Backstay problem on Gunter rig

Discussion in 'Sailboats' started by laukejas, Oct 19, 2013.

  1. gggGuest
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    gggGuest ...

    The Laser really doesn't strike me as a good model I'm afraid... The required task of this craft - basically a cruising boat - is so very different from a Laser. Basically the Laser is at its best when you're getting very wet messing about in waves: its a totally different requirement.

    I don't know if I'm just biased, but I'm really not getting away from the canoe type for this task. Its a kind of boat that is well established for the sort of task and roughly that sort of area - tremendous popularity in Sweden for example, and its a type that is of course well established for skin on frame construction.

    I really would be looking at one of the wider Canoe types. For one example of the advantages, the canoe stern is self trimming to an extent, and requires far less fore and aft trim than a modern dinghy type. The German Taifun class, for instance, is raced both singlehanded and (by juniors) two handed and would seem to be a better match for your needs. There's a drawing here: www.ic-taifun.de/fileadmin/sonst_down/Vermessungsbrief_Taifun.jpg, I did try linking it but its too large... There's far more sail area than you'd want on a cruising boat of course.

    I think I'd start by giving it a vertical daggerboard, a larger rudder than shown, leaving the jib off and making the rudder a bit bigger. But there are also lots of interesting and potentially relevant craft in Sweden, but I don't have the language to research their websites.
     
  2. Skyak
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    Skyak Senior Member

    I don't know if I am biased. My entire experience with monohull dingys is the laser and the Jet (international 14) but the laser does sail well to windward and I know there have been many boats designed that don't do as well. At any rate my recommendation is sincere and I think laukeja's result is a big step in the right direction. Now it has the shape of a fast dingy, not a keel boat.

    I for one would be very interested in discussing the merits and techniques of sailing canoe shaped boats. I understand the advantage a sharp stern has for heavy loading or a wider load range than transom hulls. I understand waterline length advantage at displacement speeds. I don't know how the transition to planing is affected (almost all planing hulls have transoms), and I don't know how sliding boards are used. Are they locked in place or do they allow you to move your butt around to adjust righting in fluky wind? If you lift off of them heeled do they slide down into the water on the lee side? Where do they run and cleat lines to?
     
  3. Skyak
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    Skyak Senior Member

    I am not sure this software has what it takes to calculate drag from eddies at the transom. It most likely just uses a formula based on hull shape properties with a term for surface area, one for waterline length, waterline beam, block coef....

    2.66% sounds like the surface area increase from the chines.
     
  4. laukejas
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    laukejas Senior Member

    gggGuest, thanks for the info and the link. I'll check it out. Until I find something, I'll work on Laser hull, but my model is dynamic, so it won't be very painful to change it later.

    Well, I wouldn't know that either. I supposed it calculates transom too. If not, how can I know if this change will affect things?

    Well, actually, I didn't add chines. I just removed the rocker, that's all. Take a look at these images from Delftship: first one is original Laser hull, other one is without rocker.

    So my guess is that this increase in resistance is either from generally worse hydrodynamics because of no rocker, or maybe DelftShip calculates transom after all.

    Including both models, in case you can open them.
     

    Attached Files:

  5. PAR
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    PAR Yacht Designer/Builder

    Lets see you want to carry a two person crew on a solo dinghy and you've removed displacement to accommodate this need?
     
  6. NoEyeDeer
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    NoEyeDeer Senior Member

    Brilliant bit of lateral thinking there. Maybe he's going to fill it with helium.

    Rubbish. Yost's SOF kayaks, as well as kayaks and other SOF boats made by other builders, often have rocker in the keel line. To take one obvious example, look at Dave Gentry's Ruth.


    No, the original Laser hull is not developable. The Deftship panel expansion tool will be fooled by some shapes. It will try to expand shapes that aren't even remotely developable. You have to use it with a bit of common sense.

    Check the hull in perspective view and "developability check" mode. That will show the hull in red and green. Green areas can be developed, red areas cannot. If everything except for the chine lines is green, split the hull into seperate layers (one for each flat panel), and set those layers as developable in the layers dialog. THEN you can expand them to see their true shape.


    Fabric is not paper. Fabric will stretch on the bias, so is far more forgiving of the sorts of shapes usually found in boats. Most SOF boats can be skinned without needing darts cut in the fabirc.
     
  7. laukejas
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    laukejas Senior Member

    No, not yet. I just compared both hulls with original Laser displacement for one person. I'm aware that with two persons numbers will be different.

    Well, if there was a clear side view, it would be easier to see, but it doesn't look like much of a rocker...

    I checked. All is red except transom. Yeah, that fooled me.

