34th America's Cup: multihulls!

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

  1. Doug Lord
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    Doug Lord Flight Ready

    AC 34 on Foils!

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    Not that I know of-just witness accounts scattered around the various threads. Sure would be good to see one and get a first hand assesment of altitude and pitch control like we have with TNZ which is generally excellent.
     
  2. Silver Raven
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    Silver Raven Senior Member

    Twist ???

    Gooday 'OZ'-Peter.

    Let me - put the cat in with the parrots ??Statement: for every 1" of twist one (1) hp is consumed. TWIST must be stopped - at all cost.

    Regardless of what the 'big tri did or didn't do - equally, regardless of how smart (& they were/are) the designers may be - TWIST consumes hp - full stop.

    Now I'll sit back & see just who jumps-in eh ?? Wish people like - blunted, Steve, Nigel, Peryon, Tom - SimonN - Doug - Gary etc etc would get into this one - just for the learning curve of all the info that should flow! ! ciao, james
     
  3. groper
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    groper Senior Member

    That explains alot, they most likely dont have very good stability and so they will not let the cat outa the bag in the form of a video...

    All the other teams got a lota catching up to do against team NZ...
     
  4. Silver Raven
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    Silver Raven Senior Member

    Tasty additives - What ???

    Gooday 'groper' - 'down-hill man'.

    'Catch-up' or is that 'Catsup' - as in sauce ??? I've noticed that once you've lost your money by dropping it & walking around the croner - you then go to work - to make-up - what you've lost - just to catch-up - your still behind.

    I'd say the big loss here - is in the sailing hours experience time-on-the-water - & with the pedal-down - as the K-1's are doing - one would think the 'other teams' may well have lost a big advantage. There are a few people in - crews.org - who don't think the K-1`'s have stolen an advantage - I'd beg to differ with them.

    I drove around 'Y-kb' the other day - sure that I could find you but had to leave to go to the Cairns Base & didn't get out for the whole day. Bugger, but I'll get there soon - I hope. ciao, james
     
  5. Doug Lord
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    Doug Lord Flight Ready

    34th AC on Foils!

    =================
    How you is, James! When I was learning about foiler design from Dr. Bradfield, one of the things he stressed is that the platform must be twist free-very stiff. However, he was talking about a foiler design with twin, independent wand controlled main foils both of which had to be in the water so the thing could use the foils for not only vertical lift but for RM. Given those constraints twist would have prevented that system from working properly.
    TNZ and Oracles foil systems are entirely different from Bradfields:
    1)the primary load is on a single main foil instead of being shared by two main foils( approx. 80% on main foil, 20% on rudder foil),
    2) the TNZ and Oracle systems do not develop all the RM for the boat in addition to vertical lift as does the Bradfield system.
    So, in my opinion, platform twist would have zero influence on the foil system on either boat, even though their foil systems are entirely different from each other: TNZ is a surface piercing system and Oracle is a fully submerged system(my analysis). Now, I haven't looked at twist and its affect on the rig and I've discounted its affect on the windward rudder t-foil( because the effect of any neg. lift would be very, very small).
    Oracles big tri twisted tremendously and it used curved lifting foils on the lee ama to no negative effect.
     
  6. Doug Lord
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    Doug Lord Flight Ready

  7. Doug Lord
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    Doug Lord Flight Ready

    Click on these pictures-they clearly show the difference between the Oracle fully submerged foils and the TNZ surface piercing foil. Oracle's foil is an "L" configuration and TNZ is a curved "S" configuration.

