Southern Cross

Hull / Deck Geometry

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Would love to be fly on the wall in their respective design offices, when they discuss the significance of the differences.

And even they would only have calculated guesses at this point.

But it's part of the fun here, so go for it!

 

 

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I think ETNZ design with both hull and sails has gone all out on achieving early liftoff.  American Magic a bit more traditional except for that bow.

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AM to my reasoning to date is the direction I would take (start with) ~ the “corrugated hull form” of the NZ design is not a direction I would go.

I think there is a large difference in the RM of the two forms below 7 to 8 degrees heel  (approx) ~ AM with a greater RM.

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12 hours ago, Southern Cross said:

AM to my reasoning to date is the direction I would take (start with) ~ the “corrugated hull form” of the NZ design is not a direction I would go.

I think there is a large difference in the RM of the two forms below 7 to 8 degrees heel  (approx) ~ AM with a greater RM.

But is that very relevant when all RM moves to leveraging off the leeward foil as soon as you hit a few knots of boat speed?  Any time there is enough breeze for that RM to really matter, surely in very short order your going to be up to a speed where the foil starts to operate, then it becomes all about getting the boat out of the water as fast as possible.

One things for sure, initial acceleration and liftoff characteristics are going to be different for the two hull forms, they are just too different for there not to be some unique characteristics of each.

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I suspect that being in the right breeze will be more key than hull design in deciding who climbs up onto the the foils the fastest, and that most of the racing after that will be won by who has the power and the best foils will set who’s the fastest once flying. Hydro for the foils, aero for everything else, but hydro the biggest factor by far. The wings are actually where the real sex appeal happens..

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13 hours ago, Boybland said:

I think ETNZ design with both hull and sails has gone all out on achieving early liftoff. 

Perhaps but how ? if you sailed boats or saiboards, which are planing and lifting first, the rounded hull or the flat ones ?

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13 hours ago, Southern Cross said:

I think there is a large difference in the RM of the two forms below 7 to 8 degrees heel  (approx) ~ AM with a greater RM.

Yes, flat hull, greater RM, faster planing, more drag in waves.

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13 minutes ago, Tornado-Cat said:

Yes, flat hull, greater RM, faster planing, more drag in waves.

Yes ~ Simplistically :  in certain conditions a bit  like comparing Wild Oates to Comanche, I would lean towards taking as much stability for the current AC class.

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46 minutes ago, Tornado-Cat said:

Perhaps but how ? if you sailed boats or saiboards, which are planing and lifting first, the rounded hull or the flat ones ?

From windsurfing, in general you would say a flat planing hull, however these do also need significant power to get started, when your lifting using a foil and that foil is placed such that leverage from the rig is lifting the hull I'm not so certain the same logic applies.  The moth which utilises a little bit of this logic appears a bit of a bet both ways, it's very narrow to get it started, but then it has a flat planing surface on the bottom (although with any heel your immediately riding on a V anyway).

The other aspect is that ETNZ is not really a rounded hull, it's a stepped hull that looks like it will be riding more on a flatish slightly concave section in normal mode, once it flattens out as the foil starts to lift it will then be riding on the narrow centre canoe which should lift out pretty easily.  I feel like American Magic on the other hand will get increasing amounts of hull dropping back into the water (albeit a flat planing surface probably) as she flattens out from the leeward foil lifting.

Alot probably comes down to the speeds at which things happen, does rotation and lifting of the hull from leverage occur at a lower speed than planing for example, or do the boats need to be planing before there is any significant lift supporting the hull weight.

The interesing part is two different computers came up with pretty different solutions to the problem!

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Also once up AM will be able to get lower before the hull touches.

But TNZ hull will get less drag from a light touch.

 

On deck AM bow is cleaner aero but TNZ has cleaner cockpit/stern and a good main endplate.

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2 hours ago, Boybland said:

From windsurfing, in general you would say a flat planing hull, however these do also need significant power to get started, when your lifting using a foil and that foil is placed such that leverage from the rig is lifting the hull I'm not so certain the same logic applies.  The moth which utilises a little bit of this logic appears a bit of a bet both ways, it's very narrow to get it started, but then it has a flat planing surface on the bottom (although with any heel your immediately riding on a V anyway).

