America's Cup

[Frogmogman]

Interesting start to the Americas Cup today. The two teams look well balanced. It should be an intriguing contest.

 

[Bobc]

The Kiwis are quicker downwind. That is already obvious. Not much between the upwind though.

Pete's never been that great on the starts, and if they lose it, that's where it'll be.

I reckon Jimmy is already rattled. Expect to see him getting increasingly aggressive, especially at the starts.

 

[TLouth7]

I might be talking out my arse, especially given the two winning times, but I thought the Italians looked a bit more comfortable out there. We saw a couple of unforced errors by the Kiwis, they were generally shy of 'match racing' situations, and the Italian manoeuvres seemed slicker.

If boatspeeds are pretty evenly matched across a range of windspeeds (yet to be seen) then having 1 or 2% better speed will count for less than winning the starts and capitalising on opportunities to pounce.

 

[Bobc]

The Kiwi boat is faster, that is now clear.

When the Italians win the start the Kiwis can pretty-much hold their gap, whereas when the Kiwis win the start, they do a horizon job on the Italians.

It's all about the starts though in the light and stable stuff, so hopefully we'll get to see a few races in shifty-puffy stuff and in some breeze.

My money's still on the Kiwis.

 

[bedouin]

Yes - in the conditions we've seen so far the Kiwis have had the edge on boat speed. They just don't seem to be quite match-fit (especially in the starts) so have allowed LR to get the jump on them.

A bit of a shame that there have been no passes so the race is effectively decided by 1 min after the start.

If the winds get lighter then the larger foils on LR may give them a big advantage

I think TNZ will win 7-5

 

[Bobc]

I think the forecast is for light winds for the next day or two, but then we get more breeze, so come Sunday/Monday, things could change.

 

[bedouin]

I think the forecast is for light winds for the next day or two, but then we get more breeze, so come Sunday/Monday, things could change.

But the impressive thing in the last race was that TNZ sailed away from LR in conditions that were probably the lightest we have seen when you would have expected LR to have an advantage

 

[Frogmogman]

A bit of a shame that there have been no passes so the race is effectively decided by 1 min after the start.

I read that it is now 95 consecutive legs without one boat passing another (96 if you include the one when the American boat capsized).

 

[Bobc]

But the impressive thing in the last race was that TNZ sailed away from LR in conditions that were probably the lightest we have seen when you would have expected LR to have an advantage

Absolutely. Smaller foils = less drag. Even thought the Italians lost by over a minute, they actually sailed a lot less distance. When the wind picks up, the high/slow mode won't help them at all, and I predict that the Kiwis will drive rings around them.

 

[Nimrod18]

Since December, I’ve been captivated by the event - love them or hate them, the boats are impressive pieces of engineering that take a lot of skill (crew and support) to make perform. It has been fascinating following the learning curve of this discipline, which has brought lots of moments of drama - witness the volume of forum discussion. However, the curve seems to have definitely flattened and, for the first time today, I only watched the start fully before forward tapping in 10 second chunks through the remaining 6 legs. I’m not trying to decry the achievements of the teams, nor am I in the “foiling will never catch on” camp. However, the format (which the teams are rightly doing their best to thrive in / tactically exploit) is starting to generate a near-absence of drama after the start. Right boats, wrong format? - (recognising the self-defeating point that the boats were explicitly designed for that format...)

 

[Bobc]

If you look back at most cups, 90% of the races have always been won on the start. I think one of the problems they've found with these boats is that the wind shadow (dirty air) is so big that you can cover the other boat from 200m away (so similar to the problems for F1 cars losing downforce as they get closer to each other). They sort of solved it in F1 by introducing the DRS flap, and are starting to get more of the aero under the car. The only way I can see them solving it in the AC75s is to move to solid wing mast rigs.

I'm impressed with the speed at which they've got to grips with these boats, and the lack of boat-handling errors is impressive.

 

[TLouth7]

Absolutely. Smaller foils = less drag.

That isn't inherently true

 

[Bobc]

That isn't inherently true

Go on then. Tell me how their smaller foils don't produce less drag.

 

[JumbleDuck]

Go on then. Tell me how their smaller foils don't produce less drag.

High drag aircraft:

Low drag aircraft of similar weight:

Basially, drag in wings and foils arises in three way. There is skin friction (or viscous) drag which matters at low speeds much more than high speeds, there is pressure (or profile) drag from the pressure distribution and there is induced drag which is much more complicated but basically arises from the production of lift by a vortex around the wing.

The longer the wing the smaller that vortex needs to be and so the smaller the induced drag. In general the longer and thinner you make a wing the better its lift to drag ratio is.

 

[Bobc]

High drag aircraft:

Low drag aircraft of similar weight:

Basially, drag in wings and foils arises in three way. There is skin friction (or viscous) drag which matters at low speeds much more than high speeds, there is pressure (or profile) drag from the pressure distribution and there is induced drag which is much more complicated but basically arises from the production of lift by a vortex around the wing.

The longer the wing the smaller that vortex needs to be and so the smaller the induced drag. In general the longer and thinner you make a wing the better its lift to drag ratio is.

I do apologise. I thought I was talking about the difference between the foils on the two AC75 contenders.

 

[Bobc]

deleted. Can't be arsed to argue with pedants.

 

[JumbleDuck]

I do apologise. I thought I was talking about the difference between the foils on the two AC75 contenders.

Aerofoil shmaerofoil, hydrofoil shmydrofoil. They work in exactly the same way. That's why racing yacht keels tend to look like this:

and it's why America's Cup contenders used to have sails this shape

but now have sails this shape

 

[JumbleDuck]

deleted. Can't be arsed to argue with pedants.

Pedants? Haven't seen any around. I'm just someone who teaches fluid dynamics for a living.

 

[newtothis]

Pedants? Haven't seen any around. I'm just someone who teaches fluid dynamics for a living.

You keep yer clever for the classroom. This is t'internet, where having once done a two-week charter in Corfu carriers far greater weight than all your book learnin'.

 

[RAI]

I vaguely remember that skin drag is much higher in hydrofoils than on aerofoils. Something to do with Reynold's number and density, IIRC. Then there's that velocity squared bit and the reference area. Maybe why ship propellers don't look like aircraft propellers.