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jmh2002 · 18-09-2019, 04:33

Quote:

Originally Posted by CassidyNZ

The deeper crew wells on ETNZ’s boat not only reduces aerodynamic drag but also reduces fatigue. Sitting in 50kn of wind will get quite tiring quite quickly.

Yes agreed.

Quote:

Originally Posted by CassidyNZ

In a video online today the bulge under the boat started to show perhaps why it’s there - the boat sank on its foils momentarily and the bulge kissed off the surface and went straight back up again.

Ah, so almost exactly how I described earlier? Can you link or tell me where the video is please, I haven't caught up with the latest yet.

Quote:

Originally Posted by CassidyNZ

Interesting “pocket” in the leach of the mainsail. Wonder what that’s all about? Looks like it goes quite a long way up.

Do you know that it's a 'double skin' mainsail? So physically two separate sails that can move independently to each other.

CassidyNZ · 18-09-2019, 04:36

Quote:

Originally Posted by jmh2002

Yes agreed.

Ah, so almost exactly how I described earlier? Can you link or tell me where the video is please, I haven't caught up with the latest yet.

Do you know that it's a 'double skin' mainsail? So physically two separate sails that can move independently to each other.

I think this link may work.

https://www.stuff.co.nz/sport/americ...sting-new-boat

Sorry, checked that video, it’s not the correct one. I’ll try and find the better one

And thanks, didn’t know about the double skin main. Curious.

jmh2002 · 18-09-2019, 04:51

The double skin mainsail is being used to try and mimic some of the benefits of previous wingsails but without their disadvantages, such as needing to step and unstep the wing each day with a crane, as well their fragility in the event of a crash or capsize.

This change has also brought sail makers and designers back into the America's Cup.

**a64pilot** · 18-09-2019, 06:59

Just from appearances it would seem the US boat has put more design emphasis into the foils, I assume their thought is to not go hull down into the water, whereas the NZ boats seems to address the possibility. personally I think having a release step is smart, cause surely there will be times when you go hull down?

The main sail using the hull as an end plate is interesting, because an end plate is rather effective, it’s not an insignificant effect.

However one would assume that with the aspect ratio that they are using and the wingspan that a boom that came close to the hull would also use the hull as an end plate.
Double surfaced airfoils are far more effective, I assume abandoning them was a rules change?
From an aerodynamics perspective they are completely different designs, if they had the same sails, I think they would be faster than the Cats, would seem to be much less drag.

I would also assume the US boat would be faster in light winds, and the NZ faster in higher winds, but that is a guess, cause it’s based on guesses on design.

jmh2002 · 18-09-2019, 07:42

Quote:

Originally Posted by a64pilot

Just from appearances it would seem the US boat has put more design emphasis into the foils.

Yes, although complex looking is not always better in this regard. TNZs foils were a leap ahead in Bermuda, being able to be carried across a broader wind range than the competition even though they still had the same simple appearance.

Quote:

Originally Posted by a64pilot

I assume their thought is to not go hull down into the water, whereas the NZ boats seems to address the possibility. personally I think having a release step is smart, cause surely there will be times when you go hull down?.

I agree with you, I think it is smart, and I'm looking forward to seeing the video of the 'touch and go' from TNZ that CassidyNZ mentioned earlier, because the video of American Magic touching down and 'parking' from the huge hull drag was awful and possibly a sign of things to come.

Quote:

Originally Posted by a64pilot

The main sail using the hull as an end plate is interesting, because an end plate is rather effective, it’s not an insignificant effect.

However one would assume that with the aspect ratio that they are using and the wingspan that a boom that came close to the hull would also use the hull as an end plate. Double surfaced airfoils are far more effective, I assume abandoning them was a rules change? From an aerodynamics perspective they are completely different designs, if they had the same sails, I think they would be faster than the Cats, would seem to be much less drag.

Yes some end plate effect must also be created even with a boom, and you can see that American Magic still has the 'cockpit hump' even with their boom mainsail.

But TNZ seems have gone 'all out and one step further' in general with the aero related effects. And their skipper Glenn Ashby (not helmsman, in fact he was the wing trimmer in Bermuda) has been doing a lot of sailing on A-Class cats which have a similar boomless end plate mainsail design.

Yes the class rules have been completely rewritten together with the new boat design and this double skin mainsail is part of that for the reasons that I mentioned earlier. So this is not a TNZ innovation, every team will have this, but there is some allowed scope for design differences.

