34th America's Cup: multihulls!

[powerabout]

I missed the last day, did Australia win?

 

[Mikko Brummer]

These "interceptors" are known as Gurney Flaps in the car racing business, where they appear frequently on racecar wings to increase downforce of a wing with given area.

They are also known as Gurney Flaps in the aero business. But they are almost never used on aircraft, because they also significantly increase profile drag, and greatly increase the nose-down pitching moment, both of which are bad.

One refinement is the Divergent Trailing Edge developed by Preston Henne at Douglas Aircraft. This is sort of like a smoothed-out Gurney Flap, but which has been properly designed rather than slapped on as an afterthought. DTEs were developed specifically for transonic airfoils, for which they are most effective. By moving the loading from the top to the bottom surface, the DTE reduces the peak velocities on the top surface and thus reduces shock-wave strength and wave drag. This can offset any viscous profile drag added by the DTE.

Reducing upper-surface velocities will also certainly benefit a cavitation-limited hydrofoil. So it's quite possible that these "interceptors" improved the Oracle boat if they were used there. If so, then a DTE would work even better. Or not -- it's a benefit/loss tradeoff problem.

A little off topic, sorry... we used a (guerney) flap in our 470 sails in the -90's - the sails won the worlds in 1997. In wind tunnel testing, the effect was dramatic for a 2D-profile carbon wing, but less so for real, sloop sails of a 470. Attached wind tunnel test pic, force measurements, and the flap in the 470 main sailing in -96 Olympics.

 

[capt vimes]

... As an indicator of true national pride ... please look at the videos:
Helmets - "USA" on one boat "Emirates" on the other
Wings - US flag on one boat some fern on the other
Hulls - US flag on one boat the other boat looks like a floating billboard ...

i do not know what you looked at?
helmets - i do not care
wings - there is a NZ flag very clearly to bee seen, just not as big as on oracle - probably you did not recognize it since it has no stripes in it...
hulls - "new zealand" is written on it in big letters.. even twice per hull!
regarding the fern:
http://en.wikipedia.org/wiki/Silver_fern_flag
it is registered and "...may be used as an unofficial flag of New Zealand. The silver fern itself is a quasi-national emblem, being used for various official symbols, including the coat of arms of New Zealand and the New Zealand one dollar coin."

it is just plain bollocks what you right here...

 

[petereng]

SAS System

OK heres a quick SAS system. It works exactly like the suspension on a car wheel. A spring and a shock absorber. The trimmer is locked out when travelling but the trimmer can unlock and trim/preset it as required. The spring acts against the foil load and the "shock absorber" is the damper. The systems neutral position is 10deg AoA so it can take off. The spring rate is set to balance at about 5deg and the boats trim creates the dynamic response. Its not fully detailed but it will show the main features required. It has no stored energy. Once they got the spring rate, damper value and preset AoA right it would look after itself just like a car wheel does on bumps. The trimmer would have a chart in front of him generated by training data. It would have speed and AoA ranges. So he can read his speedo and adjust the trim to the chart or by feel and anticipation. But if they got the values right it would nearly look after itself. Oh yes, if the heave (up movement) is too much for the SAS to handle the foil breaks the surface and the area gets smaller, so the lift drops immediately. Doing this exercise shows me that its actually OK to have a smaller foil and optimise for 30-40kn say and use a high lift foil to get it out of the water. Say a Cl=2+ Also we can make a non linear spring/hydraulic to match the load rate better. Cheers Peter S

I've just added an article I wrote for last months Australian Multihull Magazine.

 

[laukejas]

Laukejas,
nice post. You have put a lots of effort in it,

Thanks, well, 3 hours of my life written off

So, yes - I am nostalgic too, and was contrary to the adoption of these boats for the AC. But then I have realized that perhaps I am not nostalgic of those beautiful monohulls, but rather of the old times which they remind me of - which is very different. If you ask a 20-yrs old kid what does he prefer, he will imo indicate a fast foiler. Ask him the same question after 25 years time, and he will likely have changed his preference in the meanwhile, now opting for a more easy-going displacement or semi-planing mono.

Well, I'm actually 23, and I've only been into learning about sailing for about 3 years (and just a few days of actual sailing), so I guess I must be closer in age to these "20-year old kids" than to you, old salts, but even though I wasn't there to see the old AC's - I watched recordings on Youtube and elsewhere - I still feel that AC was more beautiful before.

That said, I have to say that the wall has been broken and it won't be possible to redo it the old way, in my opinion. The foiling sailboats are now here and will remain here. I believe it would be so even if Kiwis won the AC. Like I said in one of my latest posts, the sailing technology has to move forward, and will. It is a non-avoidable fact.

