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20 hours ago, MaxHugen said:

"Ground effect would prevent that."  Don't think it's relevant at all. If you look at the shape of the underside of any of the hulls, it does not fit an aircraft wing's profile. Aspect ratio is all wrong too, 0.5 compared to an aircraft's 20+ or thereabouts. I doubt you would get any "wing-in-ground" effect at all.

The hull design considerations would probably be (in no specific order):

  • Efficient Take-off
  • Maximise Laminar Airflow to Sails
  • Minimise Drag, both Parasitic (form) and Induced

IMO. :)

[Edit] I agree with a comment that if it could produce a bit of down force, it would assist RM.  This is an enormous issue for these boats, which is why they are forced to depower as much as they do. Perhaps it's why I often measure the hull AoA at around -2° ?

 Some day, when the racing is over, I’m convinced this will be established as a fact

 

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12 minutes ago, JALhazmat said:

Which boat has a hull shaped as an inverted wing profile, across its full width and sealed  to the waters surface at the edges 

lets start there. 

No inverted wing profile needed, just a narrow entrance followed by a diffuser section. Look at F1 car underbodies: works also with lousy aspect ratios and no side seals

Too bad the market for left testicles seems to be saturated, teams are bound to have static pressure pick-up points all over the boat bottom

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25 minutes ago, Xlot said:

Too bad the market for left testicles seems to be saturated, teams are bound to have static pressure pick-up points all over the boat bottom

yeah I'm convinced of that.

Water and traditional static air pressure systems arent the best of mates. wonder how they handle that ... 

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34 minutes ago, Xlot said:

No inverted wing profile needed, just a narrow entrance followed by a diffuser section. Look at F1 car underbodies: works also with lousy aspect ratios and no side seals

Too bad the market for left testicles seems to be saturated, teams are bound to have static pressure pick-up points all over the boat bottom

Inverted wing profile is utterly key to ground effect to get the low pressure center formed

where is the narrow entry section on the bow of  the boat and the diffuser? 

current F1 cars are not ground effect cars, the way they do seal the floor in front of the rear tyres is through creating a vortex of rotating air that clings to the floor 

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1 minute ago, JALhazmat said:

current F1 cars are not ground effect cars, the way they do seal the floor in front of the rear tyres is through creating a vortex of rotating air that clings to the floor 

ground effect  does not require a seal. 

it is made more efficient by the virtual "vortex fence" for sure ... but take that - or any physical fence away and ground effect is sill there 

 

 

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3 hours ago, JALhazmat said:

Inverted wing profile is utterly key to ground effect to get the low pressure center formed

where is the narrow entry section on the bow of  the boat and the diffuser? 

current F1 cars are not ground effect cars, the way they do seal the floor in front of the rear tyres is through creating a vortex of rotating air that clings to the floor 

Agree. 

Once the designers found a use for those pesky drag-inducing vortices, the F1 cars broke out in a severe rash of winglets, even quintuple sets of them, all to help direct the airflow since physical fences were banned.  Together with splitters etc.

They are used along the full length of a car body nowadays, and particularly around the front wheels. Gone are the days when an F1 car "looked" aerodynamically sleek!

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1 minute ago, MaxHugen said:

Agree. 

Once the designers found a use for those pesky drag-inducing vortices, the F1 cars broke out in a severe rash of winglets, even quintuple sets of them, all to help direct the airflow since physical fences were banned.  Together with splitters etc.

They are used along the full length of a car body nowadays, and particularly around the front wheels. Gone are the days when an F1 car "looked" aerodynamically sleek!

Theres more than one scenario in which  ground effect plays a role ..... that where the aerofoil force vector acts upwards - and that where it acts downwards. 

On an F1 car, the force vector acts downwards - towards the ground  - with a low pressure area - between the aerofoil / floor and the ground. In this scenario, ground effect contributes to the overall downforce produced allowing higher cornering speeds etc etc. 