The other aspect is that ETNZ is not really a rounded hull, it's a stepped hull that looks like it will be riding more on a flatish slightly concave section in normal mode, once it flattens out as the foil starts to lift it will then be riding on the narrow centre canoe which should lift out pretty easily.  I feel like American Magic on the other hand will get increasing amounts of hull dropping back into the water (albeit a flat planing surface probably) as she flattens out from the leeward foil lifting.

Alot probably comes down to the speeds at which things happen, does rotation and lifting of the hull from leverage occur at a lower speed than planing for example, or do the boats need to be planing before there is any significant lift supporting the hull weight.

The interesing part is two different computers came up with pretty different solutions to the problem!

Interesting, it's true that the first 2/3 of the hull has a rounded "lump" while the stern is flat in the center. It is also true that before flying they will be foil assist and they may have figured that the boat does not need all the width to plane.

OTOH all the moths I seen (but I don't sail one) have flat hull like this one.

They could also try to copy the seaplane floats, but again their floats are, unlike this boat, completely flat at the end, and the front part of the float is wider than the rear part, which not the case with this boat.

Agreed that two good design teams came to such different solutions.

oligario1.jpg

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2 hours ago, hoom said:

Also once up AM will be able to get lower before the hull touches.

But TNZ hull will get less drag from a light touch.

On deck AM bow is cleaner aero but TNZ has cleaner cockpit/stern and a good main endplate.

One explanation I can find to TNZ hull is not that they want to fly sooner but they expect their boat to fly so low (for better RM) that the hull will skim the waves from time to time. That said it's also true that, as you say, AM can foil lower without touching the water.

I repeat myself but the TNZ boat will be much less stable when touching the water. Anyway, let's see how it behaves when sailing now.

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Remember, one of those design teams has already sailed a foiling mono hull for several months and is quite familiar with the instabilities when the hull is in the water.  

If I had a sailboat that was prone to capsizing when traveling at low speeds in the water, I sure wouldn't have made it with a tipsy hull.  

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9 minutes ago, Herfy said:

Remember, one of those design teams has already sailed a foiling mono hull for several months and is quite familiar with the instabilities when the hull is in the water.  

If I had a sailboat that was prone to capsizing when traveling at low speeds in the water, I sure wouldn't have made it with a tipsy hull.  

Agreed. 

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I know scale comes into play here, but we need to remember that the Brains Trust on the ETNZ boat spent and still spend their non AC sailing time on very unstable and/or foiling dinghy's. If any team can sail a boat that's a bit less stable than your average 5 knot shitbox it's them.

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7 hours ago, Stingray~ said:

I suspect that being in the right breeze will be more key than hull design in deciding who climbs up onto the the foils the fastest, and that most of the racing after that will be won by who has the power and the best foils will set who’s the fastest once flying. Hydro for the foils, aero for everything else, but hydro the biggest factor by far. The wings are actually where the real sex appeal happens..

Quite right, Stinger. Likely be irrelevant whether the boats can go from displacement to flying quickly. The AC50's entered the start box on the fly and virtually stayed that way the whole way around the course. Probably be the same for the AC75, no?

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7 hours ago, hoom said:

Also once up AM will be able to get lower before the hull touches.

But TNZ hull will get less drag from a light touch.

 

On deck AM bow is cleaner aero but TNZ has cleaner cockpit/stern and a good main endplate.

Agree with these thoughts - the light touch effect may be critical in keeping or achieving a 95% airbourne lap or race. The AM boat looks like it will plane faster, but looks sticky should she touch down. 

The only thing I can't fathom is why the NZ Hull has the upper indents - is there a minimum hull beam that they have both built to for the sheerline in measurement mode? I understand that the upper indents allow further rotation by the arms before contact - but my puzzlement is why the hull edge extends to that point in the first place.

