PVC pipe as a SOF kayak frame material?

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

  1. mitchgrunes
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    mitchgrunes Senior Member

    I'm not going to worry too much about the different definitions that occur in different technical books. "Resonant" was the specific way the physics books I studied (in the 1970's) described the behavior of oscillating systems near their natural frequency - forced or unforced, damped or undamped. It is also used in music - e.g., when one musical string vibrates, it's harmonics often "resonates with" other strings and cause them to vibrate too. But musical halls often "resonate" describing a different phenomena - reverberation. (Though for one reason or another they often have resonances at specific frequencies, and absorb sound at different frequencies, which concert sound engineers compensate for electronically.) Again, when an electrician installs a power conditioner (which contains a resonant filter) in your building, and it happens to "resonates with" with the same frequency as the resonant filter that the power company installs on a pole - which they sometimes do if they fear the electrical load in the building (like equipment that turns abruptly on and off - e.g., large switching power supplies) would mess up power supplied to other buildings - the two filters can burn each other out, according to a master electrician I knew.

    But you may be right about the idea that the entire problem could have been caused by something other than a resonant phenomena. I assumed that because the problem only occurred in 2-2.5' waves, in oncoming wind and waves, but not in smaller or larger waves, that it was a resonant phenomena. But I looked again at my two sea kayaks. It is possible is could be interaction of the shape of the boat with the waves. In particular, perhaps the curvature of the wave trough is greater than the rocker curvature. Higher waves tend to have longer wavelengths, and PERHAPS therefore less curved wave troughs. It probably doesn't matter much for smaller waves, because they don't interact as much. And it doesn't matter much for larger waves, because the lesser wave trough curvature means the crests of the waves don't touch the boat where it overhangs much if at all. The SOF boat that does well has even less rocker curvature in the center - but it near the ends, the bottom bends upwards at a sharp angle, leading to a high overhang at the very ends - maybe enough to avoid touching those waves. In addition, its ends have less surface area to be interacted with - and its ends are basically 1/2" thick pieces of wood - so even if they interact, they would simply slice right through the waves, without a great deal of interaction - perhaps not enough to make the ends bounce off the crest.

    So perhaps the excess volume, front width, and surface area in my Caribou is causing the problem.

    I guess the obvious way to tell would be that if it is a resonant frequency problem, it would occur when the half wavelength roughly matches the boat length. If it is a curvature, overhang, and excess buoyancy problem, it would occur when the kayak fits into the trough - i.e., when the wavelength roughly matches the boat length. But I can't easily check for that, because it happens so infrequently.

    Perhaps, since people here are so convinced extreme flexibility is more likely to be a problem than a solution, I should simply choose a boat, which like my SOF, has relatively thin, relatively low volume ends. But now I have to wonder - are there conditions where rocker curvature, and overhang, actually lead to a faster boat, because the breaking waves don't interact? The Epic video where they advertise that a full length waterline is faster in waves was demoed in waves that were actually quite low. (The boat in that demo did have fairly low volume ends, and a narrow V hull at the ends, so it shared some of the characteristics of my SOF kayak, which may have helped too.)

    Also, it seems hard to find a material that will be both extremely flexible and durable. (Exception: I've been in whitewater rafts seemed to be fairly durable, but they didn't even try to be fast or efficient. They were also very heavy.)

    After all, Dyson's work on potential unintentional resonant pitching in rigid kayaks looks largely theoretical, and he goes through a lot of steps to get there. Maybe he IS wrong for lightweight sea kayaks. Maybe the boats he studied were extremely flexible because they were so lightly built that a rigid boat would have broken.

    Gonzo: I have BTW tried climbing on my SOF the way you describe. It swamps. Remember, my SOF was designed as a competitive roller. So the builder/initial user placed the top of the cockpit as low to the water as he could. I guess that gives a slightly lower center of gravity, and slightly less resistance to motion while underwater.

    BTW, I just talked to a Dupont chemist who designs polymers (e.g., plastics), who was very familiar with fiber+resin composites, such as are in fiberglass, carbon/kevlar, etc., kayaks, as well as epoxy infused wood. I was curious whether such composites can be recycled. She said it is completely impractical. She said you would have to grind the boat into tiny pieces, sort them, and chemically reduce the resins back to "monomers" (small molecules, not chains). She said the energy, cost and resources consumed in recycling would greatly exceed those required to make a new boat. So I guess old boats often have to head to the landfill.
     