On an aircraft, the force vector for the aerofoil acts upwards - away from the ground - with a high pressure area between the aerofoil and the ground. In this scenario, ground effect contributes to the efficiency of the aerofoil (in this case the hull of the yacht considered as a wing on ground effect section)

In my opinion, the force vector for the hull on the AC75's  acts upwards, helping to support the weight of the boat - less work for the foils,  faster takeoff onto the foils, and when optimised, foiling in lower windspeeds.

Its just my opinion - not being stated as fact and I'd be happy to see the facts.

Unfortunately we're unlikely to see those facts.

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15 minutes ago, MaxHugen said:

Agree. 

Once the designers found a use for those pesky drag-inducing vortices, the F1 cars broke out in a severe rash of winglets, even quintuple sets of them, all to help direct the airflow since physical fences were banned.  Together with splitters etc.

They are used along the full length of a car body nowadays, and particularly around the front wheels. Gone are the days when an F1 car "looked" aerodynamically sleek!

The current crop are a nightmare of aero features ... all of which need to work to perfection together for a car to perform. How often have we seen cars lose a piece of carbon fibre and drop off in performance because of it. 

Interestingly one of the most successful F1 ideas in recent years which was banned was the Renault (I think, from memory) active suspension , which held the car at a very specific and optimum ride height for ground effect (which, by the way was produced without the benefit of vortext fences) .

This allowed ground effect to be used very very effectively without the danger of the grip suddenly disappearing when the ride height changed because of a bump 

 

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18 minutes ago, kiwi39 said:

The current crop are a nightmare of aero features ... all of which need to work to perfection together for a car to perform. How often have we seen cars lose a piece of carbon fibre and drop off in performance because of it. 

Interestingly one of the most successful F1 ideas in recent years which was banned was the Renault (I think, from memory) active suspension , which held the car at a very specific and optimum ride height for ground effect (which, by the way was produced without the benefit of vortext fences) .

This allowed ground effect to be used very very effectively without the danger of the grip suddenly disappearing when the ride height changed because of a bump

"in recent years"...  I've always been interested in all aspects of F1 design, but the aero effects defied any "logic" my brain could come up with.

Active suspension is decades old... found a good video I saw a few months ago, discussing it's history:

 

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7 hours ago, kiwi39 said:

ground effect  does not require a seal. 

it is made more efficient by the virtual "vortex fence" for sure ... but take that - or any physical fence away and ground effect is sill there 

 

 

I made that exact point about using a vortex to seal the floor edge..

So where pray tell are the vortex being generated on a cup boat to seal the floor. Best in mind this is a downforce creating scenario not a lift force. 

again show us a boat with a positive or negative  wing profile ( fuck it any wing profile) along its full length and width and identify the features that seal the boat to the surface either  physically or through rotating vortex creation, it’s simple surely? 

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36 minutes ago, JALhazmat said:

Best in mind this is a downforce creating scenario 

In your opinion.... 

43 minutes ago, JALhazmat said:

So where pray tell are the vortex being generated on a cup boat to seal the floor

There may not be any - that’s my point. You only need an aerofoil for ground effect to exist - you don’t need a sealing end plate - either virtual in the case of a vortex or an actual physical structure like a skeg. 
 

Of course if you can put an end plate on a WIG they are more efficient - and that’s why designers do it.  But ground effect still exists without them. 
 

case in point friend bobkate’s experience in the piper Cherokee. Wing in ground effect. No seal required. 

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1 hour ago, JALhazmat said:

I made that exact point about using a vortex to seal the floor edge..

So where pray tell are the vortex being generated on a cup boat to seal the floor. Best in mind this is a downforce creating scenario not a lift force. 

again show us a boat with a positive or negative  wing profile ( fuck it any wing profile) along its full length and width and identify the features that seal the boat to the surface either  physically or through rotating vortex creation, it’s simple surely? 

 

1) I’m positing a downforce scenario too

2) IMHO we should NOT be looking at F1 “Wing Cars” initiated by the Lotus 79

 

 

FFF6D95F-2003-4010-B8BB-6D09B31A443E.jpeg

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1 hour ago, kiwi39 said:

In your opinion.... 

