Boats and foils comparison

Mozzy Sails

Super Anarchist
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United Kingdom
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)

View attachment 433595
From looking at the footage, upwind they fly with more foil out, but windward heel. Is 500mm their average flight height? Looks like less by eye. I guess we have data on this. I guess wing flex will bring more tip out as well. 

 
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MaxHugen

Super Anarchist
8 hours ago, Stingray~ said:

About ride height


My only take-away from this was...

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

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

 

MaxHugen

Super Anarchist
From looking at the footage, upwind they fly with more foil out, but windward heel. Is 500mm their average flight height? Looks like less by eye. I guess we have data on this. I guess wing flex will bring more tip out as well. 
Many variables as always. Given a probable 5% margin of error in the diagram, it could be between 475-525 mm. :lol:

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

We don't have access to ride height data.

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




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

Screenshot from 2021-03-09 13-11-47.png

 

erdb

Anarchist
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575
My only take-away from this was...

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

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

 

erdb

Anarchist
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575
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?
From looking at the footage, upwind they fly with more foil out, but windward heel. Is 500mm their average flight height? Looks like less by eye. I guess we have data on this. I guess wing flex will bring more tip out as well. 


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

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

We don't have access to ride height data.
In my race stats, I also calculate "real cant angle", which is heel + cant:

cant+heel.png

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

heel.png

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

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

height.png

 

Mozzy Sails

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

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

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

 

The Advocate

Super Anarchist
My only take-away from this was...

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

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

 

erdb

Anarchist
786
575
@ERDB so, the 67.5 or so degree cant for upwind given in the forces on an anhedral post. 

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

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

 

erdb

Anarchist
786
575
Just a few interesting bits of the straight-line performances from today:

Upwind /Downwind VMGs - race 1:

uw vmg.png dw vmg.png

Upwind /Downwind VMGs - race 2:

uw vmg.png dw vmg.png

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

  dws bs.png dw twa.png

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

favtack.png favtack.png

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

uw cant.png dw cant.png

Race 2 upwind, downwind:

uw cant.png dw cant.png

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

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

uw leew.png dw leew.png

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

 

hoom

Super Anarchist
6,398
528
Orkland
ETNZ left a little bit in the bag today by constantly sailing on the wrong tack.
In the post-race interview for Race1 PB was pretty casual about being off-phase in the 2nd beat.

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

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

 
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Mozzy Sails

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

Heading:

image.png

Foil Pos
image.png

 

MaxHugen

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

Heading:

View attachment 433806

Foil Pos
View attachment 433807
NZ's foil cant seemed excessive to me, especially in R2, so I watched some of the stern camera footage to check on heel.

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

image.png

 

Mozzy Sails

Super Anarchist
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United Kingdom
Hmm, maybe that's ETNZ's cant angles were all over the place for me, too. I have to check. I just ran my stuff on the files without looking at it in detail.
They look correct, but just out of sync. So I think your histograms are a correct reflection. I went through quite a bit of the virtual eye stuff. Also onboard communications they are talking about cant angles being 'good'. Whereas at the ACWS they never mentioned them. 

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

 

gigi

New member
17
6
Hello girls and boys, AC fans. 

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

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

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

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

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

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

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

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

Lets dive in the water and see forces there.

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

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

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

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

Screenshot_20210310_222344~2.jpg

Screenshot_20210310_221200_com.autodesk.fbd.activities.jpg

 

mauriciogfj

Member
228
122
Brazil
Hello girls and boys, AC fans. 

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

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

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

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

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

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

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

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

Lets dive in the water and see forces there.

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

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

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

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

View attachment 433846

View attachment 433847
Wow, that is a splashy entrance!
Welcome to fray, Gigi.

 




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