  2. upchurchmr
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    upchurchmr Senior Member

    Mitch,

    There is at least two common materials which would be flexible and durable in the sense you are talking about.
    Just make a solid rectangular or square longeron of graphite/ epoxy or glass epoxy.
    Not done much because to be flexible and durable it will be heavy.
    The wider the rectangle of the longeron in the direction across, the more flexibility you will have vertically.
    Fewer longerons will give more flexibility.

    Carbon/ epoxy can bend tremendously without failure if that is what you design for.

    Carbon fiber/ resin arrows of about 30" long can be bent in your 2 hands about 5+ inches without breaking. In fact that is how you test them to see if they are safe.
    And those are round and hollow and not designed to accept bending. It's just what the material is capable of.
    Making a solid rod or even a hollow one out of a +/- 45 degree winding or layup will make them much more flexible.

    You can do what you want if you are willing to pay for it, and do some destructive tests.

    Would it be worth it for the 2 times you have experienced the conditions in your life?
    You might never find the conditions again to test the issue you are concerned with.

    I won't bother you again.

    Marc
     
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  3. gonzo
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    gonzo Senior Member

    I know that we are just offering opinions, which as some particular body part, everybody has one. However, have you tried different kayaks in the same conditions? Most boaters, and kayakers, are a friendly bunch and likely to lend you their boat for you to try.
     
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  4. Skyak
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    Skyak Senior Member

    My intention was to stop you from the misguided idea that resonance is a major contributor to your control problem. If you still don't believe us do the step function test, and look up what that means in a first order system. There is a number tau you calculate from the test and another important number 'rotational moment of inertia' (proportional to mass times it's distance from the rotational center squared) that is important to minimize. BTW while you are at it, do the step function again pulling the bow under water -big damping increase.

    Your Caribou kayak is too big -YES, contributes to the problem
    overhangs are not good -YES people will tell you otherwise but look at the equations -it's mass far from center with no damping contribution.

    Smaller kayak volume, shorter, smaller overhangs, lighter, smaller wetted surface -all of these will improve your maneuverability in waves.
    When it comes to length I think you need to fully consider drag and the thrust you have to offer -longer does not mean faster. You would likely keep a faster pace in a shorter kayak with much less wetted surface.

    Rocker is another tradeoff. It increases stability by lowering your CG, it raises the ends making turning and waves easier -but it cuts tracking, and increases resonance amplitude.
     
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  5. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I forgot to mention, I've discovered that white PVC pipe can be DIY thermo-re-formed and is a poor man's Kydex. Place in about 300deg oven for about 6 minutes, take out, form over what you want (I made a nice chisel holster) with some oven gloves then dunk everything in water bath to quickly cool and freeze shape. The hot PVC will otherwise be kinda springy and want to return to original shape. No specs on if it makes it more brittle etc but my holster is holding up just fine in rough use.
    Haven't yet tried larger objects with Heat Gun but seems promising.
     
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  6. mitchgrunes
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    mitchgrunes Senior Member

    What about polyethylene (PE) or cross linked polyethylene (PEX) plastic pipe as a SOF frame material?

    I think most general market whitewater and sea kayaks are solid PE or PEX (the latter is used in the toughest whitewater boats, especially those designed to frequently boof (bump) against rocks at high speed. PE and PEX kayaks and canoes are more resilient than fiberglass, carbon/kevlar/foam core, wood, or aluminum kayaks and canoes - perhaps that means that PE or PEX pipe could do what I want?

    Unless there is too big a difference between the thick solid PE or PEX in kayaks and canoes, and my intended use as a pipe frame material under a light weight skin?

    Normal PE and I think PEX pipe is much too flexible. It is often sold in coils, like garden hose. (some garden hoses are PE.) I assume that every time I took a stroke it would bend completely out of shape, and waste most of the energy in internal friction.

    I guess I would add vertical center closed cell foam support pillars... My first kayak (a race weight Phoenix Cascade) was thin fiberglass, reinforced by foam support pillars, that someone had jammed in. (I eventually glued other foam pieces to the left and right on the top and bottom of the hull, so the support pillars wouldn't tip over.) A few whitewater boats have slots that foam support pillars fit in too.

    Foam pillars that go up to the cockpit in front would obviously eliminate one of the most common kayak re-entries, where you climb onto the back deck facing backwards, push yourself inside, then turn over to face forwards. So the boat would have to be thin enough for me to straddle it - not sure if I could do that with a 21" wide boat.

    I'm not also sure what type of foam was used. It was denser and more flexible and resilient than the polystyrene foam that is often used as thermal insulation. Does anyone know?