At the end, seaworthy vs. unseaworhy, monos vs. multis, foiling vs. planing, rigid wing vs. soft sails - they are all product of decisions and agreements which become class rules of the event. It happened that that it was decided to use foiling cats for this edition of the AC, just like it was decided at some point that cars have to drive in the right lane. Once it became a rule, people got used to it and now it would be pretty difficult to enforce driving in the left lane. After all, we are talking about a very particular sport event (on the borderline of the meaning of the word "sport") which has nothing to do with so many other regattas around the world, in which the other types of sailboats (displacement and planing monos and multis) will continue to have the main role. In the case of the AC, the rules wanted the foiling cats, and that's what we got. Period. The rest is just discussion for the sake of discussion.

Yeah, that's the sad truth. Technology in sport is something like a one way street: once you go there, it feels disappointing to almost everyone to go back to the "old ways". That's why, I believe, one should carefully consider before going further up that street, as this event might leave something valuable and irreplaceable behind as it heads down, chasing technologies of the future.
I'm sad to see that at least in current AC's direction, improvement in technology seems driven by "size matters" philosophy. Myself, I find that working in tight constrains, finding elegant and simple, compact, yet universal solutions is the true beauty of technology improvement. Everybody knows that if you throw in a longer hull, a bigger sail or a couple of foils, you'll get totally new speeds, but sticking with same general parameters and still managing to achieve better performance by tiny changes in sail shape, hull shape, boat balance, etc. is the true mastery of this art. That's why I wished AC would be raced with IACC or some similar class boats. These were simple and elegant, with much room for improvement. Subtle improvement. That's what makes technology amazing. Maximum efficiency in same constrains.

P.S.
Your English is excellent! Your writing is crystal clear!

Aw... Thanks

Some of what I said was half in jest. It is a big event and, as Jenny Giles
wrote, there is "trash talk", and laughing at failure and misfortune.
Nobody should take it too seriously.

Well, jest or no jest, I find that there is truth in that statement. That is a real issue, speaking of AC visual factor.

I have a couple comments on your post. The impressive graphics were created by Stan Honey's company and are even more impressive when you consider there are no fixed positions for reference. The technology is specifically aimed at making sailing approachable for viewers with no experience and I consider it a very expensive gift to sailing. Stan is a very accomplished sailor and navigator on numerous record attempts.

Yeah, lack of points of reference for these graphics bugged me as well. I couldn't, and still can't understand how they managed without them. I smell serious professionalism.

About the professional coverage that never said the word 'cheat', my understanding is that Oracle (Larry) had to pay not only for the coverage but also to air the race on cable TV. If Larry said all interviews would be naked spelling bees and aired in place of the nightly news it would be so.

Yeah, maybe that's the explanation. Whatever the reason, covering like that has no excuse. It degrades the audience.

About the cup, it has always been THE big bucks, no holds bared, peak of sailing technology excess. I would call this cup a great success if only in the ratio of sailing hours to lawyer hours. If you like J boats or 12 meters there are classes still out there racing -ironically not often because of rule squabbles. The AC 72 is actually reasonably seaworthy compared to most modern AC boats and considering their speed. The previous AC had much lower wind limits (an attempt by alighi to give advantage to bigger 2D sails) and the last monohull AC featured boats that broke in half and sank frequently in moderate weather. Even the original J boats were designed so light they were considered not safe to sail beyond the race series. The AC72s do over 50mph in 20mph wind, it was not a bad decision to limit the races after Bart's death. We are lucky the lawyers didn't kill the whole program.

Yeah. Poor One Australia. I saw video recording. Such a boat, right down to the bottom of the sea... Sad indeed.
You're sure about wind limit of old IACC's? I don't remember exactly where, but I saw them sailing up to 35 knot wind.
Anyways, if instead of going to wings, AC would have gone to reefable mainsails, wind limits could be lifted a lot.

You're right that we're lucky this wasn't shut down. But I really hope they'll realize that with those speeds, another death is only a matter of time.

