the righting moment comes from the weight of the boat and the distance (arm) from its weight (centre of gravity) centroid to the foil lift (centre of buoyancy) centroid. When the canting arms are maxed out and the foils are breaching, that is it, there is no more to be had, unless you get all the leeward cockpit crew to jump into the windward cockpits as well.

When you have achieved max RM, the name of the game is to reduce flap AoA to reduce drag and/or with variable flap control on the dihedral, try to convert excess uplift into extra lift to windward.

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8 minutes ago, Blitzkrieg9 said:

Gotcha!  This (the heel vs. RM) is my confusion. I don't understand it quite yet, but now I know where my error lies and what I need to go educate myself on. Thanks!

Cool, you're on your way down the rabbit hole! :D

If you're interested, there are good explanatory YT videos if you search "calculate moments". This one seems to be a simple intro:

This is a diagram I did months ago, to help me figure out the relevant forces and distances to calc RM etc:

image.png.b2fd35435aeee05356a981a9f1ffef19.png

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10 minutes ago, MaxHugen said:

Cool, you're on your way down the rabbit hole! :D

If you're interested, there are good explanatory YT videos if you search "calculate moments". This one seems to be a simple intro:

This is a diagram I did months ago, to help me figure out the relevant forces and distances to calc RM etc:

image.png.b2fd35435aeee05356a981a9f1ffef19.png

Okay.  So...  let's assume the foil is magically directly in the middle of the boat under the center of gravity.  Increasing lift would just make the boat fly higher.  I got that. 

BUT, the foil isn't in the middle. It's way off to one side.  So, increasing the lift way out there should have the same effect as rolling the boat around its axis, right?  So, how does increasing lift (fZ) on the leeward foil NOT increase righting moment?  

On an airplane, if I increase the lift on just one wing, doesn't that make the plane roll?

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17 minutes ago, Blitzkrieg9 said:

Okay.  So...  let's assume the foil is magically directly in the middle of the boat under the center of gravity.  Increasing lift would just make the boat fly higher.  I got that. 

BUT, the foil isn't in the middle. It's way off to one side.  So, increasing the lift way out there should have the same effect as rolling the boat around its axis, right?  So, how does increasing lift (fZ) on the leeward foil NOT increase righting moment?  

On an airplane, if I increase the lift on just one wing, doesn't that make the plane roll?

You need to study moments, like with the YT link and others with some exercises you can try.

You can place the "axis of rotation" on the boat wherever you like, but you have to work out which forces to include and their distances from the chosen axis.

It's "convenient" to use the leeward foil as the axis, because for all but the most involved calcs, you can ignore that foil's lift and mass forces.

If you used a central position on the hull as your axis, you would then need to include the leeward foil's forces, but now you have only about half of the boat mass contributing to RM, while the other half adds to Heeling moment and opposes the foil Lift.

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5 minutes ago, Sidecar said:

It’s all a matter of balance:

 

119E2DD2-9529-4F72-8F70-9DE51FA46232.jpeg

Looks like a Burt Rutan design?

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9 minutes ago, MaxHugen said:

Looks like a Burt Rutan design?

It is. I thought you would like it..... He does some amazing stuff.

It would be interesting to see what he could do with an AC 75!?

 

49509883-DA09-43BD-840C-A52AACD04C50.jpeg

Edited by Sidecar
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8 minutes ago, MaxHugen said:

You need to study moments, like with the YT link and others with some exercises you can try.

You can place the "axis" on the boat wherever you like, but you have to work out which forces to include and their distances from the chosen axis.

It's "convenient" to use the leeward foil as the axis, because for all but the most involved calcs, you can ignore that foil's lift and mass forces.

If you used a central position on the hull as your axis, you would then need to include the leeward foil's forces, but now you have only about half of the boat mass contributing to RM, while the other half adds to Heeling moment and opposes the foil Lift.

Alright. Thanks my friend. I'll study up some more.  I still can't grasp that increasing Fz (vertical lift) way out on the leeward foil isn't a direct counter to boat heel caused by the wind.    Its like lifting up one end of a see-saw like in the video.  My action is a directional force, but it causes a rotational moment. 

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All force is directional. Force is a vector. There is no moment for rotation as it acts directly through the pivot point. Therefore it is merely lifting the whole boat. If that force exceeds the downward force exerted on the boats mass by gravity then the boat will rise, if not it will sink. As we see neither of these as a rule then it must be equal to the boats weight.

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Just to complicate things even further, the answer depends on whether you look at a T foil or a Y foil.

What's unique in the AC75's foils is that the lateral and vertical forces are coupled. In the foiling catamarans, the vertical and horizontal surfaces of the L foils handled the vertical and horizontal forces (mostly) separately. However, in the AC75, the same foil generates both forces.

For the T foil, it's fairly simple:

445863249_Tfoil.thumb.JPG.0964c45bf2bc0b830f406985b3271064.JPG

The lift generated by the foil (L) is perpendicular to the foil wing. You can figure out the angle (a) relative to vertical for any cant angle from the geometry of the foil arm. Knowing angle (a), you can then calculate the vertical (Z) and horizontal (Y) components : tan(a) = Y / Z

If a boat is in balance, weight (W) equals foil vertical (Z), and lateral sail force (Sail_Y) equals foil horizontal (Y) force.

This means that any time the lateral sail force changes, the T foil's cant angle has to change as well. This is because Z cannot change, since weight doesn't change (and we omit rudder forces from these calculations). You can only change foil horizontal force relative to vertical, if you change cant angle.

In addition to these equilibriums in straight line forces, you also have to make sure that roll moments are the same. The foil's righting moment is arm * force: (x0 + x1) *  Z. This has to equal Sail_Y * CoE of the sails. Again, you can get x0 and x1 from the geometry of the boat and from the cant angle. This all means that indirectly the RM of the T foil depends on the side force of the sails. The higher the side force, the more the T foil is canted out and the higher the RM goes.

With the Y foil, things get more complex, because lift generated by the foil wings point to different directions:

1768682942_Yfoil.JPG.af5e07e8fe9488ec8c58f98bcd6b5c4d.JPG

 

The general principle is the same, but with differential flap settings, you can change L1 and L2 relative to each other, and since they point into different directions, that will shift forces.