    I understand, but by my common sense, if you fold fabric on two axes, it's not a question of stretch, but of extra material that needs to be cut out. Or do you suggest making skin with no rocker, and then stretch it on hull that has rocker?
     
  8. NoEyeDeer
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    NoEyeDeer Senior Member

    Probably about the same as a Laser.


    ...is not relevant. Look at what real builders have actually done with real SOF boats.
     
  9. laukejas
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    laukejas Senior Member

    Well, okay, but can you explain how that rocker is created in the skin without cutting out that extra material?
     
  10. NoEyeDeer
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    NoEyeDeer Senior Member

    That.
     
  11. laukejas
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    laukejas Senior Member

    Forgive me, but I do not understand. If one folds the fabric to this shape (bent on 2 axes), there will be extra material. Nothing is stretched, but the wrinkles appear due to excess material. I tried this with paper, cloth, fabric - the results are the same. So could you please describe how one would fold the fabric around hull with rocker without having that extra material?
     
  12. gggGuest
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    gggGuest ...

    I don't know about real cloth and skin on frame, but with glass fibre cloth you can get a suprising amount of 3d curvature by judicious stretching and distorting, most especially if the cloth is oriented with the fibres at 45 degrees to the centreline. Just because the fibres started out at 90 degrees to each other doesn't mean you can't distort it a bit to get it to drape better. Its going to depend a lot on what sort of cloth you use of course, and I know nothing about your materials.
     
  13. Skyak
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    Skyak Senior Member

    Fabric is not paper an 20 to 1 scale is not full. This is not just a skin on frame, it folds, and to do that the plan for material is PVC impregnated polyester 1000 denier or more. To my knowledge all foldable designs use impregnated fabric, not bare canvas that is sealed after it is stretched like the majority of fixed SOF hulls. The weave of impregnated fabric has far less drape and stretch. The exercise of fitting paper gives good rough indication of the cuts that will need to be made but is beyond the worst case. Fitting plastic wrap would give a much more optimistic estimate. If you know of some post-fit sealing for the canvas that can fold season after season without developing leaks or adhesions I would like to see it.

    What I hoped he would learn from the paper model is that rocker does not make the hull much harder to skin and actually helps a little. The paper would be a good guide for where to make the folds.

    About the choice of the laser -it fits the 1.33M limit on stored size very well. this boat is complicated enough without multi-piece frames. It's a successful design that is well know and widely emulated. While it is designed for one it should do well with two by just adding vertical. It had to be optimal for one or two -I think one is that better choice and the size limits made the decision. Would still be interested if anyone has a better choice and reason.

    Developable doesn't mean much in this context but the hydrostatics of the chined version should match the laser fairly well, which is important and a major improvement over the first model which lacked form stability.
     
  14. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, fair enough.


    Try it. Get a Laser and load it up with two adults. My bet is it will drag the transom quite badly.

    BTW, my opinion is that the whole concept for this boat is "too clever by half". It'd make more sense to invert in a set of roof racks and build something simpler. This seems to be a case of someone coming up with a complicated idea that hasn't been done before, just to show why it hasn't been done before. :D
     

  15. laukejas
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    laukejas Senior Member

    I red again your described experiment with paper, and no mater what I did, I couldn't make a rocker. Maybe I'm missing something here, but once the paper is folded into a cone (more or less), it becomes impossible to bend. Maybe I'm misunderstanding. Is there some kind of picture or animation of what you have in mind?

    Sorry to persist on this issue, but it is crucial in this design, so I want to decide early on whenever I should stick with Laser hull with higher gunwales, or remove rocker... Later it will become difficult to change.

    You have a point, sir. However, the roof rack limit is about 35kg's. I have yet to see a sailboat for two that fit's under such limit (at least the hull), except for very expensive designs, using very light materials. Another thing is storage - I don't have a backyard or something, so I could store the boat only in storage room (which is 2 meters high). Third reason is that if boat would travel on roof racks or on trailer, it would significantly increase fuel consumption, and if this boat is moved often (which it will), it will empty the pocket pretty soon.
    That is, considering, I believe I can make this boat for similar price of a similar non-folding stich-and-glue boat. If I can solve these technical problems, that is.

    So no, I'm not doing this to show off or something. I just want to get sailing, and if I had money and space, I would buy a used OK dinghy with a trailer, or similar. But I can't, so I'm considering options :) Maybe it'll turn out that this SOF design isn't feasible. Who knows. I have to try, at least.

    Worst case scenario, I'll abandon fit-in-trunk idea, sew a big bag, adjust parts to be folding in 2 meters, not 1.3, buy a roof rack, and put as much of the long parts on the roof as weight limit permits, with everything smaller remaining in the trunk.
     
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