tortured ply theory

Discussion in 'Multihulls' started by peterchech, May 5, 2011.

  1. peterchech
    Joined: Aug 2010
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    peterchech Senior Member

    I asked this on woodenboat forum as well but there are prob more multihullers here:

    I am in the process of designing a small proa that I hope can compete with some of the beach catamarans in my area. I intend to keep it light, 4mm okoume, etc. Prob gonna be about 16 ft long with a l/b ratio of 16/1 (any thinner and it wouldn't float my 185 lbs I think!).

    Anyway, after considering multichine S&G, I came across several designs that use tortured ply (here's just one http://home.vicnet.net.au/~mosquito/documents/building.php).

    So, the tortured ply "method" allows plywood to bend on two planes, instead of what is normally just one. Just how much can it be bent this way? Is precision reasonably attainable with this method? I mean, if I design bulkheads, will I be able to "torture" the ply to those bulkheads, or do I have to start with a precise angle at the chine and then it will only "torture" so much?

    Thanks for any feedback.
     
  2. CatBuilder

    CatBuilder Previous Member

    The 4mm Okoume will bend quite well in one direction. It's a real bear to get it bending in the other direction and it will only bend "just a bit" as you are thinking,

    Take a look at some 3mm plywood bending in one direction, while being attached together with scarf joints:

    http://www.multihulldesigns.com/pdf/cm33.pdf

    It's much easier to bend in the "bad" direction after it's securely attached along the length of the boat. It'll bend around in a tiny circle if you bend it in the good direction, like in the link.
     
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  3. peterchech
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    peterchech Senior Member

    That is a wonderful link catbuilder! I see your point.

    But I expect the ply thickness to be 4mm, or 6mm at absolute most. I expect that method would require multiple layers of ply to work, and I don't know if it would be economical to buy 3 sheets of very thin ply where 1 might do with tortured ply... Seems more like a "big boat" method... no?
     
  4. CatBuilder

    CatBuilder Previous Member

    Yes, that method does require multiple layers of thin ply, but I figured it would be a good starting point for you.

    I'm not really a "wood guy", so you have reached the limit of what I can help with. I'll let others come in with more specific information for you.

    I just know the basics on that stuff.
     
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  5. catsketcher
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    catsketcher Senior Member

    The Gougeons have a piece in the back of their book on boat construction on tortured ply.

    The problem with tortured ply is that you can only get the ply to do what it wants. I rermeber reading about a guy building a Taipan out of tortured ply and it was a testing process as he wet the ply and pushed it into a jig bit by bit.

    Then again I did build the hull of a kayak with tortured ply. It came together well

    cheers

    Phil
     
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  6. peterchech
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    peterchech Senior Member

    Cool do you have a pic of that kayak?

    I am seeing that it def has shaping limitations. I suspect that the tight radius of a narrow ama may prove nearly impossible even with 4mm. Then again here is a thread about a guy who built a proa out of 7mm fir ply using the method... http://wikiproa.pbworks.com/w/page/14592528/Te-Wheke-Proa-Hokianga-New-Zealand

    As far as precision goes, if I am an inch or so off in any direction I will find that acceptable, so long as that inch is symmetrical. Haha or even if it isn't, since these are the least symmetrical boats that exist! :p
     
  7. CatBuilder

    CatBuilder Previous Member

    Stitch and glue the keel first, then bend up the sides. Doing this in the correct grain orientation, you'll easily make the tight radius of the bilge.
     
  8. luckystrike
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    luckystrike Power Kraut

    Hi,

    I have designed and built the amas of a 30' Trimaran using Tortured Ply (TP). My information were from yellow West System Book (Gougeon Brothers). There ist a whole chapter that explanes building techniques and designing parameters. I recommend this to you.

    Its overkill to use Kurt Hughes Cylinder mould method to build a 16 foot proa hull, as 4mm ply ist flexible enough to be compatible with this hull if you build it in TP. 4mm is also strong enough for a proa hull if you do a glass sheeting.

    Here ist a link to the detailed building blog of the F16 cat "Blade", a state of the art racing machine with lots of round curves and volume. Note the extra middle panel at the keel line that alows this high volume.

    http://www.thebeachcats.com/pictures/?g2_itemId=11955

    If you want to play around with beach cats, your proa has to be longer than 16'. For my feeling.

    Greetings from the North Sea Coast, Michel
     
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  9. CatBuilder

    CatBuilder Previous Member

    Very nice link, Luckystrike. Exactly what I was suggesting.
     