There may not be any - that’s my point. You only need an aerofoil for ground effect to exist - you don’t need a sealing end plate - either virtual in the case of a vortex or an actual physical structure like a skeg. 
 

Of course if you can put an end plate on a WIG they are more efficient - and that’s why designers do it.  But ground effect still exists without them. 
 

case in point friend bobkate’s experience in the piper Cherokee. Wing in ground effect. No seal required. 

on no seal no vortex just a wing is all they need..ok

so is the hull cross section  lifting or inverted?

again wing in ground effect creates liftthe other createz down force.

can you pick a term and just stick to it as they describe different things.

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1 minute ago, JALhazmat said:

on no seal no vortex just a wing is all they need..ok

so is the hull cross section  lifting or inverted?

again wing in ground effect creates liftthe other createz down force.

can you pick a term and just stick to it as they describe different things.

Lifting.

ok, if it makes it easier WIG shall mean lifting only. 
 

just as an FYI , ground effect still exists for an inverted ( your term ) aerofoil cross section. 
 

I sincerely hope for all of our sakes that we eventually get some exposé from the teams. In reality that’ll be post event or never :( 

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The reference should be later cars, which were specified supposedly to do away with wing cars’ excessive downforce and cornering speeds: a narrow 50 cm wide flat bottom over the wheelbase, flanked by the bottom balance 5 cm higher. In fact, designers quickly found out that by raking the car forward they could still recoup a good portion of the effect. Over the years, this was greatly supplemented by coupling with rear wings and extractors/diffusers, and later still by side sealing vortices

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7 minutes ago, kiwi39 said:

I sincerely hope for all of our sakes that we eventually get some exposé from the teams. In reality that’ll be post event or never :( 

Agree. What’s maddening is that as of today, AC designers must know exactly IF ground effect exists and how it varies with boat trim: all’s needed is a few static pressure points along the hull bottom

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10 minutes ago, kiwi39 said:

Lifting.

ok, if it makes it easier WIG shall mean lifting only. 
 

just as an FYI , ground effect still exists for an inverted ( your term ) aerofoil cross section. 
 

I sincerely hope for all of our sakes that we eventually get some exposé from the teams. In reality that’ll be post event or never :( 

Yes I know ground effect exists with an inverted aero foil wing section but the effect is downforce not lift and the low pressure is between the wing and the ground and to do that you need specific shaping of the air flow

entry, sealed, and exit (diffuser) 

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1 minute ago, Xlot said:

Agree. What’s maddening is that as of today, AC designers must know exactly IF ground effect exists and how it varies with boat trim: all’s needed is a few static pressure points along the hull bottom

It will exist - regardless of whether the hulls are lifting or providing downforce. The burning question is if they’re flying low enough to be “in” the ground effect. my bet is yes. 

Static ports - yes !! I’d love to see those pressure plots. It’d be a tricky one though for sure. Static air pressure measurement systems and water aren’t the best of friends. Makes you wonder if there’s a solid state waterproof ambient pressure sensor out there. 

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3 minutes ago, JALhazmat said:

Yes I know ground effect exists with an inverted aero foil wing section but the effect is downforce not lift and the low pressure is between the wing and the ground and to do that you need specific shaping of the air flow

Agree. 
 

but it doesn’t need to be sealed. It is more effective if it is, but it’s not an absolute must. 

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1 minute ago, kiwi39 said:

Static ports - yes !! I’d love to see those pressure plots. It’d be a tricky one though for sure. Static air pressure measurement systems and water aren’t the best of friends. Makes you wonder if there’s a solid state waterproof ambient pressure sensor out there. 

What’s wrong with a few U tubes and dyed water? Showing my age, I suppose :D 

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7 minutes ago, Xlot said:

What’s wrong with a few U tubes and dyed water? Showing my age, I suppose :D 

nothing :)  would that work when the static ports are immersed on a regular basis? 
 

Probably not easily electronically measured and logged. 

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I think this image destroys the whole "ground effect" theory.

