Sail Loading on Rig, Rig Loading on Vessel

Discussion in 'Sailboats' started by brian eiland, Oct 14, 2003.

  1. brian eiland
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    brian eiland Senior Member

    Lets put this way, there are quite a few builders that believe they can simply put a BIG SLAB of stainless steel ‘chainplate’ at the side of the hull shell, and thru bolt it, and that is adequate for attaching the shrouds, etc. In many cases this is true, and it has worked out okay. It’s not exactly an ‘engineered solution’, and in the case of unballasted boats seeking to be very lt-weight, it’s certainly not a finessed solution. I don’t think Alinghi nor some of the ocean racing multihulls would be satisfied with these ‘brute solutions’, but I have seen more than a few cruising multis with these same big slabs of SS.

    I don’t really care to get into a name calling exercise here, but I might make the suggestion that you have a discussion with some of your local boat surveyors. I imagine they have quite a number of stories they can relay on any number of cases.

    And then just consider the many thousands of vessels that ‘tabbed’ their bulkheads and other reinforcements (substructures?) in with polyester resin wetted-out fabric. Polyester resin is great for sticking to shoes and skin, but it certainly doesn’t like to stick to itself. Ask your surveyors about some of these problems….how many has he seen come unbonded.

    How many production manufacturers might suggest that it’s okay to hoist their vessels up by the chainplates….this should be doable, right?
     
  2. water addict
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    water addict Naval Architect

    Maybe so- perhaps the builders aren't following the plans properly. There are lots of examples of builders short-cutting the plans without checking the engineering, in not just boats. But isn't that less rare in modern production boats where the risk of liability is a lot higher than in the past (dunno, just a query)? Also I mistakenly was thinking of monohulls as I don't frequent cruising multis often.

    I would guess with some of the cruising multis designed for charter type sailing that they are quite robustly built and that the hull skin is fine in shear and the deck skin is plenty strong to disperse the compression load developed by the shrouds. But dunno for sure. It would seem any nav.arch. would do at least a cursory check of the chainplate structure when developing the design, but maybe not. Whether the builders build to spec....
     
  3. kimper
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    kimper Junior Member

    Gutelle method

    Hello,

    see Gutelle method "FRENCH MAST VI-II.pdf" in previous post​

    Is anyone could explain me the mathematical process to find the load in genoa (noted A and D). I am not quite sure about my own result.:confused:
    kimper
     
  4. MikeJohns
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    MikeJohns Senior Member

    Kimper
    I worked through his method a while back unfortunately I seem to have deleted the mathcad file.
    I seem to remember that he didn't give a full analysis for the headsails as he did for the main. If I get time I'll I'll check my backups.

    Where do you get stuck?
     
  5. Kiteship
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    Kiteship Senior Member

    It's interesting to me that designers are willing to tackle the enormous problem of accommodating these loads, and spend so little time developing means to eliminate them altogether. Kite rigs never develop--in any part of their structure--loads in excess of their actual net thrust.

    A typical racing mainsail, say 500 sq ft, might develop a total force of some 500-750 lbs, in winds of 18-22 kts. In order to do so, it will directly develop mainsheet loads of more than a ton, forestay loads to match and shroud loads in the same range--and mast compression loads equal to the sum of all 4. If a multihull, this compression load will be taken by dolphin striker intoto water stays taking 6-8 tons of load (which is devolved back into compression of the forebeam)--all to get a lousy 500 lbs of sideforce out of the mainsail.

    Is it too obvious to think that this is kinda, well, dumb, when you can use a kite which has no weight, and develops a *total* force of 500 lbs? The kite has no interior forces higher than this (if it did--say by design--such forces could be dissipated into multiple bridle lines at the source) and the only force on the hull is simple tension--applied at a single point, very close to the boat's daggerboard. The hull structure thus needs no ballast, no beam (beyond that needed at the dock to keep the boat upright), no "56 ton" water stays, no heavy crossbeams, no mast, no boom, no bowsprit, no central pod, no shrouds, on and on.

