34th America's Cup: multihulls!

No, I am not sure. Tom Speer might tell us that info, and much more.

The analysis in my previous post was done for symmetrical foils at 3-4° AoA, but the general conclusions are equally valid for asymmetrical airfoils set at a corresponding aerodynamic AoA. For example, a symmetrical airfoil at around 3°-4° AoA will give a lift similar to an asymmetrical foil set at 0° geometric AoA. Conversely, a symmetrical airfoil at 0° AoA is like an asymmetrical foil at around -3° to -4° geometric AoA (where it gives zero lift).

So you can assume that the data previously seen are valid for a symmetrical-airfoil fairing at around 3-4° AoA, or for an asymmetrical-airfoil fairing at 0° AoA (which is what the fairings appear to be in the pics).

That's because I have used the so-called "aerodynamic" AoA in the analysis (which is the geometrical angle between the undisturbed airflow and the zero-lift line of the foil http://en.wikipedia.org/wiki/Zero_lift_axis). Once you have fixed the wing planform geometry, the aerodynamic AoA is the main factor which determines how much lift an airfoil will give, for common airfoil shapes. Want some lift? Set the foil at some positive aerodynamic AoA. Don't need the lift? Set it to 0°. The drag, and hence the L/D ratio, will change in the process, of course.

The detailed foil geometry (thickness and camber distribution) is more important when it comes to the maximum lift (which is not an issue here) and the minimum drag (which is). But I don't know the details about those fairings' construction, so I didn't dig too deep into the analysis.

Cheers

 

...

James Boyd at TheDailySail is advertising a new book today!

excerpts:

So Bob Fisher, James Boyd, Kimball Livingston, et al, actually get paid to do this? While we all do it for free?

 

I tried to make this point before AC33. It was damn obvious to me that the Oracle Tri was paying attention to drag reduction and Alinghi was not.

The boats have all the power their stability allows. The races will be won by the team that does the best job of drag management.

It really is a "drag race" ... just not in the way that most people think.

R

 

34th AC

from Scuttlebutt tonight:

America’s Cup: What you may not know about the AC72

By John Longley, 1983 America’s Cup winner(Project Manager of Australia II)
-----------------------------
After spending a week in San Francisco and having the opportunity to talk to a number of people who have actually sailed the extraordinary AC72s, I have gathered a bit of AC72 trivia to share…

* If you had an engine to power the hydraulics rather than grinders, you could sail the AC72s with 4 people rather than the crew of 11 they now sail with.

* There is really only one trimmer on board and he controls the wing. The helmsman controls the cant and rake of the board with buttons on a control pad in front of him but only has 3 seconds of stored power before he has to “throw bananas” into the grinding pit i.e. ask for more hydraulic power.

* They have seen 47 knots as the top speed so far but expect to see the 50 knot barrier broken in the Cup match.

* The boats go directly downwind 1.8 times faster than the wind. So if you let a balloon go as you went around the top mark you would easily beat it to the bottom mark.

* There is only 4 degrees difference to the apparent wind from going on the wind to running as deep as you can.

* If you lost the hydraulics while the boats were foiling they would be completely uncontrollable and would most likely capsize.

* It is faster to find the strongest adverse current going downwind because the stronger apparent that is then generated translates into more speed than if you were sailing in slack water. (Warning – this takes a bit to get your head around)

* When sailing downwind you look for the puffs in front of you not behind you.

* It is actually quite dry on the boats, unless you make a mistake and come off the foils, as you are flying a couple of metres above the water. Waves have almost no impact on the boat when foiling.

* In strong wind you carry negative camber at the top of the wing to “reef” or de-power the wing.

* All crew carry personal tackle so they can effectively rappel down the netting if the boat capsizes.

* Gennakers are only used below about 8 knots; the jibs only provide about 3% of the lift up wind.

* The foil on the rudder generates about 800 kg of lift with the rest coming from the center board foil to lift the 7 ton yachts clear of the water.

* The centre board foil’s tip comes out of the water so it effectively works like a governor on an engine i.e. as the board generates too much vertical lift it comes out of the water, the area is thus reduced so it goes back down etc until it finds equilibrium.

 

"If you remember Larry Ellison said at the beginning, 'this is going to be an American team, this is going to be American sailors who are going to win the America's Cup'. There are two Americans on the boat, the rest are New Zealanders and Australians."

 

Doug Lord, the one's that get me, of all the marvels of AC72, are these:

* The boats go directly downwind 1.8 times faster than the wind. So if you let a balloon go as you went around the top mark you would easily beat it to the bottom mark.
* There is only 4 degrees difference to the apparent wind from going on the wind to running as deep as you can.

"How daa do dat?"

