33rd America's Cup

Discussion in 'Sailboats' started by Guillermo, Sep 17, 2007.

  1. idkfa
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    idkfa Senior Member

    Travesty

    To me the America's Cup is as much about the design effort as match-racing skills, this may limit the fans but so be it. F1 has large spectator-ship cause the fans have some idea about driving but this is ultimate sailing between the billionaire boys with country bragging rights. Think Ernesto is personally to blame for the shift in emphasis to television rights/audience, had Larry been in charge we would still have a four year design cycle with a more open rule.

    As stated on Sailing Anarchy - The Rule Rules, the limit on displacement was probably due to the two year program. I would have liked to have seen a lighter but more boxy boat racing a heavier more rounded one and I would like to think the designers/teams attending the design rule meetings would have requested this, showing the guts to face the challenge.

    So, with the massive computer power available, vpp simulators and tank testing, do you think it is possible for a syndicate to explore the variations of disp/beam before settling on the basic parameters of stability and getting down to CFD refinement, in the current two year cycle? What's involved, how would a team go about this? Or does no one think a lighter boat can be faster?
     
  2. xarax

    xarax Previous Member

  3. Guillermo
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    Guillermo Ingeniero Naval

    Thanks, Xarax.
    Because of its interest, I think it's good to copy the text here:


    33rd America's Cup
    The birth of the AC90 rule, by Tom Schnackenberg
    15-11-2007

    “Factors in the rule”

    1 - Deed of Gift requirements

    For a sloop, the maximum length on load waterline is 90 feet, and the Defender is expected to provide a course free of headlands, and “practicable in all parts for vessels of 22 feet draught of water”

    These two numbers suggested that if a new “high-end” class was envisaged, a length of 90 feet and a draft of 6.5m would be plenty.

    The old ACC V5 draft was 4.1m, and there are many harbours around the world for which 4-5 metres depth is all you get, so from this came the notion of a lifting keel. The work involved in lifting the keel (and bulb) is enormous, so it made sense to require that it stay fixed in the measurement (down) position while racing.

    2 - Valencia infrastructure factors

    How was the beam decided? The original ACC yachts had beams of 5.5 metres, and it seemed that a beam like that, or even a little larger, would be in order. The obvious maximum would be 5.7 or so, as that should fit into the travel lifts.

    When I joined ACM in early September, this was the thinking, but then it was decided to check out the steel pilings around the Darsena and see if these would provide problems.

    This check revealed that most piles were about 5.8 m apart and a couple of bases had spacings of only 5.7m.

    When you allow for the necessary structure for a floating dock each side, and some padding along the sides of this dock, you quickly realize that 5.3m is maximum for the yacht’s beam, and even then it will probably be squeezing against both sides of the padding while docking.

    We could of course pick up the piles and move them apart a half meter or so, but that doesn’t fit in too well with cost containment

    How about weight? When I got into the loop, no firm decisions had been made. The existing travel lifts can pick up 25 tonnes, and perhaps a bit more. A new class could be developed around any weight within quite a broad range. This range would run from say 21 or 22 tonnes all the way to maybe 30 tonnes, for the same length and beam, and all these would be valid boats with their own strengths and weaknesses.

    If the boat is very light, (down around 21 tonnes) it tends to lack power upwind, and in fact might not go much faster than the old V5 boats, which would be embarrassing, but not the end of the world. With the light weight you do get a boat that is very quick downwind, surfs easily, and probably is lot of fun in that direction.

    As you add weight, the boat picks up power and stability faster than it gains drag, so it goes better upwind, loses less distance while tacking, and is slower downwind. If we wanted pure match-racing boats, we would tend for boats that were simply bigger and faster but similar in behaviour to the V5 boats. In that case we would pitch the weight somewhere between 25 and 30 tonnes.

    It seemed that 25 tonnes was a pretty good compromise. It caused no problems with travel lifts, the sailing world was ready for more performance downwind and the crew could make it work upwind OK.

    The next thing to consider is sail area. People looked at a boat of 25 tonnes and 5.7m and calculated how much sail force would match this sort of boat. The stability would be about 50% more than the old V5 and so for a first guess the sail heeling moment could also go up 50%.

    In fact, with the desire to make the boat more exciting and demanding it was thought that we should increase the sails by at least 50% (and therefore the heel moment more like 70%) just to keep things interesting.

    Then the idea came out that perhaps spinnaker area could be unlimited.

    It is restricted for all practical purposes by the distances between the masthead, bow sprit, and sheeting blocks, so why bother to curb them further, with all the effort involved in measurement?

    Initial report to Challengers
    When we met with the challengers on 13 September we suggested 25 tonnes, 5.3m beam, and the length and draft exactly as announced in July. Sail area about 510- 530 sq m upwind with a rig height of 39.4 m above sheer, and unlimited spinnaker area.

    As far as spinnaker girths were concerned, it didn’t serve any useful purpose to limit or measure sail girths, so light air code 0 spinnakers could be triangular in shape and the biggest ones would be a bit like giant versions of the code 4 gennakers we saw in the last Cup.