The approach is still the same though: you can calculate vertical and horizontal components of L1 and L2 lift forces, and when you add these together, you still have to match the weight of the boat and the lateral sail force:

W = Z1 + Z2  and Sail_Y = Y1 + Y2

Vertical and horizontal forces are also coupled for each wing just like with the T foil:

tan(a1) = Y1 / Z1  and tan(a2) = Y2 / Z2

You can calculate the angles (a1 and a2) from the geometry for any foil cant angle. Then, you have four variables and four equations. If you put all that together, you get this beauty: 

Z2 = (sail_Y - tan(a1) * W) / (tan(a2) - tan(a1))

From here, you can get all the others, Y2, Z1 and Z2 using the above equations.

To get the righting moment generated by the Y foil, just add the RMs of the two wings together:

(x0 + x1) * Z1 + (x0 + x2) * Z2 = sail_Y * CoE

We have to establish some limits on how much lift the wings can generate by changing flap angles (max L1 and L2), and we get these curves showing how angles, forces and RM change with increasing lateral sail force:

top2.thumb.png.c4a0e8b2c1bf64c1ff9d1d59d3c04121.png

rm.png.94a49b69d9c252a38649c3ce5deea531.png

The T foil (red line) can only have one specific cant angle for a given lateral force. The Y foil on the other hand, has a range of possible cant angles, because you can fine-tune the direction of combined foil forces with the flaps.

The foil area only comes in when establishing the max lift of the wings. In the case of the T foil, area has no effect on RM at all. If you don't have enough lift, the boat doesn't foil. Once the boat is flying, the RM is determined by the cant angle, not the foil area.

On the other hand, the Y foil wing area will affect max L1 and L2 forces, which will determine how wide the cant angle range is for the Y foil for a given sail side force.  So indirectly, the foil size does affect RM here, but it's not like the higher lift equals higher RM, but if you can generate more lift with a wing half, you have a wider range of possible cant angles for a given side force, and more cant = higher RM. In practice, I doubt LR deviated too much from symmetrical loading of the two wings, because it would increase drag by a lot. Plus, there is the issue of sticking the tip of the foil into the air if you try to cant it out too much.

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38 minutes ago, kiwin said:

Therefore it is merely lifting the whole boat. If that force exceeds the downward force exerted on the boats mass by gravity then the boat will rise, if not it will sink. As we see neither of these as a rule then it must be equal to the boats weight.

This just can't be true.  The leeward foil is not the center of gravity/ mass.  If Neptune were to rise from the ocean and exert 20,000 lbs of upward force on the leeward foil of an AC75 in flight, it wouldn't just lift up the whole boat. Rather, it would capsize the boat to windward. 

Right?!?

EDIT: balance a pencil across your finger. Now swipe up on one side. It doesn't "lift up the whole pencil".  Rather, rolls the pencil

Edited by Blitzkrieg9
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31 minutes ago, erdb said:

Just to complicate things even further, the answer depends on whether you look at a T foil or a Y foil.

What's unique in the AC75's foils is that the lateral and vertical forces are coupled. In the foiling catamarans, the vertical and horizontal surfaces of the L foils handled the vertical and horizontal forces (mostly) separately. However, in the AC75, the same foil generates both forces.

For the T foil, it's fairly simple:

445863249_Tfoil.thumb.JPG.0964c45bf2bc0b830f406985b3271064.JPG

The lift generated by the foil (L) is perpendicular to the foil wing. You can figure out the angle (a) relative to vertical for any cant angle from the geometry of the foil arm. Knowing angle (a), you can then calculate the vertical (Z) and horizontal (Y) components : tan(a) = Y / Z

If a boat is in balance, weight (W) equals foil vertical (Z), and lateral sail force (Sail_Y) equals foil horizontal (Y) force.

This means that any time the lateral sail force changes, the T foil's cant angle has to change as well. This is because Z cannot change, since weight doesn't change (and we omit rudder forces from these calculations). You can only change foil horizontal force relative to vertical, if you change cant angle.

In addition to these equilibriums in straight line forces, you also have to make sure that roll moments are the same. The foil's righting moment is arm * force: (x0 + x1) *  Z. This has to equal Sail_Y * CoE of the sails. Again, you can get x0 and x1 from the geometry of the boat and from the cant angle. This all means that indirectly the RM of the T foil depends on the side force of the sails. The higher the side force, the more the T foil is canted out and the higher the RM goes.

With the Y foil, things get more complex, because lift generated by the foil wings point to different directions:

1768682942_Yfoil.JPG.af5e07e8fe9488ec8c58f98bcd6b5c4d.JPG

 

The general principle is the same, but with differential flap settings, you can change L1 and L2 relative to each other, and since they point into different directions, that will shift forces.

The approach is still the same though: you can calculate vertical and horizontal components of L1 and L2 lift forces, and when you add these together, you still have to match the weight of the boat and the lateral sail force:

W = Z1 + Z2  and Sail_Y = Y1 + Y2

Vertical and horizontal forces are also coupled for each wing just like with the T foil:

tan(a1) = Y1 / Z1  and tan(a2) = Y2 / Z2

You can calculate the angles (a1 and a2) from the geometry for any foil cant angle. Then, you have four variables and four equations. If you put all that together, you get this beauty: 

Z2 = (sail_Y - tan(a1) * W) / (tan(a2) - tan(a1))

From here, you can get all the others, Y2, Z1 and Z2 using the above equations.

To get the righting moment generated by the Y foil, just add the RMs of the two wings together:

(x0 + x1) * Z1 + (x0 + x2) * Z2 = sail_Y * CoE

We have to establish some limits on how much lift the wings can generate by changing flap angles (max L1 and L2), and we get these curves showing how angles, forces and RM change with increasing lateral sail force:

top2.thumb.png.c4a0e8b2c1bf64c1ff9d1d59d3c04121.png

rm.png.94a49b69d9c252a38649c3ce5deea531.png

The T foil (red line) can only have one specific cant angle for a given lateral force. The Y foil on the other hand, has a range of possible cant angles, because you can fine-tune the direction of combined foil forces with the flaps.

The foil area only comes in when establishing the max lift of the wings. In the case of the T foil, area has no effect on RM at all. If you don't have enough lift, the boat doesn't foil. Once the boat is flying, the RM is determined by the cant angle, not the foil area.

