PVC pipe as a SOF kayak frame material?

Discussion in 'Boat Design' started by mitchgrunes, Jul 14, 2021.

  1. gonzo
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    gonzo Senior Member

    I don't see how splicing wood makes it substantially heavier. The weight of the glue will be a few grams. There are usually a few good boards in the pile at the store. It takes a bit of time to sort through them, but you only need a few for a SOF.
     
  2. mitchgrunes
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    mitchgrunes Senior Member

    I'm obviously not an expert, but from what I have read, you splice wood by creating joints, e.g.,
    https://en.wikipedia.org/wiki/Splice_joint
    https://en.wikipedia.org/wiki/Finger_joint

    I presume all of the joints shown in those articles would create a weaker or less durable boat than solid wood, in a SOF boat, in that if you bend the wood, the pieces are only held together by the glue joint. While the glue itself may be stronger and more durable than wood, I presume the wood/glue joint or the wood around the joint would not be, for the reasons I mention below - when used in a lightweight somewhat flexible SOF boat.

    A major basis of lightweight SOF construction, according to many sources, is that the boat needs to bend to be able to take impacts, from the ground (during put-ins, take-outs, bumps and bottom scrapes), and from waves.

    I assume that to be fully waterproof during long immersion, you would use epoxy glue. But epoxy doesn't bond strongly to wood - you have to roughen the surface (e.g., with coarse sandpaper), to create nooks and crannies that the epoxy can seep into. After the epoxy hardens, it holds the pieces together because the epoxy or wood has to break or bend, or compress, to come apart, much as it would in a dovetail joint. West Systems Epoxy has an article on their website mentioning this.

    I assume wood that is roughened up has many of its fibers broken or separated, and is therefore weaker than solid wood.

    On top of that
    https://craftknights.com/how-strong-is-wood-glue-really-we-tested-it/
    claims that epoxy/wood joints come apart over time, as the wood moves. Obviously, in a lightweight somewhat flexible boat, the wood moves.

    Other sources, e.g.,
    Is Finger Joint Lumber Worth It? - The Craftsman Blog https://thecraftsmanblog.com/is-finger-joint-lumber-worth-it/

    also talk of joined wood being weaker than solid wood when the wood moves.

    And some, e.g.
    finger joint vs.solid wood studs - Fine Homebuilding https://www.finehomebuilding.com/forum/finger-joint-vs-solid-wood-studs
    say that it is stiffer (bends less). For that reason jointed wood is often actually preferred for home building. But in a flexible craft, if it can't bend much at the joint, it needs to bend more outside the joint, so it would be more likely to break there.

    My assumption is that for all these reasons, you need to use thicker or wider material to attain the same strength and durability as solid wood, when used in a lightweight flexible craft - increasing weight.

    Am I wrong? I obviously could be - I'm extrapolating from many sources, based in part on other uses, other joints, and sometimes other glues.
     
    Last edited: Jan 14, 2022
  3. gonzo
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    gonzo Senior Member

    Yes you are wrong ;). A proper splice will work fine and won't weaken the wood as you presume. Also, I don't know where you get the idea that epoxy does not bond strongly to wood. The wood fibers will break before breaking the bond. The millions of objects held together by glue clearly show that it is an effective method for joining wood. The links you posted show unsubstantiated claims that epoxy glue joints fall apart. There is not a single documented test to back the claim. It also not true that a wood surface has to be roughened for the epoxy to adhere. In fact, breaking the fibers will weaken the joint. You are making a lot of assumptions that are not based in any experimental data. There is absolutely no reason to use a thicker or wider material in wood that was scarfed than if it was a single piece. You are driving yourself crazy by compiling a lot of random opinions from the internet. An SOF can be built with thin branches and polyethylene sheet. I have done in the field. The branches are not even scarfed, but overlapped and lashed.
     
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  4. mitchgrunes
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    mitchgrunes Senior Member

    As far as where I got the idea that epoxy does not (chemically) bond to wood, see page 12 of West System Epoxy's user manual

    https://www.westsystem.com/wp-content/uploads/British.pdf

    which says

    It seems reasonable that wood/epoxy joints use secondary (mechanical) bonding, because the wood is already formed.

    In addition, all the sources I have found, including West Systems Epoxy, say you have to roughen wood surfaces for epoxy to work properly.

    I presume West Systems Epoxy is a reasonably authoritative source of info on their own epoxies.
     
    Last edited: Jan 14, 2022
  5. upchurchmr
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    upchurchmr Senior Member

    ++ for Gonzo's reply.