    Because the stability had not gone up as much as the heeling moment, the boat would be de-powering in lighter winds, so we would be very unlikely to be able to race in winds much over 20 knots. Even 18 knots will be plenty. This will be fine for Valencia, but in a heavy air venue there will need to be quite a range of mainsail sizes to suit the different wind strengths.

    With these dimensions in place, it was time to think about the other features of the boats.
    - How many crew?
    - Is there any reason for genoa overlap?
    - Hollows in the hull? Banned or allowed?
    - Rotating or fixed masts?
    - How much freedom for the bowsprit?
    - One rudder or two?

    Speaking of crew, how many? To sail the boat with comparative ease, similar to V5, 24 could be a good number. However it has also been decided that this boat should be more demanding, so the number was tightened to 20 and the weight limit removed. The average crew weight last time was about 92 kg, and it may well be that this will lift by a few kg for the new AC90 boats.

    Some people worried that it will lead to fat giants sitting on the weather rail, but it is obvious that every crew member will need to be athletic and they are not permitted to sit on the windward side to increase stability. They will need to be fit, strong, and superb yachtsmen.

    Many of the handling problems that you see on boats start with mistakes, or breakages, not with lack of strength!

    Genoa overlap is really only useful in an underpowered boat, and also tends to shorten the life of the headsails. It can help the balance a little, but there are other ways of achieving that.

    Having no overlap allows much longer spreaders, which makes the masts and rigging lighter.

    Masts we decided to keep simple, at least for the first version of the rule. We were advised that masts needed to be less than 41m in their packing cases for air freighting from NZ. After hearing this, we settled on max length of just under 39m above the deck and built the sail plan around that.

    Hollows in the hull took on a life of their own. We had a philosophy that we should try not to have rules just for rules’ sake, and with the lack of overhangs, the need for a no-hollows restriction was removed.

    In the end we were all persuaded by the argument that allowing transverse hollows would lead to “aircraft carrier” shapes for the after part of the boat, and so for aesthetic reasons, we decided to ban transverse hollows, but still to allow longitudinal ones.

    The bowsprit went through a few rule gyrations as well, It started out in the rules as a simple spar strut, fixed during the course of the race. Then we wondered whether we would need spinnaker poles, so that the boats could change modes downwind, and open up more passing possibilities on the run.

    The sheer size of a useful spinnaker pole was daunting, so we investigated a bowsprit that pivoted about a vertical axis. We thought that perhaps it would fold back alongside the hull upwind. This also had problems, so we investigated folding it back the other way to sit on top of the deck upwind. Subsequently for a while it was seen as not pivoting, but simply retracting for pre-starts and upwind. In the end we wound up back at the beginning, with a simple fixed bowsprit.

    The question of one rudder or two is too hard to predict and so the rule allows both choices. It will be interesting to see where we end up.

    One thing we decided to allow was changing the shape with filler, and trying out slightly different shapes without restriction. We put a 200m limit on the thickness, which should be plenty. In this way teams can try out some different shapes, and once they settle on something they like, they can apply to the Technical Director for permission to change the rule-legal structural hull shape (within the 50% allowable limit) and get back to a light fast hull.

    The thinking was that this is pretty cheap and quick and allows people to get a boat in the water more quickly than if you have to commit more seriously and finally before starting.

    There were lots of lively discussions all along the way. What was great was that these were out in the open. People were very frank “that won’t work for us” -- “We think that will lead to an ugly boat” “If we leave the rule like that it will lead to all sorts of weird ideas” etc. etc.

    We didn’t have people retiring to a corner to have a confab and then returning with a cryptic “our position is that the rule should say the following”

    One interesting discussion was on the J dimensions. It affects balance and mast position and of course people were working very hard to get the right answer. (Nobody wants the mast to end up too close to the lifting keel!)

    In the end we settled on the average of several recommendations. We could have specified a range of J’s and let people experiment, but in the end decided that in the context of a short time frame and tight sail limit, it would be better to settle fixed numbers for max J and maximum mainsail area, and live with these values.

    Another problem which was peripheral to racing, but very important for practicality was the keel lifting situation. After some study, it was decided to require that the keel be lifted to 4.7m draft, as this is much better from an engineering point of view. We spent hours discussing how to word the rule on this and it is still perhaps one of the least satisfactory pieces of writing in the document.

    About half way through the process it was determined that we would reduce the displacement to 23 tonnes as a concession to legal wrangling in the hope that this change would end the situation. The change in weight lowered the righting moment by about 10% and the mast height and sail area were also reduced a little to keep the boat on its feet.

    This lighter weight will make the yacht a little slower upwind, and a little livelier downwind. The traveller team is going to be more active than they were on the old boats!

    So in summary, how will the new yacht go? It will be fast upwind and down compared to ACC V5 boats.
    It will be a real handful to sail. Probably quite twitchy at first and difficult to keep in the groove, as people come to grips with helm balance, sail shape and trim

    It will definitely “light up” downwind in a breeze. In fact, even in the light Valencia conditions, it is much more responsive to wind strength and so there will be more passing opportunities caused by variations in wind strength.

