America’s Cup - ETNZ launches 1st AC75 (new)

[CassidyNZ]

Originally Posted by MicHughV

We can go round and round on this issue forever.

My contention is that these foils and rudder throw off considerable spray, denoting that they are not aligned with the forward motion of the boat.

Whilst not claiming to be skilled in hydrodynamics or a structural engineer, a purely practical (perhaps naive) understanding of the alignment of the foil with the boat centreline would have me believe that if misaligned, the foil, which has a very much larger cross section than the rudder, would overpower the rudder’s ability to keep the boat on track. The rudder would have to work very hard to counter this and consequently would also generate a lot of spray which it doesn’t appear to.

When you say that the foil is not aligned with the forward motion, are you suggesting that the rudder and the foil are in competition regarding forward direction? I can’t believe that millions of $$$ of design, research and computer simulation would miss something that fundamental.

[a64pilot]

No. I think he is correct and that the designers didn’t miss anything at all.
There is of course a sideways force applied to any boat under sail, the keel of course resists this force in a normal boat.
The foil itself and or the support leg must function as the keel, there isn’t anything else that can.

From a post earlier, these boats may have more in common with ice boats than our sail boats?

[CassidyNZ]

Originally Posted by a64pilot

No. I think he is correct and that the designers didn’t miss anything at all.
There is of course a sideways force applied to any boat under sail, the keel of course resists this force in a normal boat.

What about the average cruising cat? Anything misaligned underwater to keep it on track? Don’t think so.

Originally Posted by a64pilot

The foil itself and or the support leg must function as the keel, there isn’t anything else that can.

From a post earlier, these boats may have more in common with ice boats than our sail boats?

I would respectfully suggest that a foil arm that is probably a metre or more wide would have way more resistance to leeway at 40kn than my boat’s keel at 7kn.

No, I can’t believe that designers would build in speed-sapping conflict in the underwater elements. But, as I say, uninformed rhetoric, nothing more.

[a64pilot]

Originally Posted by CassidyNZ

What about the average cruising cat? Anything misaligned underwater to keep it on track? Don’t think so.







I would respectfully suggest that a foil arm that is probably a metre or more wide would have way more resistance to leeway at 40kn than my boat’s keel at 7kn.

I don’t believe you understand what I am saying, any boat under sail that is say on a beam reach or upwind, the boat in order to maintain a course is “crabbing” to windward, the whole boat is misaligned, if it’s not then to use a helicopter term, it translates to windward. Tail rotor thrust is very similar to the force from a sail, they both push the vehicle sideways.
Whether this angle is built in or not is irrelevant, you generate it by steerage.
Boats keels generate lift based on angle of attack, that is the boat has to be pointed into the wind and not directly in the direction of travel to generate the angle of attack required to generate lift.
It’s not about resisting leeway, it’s about generating lift to cancel out leeway.
This is due to any keel that has an airfoil shape is a fully symmetrical, airfoil or you would have to have two keels, one you used when the wind was to port and the other when the wind was to starboard.

Guess what these boats have? Yep two keels, one they use for wind to port and the other when it’s to starboard, this enables them to have asymmetrical airfoils that can generate significant lift with no apparent angle of attack, but because your generating lift, your going to have spray as if the airfoil wasn’t aligned with the direction of travel, whether it is or not.
Now assuming they are asymmetrical foils and I’d bet lunch they are, but if they are with both down the lift from one is cancelled by the other and you just have excess drag.

Now assume if you will that these foil keels could generate enough lift to cancel out the translating tendency, and I think they could maybe, but only at one sweet speed, above and they have excess lift, below that and not enough.
Enter high lift devices, like flaps and slats, but I don’t know if they are allowed, probably I’d guess not, I’m talking adjustable surfaces that control lift and of course drag.
But someone said they were allowed to have several fairings, which would I assume be tailored for different speeds, so you pick a fairing that gives enough lift to cancel out the translating tendency at the speed you expect to be racing today.

Now all this is just an assumption of course, and could be way off, sometimes 2+2 isn’t 4.

