America's Cup 2010: Race Thread

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

  1. Gary Baigent
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    Gary Baigent Senior Member

    Phil, you had a Twiggy -I thought Lock was close to producing a very good tri there, wide platform, lightweight, looked very good, was not expensive (sort of in line there with his very popular B24) BUT the rig was off course IMO, should have been antipodean 3/4 with a larger main, (still stepped well aft though) smaller headsail. Also about the same time, Malcolm Tennant was drawing his overlength forward amas - maybe that configuration would have helped the Twiggy's keep there bows up when going downwind in wave ..... or, very quietly, foils.
    On the point of square cats, none exist. The problem with such a beast, if there was one, compared to the superior platform of a trimaran, would be you can't get any dihedral with a cat (stating the obvious, the central pseudo-vaka is no help there)... and that is their failing. BMW-O has revealed to everyone that, for an all round boat, the tri is the way to go.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    whot?

    -------------------------------
    If this is true how can this be true??
     
  3. Gary Baigent
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    Gary Baigent Senior Member

    Well. Doug, old cobra, there are no square cats that I know of, plenty of 40-50-60% versions plus a small few of around 75%, but no 100%'ers. The fact that a tri has dihedral in its amas allows it to tack better than a cat plus allows it to have greater beam (not counting racks) and with foils, better downwind in hard conditions than a cat, a cat without foils that is. Previously this is one area where cats have been considered superior. Dihedral is the major and key difference and a tri is better in light conditions, better structurally, better mast base load carrying, a faster boat etc. etc, because of the central hull and the dihedral - F40's proved this and it is common knowledge now. We're talking about performance multihulls, not cruiser designs, where to date, the cruising cat has proven the better design for those purposes ... but even that could change.
     
  4. Gary Baigent
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    Gary Baigent Senior Member

    And, sorry, on your point about 100% LB cats, you, Doug, will have to build one and show us the superiority of it over a square tri. Not saying it can't be built ... but you'll be pushing defecation uphill, so to speak. Just IMO of course.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    Square cats

    Model testing tells an entirely different story about tacking ability for square cats as long as the foils are long high aspect blades. I wonder why-that is an area where model testing has scaled up very well.....
     
  6. Doug Lord
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    Doug Lord Flight Ready

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    I'm not at all sure a square cat will be superior to a tri just that it will tack as quickly as a tri. If the reason for not doing a square tri is tacking ability then my model testing suggests that is an invalid reason.
     
  7. catsketcher
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    catsketcher Senior Member

    Models may not bear this out

    I can't buy the square cat tacking well Doug. Cats have a greater roll moment of inertia - it takes more to start turning them and more to stop them. This because their hulls are heavy and deeply immersed.

    In ocean going multis increasing roll moment is a good thing. It is one reason why cats are better in lying ahull than tris. A tri can easily get flipped over its fore and aft axis compared to a cat. In tacking it is not a good thing. I can get my little 6 metre cat to tack well and it has large rudders but I would think that a tri with only one hull to turn (during much of the tack) and most of its mass over the centre of gyration would tack better.

    Scale is important here. Roll moment becomes very important as you increase in size. Roll moment and stability go up by the fourth power ( I may be wrong on roll moment) as you increase in size. Inertia problems may not show up in a model. I seem to recall an early (and heavy) wing on a C class caused big problems at the bottom mark because it just wanted to go straight when the cat turned uphill.

    The gist of this is that a cat will always be safer in beam seas because of roll moment but that this takes away nippy tacking. You can't get both.

    cheers

    Phil
     
  8. Doug Lord
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    Doug Lord Flight Ready

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    Fairly sweeping statement there, Phil....do you think that is accurate in regard to Alinghi?
    I'm not sure I follow you here-do you mean yaw moment?
     
  9. catsketcher
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    catsketcher Senior Member

    Very sweeping Doug and yes probably mistaken in the case of Alinghi but they will certainly be more deeply immersed than a tris floats (when the tri is built to a similar standard).

    Don't get me wrong - I love cats - I own 2 and a half largish ones its just I worry about going square. I use a low down flare like Shuttleworth to try and get the CB going forward when pressed. It works nicely for a cruising style cat. Strangely in the case of Alinghi the CB will go sternwards as they press due to the bow style - this is obviously fast but shows that drag reduction seems more important than volume on its own. (Okay there is more to this due to stern shape and sections too but the A class bows themselves will tend to shift CB aft rather than a traditional raked bow that will shift it forward when pressed)

    The thing about going square had its first origins with me when I read an article about Conner's Stars and stripes by Britton Chance. As I recall BC was one of the designers of the hulls of S and S. He wrote an article about how you wanted less sideways stability in this wing mast boat. It seemed sensible to me after my Twiggy sailing and after the Aussie multihullers had shoved numerous multis over their bows in the 80s. There was a good reason monohullers disliked us - multis really stuffed up a few mono races in the early 80s with capsizes, rudder breakages and the like. Wistari the mono was going well, went to help a multi in trouble, got time allowance, got protested for too much allowance and had to settle for second. Not good.