AM.thumb.png.279c95baedb99fb028724722e443663b.png

The spray being generated on the leeward side of the keel shows there's a significant venturi effect from air going from a high pressure zone on the windward side and escaping under the keel, creating a low pressure zone on the leeward side.The velocity on the leeward side is sufficient to lift spray off the surface.

So the airflow on the windward and leeward sides is totally different, and so are the air pressures, which are likely contributing (relatively minutely) to overturning moment, not righting moment. The benefit being sought is to force more air up over the deck and sails and not let it escape under the hull, increasing the efficiency of the sails.

Another other nail in the "ground effect" theory is that proponents can't make up their minds whether it's lifting or pushing down, e.g. F1 cars vs aircraft. The amount of trickery required in F1 to create useful ground effect highlights just how difficult it is where there is severely limited span and no skirts.

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Without really thinking I just assumed that there would be an advantage to producing lift from the hull from the airflow - reducing the amount of lift required from the leeward foil and therefore reducing drag. 

However, on actually thinking about it, I don't think there would be an advantage to doing this as:

1.  At takeoff speeds it would be ineffective due to low airspeed (when you would need the lift)

2.  At flying speed a lifting hull would reduce overall RM by trying to lift the hull up and rotate it up and around the leeward foil

So, without getting into the details of airflows / ground effect etc, I don't think there is a motivation for the designers to create lift from the hull, possibly the opposite; pushing the hull down at higher speeds to increase RM. 

Or I could be totally wrong. Who knows...

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From an interview in Tip&Shaft of Benjamin Muyl (Chief ingeneer of INEOS) and Martin Fischer (PRADA):

Quote

Tip and Shaft: What are the advantages and disadvantages of putting more or less volume in these famous “bustle”?


M.F .: If you put more volume, you need less lift from the foil, while with less volume, the acceleration phase at low speed is a bit harder. But the advantage with less volume is that when you touch the water during the "touch down" phases, you have very little extra wet surface, so you can afford to touch more often and take more risk by flying lower. Because the lower we fly, the faster we go; ideally, the hull is practically touching the water.
B.M .: The other issue of the "bustle" is aero: the goal is to close the hull under the boat to create a plaque effect and limit air circulation. We did it with a square shape, very vertical faces that contain more volume than all the others. Luna Rossa has been on a much rounder shape.
MF: It's all a matter of compromise: you calculate how much it costs you one centimeter more of the difference between the hull and the water in aero, and conversely, how one centimeter less will increase the probability of hitting a wave , based on that, you decide on the size of the "bustle".

 

The idea is to have a clean aerodynamic flow rather than a lift.

 

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Can we just stop revisiting this Debunked and Fallacious concept.

There is no steady state that these boats operate within for them to chase this as an aid, that would out do the refinement that would be compromised elsewhere to make it actually happen.

The Hull is too long and narrow to create a substantial effect. These are not Piper Cherokees or whatever. Nor are they Jet Fighters. And certainly are not Hovercraft or Ekranoplanes.

All Ground effect concepts whether seeking lift or downforce are much squarer in planform and utilise downtips, or skirts, or fences to maximise the pressure differential needed to achieve the lift.

By the time these hull shapes could be contributing a lifting force (i.e. high hull speed) they are either actively seeking increased RM (downforce to the challenged here) or are butting into the window of foil cavitation. So a Lifting Hull is the last scenario that is being sought. Whenever Lift is generated - remember that it comes at a price of drag. Lowest Drag solution will win this Cup because it will maximise VMG. Discuss VMG gains instead - I implore you.....

The Keel/Skeg/Bustle (I dont care what term is used)  - is to endplate the whole rig to a reflection plane status which boosts the K value of Rig efficiency - it also deepens the I-Beam effect of the hull making it a stiffer global platform to reduce hull flex. 

Bow down allows wider foil geometry to be achieved with what is otherwise a simple and resticted foil articulation.

