America's Cup 2010: Race Thread

Discussion in 'Multihulls' started by Doug Lord, Feb 12, 2010.

  1. CT 249
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    CT 249 Senior Member

    Dunno Sandy - the later F40 tris were designed to fly two hulls in those conditions and their centre hull stayed DRY most of the time, and they seemed to show that the tri was faster within the constraints of the class rules. And in the case of the F40 (which had minimal jibs) and AC boat (which often sailed without a jib) one wonders how vital any improvement in forestay tension over a 'spine' actually was.

    Certainly, there was a lot of talk in F40 days about the fact that a tri designed to fly two hulls works in quite a different manner to a cat, and that the pitch dampening can be quite different.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    There are a few things that I think have been proven or at least a long way toward that between these boats:
    1) a wing is a better source of power,
    2) if you're going to use curved lifting foils you best have the beam to allow their use w/o an untoward reduction in RM.
    a.I think that Alinghi didn't use their "S" foils for lift because the
    reduction in RM was proved to be too great.
    3) I believe that if Alinghi had been as wide as USA, used lifting foils and a wing that they might have won.
     
  3. Gary Baigent
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    Gary Baigent Senior Member

    A5 had extreme 75foot beam as it is Doug, to go to 90 foot .... that would have been very difficult to construct, keep the platform stiff and not be overweight, plus it would have been an absolute ******* to tack. To have a square catamaran .... is maybe a bridge too far.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    I don't know Gary-I've sailed square rc cats and they can tack very well.
    Have you ever sailed a square full size cat? And I think it is the Decision 35 cats that have a module down the middle to stiffen the rig...
     

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  5. Gary Baigent
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    Gary Baigent Senior Member

    The RC cats Doug, I thought they were 32 x 18 or something near that, way less than square - and the D35's (trimaran platform?) without their racks (you can't count racks) aren't near square either.
    The widest cat I've sailed on was my old Supplejack and that was 32 x 20 - and that was definitely a slow tacker - and where I learned the necessary sailing in reverse, reverse rudder, skills. Now talk about 90x90, holy ****!
     
  6. Doug Lord
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    Doug Lord Flight Ready

    ----------------------
    Gary, I didn't refer to the D35 because of its beam-only because of the module running down the middle. I built the D4Z cat at 35" X 48 " and 48" X 48". Both tacked very well. The foiler below was 59" LOA X 72" wide and tacked faster than any leadbelly her length when on foils and about the same off foils.
     

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  7. sailor2
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    sailor2 Senior Member

    If you take a tri, and then lift the hull & rig up together with respect to other parts of the boat during it's consruction, why would it be any more difficult to construct, keep the platform stiff or not be overweight, than the tri ?

    On the contrary, the loads transmitted from the mainhull to the beams are mostly vertical in unheeled condition, and tilt as the boat heels, meaning that those are mostly shearloads for the mainhull, and thus the loads are carried by the sides of the hulls, so that it's more easy & efficient to transmit the loads when beams go through the sides instead above the deck. And that happens automatically, when what could have been the mainhull is lifted to make it a cat.
    For the torque from the beams, the position of beams going through the hull sides gives even more advantage for the cat in structural weight reduction.

    In a tri there is extra weight from the structure needed to transmit the loads from the beams into hull sides, that's the bulkheads, that can be omitted or at least be made lighter with the cat with the central hull like structure, and the structure can be better shaped for the loads in the cat case, when it's hydrodynamic shape is of no issue, it being airbourne all the time in a cat.
    The only differencies that matter would be the higher center of gravity of the cat in measurement condition due to the lifting up, and less heel angle needed to lift all but one hull, and the differencies in turning ability you mentioned as long as both boats are flying hulls. All that assuming the cat hulls have identical shape to the amas of the tri and have identical construction & loads.

    If not fully powered up, then different wetted areas with same proportion of righting moment, and different longitudinal & diagonal stability would be an issue as well with the main hull bow contributing to that more than windward hull of the cat at the same heeling moment.
     
  8. Gary Baigent
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    Gary Baigent Senior Member

    Sailor2, BMW-O WAS like a square platform version of a Decision 35, except a square D35 does not exist; BMW-O's vaka, main hull was only there for DoG measurements, superior base mounting area for the wing, load spreading for the tri's greater overall beam, for winches, crew and aerodynamics, much cleaner than the cluttered, inverted fence line tangle mid beam area on A5 - and our eyes popped (well mine did) when we observed the monster tri sailed damn near all the time on her leeward ama, a much sweeter, less draggy, designed ama than A5's ugly versions too by the way. A5's configuration problem was it wasn't wide enough - but that is Catch 22 - because of tacking and construction problems already covered here.
    I bet that if the weight figures do emerge, that the the tri's stripped down central hull as compared to the compound bow setup on A5, would be not that much different. Agreed, if A5 had gone to a Decision 35 type platform with central long pod instead of the fence line, maybe she could have been built to be what would be the same configuration platform as BMW-O, the same 90 foot beam - the only difference being that the cats central vaka? pod, whatever you want to call it, would not touch water at rest. Still got a tacking problem though.
     
  9. catsketcher
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    catsketcher Senior Member

    Differences between cats and tris

    Although one could say that a tri is just a cat with a big nacelle the hulls are designed very differently.

    A tris amas are usually flatter than deep although this has not always been the case. This is because the tri can have very clean amas free from winches, hatches cockpits etc. They can easily be designed for wave piercing. This allows the designer to reduce hull volume and therefore pitching moments. In fact it is essential that safe tris do have wave piercing bows.