On the other hand, the Y foil wing area will affect max L1 and L2 forces, which will determine how wide the cant angle range is for the Y foil for a given sail side force.  So indirectly, the foil size does affect RM here, but it's not like the higher lift equals higher RM, but if you can generate more lift with a wing half, you have a wider range of possible cant angles for a given side force, and more cant = higher RM. In practice, I doubt LR deviated too much from symmetrical loading of the two wings, because it would increase drag by a lot. Plus, there is the issue of sticking the tip of the foil into the air if you try to cant it out too much.

Please, no...  let's not "complicate further".  :) :) :)  i still can't grasp the basics. 

Seriously tho, thanks!  I'm about 6 beers deep so I just scrolled straight part your dissertation.  But I will read it about 4 times tomorrow. 

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22 minutes ago, Blitzkrieg9 said:

The leeward foil is not the center of gravity/ mass.  If Neptune were to rise from the ocean and exert 20,000 lbs of upward force on the leeward foil of an AC75 in flight, it wouldn't just lift up the whole boat. Rather, it would capsize the boat to windward. 

The entire boat would “fly” (very rapidly) upwards..... kept in balance by the heeling (and driving) force of wind on the the sails.

Sleep well, and don’t have any beer for breakfast.....

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The red is the sail and the foil arm/foil. They make a rigid tandem.  What is the difference between the blue arrows?  One pushes the sail to windward, one pushes the foil up.  It doesn't matter which blue force vector you apply. The result is the same.   

Right?!?

20210318_192111.jpg

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21 minutes ago, Blitzkrieg9 said:

This just can't be true.  The leeward foil is not the center of gravity/ mass.  If Neptune were to rise from the ocean and exert 20,000 lbs of upward force on the leeward foil of an AC75 in flight, it wouldn't just lift up the whole boat. Rather, it would capsize the boat to windward. 

Right?!?

EDIT: balance a pencil across your finger. Now swipe up on one side. It doesn't "lift up the whole pencil".  Rather, rolls the pencil

But it is true. The centre of gravity is NOT the pivot point. The couple between COG and the lifting centroid of the lee foil, the pivot point is the righting moment. 

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2 minutes ago, Blitzkrieg9 said:

The red is the sail and the foil arm/foil. They make a rigid tandem.  What is the difference between the blue arrows?  One pushes the sail to windward, one pushes the foil up.  It doesn't matter which blue force vector you apply. The result is the same.   

Right?!?

20210318_192111.jpg

The sail force arrow is acting the wrong way. Sail force acts to leeward and forward. Hence the two forces balance with the addition of mass acting down

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8 minutes ago, Blitzkrieg9 said:

The red is the sail and the foil arm/foil. They make a rigid tandem.  What is the difference between the blue arrows?  One pushes the sail to windward, one pushes the foil up.  It doesn't matter which blue force vector you apply. The result is the same.   

Right?!?

20210318_192111.jpg

You have the blue horizontal arrow the wrong way round.... And you are missing a downward vertical blue arrow representing the weight of the boat. 

Get some sleep.

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11 minutes ago, Sidecar said:

You have the blue horizontal arrow the wrong way round....

Get some sleep.

No. No.  I drew it as I meant to draw it.  But I think I might see my confusion.  I believe that regardless of which blue force I increase on a foiling AC75, the result will be to pivot around the intersection of the two red lines. The "axis of rotation".  

But, I think you are telling me that this is wrong. If I apply force on the foil arrow it will lift the boat up.  If I apply force to the sail arrow it will scoot the boat sideways.  But in neither case will the boat pivot around the axis.  Is this your contention?

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10 minutes ago, Sidecar said:

You have the blue horizontal arrow the wrong way round.... And you are missing a downward vertical blue arrow representing the weight of the boat. 

Get some sleep.

Your diagram is also missing a horizontal blue arrow representing water resistance at or near foil lift which is opposite to the wind pressure above.

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6 minutes ago, Blitzkrieg9 said:

No. No.  I drew it as I meant to draw it.  But I think I might see my confusion.  I believe that regardless of which blue force I increase on a foiling AC75, the result will be to pivot around the intersection of the two red lines. The "axis of rotation".  

But, I think you are telling me that this is wrong. If I apply force on the foil arrow it will lift the boat up.  If I apply force to the sail arrow it will scoot the boat sideways.  But in neither case will the boat pivot around the axis

You drew it as you meant to, but it does not reflect reality

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7 minutes ago, Blitzkrieg9 said:

No. No.  I drew it as I meant to draw it.  But I think I might see my confusion.  I believe that regardless of which blue force I increase on a foiling AC75, the result will be to pivot around the intersection of the two red lines. The "axis of rotation".  

But, I think you are telling me that this is wrong. If I apply force on the foil arrow it will lift the boat up.  If I apply force to the sail arrow it will scoot the boat sideways.  But in neither case will the boat pivot around the axis

Your "axis of rotation" is one you have arbitrarily imposed. The vectors will still resolve out, but when the boats are sailing, they actually rotate largely around the centre of the leeward foil.

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3 minutes ago, kiwin said:

Your "axis of rotation" is one you have arbitrarily imposed. The vectors will still resolve out, but when the boats are sailing, they actually rotate largely around the centre of the leeward foil.

Okay.   Hmmm...  I think I'm catching on...  I'm close to grasping this.  Thanks for your input!!!

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9 minutes ago, kiwin said:

Your "axis of rotation" is one you have arbitrarily imposed. The vectors will still resolve out, but when the boats are sailing, they actually rotate largely around the centre of the leeward foil.

Okay. I might have had the eureka moment. I have two simple scenarios and I need two simple answers. 

1) An AC75 is sitting still in the water. Neptune pushes up on one foil. What happens?  

2) an AC75 is foiling along at 30 knots. Neptune pushes up on the leeward foil. What happens?

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16 minutes ago, amc said:

In the absence of anything else 

1. boat capsizes 

2. boat lifts off and flies, drifting faster sideways until air pressure resistance negates heeling moment and then the Neptune force and RM couple will capsize the AC75 to windward.

Fixed.

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Yes.

And your “Neptune” force will only ever be limited to the mass of the boat, because any more than that and the foils will lift and breech and lose lift.

It is almost self regulating, but get it wrong and you have seen the results.

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7 minutes ago, Blitzkrieg9 said:

"Drifting faster sideways"

To leeward?