Bow down also allows the Jib and mainsail base to "see" cleaner and scoop more air. The ETNZ B2 has clearly been designed to allow as clean an airflow around the deck structure as possible - lower drag improves Lift Coefficient.

The slim profile that the deck surface and hull undersides allow for best possible drag characteristics. No Flat (vertical) transom removes another drag point. The full length Skeg allows the transference of Runner loads into the hull more efficiently. The Skeg may also contribute leeway resistance at take off. The Horse Blinkers that have been added to the Foil Arms are I suggest both Spray Control but possibly also extra Lateral surface for leeway resistance whilst immersed, at that critical point of take off where loads are rising exponentially yet a slippery hull aids and assists with acceleration and lift off. I cannot find any reason why thy cannot be assymetrically shaped to assist in this role as they fall outside the Foil Control box which is where symmetry is required under the class rules.

Study the take off video sequences, there is no flow of air under the boat whilst it is still floating - so the platform is not assisting in providing any lift. There is only a few seconds of this eagerly claimed Tunnel effect, but it really does not add any sustained or steady state effect..... The T profile that the Hull presents allows easier water release and non catastrophic touchdowns as well as enplating of the whole platform to the waters surface.

IF, the hull structure was creating lift - there would be additional interference drag by the competition of effects that the deck structure was trying to create by endplating the sails and creating distinct high and low pressure zones to windward and leeward of the sail skins, thereby lowering the Rigs efficiency. Not a situation that would be actively encouraged or sought.

As, written about elsewhere - the acceleration of the boat speed is so rapid that by 15 knots of windspeed you are rapidly approaching the Boat speed limitations caused by Foil cavitation. So to propose that these boats actively seeking Ground effect lift over an 8 to 8.5 knot windspeed increase range (6.5 or 7 to 15 knots of windpeed) is just too far fetched......

The reverse is more likely - with additional RM sought from the orienation of the hull to at least fly neutral or possibly gain RM - but certainly to wash out any lift that that the platform is generating.

Windward heel is widely witnessed. It throws the whole weight of the rig into a beneficial pose. The Foils become slightly unweighted, requiring less lift, and associated drag. The heel of the boat (upwind) allows the boat to crab to windward as the heel axis is diagonal across the direction of movement.

Streamlining seems to be the mantra of both ETNZ and LR, with ETNZ a half generation further down the development evolution.

Impressive they are. Hovercraft or Ground Effect vehicles they are not........

Please feel free to contradict everything here with Piper Cherokee anecdotes, Private Pilot Licence Snobbery and Fighter Jet stories. Do not forget to add in lots of Lotus 79 pics and Wright Brothers parallels. You are all unique and valued and highly paid members of Team New Zealand Design Inner sanctum. And Dan is completely Lost, without you all.......

Miranda Kerr and Scarlett Johannson both need their Massages with Happy Endings - Do excuse me ;)

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3 hours ago, Boink said:

Discuss VMG gains instead - I implore you.....

Not disagreeing with any of the rest of what you wrote, but this is the key I think. All the boats are balancing by (upwind) luffing in the gusts and bearing away in the lulls to balance the lateral force against RM. Nothing original in that thought. But it means that top speed is not the issue, they just go for better VMG. The same is largely true with downwind , albeit with the actions reversed. 

So I'm not even sure that RM is the factor I thought it was. In any wind they just bleed off excess lateral force over RM by improving angles. 

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Thank you! 

The possibility of this having any affect has always seemed dubious, and complicated by the fact that many proponents of "ground effect" being relevant use that term to describe vastly different concepts.  

I think you all should have been arguing about the relative gravity of the foil arms or something. 

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14 minutes ago, RMac said:

Thank you! 

The possibility of this having any affect has always seemed dubious, and complicated by the fact that many proponents of "ground effect" being relevant use that term to describe vastly different concepts.  

I think you all should have been arguing about the relative gravity of the foil arms or something. 

Or the holes in Rita that are obviously there for ducting air instead of some flimsy excuse about drainage.. 

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2 hours ago, JALhazmat said:

Or the holes in Rita that are obviously there for ducting air instead of some flimsy excuse about drainage.. 

