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Thread: Will plywood do this?

  1. #1
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    Default Will plywood do this?

    I've been plying with a design for a fantail catboat in stitch and glue, and I've got the panels developing with great precision, to the point where one could easily build the boat in aluminum, but I'm not so sure plywood will bend that much.

    14' fan tailed catboat 1-29-2018 3-02-58 PM 1607x784.bmp.jpg


    I'm thinking 4 mm Okume for this 14' boat, but the round stern is a pretty tight curve, and some of the bends in the forward sections are extreme as well. What do we reckon? Will I just be breaking a lot of perfectly good plywood?

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    Default Re: Will plywood do this?

    4mm okoume bends well. You should not have any problem with the bend as long as you bend with the grain vertical. If you try it with the grain horizontal, it might work if you wet it before you try bending the hard way. Testing a scrap strip is not so expensive.

    I can't read the numbers. It looks close to a 2' radius, which should be OK up to 5/16 (~8mm) thick with the face grain running vertically.
    http://www.schmeling.com/reference/p...ing_radius.pdf BENDING PLYWOOD
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    Default Re: Will plywood do this?

    Quote Originally Posted by MN Dave View Post
    4mm okoume bends well. You should not have any problem with the bend as long as you bend with the grain vertical. If you try it with the grain horizontal, it might work if you wet it before you try bending the hard way. Testing a scrap strip is not so expensive.

    I can't read the numbers. It looks close to a 2' radius, which should be OK up to 5/16 (~8mm) thick with the face grain running vertically.
    http://www.schmeling.com/reference/p...ing_radius.pdf BENDING PLYWOOD
    Hey, thanks for the advice and the link!

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    Default Re: Will plywood do this?

    The kids boat I did in 4mm okoume involved some pretty tight bends and I was really surprised I didn't hear a crunch. The bends were definitely tighter than your stern.

    The bow on the other hand gives me pause. That hard turn at the bilge in front strikes me as a difficult to build in any method of wooden construction. I think there you have two options: a) run the chine where the sharp bilge turn is (you can always put another one rising higher up for appearance) or b) flatten out those tight bends at the bow. This would produce a finer entry and more flare up front, which probably wouldn't hurt to shed the water.

    I have never seen a catboat in other than pictures and don't sail at all (yet), so I cannot speak to any functional qualities of your design. If it were my own boat I'd chose option b based on aesthetics and would probably widen and flare the topsides of the bow also. But first I'd research how other catboats deal with this part of the hull and not try to deviate too far from that. I recently bought 87 Boat designs https://www.woodenboatstore.com/prod...Designs/design . It features a number of catboats but I haven't had a close look at their lines yet.

    I'm trying to work out myself where the "boat size boundary" is for going from 4mm to 6mm. Fourteen foot rowboats are sometimes 4mm but most sailboat designs that I have seen in that size are 6mm or even 9mm for flat bottom panels. Or they are reinforced with a bunch of stringers.

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    Default Re: Will plywood do this?

    If it worries you do the bottom in two layers vacuum bagged.
    I did a similar boat at 12 foot, the bow bent round OK with the ply I used, but it was too thin for the flatter panel in the run, so I had to add stringers and frames.
    It really is quite difficult to build an ugly wooden boat.

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  6. #6
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    Default Re: Will plywood do this?

    Quote Originally Posted by BOI View Post
    The kids boat I did in 4mm okoume involved some pretty tight bends and I was really surprised I didn't hear a crunch. The bends were definitely tighter than your stern.

    The bow on the other hand gives me pause. That hard turn at the bilge in front strikes me as a difficult to build in any method of wooden construction. I think there you have two options: a) run the chine where the sharp bilge turn is (you can always put another one rising higher up for appearance) or b) flatten out those tight bends at the bow. This would produce a finer entry and more flare up front, which probably wouldn't hurt to shed the water.

    I have never seen a catboat in other than pictures and don't sail at all (yet), so I cannot speak to any functional qualities of your design. If it were my own boat I'd chose option b based on aesthetics and would probably widen and flare the topsides of the bow also. But first I'd research how other catboats deal with this part of the hull and not try to deviate too far from that. I recently bought 87 Boat designs https://www.woodenboatstore.com/prod...Designs/design . It features a number of catboats but I haven't had a close look at their lines yet.

    I'm trying to work out myself where the "boat size boundary" is for going from 4mm to 6mm. Fourteen foot rowboats are sometimes 4mm but most sailboat designs that I have seen in that size are 6mm or even 9mm for flat bottom panels. Or they are reinforced with a bunch of stringers.
    I have spent a fair amount of time sailing small catboats, and I hope I understand how they behave. I've designed and built one so far that turned out okay.

    It's pretty tricky, trying to get a catboat bow in stitch and glue, but this develops as a section of a cone to a high degree of precision. I think the bend forward is within the 2' radius mentioned in Dave's link.