  10. luckystrike
    Joined: Feb 2010
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    luckystrike Power Kraut

    Sometimes its good to have a large paper archive....

    Here is a scan of a 33 foot racing proa to be used on the great european lake races like the "Centomiglia" or the "Bol D Or".

    The proa ist designed with a wavepiercing hull to be built with non-exotic materials as a low budget boat. Of course the gibbons-rig shown here will be not powerful enough for a racer, the same counts for the "classical" crabclaw.

    I think the concept would work for a 21' proa as well. In my opinion this is the size needed for a proa to be competitive with the beach cats.

    Perhaps its an inspiration for your project. Sorry for the waterblot.

    And another link to the german monocat site that uses some advanced sort of Tp too. Look at "Bauverfahren" that translates construction-method in german.

    http://www.mono-cat.de/index.html#r%C3%BCck



    Grreeetings from the North Sea Coast, Michel
     

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  11. DCockey
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    DCockey Senior Member

    In keeping with the title of this thread here's some theory which I'll keep non-mathematical.

    First consider a flat sheet which is very, very, very thin compared to the minimum radius of curvature it will be bent to. Such a sheet can bend to any developable surface shape without any stretching or shrink. If the sheet has a grid on it before bending then it after bending to a developable shape each segement of the grid will be the same length as it was before being bent, and at intersections of the lines the angles between the lines will be the same. Since there isn't any stretching or shrinking happening no work is needed to bend the sheet to a developable shape.

    If the same very, very, very thin sheet is bent, pushed and pulled to a non-developable surface then it will have to stretch or shrink. Some or all of the grid line segments will be different lengths. Since it is stretching and/or shrinking work is required to force it into the non-develobable shape. Try to stretch or shrink it too much and it will fail.

    Part II discussing sheets which are not quite so thin.
     
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  12. DCockey
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    DCockey Senior Member

    Part II: More on plywood bending theory.

    What if the sheet to be bent isn't very, very, very thin, like real plywood, and is bent into a developable shape? In that case the the side of the sheet on the outside of the curve will stretch and the side on the inside of the curve will shrink/compress. The mid-surface between the two exterior surfaces will not stretch or shrink. Think about a sheet of plywood wrapped around a cylinder. The distance around the outside is longer than around the inside so the outside surface of the plywood stretches and inside surface shrinks/compresses. The amount of stretching and shrinking within the sheet is proportional to the distance from the mid-surface, and is inversely proportional to the radius of curvature. Double the thickness and the amount of stretching and shrinking will double. Likewise if the radius is half the amount of stretching and shrinking doubles. Too thick for the radius of curvature and the amount of stretching will be too much and panel will start to crack on the outside. Also work needs to be done to bend the panel due to the stretching and shrinking.

    So how about "torturing" plywood into a non-developable, compound curvature? In that case the total stretching and shrinking will be equal to the sum of the stretching and shrinking/compressing needed for the mid-surface to adapt the compound curvature plus the added stretching away from the mid-surface on the outside of the curvature, and the shrinking/compressing on the inside of the curvature. Work is needed to do both. As long as the total stretching or shrinking/compressing isn't too much anywhere the plywood won't crack or wrinkle. Work is needed to do the stretching and compressing/shrinking and it can become large, particularly on a thicker panel.
     
  13. catsketcher
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    catsketcher Senior Member

  14. peterchech
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    peterchech Senior Member

    I was hoping not to sheath the vaka in glass, except at the keel. I have no experience with tp, would unsheathed 4mm be strong enough if I decked it? I often drag the canoe up on the beach, would this be an issue with the 4mm?

    I may just stretch out the hull to 22' I'm not sure yet, when I stretched out my wa'apa outrigger canoe just 4' from 20' to 24' it was a HUGE difference in not just speed but also general seaworthiness and comfort.
     

  15. bruceb
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    bruceb Senior Member

    practical torture (of ply)

    I have a crowther tri that I have been doing some repair on. It was designed to use 4mm but was built in 6mm 3 layer ply. Most panels have about a half inch to the foot bend the "good" way and around half inch to four feet the long way. it works- there are a lot of these tris, but the ply used makes quite a difference. Good marine grade fine grained ply works well, cheap soft wood material opens and cracks on the outside surface. I epoxy coat the inside surface the day before and then clamp it bent the short way overnight. It makes assembly much easier. It still takes lots of straps and clamps to pull the panels into place. Strong, well supported frames are necessary to take the loads until everything is glued together. The 4mm is strong enough for a 24 foot tri main hull, it is probably overkill for a 16 foot hull. The tortured panels are very strong when finished. B
     
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