Are multiple 1/8" plywood strips as flexible, strong & durable as 1/4" bending wood strips

Discussion in 'Boat Design' started by mitchgrunes, Sep 5, 2021.

  1. mitchgrunes
    Joined: Jul 2020
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    mitchgrunes Senior Member

    I'm at the early stages of trying to make a SOF (Skin-On-Frame) sea kayak, with a wood frame. I have no carpentry experience, and am trying make it as foolproof as possible. My boat should be functional, lightweight, durable, and should fit me. It does not need to be authentic or beautiful.

    Some people, like Build Your Own Skin-on-Frame Kayak https://www.capefalconkayaks.com, favor traditional techniques, in which a fairly flexible but strong bending wood, like green (newly cut, high moisture content) or steamed or multi-week-soaked white oak strips are cut, and then bent into curved shapes, to form the ribs.

    Unfortunately, such woods are sometimes hard to get at modern lumber yards, expensive, and most of the strips have significant imperfections that would weaken them, or have grain in the wrong directions, or don't work out for other reasons, giving low yield.

    So many people, like SOF Kayak Builders Manual http://yostwerks.org/SBSectionsA.html, prefer to cut complicated shape cross section pieces out of plywood (though some use traditionally shaped strips), which is cheaper and more available, but which is less flexible, and denser, though that is complicated, because there are many types of plywood, and "marine plywood" is not cheap or readily available.

    The traditionalists say, to make a very lightweight frame that is durable in storms and surf, use flexible woods instead of plywood. They also prefer lashed joints, which are more flexible than peg joints, screws or glue. Also, bending woods save weight over plywood, and the flexibility allows more margin for error when fitting pieces together.

    Plywood advocates say it rots less quickly the high moisture content wood, and cutting plywood to shape is faster and easier than bending good bending wood, and gives 100% yield cross pieces.

    There are many places the merits of these techniques have been endlessly debated, in many forums. Some people also debate terminology, whether boats with plywood cross sections should be called "fuselage boats" rather than "skin-on-frame boats". I do not expect you folks to resolve or rehash such debates for an idiot like me.

    But, here is my question: if rib strips were cut out of thinner plywood - e.g., half the thickness of the usual bending wood strips (about 1/4", based on the specified router bits at Cape Falcon), but were to use a little less than twice as many strips, to get about the same weight, and used lashed joints, could I achieve comparable flexibility, strength and durability to using bending wood strips?

    Has anyone tried this? What were the results?
     
  2. fallguy
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    fallguy Senior Member

    The only way to know is to test your variables using something like a point load on a piece fo each and a deflection measurement.
     
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  3. Blueknarr
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    Blueknarr Senior Member

    No.
    Plywood has grain going in the wrong direction. You would have to discount this grain. Your piles would need to be almost twice as thick as solid stock.
     
  4. gonzo
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    gonzo Senior Member

  5. upchurchmr
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    upchurchmr Senior Member

    If you were to use thin plywood but 2 pieces to try to make a bent wood thickness, you would have to glue the 2 pieces together.
    The reasons to not use 2 strips given above are completely true, if you don't glue them together then it gets even worse.

    You are over thinking this. Don't try to re-invent the wheel.
    Thousands of people have made kayaks with known methods.
    If you don't want to find green wood, then use another method.

    Do something, learn to do the normal methods instead of trying to invent something about which you say you know nothing.

    I've built 8 Yost kayaks.
    They all worked good.
    They all lasted.
    They were fairly simple to build.
     
  6. fallguy
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    fallguy Senior Member

    Another thing to be mindful about. Raw edges of plywood rot easy and delam.
     
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  7. mitchgrunes
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    mitchgrunes Senior Member

    I will take it from that that though the wood Yost kayaks use plywood, they are durable enough to handle launching and landing through ordinary breakers, a few feet high, perhaps up to 5 or 6' high, on a beach? (That's about as high as it could go where I plan to move, near the Finger Lakes and one Great Lake in upstate NY.) And that I also need not worry about lash joints?
     