The reality is that this is a very complex set of forces, if you change any one then a whole bunch of other things must change with it or boat capsizes 

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Also you are close to understanding what a knife edge balancing act sailing one of these monsters is. I presume that like some cars I have driven that they are basically trying to kill you. Yes I survived a 70's  TVR.

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2 hours ago, Blitzkrieg9 said:

This just can't be true.  The leeward foil is not the center of gravity/ mass.  If Neptune were to rise from the ocean and exert 20,000 lbs of upward force on the leeward foil of an AC75 in flight, it wouldn't just lift up the whole boat. Rather, it would capsize the boat to windward. 

 

I don't think it's helpful to consider the effects of a single theoretical force. Excess lift on the leeward foil greater than the weight of the boat would simply lift the boat up, it wouldn't necessarily capsize it until it leaves the water, at which point leeway resistance is lost so the boat will start moving sideways and could just continue to rise as RM and capsize moment remain in balance. But pitch likely isn't balanced (no rudder up/down once out of the water) so rather than capsize to windward, the boat will probably pitch forward and pitchpole, or go bow up and pitchpole backwards. It all depends on where the centre of lift is in regard to the boat's centre of gravity.

The sail produces a force that pushes the boat forwards at an angle that we resolve into the linear and rotational forces described by Max. The unhelpful components are balanced out (drag, lift, capsizing moment, etc.) so that what's left over goes to forward motion at a speed that is limited by drag. Increasing or reducing any single force means rebalancing, if you go outside the ability of the model to deal with the change, you reach a point of inflection or a discontinuity beyond which the results are meaningless.

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3 hours ago, Blitzkrieg9 said:

Okay.  So...  let's assume the foil is magically directly in the middle of the boat under the center of gravity.  Increasing lift would just make the boat fly higher.  I got that. 

BUT, the foil isn't in the middle. It's way off to one side.  So, increasing the lift way out there should have the same effect as rolling the boat around its axis, right?  So, how does increasing lift (fZ) on the leeward foil NOT increase righting moment?  

On an airplane, if I increase the lift on just one wing, doesn't that make the plane roll?

You can take any point you want as the moment reference center and balance the moments about that.  If there is a force that is unknown, then that is a good place for the moment reference center because the arm then is zero and the moment from the unknown (or variable) force is zero.

So consider taking moments about the center of effort of the leeward foil.  Now it doesn't matter what the lift on the foil is.  The righting moment about this reference center comes from the weight of the boat to windward of the immersed foil.  That is largely set by the Design Rule.  And it is independent of the design or operation of the leeward foil.  This righting moment will dictate the sail trim. 

As long as the moment about the leeward foil is balanced, you can vary the lift on the leeward foil and all it will do is accelerate the boat up or down.

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1 hour ago, Sidecar said:

The entire boat would “fly” (very rapidly) upwards..... kept in balance by the heeling (and driving) force of wind on the the sails.

Sleep well, and don’t have any beer for breakfast.....

@Sidecar i went back and reread a bunch of comments. Thanks again to you and @erdband @MaxHugenand a few others.  I think I have a much greater understanding (I'm still naive, but no longer ignorant per se)...  

I believe my initial assertion is still correct insofaras an increased upward lift on the leeward foil WILL mathematically result in a greater righting moment.  Let's call this "Posit A".  

But my error was not understanding the insignificance of "Posit A".  

In fact, the forces from every direction are so great on a foiling AC75 (at say, 35kn) that in reality increasing the Fz vector at the leeward foil will miraculously just plain lift up the ENTIRE vessel and barely affects the righting moment at all.  (I.e. the center of gravity is basically on the foil itself).

This is my current understanding. 

 

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10 minutes ago, Blitzkrieg9 said:

@Sidecar i went back and reread a bunch of comments. Thanks again to you and @erdband @MaxHugenand a few others.  I think I have a much greater understanding (I'm still naive, but no longer ignorant per se)...  

I believe my initial assertion is still correct insofaras an increased upward lift on the leeward foil WILL mathematically result in a greater righting moment.  Let's call this "Posit A".  

But my error was not understanding the insignificance of "Posit A".  

In fact, the forces from every direction are so great on a foiling AC75 (at say, 35kn) that in reality increasing the Fz vector at the leeward foil will miraculously just plain lift up the ENTIRE vessel and barely affects the righting moment at all.  (I.e. the center of gravity is basically on the foil itself).

This is my current understanding. 

 

The COG cannot be on the foil itself or there is no righting moment. The distance of the COG from the lee foil is what creates the righting moment

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26 minutes ago, kiwin said:

The COG cannot be on the foil itself or there is no righting moment. The distance of the COG from the lee foil is what creates the righting moment

You are less helpful than others.  Your comments rarely edify. 

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I am sorry, I am trying to help. You seem to constantly conflate the COG with the pivot point. They are not the same thing. When in flight the boat operates around the centre of lateral & vertical resistance. This is roughly the centroid of the leeward foil, this being the only part in the water.

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23 minutes ago, kiwin said:

I am sorry, I am trying to help. You seem to constantly conflate the COG with the pivot point. They are not the same thing. When in flight the boat operates around the centre of lateral & vertical resistance. This is roughly the centroid of the leeward foil, this being the only part in the water.

You're 100% right. I'm struggling to differentiate between the vertical component of the  COG and the pivot point.  To my mind, these are on the same vertical axis. 

Plus, I do not understand the concept of "the centroid of the leeward foil".  I must needs further my research and understanding. Thank you!

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When the boat is foiling it is supported by the leeward foil and the rudder. If you draw a line from the rudder through the lee foil, then this is the actual pivot around which the rig and ballast operate. Note that this is not parallel to the centre line. Also note that the rig exerts a force which is forward as well as to leeward. This certainly results in trying to press the nose of the boat down, so the rudder foils may well be pulling the stern down to compensate and increasing righting moment.

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8 minutes ago, kiwin said:

When the boat is foiling it is supported by the leeward foil and the rudder. If you draw a line from the rudder through the lee foil, then this is the actual pivot around which the rig and ballast operate. Note that this is not parallel to the centre line. Also note that the rig exerts a force which is forward as well as to leeward. This certainly results in trying to press the nose of the boat down, so the rudder foils may well be pulling the stern down to compensate and increasing righting moment.

Same as planes, the elevator actually holds the tail down. The centre of gravity is in front of the centre of lift. As witnessed by a stalled plane falling nose first

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Great discussion here. Thanks to all from a dedicated lurker.