You may be being just cheeky but fwiw: when asked about this possibility Ward discounted it for the reason that altering the boundary layer on the hull is typically not allowed, he gave riblets and other surface coatings as examples. 

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5 hours ago, Stingray~ said:

You may be being just cheeky but fwiw: when asked about this possibility Ward discounted it for the reason that altering the boundary layer on the hull is typically not allowed, he gave riblets and other surface coatings as examples. 

Who is "Ward"?   Did I miss a design discussion in a video somewhere?

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15 hours ago, Boink said:

Can we just stop revisiting this Debunked and Fallacious concept.

There is no steady state that these boats operate within for them to chase this as an aid, that would out do the refinement that would be compromised elsewhere to make it actually happen.

The Hull is too long and narrow to create a substantial effect. These are not Piper Cherokees or whatever. Nor are they Jet Fighters. And certainly are not Hovercraft or Ekranoplanes.

All Ground effect concepts whether seeking lift or downforce are much squarer in planform and utilise downtips, or skirts, or fences to maximise the pressure differential needed to achieve the lift.

By the time these hull shapes could be contributing a lifting force (i.e. high hull speed) they are either actively seeking increased RM (downforce to the challenged here) or are butting into the window of foil cavitation. So a Lifting Hull is the last scenario that is being sought. Whenever Lift is generated - remember that it comes at a price of drag. Lowest Drag solution will win this Cup because it will maximise VMG. Discuss VMG gains instead - I implore you.....

The Keel/Skeg/Bustle (I dont care what term is used)  - is to endplate the whole rig to a reflection plane status which boosts the K value of Rig efficiency - it also deepens the I-Beam effect of the hull making it a stiffer global platform to reduce hull flex. 

Bow down allows wider foil geometry to be achieved with what is otherwise a simple and resticted foil articulation.

Bow down also allows the Jib and mainsail base to "see" cleaner and scoop more air. The ETNZ B2 has clearly been designed to allow as clean an airflow around the deck structure as possible - lower drag improves Lift Coefficient.

The slim profile that the deck surface and hull undersides allow for best possible drag characteristics. No Flat (vertical) transom removes another drag point. The full length Skeg allows the transference of Runner loads into the hull more efficiently. The Skeg may also contribute leeway resistance at take off. The Horse Blinkers that have been added to the Foil Arms are I suggest both Spray Control but possibly also extra Lateral surface for leeway resistance whilst immersed, at that critical point of take off where loads are rising exponentially yet a slippery hull aids and assists with acceleration and lift off. I cannot find any reason why thy cannot be assymetrically shaped to assist in this role as they fall outside the Foil Control box which is where symmetry is required under the class rules.

Study the take off video sequences, there is no flow of air under the boat whilst it is still floating - so the platform is not assisting in providing any lift. There is only a few seconds of this eagerly claimed Tunnel effect, but it really does not add any sustained or steady state effect..... The T profile that the Hull presents allows easier water release and non catastrophic touchdowns as well as enplating of the whole platform to the waters surface.

IF, the hull structure was creating lift - there would be additional interference drag by the competition of effects that the deck structure was trying to create by endplating the sails and creating distinct high and low pressure zones to windward and leeward of the sail skins, thereby lowering the Rigs efficiency. Not a situation that would be actively encouraged or sought.

As, written about elsewhere - the acceleration of the boat speed is so rapid that by 15 knots of windspeed you are rapidly approaching the Boat speed limitations caused by Foil cavitation. So to propose that these boats actively seeking Ground effect lift over an 8 to 8.5 knot windspeed increase range (6.5 or 7 to 15 knots of windpeed) is just too far fetched......

The reverse is more likely - with additional RM sought from the orienation of the hull to at least fly neutral or possibly gain RM - but certainly to wash out any lift that that the platform is generating.

Windward heel is widely witnessed. It throws the whole weight of the rig into a beneficial pose. The Foils become slightly unweighted, requiring less lift, and associated drag. The heel of the boat (upwind) allows the boat to crab to windward as the heel axis is diagonal across the direction of movement.