  8. upchurchmr
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    upchurchmr Senior Member

    I have never done 5-6' high breakers, but others have said they have.
    You should look for SOF skin destruction tests on video.
    There are usually a couple.
    From my tests, a newly sharpened knife goes right thru Polyester cloth/ paint easily.
    Anything not razor sharp does not cut the skin. It mostly just bounces off.
    Apparently ballistic nylon is somewhat tougher.
    I don't lash the frame. I put a single deck screw thru the stringer and into the plywood bulkhead at each joint.
    Then I make epoxy/ filler fillets on each side of the joint. Remove the screw and insert a small wood dowel with epoxy.
    I also epoxy the edges of the plywood for rot resistance.
    Don't use Fir marine plywood, the fir is too brittle and chips away from the inside and outside cuts - resulting in a very thin single ply left with no strength.
    Lots of guys do lash, no one that I know has reported problems.

    Dive in, any of the methods have made good boats.
     
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  9. Rumars
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    Rumars Senior Member

    You have forgotten the obvious third choice, laminate the ribs from solid wood. Cut the dry wood to a thickness that takes the bend (or buy thick veneer), laminate as many layers as required for the desired thickness. All you need is clamps and some epoxy. Ash, black locust, osage orange are all good woods for this. Laminating can be done in place, same thing as steam bending, just a lot more clamps.
     
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  10. Blueknarr
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    Blueknarr Senior Member

    Twisted string clamping.
    Lots cheaper than hard Clamps.

    Tie string snugly around glued bundle. Insert a dowel thru the loop and twist until enough pressure is applied. Tie the dowel in place. Repeat often along the bundle. Cut the string loops away when cured.
     
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  11. upchurchmr
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    upchurchmr Senior Member

    I assumed that "bending wood" was the same as laminating from solid wood.
     
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  12. KeithO
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    KeithO Senior Member

    1/8 strips should bend better than 1/4 the only difference is that one should expect to pay more for them due to higher labor input and greater loss to the re-sawing operation. Any glue that you should be using will be stronger than the wood fibers, so assuming proper prep and clamping pressure the strength of the wood itself should be the limiting factor.
     
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  13. upchurchmr
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    upchurchmr Senior Member

    What an idiot. I mixed this thread with another.
    Nothing I said about big thick strips has anything to do with this thread.
    My apologies.


    If I remember right one of the boats had 3/4 x 3/4" strips.
    The design did not require glass, just a coat of epoxy.
    A coat of epoxy with no glass will be easily damaged - not for me.

    So I thought to use 1/2" strips with inside and outside glass. How much I don't know yet. I'd probably make some test samples and break them.

    On a kayak it is easy to bend and twist the strips with a heat gun - but that is 1/4" thick.
    What it takes to bend 1/2" with heat only I don't know - another test.

    If you do a glass coating, the strength is in the glass for 1/4". Still going to add strength to 1/2 but how much is a question.

    You might understand that I'm not much for "traditional" building. So my comments might not be useful to the OP.
     
    Last edited: Sep 8, 2021
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  14. fallguy
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    fallguy Senior Member

    It happens to me now and then as well. So far, haven't had to eat shoe!
     

  15. mitchgrunes
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    mitchgrunes Senior Member

    I have no interest in being traditional. If you recall, I started out asking about using a PVC pipe frame.

    I'll experiment with making ribs out of laminated strips. I bought a jig saw with a strip cutting attachment - though I guess I could have taped or clamped a piece of straight wood to the bottom to do the same thing. I may use a steam iron and heat gun to help. If that works, I won't bother with plywood cross pieces, and may try to make the whole frame out of a wide 16' board. Because I am new to carpentry, I will use lash joints to re-enforce even if I glue the stringers to the cross pieces or ribs.