Just wondering if variable incidence of the rudder foil has been considered in RM discussion. A change in AoA of the rudder foil through a change in rudder vertical axis angle would have a much bigger effect than roll through differential flap angle on the arm foil. It was very interesting to watch the different wakes from the rudders at different times. They are certainly availing of negative lift from the rudder foil.

Also, and this may have been covered earlier, the change in height to the CoE through ride height adjustment will have as significant an effect on RM as the cant angle effect discussed above. The higher the ride height the more RM required. It was noticeable that ETNZ flew lower more often than anyone else.

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21 minutes ago, SpudH said:

Great discussion here. Thanks to all from a dedicated lurker.

Just wondering if variable incidence of the rudder foil has been considered in RM discussion. A change in AoA of the rudder foil through a change in rudder vertical axis angle would have a much bigger effect than roll through differential flap angle on the arm foil. It was very interesting to watch the different wakes from the rudders at different times. They are certainly availing of negative lift from the rudder foil.

Also, and this may have been covered earlier, the change in height to the CoE through ride height adjustment will have as significant an effect on RM as the cant angle effect discussed above. The higher the ride height the more RM required. It was noticeable that ETNZ flew lower more often than anyone else.

G'day "lurker"! Yes, when you get down to the "fine detail", the ± force from the rudder foil also has to be accounted for. Until you get to higher boat speeds, that force is a positive Lift, which actually reduces RM a bit.

The rudder foil has to balance the Longitudinal moments of sail force, as well as adjust the main foil AoA... these are all interconnected. Changes in rudder foil force also have to be compensated by the main foil, so that total vertical lift balances the total boat+crew mass.

Ride height has only a small influence on RM.  Flying lower does allow the foil to be canted further, and that does move the centre of vertical lift outboard, but the difference between say 20° and 22° cant is not much. The extra cant is more helpful in increasing Lateral force.

IMO :)

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9 hours ago, Blitzkrieg9 said:

Please, no...  let's not "complicate further".  :) :) :)  i still can't grasp the basics. 

Seriously tho, thanks!  I'm about 6 beers deep so I just scrolled straight part your dissertation.  But I will read it about 4 times tomorrow. 

Hahaha... welcome to the rabbit hole. We've probably all been there at one time or another.   I've asked the wabbit to explain this in simpler terms... let's see how he does. Carry on, wabbit :

A force is measured in Newtons, and 1000 Newtons is a kilo-Newton (kN). Let's say the boat+crew exerts a downward force of 80kN.  We'll use this in the examples.

A see-saw is a good place to start. A moment is a turning (rotational) force. In Figure 1,  Forces A & B are the same, as are their distances from the axis. As these forces are turning in opposite directions and cancel each other out, the see-saw is balanced.

In Fig 2, by halving Force B but doubling it's distance to the axis, the turning forces of the see-saw are still equal and balanced.

1283042344_RabbitsMoments1.thumb.JPG.544714d03c566dcbfed59593142c06f9.JPG

Since a moment is only concerned with forces that rotate around an axis, we're not interested in the Total up and down forces when calculating moments. You can see that these are not the same in Figures 1 and 2.

For moments, the "distance" is measured perpendicular to the direction of the force, from the force's centre of effort (CE) to the chosen axis.

In Fig 3 and 4, the turning forces have remained the same, but Force B has been rotated. So the turning forces - moments - are exactly the same in Figures 1 through to 4.
1151184437_RabbitsMoments2.thumb.JPG.7c8ea2f848543624faf4da99d644512b.JPG

Figure 5 just gives the forces some "boaty" names - "Heeling Force" and the "Boat Mass" which provides the Righting Moment. These turning forces haven't changed, and are still balanced.

How can you increase the sail force to increase boat speed? The turning forces still have to balance, so if the Heeling Force increases, the Distance to the axis must decrease, as shown in Figures 6 and 7.

1274447681_RabbitsMoments3.thumb.JPG.39385c33944688706a76b9bcd8008fac.JPG

How do the moments stack up if the axis is moved to the centre of the boat? If we assume the CoG of the boat is also on the centreline for simplicity,  Figure 8 illustrates that the Boat Mass is no longer a turning moment, and instead the Vertical Lift from the foil must be accounted for, and becomes the Righting Moment.

So if the foil Force is increased, then the Heeling Force can also increase, and the moments can still be balanced.      Yes...   but NO.

679432573_RabbitsMoments4.thumb.JPG.449d3bf85faff83a91a757b0a0aae4e6.JPG

The problem is that the "directional" forces, which are Up versus Down in this case, also have to balance at the same time.

Now we're not interested if the forces are turning, only if their direction is Up or Down. In Figure 9, since the Heeling Force is sideways, it becomes irrelevant. So only the downward Boat Mass versus the Lifting Force are assessed. As long as the Up force (Vertical Lift) from the foil equals the Down force of the boat mass, the boat will not rise or sink as it's balanced.

Ignoring other factors, the Vertical Lift cannot exceed the Down force of the Boat Mass, or the boat will simply rise until the foil breaches the surface and loses lift.

Well, how did the wabbit do - does he get a carrot?          592341560_RabbitsMoments5.JPG.1d109e45f5abcaeefd7bbe6c7957d839.JPG

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5 hours ago, SpudH said:

Great discussion here. Thanks to all from a dedicated lurker.

Just wondering if variable incidence of the rudder foil has been considered in RM discussion. A change in AoA of the rudder foil through a change in rudder vertical axis angle would have a much bigger effect than roll through differential flap angle on the arm foil. It was very interesting to watch the different wakes from the rudders at different times. They are certainly availing of negative lift from the rudder foil.

Also, and this may have been covered earlier, the change in height to the CoE through ride height adjustment will have as significant an effect on RM as the cant angle effect discussed above. The higher the ride height the more RM required. It was noticeable that ETNZ flew lower more often than anyone else.

Yes, when I made my VPP, I included rudder vertical force, too. Rudder downforce increases righting moment, but it also increases drag. Initially, I always had downforce on the rudder, but as I played with parameters like location of CG and pitch angle of the boat, I found that upwind performance was best when the rudder produced a slight vertical lift. Reaching angles (mark rounding) however require a lot of rudder downforce, and downwind as well depending on wind strength.

