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There was quite a lot of interest in AC36 design topics, and it was great to have some very knowledgeable people contribute, as well as those who like to speculate like myself.

So I thought it might be useful to add this topic specifically for the F50. Hopefully, others will also share their thoughts on wingsails, foils, VPPs, performance, etc.  :)

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CAD.  I don't know if anyone else is interested in this, but I enjoy drawing up designs as well as testing out aero and hydro profiles in tools such as XFoil and JavaFoil.  I began with 2D, using Inkscape which is a free and open-source vector graphics editor used to create vector images, primarily in Scalable Vector Graphics (SVG) format.

I've been keen to try out 3D for many years, and finally motivated myself to give CAD a go, despite expecting that the learning curve would be significant - I was correct. The tool I settled on was the free (for one year) "Personal" version of AutoCAD's Fusion 360.

From a lot of images gleaned from videos plus some very basic info, I drafted the lines of the wingsail and hulls using Inkscape.  Then I plunged into CAD, and have made some progress.  Fortunately I've had great help from members on the Fusion 360 forum when I got bogged down. I did the wingsail first (18m variant shown), and now have the hulls done:

image.png.26a08e524b882dcfe80927062171f73e.png

Next will be the platform, then foils and rudders etc.

If anyone is using CAD, or wants to give it a go, I'm happy to share my files and/or what I've learnt so far.  ;)

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Wingsail.  I've drafted the profile of the the wing and flap, from an image:

image.png.448df72e839f2c71d3e9a33a3727f571.png

It was interesting to hear in a video about the new wingsail that it takes as long to produce one wingsail as one platform (hulls, cross beams etc etc)!

From some images supplied by @Basiliscus I had a look at the evolution of the windsail profiles:

image.png.9dc5fcd2476b4830b3e233bf8c50acbd.png

I created some airfoil DAT files - available to anyone who wants to play with them - which I had a look at in JavaFoil:

image.png.5e95a2af5b5273de1354c727d52c37c2.png

Used the profiles to loft the wingsail in 3D:

image.png.7f684bb2a99687166e9d221e822254af.png

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I wasn't involved with the section design of the F50, but I can give some background on the BOR90 and AC72 wingsail sections.

The BOR90 wingsail was originally intended as the light wind rig.  The idea was the boat would use the wingsail in light winds and go to the soft sail rig for medium and heavy winds.  That allowed the mainsail area to be varied for the wind strength, much as the new wingsail does for the F50.  Then Mast3 broke on the BOR90 about two weeks before they were going to start sailing the wingsail.  There was a crash program underway to build Mast4 as a replacement for Mast3 over the holidays while the boat was shipped to Valencia.  Once they got the wingsail sailing, they discovered it could be used uprange from its design wind speeds, the team decided they could go with just the wingsail, and the build of Mast4 was terminated.

As a result of being designed for power in light winds, the BOR90 section was optimized for high lift/drag ratio at high lift.  A range of flap sizes was investigated, and the 50% chord flap was selected.

At the time, only C-class catamarans, landyachts and iceboats used rigid wingsails.  The C-class wingsails included a tab on the main element, but landyacht wingsails did not.  Based on the landyacht experience, BMW Oracle decided not to include a tab on the BOR90 in order to simplify the design and construction.  Although they were not used, there were multiple flap pivot locations built into the structure so the pivot could be moved fore or aft to open or close the slot for the purpose of tuning.  But all the sailing was done with the original design pivot location.  

The AC72 section was designed for more all-round performance, especially in the heavier winds of San Francisco.  The section was designed to have a moderate amount of laminar flow for low drag, but not to suffer if the boundary layer was fully turbulent.  In this regard it was probably in between the first Artemis wing, which appeared to be designed for more laminar flow, and the ETNZ wing, which appeared to put more emphasis on high lift with a shorter run of laminar flow.  

The BMW Oracle AC72 wing included a tab, because the best lift/drag ratio was slightly higher.  This turned out to be useful during the regatta because it allowed them to open the slot, which moved the center of effort of the wingsail aft, and improved the balance of the helm for better upwind performance.

The flowfield shown in the Javavoil plot is real.  I remember an early sail of the BOR90 wingsail that had tufts running along the chord at several stations up the wing.  The tufts on the D tube were all laying flat and pointing forward.  The stagnation point was back at the main spar!  

Although Javafoil is capable of calculating the flow around these sections at low to moderate angles of attack, it is not capable of properly predicting maximum lift.  The reason is stall for these wingsails often occurs through the mechanism of wake bursting, in which the flow separates in the wake of the main element as it passes by the flap.  This results in attached flow on the surface of the flap itself, with the flow through the slot forming a wall jet along the lee side of the flap, but a huge wedge of turbulent air between the jet on the flap and the outer flow around the main element.  Javafoil cannot represent this kind of mid stream separation.

As for the F50 wingsail, you ain't seen nothin' yet!

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I had noticed that zone you discuss on the lee side of the flap when I looked at the flow field in higher resolution, and wondered what was going on there:

image.png.82140eef40d1fb281814f2f39818233c.png

It will be interesting to see the F50 in action, the 18m config in higher winds should reduce unnecessary drag I think?  They've also improved the control systems so the the flaps can auto-rotate through tacks etc at the push of a button.

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I wonder if the ribs' distortion on the wingsail due to using shrink wrap affects performance?  Doesn't exactly look optimal...

image.png.3007d11cb7574193c08535e590bccf2c.png

@Horn Rock

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l Love the way these SailGP threads go into the AC forum, just like how Larry and Russel intended!

 

I'm looking forward to seeing these boats, and hopefully more background into how they work as it is owned by the series and there's no secrecy crap we get from AC.

 

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8 minutes ago, shebeen said:

l Love the way these SailGP threads go into the AC forum, just like how Larry and Russel intended!

 

I'm looking forward to seeing these boats, and hopefully more background into how they work as it is owned by the series and there's no secrecy crap we get from AC.

 

When there were lots of AC sailors/teams sailing in the MedCup and Volvo, we had respective threads in SAAC as well. Nothing special, actually, and nothing of Larry's or Russell's doing. 

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9 minutes ago, shebeen said:

I'm looking forward to seeing these boats, and hopefully more background into how they work as it is owned by the series and there's no secrecy crap we get from AC.

Yes, I'd like to see some design data... but it's been pretty scarce so far.

I've scrounged far and wide to find just a small amount of info!

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1 minute ago, Rennmaus said:

When there were lots of AC sailors/teams sailing in the MedCup and Volvo, we had respective threads in SAAC as well. Nothing special, actually, and nothing of Larry's or Russell's doing. 

Not knowing the dynamics of this website, I did ask the owners if they'd start a separate SailGP section, but the reply was "not unless they pay for it".

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7 minutes ago, MaxHugen said:

Not knowing the dynamics of this website, I did ask the owners if they'd start a separate SailGP section, but the reply was "not unless they pay for it".

Actually, there would be a place for this thread in Multihull Anarchy, but as I wrote, SAAC sometimes becomes a place for anything of interest, AC or not, so don't worry. 

If you compare the SGP collective monologue thread in MH Anarchy with the one hereabouts, you will see that the latter is a lot more vivid, hence it's definitely a good decision to stay here :)

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1 hour ago, Rennmaus said:

Actually, there would be a place for this thread in Multihull Anarchy, but as I wrote, SAAC sometimes becomes a place for anything of interest, AC or not, so don't worry. 