 

[rogerf]

Stan Honey discusses ocean going mono and multihulls, "multihulls are much more sensible"

http://www.nemasail.org/pdf/Spring2011Newsletter.pdf

 

[Doug Lord]

First Foiling America's Cup

I picked up a little tid bit that Oracles foils had "interceptors" added to them.
Supposedly done by Paul Bieker. The following rough sketch shows one on top and one on the bottom of the foil. It was likely placed one side or the other.
PeterEng, Slavi-what do you think?
NOTE-Slavi's sketch below seems to show some benefit for a foil-could it tend to retard cavitation, Slavi?

click(Ps I couldn't draw the foil level or nose up-the front is to the left) :

About interceptors:

For decades trim tabs have been the popular way to adjust a boat’s running angle. They work well on boats up to about 45 feet but on larger boats, you need some seriously big plates, which can create drag and don’t always deploy quickly. That’s why lately many manufacturers are turning to interceptors, such as those manufactured by Humphree and Naiad Dynamics. These blades mount to the transom and drop down vertically into the flow of water coming from beneath the hull to create lift without producing the drag of conventional tabs.

[Interceptors are] an upgrade because you don’t add surface friction [like you do] with trim tabs,” says Dave Newcomb, engineering manager at Sabre Yachts in Casco, Maine. “They’re more sophisticated, and they’re precise.”

Sabre uses Humphree’s 850-mm-wide (about 33½ inches) Interceptors on its 54 because the Volvo Penta IPS II pods that are standard power on the boat don’t have integrated tabs. They are mounted in the same position as trim tabs. The standard series lowers two inches while the extended series drops three inches. The farther the Interceptors extend, the more lifting force they generate.

They create more lift efficiently with less drag,” explains Greg Platzer, owner of Platzer Marine in Fort Lauderdale, Florida, the exclusive U.S. Humphree distributor. Platzer says that because they can be custom designed, Interceptors can be installed in locations where trim tabs don’t always work, such as at the aft end of a propeller tunnel. They can also be retrofitted to a boat that has conventional trim tabs or had none at all. While some critics question the effectiveness of interceptors at slower speeds, Platzer says he has installed them on trawlers where they work just fine.
===========

Pictures,L to R --1) sketch and info of a foil using an interceptor(hard to see in sketch) by Slavi(found under "interceptors" on google) .
2) illustration of an interceptor used to replace trim tabs on a powerboat(see above), 3) my rough sketch of how it might be used :

========================
After some research , I found that "inteceptors" or gurney flaps are illegal in the AC 72 class according to rule 5.14.....

 

[markdrela]

========================
After some research , I found that "inteceptors" or gurney flaps are illegal in the AC 72 class according to rule 5.14.....

I don't see how rule 5.14 prohibits DTE's, or Gurney flaps for that matter. These are not boundary layer control devices.

 

[capt vimes]

Stan Honey discusses ocean going mono and multihulls, "multihulls are much more sensible"

http://www.nemasail.org/pdf/Spring2011Newsletter.pdf

there are also some other statements in this pdf worth quoting:
"...if G3 could find flat water and survive the southern ocean, she might prevail in the atlantic."
there are many conditional forms in only this small sentence...
"the challenge is to find enough wind to sail fast, without damaging the boat in stormy seas."
well - that is valid for anyone, but:
if you cannot stay out or sail away from stormy seas, in a mono it does not matter that much while on a multi you are in really big problems...

sure enough - they are a lot more sensible for record breaking purposes but not at all, if it boils down to get around as safely as possible.

 

[Skyak]

Stan Honey discusses ocean going mono and multihulls, "multihulls are much more sensible"

http://www.nemasail.org/pdf/Spring2011Newsletter.pdf

Stan is my hero!

 

[RHough]

i do not know what you looked at?
helmets - i do not care
wings - there is a NZ flag very clearly to bee seen, just not as big as on oracle - probably you did not recognize it since it has no stripes in it...
hulls - "new zealand" is written on it in big letters.. even twice per hull!
regarding the fern:
http://en.wikipedia.org/wiki/Silver_fern_flag
it is registered and "...may be used as an unofficial flag of New Zealand. The silver fern itself is a quasi-national emblem, being used for various official symbols, including the coat of arms of New Zealand and the New Zealand one dollar coin."

it is just plain bollocks what you right? here...

You know I don't really care, you know that I am against nationality rules for crew. With all the expressions of patriotism from the NZ crowd and all the sniping about the "American" boat only having 1 US sailor aboard, it was time to make some points and have some fun.

It is about branding vs pride. There were two big black catamarans, both designed in the USA. Both had multinational crews dressed in black outfits. When the video showed the crews, one crew was wearing Emirates helmets, the other USA helmets. Most importantly the $$$ that funded OTUSA came from a US company, not an Arab airline.

I'm just pointing out that for all the chest pounding from NZ, they put their sponsor Emirates before national branding.

Are you saying the 44° pass was so we could read "New Zealand" on the bottom of the boat?