    Sure, kites aren't yet developed sufficiently to replace mainsails. This I grant. Once upon a time *mainsails* were not sufficiently engineered to take the loads they routinely accept today; the science has progressed. My question is why is nobody working on the kite angle, despite the astonishing reduction in weight, inertia, windage, and cost they can offer? Alinghi cost an absolute bomb; for a small fraction of that sum, really high performance kites could have been developed, and these sort of problems would disappear for all time. I just don't get it...

    Dave
     
  6. Kaa
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    Kaa Wanderer

    One word: "windward".

    Kaa
     
  7. kimper
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    kimper Junior Member

    Hello sorry for the late reply

    i get stuck there

    I would like to set up a spreadsheet but, even if i can do it graphically, i can't do it mathematicaly.
    regards
    kimper
     
  8. water addict
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    water addict Naval Architect

    I think it is more of a practical issue. For long voyaging, messing with a kite in trying conditions like green water pounding on deck, seems a tough one to get over. How to shorten sail if a squall comes up in 20 foot waves?

    Maybe for shorter passages in conditions that are predictable it could be a good solution. Perhaps something like day charters in the Carribbean for some of the excursion cats I could see that working. Steady 15-20 knot trades, you could put a kite on a powered winch to reel it in and out, might work there?
     
  9. Kiteship
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    Kiteship Senior Member

    You miss my point, Water. You're speaking about *today's* kites. Kites today are where sails were 50, 80- years ago. My point was, if a fraction of the development work were put into kites that's been put into sails, they'd do anything sails can do today--at a fraction of the "cost" (not just dollars, but every possible definition of "cost")

    This is a very common misconception, Kaa. I've personally been sailing with kites--as high upwind as any sailboat--since 1978. Kites go to windward as well--and as fast--as any soft sail--today--and have the potential to go as well as any wingsail if optimized.

    FWIW, I have a book about kite sailing published in 1827. The only engineering drawing in it describes precisely how to sail *upwind* with a kite. 1827.

    Cheers,

    Dave
     
  10. Kaa
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    Kaa Wanderer

    Interesting, I didn't know that. I've seen kite-propelled kayaks and canoes, but their rigs were billed as pure downwind ones. How does it work? Is the kite a wing, basically?

    What's the biggest kite-powered boat that you know of?

    Kaa
     
  11. Mychael
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    Mychael Mychael

    Dave,
    Can you post some pics of these larger kites. (with ship attached).

    Mychael
     
  12. Kiteship
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    Kiteship Senior Member

    No "ships," Mychael. As I said, our records are on smaller boats--to 50 tons. You can see photos at www.kiteship.com. You can see our competitors' work at www.skysails.info

     
  13. Kaa
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    Kaa Wanderer

    Ah, I am sorry, I expressed myself unclearly.

    What is the largest boat that you know of that uses kites as her ONLY method of propulsion? A boat that's used regularly for recreational or commercial purposes and is driven by nothing but kites?

    You were making a point that in your view the conventional arrangements of mast(s), standing rigging, sails, etc. are a very inefficient way of generating propulsion from wind and that kites allow you to get as much force in a much simpler way.

    That may or may not be true -- I don't know enough about kites to have an opinion -- but certainly getting enough force is not the only design objective of a wind-driven propulsion system. Among other things, there's also flexibility (aka ability to deal with different situations on the water) and it seem to my uneducated eye that this is the trade-off that kites make.

    Yes, a kite will propel a boat across the water "simply". But the range of wind and wave conditions that a "conventional" sailboat can adapt to is much wider than what a pure kiteboat can tolerate. And if you want both conventional sails and kites, to get the best of both worlds, you're still stuck with the masts and rigging and all that :)

    Kaa
     
  14. Kiteship
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    Kiteship Senior Member

    There are none of any size (larger than, say, hobiecats). This is my point. The genre has such great potential it surprises me that almost no one is exploiting it.
     

  15. Stephen Ditmore
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    Stephen Ditmore Senior Member

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