If you consider that the hinge points are are set by class rules to a 2mm tolerance - see attached outline of rules.  As is the ram connection points - then both boats have identical geometry in this respect. But the NZ hull has indents, where as AM does not need such sculpting as the sheerline does not put the hull face in conflict with the foil arm. 

So my puzzlement is about why the NZ hull even extends to this point in the first place. Is it making the foredeck wider and flatter from an aero endplate consideration? does it give the hull the buoyancy that would be required from a capsize recovery position - nobody is perfect, and capsizes will happen..... Or does the hull just need to be this wide to measure against the rule?

Because if none of these requirements are stipulated, then the hull could be built phsically smaller, with ultimately less cross section and aero drag and result in a more concentrated COG - I have also wondered whether the Cut away Hull Deck joint - up front - could not have been done more aggressively. Its a purposeful looking boat - but actally quite conservative in some regards.

Having said all that - if they have developed a Deck Sweeping Mainsail then they have definitely one upped AM at this stage......

slwfa18_stl_technology-ferguson-2.jpg

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4 hours ago, Tornado-Cat said:

One explanation I can find to TNZ hull is not that they want to fly sooner but they expect their boat to fly so low (for better RM) that the hull will skim the waves from time to time. That said it's also true that, as you say, AM can foil lower without touching the water.

I repeat myself but the TNZ boat will be much less stable when touching the water. Anyway, let's see how it behaves when sailing now.

They were the best in the business at keeping the boat airborne last time. It’s a key measure of winning the big dance.

Keeping part of the hull very close to the water should create some ground effect, further improving stability.

 

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36 minutes ago, Boink said:

is there a minimum hull beam that they have both built to for the sheerline in measurement mode?

Rule 11.5 a & b

2.4m from Longitudinal Centre Plane.

2.5m max.

and at least 2m on Transom Reference Plane.

 

There is also a 70m3 minimum enclosed volume (rule 11.8) which requires a certain bulkiness.

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5 hours ago, Tornado-Cat said:

 

I repeat myself but the TNZ boat will be much less stable when touching the water. Anyway, let's see how it behaves when sailing now.

Yip. We’ve all got the picture. 

If you’re right you can have all the credit, because you said it first. 

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1 hour ago, hoom said:

Rule 11.5 a & b

2.4m from Longitudinal Centre Plane.

2.5m max.

and at least 2m on Transom Reference Plane.

 

There is also a 70m3 minimum enclosed volume (rule 11.8) which requires a certain bulkiness.

Thanks for that.

I take it that it is the 70m3 that is driving the overall beam then and would also explain the high central deck down the spine through the cockpit - which deepens the side walls of the side of that pod - making a stiff/strong Hull structure, by way of a Box Beam - and also gives a good surface to endplate the Mainsail against - are those crew cockpits allowed to have slide back lids or sail cloth roofs to further smooth airflow?

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44 minutes ago, Boink said:

re those crew cockpits allowed to have slide back lids or sail cloth roofs to further smooth airflow?

Not allowed to cover crew from above.

Quote

12.11 When viewed from above and orthogonal to MWP, neither the crew nor the guest racer shall be covered
at any time by any part of the yacht except the mast, sails and rigging, or occasionally by other components
provided that only a small part of any crew member is covered, and this covering is not designed to provide
aerodynamic fairing of the crew.

There's possibly some leeway for bits where crew aren't but very limited by the caveat that if crew will be partially under it at any point the cover can't be there for aero.

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Thinking twice, the rounded part under the hull could not only be for skimming the waves or "light touches", but they may find that a flat hull hitting a wave at 50 kts could desequilibrate the boat, thus a kind of motor boat hull. 

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2 hours ago, Tornado-Cat said:

Thinking twice, the rounded part under the hull could not only be for skimming the waves or "light touches", but they may find that a flat hull hitting a wave at 50 kts could desequilibrate the boat, thus a kind of motor boat hull. 

Wow, now you are starting to make up words.... desequilibrate

The smaller torpedo hull appendage of the NZ boat could toss the boat around depending upon which radius of the surface hits the water.  A flat surface will tend to flatten the boat when it hits the water.