    PE and PEX are not very UV resistant. I guess I could use a UV protectant, even though it is inside the skin. It also deforms over long periods under load, especially if stormed in the sun on the ground - hopefully foam pillars would stop that, or I could store it upside down, so the deformation was less importtant. (People speak of boats "oil canning" when the center deforms. I never worried about it in the thick whitewater boats, but some people did. But I always stored and transported mine upside down.)

    Obviously, foam pillars would eliminate the possibility of the extreme flexibility needed for the deliberate end-flexing that Dyson speaks of. So I would have to hope low volume ends would be enough that I wouldn't need that flexing.

    I was once told by a whitewater kayak designer that he made early prototypes out of (thick) fiberglass. The final designs had to be modified to take out the sharpest corner edges for the PE or PEX production version. I don't know if that is true because of a weakness in the material itself, or if it was just a consequence of mass production rotomolding.

    (Incidentally, the polymer chemist confirmed that PVC is totally unsuitable, because it cracks too easily, and abruptly. I didn't think to ask about PE or PEX.)
     
  7. Mark C. Schreiter
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    Mark C. Schreiter Junior Member

    as an experiment is elementary school we built a small bridge out of Elmer's glue and popsicle sticks. No one thought it could support the weight of a fly but 70 lbs later it was still standing tall.

    imagine the guy who designed the internal combustion engine. I think about him/her going to the bank for a cash advance and explaining his idea and quickly getting laughed out of the place.

    a sky scrapper built from concrete and steel? what????? an airplane built from aluminum? not a chance, that thing will fall from the sky. boeing building an all composite airliner? what, a plastic plane won't work.

    I don't discredit anyones experience here, but I think the question should be "how can I make this work?" opposed to "will this work?"

    design, built, test and repeat because I'm sure there is some combination of designs and ideas that will satisfy at least a few of your requirements.

    -Mark
     
  8. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Now I'm thinking about cutting 3" diameter PVC into 8 45deg strips then inserting the strips into slits cut into plywood or flat PVC cross frames such that the strips would have narrow cut side facing outward. Then using wire ties similar to Stitch and Glue other strips would be tied to those strips with rounded side facing out to form structural "T" with rounded top (to go easy on the fabric skin and streamline). Then after everything is adjusted, all joints get painted with PVC solvent welding glue and after glue dries all the wire ties are removed. The glue dissolves PVC so should anchor PVC strips into plywood pretty well.

    You will have a bunch of curly PVC sawdust from all the cutting and combined with solvent could be used to....do something and create shapes or filling as needed for anchor points etc on the frame.
     
  9. mitchgrunes
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    mitchgrunes Senior Member

    It may amuse you guys to know that "custom" PVC core whitewater kayaks are being made by Soul Waterman, Corran Addison's company.

    See also.

    He does not give full details of the manufacturing process.
     
  10. KeithO
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    KeithO Senior Member

    PVC foam cores, composite skins.
     
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  11. mitchgrunes
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    mitchgrunes Senior Member

    In any event, it appears that those PVC core kayaks, like all high end composite whitewater freestyle kayaks, are intended for use by experts who can avoid hitting rocks too hard, and are willing to accept limited lifetime.

    I looked a little more into PE, PEX and HDPE pipe as a kayak frame material. No one is doing it! That is fishy, given that it looks much easier to put together than wood or aluminum frames.

    The material used in the most resilient and abrasion resistant kayaks is actually HDPE.

    From what I have found HDPE pipe is cheap and extremely durable - but is less strong/weight than PVC. Given that I calculated that adequate strength PVC would have been about the same weight as wood (but, unlike HDPE, not nearly as durable), it is clear that an HDPE pipe frame for a SOF kayak, would be significantly heavier (twice as heavy??) than a wood (or aluminum) frame kayak. Given HDPE's very high resilience and durability, I assume that, is the main reason why HDPE pipe is not commonly used as a SOF framing material. Then again, I think the strength figures I required for PVC were slightly overkill.

    HDPE also has a high coefficient of thermal expansion. So, if one used vertical foam support pillars to overcome its relatively high flexibility, the boat width would vary somewhat. In addition, it could still bend laterally under load, which would probably make paddling somewhat less efficient - perhaps that could be overcome by using a larger diameter pipe.

    HDPE joints are usually welded (melted), which sounds complicated. I would either have to use the special glue that bonds HDPE pipe to PVC pipe fittings, or the mechanically tightened fittings. That still sounds easier to me than learning to work with wood or metal.
     
