34th America's Cup: multihulls!

Discussion in 'Multihulls' started by Doug Lord, Sep 13, 2010.

  1. powerabout
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    powerabout Senior Member

    I missed the last day, did Australia win?
  2. Mikko Brummer
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    Mikko Brummer Senior Member

    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.

    Attached Files:

  3. capt vimes
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    capt vimes Senior Member

    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:
    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...

    Attached Files:

  4. petereng
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    petereng Senior Member

    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.

    Attached Files:

    Last edited: Sep 27, 2013
  5. laukejas
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    laukejas Senior Member

    Thanks, well, 3 hours of my life written off :D

    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.

    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.

    Aw... Thanks :)

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

    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.

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

    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.
  6. rogerf
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    rogerf Junior Member

  7. Doug Lord
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    Doug Lord Flight Ready

    First Foiling America's Cup

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

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

    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.
  10. Skyak
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    Skyak Senior Member

  11. RHough
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    RHough Retro Dude

    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? :p

    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?
  12. xarax

    xarax Previous Member

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

    Fly Emirates? :D
  14. daiquiri
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    daiquiri Engineering and Design

    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):

    Gourney flap.jpg

    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.


  15. SteveMellet
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    SteveMellet Senior Member

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

    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.
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