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Thread: Best way to bend plywood

  1. #1
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    Default Best way to bend plywood

    In the interest of time and money, I've decided to use marine plywood to plank my lapstrake dingy.

    I've just cut out a pattern for the garboard, and when trying to bend it around the station closest to the bow (the point of maximum bend), the outside layer of the cheap 1/4" plywood I was using for the pattern cracked.

    So, now I am thinking I may need to steam bend the marine plywood plank, so it doesn't crack...

    I'm pretty sure the plank just needs to bend at this one forward station... the run is pretty straight right to the transom from there.

    Question: can I use the 'wrap board in rags and pour on boiling water' method of spot bending this bit of plywood? Or will boiling water cause internal glue failure, or some other dramatic failure that will ruin my day?

    Thanks!
    -- John

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  2. #2
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    Default Re: Best way to bend plywood

    I've often read about the "Boil" test for marine ply's so I don't think a brief exposure to your boiling water will affect most marine ply's. On smaller radii you might have to kerf the backside to get it to conform to your curve.
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    Default Re: Best way to bend plywood

    Quote Originally Posted by knottyBuoyz View Post
    I've often read about the "Boil" test for marine ply's so I don't think a brief exposure to your boiling water will affect most marine ply's. On smaller radii you might have to kerf the backside to get it to conform to your curve.
    Using hot rags and boiling water should help, definitely stay away from kerfing the panel, that will dramatically weaken or ruin the plywood.

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    Default Re: Best way to bend plywood

    It's 3/8" marine plywood
    -- John

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    Default Re: Best way to bend plywood

    The cheap temp broke, don't worry yet about the marine. it has no voids. The voids in the other ply were probly what caused the break. Wetting ply should not be nessessary. Another potential problem is you may be attempting a compound curve (torture)?Sure the station is correct? No kerfing.

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    Default Re: Best way to bend plywood

    Quote Originally Posted by jalmberg View Post
    It's 3/8" marine plywood
    Marine what? Marine fir? Marine okoume? Marine meranti? None of the above? The devil is in the details.
    Also, how big is this dinghy? The shorter the span where the girations have to be accomplished the harder it will be. Especially with 3/8" ply.
    Is this glued lap or metal fastened? With glued lap you could use " for the garboard and overlay it with 6 oz. glass and nobody would be the wiser. And the laminated garboard would be as strong or stronger.

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    Default Re: Best way to bend plywood

    Quote Originally Posted by jalmberg View Post
    It's 3/8" marine plywood
    Marine what? Occume will take the biggest bend, much easier than meranti or fir. Keep in mind if you use hot water to soften the wood it will leave water stains. Going to paint it?

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    Default Re: Best way to bend plywood

    No one expects....


    ...the Wooden Boat Inquisition!!!



    This can be done the easy way, where you tell us what design, exact materials and their dimensions, etc ---- or the hard way, where we attempt to pry the information outa you bit by painful bit.

    You decide....

    Just don't make us bring out...THE COMFY CHAIR !!!
    Last edited by Thorne; 02-12-2010 at 11:11 AM.
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

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    Default Re: Best way to bend plywood

    A small glued lapstrake dinghy shouldn't need plywood planking thicker the 1/4 inch. Starting with 3/8 material of any kind may be the source of all your problems.
    Goat Island Skiff and Simmons Sea Skiff construction photos here:

    http://s176.photobucket.com/albums/w...esMan/?start=0

    and here:

    http://www.flickr.com/photos/37973275@N03/

    "All kings are not the same."

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    Default Re: Best way to bend plywood

    The boat in question is Atkin's CABIN BOY.



    I thought you were going to use 1/4" for the sides and 3/8" for the bottom.

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    Default Re: Best way to bend plywood

    best to bend it before it becomes plywood - before the glue between the veneers dries
    "Twenty years from now you will be more disappointed by the things you didn't do, than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover." - Mark Twain

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    Default Re: Best way to bend plywood

    I agree with
    the
    fellow above who said you won't have the same problem with good (I like okoume) plywood as with cheap stuff. Also what the guy said about 1/4" for the sides sounds
    righton
    .And the guy who implied that you are being stingy with details nailed it. If you can't make the required bend with the ply you need ,do it with 2 layers of thinner ply. David

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    Default Re: Best way to bend plywood

    Quote Originally Posted by holzbt View Post
    The boat in question is Atkin's CABIN BOY.



    I thought you were going to use 1/4" for the sides and 3/8" for the bottom.
    And I thought this was a simple question ;-)

    Roger, you are 100% correct. I mis-remembered what I bought. It is 1/4" okoume (made by Joubert, if that matters).

    -- John
    -- John

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    Default Re: Best way to bend plywood

    Quote Originally Posted by wizbang 13 View Post
    The cheap temp broke, don't worry yet about the marine. it has no voids. The voids in the other ply were probly what caused the break. Wetting ply should not be nessessary. Another potential problem is you may be attempting a compound curve (torture)?Sure the station is correct? No kerfing.
    Actually, the bend isn't all that much. I was pretty surprised when the pattern cracked. I was bending it around the chine logs, which are made of white oak. The white oak bent around that curve with no problem at all.