 

Michael, I know that Marcel Fachler was quoted saying that about Ellison yesterday. But I don't know that he was accurately repeating anything Ellison has said. He may have been putting words in Larry's mouth?

Ellison hasn't said it during the interviews I've watched, at least.

 

...

Dean Barker updated his blog today:

...

I think it must be the first time that I bother to quote an inveterate monohull die-hard.

 

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It's a slow news day in the America's Cup, while we wait for the hammer to come down on Oracle. My vague understanding of it is that the jury will perhaps make public their decision on the Article 69 procedures today, before the meeting tomorrow, which will deal with the Article 60 complications. Meaning perhaps the Article 69 stuff gets released tonight so the lawyers can bill an all-nighter ahead of tomorrow's face-off?

Coutts, Simmer, Spithill, Turner, Slater, Henderson: The list alone, of witnesses ordered to appear before the Jury, tells you that this is back-to-the-wall time for the Oracle Racing Team. Rumors are flying around that Ellison has now bought a section of Alcatraz prison, to which select members of his team will receive complimentary tickets.

lol


Is there any good news? I was surprised to find that Bruno Troublé actually said something interesting a couple of days ago!

Gary Jobson has apparently watched Oracle's 2-boat racing 14 times, and he thinks it's very close but gives the edge to ETNZ on account of their boat-handling and strong team ethic. But then again, Jobson expressed surprise the other day that a multihull was able to point quite so high, revealing his imperfect knowledge to all us initiates.

Nathan Outerridge on the other hand thinks Oracle has a speed advantage:
"Oracle's boat, to me, seems like a faster boat - with a better chance to get top speeds, but it's a harder boat to sail,"

LINK

 

ETNZ goes SR71

ETNZ has now gone and installed jet fairings on the aft beam.

http://etnzblog.com/#!2013/08/back-on-the-water-with-more-modifications

It is very interesting to see that I was wrong and that is substantial laminar flow back there according to the tufts on the fairing (shown for a very short time). What these things will do I can't say. I can get the extra fairing on the bowsprit.

It does seem that they are trying very hard to minimise drag. The aft beam fairings must somehow direct the breeze somehow but I have never seen anything like them anywhere before - a nozzle on a prop but where is the prop?

Are the boats so fast that a permanently mounted foil at 0 AOA will generate lift around the course?

Black arm band day though - Thanks Dick for everything - 3 Cheers, Rogue Wave, Val,

Phil

 

34th AC

Thanks for pointing this out Phil! I don't get the big box things aft?
http://www.youtube.com/watch?v=anSD-ShdIGQ

 

You probably mean that the flow is attached, not that it is laminar. The flow is probably turbulent but attached. It is very difficult and usually impossible to determine whether flow is laminar or turbulent by observing tufts. Tufts show if the flow is attached or separated. It's a common misconception that all attached flow is laminar and all turbulent flow is separated.

Zero aerodynamic angle of attack is the angle at which zero lift is generated and is not speed dependent at low subsonic speeds. Its difficult to judge what the aerodynamic angle of attack is for an airfoil mounted near another body since the other body will alter the flow around the airfoil. So the angle of attack of the airfoil relative to the local flow can be different than the angle of attack relative to the "freestream".

 

Obvious the purpose is to redirect airflow. For what reason is a guess. I would venture a stability/balance purpose for when they are foiling. Just a guess.
Examining further, their proximity to the trailing edge of the mainsail wing might indicate some kind of redirection of the vortices coming off the sail.

 

I wouldn't bet too much on it, but at 0:58 of that video it looks to me that the aft fairing has a negative camber or AoA. If it is actually so, it is designed to give a downwards-directed lift, which makes sense. In which case we shouldn't talk about a fairing anymore, but rather about a horizontal wing. So at this point perhaps we should start talking about the vertical and horizontal wings on these machines?

As about the box-shaped additions on top of the horizontal wings, the upper plate appears to be very thin, which means that it's the vertical parts which actually work, probably in the way Rumncoke has suggested. Their function might be comparable to winglets on airplane wings.

Winglets work by widening the wake behind the airplane wing and thus, by doing so, reducing the induced drag. Here we have vertical wings whose wake cannot be pushed downwards - there's the water surface below. So the boxy ducts might be there (imo) to redirect the flow induced by the large vortices at the vertical wing's bottom, making the wake shallower (in the vertical plane) and hence reducing the induced drag of the vertical wing.

It is starting to become very interesting from the technical point of view.

Cheers

 

With the kind of speeds these machines are running at, the boat could be pulling out of a Vacuum. Those flow directors could push that vacuum father aft of the boat, allowing regular static air pressure more time to fill in behind. Every last percent of drag is what they are searching for and it seems they leave no stone unturned.