Streamlining seems to be the mantra of both ETNZ and LR, with ETNZ a half generation further down the development evolution.

Impressive they are. Hovercraft or Ground Effect vehicles they are not........

Please feel free to contradict everything here with Piper Cherokee anecdotes, Private Pilot Licence Snobbery and Fighter Jet stories. Do not forget to add in lots of Lotus 79 pics and Wright Brothers parallels. You are all unique and valued and highly paid members of Team New Zealand Design Inner sanctum. And Dan is completely Lost, without you all.......

Miranda Kerr and Scarlett Johannson both need their Massages with Happy Endings - Do excuse me ;)

I was just going to say that...:P

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1 hour ago, MaxHugen said:

Who is "Ward"?   Did I miss a design discussion in a video somewhere?

Fuck me, you must have missed stink ray regurgitating a TE interview for a week or so. I only see quotes luckily.

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1 hour ago, Stingray~ said:

Britt Ward is very smart, he is Senior Naval Architect, Farr Yacht Design, and is on from around :35 to 1:45 of this 

Thanks, but now I know why I missed it... after 5 painful minutes I'd had enough, and wasn't prepared to spend 2hrs 17m on listening.

Mark didn't really have anything to contribute, he was speculating no less than everyone on this forum.

Not suggesting Mark isn't a smart bloke, I'd been impressed with Farr designs from... well, decades ago, and I'm sure they don't hire dummies.

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On 12/1/2020 at 8:42 PM, Boink said:

Can we just stop revisiting this Debunked and Fallacious concept.

I think there is a lot of aero happening that is being confused as ground effect. Consider the following image posted in The new sailing twin skin setup thread by @MaxHugen:

image.png

It's pretty clear that B2 has been hugely optimised for aero over B1, even dropping the chic reverse sheer bow for a totally passé plumb bow. The profile has maximum camber near the stern and combined with the skeg, should at least minimise turbulence off the transom and possibly provide a bit of downforce on the stern. No skirt, but the (relatively) sharp chine over the last 3 metres or so might help to contain the flow? Similar shapes can be seen on other B2s.

 

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Intuition suggests that if you want to end-plate the rig then the last thing you’d want is to intentionally suck any of the windward pressure towards under the hull where it can escape to the lee. I think of it a lot like deck skirting. 
 

My intuition also thinks that the AM hull, the most ‘soap-shaped’ is the slippierest one aero-wise. 

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17 minutes ago, RobG said:

I think there is a lot of aero happening that is being confused as ground effect

Correct because those promoting "ground effect" don't understand it.

Ground Effect was termed to describe a phenomena experienced in aircraft when landing and taking off where the predicted outcomes didn't occur.  In some cases with disastrous consequences.

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3 hours ago, RobG said:

I think there is a lot of aero happening that is being confused as ground effect. Consider the following image posted in The new sailing twin skin setup thread by @MaxHugen:

image.png

It's pretty clear that B2 has been hugely optimised for aero over B1, even dropping the chic reverse sheer bow for a totally passé plumb bow. The profile has maximum camber near the stern and combined with the skeg, should at least minimise turbulence off the transom and possibly provide a bit of downforce on the stern. No skirt, but the (relatively) sharp chine over the last 3 metres or so might help to contain the flow? Similar shapes can be seen on other B2s.

I think they've changed the bow sections to better deal with a "dive". B1 had a lot more volume there, which tends to bring them to a dead stop - as they've noticed! The flare seems a better alternative for getting the bow back up without totally sacrificing all speed, and to reduce the "pitch-poling" moment.

They also significantly increased the gunwale curvature, presumably to improve airflow and regain laminar flow over the deck for the sails etc.

I like it. But as an Aussie, I'm biased towards our "cousins".