Rudder and foil vertical forces:

rud_foil_z.png.82cc1dc8f7d1ac97f767a382ef43c992.png

(As the rudder produces more downforce, the foil needs to produce more vertical lift)

Roll and pitch moments:

heel_pitch_ms.png.a702c68326c58af78968c64461f26570.png

 

 It's still far from ideal, and there's a lot to optimize here, because any change in lift / drag ratios of the foil or rudder horizontal would change where the optimum performance is.

 

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Ask yourself one simple question.

 

If they could change out foils each day for different conditions what would the foil be each time.

Me thinks big in low winds and small in high winds as they go from a shortage of lift to a surplus of lift. Its all about drag.

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While there is a link between wind speed and boat speed, I think what boat speed you do changes the foil size.  Most of the time you are travelling fast (even in low wind speeds) so small foils all the time and very small for very fast?  It is only for lift off that you need large foils and if you can solve that using other boat changes then better because it is a very small time in the whole race picture?

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3 hours ago, Kiwing said:

 It is only for lift off that you need large foils and if you can solve that using other boat changes then better because it is a very small time in the whole race picture?

I still think that hull aerodynamics had a role to play.... And I think TR overall, did it best, resulting in the smallest foils.

Over in the “New sailing twin skin setup” thread, I opined:

“For me the mast stump was about lowering the deck to minimise the frontal area facing 40 to 50 knots AWS. Plus channeling the airflow, which in itself means that the (additional) sail area [created] low down is even more effective.

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

DF78887E-B74D-44B0-A6FC-1D8B989EF344.thumb.jpeg.25fc71eef8f0f13a6cf6a4814680f8a9.jpeg

We all can understand that “hull lift” can reduce RM max, but if it is only really effective at lift off and in soft conditions, when the hull underside is sufficiently close to the water, it must help. It also would help to keep “flight”steady, hull too high, the effect is negligible/non existent due to increasingly excessive cross flow, hull drops low and it increasingly kicks in. With a canoe central underbody (plus keel runner underneath) half a tunnel hull is enough for an advantage, because a full tunnel hull is unachievable under the rules?

BTW, did anyone else also notice that LR add on their own keel runner late in the day?

I also think TR possibly could have been improved if they had extended their canoe skeg right up to the rudder like LR did....

And then there is this:

 

A73F5921-3C5E-45AA-9FDF-C0A712AF0271.jpeg

Edited by Sidecar
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On 3/19/2021 at 5:34 AM, MaxHugen said:

You need to study moments, like with the YT link and others with some exercises you can try.

You can place the "axis of rotation" on the boat wherever you like, but you have to work out which forces to include and their distances from the chosen axis.

It's "convenient" to use the leeward foil as the axis, because for all but the most involved calcs, you can ignore that foil's lift and mass forces.

If you used a central position on the hull as your axis, you would then need to include the leeward foil's forces, but now you have only about half of the boat mass contributing to RM, while the other half adds to Heeling moment and opposes the foil Lift.

 

On 3/19/2021 at 7:43 AM, kiwin said:

Your "axis of rotation" is one you have arbitrarily imposed. The vectors will still resolve out, but when the boats are sailing, they actually rotate largely around the centre of the leeward foil.

 

On 3/19/2021 at 10:50 AM, kiwin said:

When the boat is foiling it is supported by the leeward foil and the rudder. If you draw a line from the rudder through the lee foil, then this is the actual pivot around which the rig and ballast operate. Note that this is not parallel to the centre line. Also note that the rig exerts a force which is forward as well as to leeward. This certainly results in trying to press the nose of the boat down, so the rudder foils may well be pulling the stern down to compensate and increasing righting moment.

I wanted to thank the general level of input here on this forum, especially to @erdb & @Basiliscus for their insight and input.

I was getting frustrated however with the lack of acknowledgement on the curious nature of these beasts being a three legged stool (outboard foil arms and rear but centreline mounted rudder foil) and it is only in @kiwin post above (@ 10.50am) that someone finally hints at how dynamic this attribute makes these craft when you consider how small changes in Roll (Heel) will have on both Nose Up or Down attitude, as well as changing AOA on foil surfaces simultaneously. So Roll can be affected by Flap angle, foil arm cant, mainsheet or jibsheet trim, wind pressure on (or off), compettitors windwash, and rudder flap angle - to name just a few of the inputs that could set the whole show off kilter.

Think about how the Axis being offset from the boats centreline makes the chase for dynamic stability just so much more complex than the Foilng Cats or even Canting Keels ever had........ Its a wonder that these boats were ever capable of being flown so accurately and repeatedly and in close proximity to each other. Bravo to the concept, Wonderment at the real matchracing that we witnessed and now finally we will have a Mea Culpa to all the naysayers that were writing such crap when this rule was first mooted? You know who you are, and we will not forget......

So Add that to your mix of going down the rabbit hole if you want a truly disturbed nights sleep......

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2 hours ago, oatsandbeans said:

Ok so if you were building a AC boat for the next cup what foil design would you use? Answers of “just copy ETNZ foils” are not allowed as sure as hell they won’t be using them next time round! 

The fastest ones right from the beginning and then learn how to sail them.

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10 minutes ago, I ride bikes said:

The fastest ones right from the beginning and then learn how to sail them.

These boats are a set of compromises.  My belief is you must learn to get the sails working to maximum low speed grunt.  Then you have to design some minimum foils that will get you up.  Once foiling all the drag factors start being most important. Mostly the sails IMHO.

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On 3/11/2021 at 9:44 AM, MaxHugen said:

I understand that, but "both foils start at moderate cant and then gradually increase throughout the [entire] tack".

image.png.6ad71420ae4c543baa2b29314c2ce47c.png

 

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Max, coming out of a tack the 75 takes some time to reach the maximum speed it can achieve on the new tack. Initially the AWA is high and AWS low, however the heeling moment remains constant. As the speed increases the power available from the wind increases, hence to maintain constant heeling moment the sails have to be eased or twisted. twist is best as it lowers the CofE and increases the forward force. Lowering the CofE requires a higher cant angle to avoid side loads on the strut. My guess is that AofA of the strut is measured and the cant is increased to keep it near zero as the boat accelerates

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3 hours ago, alanjs said:

Max, coming out of a tack the 75 takes some time to reach the maximum speed it can achieve on the new tack. Initially the AWA is high and AWS low, however the heeling moment remains constant. As the speed increases the power available from the wind increases, hence to maintain constant heeling moment the sails have to be eased or twisted. twist is best as it lowers the CofE and increases the forward force. Lowering the CofE requires a higher cant angle to avoid side loads on the strut. My guess is that AofA of the strut is measured and the cant is increased to keep it near zero as the boat accelerates

The image you referenced is not typical of cant angles, I posted it as I suspected at the time that they may have had a hydraulic pressure leak, although I don't know if that was actually the case.