    There are lots of uninformed idiots who think that wood will break if it "moves".
    There are also lots of "people" who will not accept instruction from a knowledgeable source - the Gougeons.
    So many people will improve on good advice and ruin what they are doing.

    The quote you gave was talking about fiberglass/ epoxy.
    Not the same as wood/ epoxy.

    But you can test this for your self.
    Cut a 8/1 slope on two pieces of wood, doesn't matter what wood except for possibly teak.
    Give it a week to fully cure, then bend the joint until it breaks.

    I'll eat my hat and boots if the joint does not show wood fibers on both sides of the joint.
    FYI, I've done this test for myself - using pine and cedar.

    If you want to do this yourself, PM me for a few additional directions.

    Since you don't have facilities available, I'll cut you some pieces, if you will pay shipping.
    Epoxy joints for wood if done right are stronger than the original wood.

    Done wrong anything will break.
    I originally assumed a 45degree joint (1/1) would be strong enough. In trying to bend a stringer for a SOF kayak, the stringer broke at the joint, but there still was wood fiber on the glue surface.
    8/1 is necessary - approximately.
     
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  6. upchurchmr
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    upchurchmr Senior Member

    MrNewman,

    It would help to know where you are from. Don't be ashamed.

    There have been very good SOF kayaks made from aluminum tube - as far as I know available anywhere.
    The comment above about people making "kayaks" from locally sourced wood is very appropriate.
    It might weight a little more, but you can still have a sound boat.
     
  7. Waterwitch
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    Waterwitch Senior Member

    I don't know if you have seen these sorts of videos, materials are "free" if you have access to a wood lot to gather some branches and splints.

     
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  8. mitchgrunes
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    mitchgrunes Senior Member

    Are you saying the Gougeons are a knowledgeable source, or that they won't accept instruction from knowledgeable sources?

    Do you mean that the part that breaks is right next to the joint? (Which would imply the failure could be the fault of the joint, or of the material preparation req1uired for it.)

    The manual I quoted, is from the Gougeons, and it discusses using epoxy with many materials. The part about primary [chemical] bonding is somewhat specific to fiberglass/ epoxy. The part about secondary [mechanical] bonding, is not.

    If I only wanted to test maximum flex, PVC pipe would have done better than many woods. And a boat made like PVC pipe furniture would have probably worked fine - in flatwater, and/or for a short while. When I went looking for PVC pipe failure tests and examples online, I found many studies saying that it has a fair amount of initial flex and strength, but fails over the long term under conditions far below initial tested flex and strength limits, after only a few tens of thousands of flexes - and unlike many wood or fiberglass/resin boats under similar conditions, it often fails abruptly, without obvious visible warning. That would make a PVC kayak dangerous to use far from shore. I need sufficient warning to paddle the boat to the take-out, and effect repairs at my leisure. And I really want it to survive hundreds of thousands or millions of wave flexes.

    I.E., an equally important question to immediate failure modes is what happens under much lower but more typical conditions of stress, over a period of years. That sort of long term study isn't as easily done.

    Perhaps that is a strong reason for using materials and techniques that have been well tested for the intended purpose over decades or longer, like wood, lashed joints, and to some extent epoxies. (My old SOF, whose wood and skin are gradually failing, after maybe 10 years of mostly light use, still has intact lash joints. Maybe lash joints can work pretty well, and are less trouble prone? The rather elaborate instructions associated with epoxy are scarring me - a lot of possible mistakes are discussed.)
     
  9. MrNewman
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    MrNewman Junior Member

    Yep, sure - I've seen some folding kayaks made of alu, and even paddled one very old two-seater.

    I was a bit pessimistic about the timber availability, had to search better, and not in the nearby department store :)

    Generally, was just intrigued by the original topic about PVC pipes, and then realized that hey - there are also polypropylene ones, and many grades have an enforced layer (aluminum or fiberglass).
    Certainly, its primary purpose is to hold the pressure from inside, and usually length doesn't exceed 4m, and some other concerns. But for limited conditions - why not?

    ...and the given answer about the stiffness completely ruined that my idea :)
     
  10. upchurchmr
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    upchurchmr Senior Member

    Mitch.

    The Gougeons are THE authority for this crowd, and also for more sophisticated ones.

    Do you want to do a test or do you just want to talk, and talk, and talk.

    The only reason I post here is so that others might get some value.
    I've really given up on you.
     

  11. gonzo
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    gonzo Senior Member

    At the end, the proof is in the pudding. You can build a SOF kayak for less than $60. If you have a tablesaw, the job is easier. However, a circular saw will do.
     
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