If you compare the SGP collective monologue thread in MH Anarchy with the one hereabouts, you will see that the latter is a lot more vivid, hence it's definitely a good decision to stay here :)

Sure, noticed that, no probs.  I actually asked the site owner quite a while ago - probably just as Topics were being created! :lol:

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23 minutes ago, MaxHugen said:

Sure, noticed that, no probs.  I actually asked the site owner quite a while ago - probably just as Topics were being created! :lol:

Are you confusing "forum" and "thread/topic"?
If you ask Scot for a "forum", he wants to see money, and rightfully so, as someone has to pay for the fun we enjoy here. But of course you are allowed to open threads/topics in the forums as many as you want. For SGP the "correct" forum would be "Multihull Anarchy - Brought to you by Aeroyacht", the "correct" SGP thread in this forum is this rather boring topic https://forums.sailinganarchy.com/index.php?/topic/220551-sailgp-2021/.

You could open a SGP Design topic there as well, as you did here.

If you look at the forum index you will see that the only "class" (for lack of a better word)/manufacturer that has an own forum are the J Boats - paid by J Boats.

 

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10 minutes ago, Rennmaus said:

Are you confusing "forum" and "thread/topic"?
If you ask Scot for a "forum", he wants to see money, and rightfully so, as someone has to pay for the fun we enjoy here. But of course you are allowed to open threads/topics in the forums as many as you want. For SGP the "correct" forum would be "Multihull Anarchy - Brought to you by Aeroyacht", the "correct" SGP thread in this forum is this rather boring topic https://forums.sailinganarchy.com/index.php?/topic/220551-sailgp-2021/.

You could open a SGP Design topic there as well, as you did here.

If you look at the forum index you will see that the only "class" (for lack of a better word)/manufacturer that has an own forum are the J Boats - paid by J Boats.

 

Yes you're right, I probably should have referred to the term "forum". Originally found the topic in "Multihulls", when I questioned the lack of interest I was pointed here! :)

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..

The scalloping of the shrink film is the inevitable result of pulling a membrane over a curved surface. However on the important lee side , the film is sucked out so the surface is smooth.  I always wondered what the shape really was as there is nothing holding the film to the ribs.   On C Cats the focus on weight reduction made accepting this distortion a reasonable trade off.  For wings with minimum weights, it would be an area to improve.  One strategy is to limit the shrink film to areas where there is minimum curvature, and in some cases, designing airfoils which have relatively flat sides.  This always seemed like the tail wagging the dog, but design is always limited by what you can build, which is often driven by how you build.

In the pre foiling days of C Class, down wind speed was what dominated racing.  This was mostly driven by weight and maximum lift coefficient.  Relative to F50s the C Class sail area limit is pretty modest, so we needed plenty of grunt.  However, we were also overpowered in 8 knots TWS..... We never found twist as potent as the F50s seem to find.  It always seemed faster to set the flaps to minimum camber ( smooth lee side - about 7 degrees) and let the sheet out.  Unlike a soft sail, a wing will still deliver lift at AOA approaching zero degrees.  As Cl and Cd are both functions of AOA, this seemed to make sense.  The wing got more efficient the harder it blew. We thought we could sail upwind in almost any amount of wind, until windage just became so much that we were blown over backwards.

SHC

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Could I suggest we summarise our thoughts on the cat verse AC75?

Why do cats lack an ability to have a good VMG upwind?  Is it me or is this a generally  held opinion?

Are cats greatly better at any other point of sailing?

Can these F50s improve their ability to sail up wind? or do we have no directly upwind legs in races?

Max tell me if I am out of place here.

Fancy coming in after that great info from SHC  !!?

 

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I'm not proposing any answers here however the AC-75's must have suffered the same issues as multihulls in that the faster you go the further forward the apparent wind goes, and therefore the perceived lack of height upwind shown by fast multihulls.

AC-75's probably had an aero advantage that the cat's could improve on to reduce drag. It all comes down to sailing the closest AWA before the increasing excess drag becomes a factor. A blank chequebook AC-50 cat would definitely find improvements. Whether these improvements got them close to an AC-75 we'll probably never know...

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56 minutes ago, Steve Clark said:

..

The scalloping of the shrink film is the inevitable result of pulling a membrane over a curved surface. However on the important lee side , the film is sucked out so the surface is smooth.  I always wondered what the shape really was as there is nothing holding the film to the ribs.   ...

Why do you think nothing was holding the film to the ribs?  IIRC, they used double-sided tape to attach the film.

I was wondering about the scalloping, too.  When I viewed the lee side of the AC72 wingsail from the chase boat, I was surprised to see how smooth it was.  There was just a hint of convex scalloping visible when the light was glancing off it. So the shape was pretty close to the designed shape.

The windward side was pushed in significantly.  That may have been helpful, as it added camber and reduced thickness.

I think of more significance were all the water droplets adhering to the wing from sailing in San Francisco fog.  You could forget any hope of laminar flow in those conditions.

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1 hour ago, Kiwing said:

Why do cats lack an ability to have a good VMG upwind?  Is it me or is this a generally  held opinion?

 

I don't think it's a question of cat, the AC75 is just much heavier and powerful upwind.

The other factor is the perhaps lower aero drag of one hull vs two + tramp

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3 hours ago, Kiwing said:

Could I suggest we summarise our thoughts on the cat verse AC75?

Why do cats lack an ability to have a good VMG upwind?  Is it me or is this a generally  held opinion?

Are cats greatly better at any other point of sailing?

Can these F50s improve their ability to sail up wind? or do we have no directly upwind legs in races?

Max tell me if I am out of place here.

Fancy coming in after that great info from SHC  !!?

 

This is the place to pose the more difficult questions. Not that I really have any worthwhile speculation yet... but stay tuned, we'll see how the cats do with their improved solid wing!  ;)

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On 4/21/2021 at 4:57 AM, Rennmaus said:

Actually, there would be a place for this thread in Multihull Anarchy, but as I wrote, SAAC sometimes becomes a place for anything of interest, AC or not, so don't worry. 

If you compare the SGP collective monologue thread in MH Anarchy with the one hereabouts, you will see that the latter is a lot more vivid, hence it's definitely a good decision to stay here :)

There is a thread in MHA also. Funny how now that NZ won the AC and has a SagP team people stopped complaining about the one here.

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@Basiliscus, I'm in the process of lofting an F50 foil in CAD, bit of a challenge for a greenhorn. I'm treating the vertical area much as a symmetric strut, with a higher efficiency asymmetric profile for the horizontal lifting area. 

The symmetric foil profile is representative of what it actually looks like at the very top of the foil from images, don't know about a bit lower though.   And the lifting horizontal area is pure imagination, so disregard the actual shapes I used.

image.png.8bca90b71c272f0a63d6869bfd7383df.png

My question is, from your past experience, would the F50 possibly use an asymmetric profile in the lower section of the vertical component, to increase lift to windward?

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15 hours ago, Steve Clark said:

..

The scalloping of the shrink film is the inevitable result of pulling a membrane over a curved surface. However on the important lee side , the film is sucked out so the surface is smooth.  I always wondered what the shape really was as there is nothing holding the film to the ribs.   On C Cats the focus on weight reduction made accepting this distortion a reasonable trade off.  For wings with minimum weights, it would be an area to improve.  One strategy is to limit the shrink film to areas where there is minimum curvature, and in some cases, designing airfoils which have relatively flat sides.  This always seemed like the tail wagging the dog, but design is always limited by what you can build, which is often driven by how you build.