    Pictures and sketches, L to R : 1) Oracles AC 45 in steady flight on the lee main, "L", (80%) and rudder, "T", (20%) foils, fully submerged foils, 2) sketch of Oracles main foil, 3) TNZ surface piercing foil, curved "S" , 80% and rudder "T" 20%, 4) sketch of TNZ foil:
    Note: curved lifting foils used on modern trimarans are designed to operate as fully submerged foils. These are not. In the past when tri's with curved lifting foils went fast enough to lift the ama while flying the main hull there was likely to be a crash when the foil all of a sudden became a surface piercing foil with the highest load near the surface. The tip and how it works is probably critical to TNZ's system working well but I don't quite get it yet though it appears the foil tip is possibly acting like a part of a "V" foil. One could reasonably expect to see the tip broach the surface at some point-maybe like in the previous video(post 531) where the boat goes severely nose up:

    Click on these images:
     

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  8. Doug Lord
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    Doug Lord Flight Ready

    34th AC on Foils!

    I just noticed that this thread has lost one of its rating stars! Amazing-I think its one of the best on this forum......Somebody click on the "RATING" icon above and please add back our missing star.....
     
  9. petereng
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    petereng Senior Member

    Platform Twist
    There have been many cats designed with intentional twist. Francis Herreshoff made all his early cats with "suspensions' so they could ride over the waves vs through them. Cats with platform twist generally are designed for small waves not ocean type waves. We are really early on in the development of this type of craft. The Bradfield type arrangment needs platform stiffness to keep the foils in control. Same as a car body needs global stiffness so its suspensions can react independantly of the body. The requirement for rotating wing sails means the rigging can't be done up too tight. The requirement for forestay tension is in direct opposition to this. The requirement for platform stiffness is driven from the rig tension so we are in a three ringed circus affair when it comes to design the entire structure. I have been through a general design cycle of an AC72 so know some of the problems facing the teams. In the AC72 case I don't see it as desirable for platform twist. So I think the twist seen in USA17 is just teething problems and settling in. I think all the teams will be sailing "lines" and dveloping their real Velocity Profiles and Polar Plots. They will be comparing these to the design data and either drinking champage or the bosses will be unhappy and getting the whips out. Seems NZ is hitting its plots as Dalton is happy. If they hit their data then its onto boat 2 pronto. Boat 2 will be a bit more radical perhaps as they now have confidence that thier design tools are up to the task and can put them into the design space they want to be in. Boat 1 is really the "basic or test" boat covering all bases. Boat 2 maybe a bit more specialised for San Fran. In regard to foiling they will be comparing speed and pointing on and off foils. Some pundits have said foiling is 25%faster which is quite wrong. The videos so far don't show the boat speeding up much at liftoff but its early days yet. We have independant boats sailing in differnet parts of the world. When the Italians arrive in NZ or SF and we see some side by side action that will be the exciting bit! Peter
     
  10. Doug Lord
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    Doug Lord Flight Ready

    34th AC on Foils!

    ===================
    Peter, one of the videos I saw clearly showed dramatic acceleration as the boat got up on foils. I think it was the one that starts out with the guy on the boat with 4 outboards-a tv reporter I think. The acceleration was obvious and you could hear someone saying "Oh my God!". That video is on this thread somewhere. There is also other comment by witnesses saying the same thing.......
     
  11. groper
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    groper Senior Member

    [​IMG]


    This is the resistance output of a slender catamaran hull i modelled in Michlet, with a form very similar to an AC45, probably not as extreme in form, it only had a l/b of 18:1 and a Cp = 0.67, transom stern, mostly U shaped section with very fine entry and shallow forefoot, but a very similar form none the less - i was deliberately trying to get as close to an AC45 as possible. I assumed a pressed displacement of 1.7tonnes (flying a hull) etc.

    The speed is along the x-axis in meters/second - to translate that into knots, is very close to simply doubling the number, so the axis goes from roughly 5 kts upto 26kts of boat speed. The y axis, is resistance in kilo-newtons.