[MicHughV]

...Yes, A64 Pilot, this would be easier to explain with a pen and sketch pad in hand....but really what is the point ??

I have since read (and seen) that the different teams have quite different views on the matter of what makes an AC boat tick, never mind the views and assumptions of people on this chat forum.

One needs only to look at the different hull designs as well as the different foil designs as well as different sail setups to see that each team have opted to implement what they think is best on different elements of design. Notably, the American team has been working with Airbus, who undoubtably have access to some serious airfoil computing power or other testing means.
It is thus obvious, that even the very smart people involved with these boats have conflicting and different opinions on the matter as to what will produce the winning boat.

I would like to think that these forums present an opportunity to present different points of view and not sink into some abyss of verbal conflict.

I have even seen a video of one of these AC boats laying on it's side, so even the crew sailing these boats have to " figure it all out".....it would be lovely to know what transpired there, but the crews have several months to learn to learn the faults and foibles of these craft and it is not likely that they will share with us until the last mark is passed.

[MicHughV]

...from my understanding, the modern day cruising catamaran, pretty much (motor) sails on both hulls all the time, therefore they have symmetrical hulls. Additionally, it is well known that the modern day catamaran caters to interior cabin volume so as to have 4 bathrooms, rather than sailing prowess and goes to windward more or less like a brick.
... a beach hobie cat, typically sails with one hull out of the water, so they have asymmetrical hulls.
...not quite sure what catamarans have to do with the AC 75 boats, but....like the AC boats there are numerous different cat designs out there, with each designer showcasing what he/her thinks is best solution to a cat design.

[cal40john]

Originally Posted by MicHughV

...from my understanding, the modern day cruising catamaran, pretty much (motor) sails on both hulls all the time, therefore they have symmetrical hulls. Additionally, it is well known that the modern day catamaran caters to interior cabin volume so as to have 4 bathrooms, rather than sailing prowess and goes to windward more or less like a brick.
... a beach hobie cat, typically sails with one hull out of the water, so they have asymmetrical hulls.
...not quite sure what catamarans have to do with the AC 75 boats, but....like the AC boats there are numerous different cat designs out there, with each designer showcasing what he/her thinks is best solution to a cat design.

Yes the 1970 designed by a surfboard maker Hobie 16, which is designed to run up a beach so he didn't want daggerboards is asymmetrical.
I can only think of the Prindle 16 and Prindle 18-1 that are the same. Most every performance cat out there has symmetrical hulls.
Some have asymmetrical daggerboards though, so you have to lift the weather board up.
Some have toe in. (The E-scow with bilge boards have toe in as well.) This is for the board to provide lift before the hull has to slide sideways, so the hull is not making leeway.

Seems like the AC75 should have asym foils or toe in or both, but since the hull is out of the water the additional drag due to leeway would be tiny compared to a boat in the water.

[jmh2002]

Originally Posted by MicHughV

I have even seen a video of one of these AC boats laying on it's side, so even the crew sailing these boats have to " figure it all out".....it would be lovely to know what transpired there, but the crews have several months to learn to learn the faults and foibles of these craft and it is not likely that they will share with us until the last mark is passed.

Please share a link to this video because I am completely unaware of this.

Or maybe this is again slightly incorrect and you are referring to American Magic's half scale test boat, not an AC75?

If yes, you can see the capsize here just after the 3min mark. This happened months ago during summer testing.

https://www.youtube.com/watch?v=eTVKkv9xtAo

In any case, yes that type of incident will be possible with the AC75 too. The boats are not form stable and even a simple error with the deploying the foil arms incorrectly may cause this.

A failure at high speed will be much more serious.

These are race boats at the very edge of what is currently possible. They are not designed to be forgiving. In effect they are a big fast dinghy handling wise.

That being said they are meant to be designed to be self righting. Let's see how that works out in reality.

I expect the Teams to actually capsize the boat on purpose at some stage and practice the recovery technique. Maybe once they have their second boats launched?

[jmh2002]

Here is a new article and photos commenting on the different design approaches from the Teams.

