Plywood bending question, replacing skiff bottom panels

Discussion in 'Wooden Boat Building and Restoration' started by Paul_A, Aug 20, 2013.

  1. Paul_A
    Joined: Aug 2013
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    Location: Pohnpei Micronesia

    Paul_A Junior Member

    I am in the process of replacing most of the bottom of a 20' Tolman skiff. I will be using ACX covered with glass and epoxy. The plans for this boat call for 2 pieces of 1/4" ply glued one at a time in order to allow the plywood to conform to the shape of the bow.

    The rearward pieces are 1/2" plywood. I am not replacing the very forward portion of the bottom as it is still sound which leads me to a couple of questions.

    I was planning on scarfing the new to the old as I am thinking it will be easier to get the plywood to conform to the curve of the remaining bottom. Is this the best way?

    Also I am hoping to get away with bending 1/2" ply to conform to the curved portion. When I fit a piece a couple inches over the existing bottom I am able to bend the ply down although it does take a fair amount of pressure.

    The new piece at that point will be 26.5 inches wide and the gap before the plywood is flush is 4.5 inches The stringers are also sloping at that point so the ply will also be bent downward 2.5 inches in that direction.

    Is this too much to ask of 1/2 plywood? I don't want to cut and scarf and glue only to have the joint fail when I pull the screws out to glass. Some pics to help explain what I'm attempting to describe.

    [​IMG]

    [​IMG]

    [​IMG]
     
  2. El_Guero

    El_Guero Previous Member

    Tell Joe Wayne said hello!
     
  3. El_Guero

    El_Guero Previous Member

  4. Paul_A
    Joined: Aug 2013
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    Location: Pohnpei Micronesia

    Paul_A Junior Member

    Who is Joe???

    Looked at table 1.2. Searched "minimum bend radius" as I have never heard that term until now. Math was never and will never be my strong suit.

    I appreciate you giving me the link on airplane repair and introducing me to minimum bend radius but I don't understand it.
     
  5. El_Guero

    El_Guero Previous Member

    Minimum is really the 'most' you can bend the wood. Minimum is the smallest curve you should bend the plywood to safely.

    Let us use .10 (that is close to 3mm if I remember right).

    Dry, we should be able to bend along the grain to a 9 inch radius (18 inches from one side of a complete bend to the other side, with the middle being 9 inches ).

    Wet, we should be able to bend a 3 inch radius. 6 inches across if we bend a complete circle.

    I rounded both examples.

    I am tired, but I hope that helps.

    After some sleep, I will try to find my notes, and give you more information about bending the grain length wise, or bending across the grain.

    Unless someone who is more awake beats me to it.

    Wayne
     
  6. El_Guero

    El_Guero Previous Member

    Is that wbp glue plywood? You will have major voids internally if it is not bs 1088 grade plywood.
     
  7. El_Guero

    El_Guero Previous Member

    I went to College with Joe from Ponapei ....
     
  8. Paul_A
    Joined: Aug 2013
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    Location: Pohnpei Micronesia

    Paul_A Junior Member

    Thank you Wayne.

    Are you referring to Dr. Joseph Daisy?

    And regarding plywood, I am afraid the best grade of plywood available here is exterior grade.

    Interestingly enough, when I ripped the first panel today there were no voids so hopefully the quality is good enough for my needs.
     
  9. Paul_A
    Joined: Aug 2013
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    Location: Pohnpei Micronesia

    Paul_A Junior Member

    Couple more pics to show the entire project.

    [​IMG]

    [​IMG]

    I am ready to buy the rest of the wood so any more input would be greatly appreciated.
     
  10. El_Guero

    El_Guero Previous Member

    Looking at your pictures, I do not see a problem getting the wood to bend that much.

    But, use a real good coat on both sides of your ply. Outdoor plywood has ok water resistance, but it can also fall apart quickly if left wet.

    No, Joe's last name escapes me now. And he was actually Micronesian.

    From your pictures, the rest of the wood looks to be in good shape and that is always reassuring.

    One concern of mine would be, how is the keelson arranged? It looks like two stringers are supporting the hull, and the shape of the hull is being used to provide the strength usually given by the keelson?

    The keelson is the internal support often called the keel, but keelson is on the inside, and keel on the outside.

    wayne

    This photo is for building model boats, I would expect a keelson to be slightly bigger (height) in an actual boat.

    [​IMG]
     
  11. Paul_A
    Joined: Aug 2013
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    Location: Pohnpei Micronesia

    Paul_A Junior Member

    Thank you Wayne.

    The keelson is made by forming a fillet of thickened epoxy 3" wide and narrows as the deadrise increases. The fillet is then covered with 3 layers of 10 oz glass inside and out. The chine also receives 3 layers inside and out.

    If you are curious as to how I happened upon this project you can read the back story here: http://www.fishyfish.com/boards/index.php?topic=2762.0

    My next challenge is determining if after I put the bottom on and glass and paint is will just the exterior glass be strong enough to support the boat when I roll it over, or should I after cutting and fitting the bottom screw it to a jig and poor and glass the keelson prior to attaching the bottom to the boat.

    Any thoughts?
     
  12. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    If you have trouble bending the ply you might try kerfing. Kerf the OUTSIDE then cover in structural eglass like 12 oz 45 45 biax.

    Im not familiar with tolman construction. Kerfing is common with plywood as a core construction..... Eglass skins on both sides of the kerfed panel.
     
  13. Paul_A
    Joined: Aug 2013
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    Paul_A Junior Member

    The Tolman is built with stitch and glue construction. I can bend the ply without hearing any cracking but in doing so I had to apply a fair amount of pressure.

    By fair I mean I had to press down with most of my weight, (175lbs) to bend it fully.

    I don't have access to E glass unless I order it from the States which is a 3 week proposition. I have been at this thing for 2 months (daily 6 hour power outages don't help) and am anxious to get it finished so I can go fishing...
     
  14. Grady300
    Joined: Jan 2013
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    Location: Bend Oregon

    Grady300 Junior Member

    Paul
    It looks to me you need to run the two layers of ply back a little further before going to the full 1/2". Personally it sounds like too much pressure to make the bend and be confident it will hold.
     

  15. Paul_A
    Joined: Aug 2013
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    Location: Pohnpei Micronesia

    Paul_A Junior Member

    Possibly and have not ruled that out. I am hoping to not use up a ton of expensive hard to get epoxy joining two 1/4" ply pieces if I don't have to.

    I posted here because I was hoping one of the experts could do the math and tell me with certainty yea or nay.
     
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