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    Default Re: Will plywood do this?

    It strikes me, too, that you want the chine in the forward section to be right at the waterline you've shown. As it is, if I'm looking at it correctly, it is running right through the middle of a flat panel where no angle change is involved at all. If that's the case, why not just go with a flat panel there?

    You might build a model out of stiff cardboard first. It should behave close enough to plywood at that scale that you'll see where you're forcing it.
    Alex

    "A man who is not afraid of the sea will soon be drowned, for he will be going out on a day he shouldn't. We do be afraid of the sea, and we only be drowned now and again" Aran Islands Fisherman

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    Default Re: Will plywood do this?

    I started out with the chine at the waterline, but didn't get anything like as neat a shape. I think this will produce a dry boat that will slice through waves rather than bashing into them, like a Beetle Cat.

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    Default Re: Will plywood do this?

    The chine at the bow could give you some difficulty with 5mm. You can just cold mold that area with thinner stock and scarf into it or set up your layers to accept the 5mm.

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    Default Re: Will plywood do this?

    Use a good quality plywood and steam it.

    My canoeyawl is planked with 12mm Sapele and the planks at the stern were all steamed.
    (12mm is pretty stiff!)

    example "A"


    Last edited by Canoeyawl; 01-29-2018 at 08:34 PM.

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    Default Re: Will plywood do this?

    I'm thinking of building in the back room of my bookstore, not sure my stock would handle steam very well.

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    Default Re: Will plywood do this?

    Quote Originally Posted by navydog View Post
    The chine at the bow could give you some difficulty with 5mm. You can just cold mold that area with thinner stock and scarf into it or set up your layers to accept the 5mm.
    I was planning to use 4 mm. Still a problem?

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    Default Re: Will plywood do this?

    John you need to test it. Looking at a little picture doesn't cut it.

    I think you need to cut narrow planks and cold mold it no matter what ply thickness you use. The compound curves won't let use use ply as a sheet.

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    Default Re: Will plywood do this?

    I think that the turn of the bilge in the forward sections is too severe. Yes, you can torture plywood into fantastic shapes with the right technology, but forming jigs are expensive to build and you will have to go through expensive trials to get it right. The laminated chair that Canoeyawl posted is formed from dry laminates in a hot-glue press at about ten tons of pressure (I did work in a factory that made such things). Sorry, Canoeyawl; no insult intended. I think that the hull form you are contemplating can be approximated reasonably well in plywood, but not that form, and possibly not with a single chine.
    Hope for the best, but plan for the worst.

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    Default Re: Will plywood do this?

    Quote Originally Posted by navydog View Post
    John you need to test it. Looking at a little picture doesn't cut it.

    I think you need to cut narrow planks and cold mold it no matter what ply thickness you use. The compound curves won't let use use ply as a sheet.
    These are not compound curves. This is not tortured plywood. The panels develop as a section of a cone, with very little surface stress. Because there are no compound curves, a sufficiently ductile material will bend to these shapes without a lot of stress. The question remains, is 4 mm plywood ductile enough?

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    Default Re: Will plywood do this?

    What is the radius of the chine curve at the first station?

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    Default Re: Will plywood do this?

    Quote Originally Posted by navydog View Post
    What is the radius of the chine curve at the first station?
    Station 1 has hardly any curve. Station 11, near the bow, has I think about a 2' radius.

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    Default Re: Will plywood do this?

    There's always this option where you could cut your own kerfs:
    27827-02-1000.jpg
    Butter some epoxy into the gaps, bend it into place, clean out the squeeze-out.

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    Default Re: Will plywood do this?

    John,
    Well I cant read those little numbers. Who numbers the stations from the stern anyway?

    So I imported your drawing into a drawing program and measured the the radius by over laying a circle on the radius. I scaled tge boat design to 14:1. A one inch circle matches your curvature. Your radius is 6". That's pretty tight for 4mm. I just tried some cheapo birch, it can maybe do a 2' radius but starts making noises. With a 6" radius I think your down to 2mm to make those bends.
    Last edited by navydog; 01-30-2018 at 10:04 AM. Reason: Correction

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    Default Re: Will plywood do this?

    Or just cold mold the bow with veneers back to the point where the radius is easy for the ply to take.
    -Dave

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    Default Re: Will plywood do this?

    I could cut 24" lod to scale model stations out for you with a profile and deck. You can assemble it and then inspect the bottom to see exactly what is required to bend the bottom on using 1/16" basswood.

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    Default Re: Will plywood do this?

    I know you think the bow is a cone, but is ready isn't. If you look at the bow sections you can see that the bottom material needs to be pushed up to the point where it meets the stem and chine. After it is pushed upward to the stem, the sides are bent up to the chine. 2 bends. Make your life easy and cold mold the first 3-4 stations along the chine.