We have already covered most of the points here earlier in the thread, apart from the rule change regarding hull modifications:



https://www.sail-world.com/news/223235/?source=rss

Originally Posted by www.sail-world.com

The four AC75's yachts now launched fall into two categories - the Skiffs and Scows.

The Italian Luna Rossa Pirelli have, along with with Emirates Team New Zealand gone for the skiff option, with both using a skeg in the Italian AC75, and a bustle with the Kiwi Defender to provide some low drag buoyancy to help get the AC75 foiling and then soften the touchdown.

As has been seen on the Kiwi boat the reverse bow and increased hull volume performs a vital function in a breeze in a splashdown situation, where the bow kicks in and lifts the boat back onto her foils without too much impact on speed.

We haven't seen how the scow approach reacts in this situation, and whether the flat bottomed hull is as forgiving as the skiff... The scow designs would be expected to have an edge in the aerodynamic stakes, which results in lower aero drag.

Indeed we are waiting to see this because so far the Scow seems very high drag during any transition modes. We won't know if the aero advantage is enough to overcome this until they race each other. I also expect that the relative advantage may go back and forth depending on the exact conditions of the day.

As I mentioned earlier in the thread TNZ seem to have gone for a more 'all round' type hull design, looking to make up any aero disadvantage via their smaller, lower drag foil package which will be faster in more breeze.

In very basic terms, so far it seems:

TNZ = higher drag hull, lower drag foils
AM = lower drag hull, higher drag foils

Originally Posted by www.sail-world.com

A rule change allowing the teams to alter both AC75's by 12.5% of the hull surface area could well be significant if changes are to be made after Cagliari ACWS... the old rule allowed a 25% hull surface area change to the first boat only. But even that degree of permitted change is not sufficient to turn a skiff into a scow, or vice versa. Now it is split evenly between the two boats built.

The conundrum for all teams is whether their second AC75 is a Skiff or a Scow - and that is a decision that can now only be made in the simulator.

So it's possible that we may see some hull tweaks.

And it's also possible that Hull #1 may be further modified based on Hull #2's performance, with Hull #1 therefore becoming the faster evolution and the final boat to be raced in the Cup.

[MicHughV]

yes, jmh2002, you are correct, it was the half scale model.

In some past time, I was invited to do some tank testing, but this required a tank, which off course we did not have, so we had to build one.
The tank was about 75' long x 10' wide x 6' deep. At one end were fitted some turbine pumps which in turn were fitted with propriety flow straighteners, velocity stacks, etc, to ensure water was being drawn into the turbines across the entire cross-sectional area of the tank. This was pumped to the far end of the tank, where it first had to go thru' a stilling well to quiet the turbulence from the pumps and again thru' a variety of flow straighteners to ensure a uniform flow of water along the long section of the tank.
The speed of the turbines could be infinitely adjusted to modulate the flow velocity.
Every corner, every joint, every curve, etc, in the tank were given due consideration to ensure a uniform flow of water.
The tank was additionally fitted with viewing windows so one could view any experiment from the bottom.
The tank was made from concrete but was covered in a propriety teflon type paint to ensure smooth flow.
Along the length of the tank were fitted a number of testing equipment, gauges, etc.
Typically, a team of Ph.D's would arrive with a variety of things they wanted tested, included in these were a variety of foil and other shapes.
Ironically, the Ph.D's freely admitted that the tests were " simulations" and that real life results would likely differ, but real life experimentation would be unrealistic and cost prohibitive to conduct, so the simulations gave a starting point.

This brings me back to the AC75 boats. At present time, teams are conducting " field tests" to validate their computer simulations. There is no telling what is going on here as teams are unlikely to divulge anything.

Interestingly, there are some smart posters here on this forum and it's fun to read all the different thoughts, opinions, facts, ideas, etc.

I, myself, have my own opinion based on my own experiences and it will be interesting to see if any of these will hold water as testing continues with these AC75 boats.

[jmh2002]

Originally Posted by MicHughV

This brings me back to the AC75 boats. At present time, teams are conducting " field tests" to validate their computer simulations. There is no telling what is going on here as teams are unlikely to divulge anything.