The AC75 monohull is already predicted to be faster than the AC50 catamaran regardless.

Quote:

Originally Posted by a64pilot

I would also assume the US boat would be faster in light winds, and the NZ faster in higher winds, but that is a guess, cause it’s based on guesses on design.

Well, as I mentioned in some earlier posts when American Magic was launched I certainly think that their foils are designed for more lift in light air, but whether that translates to them being faster then I'm not sure.

AM: Bigger foils, more lift, but more drag = slower
TNZ: Smaller foils, less lift, but less drag = faster

But with less lift TNZ still needs to be able to get up onto the foils in the first place, and this is maybe where the centerline bulge / canoe body / release step may also come into play.

This may enable smaller lower lift foils to be used that lower drag and faster once flying. At least that is my thinking, especially since TNZ seem very focused on a low windage low drag design overall.

What I'm not sure about if it is still allowed to change foils for race days. In Bermuda the AC 50s had both heavy air and light air foils to choose from. I need to check this.

barnakiel · 18-09-2019, 08:05

Quote:

Originally Posted by CassidyNZ

I think this link may work.

https://www.stuff.co.nz/sport/americ...sting-new-boat

Sorry, checked that video, it’s not the correct one. I’ll try and find the better one

And thanks, didn’t know about the double skin main. Curious.

Amazing.

You will see how the main reaches the deck. If I remember well, NYYC main does not sweep.

It shows they will be fighting for every .001 of a knot in the development.

GOOD.

b.

jmh2002 · 18-09-2019, 08:09

Yep, see the pics on page 4 for a side by side comparison.

cj88 · 18-09-2019, 21:15

A short video from today.

https://www.stuff.co.nz/sport/americ...t-counterpunch

jmh2002 · 18-09-2019, 21:47

THANKS!

CassidyNZ · 18-09-2019, 22:37

Quote:

Originally Posted by jmh2002

I agree with you, I think it is smart, and I'm looking forward to seeing the video of the 'touch and go' from TNZ that CassidyNZ mentioned earlier, because the video of American Magic touching down and 'parking' from the huge hull drag was awful and possibly a sign of things to come.

Sorry, can’t find the video I watched - maybe it’s been pulled off the net. The one posted by CJ88 is next best at this time. Seems that the “belly” of the boat touching doesn’t slow the boat much.

jmh2002 · 18-09-2019, 23:27

I have more video on my news feed this morning and I'll post it a little later

jmh2002 · 19-09-2019, 01:45

Here is the slightly better video of TNZ showing the centreline canoe body in action and the boat actually sailing on this for an extended period.

Having seen this I'm going to say that I think that American Magic could be in trouble if the breeze is light. That looks like a very fast mode on TNZ.

But there are much greater minds on the American Magic design team than me of course...

So very interesting

And some comment from Sail World:

https://www.sail-world.com/news/222183/?source=rss

Quote:

Originally Posted by www.sail-world.com

The video below reveals the answers to a couple of key questions for the team. The first is whether the bustle which runs down the centreline of the AC75 would come into play as the boat at minimum foiling speeds. The video tends to indicate that the bustle certainly plays its intended role, creating a virtual catamaran around the bustle and the leeward foil, and keeping the windward foil clear of the water for as long as possible.

Many of the video and still images of the day show the AC75 being sailed in the same mode, or an even more extreme stance than the AC50 - bow down and heeled slightly to windward creating a lifting force on the AC75 in a similar way to a windsurfer. The fact that the crew were able to maintain this stance in a sustained way, points to a close correlation between what the team are seeing on their practice simulator and what is happening, in the real-world, on the water.

... Te Aihe appears to be sailed in a more heeled to windward bow-down stance, while Defiant is more upright, lending support to the theory that her scow-like design is predicated around maximising ground effect - allowing the AC75 to sail on a cushion of air trapped and compressed below her wide flat hull.

https://www.youtube.com/watch?v=Kw5xyAxgrL8

jmh2002 · 20-09-2019, 00:47

A little more video here:

https://www.youtube.com/watch?v=elSDgQKvc7I

jmh2002 · 25-09-2019, 03:04

Test sail update article and photos (but no video) here:

"Emirates Team New Zealand started the third week since her launch with a training session in conditions that were reported at both the top and bottom end of the course as being 18-20kts, gusting over 25kts."

https://www.sail-world.com/news/222352/?source=rss