I'd recommend you have a look at @dorox website at  https://ac36.herokuapp.com/stats_app  where you can select foil arm cant angle stats for any race of any of the AC series.

FYI, to get the foil angle from the reported foil arm angle , subtract 42°.

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9 hours ago, oatsandbeans said:

Ok so if you were building a AC boat for the next cup what foil design would you use? Answers of “just copy ETNZ foils” are not allowed as sure as hell they won’t be using them next time round! 

There is no easy answer to this, as the fastest configuration at one TWS is not the same as at a higher TWS etc.  ETNZ has shown that with good control systems the narrow foils work OK at lower TWS.

At higher TWS there are two major problems (besides plenty of others too).

Drag. At high boat speeds, they carry too much sail. So they have to twist off the top to reduce heel by lowering the CE, but the top section still contributes to drag with no benefit. Allowing a mainsail that has a shorter luff for high wind speeds may be a possibility.

Foils comprise around 50% of total drag. Allowing teams to select foils on the day for the TWS will definitely help, as they could use higher speed foils that are not only narrower but also thinner to minimise drag, whilst using thicker high-lift foils at low wind speeds. Or could a brilliant designer invent a foil that would vary foil profile?

Cavitation. If the drag issues can be addressed, foil cavitation becomes a problem. This can occur with any foil, although at different speed depending on profile. @Basiliscus did a 6 part series which explains this, starting at Part 1.

Again relying on a brilliant designer, perhaps a foil can be designed that provides sufficient sub-cavitation lift for take-off at high TWS, then "morphs" into a ventilating super-cavitating section?

 

Another possibility may be by changing the boat design to dynamically increase RM. The latest boats in the IMOCA class use a curved design with their surface piercing foils. As they heel, the foils are lowered, and the curved design moves the centre of lift further outboard. Is this even a possibility for the AC75?

But all this may be wishful thinking from the rabbit hole.  :rolleyes:

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The thickness is driven by other than hydrodynamic considerations.  On the AC50, the foil span could be allowed to extend very far inboard, and the foil wings were cantilevered, so the foil thickness was driven by structural strength and stiffness.  On the AC75, the foil span is much more limited, and the T foil results in a much smaller unsupported span.  But the ballast and foil control requirements drive the thickness.

I think in a Version 2 AC75 you'll see more convergence of the designs.  Since the foil arms are supplied items, I think it's unlikely there will be significant changes to the Design Rule with regard to foil size or weight, as that would change the specifications for the production arms.

Atmospheric pressure is greater than vapor pressure (or else the oceans would boil) so ventilating the top of a foil results in a greater loss of lift than if the top of the foil is cavitated.  I don't see intentionally ventilating the foils is a good way to go.

As for morphing foils, you can view the flap as a form of morphing.  The Design Rule is pretty restrictive with regard to other means of morphing the foils.

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2 hours ago, Basiliscus said:

I don't see intentionally ventilating the foils is a good way to go.

Understood.  I was thinking more in terms of ventilating the cavitation bubble of a super-cavitating foil, as Peter Larsen did.

Perhaps an A75 v2 might include some mods to the Design Rules? And/or allowing a change of foils within a few hours of a race...

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3 hours ago, MaxHugen said:

Drag. At high boat speeds, they carry too much sail. So they have to twist off the top to reduce heel by lowering the CE, but the top section still contributes to drag with no benefit. Allowing a mainsail that has a shorter luff for high wind speeds may be a possibility.

Could be a Rule change for AC75 V2 which allows for light and heavy weather rigs. Shorter mast and smaller flatter cut sails for the heavy weather version.

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6 minutes ago, Sailbydate said:

Could be a Rule change for AC75 V2 which allows for light and heavy weather rigs. Shorter mast and smaller flatter cut sails for the heavy weather version.

That sounds like a sensible move, assuming they reduce the 120 hr pre-race measurement rule.

It will be interesting to see how this evolves. :)

Ps: SailGP took this route, with a segment of the wing sail that could be omitted, reducing wing from 24m to 18m in height.

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2 minutes ago, coercivity said:

can the flaps be adjusted in opposite directions so as generate rotational force.

Yes, they could use differential flap settings. However, the downside is additional drag.

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15 minutes ago, MaxHugen said:

That sounds like a sensible move, assuming they reduce the 120 hr pre-race measurement rule.

It will be interesting to see how this evolves. :)

Ps: SailGP took this route, with a segment of the wing sail that could be omitted, reducing wing from 24m to 18m in height.

I think also, a shorter rig would also improve jib efficiency (by closing the slot, slightly) in the AC75.

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But two small foils concurrently in the water with righting moment could get the boat up and away then lift upwind foil as speed builds  Then you have less drag as the surplus lift of the current foils is not there. 

As the speed runs up the the lift rises by the square of the Velocity so they must be killing lift with the flaps inducing even more drag. The nose down attitude is to reduce the fixed AOA of the foil without to much flap.

If you had a perfect foil for say ETNZ@40knts it would be less than half the current area but they would be stuck on the water in all winds other than north of 20Knts 

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2 hours ago, Sailbydate said:

I think also, a shorter rig would also improve jib efficiency (by closing the slot, slightly) in the AC75.

Not so sure about that. The jibs on the AC75 can already "close the slot" with their traveller systems, probably well past what is optimal.

There's a good discussion on the slot, with helpful diagrams, by A. Gentry on p14 of A Review of Modern Sail Theory.

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3 hours ago, coercivity said:

But two small foils concurrently in the water with righting moment could get the boat up and away then lift upwind foil as speed builds  Then you have less drag as the surplus lift of the current foils is not there. 

As the speed runs up the the lift rises by the square of the Velocity so they must be killing lift with the flaps inducing even more drag. The nose down attitude is to reduce the fixed AOA of the foil without to much flap.

If you had a perfect foil for say ETNZ@40knts it would be less than half the current area but they would be stuck on the water in all winds other than north of 20Knts 

At 30 knots boat speed, NZ's foil, allowing for 0.5° of leeway and 22° cant, would have 0° AoA and ~ flap to produce ~70kN of vertical force (Fz).