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On 12/1/2020 at 9:53 PM, enigmatically2 said:

Not disagreeing with any of the rest of what you wrote, but this is the key I think. All the boats are balancing by (upwind) luffing in the gusts and bearing away in the lulls to balance the lateral force against RM. Nothing original in that thought. But it means that top speed is not the issue, they just go for better VMG. The same is largely true with downwind , albeit with the actions reversed. 

So I'm not even sure that RM is the factor I thought it was. In any wind they just bleed off excess lateral force over RM by improving angles. 

Not sure where you are going with all of this. It is probably easier to consider these Hydrofoil craft in Ice Boat terms. Go back to your High School Vector Diagrams to better consider what you are ultimately trying to achieve - namely maximise Rig Power, orientate Rig power by minute increments towards a forward facing direction and yet minimise side slip (leeway) to allow the Forward Thrust to be maximised. Then get creative as to how to minimise drag......

But if you dont maximise RM to resist the large sideways component of Rig Thrust then you will never be as fast as the competition - so RM is vital.

On 12/2/2020 at 7:33 AM, Kiwing said:

It is a shock wave, below a certain height it produces lift and above a certain height it produces down force.  Wouldn't it be good to get dead right?

Not when to get it "dead" right you compromise all the other things that are really driving the show. That is a classic case of being Penny Wise and Dollar Dumb.....

There is talk elsewhere about ratios of TWS to outright Boat speed - some claiming as high as 3.6 or 3.7 multiples of windspeed translating to velocity. So why would they chase so hard to gain a Ground effect of Lift when you are almost immediately trying to reduce global lift becuase to reduces overall RM as discussed immediately above?

On 12/3/2020 at 7:32 AM, Stingray~ said:

Intuition suggests that if you want to end-plate the rig then the last thing you’d want is to intentionally suck any of the windward pressure towards under the hull where it can escape to the lee. I think of it a lot like deck skirting. 
 

My intuition also thinks that the AM hull, the most ‘soap-shaped’ is the slippierest one aero-wise. 

Simply put, they are chasing the best aero solution - highest rig efficiency requires low flight height to reflection plane and preservation of trying to separate high pressure from low pressure - whether that be undre the Hull or across the deck - that concept is not disputed and widely accepted. So why chase Ground effect that will compromise those keenly sought effects? 

With respect to AM - yes they are slippery - but so are all the boats - but I believe their hull/skeg camber profile is still not as extreme as say TNZ or Inoes - its as if the designers have said - well touchdowns and take off still need consideration so we will go less extreme. The others have gone "all-in" to use a poker analogy......

I like the deck treatment used by TNZ to both encapsulate the crew and provide a great piece of wind tunneling for the sails to sit within.  The anorexic skinniness that the whole hull provides for it to slice the air must count for something. The others are slippery too, but are more towards the slim, natural or plump status depending on where your allegiances or favouritism sits. It even has the protruding Hip Bones that Skinny girls display in their Bikini Bridges..... Look it up (Discreetly if you are at work).

Just for the Record, I dont have carnal desires for any of these Boats.........

My Heart remains firmly with Scarlett and Miranda! :wub:

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On 12/12/2020 at 2:57 PM, Man Overboard said:

My calculations indicate that ground effect on a water-based vessel would quite probably suck the sea out of the ocean so I don't think we should try that.

Think of the poor fish.

 

Are your calculations also from the same mind that drew the sailboat on your Avatar???

Asking for a Confused Friend...... ;)

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Flattest bottom wins the cup.
This was the finish between the two contenders.
2118795505_ETNzAmericanMagicfirstracefinish.thumb.PNG.48568b1bc60311f44facf9d4a9f80c72.PNG
This is ETNZ bottom:
DBEC744D-0B34-464E-8944-E3C3C1BCB555.thumb.jpeg.2e46bd7a33104e5f31f5748f1179a22e.jpeg.6aa329178e153077d4d33f7c73c45b39.jpeg

This is American Magic:
1422119502_americanMagicflatbottom.thumb.PNG.30b0c65fef5843ef8c94eff7bba3a1dd.PNG
Sliced and diced on a very simple design feature.
I am getting too old for this..
Airbus designing the american boat. This is shitty.

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