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36 minutes ago, Herfy said:

Wow, now you are starting to make up words.... desequilibrate

The smaller torpedo hull appendage of the NZ boat could toss the boat around depending upon which radius of the surface hits the water.  A flat surface will tend to flatten the boat when it hits the water.

Interesting point.

The way I remember Brit Ward explaining the Dolphin’s belly on TE’s show is that it’s all about the transition. Its purpose (porpoise?) is to gradually reduce the hull’s surface drag during the transition to lift-out. But yes, there could be trade offs, as you suggest, ~if~ you’re going to be skimming or even crashing all the way down from out of the air.

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1 hour ago, Herfy said:

The smaller torpedo hull appendage of the NZ boat could toss the boat around depending upon which radius of the surface hits the water.  A flat surface will tend to flatten the boat when it hits the water.

As I said before I think that the NZ hull will toss the boat when hitting the water, I have a preference for the AM flat bottom, more stable and will allow the boat to fly lower.

But what do we know ? Dalts and Deano themselves don't admit they don't know yet who will have a better design.

I can't wait to see the other two design and watch boats sailing in stronger conditions.

Don't know why TNZ is so silent since trying their sails, today perhaps ?

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17 minutes ago, Tornado-Cat said:

As I said before I think that the NZ hull will toss the boat when hitting the water, I have a preference for the AM flat bottom, more stable and will allow the boat to fly lower.

But what do we know ? Dalts and Deano themselves don't admit they don't know yet who will have a better design.

I can't wait to see the other two design and watch boats sailing in stronger conditions.

Don't know why TNZ is so silent since trying their sails, today perhaps ?

All the pictures with the main up still had guy at top of rig so possibly it’s not all perfect

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Remember our sailors have a huge input in design and they will have a list of areas the hull would need to perform in.

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21 hours ago, Boybland said:

From windsurfing, in general you would say a flat planing hull, however these do also need significant power to get started, when your lifting using a foil and that foil is placed such that leverage from the rig is lifting the hull I'm not so certain the same logic applies.  The moth which utilises a little bit of this logic appears a bit of a bet both ways, it's very narrow to get it started, but then it has a flat planing surface on the bottom (although with any heel your immediately riding on a V anyway).

The other aspect is that ETNZ is not really a rounded hull, it's a stepped hull that looks like it will be riding more on a flatish slightly concave section in normal mode, once it flattens out as the foil starts to lift it will then be riding on the narrow centre canoe which should lift out pretty easily.  I feel like American Magic on the other hand will get increasing amounts of hull dropping back into the water (albeit a flat planing surface probably) as she flattens out from the leeward foil lifting.

Alot probably comes down to the speeds at which things happen, does rotation and lifting of the hull from leverage occur at a lower speed than planing for example, or do the boats need to be planing before there is any significant lift supporting the hull weight.

The interesing part is two different computers came up with pretty different solutions to the problem!

Yep fully agree.

Moths with time have started foiling well before planing speed, so hulls got rounder and with more rocker, so that when foils start to lift wetted surface decrease more rapidly.. and that sounds closeer to etnz thinking.

The fact that on the 75 more lift equals to more RM makes the whole problem even more interesting to solve. Fun times

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AM seem to have compromised aero for the sake of ergonomics, while ETNZ have compromised ergonomics for the sake of aero. Different philosophies, but I don't see AM sticking with their concept as is for too long. I fully expect it to get more aero while sticking to the more open cockpit design. 

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40 minutes ago, Raptorsailor said:

AM seem to have compromised aero for the sake of ergonomics, while ETNZ have compromised ergonomics for the sake of aero. Different philosophies, but I don't see AM sticking with their concept as is for too long. I fully expect it to get more aero while sticking to the more open cockpit design. 

Am Looks pretty aero in the bow and overall 

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20 minutes ago, hairyharford said:

Am Looks pretty aero in the bow and overall 

I think AM wins out in the fore-body treatment (geometry) compared to NZ.