    Yes, the station is correct.
    -- John

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    Default Re: Best way to bend plywood

    Sorry if these are dumb questions, but I have zero experience with plywood... I'm also trying to figure out the best way to cut the curving planks out of the sheet, without wasting too much of this valuable stuff. My best guess is a jig saw. Any other type of saw I should consider?
    -- John

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    Default Re: Best way to bend plywood

    Quote Originally Posted by TimH View Post
    best to bend it before it becomes plywood - before the glue between the veneers dries
    I think you're on to something there, Tim...
    -- John

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    Default Re: Best way to bend plywood

    I built an 11' Shellback dinghy with 1/4" doug fir marine grade for the lapstrake sides. I made the bottom with 1/2" doug fir marine grade. I don't remember anywhere that it was especially difficult to bend.

    When I later built the Elver I had a hard time bending the shear clamp. After looking things over for a while I discovered that I had built the stern post 1" too high. I trimmed it down and it was a much more natural fit.
    Will

  18. #18
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    Default Re: Best way to bend plywood

    Hi,
    Try using a 1/4" okume, and then a1/8 veneer ply over that.

    Jack.

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    Default Re: Best way to bend plywood

    Oh well, I guess I'll just try it and see what happens.
    -- John

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    Default Re: Best way to bend plywood

    John, this should not be a problem. 1/4" okoume should fly round that.

    Jigsaw is best.
    Keep It Simple: KISS it better.

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    Default Re: Best way to bend plywood

    Quote Originally Posted by jalmberg View Post
    Sorry if these are dumb questions, but I have zero experience with plywood... I'm also trying to figure out the best way to cut the curving planks out of the sheet, without wasting too much of this valuable stuff. My best guess is a jig saw. Any other type of saw I should consider?
    The big thing with curbing watage id nesting the pieces to get the most out of what you have. And with planks that tend to be simitar shaped that can be troublesome.
    In Tom Hill's book. "Ultralight "Boatbuilding" he demonstrates a technique that will cut waste about as far as is possible given the difficulties.

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    Default Re: Best way to bend plywood

    Quote Originally Posted by Candyfloss View Post
    John, this should not be a problem. 1/4" okoume should fly round that.

    Jigsaw is best.
    Will soon find out. I'm not going to bed tonight until that garboard is cut out, bent on, and fastened.
    -- John

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  23. #23
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    Default Re: Best way to bend plywood

    Your Japanese pull saw will work fine for this if you'd rather not use power tools. If you must use something with a trigger a circular saw should be your first choice if you are comfortable using one. It's difficult to get a nice fair cut with a sabre saw. Not a big deal on an 8' plank, just a little more planing on the edges to get things fair.

    You are planning to cut the gains (lap width rabbets at each end of plank to make the planks flush at the stem and transom) before fastening the planks permanently.
    Last edited by holzbt; 02-12-2010 at 06:47 PM.

  24. #24
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    Default Re: Best way to bend plywood

    Quote Originally Posted by holzbt View Post
    You are planing to cut the gains (lap width rabbets at each end of plank to make the planks flush at the stem and transom) before fastening the planks permanently.
    Yes... but I'm going to try your bevel-less trick for the laps. I think that will save a lot of time. My main goal at this point is to finish up so I can get back to FL.
    -- John

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  25. #25
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    Default Re: Best way to bend plywood

    One way to avoid tear out is to use a razor knife and cut the surface veneer on or just outside the line then cut with your saw. This leaves a nice clean edge to either use or plane from.

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    Default Re: Best way to bend plywood

    Laying very wet towels on the plywood overnight has always worked great for me. Sometimes, with a considerable curve, I might take a few days to slowly bend in the full curve. If you try to do all of a considerable curve all at once and too fast, you risk breaking the wood. If you sense that you're trying to take the wood too far at one time, just make sure to keep it wet and to take your time bending it.
    Last edited by mackaye; 02-12-2010 at 11:36 PM. Reason: sp probs

  27. #27
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    Default Re: Best way to bend plywood

    Perhaps not relevant here, but Designer John Welsford in one of t'other of his writings suggests that for areas of serious bend in a plywood plank it pays to cut a sheet of ply corner to corner then take the plank out of the angled edge, it then has the grain in all of the veneers angled and will bend both more easily and further before it lets go.

    Cedric Rhyn

    Quote Originally Posted by jalmberg View Post
    In the interest of time and money, I've decided to use marine plywood to plank my lapstrake dingy.

    I've just cut out a pattern for the garboard, and when trying to bend it around the station closest to the bow (the point of maximum bend), the outside layer of the cheap 1/4" plywood I was using for the pattern cracked.

    So, now I am thinking I may need to steam bend the marine plywood plank, so it doesn't crack...

    I'm pretty sure the plank just needs to bend at this one forward station... the run is pretty straight right to the transom from there.

    Question: can I use the 'wrap board in rags and pour on boiling water' method of spot bending this bit of plywood? Or will boiling water cause internal glue failure, or some other dramatic failure that will ruin my day?

