Idea: Simple, cheap bi/triplane rig conversion using a 'yard' - would it work?

Discussion in 'Multihulls' started by chris14679, Jan 7, 2010.

  1. gonzo
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    gonzo Senior Member

    The box is a wrong analogy. Put the box against something that won't let it slide and it will tip either way. Do some force vector diagrams and it will be more clear to you.
     
  2. chris14679
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    chris14679 Junior Member

    If you find me some water that isn't slippery, I'll put the box against something.

    ... but the box STILL won't tip unless you cheat and push upwards - even if you put it against something you would squash the box before you would tip it, but you would tip it easily pushing on the inside edge. I suggest you do some force vector diagrams yourself, or just go get a box and try it! This assumes the box has some weight of its own, as a boat does. Maybe that's what you missed from your force vectors.
     
  3. gonzo
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    gonzo Senior Member

    They are the same. If you push a box at the same angle on both sides, the reaction is the same. Your pet idea is flawed.
     
  4. chris14679
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    chris14679 Junior Member

    Not my pet idea - it comes from the 'Schionning designs' website, the most popular designers and builders of biplane cats. I guess you must know more about biplane cat stability than them.

    http://www.schionningdesigns.com.au/login/pages/images/RadicalBay.pdf
     
  5. gonzo
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    gonzo Senior Member

    diagram
     

    Attached Files:

  6. CT 249
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    CT 249 Senior Member

    I just realised that one thing got accidentally erased from by earlier post. The load on the forestay would be a combination of static rig tension and the loading exerted by the sail. That means that the forestay of a jib in use would exert more pressure on the yard and therefore the yard would be under twisting forces, around the axis of the mast. This twisting force would vary routinely, according to the comparative pressure on each forestay, which is a combination of sheeting angle, blanketing, amount of sail set etc. Given the inherent stretch in any system, one would guess that there would be a lot of torsion in the rig, which would be hard to control without very stout gear and high rigging loads.


    I just tried your cardboard experiment, using a wine box and a set of kitchen scales to measures the heeling moment.

    If the box was allowed to slide sideways, of course no amount of heeling force on the 'leeward' OR 'windward' side caused it to heel. However, unless you have lateral resistance no boat can experience heeling forces, so that experiment is invalid.

    And of course, the heeling force has to be applied above the part of the box that is restrained from sideways movement, just as the heeling force in a rig is applied above the keel. If you push directly opposite the area where the box is restrained, it doesn't matter where you push - if the heeling force was applied directly opposite to the restraining point, even an infinitely slim unballasted mono model won't heel.

    Once I stopped the box from sliding (just like a centreboard or keel stops a boat from sliding) the measured power that was required to lift the box to a certain angle of heel was exactly the same (within the accuracy of the measurement system) whether I exerted the heel on the windward side or the leeward side.

    Re "Not my pet idea - it comes from the 'Schionning designs' website, the most popular designers and builders of biplane cats. I guess you must know more about biplane cat stability than them."

    Couldn't we just use the same response to your idea? We could just say "the idea (of using a conventional sloop rig) comes from Lagoon, Hobie, Seawind, Gemini etc, the most popular designers and builders of cats. I guess you must know more about cat rigs than them".

    By the way, the 'rant' about the jib v main issue was in response to your claim that your rig would be more efficient as the jib luffs did not have a mast at the leading edge.

    I expressed my views politely. If you feel that polite feedback is 'ranting', please do not ask people for their views.

    PS - a lot of my thousands of miles offshore have been spent on a beam reach.
     
  7. Doug Lord
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    Doug Lord Flight Ready

    In the book, "Hydrofoil Sailing" by Alexander,Grogono and Nigg the "Exactly compensated foil stabilization system" is described on page 34. The description is for a low aspect single rig though I imagine a bi-plane rig would work better because the aspect ratio of the bi-plane rigs would be better than the single sail version of the system. According to the authors " no net roll moment shall exist when the center of gravity of the boat and crew is on the centerline of the boat. Theoretically such a boat will not heel under steady state conditions, and this will hold true on all points of sailing."
     
  8. gonzo
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    gonzo Senior Member

    Is that a theory from an alternative Universe? A rig that produces no heeling moment is a fantasy. This is another crackpot thread.
     
  9. DarthCluin
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    DarthCluin Senior Member

    While I would like to see this rig built, I question the validity of the box analogy. Perhaps you mean there is no lateral reistance if the centerboard/daggerboard/foil is up.

    http://www.youtube.com/watch?v=f8j5FSCuJvI
     

  10. rayaldridge
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    rayaldridge Senior Member

    This is a little hard to believe. In a fast multihull, reaching is very common. While speed causes the apparent wind to go forward, whether or not you end up sailing close-hauled depends on whicht point of sail you started sailing. For example, if you are broad-reaching, you may end up close-reaching in a good breeze. If you tack downwind, as you should in a fast multi, you'll be reaching.

    The Schionning biplane cats do suffer from the lee blanketing problem.

    As far as I know, the only biplane cat that has solved this problem ( according to the owner and designer) is Tony Bigras' Miss Cindy. He did this by sheeting the windward sail to the bow.

    I don't know why you think a light yard would suffice. Even if you treat it as a diamond stay spreader, it will still have an enormous amount of downward pull, as it is holding up a whole rig on each end. To withstand the compression, it will need at least as big a section as a mast designed to hold up a similar rig, I think.

    Another problem is what to do with the rigging needed to hold up a centerline mast on a cat. A freestanding centerline mast on a cat is impractical for a number of reasons. The shrouds needed to hold the mast up will interfere with sheeting the rigs that hang from the yard.

    I have nothing against experimental rigs (I put a sprit sloop rig on a cat) but I'm a little dubious about this idea. You might want to try making a sailing model before you attempt a real boat.
     
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