In the pre foiling days of C Class, down wind speed was what dominated racing.  This was mostly driven by weight and maximum lift coefficient.  Relative to F50s the C Class sail area limit is pretty modest, so we needed plenty of grunt.  However, we were also overpowered in 8 knots TWS..... We never found twist as potent as the F50s seem to find.  It always seemed faster to set the flaps to minimum camber ( smooth lee side - about 7 degrees) and let the sheet out.  Unlike a soft sail, a wing will still deliver lift at AOA approaching zero degrees.  As Cl and Cd are both functions of AOA, this seemed to make sense.  The wing got more efficient the harder it blew. We thought we could sail upwind in almost any amount of wind, until windage just became so much that we were blown over backwards.

SHC

That's a very flat wing profile!    Will be interesting to see how much the F50s use when we get a good look at them racing.   B)

image.thumb.png.bf665e75b0f4f12503adb22c0de67aec.png

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19 hours ago, Kiwing said:

Could I suggest we summarise our thoughts on the cat verse AC75?

Why do cats lack an ability to have a good VMG upwind?  Is it me or is this a generally  held opinion?

Are cats greatly better at any other point of sailing?

Can these F50s improve their ability to sail up wind? or do we have no directly upwind legs in races?

Max tell me if I am out of place here.

Fancy coming in after that great info from SHC  !!?

 

Personal opinions.

At this point there are probably only a very small number of points of sail (if any) where an AC50 an out perform an AC75 and most of those are probably only in certain very light (basically unracable) wind conditions.

One: The mono is way more aerodynamic than a messy old cat. Even if you make everything as clean as possible on the cat your still going to get air off the two seperate hulls interacting and your always going to have to connect them using something...

Two: The twin skin main looks to be much better than first anticipated. It's incredibly dynamic allowing very good shapes regardless of conditions, point of sail etc... It also looks extremely slippery once powered up when you need to bleed power with a minimum drag penalty.

Three: I don't think any traditional mono versus cat comparisons are relevant at all. Once airbourne the two platforms are essential the same and it simply comes down to power, weight, drag and the ability to control and balance them. Given how easily both platforms can get airbourne, we can probably call any none foiling aspects of the boats basically edge cases.

 

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4 minutes ago, Boybland said:

Personal opinions.

At this point there are probably only a very small number of points of sail (if any) where an AC50 an out perform an AC75 and most of those are probably only in certain very light (basically unracable) wind conditions.

One: The mono is way more aerodynamic than a messy old cat. Even if you make everything as clean as possible on the cat your still going to get air off the two seperate hulls interacting and your always going to have to connect them using something...

Two: The twin skin main looks to be much better than first anticipated. It's incredibly dynamic allowing very good shapes regardless of conditions, point of sail etc... It also looks extremely slippery once powered up when you need to bleed power with a minimum drag penalty.

Three: I don't think any traditional mono versus cat comparisons are relevant at all. Once airbourne the two platforms are essential the same and it simply comes down to power, weight, drag and the ability to control and balance them. Given how easily both platforms can get airbourne, we can probably call any none foiling aspects of the boats basically edge cases.

Difficult to compare a 50' cat against a 68' mono. My guesses:

  1. The F50 with crew weighs less than 3t, the AC75 comes in at 7.8t.  That means a lot less hydro drag, which is well over 50% of total drag.
  2. The twin skins did perform well, and a designer early on stated they were almost as good as the wings. However, having to twist off a lot is draggy. The cats will benefit in a good breeze now they can go gown from a 24m wing to 18m.
  3. Agree. Both boats are now platforms onto which they stick foils and sails/wings.

I've little doubt that if the F50s get the breeze they will do sustained bursts of 50+ knots.

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So... F51.5s now? Not unlike many, the F50 seems to have put on extra bit of derriere during lockdown. 

Seems like an odd addition. Bit longer waterline length for take off? 

176918112_1419005401790164_1541008390351465266_n.thumb.jpg.4d6c58ea2e0efdca423b675813c1d450.jpg

The new wings seem to have a more pronounced increase in chord from deck to the top of the third section. However, that third section must have the same chord top and bottom as that's the one that is removed? So maybe it's an optical illusion. Each section has a very straight trailing edge ratehr then teh slight curves we saw on the previous Oracle / Artimis wings. 

Image_1.thumb.jpg.e3959ec0ad5e509ac891cd60c49c2e13.jpg

174761175_1042989532774730_7702309214924780867_n.thumb.jpg.f083ed54cf06d8323ef496b6390db033.jpgimage.thumb.png.0bc4f121b50cab35bc77cba6dcef0c96.png

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Yeah, I noticed the longer bum... and I'd just finished a CAD of the hull and also noticed this today!  It doesn't look the same on DEN as GBR though?

image.png.aab1d3727c9f8f7b537e094d463ad670.png

The wing has straight trailing edges on the 1st and 2nd flap sections (from the deck) then curved. I have yet to fix the leading edge of the 3rd and 4th sections of wing as I think they have some curve as well.

image.thumb.png.28244c81f02c0f0078cc73361d5d4318.png

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Yeah, I'd seen your post above with the drawing of the new wing, but only realised how different it was when looking back. I had assumed the new OD wings would be very similar term of shape, but with new internals and obviously modular. I wonder how much the new shape is an improvement in aerodynamics compared to the old wing, or if the more angular shape is just a side affect of the modular nature. 

Does the new wing (full 4 sections) compare to the old wind in terms of area and how high up the geometric centre is? 

Regarding the transom, the picture above of Denmark has the old transom. I think this was an early picture from Bermuda when they were testing the new wing. In picture from practice racing it looks like Denmark have had their extension added on now too. 

Looking at the GBR picture it does look very 'stuck on'. So possibly a part that comes off for transport. It is odd though. I guess it will have a small hydro and aero advantage, but if they're all the same seem like it wouldn't really add anything to the racing. Maybe it frees up more space for rudder linkages or something. I just fee there must be more to it than just hydro or aero. 

176948945_1419005358456835_8894604144214495728_n.thumb.jpg.f8f87b7afdcbfe444971fc32c2a5f21c.jpg

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SailGP has a brief bit on the new wing as being more efficient, less drag, lighter etc.  They also mention (again) that a 28m sail will be created for very light wing.

"The light air foils and light air elevators have increased surface area, which results in increased lift to make the most of weaker winds. The light air wing also boats a larger surface area, standing at 24m to catch more wind to power the boat."

As for the transom, I'll have to glue on an extra piece! :D

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21 hours ago, MaxHugen said:

@Basiliscus, I'm in the process of lofting an F50 foil in CAD, bit of a challenge for a greenhorn. I'm treating the vertical area much as a symmetric strut, with a higher efficiency asymmetric profile for the horizontal lifting area. 

The symmetric foil profile is representative of what it actually looks like at the very top of the foil from images, don't know about a bit lower though.   And the lifting horizontal area is pure imagination, so disregard the actual shapes I used.

image.png.8bca90b71c272f0a63d6869bfd7383df.png

My question is, from your past experience, would the F50 possibly use an asymmetric profile in the lower section of the vertical component, to increase lift to windward?

I doubt it.  I suspect the AC75 shaft section is symmetric all the way down.