    Do we need to change any rules to suit the boat? It is too early to say, but areas of concern would be dial-ups in pre-start and the likelihood of trawling at gates. We don’t want crews to get in the habit of casting off sails at the bottom mark!

    All in all, it is going to be an exciting couple of years!

    Tom Schnackenberg
     
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  4. BOATMIK
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    BOATMIK Deeply flawed human being

    Howdy Guillermo,

    thankyou hugely for putting this thread together.

    I think the new boats do represent a big step forward as far as making the design side more interesting.

    From my view it may have been more interesting to stipulate a mast height and hull draft and allowed displacement and sail area end up wherever they end up to make the fastest boat around the course.

    Schnackenberg is right of course, upwind favours a heavy boat. And there needs to be an upper limit to stop things getting too heavy, but it would have been way more interesting to leave the bottom end of disp open under the box rule to allow people to go ahead and work out solutions to the problems with light displacement that he points out.

    Maybe someone has shares in a lead company? (I am not being serious about this - it is supposed to be funny)

    So maybe an even less prescriptive rule might be nice should this current thinking all work for next time round.

    To try and push a light boat to perform differently from the way they do now might be a really interesting technical problem and a real breakthrough with broader implications - and lighter would mean that the downwind legs would become less tedious to watch.

    However, I am at least 4.6 times more interested in this AC compared to the last few.

    Thanks again, and I will be dropping by for further updates.

    Michael Storer
     
  5. Guillermo
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    Guillermo Ingeniero Naval

    Attached Files:

  6. BOATMIK
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    BOATMIK Deeply flawed human being

    Howdy,

    When I saw this image a few weeks ago I thought it certainly does look a lot more relevant to everything else that is going on at the moment.

    Would be interesting to see a set of parametric VPP polars for something approximating the two sets of proportions.

    Michael
     
  7. Guillermo
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    Guillermo Ingeniero Naval

    Michael,
    I'll try to get comparative polars from Manuel, but I can promise nothing.
    Cheers.
     
  8. lazeyjack

    lazeyjack Guest

    I was always impressed by Tom S guillo, I met him when he talked to the NZ export group of which I once was a member
     
  9. BOATMIK
    Joined: Nov 2004
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    BOATMIK Deeply flawed human being

    Cheers Guillermo,

    See how what happens - to get something from manuel would be very interesting, however, I was really seeing if someone here had a vpp and a spare half hour.

    I'm really just interested in a very rough idea based on proportion. Of course we don't know how things will end up at this point - after all the final hullshapes wil be going through a few hundred iterations before anyone reaches for a roll of prepreg.

    So no detail - just proportion.

    I think there might be some interesting methods to try and keep the wetted surface down for the light stuff. You can see by the comparison of waterline lengths what a struggle that is going to be.

    The old boats were somewhat "wetted surface minimal" at some sacrifice to form stability. I doubt these boats will turn out anything like a skinnier version of the Open 60s (or do I mean minitransats - or Pen Duick 5?) which seem to be driving a lot of people's thinking in general design trends.

    And Schnackenberg's idea about spinnaker measurement is spot on - Good Creativity!

    Interesting, interesting, interesting

    MIK
     
  10. Guillermo
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    Guillermo Ingeniero Naval

    Hi Stu!
    How's your channel crossing thing going on?
    (Off topic, sorry)

    MIK,
    I'll see what I can do.

    Cheers.
     
  11. TimClark
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    TimClark Senior Member

    I decided to mess around with a design for a little bit and the following attachment(s) is what I came up with. I would list all the hydrostatics but the text for some reason is not working on the program, so that limits it. However, the boat is completely conforming to the rules. However, the beam is slightly narrower than the max allowance, and if I could do it again I would for a wider beam to increase the form stability.

    TC

    [​IMG]

    [​IMG]

    [​IMG]
     
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  12. TimClark
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    TimClark Senior Member

    One last one....

    [​IMG]
     
  13. Guillermo
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    Guillermo Ingeniero Naval

    Excellent, Tim!
    The missing part now is knowing average wind and sea conditions at the regatta field, to optimize for those conditions with the help of a VPP program (leaving aside structural matters).
    Perhaps it wouldn't be necessary to increase beam, depending on speed prediction.
    A pity the hydros thing. Can you post main data, at least?

    Cheers.
     
  14. RHough
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    RHough Retro Dude

    The average wind during racing hours in Valencia should be about 12knots IIRC. 2007 was a light year.

    I'd design to be fully powered up at 10 knots true and hope to never have to sail the beast in 20+ :D
     

  15. RHough
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    RHough Retro Dude

    The ruling is in, CNEV is out ...

    The ruling came in today.

    GGYC's website says they would like to negotiate AC racing in 2009 with all challengers and Alinghi. Failing that they would like to get Alinghi to race the DoG match in the new AC90's.

    SNG says they look forward to talking with GGYC and getting the event back on track.

    It looks like the new boats could be a go for 2009 after all.
     
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