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10 hours ago, Tornado-Cat said:

As I said before I think that the NZ hull will toss the boat when hitting the water, I have a preference for the AM flat bottom, more stable and will allow the boat to fly lower.

Yip. WE GOT IT!

 

10 hours ago, Tornado-Cat said:

Don't know why TNZ is so silent since trying their sails, today perhaps ?

Because it was forecast to be windy. 

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15 hours ago, 17mika said:

Moths with time have started foiling well before planing speed, so hulls got rounder and with more rocker, so that when foils start to lift wetted surface decrease more rapidly.. and that sounds closeer to etnz thinking.

The fact that on the 75 more lift equals to more RM makes the whole problem even more interesting to solve. Fun times

Do they get foiling with windward heel or horizontal ?

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External sources (not reviewed)

The mouth and dentition as an organ for
 chewing take damage not onlyby desequilibrated overload, but also by lack of exercise 
due to too soft food.
 
 
 
L?organe masticatoire ne s?use pas
 seulement par une surcharge désequilibrée, mais se rabougrit aussi par un manque d?exercice, 
soit de mastication.
 
 
 

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15 hours ago, Tornado-Cat said:

Do they get foiling with windward heel or horizontal ?

Typically a bit of windward heel (not too much) seems to help, i guess because of rig lift.

For the 75 I suspect optimizing wetted surface and righting moment at liftoff would be more important than the sail lift. My bet is that we'll see the 75 flat at all times.

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5 hours ago, 17mika said:

Typically a bit of windward heel (not too much) seems to help, i guess because of rig lift.

For the 75 I suspect optimizing wetted surface and righting moment at liftoff would be more important than the sail lift. My bet is that we'll see the 75 flat at all times.

If you say that it is easier to get foiling at with windward heel then Flipper can do it with its hull and some push of the lw foil,

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6 hours ago, 17mika said:

Typically a bit of windward heel (not too much) seems to help, i guess because of rig lift.

For the 75 I suspect optimizing wetted surface and righting moment at liftoff would be more important than the sail lift. My bet is that we'll see the 75 flat at all times.

My guess is a bit of windward heel when floating, then bear away and accelerate. As the rig heels back upright, the boat lifts off and stabilises flat. The initial windward heel provides some reserve so they can stay overpowered while accelerating without heeling to leeward (which would cause then to back off just when they don’t want too).

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Looking at the new gallery from the other thread

https://farevela.net/2019/09/14/american-magic-le-foto-dei-dettagli-dellac75-americano/

I'm quite astonished at just how different the two boats are and not just slightly different as you would expect, but radically different!

Bows, totally different concepts!

Hull shapes, totally different concepts!

Foils, totally different concepts!

Even the fairing on the one design supplied arms, totally different (man the American Magic ones have long chord in the middle section!)

Mainsails and booms on supplied one design mast, completely different!

 

They could just about be from two different rulesets!

What would be really interesting would be if they got on the water and performance was pretty similar, I suspect that is not the case, there will be crossover points, but at any given windspeed one of them has to be significantly better!

 

Heck and we haven't even seen INEOS of LR yet!  But we know INEOS is probably yet another completely different bow shape!

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Yep, thats why one-design of the key structure >> one-design the exterior surfaces.

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On 9/12/2019 at 11:19 AM, Tornado-Cat said:

Perhaps but how ? if you sailed boats or saiboards, which are planing and lifting first, the rounded hull or the flat ones ?

 

On 9/12/2019 at 12:17 PM, Boybland said:

AA) From windsurfing, in general you would say a flat planing hull, however these do also need significant power to get started, when your lifting using a foil and that foil is placed such that leverage from the rig is lifting the hull I'm not so certain the same logic applies.  <SNIP>

BB) A lot probably comes down to the speeds at which things happen, does rotation and lifting of the hull from leverage occur at a lower speed than planing ... or do the boats need to be planing before there is any significant lift supporting the hull weight.

<SNIP>

AA) Yes, and

BB) yes (but it may depend on TWS.

Becoz twist vs drag ...)

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