    Thanks!

  28. #28
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    Default Re: Best way to bend plywood

    And if you don't already have enough on your mind .. sheesh.
    You want to make sure your boat framing is well braced. When bending and torturing plywood into shape you inadvertently apply alot of pressure on the framing, making it want to rack, or worse blow apart.

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    Default Re: Best way to bend plywood

    Quote Originally Posted by Cedric Rhyn View Post
    Perhaps not relevant here, but Designer John Welsford in one of t'other of his writings suggests that for areas of serious bend in a plywood plank it pays to cut a sheet of ply corner to corner then take the plank out of the angled edge, it then has the grain in all of the veneers angled and will bend both more easily and further before it lets go.

    Cedric Rhyn
    Xlnt point. Still, if you must bend either parallel or perpendicular to the face grain, ply theoretically bends easier in the direction parallel to the face grain. Don't know why, but that's what the books say.....
    When you're chewing on life's gristle
    Don't grumble, give a whistle...

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    Default Re: Best way to bend plywood

    Quote Originally Posted by cookie View Post
    Xlnt point. Still, if you must bend either parallel or perpendicular to the face grain, ply theoretically bends easier in the direction parallel to the face grain. Don't know why, but that's what the books say.....
    Plywood is made (mostly) of uneven #s of layers...with the outer sheets grain direction being the more numerous. If you bend parallel with the grain you bend along the axis that is naturally weaker and, with plywood, against the fewer veneers that compose the inner perpendicular layers.

  31. #31
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    Default Re: Best way to bend plywood

    Quote Originally Posted by Cedric Rhyn View Post
    Perhaps not relevant here, but Designer John Welsford in one of t'other of his writings suggests that for areas of serious bend in a plywood plank it pays to cut a sheet of ply corner to corner then take the plank out of the angled edge, it then has the grain in all of the veneers angled and will bend both more easily and further before it lets go.

    Cedric Rhyn
    That would be one expensive plank.
    -- John

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  32. #32
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    Default Re: Best way to bend plywood

    Quote Originally Posted by Mike Vogdes View Post
    And if you don't already have enough on your mind .. sheesh.
    You want to make sure your boat framing is well braced. When bending and torturing plywood into shape you inadvertently apply alot of pressure on the framing, making it want to rack, or worse blow apart.
    That's not really a problem in my case. The framing is strongly built and reinforced by the keelson and chine logs.

    No, the problem at the moment is that the first pattern I developed by spiling did not fit at all. I suspect that my spile batten was edge set. I hate to do it, but I'm going to start over from scratch and use a three part spile batten.

    The good thing is, I've learned a few things from doing it wrong once. Hopefully, the next attempt will be a bit more successful.

    Back into the shop!
    -- John

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  33. #33
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    Default Re: Best way to bend plywood

    John,

    1) Hard to beat 1/4" luaun floor underlayment ($10/sheet) for pattern stock.

    2) Getting done fast when you are learning and getting done good are mutually exclusive.
    Denny Wolfe
    www.wolfEboats.com

  34. #34
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    Default Re: Best way to bend plywood

    Quote Originally Posted by mcdenny View Post
    John,

    1) Hard to beat 1/4" luaun floor underlayment ($10/sheet) for pattern stock.

    2) Getting done fast when you are learning and getting done good are mutually exclusive.
    Well, I have definitively discovered the problem with bending my pattern stock -- it is the plywood I was using. It cracks waaaaay too easy to be used for this purpose. In fact, it cracks if you look at it too hard.

    I've heard of luaun, and I'm going to go out tomorrow and buy a sheet to try. I need something decent to work with.

    To your second point, I am learning this truism the hard way, I guess. In fact, it is not possible to rush this stuff, anyway. I would *like* to go faster, but its not happening.

    My first attempt at making a pattern for my garboard was a total, dismal failure, but I know a heck of a lot more as I start on the second try.

    Someone on another thread was talking about how a photo record of your build is a good thing to have. I'm actually discovering that lots of photos/videos are invaluable for figuring out what you did wrong. I am just putting together a video of my recent failure, and can see the root cause of the failure in the very first clip. Everything I did after that was doomed to failure, but I didn't know it.

    Well, live and learn, as they say.
    -- John

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  35. #35
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    Default Re: Best way to bend plywood

    Quote Originally Posted by cookie View Post
    Xlnt point. Still, if you must bend either parallel or perpendicular to the face grain, ply theoretically bends easier in the direction parallel to the face grain. Don't know why, but that's what the books say.....
    Whoops, did I say parallel
    Checked the books and should have said that plywood bends easier in the direction perpendicular to the face grain. Sorry...
    When you're chewing on life's gristle
    Don't grumble, give a whistle...

  36. #36
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    Default Re: Best way to bend plywood

    Quote Originally Posted by cookie View Post
    Whoops, did I say parallel
    Checked the books and should have said that plywood bends easier in the direction perpendicular to the face grain. Sorry...
    The books can say what they want. In my (considerable) experience you were correct the first time.

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