    I think I just went off thread

    Cheers

    Phil
     
  10. Gary Baigent
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    Gary Baigent Senior Member

    Phil, I'm not flaming, oh, no, no - but you bloody aussies kept arsing over because you had macho fixations about carrying masthead kites, flat bloody off downwind in large seas. Is that not so?
    Britton Chance's comments about sideways stability on the 50% B/L S&S 88 came from the extra forward drive created by the hard wing.
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    I'm not so sure that a wingsail is more powerful than a fabric sail. From feedback from folk who were experimenting with the wings, admittedly a few years back now, I gathered that for a contest between two similar hulls the fabric sail was found to out-accelerate the wingsail consistently and the wingsail experiments were frequently discontinued due to this.

    Where the wing will always score is its efficiency, i.e., low drag. Matched with a hull with similar low-drag characteristics and a crew that understand how to use it, this should deliver a vessel that is able to sail faster on upwind legs and sail closer to the wind for significantly improved WMG. Of course if the hull cannot deliver then the fabric sail, with (perhaps) more thrust and certainly a crew with more experience, can turn the trick.

    That's the theory, anyway. Reality often has a nasty way of confounding the theorists but in this case it seems that theory was made good. However there were so many other differences between the AC boats, rig, hull and crew, that it is hard to be sure either way.

    The best boat, skipper, crew and legal team combination will almost always win the race!
     
  12. catsketcher
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    catsketcher Senior Member

    Interesting you say that Terry, about drag I mean. I have just come across a treasure trove of old mags from 1966 onwards. In it the designer of Miss Nylex's wing talks about how she could point at about 30 degrees true. Thats sounds amazing. I should just ask Lindsay Cunningham about Victoria 150, the Edge and the like.

    Gary - I have never really raced multis. As I have said on other threads I like them for cruising but after racing the Twiggy a bit I didn't like the drive at all costs attitude I found. Dinghies and monos are more my scene for racing - I never got the hang of driving a boat I can't afford to break and I can only afford to break cheap ones or ones that come back up.

    cheers

    Phil
     
  13. Gary Baigent
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    Gary Baigent Senior Member

    You know Phil, that Miss Nylex early wing was very clean, just a couple of flaps but otherwise a very clean foil ... and it's no wonder that boat could point high. But because they couldn't get much camber offwind, was their Achilles heel. That changed with Cunningham's Victoria because that wing was all camber and slots - but with a loss of the high windward pointing ability of the Martin/Buzaglo boat. All very interesting and a number of decades ago. Have you any photographs of your Twiggy?
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Phil & Gary: I have been following wingsail development in a half-assed way for a couple of years and often wondered why boat designers seemed slow to adopt them. Although there are many examples the AC still took me by surprise; I thought wingsails were still largely experimental.

    The Miss Nylex story was of interest to me; even using fabric sails AC boats in the past routinely could achieve 35 deg of the wind. I took a quick look at the numbers. If I understand it right, the ability to point high primarily involves 3 factors:

    1) wingsail lift/drag ratio: a good profile beats 20:1 in theory but let's say 10:1 to allow for other above-waterline drag or 6 deg.

    2) The hull's leeway angle is similar, I'll assume 4 deg for that, may well be better in practice.

    3) That leaves the vector relationship between total sail force and thrust. To sail 30 deg off the wind thrust = Fsin (30-6-4) = 0.34B

    This thrust is available to overcome hydrodynamic drag. Undoubtedly a good boat can beat these numbers but VMG is more important. While it is not usually worth sailing as tight as possible the ability to do so translates into good VMG at other sailing angles. These numbers are not necessarily typical, by the way, just pulled out of the air to aid my own understanding.

    All this might have been revealing to pre-Nelson sailors but it’s old hat now. However, it was useful to me, as I had not appreciated before that a multihull is putting the cosine vector from the sail's lift to good use when it lifts the windward hull out of the water. Also it does reveal how much less important the sail's lift/drag is than its thrust; obviously it's the one-piece wings that lack power and slotted wings with variable camber are another matter altogether. I have been looking at a scheme to deploy a fabric sail from the trailing edge of an unslotted wing to do the same thing.

    Creating the math to optimize VMG is not so trivial; it would need the characteristics of the hull and sail as math expressions. No doubt it already exists at least as a computer model, but it's different for every boat and finally it rests in the mind of the skipper. Maybe computers should be banned for boat races the way they are already at casinos.
     
    Last edited: Feb 25, 2010

  15. Doug Lord
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    Doug Lord Flight Ready

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