I'm just poking fun here. I'm a supporter of the AC, not of any team. The better boat won as they almost always do. I find kiwi nationalism just a bit silly. I guess there are no designers in NZ, so they hired a US design firm?

 

[xarax]

I don't see how rule 5.14 prohibits DTE's, or Gurney flaps for that matter. These are not boundary layer control devices.

They "alter" the boundary layer at some (small) area along the surface of the foil - does this mean they "control" it ? ? ?

 

[RHough]

well - that is valid for anyone, but:
if you cannot stay out or sail away from stormy seas, in a mono it does not matter that much while on a multi you are in really big problems...

sure enough - they are a lot more sensible for record breaking purposes but not at all, if it boils down to get around as safely as possible.

Fly Emirates?

 

[daiquiri]

They "alter" the boundary layer at some (small) area along the surface of the foil - does this mean they "control" it ? ? ?

Rule 5.14 is very badly written, imo. Anything one puts into a water flow will alter the velocity and pressure field around it, and will hence alter the boundary layer. In the same moment they have put the horizontal elevator to the rudder, it has modified the flow field around the hull and the rudder. So, the rule 5.14 is practically very hard to enforce in a literal sense.
However, if one wants to read it literally, any device on the surface of the airfoil or of the hull, the purpose of which is to modify the water flow (and hence the boundary layer) around the foil or the hull, is prohibited.

But... What if it is not ON the surface but IS the surface or an integral part of it?

So I can design an airfoil with a shape which mimics the gurney flap (or an interceptor) and nobody could say anything about it. It is not an interceptor, it is the airfoil's shape - as odd as it could be. If we assume that this is legally possible, than an interceptor would have a pretty big effect on the pressure field around the foil.

Since one picture is worth a thousand words, I have prepared this comparative graph (made with a software based on Mark Drela's Xfoil):



It shows two identical airfoils, the above one in clean configuration, the bottom one with a small (1.5% of the chord) trailing-edge gurney flap set at 65°.
For the same lift coefficient (0.7 approximately) the foil with a gurney works at a much lower angle of attack, has a much lower peak pressure coefficient (Cp) and the distribution of Cp along the chord is nearly flat and low-value. A low (absolute value of) Cp means that cavitation (and a ventilation too) is delayed for a given speed, thus increasing the foil efficiency.
For the same angle of attack, a foil with a similar gurney flap has a considerably higher Cl (i.e. produces more lift), at a very small drag expense.

For the above reasons, I see the interceptor mostly valuable at low speeds, where a high lift coefficient is required for a rapid take-off to a foiling mode. At high speeds, and close to the maximum speed, a foil with an interceptor will produce a higher pitch-down moment and will hence require more counter-balancing negative lift by the rudder elevator. That is a straightforward drag. The final result, in terms of maximum speed, would be the sum between less drag due to delayed cavitation and ventilation, and more drag due to the elevator working harder. Hard to say more with no real-world telemetry data from the boats.

A few remarks:
- Please note that when I say "low Cp", it is referred to its absolute value, |Cp|. Example: |-2| = 2 and |-4.5|=4.5
- Also, please note that Cl and other hydrodynamic values shown on the graph are not directly related to the actual AC72 foil data (we don't even know for sure whether it was the solution used by the Oracle); they serve to make a visual explanation of what a trailing-edge interceptor does to the pressure field around the foil.
- Furthermore, this is a numerical analysis made with a computer software, hence based on a mathematical model of airfoil aerodynamics. So don't take the numerical values shown in the picture as the absolute truth. By seeing the results, I'd say that the software is a tad optimistic regarding the influence of the interceptor. If I could compare these results with the experimental data for a similar configuration, I wouldn't be surprised to find out there's a difference of 10-15% between the two data sources, in this range of angles of attack.
- Finally, the above graphs show a small-scale saw-toothed irregularities, which is probably due to a numerical instability coming from the very high Reynolds number used for the calculations. But for the purpose of a qualitative comparison, they are more than adequate.

Cheers

 

[SteveMellet]

I missed the last day, did Australia win?

Why am I the only one who thinks this is the best post in this thread ?

Nationality rule is a bad idea anyway, all those who are crying for it to be re-instated probably don't know that it was first introduced around 1980, and abolished 7 years later as an unworkable rule that just drove costs up, so in fact it has nothing to do with historical AC events or the Deed of Gift. The very first event had, I believe, no less than six Englishmen on the crew of the yacht America. So get over it.
Want Jimmy Spithill on your team ? just buy him a house in your country. Nationality rule sorted.