Boats and foils comparison

MaxHugen

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

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

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

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

The thing that still has me confused is thay the upwind cant angles seem very high. I'm not sure they cant be achieved without the hull touching down or the foil surfacing. Os thay becuase the VPP was just left to keep getting faster VMG without limit on maximum cant?
I don't quite follow erdb's diagrams. In the second from left, the point at which Vertical force should be equal for both wings of a Y foil is when the foil is at 0° cant, ignoring leeway for the moment. That means the foil arm would be at a cant of 42°, not at 60°. (foil arm cant - 42 = foil cant)

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

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

Mozzy Sails

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

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

  • Foil cant = 22°
  • Foil AoA = 0°
  • Foil anhedral = 16°
  • Windward wing cant = 38°
  • Leeward wing cant = 6°
     
  • Leeway = 2°
  • Windward wing AoA = 1.23°
  • Leeward wing AoA = 0.21°
0 degrees foil cant is foil symmetry line vertical (in line with big grey gravity arrow)?

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

 

Mozzy Sails

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Yes, that's why I added the caveat " ignoring leeway for the moment ".
Yeah, got that. But is ERDB ignoring leeway? He's just saying how much load and lift each wing would have to produce to balance forces?

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

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

20210308_104436.png

 
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Mozzy Sails

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United Kingdom
I don't think you can really separate out vertical and horizontal in this way. 
I few days later, I realise I am wrong.

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

 
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Paddywackery

Super Anarchist
1,112
438
Ireland
I few days later, I realise I am wrong.

Ironically I did argue the CoL in  vertical shifted outboard to my friends but was was persuaded differently. Should have stuck with my gut for the video.
It's okay to be wrong Mozzy and to make the occasional mistake, it gives the rest of us hope  :D

 

MaxHugen

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

At 0 degrees cant the outboard foil would be producing very little lift. And AoA may be negative (depending on pitch).
I don't know if erdb is using a Leeway value to calc the relevant forces for each wing of an anhedral foil.

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

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

 
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Mozzy Sails

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

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

If someone with a VPP doesn't respond, I might give the calcs a go tomorrow - there's a fair bit to it.
Guess we should wait for erdb to answer. 

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

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

 

erdb

Anarchist
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575
I don't know if erdb is using a Leeway value to calc the relevant forces for each wing of an anhedral foil.

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

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


Guess we should wait for erdb to answer. 

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

Not sure if that is leeway prediction or just a result of force balancing for a given CoE. 
Sorry guys took some time off  :D . There are obviously several ways to look at it, and as with all my other posts, there's a good chance I'm wrong.

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

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

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

 

MaxHugen

Super Anarchist
What’s the direction of travel of this foil - out of the page or right to left? If it’s out of the page I don’t understand the water jet or why it appears to have a stagnation point? If right to left that’s more representative of planing than foiling. 
That's a stock hydrofoil pic, looking at it from front or rear.

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

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

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

 

erdb

Anarchist
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575
How are you calculating the leeway angle to get the AoA of each wing of the foil, and then the forces for each?
I didn't calculate it here. I just solved the geometrical questions of how with a given cant angle, you can produce the same vertical and lateral forces as a T foil. Whether and how these forces can be generated is a completely different question depending on speed, yaw angle, pitch, speed, flap angle etc.

 

barfy

Super Anarchist
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1,410
Ventilation drastically reduces Lift, because air is ~1000 times less dense than water.

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

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

View attachment 433418

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

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

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

Or mozzys RM calculations?

 

coercivity

Member
63
6
Auckland
Ventilation is a big problem for rudders the stern flies out of the water when it happens, ETNZ had such an experience in Burmuda. 

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

 

barfy

Super Anarchist
5,134
1,410
Ventilation is a big problem for rudders the stern flies out of the water when it happens, ETNZ had such an experience in Burmuda. 

The foils that support the boat are sort of self-correcting ventilation leads to a drop-down into more water, not a catastrophe. 
Or the stern sinks in this class and the boat flies out of the water.

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

 

MaxHugen

Super Anarchist
The thing that still has me confused is thay the upwind cant angles seem very high. I'm not sure they cant be achieved without the hull touching down or the foil surfacing. Os thay becuase the VPP was just left to keep getting faster VMG without limit on maximum cant?
I originally calculated the maximum practical foil cant, with just the tip out, to be 25°. But that was from the Rules diagram, as there was nothing better to use.

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

image.png

 




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