    Rt (majenta) = total resistance ie. sum of all components
    Rw (green) = total wave resistance = sum of transverse + divergent wave resistance
    Rdiv (blue) = divergent wave resistance
    Rtrans (yellow) = transverse wave resistance
    Rv (Red) = viscous drag
    Rh (cyan) = transom drag

    You can see from the resistance components, that as the speed increases, by far the largest component of drag is the viscous drag (red), and you can see the slopes of the resistance curves at the 26kts point. Look how much of the total resistance this component is contributing at 26kts! If you can lift the boat out of the water at these speeds and reduce the wetted area, guess what happens to the viscous drag? its not difficult to see where the gains are to be made at the speeds these boats are making...
     
  12. Doug Lord
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    Doug Lord Flight Ready

    34th AC on Foils!

    Great stuff, Groper-thanks!
     
  13. Silver Raven
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    Silver Raven Senior Member

    *** Doug - Let us deal with some basic facts - very basic facts - please - as of today: TNZ's foil systems are entirely different from Bradfields. YES, OK

    Oracle - DOES NOT have any foils - of any kind - that work or are in a working condition - attached to a working model sailing graft ! ! FACT

    I put to all persons out there - regardless of which type/style/size etc of multihull - 'twist' is slowing the vessel.
    'A', 'B', 'C, 'D' class - Paper-tiger, Hobie class - etc - any multihull you wish to mention - 'Twist is slow' - 'Twist costs boat speed' - 'Twist must be avoided at all cost' - 'Twisting vessels will always loose to 'stiff' vessels.

    If you have another thought - first - go sailing a sloppy (twisting) cat'/tri - - then back to the beach - tighten-it-up - - then go back out sailing - can't take more than a few hours - & you will jave just proved that - 'Twist is a no go area' if you wish to cross the finish line in first place.

    O.K. so there's a part of it - - Go sailing a 'sloppy' & prove me wrong ! ! !

    Next - just to make my point - even stronger - - Oracle broke its foil because the over-twisting 'frame' induced an extreme angle on the lifting part of the foil - drastically increased its negative angle of attack & CRACK there you have it.
    Again - go & do a simple - drag-test - in your local creek/pond/lake & watch what happens. Pull the hulls attached to frame & attach a simple set of 'fish scales' & read the numbers - easy-peasy.

    Back in the 60's & 70's the multihulls - that were winning their calss races & the 'world championships' built here in OZ were stiff - very very stiff with much attention being given to little to no 'twist' - what part of design practice seems to have changed ??? Someone invented a - newgravity ??? I think not ! ! !

    By the way - everyone - do have a great weekend - go sailing - enjoy - smile & stay-stiff (just can't help it) - ciao, james
     
  14. Doug Lord
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    Doug Lord Flight Ready

    34th AC on Foils!

    James, don't forget USA 17 that twisted a lot -and won. Don't forget the "revolutionary" Formula 40 tri Adrenaline that had flexibly mounted amas-and beat most cats before they changed the rules. Don't forget "Victor T" that in the late 60's using a flexible platform whupped all the C Class cats.
    Don't forget, James......
    I think the case for twist-or not- has got to be taken on a case for case basis but for the AC I don't see the disadvantage. Can you explain to me what that might be? I fail to see how a twisting platform could exert any force on a single main lifting foil-the main foil and rudder t-foil would keep the lee hull going along in great trim unless you're thinking the rig could move forward enough to cause a pitchpole-but there is no evidence of a pitchpole in any of the Oracle video at the time of the failure.
    I'll keep thinking about it ,James-hope you have a great weekend!
    PS-only my back is stiff, damnit!

    Picture #1=Winner of the last Americas Cup USA/BMW 17-twisted-Giles Martin-Raget photo; Pictures #2-#4 are of Oracles AC 45 on foils:

    click-
     

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  15. petereng
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    petereng Senior Member

    A thought on Twist - TNZ AC72 has a bridle at the bottom of the shrouds. Whereas USA17 has an inner and outer shroud terminating at near the same place. As the rig load comes on the shrouds try to lift the stern of the windward hull. Perhaps the bridle is designed to lower the torsional load of the shrouds? I'd have to model this to confirm but the rigging is quite different beween the two boats & its the rigging that generates the torque.. Peter
     
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