They can use cant to adjust the ratio of vertical to horizontal lift so as to maintain near zero load on the shaft and maximize the effective span.

With regard to the L foil sections, it would be typical to use many different sections along its length, as you've indicated. You have about the right number of sections, too, although you probably need to have some more closely spaced near the elbow and maybe one or two more along the wing.

At the top, the sections are thick because of the need to react the bending moment, and because those sections are out of the water at high speed.  The top sections are still cambered - there is no need to make them symmetric because each board is only used on one tack and the shaft has to provide side force even when hullborne. The bearings in the board trunk can easily be designed to handle a cambered section.  And don't forget that because of cavitation considerations, even the cambered sections on the board are nearly symmetric from the leading edge past maximum thickness.  You might want to thicken the trailing edge of the upper shaft sections because of the bearing at the aft end of the board trunk that takes the drag loads.

The section between your two lower red arrows would be about where the principal design section for the wing would be.  That area is a difficult compromise between thickness for structure, lift, and cavitation.  The tip section could be similar, but thinner because it doesn't carry much load.

The section in the elbow will be a bit weird.  The elbow is where cavitation begins first at high speed, due to mutual interference between the shaft and wing.  The upper surface needs to be flattened in order to reduce the local velocities.  But the lower surface can be bulged outward to get the meat for structure, because that area acts more like an axisymmetric body than a 2D wing and the interference effects are much weaker.  As a result the elbow section tends to look like a pregnant guppy, with negative camber in the mid section.  Despite this, it still has the same rooftop pressure distribution as the wing, because the distortions are designed to compensate for the interference and bring the pressures back to where they would be if the foil were straight. 

You can only design the elbow section with the help of a 3D hydrodynamic code.  You need a panel code, at least.  The panel code will give you the 3D surface pressures and you can take the difference between the pressures in 3D and the pressures for the same shape in 2D to see the interference effects.  You can then subtract the interference delta from the design pressures to get a new design pressure distribution.  From that you can use a 2D inverse code like Xfoil to design a new section shape.  Then when that shape is placed in the 3D context, you'll get closer to the intended pressure distribution.  The interference effects are not a strong function of the shape, so the process converges rapidly.

I suspect a similar process would be used to design the sections of a T foil in the vicinity of the junction.  The interference from the unloaded shaft would be much less than for the L foil, but there would still be interference from the thickness of the shaft.

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21 hours ago, MaxHugen said:

That's a very flat wing profile!    Will be interesting to see how much the F50s use when we get a good look at them racing.   B)

image.thumb.png.bf665e75b0f4f12503adb22c0de67aec.png

FWIW, judging camber in that manner is not very useful.  SailGP uses the main element chord as the reference and measures flap deflection as the angle between the main element chord and the flap chord.

If you want to visualize the effect of flap deflection on lift, it is much better to simply look at where the flap trailing edge is pointed.  Since the whole purpose of the wing is to bend the flow, the flap chord points in the direction the flow will be moving when it leaves the trailing edge.  For a wing with a 50% flap chord, the change in lift due to a change in flap angle (measured between the main element chord and flap chord) is 80% of the change in lift due to a change in angle of attack (measured to the main element chord).  In other words, if you use the flap angle of attack to estimate the lift, you're only off by 20%!  Viewed in this way, the main element is effectively a leading edge device for the flap.

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I read somewhere sorry no linky that the new rudders can be lifted making the launches easier. So perhaps the transom has that mechanism. Also room for the guest female to sit. Be interesting to see if they each with 6 bodies on board.

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Thanks for the info Mr B, very interesting as always.

With the complexity of the foil shape I won't be trying to do any 2D lift calcs!  Besides having to lift the boat, Freddie Carr mentioned that they can easily put a tonne of downforce on the windward rudder for RM as well, so there is a trio of hydro surfaces. Quite a balancing act.

Your explanation of the "camber" for a wingsail was useful.  Now I understand why @Steve Clark was referring to flap angle as camber. Should I get to doing some calcs on it, I'll have to use JavaFoil. I think I read in the documentation that JF adds in a 3D calc for drag as well, need to read up on that.

In the meantime I shall persevere with a steep CAD learning curve.

Cheers, Max

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1 hour ago, NeedAClew said:

I read somewhere sorry no linky that the new rudders can be lifted making the launches easier. So perhaps the transom has that mechanism. Also room for the guest female to sit. Be interesting to see if they each with 6 bodies on board.

SailGP mentioned that they can swap rudders as well as foils depending on wind strength. But I'd find it very odd if they hadn't planned for this, and had to extend the transom to cope with it.  Good question for a savvy reporter to ask - hope Matt Sheahan does some commentary during the series, he asks worthwhile questions and knows Freddie Carr well.

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15 minutes ago, MaxHugen said:

Thanks for the info Mr B, very interesting as always.

With the complexity of the foil shape I won't be trying to do any 2D lift calcs!  Besides having to lift the boat, Freddie Carr mentioned that they can easily put a tonne of downforce on the windward rudder for RM as well, so there is a trio of hydro surfaces. Quite a balancing act.

Your explanation of the "camber" for a wingsail was useful.  Now I understand why @Steve Clark was referring to flap angle as camber. Should I get to doing some calcs on it, I'll have to use JavaFoil. I think I read in the documentation that JF adds in a 3D calc for drag as well, need to read up on that.

In the meantime I shall persevere with a steep CAD learning curve.

Cheers, Max

Defining camber as the angle between the flap and main element chords also has the advantage that it is easily measured onboard.

You can use a lifting line or vortex lattice to get the induced drag of the L foil, including both side force and vertical force.  It's the surface velocities that require a panel code.  If you look around you may be able to find free panel codes.  I've written a plug-in in Java that I use to create PMARC formatted input files directly from a Rhino CAD model.

Most CAD packages have a command for creating a surface by sweeping along one or two curves, interpolating between successive section shapes.  That is ideal for lofting foil shapes.

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Was bugging me, so I asked a friend.

"Because we have lifting rudders now.. the transom opens up and the rudder gantry can swing out and up.. saves on crane timings.. which with 8 boats is a must."

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On 4/23/2021 at 6:46 AM, MaxHugen said:

Difficult to compare a 50' cat against a 68' mono. My guesses:

  1. The F50 with crew weighs less than 3t, the AC75 comes in at 7.8t.  That means a lot less hydro drag, which is well over 50% of total drag.
  2. The twin skins did perform well, and a designer early on stated they were almost as good as the wings. However, having to twist off a lot is draggy. The cats will benefit in a good breeze now they can go gown from a 24m wing to 18m.
  3. Agree. Both boats are now platforms onto which they stick foils and sails/wings.

I've little doubt that if the F50s get the breeze they will do sustained bursts of 50+ knots.

It will definitely be interesting to see an F50 with a smaller wind in breeze!

I would also be interesting to see a 50 foot (or a little longer now the cats looked to be extended) version of the AC75. We might find the weight is rather similar once you scale down to the same size. The AC72 was around 5.9t before crew right?

 

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11 minutes ago, Boybland said:

I would also be interesting to see a 50 foot (or a little longer now the cats looked to be extended) version of the AC75. We might find the weight is rather similar once you scale down to the same size.

Battery-powered canting ballast is what gives the AC75 beasts their power. RM is king but it’s a bit of a ‘cheat’ to achieve it.

Cavitation is still where both Classes hit the wall downwind but the AC75 will always beat the F50’s upwind. 