    I have just spent much more money than I expected (close to $300) on tools (cheap tools - I probably won't do this again), West Systems epoxy, and paint - from thrift stores, dollar stores, Harbor Freight, Walmart, and a marina store. (I looked at Lowes and Home Depot too, but their stuff costs more.) That doesn't include the wood and skin for the final boat, and a few more materials. To make matters worse, I didn't plan perfectly, wasn't exactly sure what method I will use to do things, and won't use everything I bought. Also, I wasted a lot of time and money driving back and forth between various stores to see what I could find at a reasonable price. Oh well - some people spent even more setting up a real wood shop. And a custom boat by someone else would have cost me $3000 - $5000.

    I'm slowly gaining experience cutting, drilling and gluing scrap wood. Maybe I will make a small scale kayak model first - though I know strip flexibility doesn't scale with thickness.

    Spruce is supposed to have a good strength/weight ratio for stringers, and white oak stiff enough for cross pieces or ribs - but I'm just going to use cheap (but prime, knot free) pine from 84 Lumber for this try. I hope that is good enough. I will also use a straight keel, and straight top stringers for the foredeck and aft deck, so I don't need to bend them. Obviously the gunwales and chines need to bend - but each can be kept co-planar, to reduce the tendency to impose a bend (rocker) on the keel. The advantage is that I probably won't need to build a table or strongback or jig, though I will need to lay the wood across short 2x4s or palettes (which are free from some stores) so the jig saw blade and drills don't touch the ground. Given my lack of space for a woodshop, that would be a huge advantage. It means getting down on my knees to work on the ground, but I can still do that.

    As one person at this site suggested, I will start with a polyester tarp for a skin, which are available locally. I will experiment with a small tarp to see if a 4 mil tarp can hold a foot with my full weight. If it can, I will use a 7 or 10 mil tarp for safety, and glue an extra piece in the middle of the boat so there are two layers where I sit or step, or lay my body across the back. I may also glue an extra strip down the keel to slow abrasion there. I will experiment with a heat gun to see if it can shrink the tarp to be taut.

    I'm aware that weight support across a taut skin is complicated. I recall from a book on rope climbing for cavers called "On Rope" that if a rope hangs straight down, its tensile strength only need support its own weight and the climber's weight, whereas if it is strung across a pit at an angle, it needs much more strength, because it is only the vertical component of the rope direction that supports the climber's weight - in fact if it is straight across, you need infinite tensile strength. (Assuming the rope has no resistance to bending - i.e., sheer resistance and strength.) By analogy, a taut skin that didn't bend or stretch at all would need infinite tensile strength to support my weight. So I'm not sure how to make sure that the skin is strong enough to support me at a given level of tautness, especially since I can't guess how taut it will be in the final product. All I can do is guess, and run a simple test.

    Whether or not I use plywood or laminated cross pieces, and whether or not I attach the wood with epoxy or only lash joints, I will mostly base my design on the Yostwerks Roldarka LC. With some changes, including the straight keel, with other lowered vertical heights above baseline to compensate and keep about the same volume. I will eliminate almost al the overhang, following Epics theory that that is faster, and it hopefully has fewer interaction problems with wind and waves.

    I will also scale the half-widths so as to make it only a little wider than my hips, which will reduce the displacement. That change is actually about right in terms of displacement- I am lighter than the weight Yost designed the Roldarka LC for. If that proves too thin for stability, I may have to add PVC pipes + floats that fit under the bungies to stabilize it during entry and re-entry, that I can take out of the water once I am inside.

    However, I want about a 2" higher back deck top stringer next to the cockpit, so it is easier to re-enter the boat without swamping (a major problem with my current SOF), and a 4-5" higher foredeck top stringer next to the cockpit for rapid re-entry. I would linearly interpolate the resulting heights so as to keep each of those top stringers straight. That will increase the total volume above-water - but it will strongest near to the center, so hopefully wind and wave won't affect tracking too much.

    I like keyhole cockpits, because I think they rapidly entry and re-entry easier, and speed pray skirt mount while underwater during a re-enter-and-roll (minimizing water entry), or to put it only rapidly between breakers when launching into surf. I'm not a serious surf boater, but weather and conditions aren't always predictable.
     
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