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    Default Re: Will plywood do this?

    Quote Originally Posted by navydog View Post
    John,
    Well I cant read those little numbers. Who numbers the stations from the stern anyway?

    So I imported your drawing into a drawing program and measured the the radius; it's one foot. That's pretty tight for 4mm. I just tried some cheapo birch, it can maybe do a 2' radius but starts making noises. With a 1' radius I think your down to 2mm to make those bends.
    Thank you, that's valuable information.

    Delftship numbers them from the stern.

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    Default Re: Will plywood do this?

    Quote Originally Posted by navydog View Post
    I know you think the bow is a cone, but is ready isn't. If you look at the bow sections you can see that the bottom material needs to be pushed up to the point where it meets the stem and chine. After it is pushed upward to the stem, the sides are bent up to the chine. 2 bends. Make your life easy and cold mold the first 3-4 stations along the chine.
    I'm relying on the Delftship program for the numbers I'm getting on the development of the panels, but it sounds like even if they develop properly the material isn't ductile enough.

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    Default Re: Will plywood do this?

    Quote Originally Posted by cracked lid View Post
    There's always this option where you could cut your own kerfs:
    27827-02-1000.jpg
    Butter some epoxy into the gaps, bend it into place, clean out the squeeze-out.
    I had considered that for the stern, but I think the bow would be a bugger. How do you cut all those kerfs so precisely? Table saw?

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    Default Re: Will plywood do this?

    John, Don't do that, you will not be happy. I't wont bend around the shape in 2 directions.
    Last edited by navydog; 01-29-2018 at 10:38 PM.

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    Default Re: Will plywood do this?

    Quote Originally Posted by navydog View Post
    John, Don't do that, you will not be happy. I't wont bend around the shape in 2 directions.
    Doesn't have to, because it's a section of a cone, believe it or not. But I have no idea how to do those massively parallel kerfs, and in any case, the whole attraction of doing stitch and glue is that you don't have to do a lot of work, so unless it's a lot easier to do than it looks, I won't be doing it.

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    Default Re: Will plywood do this?

    A pity not to pursue one's dreams. Maybe set up the framing and molds then give it a try. If it doesn't work with the ply maybe you could replace the forward molds to build hard chine. That would make approximately zero difference in performance but allow you to use flat panels.

    Looks a bit narrow for a catboat to me John.

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    Default Re: Will plywood do this?

    Looks to me like the combined sides and bottom in way of the forward 6 frames would form a cup shape, despite what your software is telling you. If that's the case it can't be done with one panel.

    If you were to do it with hard chines you could make the first planking with pattern stock and keep the patterns for future hulls then whip up a few of them on the same building jig with sailing friends and race or cruise on the weekends. They'd make nice gifts as well, something to do in your old age.

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    Default Re: Will plywood do this?

    I use a bench mounted chop/cross cut saw to cut kerfs in anything up to 18in wide. If you can purchase it over there, there is a product called flex-ply.....you can virtually roll it up like a carpet, though that would need some sort of sheathing when finished.

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    Default Re: Will plywood do this?

    Quote Originally Posted by Peerie Maa View Post
    If it worries you do the bottom in two layers vacuum bagged.
    I did a similar boat at 12 foot, the bow bent round OK with the ply I used, but it was too thin for the flatter panel in the run, so I had to add stringers and frames.
    Quote Originally Posted by johnw View Post
    These are not compound curves. This is not tortured plywood. The panels develop as a section of a cone, with very little surface stress. Because there are no compound curves, a sufficiently ductile material will bend to these shapes without a lot of stress. The question remains, is 4 mm plywood ductile enough?
    What John said. The boat I mentioned was also a conic projection with the bottom panel seam running high up the stem like that. It has been done.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Will plywood do this?

    You wouldn't be able to build this boat from ally either. Its got double curvature. Better to build the bottom and topsides as flat plates and then strip plank the radius in the forward section along the "chine"

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    Default Re: Will plywood do this?

    Quote Originally Posted by Mark Bowdidge View Post
    You wouldn't be able to build this boat from ally either. Its got double curvature. Better to build the bottom and topsides as flat plates and then strip plank the radius in the forward section along the "chine"
    There is no double curvature. I know this, see post #31 just above yours.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Will plywood do this?

    Since you have designed your project with Freeship (or Delft ship?) go to "develop plates" function, check edge and area errors and print our developed panels to scale. Then follow Gougeon brothers method as described in their Boat construction book, chapter 23. Design guidelines are quite reliable and helped me to build successfully several hulls between 25 to 16 feet long. But looking at your hard screen copy, it seems that your first buttock shows a double curvature close to the bow, meaning there is a hollow there. Same thing with your second waterline which doesn't seem to be well faired along its rear end...

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    Default Re: Will plywood do this?

    Im not talking about the stern. I'm talking about the bow sections.

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