Twisting 8mm ply?

Discussion in 'Wooden Boat Building and Restoration' started by Antagon, Aug 12, 2018 at 5:47 PM.

  1. Antagon
    Joined: Jul 2018
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    Antagon Junior Member

    As for my first post, it’s time to introduce myself, I guess. I live in Bavaria, 50y.o. and have some little ocean/yachting experience, that is unfortunately 20 years past. My goal is to revive that big time in the next years. Obviously I’m not a native speaker/writer, so I apologize for eventually being cruel to you language beforehand and highly appreciate that you (hopefully) share your experience with i.e. mountain folks like me.
    To start my remarinisation gently, I build a 12ft general purpose dinghy from plywood, stich & glue.

    In order to make it ultimately bullet-proof I want to use 8mm for the hull panels instead of 6mm as specified. The bottom panel will be twisted some 170° from bow to stern.

    1. Am I going to really hate me for that idea when trying to unfold, going 3D, due to the thicker ply?
    …. or worse, could it proofe impossible?

    2. How much worse will it become, when 25g/m² glas is glued to the inside before bending it, because it’s more convenient, flat on the bench?
     
  2. Blueknarr
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    Blueknarr Senior Member

    Welcome
    Do not worry about language. Translator program is awesome, most of time.

    Great first project
    In order to make it ultimately bullet-proof I want to use 8mm for the hull panels instead of 6mm as specified. The bottom panel will be twisted some 170° from bow to stern.
    Is 170° error? That is a lot of rocker. Degree of turn is less problem than radius of turn. 4 M radius no problem; 1 M radius very difficult.

    1. Am I going to really hate me for that idea when trying to unfold, going 3D, due to the thicker ply?
    …. or worse, could it proofe impossible?

    2. How much worse will it become, when 25g/m² glas is glued to the inside before bending it, because it’s more convenient, flat on the bench?
    Bend and assemble first. The glass will be relaxed. If pre-glassed the glass will be stressed for a long time.

    Good luck
     
  3. Antagon
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    Antagon Junior Member

    No Sir, 170° is not an error, only a little exagerated perhaps. It's a single chine. The bottom panels start near vertical at the bow, beeing cork-screwed to a dead rise around 10° at the stern.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    A drawing of the 12 foot dinghy in question, would be helpful.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    That would be 80 degrees ?
     
  6. Antagon
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    Antagon Junior Member

    This is the boat
    [​IMG]
     
  7. Antagon
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    Antagon Junior Member

    Ooophs...yes you are right!
    Still a lot of strain problably
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Bending of ply depends on a few factors, timber species, ply orientation, number of plies, thickness, etc. I note one source gives 2 feet radius across the grain, and 6 feet along the grain, for 5/16 inch ( 8mm) ply. I would contact the provider of the plans, to get an opinion, the designer has created a shaped using developable surfaces, and the generating lines of the developed surface would be a good clue to the sharpest curvature. But that is usually not available to be seen.
     
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  9. JamesG123
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    JamesG123 Senior Member

    ^ This. You aren't building an ice breaker.
     
  10. Antagon
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    Antagon Junior Member

    The architect, a wise man, retired and went off cruising.
    Wood will be okoume, 7 plys problably.

    I do not understand the term "radius" in this context?
     
    Last edited: Aug 12, 2018 at 10:04 PM
  11. Antagon
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    Antagon Junior Member

    Isn't ice breaking covered by "general purpose"?
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    It is half the diameter of the circle that it would require for your ply to bend 360 degrees, head to tail ! I think your best plan is, stick to the plans specification, and apply some epoxy and glass afterward, to the bottom, if you want more strength. That will save a lot of cursing trying to bend something that is resisting your efforts.
     
  13. JamesG123
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    JamesG123 Senior Member

    Not to mention imparting stresses that will cause your boat to fail.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    It does go against the nature of stitch and glue. I think it would be silly to try 8mm, when the glass/epoxy option is available, which will add strength and water-tightness.
     

  15. Blueknarr
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    Blueknarr Senior Member

    Radius and his sister refer to how big\small of barrel matches the curve you are making. While degrees measures how far around the barrel the plywood is forced. The smaller the barrel or tighter the bend, the harder it is to bend.
     
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