tortured ply theory

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

  1. Ilan Voyager
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    Ilan Voyager Senior Member

    Luckystrike is very right. No need of a cylinder mold, several plies and other complications. The Gougeos bros book 24th chapter has all the info needed.

    Go to 18 feet as several catamarans of this size has been designed for the compounded plywood, and maybe plans are avalaible for very cheap. The Gougeon bros used to sell the plans for a class A called Michigan, ask them. It's an excellent start as it covers also the centerboard and rudder fabrication and gives all the internal framework. You may add 1 to 2" for a higher freeboard. So you have the main hull of your proa if it's monodromic ie tacking like a normal sailboat. Remains the ama: rule of thumb volume about 95 to 110 % of the total displacement, no need for bigger or you'll have a cat. No fun. Same plywood just a thinner hull a bit shorter but about the same shape of the main hull. Do not make it V shaped, to much wetted surface.

    Use 3mm okoume plywood 3 equal plies, marine best sort. 4mm will give you trouble to bend, and is needed only for a 20 feet Tornado catamaran (which were also designed for compounded plywood...)

    If you want extra abrasion resistance add glass fiber outside, 4 o 6 ounces of satin with the smallest weave you can find. Working well you can keep it under 75 kg (160 pounds) easily. If very good under 70 kg.

    I made one monodromic proa in 1983. Very funny to sail but not faster than the 18 feet cats, except that this thing points to weather like an America class.
    I made also several cats of 18 feet long, with 18 square meters surface sail. No jib. On very light weather a 16 to 20 m2 gennaker could be added but it's a lot of trouble for a poor lone guy.
    The last one was weighting complete ready to sail (and with 2 wooden paddles....) 89 kg. It's a class A with lots of steroids...
    Believe me 3mm plywood is largely enough for a strong proa.

    Plywood can be compounded because the compression side actually crushes in a microscopic way. The cellulose cells crumble down and after are stabilized by the epoxy resin. The limit is the modulus traction (stretching) of the outer and middle ply. If you add a light glass fiber you can bend more without splitting the plywood. Is the Stressform technique of the Gougeon bros. For a 18 feet proa or cat is a useless complication but you can make 30 feet cats and tris with 6mm plywood.
     
  2. peterchech
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    peterchech Senior Member

    Thanks everyone! I have ordered the gougeon brothers book. I think after reading it i will try the ama first to get some experience without wasting too much expensive ply. I guess if i make it longer than 20 ft ill go with 4mm, if smaller ill use 3mm. Lots to consider here, not the least being what kinds of shapes i can get this ama into...

    Ilan i have a three board outfitted canoe that is double ended, and theoretically could be a shunted. I wind up tacking it because a proa arrangement is rather a pita. However it may be the only way to compete speed wise with the cats. It allows a small, light ama, thin scantlings all around and therefore a very light build, and with some practice flying the ama, would be able to get up on one hull in much lighter conditions than a cat.
     
  3. dstgean
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    dstgean Senior Member

    Peter,

    Although I admire your gumption, there's almost no way you'll keep up with the cats unless you go with a serious high aspect rig. Ask Laurent about his A cat rig experience on his proa. Are you talking about ripping along on a reach? Light air? Heavy air? Give some performance parameters & you'll likely get a better response. Given a boat without restraints, (like the AC boats) the tri seemed to develop the most righting moment. In light air venues something like an old school moth might do well. A pacific proa could set more sail than that and thus do well in that set of parameters, I think. But generally, it's tough to beat the beachcats as they've been optimizing for over 50 years on that platform.

    Lots of other reasons to build a proa though.

    Dan
     
  4. peterchech
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    peterchech Senior Member

    I know dan, 50 years of industry development vs a guy who hangs out on internet forums for most of his" research" :) where can i contact laurent btw?

    The advantage i can see over the guys at my club is that of weight. The average hobie here weighs about 400#. I think i can get a skinny proa to about 200# at 18-20 ft. Yes its kind of cheating, but i just want to be in the pack at our races. Hey at least i should be able to pass a wave!

    As for rig, i def wont be going for oceanic lateen. I think ill keep my eyes open for some used beach cat sails. If i dont find any, i may buil some leg of nuttin sails and arrange them in schonner form. That way they would tighten up and flatten close to the wind, but loosen and get some camber downwind. Theoretically:)

    Hull shape would be symetrical, relying on boards for lat resistance.

    Anyway, those are my thoughts on it for now...
     
  5. upchurchmr
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    upchurchmr Senior Member

    Peterchech,

    I had a wooden Tornado (tortured construction). From the point of having to fix things every other week, go ahead and fiber glass the boat, you will be much happier. Epoxy by itself takes very little abrasion.

    The Gougeon book will tell you everything you need about tortured ply. Mostly they tell you to get some very thin aircraft ply to make a model (there is a chart showing you what thin ply equals 4 and 6 ply). If the model can be made the full size boat will most likely work. They worked at 1" = 1' for the model. There is somebody on line who will let you have a free download for the plans for the original Tornado catamaran, just for comparison. The Tornado was 20 feet and used 4mm marine ply. The only bulkheads were at the crossbeams, along with some small deck beams to help with walking loads. Sorry I no longer have the link.