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7 hours ago, Mozzy Sails said:

Was bugging me, so I asked a friend.

"Because we have lifting rudders now.. the transom opens up and the rudder gantry can swing out and up.. saves on crane timings.. which with 8 boats is a must."

Yep, snagged some images in the video earlier on, in the background you can see the rudder shaft swivelled up, foreground has some detail too:

image.thumb.png.b7880953bc05457cf752bf7903ec7cf3.png

image.png.f1eb62d7410e4c87b519feedbf06c42f.png

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5 hours ago, Boybland said:

It will definitely be interesting to see an F50 with a smaller wind in breeze!

I would also be interesting to see a 50 foot (or a little longer now the cats looked to be extended) version of the AC75. We might find the weight is rather similar once you scale down to the same size. The AC72 was around 5.9t before crew right?

 

I don't know the details of the AC72. 

The AC75 is ~6.7t without crew, but it is also carrying around 1.8t in ballast! Can't do much about that apart from scaling down.

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5 hours ago, Stingray~ said:

Battery-powered canting ballast is what gives the AC75 beasts their power. RM is king but it’s a bit of a ‘cheat’ to achieve it.

Cavitation is still where both Classes hit the wall downwind but the AC75 will always beat the F50’s upwind

A lot of racing sailing craft use power nowadays, canting keels sure aren't winched up by hand etc. So maybe not really "cheating".  The F50 is 95% electro-hydraulically powered, only the mainsheet is human powered.

I don't see why the AC75 should be faster upwind?  Comparing the two boats as "platforms" regardless of number of hulls:

  • F50 uses a wingsail, the AC75 a twin skin sail that's apparently almost as efficient
  • F50 uses 3 foils, the AC75 uses 2
  • F50 uses crew weight pus windward rudder foil for RM, the AC75 uses ballast mass, and both try to lower CE

Where do you see a clear difference for upwind performance?

Cavitation will still be an issue as you say... no easy way around that. But I think we'll see the F50 manage 50+ knots in a sustained  run in the right conditions whilst racing, something he AC75 didn't get to do - accepting conditions may not have been ideal for them.

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1 hour ago, MaxHugen said:

A lot of racing sailing craft use power nowadays, canting keels sure aren't winched up by hand etc. So maybe not really "cheating".  The F50 is 95% electro-hydraulically powered, only the mainsheet is human powered.

I don't see why the AC75 should be faster upwind?  Comparing the two boats as "platforms" regardless of number of hulls:

  • F50 uses a wingsail, the AC75 a twin skin sail that's apparently almost as efficient
  • F50 uses 3 foils, the AC75 uses 2
  • F50 uses crew weight pus windward rudder foil for RM, the AC75 uses ballast mass, and both try to lower CE

Where do you see a clear difference for upwind performance?

Cavitation will still be an issue as you say... no easy way around that. But I think we'll see the F50 manage 50+ knots in a sustained  run in the right conditions whilst racing, something he AC75 didn't get to do - accepting conditions may not have been ideal for them.

The clear difference is in the wind resistance.

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41 minutes ago, Terry Hollis said:

The clear difference is in the wind resistance.

No too sure about that...   

Hydro drag amounts to something like 2/3 of total drag.  Of aero drag, my guess is that on a size for size basis, the AC75 hull plus windward foil might produce less drag, but the F50 wingsail would produce less than the twin-skin.  Could be more even than one might think.

Maybe@Mikko Brummer would have a better idea, given his experience with CFD etc?

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11 hours ago, MaxHugen said:

A lot of racing sailing craft use power nowadays, canting keels sure aren't winched up by hand etc. So maybe not really "cheating".  The F50 is 95% electro-hydraulically powered, only the mainsheet is human powered.

I don't see why the AC75 should be faster upwind?  Comparing the two boats as "platforms" regardless of number of hulls:

  • F50 uses a wingsail, the AC75 a twin skin sail that's apparently almost as efficient
  • F50 uses 3 foils, the AC75 uses 2
  • F50 uses crew weight pus windward rudder foil for RM, the AC75 uses ballast mass, and both try to lower CE

Where do you see a clear difference for upwind performance?

Cavitation will still be an issue as you say... no easy way around that. But I think we'll see the F50 manage 50+ knots in a sustained  run in the right conditions whilst racing, something he AC75 didn't get to do - accepting conditions may not have been ideal for them.

Am not ever going to try argue your far-better wisdom, it’s just my very-basic ‘understanding’ that RM is King, and the the AC75’s are therefore the much-more powerful boat. 
 

As you say, the F50’s do benefit a little by movable crew ballast, and a lot by the windward rudder producing significant RM, but that rudder also creates drag. 

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32 minutes ago, Stingray~ said:

Am not ever going to try argue your far-better wisdom, it’s just my very-basic ‘understanding’ that RM is King, and the the AC75’s are therefore the much-more powerful boat. 
 

As you say, the F50’s do benefit a little by movable crew ballast, and a lot by the windward rudder producing significant RM, but that rudder also creates drag. 

You're quite right, there's always a trade-off, so the cats using the rudder for RM certainly creates extra drag itself.  Plus more drag from the other 2 foils that have to balance the downforce.

So does the ballast in the AC75. That weight has to be lifted by a foil which also results in extra drag.

We might get a better idea during the SailGP series, but it will always be difficult to really compare the boats with very limited resources.

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17 hours ago, MaxHugen said:

A lot of racing sailing craft use power nowadays, canting keels sure aren't winched up by hand etc. So maybe not really "cheating".  The F50 is 95% electro-hydraulically powered, only the mainsheet is human powered.

I don't see why the AC75 should be faster upwind?  Comparing the two boats as "platforms" regardless of number of hulls:

  • F50 uses a wingsail, the AC75 a twin skin sail that's apparently almost as efficient
  • F50 uses 3 foils, the AC75 uses 2
  • F50 uses crew weight pus windward rudder foil for RM, the AC75 uses ballast mass, and both try to lower CE

Where do you see a clear difference for upwind performance?

Cavitation will still be an issue as you say... no easy way around that. But I think we'll see the F50 manage 50+ knots in a sustained  run in the right conditions whilst racing, something he AC75 didn't get to do - accepting conditions may not have been ideal for them.

A couple of things to add here.

The AC75 uses flaps (of course you could put flaps to the cats if you wanted).

The AC50 is breaking the water at three points instead of two.

The AC75 rudder is lifting not pulling downwards.

It sort of feels like once your air bourne this is likely more efficient that deliberately creating drag to create power and doing it with less foils also should be better (if harder to control).

I wonder what the cost of having the foils out of alignment is or whether it's a benefit as the wake from the main foils can't affect the rudders.

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3 hours ago, Boybland said:

A couple of things to add here.

The AC75 uses flaps (of course you could put flaps to the cats if you wanted).

The AC50 is breaking the water at three points instead of two.

The AC75 rudder is lifting not pulling downwards.

It sort of feels like once your air bourne this is likely more efficient that deliberately creating drag to create power and doing it with less foils also should be better (if harder to control).

I wonder what the cost of having the foils out of alignment is or whether it's a benefit as the wake from the main foils can't affect the rudders.

Just a small point, at times the AC75 rudder foil does in fact exert downwards force. It has to balance the longitudinal moment of the sail, and at higher speeds needs to exert a countering force.