  12. mitchgrunes
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    mitchgrunes Senior Member

    Maybe I am trying too hard to avoid learning to work with wood. A lot of people do figure it out for themselves, and it is a well tested material for this purpose.

    And used woodworking tools are relatively cheap.
     
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  13. Skyak
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    Skyak Senior Member

    Several pages back I explained why unreinforced plastic is a poor choice for SOF stringers (deflection, creep, glass transition temp). Since then you have piled on even higher requirements for safety to the point I can't even recommend you trust an amateur first-time build.
    Wood is a worthy, known, quality material. Making a folding kayak from it is still going to be a fairly advanced project. Joining many pieces into a smooth strong length with continuous properties is easier in aluminum tube than wood. A non-folding SOF would be a better first time project that would be a quality boat to your custom design needs. A stich and glue kayak would be even more certain to result in a high quality long lasting boat.
     
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  14. KeithO
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    KeithO Senior Member

    Remember, wood is natures composite. Having said that, it is a bit of a dying art and since everything today comes from plantations (that is legal) the quality of the product is not comparable to what was available in the WW1 WW2 eras when much was being made from wood. Its not easy to get and certainly not cheap. Marine ply was pretty astronomical and that was before the covid related material shortages.

    So, its 2021 and you dont have to use wood anymore. I suggested you use the road marker pins which are fiberglass. You can of course make your own fiberglass wood substitute from fiberglass roving and cloth. You wont need to get all the special tools for re-sawing and planing fancy wood. You will need to make molds to make your structural elements in, could be as crude as hardboard formed around some blocks glued to a sheet of plywood. You can make up a machine that lets you feed roving through a resin bath so it is automatically saturated with just the right amount of resin and then you apply that roving to your form and within reason you can make just about any practical shape with unidirectional roving. If needed at the end you can add a wrap or wraps of bidirectional cloth if it needs compressive strength. So you dont need to learn to work with wood, you dont need to deal with the problems of wood getting wet or rotting. But resin will need protection from UV. So you will need to paint it at the end. There is very good 2 part epoxy paint available today that the old timers could only dream of. And your frame structure would not need much paint, which is a good thing because fancy paint is not cheap.

    So, you have a LOT of options. Pick a material, figure out what it will do, master it. Then build whatever it is you want.

    Perhaps start here : http://seemanncomposites.com/cflex.htm
    It may be an ideal material for your project. There are 2 different physical sizes available depending on how stiff you want the part to be.
    Since the maker of C-flex is so daft as to not include a single picture of his material or how to use it, I will provide the following graphical reference:
    Building Methods: C-Flex page 1 https://www.glen-l.com/methods/mthdfg31.html
     
    Last edited: Aug 18, 2021
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  15. mitchgrunes
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    mitchgrunes Senior Member

    I guess I was trying for perfection, the first time, which may be impossible, for a first time builder. E.g., In truth, I don't know anyone who has a SOF boat that they feel can safely give or receive T-rescues, or that they can enter at sea from above the water. I can sacrifice those, and I can sacrifice folding.

    In all likelihood, I will find that my first attempt doesn't quite please me in any event. I've owned a lot of kayaks over the years, and have learned from each of them that I wanted something to change. So maybe I should just try, using the cheapest wood or plywood I can find, and go from there. Plus, maybe I should make a scale model out of the same materials, to learn how to do it.

    The stitch and glue boats I have seen (I belong to a club at whose sessions Chesapeake Lightcraft used to demo boats, and, before Covid, we often let people try each others' boats) are about as heavy as my Caribou (42 lbs), and were designed to take heavier, wider and taller people than me, and had huge cockpits - so weren't as good as my Caribou, I guess. Obviously, if I build from a plan, I could re-scale someone else's plans, and a smaller-person boat would be a bit lighter, but the leaning problem and cockpit problem would remain. But I guess I can look around a bit more, to see if someone has better plans available.

    In almost every respect my SOF is perfect, except the short waterline length, the low round cockpit which makes it hard to re-enter and doesn't give my knees enough bending room so I can lean forwards. And it is at end of life.

    I liked the plastic pipe SOF idea because it was cheap, fast to make, and so I could play with size and shape - but maybe I should just guess what I need, and go with it with conventional materials for my first attempt.

    In theory, I guess a plastic pipe SOF + tarp could be coated with a release agent and used as a mold for a fiberglass boat, which could be reasonably light, because it was sized for me. I'm not sure how hard that is - probably too hard for now.
     
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