    Marc
     
  6. oldsailor7
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    oldsailor7 Senior Member

    Part of the beauty of Crowthers Buccaneer tris was the fact that although they had multi-chine bottoms, the individual panels were two dimensionally bent just enough to accommodate 1/4" ply without excess stress on the surfaces of the ply. Although the Buccaneer 24 ( for instance), was specified for 4mm ply, many have been built with1/4" Douglas Fir Plywood, with no problems.
     
  7. Ilan Voyager
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    Ilan Voyager Senior Member

    upchurchmr : yes epoxy is not very good for abrasion so with glass the surface becomes harder. Other advantage with a thin layer of glass you can control the thickness of the final layer. The con is you have to be very careful: 4 oz satin with a very small weave or the weight will be prohibitive on a small boat.
    I helped on the construction of 2 Tornados made in conpounded plywood. One was the classic one: there are only 2 bulkheads but there is a complex bracing made of tiny pieces of wood like in some planes' wings. Very time consuming as you have to coat every piece. Horrendous...sticks of pultruded fiberglass (carbon is too stiff) made life easier.
    The second tornado was made outside of jauge, for raid, with a different mast and bigger sail plan (plus gennaker). Wider also. Very fast beast. The hulls were in 3mm ply with 6oz fiberglass inside and outside, plus carbon UD strips. No bracing but stringers and ribs plus a horizontal bulkhead and crash boxes. Heavier than the original plan but very strong, and that was needed for raid and fast coastal cruising. The cat had to be unsinkable, and easy to right up by the crew (that's another story).
    The scale 1/12 (1" to 1') and aircraft ply is rather accurate, and gives a lot of infos (centers, volumes etc) in a fast and simple way instead of spending hours learning a program on a computer...and struggling after with. Design programs use NURB curbs and that's the problem.
     
  8. BalatonProa
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    BalatonProa Junior Member

    About the 33 ft racing proa mentioned by Lucky Strike

    Dear Lucky Strike, please let me know more about the 33 ft racing proa that you mentioned. We start to build a 40 feet racing proa for the next sailing season and would be glad to learn from any analogue experience and this seems the only similar project in Europe.
    The link of the scanned picture (www.uni-karlsruhe.de/~ubwp/pix/p2_hut1.html) is not working any more. How could I contact the people involved in this German project? I know this is a bit off topic here, sorry about thet, but we are also considering tortured ply construction... that's how I found this forum.
    All the best
    Gabor (Lake Balaton, Hungary)



     
  9. lesburn1
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    lesburn1 Junior Member

    Just a quick note. About 30 years aga I built an 18Sq using the tortured method. The plans came from the Gougeon Bro. by way of John Lindahl. The method worked well for me, I used 4mm okume ply.
    Check this site for some recent developments.

    http://www.gust.ax/gallery/blade/

    good luck
     
  10. Ilan Voyager
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    Ilan Voyager Senior Member

    Very nice site...with beatiful pics...Compounded plywood is a very good method. Unhappily like the cylinder mold it's a bit beyond a lot of people capabilities. That needs some woodworking ability and visualisation of surface transformation in 3D.

    The Lindahl was a modification of the class A Michigan of the Gougeon. We are getting old... it was in 1981 or 82. I bought the offsets and pics but never made it, as I made 2 monodromic proas. However after the proas and their limitations, I made a first 18m2 (I kept the surface just because the cipher sounded good, and it's almost the max that one guy could handle in solo in a 15-20 knots wind) in 1986, followed by 4 other. The French sailing federation practically forbade our prototype in the cat regattas as the importers of Nacra, Bims and other were very angry; can you imagine a plywood cat winning in front of the very expensive cats advertised as high tech composite?
     
  11. Gary Baigent
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    Gary Baigent Senior Member

    "The French sailing federation practically forbade our prototype in the cat regattas as the importers of Nacra, Bims and other were very angry; can you imagine a plywood cat winning in front of the very expensive cats advertised as high tech composite?"

    Yes, and that is a great point, Ilan Voyager, many don't realise that this method actually allows you to have a very performance, stiff (and if looked after sensibly), long lasting boat ... at a fraction of the cost of high technology laminate designs ... a perfect boat for the 99%?
    But then, tensioned ply has connotations of cheapness ... and many here are 1%'ers. Sorry, poor taste, apologies.
     
  12. oldsailor7
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    oldsailor7 Senior Member

    Gary. I don't think you have to apologize. :cool:
     
  13. Gary Baigent
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    Gary Baigent Senior Member

    Thanks OS7, thought for a second I'd overshot the turning mark.
     
  14. upchurchmr
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    upchurchmr Senior Member

    Gary,

    Do you have any pictures and specifications of the 18m2 you built? I keep thinking about doing something with compounding again, mostly aimed a quick and light. If you did not do the Lindhal, what was different. I was surprised to see the flat narrow plank on the keel of Lesburn1's boat. I thought about it a lot but had not seen anyone succeed.
     

  15. Gary Baigent
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    Gary Baigent Senior Member

    upchurchmr, I haven't built an 18m2; you've got the wrong bloke. But I built 6 x 6 m foiler Flash Harry (left) in tensioned ply which had around 18m2 sail area. Right now I'm finishing off an 8 x 8 metre tri foiler, also in tensioned ply; platform, no rig, weighs 150kgs.
     

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