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11 hours ago, Stingray~ said:

Am not ever going to try argue your far-better wisdom, it’s just my very-basic ‘understanding’ that RM is King, and the the AC75’s are therefore the much-more powerful boat.

RM is king for most sailboats, but these ones isn't drag the real king ? Iceboats are the fastest sailing "boat" even though they have a lower RM but amazing low drag.

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13 minutes ago, Tornado-Cat said:

RM is king for most sailboats, but these ones isn't drag the real king ? Iceboats are the fastest sailing "boat" even though they have a lower RM but amazing low drag.

Good point.  Hydro drag is the real killer, so any improvement here is a major advantage.

I expect we'll see ongoing developments in the reduction of drag, but it will take time, and probably a relaxation of some class design rules. Aerated boundary layers, hydrophobic surfaces, super-cavitation... several possibilities.

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16 minutes ago, MaxHugen said:

Good point.  Hydro drag is the real killer, so any improvement here is a major advantage.

I expect we'll see ongoing developments in the reduction of drag, but it will take time, and probably a relaxation of some class design rules. Aerated boundary layers, hydrophobic surfaces, super-cavitation... several possibilities.

AC75s are limited by 1) weight in light wind conditions, 2) drag as any fast boat, 3) width of the hull in case of waves. Prada commented before the CSS that wind was not a limit, which implicitly tells it was the sea state. That said they lowered the wind limits for the match after a few capsizes.

I think new developments will include a smaller beam.

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After a number of failed attempts I've managed to loft a foil in CAD. The shape is reasonably accurate, from recent videos.  It has a foil profile that transitions from the vertical area to the horizontal.

image.thumb.png.d54c16b1760f507868551c3c5588b33f.png

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On 4/23/2021 at 9:27 AM, Boybland said:

Personal opinions.

At this point there are probably only a very small number of points of sail (if any) where an AC50 an out perform an AC75 and most of those are probably only in certain very light (basically unracable) wind conditions.

One: The mono is way more aerodynamic than a messy old cat. Even if you make everything as clean as possible on the cat your still going to get air off the two seperate hulls interacting and your always going to have to connect them using something...

Two: The twin skin main looks to be much better than first anticipated. It's incredibly dynamic allowing very good shapes regardless of conditions, point of sail etc... It also looks extremely slippery once powered up when you need to bleed power with a minimum drag penalty.

Three: I don't think any traditional mono versus cat comparisons are relevant at all. Once airbourne the two platforms are essential the same and it simply comes down to power, weight, drag and the ability to control and balance them. Given how easily both platforms can get airbourne, we can probably call any none foiling aspects of the boats basically edge cases.

 

Interesting thoughts. Also, how much extra drag comes from a second rudder and especially, negative AoA set elevator?

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2 hours ago, Sailbydate said:

Interesting thoughts. Also, how much extra drag comes from a second rudder and especially, negative AoA set elevator?

I don't think anyone has run any numbers yet, but it will be quite significant.

Freddie Carr mentioned they can get 1t of downforce on the windward rudder, which creates a lot of drag.  But that's not all, now the main foil plus the leeward rudder have to balance this force as well as the mass of the boat+crew which is a bit under 3t. That's really going to add up.

This is one scenario of drag distribution of an AC75:

image.png.dc6134234708e8765631033502b0f67d.png

The Stabilator (green bit) is the rudder foil, and it's probably in a state of providing a bit of lift, but not a lot.  I think that between the two F50 rudder foils this percentage would be multiplied many times over. I wouldn't be surprised if it was 20% of total drag, maybe even more.

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Can't do much about spray, wave and hull but the foils wetted area is dominant and the ETNZ outcome was driven by less the question was could they sail it all the way through a race, remarkable that ETNZ survived the light winds but also sad that the maximum speed was never revealed.

The discussion around the dock was that they had achieved 57knts I suppose they want to keep something in the box for the next event. 

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Would love to see the F50’s get main-foil flaps in the next evolution. The ultimate would be ‘morphing’ sections too, to transition through the cavitation wall. 

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1 hour ago, Stingray~ said:

Would love to see the F50’s get main-foil flaps in the next evolution. The ultimate would be ‘morphing’ sections too, to transition through the cavitation wall. 

As water is 1000x as dense as air, morphing is even more difficult for hydrofoils as the profiles get so thin for high speed they become a very difficult structural challenge for engineers.

Not to mention the vastly different profiles used between sub-cav and post-cav foils.

But hey, who knows, it wasn't even 120 years ago that most people wouldn't have believed man would fly.

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3 hours ago, Stingray~ said:

Would love to see the F50’s get main-foil flaps in the next evolution. The ultimate would be ‘morphing’ sections too, to transition through the cavitation wall. 

They will. And, why not, AC75 foils on the cats.

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13 minutes ago, Tornado-Cat said:

They will. And, why not, AC75 foils on the cats.

Since on Sunday the F50s were topping 50 knots with regularity sporting that 18m wing setup and the ‘fast’ foils, well... maybe they are fast enough? 

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Just now, Stingray~ said:

Since the F50s were topping 50 knots with regularity sporting that 18m wing setup and the ‘fast’ foils, well... maybe they are fast enough? 

I am not sure they tried all the new foil profiles yet, and as you mentioned, I am sure they will pass the cavitation wall with morphing sections or supercavitating profiles.

The AC is stuck in budget requirements while SailGP can try what they want.

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1 hour ago, Tornado-Cat said:

They will. And, why not, AC75 foils on the cats.

The D35s have gone that way.

I think the current system (ie tilting the whole foil) is a better solution to altering lift (simpler foil construction, no extra drag at foil/flap interface).

One of the f50 designers mentioned if they had a computer control the flight, the boats would stay flat the whole way around the course.

Appears they have kept a human in the loop to introduce a degree of fallibility /uncertainty. (I like it, measure of skill)  

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1 hour ago, Stingray~ said:

Since on Sunday the F50s were topping 50 knots with regularity sporting that 18m wing setup and the ‘fast’ foils, well... maybe they are fast enough? 

What I found interesting is that they also have a high (really high) mode upwind (slingers seemed to use a couple of times to pinch to the mark and avoid several tacks)

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20 minutes ago, Wandering Geo said:

D35 foiling

 

Cool!   Apparently sans-hydraulic, lots of string everywhere.  :D

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Question:  The winches on the F50, are they controlling the mainsheet or traveller?

I recall someone (Freddie?) saying that they pretty much set the mainsheet, and just play the traveller?

image.png.3ca494a4cc28fe03ffd70402ce217830.png

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1 hour ago, MaxHugen said:

Question:  The winches on the F50, are they controlling the mainsheet or traveller?

I recall someone (Freddie?) saying that they pretty much set the mainsheet, and just play the traveller?

image.png.3ca494a4cc28fe03ffd70402ce217830.png

Wing sail = no traveller.

One of the commentators noted that slingers was playing the flaps, not the mainsheet like all the rest (Day 1 when noticeably quicker than everyone else)

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59 minutes ago, Wandering Geo said:

Wing sail = no traveller.

One of the commentators noted that slingers was playing the flaps, not the mainsheet like all the rest (Day 1 when noticeably quicker than everyone else)

Right, just realised there was no traveller from stern shot.  My understanding is that the flaps are also hydraulically operated.  Also heard a commentator say they were playing the flaps for twist, I guess to balance heel?  Wonder if the wingman has foot pedals to control the flaps, he's got his hands full with the sheet.

image.png.e44d10f13bfe4aaefe87ad513db0aa53.png

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7 hours ago, Wandering Geo said:

The D35s have gone that way.