Agreed. Although like all technology the simulator has become very advanced now too. TNZ seem confident and rely heavily on this, but it's also budget related - TNZ don't have as much money, and the simulator is still cheaper than multiple test boats.

I'm not sure if you picked up on comments from the design team that the most difficult aspect to simulate is the Code Zero and the related performance.

This is because this sail will cause the boat to operate over a vast range.

It's a very light air sail, so generally this would be non foiling conditions. But the Code Zero generates so much power that it then causes the boat to foil, which then causes a huge jump in boat speed AND apparent wind speed.

And all of a sudden you are now foiling and going fast in an apparent wind speed that the Code Zero wasn't designed for.

That appears to be a complicated balancing act.

TNZ already broke some part of the tack fitting while testing the Code Zero.

I'm sure we will see more breakages with these new boats until they validate their baseline numbers.

[admiralslater]

Originally Posted by cal40john

Yes the 1970 designed by a surfboard maker Hobie 16, which is designed to run up a beach so he didn't want daggerboards is asymmetrical.
I can only think of the Prindle 16 and Prindle 18-1 that are the same. Most every performance cat out there has symmetrical hulls.
Some have asymmetrical daggerboards though, so you have to lift the weather board up.
Some have toe in. (The E-scow with bilge boards have toe in as well.) This is for the board to provide lift before the hull has to slide sideways, so the hull is not making leeway.

Seems like the AC75 should have asym foils or toe in or both, but since the hull is out of the water the additional drag due to leeway would be tiny compared to a boat in the water.

Not a cat but Newick tris had asymmetrical out riggers

[MicHughV]

Yes, much has been opined about the hulls and foils, but little about the sails.

Again, these boats appear to be mostly crewed (safely ensconced in their respective holes) from the stern due to the inherent speed of these craft, so sail management will likely be challenging, as I don't see crew running around the deck to hoist, lower or stow a sail.

These boats all sport a sprit of sorts extending out from the bow, so I imagine any screw up here will be difficult to address.

I'm confident that we will see these boats sail on both foils at times during the race for any number of reasons.

There was a time when I took an interest in radio controlled sailboats. These latest iterations now sport a spinnaker, all controlled by r/c. the spinnaker is pulled out of a tube near the bow and also drawn back into this tube when not in use. I imagine we will see some ingenious methods to control a code zero on the AC 75 boats.

Wally boats also revolve around a single person being able to control all the sails from his station behind the wheel.

Lastly, I'm interested to see what kind of course they will lay for the race. For one, I would imagine the course will have long legs considering the speed of these boats.

We'll see.

[jmh2002]

Let's not get too far off base again, since a lot of these points are already defined. They are not making it up as they go along.

Course options are available already, and in fact there was a diagram in the previous link that I posted too:



Code Zeros are free hoisted, no launch tubes, etc.

Here are some images of the Code Zero tack failure on TNZ:

https://www.sail-world.com/news/2228...the-first-time

[jmh2002]

Originally Posted by MicHughV

I'm confident that we will see these boats sail on both foils at times during the race for any number of reasons.

TNZ agrees with you in general (and with what I posted very early in this thread), but since the 2nd (windward) foil causes higher drag and lower righting moment when immersed, instead they have specifically designed their hull with a centre line canoe body, the 'bustle' as some call it, for this very purpose.

http://www.cruisersforum.com/forums/...ml#post2991661

Here is a screenshot of TNZ coming down off the foils and sailing in (what I am calling) semi-displacement mode. Some are calling this a 'virtual catamaran' - on one foil + the narrow centre line canoe body. Video of this at the previous link.



The Scow designs from American Magic and Team GB can't do this with their very flat bottoms, or if they do it appears that their immersed wetted surface will much greater in this mode.

It seems clear that they have not planned to do this in general, either in light air, or during a 'touch and go' in more breeze.

TNZ on the other hand have, very specifically, planned to do exactly this, and they were already doing it on day 1 of testing.

There really are two totally different approaches here design wise.