    On top of this the designer can make the main hull good for fewer jobs. In BMW I saw the main hull was shorter than the amas. It was also a better manouvring shape than the amas with greater rocker and more rounded sections. It is hard to see how a cat designed with 2 low rocker hulls could ever approach the tacking ability of a well designed tri. Although the amas are wider most of the mass is on the main hull so the rotational inertia could be the same but the rotational drag much less.

    You don't often see cats made from tri floats. It has been done but when you look at the boats they seem a tad strange. The reverse is also seldom done (except with Newick and the Tremolino and Hobie 16s aren't normal cat hulls anymore)

    Going too wide won't help in the end - we have been down this road before. Any multi is easier to sail if it has less stability sideways than fore and aft. This means you can ease sheets rather than pitch pole in the boats least stable condition. The Twiggy was an example of a tri that flipped over its bows cos it was too wide. It was cured by adding volume up front but you can only go so far this way before drag increases too much.

    A cat has a problem in that it can't generate fore and aft stability as well as a cat. We have a problem here if we use the old fashioned notion of fore and aft stability - we don't want just volume up front - we need volume and very little drag when underwater. This is the reason that A class cats and Alinghi have their bows the way they do. But you can't do it as well as in a tri. Irens started this off with pushing the volume way forward in the amas and reducing deck drag so that the tris after 1986 were faster than the cats on a reach. It has been tris ever since (when same sized). When the wind is light the main hull is there with its CB way back. You can't do this on a cat. The cat would probably always be lighter but going too wide (like square) will be very tricky.

    In fact in the AC you could be very scared in a square cat. If your competitor found out that you had a black spot in your angles you could be very compromised in pre-starts etc. Okay with wind forward of the beam but worrying on a run/gybe. The garda boats probably do not have that problem as they don't get hunted pre start.

    On another stability thing. With these things going so fast what is a large change in windspeed - 6 knots to 8 knots (increase by 33%) is much less for them - they probably go from 22 knots apparent to 27 knots apparent (guesses there). This means they would be operating in a quite narrow wind range. This still doesn't answer the question as to why BMW was so much more stable than the cat. I guess the wing has more secrets within.

    cheers

    Phil
     
  10. catsketcher
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    catsketcher Senior Member

    (Tris amas flatter than deep)

    Oops - this should have been - tris amas are getting flatter and less deep
     
  11. sailor2
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    sailor2 Senior Member

    Usually yes, and for a very good reason, but for rtw records, that doesn't have to be so, if a central structural pod is used. In such a case, time to tack is mostly not much of a consern, and cat can have same hull shapes that is typically used on amas of a tri just fine.

    I agree, unless someone comes out with a cat with hulls capable of bending to create camber like the 2-element wing BMWO used. Propably a nightmare on structural engineering point of view though, but still doable with some weight penalty with big budget & enough time. That would allow a cat with ama like hulls to tack like a tri.
    There are very few multis around having more righting moment fore/aft than sideways. Loa/Boa > 1.8 needed for that and crew weight or ballast must be very small fraction of total weight to make that happen. Only production performance multi meeting that criterion that comes to mind is Reynolds33 with 14ft beam.

    I guess the latter underlined cat should have been a tri. ;)

    AC multis seem to be so fast, that the sailforce vector is never much forward of the beam in light conditions where they sailed. That means fore&aft stability is less of a consern than in most other (slower) multis.

    - If by more stable you meant in roll dynamics, it's clearly 99% of the result of easy & fast adjustability of the wing & 1% due to all other things brought together.
    - If you meant in pitch dynamics, it's about less rocker in amas & waterpiercing shape up- & downhill, and aided by a lifting foil while reaching.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    =====================
    Phil, just curious-have you ever sailed a square full size cat?
     
  13. sailor2
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    sailor2 Senior Member

    Loa = 113.3ft Boa=90ft means Loa/Boa = 1.25, quite a common value for tris, not at all square like orma60 with L/B=1.05 which have some real difficulties due to being so close to square. One of the reasons that class is finished by now.
    I agree, except the ledged construction problems, I don't think that is any problem at all, just the same structural case as for the tri with same possible solutions and nothing else.
    The biggest reason 90ft beam cat don't work for AC is the limitation on wl length to 90ft, requireing too much rocker compared to amas of a tri.
    With 90ft waterline, the tacking problem means bigger rudders have to be used making it slower out of a tack, but not preventing it. Reduction of fore & aft stability is a bigger consern, resulting more hobby horsing motion. And that when combined with more rudder drag means slower boat with less driving force even with a wing. I don't expect cats being able to win DoG default matches due to that even if they can be made 10% ... 15% lighter.

    Ps. some numbers quoted on BMWO at SAAC forums less than 24hr ago.
    15 tons in racing configuration, speed/WS ratios 2 for upwind, 2.5 downwind, 3+ for reaching. VMG up 1.4 times tws, and down 1.8 times tws, tws taken at top of wing.

    No max vmg info so far given.
     
    Last edited: Feb 24, 2010
  14. sailor2
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    sailor2 Senior Member

    Are you suggesting there exists any ?
    I have never heard such cats in full size, just think there is no reason such couldn't be built, and can't see why if not for an rtw route.

    Small RC-multis are very different, because they naturally need proportionally larger rudders anyway due to RE-number effects. Big full scale boats would need oversize rudders for good tacking behavior, making them slower due to added wetted area, or more complexity if capable of being lifted partially up to counteract that.
    And even then, more draggy during turns mean coming out of a tack with less speed, not a winning solution for close course racing.
     

  15. catsketcher
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    catsketcher Senior Member

     
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