I think the current system (ie tilting the whole foil) is a better solution to altering lift (simpler foil construction, no extra drag at foil/flap interface).

One of the f50 designers mentioned if they had a computer control the flight, the boats would stay flat the whole way around the course.

Appears they have kept a human in the loop to introduce a degree of fallibility /uncertainty. (I like it, measure of skill)  

Keeping the human input is key, we don't want boats ran by computers at distance.

An exterior foil with flaps would be a create a bit more drag but would be more powerful. It would be interesting to compare the F50 with the D35 in different conditions.

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On 4/22/2021 at 2:24 AM, Kiwing said:

Could I suggest we summarise our thoughts on the cat verse AC75?

Why do cats lack an ability to have a good VMG upwind?  Is it me or is this a generally  held opinion?

Are cats greatly better at any other point of sailing?

Can these F50s improve their ability to sail up wind? or do we have no directly upwind legs in races?

Max tell me if I am out of place here.

Fancy coming in after that great info from SHC  !!?

 

The AC75's are achieving positive leeway upwind through their foil cant angle.

This on its own is going to make it difficult for anything else to live with them upwind even if they can match/exceed boat speed.

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2 hours ago, SpudH said:

The AC75's are achieving positive leeway upwind through their foil cant angle.

This on its own is going to make it difficult for anything else to live with them upwind even if they can match/exceed boat speed.

@Basiliscus pointed out that they would have an asymmetric foil profile on the vertical section of the L foil as well, so that would be providing windward lift similar to the AC75.

Perhaps we can talk @doroxinto having a look to see if there's anywhere the boat data can be obtained from.  This could shed some light on comparative VMG and AoA etc between the AC75 and F50.

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2 hours ago, SpudH said:

The AC75's are achieving positive leeway upwind through their foil cant angle.

This on its own is going to make it difficult for anything else to live with them upwind even if they can match/exceed boat speed.

Leeway in itself is irrelevant to performance.  Leeway only determines which way the bow is pointed, not which way the boat is moving.  It is of interest when dealing with the wake of a daggerboard impinging on the rudder, and it affects sail trim.  But camber and cant do not drive the boat to windward because the boat's heading angle is not fixed.

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14 hours ago, MaxHugen said:

As water is 1000x as dense as air, morphing is even more difficult for hydrofoils as the profiles get so thin for high speed they become a very difficult structural challenge for engineers.

Not to mention the vastly different profiles used between sub-cav and post-cav foils.

But hey, who knows, it wasn't even 120 years ago that most people wouldn't have believed man would fly.

I sense a swing wing variable geometry foil inspired by the F-14 Tomcat on the way:P

In all seriousness, something like that would deal with aspect ratio conflicts between high lift take off requirements and high speed cavitation issues. The aspect ratio reduces dramatically as sweep angle increases

It's a bit like the battle aeronautical engineers fought with the sound barrier. Except they had unlimited funds to fight it with. But I bet there's a lot to be learned from area ruling etc.

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4 minutes ago, SpudH said:

I sense a swing wing variable geometry foil inspired by the F-14 Tomcat on the way:P

In all seriousness, something like that would deal with aspect ratio conflicts between high lift take off requirements and high speed cavitation issues. The aspect ratio reduces dramatically as sweep angle increases

It's a bit like the battle aeronautical engineers fought with the sound barrier. Except they had unlimited funds to fight it with. But I bet there's a lot to be learned from area ruling etc.

Aspect ratio?  Cavitation is caused when a foil creates a low pressure that is lower than water vapour pressure.

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5 minutes ago, Basiliscus said:

Leeway in itself is irrelevant to performance.  Leeway only determines which way the bow is pointed, not which way the boat is moving.  It is of interest when dealing with the wake of a daggerboard impinging on the rudder, and it affects sail trim.  But camber and cant do not drive the boat to windward because the boat's heading angle is not fixed.

Watching the AC75'S, it seemed they were making gains to windward instead of losing ground to leeway.

I haven't sat down to consider it fully but if the AWA becomes a limiting factor to VMG as boat speeds go beyond 50kts then positive leeway (if that is what is happening and not just a boat attitude illusion) must be significant.

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2 minutes ago, MaxHugen said:

Aspect ratio?  Cavitation is caused when a foil creates a low pressure that is lower than water vapour pressure.

Yes and you would expect to reach that pressure sooner (lower speed) on a high aspect ratio foil than a low aspect ratio foil.

If you take a straight foil section that say cavitates at say 50kts. As you sweep that foil back the relative chord the water sees is reduced (longer, thinner wing section) so it should stall later.

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39 minutes ago, SpudH said:

Watching the AC75'S, it seemed they were making gains to windward instead of losing ground to leeway.

I haven't sat down to consider it fully but if the AWA becomes a limiting factor to VMG as boat speeds go beyond 50kts then positive leeway (if that is what is happening and not just a boat attitude illusion) must be significant.

I think it helps to consider the non-foiling example of a gybing centerboard.  Picture the centerboard, rudder, and rig sailing to windward all by themselves without any hull.  The angle of attack of the centerboard is whatever is required so that the lift on the board exactly matches the applied load from the sail rig.  The angle of attack on rudder is determined by the yawing moment from the sail rig.  Now place the hull on top of the board and rudder.  For a normal centerboard, the hull will be aligned with the chord of the board and see a leeway angle.  If the board is gybed to weather, the hull can be aligned with the velocity vector and see no leeway.  But the board and rudder are still going through the water as they were.  It's the boat's heading that is different.

The same thing is true for a canted T foil or an asymmetrical L foil.  The lift on the foils has to match the applied loads of the sails and gravity.  This determines the angles of attack relative to the zero lift lines of the foils.  Adding camber, changing the flap, or canting changes the angle between the zero lift line and the chord, but the lift still has to be the same.  Changing the incidence of the foils acts like gybing a centerboard - it really rotates the hull relative to the velocity vector.  

To predict a yacht's performance, all you need to know are the lift and drag forces in the air and in the water.  How those components are produced does not change the performance.  It is only indirectly through its effect on drag that leeway affects performance.  The same lift can be produced with no camber and more angle of attack, or it can be produced with camber and zero angle of attack.  The leeway angles will be different in the two cases.  But if there is no difference in drag when producing the same lift, there will be no difference in performance.

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40 minutes ago, SpudH said:

Yes and you would expect to reach that pressure sooner (lower speed) on a high aspect ratio foil than a low aspect ratio foil.

If you take a straight foil section that say cavitates at say 50kts. As you sweep that foil back the relative chord the water sees is reduced (longer, thinner wing section) so it should stall later.

You might find this paper useful.  Modest amounts of sweep are effective at extending the cavitation speed.  Larger amounts of sweep, on the order of 30 - 45 deg, can perform better at high speed but their performance suffers at lower speeds.

AIAA-48104-766_EffectsOfSweep.pdf

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13 hours ago, Basiliscus said:

You might find this paper useful.  Modest amounts of sweep are effective at extending the cavitation speed.  Larger amounts of sweep, on the order of 30 - 45 deg, can perform better at high speed but their performance suffers at lower speeds.

AIAA-48104-766_EffectsOfSweep.pdf 1.54 MB · 6 downloads

Thanks Basiliscus,

That's much as I expected with some interesting wrinkles, I will be digesting that for a while.