This is based on a foil profile that is producing a CL of ~0.47, and a L/D ratio of 59.

At 40 knots, the flap angle is changed to -1.7° for the same Fz.   CL dropped to 0.26 and L/D to 37.

As you can see, modest changes in flap angle considerably change the CL, and maintains the required Fz without changing NZ's foil area of 1.32 m^2.

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3 hours ago, coercivity said:

But two small foils concurrently in the water with righting moment could get the boat up and away then lift upwind foil as speed builds

I'm not sure this works. Having both foils in the water can indeed double the lift, but only at the cost of cancelling all the righting moment.

So as soon the hull leaves the water, the boat would have no righting moment at all. Perhaps changing the windward flaps from positive to nuetral/negative lift at the moment the hull lifts would work?

But could these boats get to take off speed with only the righting moment of the hull? Again perhaps quick adjustments of flaps may help, but it sounds like making an already very dynamic transition even more so.

If possible, it would be a good thing!

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8 hours ago, MaxHugen said:

Understood.  I was thinking more in terms of ventilating the cavitation bubble of a super-cavitating foil, as Peter Larsen did.

Perhaps an A75 v2 might include some mods to the Design Rules? And/or allowing a change of foils within a few hours of a race...

Ventilating the top of a supercavitating foil will reduce the lift.

If you are referring to Sailrocket, I believe that boat used a base-ventilated foil.  That is different from a supercavitating foil because the upper surface is wetted.  Ventilating the base of such a foil reduces the drag compared to a cavitated base because the lower pressure of the cavitated flow pulls back more on the foil than does atmospheric pressure.  

Base ventilated foils are subject to the same pressure limitations as sub- or trans-cavitating foils.  However, the flat "rooftop" portion of the pressure distribution can extend further aft because the pressure doesn't need to recover to ambient pressure at the trailing edge.  Instead, it only needs to come down to atmospheric pressure.  

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8 hours ago, coercivity said:

But two small foils concurrently in the water with righting moment could get the boat up and away then lift upwind foil as speed builds  Then you have less drag as the surplus lift of the current foils is not there. 

...

The Design Rule and the Foil Control System design were specifically intended to prevent the windward foil from pulling down to create righting moment.  When the windward foil is in the water and lifting upward, the righting moment is reduced.

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4 minutes ago, Basiliscus said:

Ventilating the top of a supercavitating foil will reduce the lift.

If you are referring to Sailrocket, I believe that boat used a base-ventilated foil.  That is different from a supercavitating foil because the upper surface is wetted.  Ventilating the base of such a foil reduces the drag compared to a cavitated base because the lower pressure of the cavitated flow pulls back more on the foil than does atmospheric pressure.  

Base ventilated foils are subject to the same pressure limitations as sub- or trans-cavitating foils.  However, the flat "rooftop" portion of the pressure distribution can extend further aft because the pressure doesn't need to recover to ambient pressure at the trailing edge.  Instead, it only needs to come down to atmospheric pressure.  

OK.  So does cavitation only occur at the top and bottom edges of the flat "base" in a base-ventilated foil?

And does "super-cavitating" refer to a foil that employs cavitation on only the upper surface, but from the LE?

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Cavitation occurs anywhere the local pressure drops below the vapor pressure.  Sharp edges cause a local pressure peak that initiates flow separation and cavitation.  A base ventilated foil is designed to maintain attached flow back to the blunt trailing edge.

The definition of a supercavitating foil is one in which the vapor cavity starts forward on the foil and extends aft beyond the foil.  They typically have a sharp leading edge that results in the cavity beginning there, so the whole upper surface is covered by vapor. 

There are several reasons for wanting to do this.  One is to avoid the erosion that occurs when bubbles collapse near the foil before they reach the trailing edge.  Another is to reduce the skin friction of the upper surface by not having it wetted.  The sharp leading edge also provides consistent and predictable characteristics compared to allowing cavitation to start at a variety of locations depending on the operating conditions.  The sharp leading edge also leads to clean sheet cavitation as opposed to cloud cavitation.  So most supercavitating foil sections are more or less wedge-shaped.

I suppose a foil that had a flat base, leading to a cavitated bubble extending into the wake with wetted top and bottom surfaces, could be considered a supercavitating foil because the cavity closes behind the foil.  But that is kind of the worst of all worlds from a drag standpoint.

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15 hours ago, MaxHugen said:

........
Drag. At high boat speeds, they carry too much sail. So they have to twist off the top to reduce heel by lowering the CE, but the top section still contributes to drag with no benefit.
..........

@MaxHugen We have a picture of TNZ with the top of the sail inverted providing RM.  So they can use far more power (which I think we agree they have?) to overcome increased drag from cavitation etc??
I don't know why this is not considered and evaluated more?  It seems the analysts on here have written the inverted top off for some reason?

Am I missing something?

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7 hours ago, Basiliscus said:

Cavitation occurs anywhere the local pressure drops below the vapor pressure.  Sharp edges cause a local pressure peak that initiates flow separation and cavitation.  A base ventilated foil is designed to maintain attached flow back to the blunt trailing edge.

The definition of a supercavitating foil is one in which the vapor cavity starts forward on the foil and extends aft beyond the foil.  They typically have a sharp leading edge that results in the cavity beginning there, so the whole upper surface is covered by vapor. 

There are several reasons for wanting to do this.  One is to avoid the erosion that occurs when bubbles collapse near the foil before they reach the trailing edge.  Another is to reduce the skin friction of the upper surface by not having it wetted.  The sharp leading edge also provides consistent and predictable characteristics compared to allowing cavitation to start at a variety of locations depending on the operating conditions.  The sharp leading edge also leads to clean sheet cavitation as opposed to cloud cavitation.  So most supercavitating foil sections are more or less wedge-shaped.

I suppose a foil that had a flat base, leading to a cavitated bubble extending into the wake with wetted top and bottom surfaces, could be considered a supercavitating foil because the cavity closes behind the foil.  But that is kind of the worst of all worlds from a drag standpoint.

Does chord length (and Re) have any effect on the onset of cavitation or on how cavitation affects drag?

Just wondering if the real benefit of the T foil may have been the simpler flap mechanism allowing reduced thickness and therefore chord length -> less adverse effects from cavitation.

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