I'm new to foiling but have spent the last 30 years studying fighter wing design as a hobby. The whole history of high speed flight is fascinating and fighters in particular have to be able to perform in all corners of the speed/maneuver flight envelope so their wing design is critical. And the solution rich and diverse.

Most people seem to think that hydraulic design and aeronautical design are two distinct disciplines with little overlap in skillset but it is not widely known that at high Mach numbers air starts to behave more like a liquid than a gas. The issues designers faced as fighters got faster at the end of WW2 with compressibility ( detached flow over the control surfaces) is akin to the issue foils are having with cavitation.

Obviously the difference is that cavitation is where the pressure in the water flow over the foil is so low that the water vapourises as against becomes detached per se. But the solution will involve managing the pressure gradient to prevent/delay cavitation in a similar fashion as aero designers managed shock wave propagation.

The difference here is that boat industry will only be able to spend millions on R&D. The military industrial complex has thrown billions at wing design so any foil designer who can find the cross over research will have a major short cut.

I guarantee that there is a rich amount of research done in the late 1940's where they used water instead of air to simulate the high speed regime when they didn't have acces to supersonic wind tunnels. There will be NACA wing profiles that have all the research complete to gain a few kts on delayed cavitation, thats all you need, if your foil is 2 kts faster, game over until the others catch up.

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1 hour ago, SpudH said:

Thanks Basiliscus,

That's much as I expected with some interesting wrinkles, I will be digesting that for a while.

I'm new to foiling but have spent the last 30 years studying fighter wing design as a hobby. The whole history of high speed flight is fascinating and fighters in particular have to be able to perform in all corners of the speed/maneuver flight envelope so their wing design is critical. And the solution rich and diverse.

Most people seem to think that hydraulic design and aeronautical design are two distinct disciplines with little overlap in skillset but it is not widely known that at high Mach numbers air starts to behave more like a liquid than a gas. The issues designers faced as fighters got faster at the end of WW2 with compressibility ( detached flow over the control surfaces) is akin to the issue foils are having with cavitation.

Obviously the difference is that cavitation is where the pressure in the water flow over the foil is so low that the water vapourises as against becomes detached per se. But the solution will involve managing the pressure gradient to prevent/delay cavitation in a similar fashion as aero designers managed shock wave propagation.

The difference here is that boat industry will only be able to spend millions on R&D. The military industrial complex has thrown billions at wing design so any foil designer who can find the cross over research will have a major short cut.

I guarantee that there is a rich amount of research done in the late 1940's where they used water instead of air to simulate the high speed regime when they didn't have acces to supersonic wind tunnels. There will be NACA wing profiles that have all the research complete to gain a few kts on delayed cavitation, thats all you need, if your foil is 2 kts faster, game over until the others catch up.

Very interesting discussion.  Now you've got me reading up on sweep too. :)

I guess another issue with variable sweep for a hydrofoil is the movement of the centre of lift aft?  Maybe couple it with a strut that rotates forward (reverse sweep?), but that would come with it's own set of problems besides engineering, such as reduced height of hull to foil?

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16 hours ago, Basiliscus said:

You might find this paper useful.  Modest amounts of sweep are effective at extending the cavitation speed.  Larger amounts of sweep, on the order of 30 - 45 deg, can perform better at high speed but their performance suffers at lower speeds.

AIAA-48104-766_EffectsOfSweep.pdf 1.54 MB · 6 downloads

While we're on the subject of foils, can you offer any insight into the unusual shape of the F50 foils?

image.thumb.png.5e53ccaa35c214a9528913357ac7805a.png

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5 hours ago, MaxHugen said:

Very interesting discussion.  Now you've got me reading up on sweep too. :)

I guess another issue with variable sweep for a hydrofoil is the movement of the centre of lift aft?  Maybe couple it with a strut that rotates forward (reverse sweep?), but that would come with it's own set of problems besides engineering, such as reduced height of hull to foil?

Here's the basis for simple sweep theory.  Imagine an unswept wing with infinite span, in inviscid flow (no boundary layer).  Now pull on the wing so it is sliding sideways in the spanwise direction.  The flow over the wing hasn't changed (no boundary layer, remember) because the fluid still follows the same contour over the wing.  But if you consider the fluid velocity relative to the wing, it is now flowing over the wing at an angle.  So the unswept wing sliding sideways is equivalent to a stationary swept wing.

What this means is simple sweep theory says ignore the spanwise component of the flow and just consider the flow normal to the wing's axis.  The lift will drop with cosine squared of the sweep angle.  At least in 2D.

For a 3D wing, things are different because the wake trails at an angle.  So the vortex filaments trailing from the forward parts of the wing have an effect on the bound vortex of the aft parts of the wing because they have a component that is parallel to the bound vortex.  This tends to load the aft parts more than the forward parts, which means the spanwise lift distribution is affected.  

Lifting line theory can't handle sweep.  Although if the swept and unswept wings have the same spanwise lift distribution (and therefore the same wake wash distribution) the induced drag will be the same.  The planform shapes required to produce that lift distribution will be different, however, for the swept vs unswept wing.  For example, a constant chord wing swept forward by 18 degrees (IIRC) will produce a spanwise lift distribution that is nearly elliptical.

In order to analyze a swept wing you need a lifting surface method.  Vortex lattice is the simplest, and panel codes are a step up from that.  A panel code is needed to predict cavitation on a 3D foil, so that is the way to go for designing a swept hydrofoil.  It is much quicker to iterate the design with a panel code than with RANS, so panel codes are used even if more sophisticated CFD is available.  It's also much more feasible to use a panel code to generate the data needed for a VPP.  RANS is good for validating the final design and panel code predictions.

Variable sweep is certainly possible - look at planes like the F-111, F-14, MIG-23.  But I don't think it's necessary for a hydrofoil.  There are more important things to actuate than sweep and the structural challenges for a variable sweep foil would be quite difficult.  If you wanted to go variable sweep, I think a T foil with an oblique wing would be worth investigating.

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38 minutes ago, Basiliscus said:

Here's the basis for simple sweep theory.... 

If you wanted to go variable sweep, I think a T foil with an oblique wing would be worth investigating.

Thanks, that's easier to understand than the technical paper. 

I came across the oblique wing whilst reading about sweep...  such a weird looking wing!  It does look simpler to implement than variable sweep though. I'd assume it also doesn't alter the centre of lift either.

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The big advantage of the oblique wing is the structure is continuous from tip to tip, so it carries the bending loads across the pivot. th This makes the pivot a lot easier to implement.

You still get some shift in the center of lift, and moment across the pivot, because the aft half of the wing is in the upwash from the wake shed by the forward half of the wing and this makes it more heavily loaded.  But it's not as much as with symmetrical sweep.

In an airplane, oblique wings have some interesting handling qualities.  There is a pitching moment due to roll rate, and a rolling moment due to pitch rate.  I once had the opportunity to fly NASA's simulation of the AD-1 .  Right turns were easy.  When I rolled right, the nose came up. When I pulled back, it rolled right.  So it laid right into the turn.  Turning left was a different story.  When I rolled left, the nose dropped. When I pulled the nose up, it rolled right.  And the dynamic modes were lightly damped - about 0.1 damping ratio - so I was fighting it the whole way. 

299862main_ECN-15846_full.jpg

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