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 Junior Member

    :) was meant to hint that I was being a bit tongue in cheek about the CAD/3D structural force field printing program. Sorry if that wasn't clear.

    For that matter, I imagine an animated cartoon paddler taking a force field boat far out to sea, and the battery dies. The animated paddler says "Woops!", and a shark smiles. :) (Note - another :) - please don't take that animation too seriously.)

    However, there is a kind of semi-automation CAD that I would like. Several kayak kit companies can provide full size plans and/or cut pieces using computer controlled cutting devices. I would like them to adapt their plans to my body dimensions, weight and desires, at a reasonable cost.

    I'm well aware that it is possible to make mistakes using a computer. I used to do "scientific programming". Sometimes I made mistakes. Sometimes I fixed other people's mistakes. Sometimes computers malfunction and make mistakes. Sometimes system tools (like compilers) have mistakes or provide unexpected results.

    I am not sure what to do about the PVC boat. I had hoped it could be the final product, and maybe to enjoy showing other people how easy it was to do. But you guys have convinced me it would be extremely unsafe except under very controlled conditions. :( I might just make a ridiculously crude prototype to determine how well I personally can control a tippy boat with my knees.

    And I am now convinced that I should at least do some reading of intro engineering texts. If I can find them in a library, I'll start with the two that clmanges listed - do you have another suggestion?

    But will introductory level engineering texts just get me into more trouble, if I assume what they provide is precisely correct?
     
  2. Iridian
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    Iridian Junior Member

    I tried building a 14' rowboat with skin on pvc when I was 14. As other posters have said, it's too fragile and not stiff enough.

    I even tried screwing on aluminum stringers to stiffen it.
     
    Last edited: Jul 21, 2021
  3. upchurchmr
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    upchurchmr Senior Member

    Introductory courses are of course simplified.
    And the first issue is learning a new language.
    Not precisely like a foreign language, but common meanings do not match a very limited and precise meaning to a seemingly common word for the purposes of engineering.
    And most often the words have a very specific meaning when relating the associated math to the concepts.
    Good luck if you start.
    Send me a private message if you start and need help with structures/ mechanical engineering concepts.
    PROBABLY I can help, but it may be a struggle for both of us.
    The basic explanations are way back in ancient history for me.

    I've ordered the second book. If I can stay awake to read it I'll let you know what I find.
    It does look interesting.
    PS: I was a structural designer for Aerospace for 35 years. Not the same thing as an analyst at all - at my company.
    Did you look thru Yost's entire catalog? To me, there were really tippy boat designs, but you are obviously better in a kayak than I ever pretended.

    IRIDIAN - thanks for the report on actual experience. That's worth more than all the theories in the world. Sometimes a clever practical approach succeeds where all the narrowly focused theories predict failure. Heavier than air flight is a good example.
     
  4. clmanges
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    clmanges Senior Member

    I can't think of any others, but those two are very good. They don't go into any math worth mentioning, but they explain the basic principles of strength of materials and the way structures are designed to resist stress. Gordon spends considerable time explaining how different materials deform under stress, going down to the molecular level when needed. He talks a lot about cracks, how they propagate, and what to do about that. He also commented that perhaps engineers should be thinking less about exotic materials and instead think about inventing better holes. for the importance of holes, just think of feathers and bird bones, which are largely hollow.

    For the first time in human history, we can now make full use of holes in artificial structures, by the use of 3-D printing. Too bad it's slow and expensive, and I don't see that changing much any time soon (or maybe ever).

    You should have no trouble getting these from the library, but once you've read them you'll probably want to buy copies for yourself. They're quite entertaining, including various anecdotes about aircraft whose wings break off and ships that break in half. You'll also learn to be cautious in thinking about patches; if a thing breaks and you repair the break too well, you could cause the item to break somewhere else.

    Good luck, and enjoy the journey.
     
  5. upchurchmr
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    upchurchmr Senior Member

    We have been thinking about holes for a long time.
    Foam sandwich might come to mind.
    The problem with 3D with holes is that it is very difficult to analyze such a structure - to insure clever use of holes doesn't prematurely break the boat.
    Bird bones are just another version of foam sandwich.
     
  6. lobsterman
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    lobsterman Junior Member

    I was wondering if anyone has ever tried building a "skin on panels" type of ultra-light boat ?. Not a folding boat, but sort of like building a stich and glue skiff, without the stitches, or the fiberglassing.
    Using something like this 20oz white billboard waterproof fabric material, New Vinyl Tarp: 20oz, White/White - Click for Sizes - billboardtarps.com https://billboardtarps.com/shop/new-vinyl-tarps/new-vinyl-tarp-20oz-white-white/
    ( i regularly use that material as a roofing material on R/V sunrooms, and additions), glued onto the insides of say precut 1/4" marine ply panels with about 4" of extra material around the edges, for gluing the panels together into the hull shape, then glue on the outer skin, and double up the coverage on the seams for added strength, and waterproofing.
    It would not be a folding boat, and it would not be an entirely rigid boat, but it would be fairly durable, easily patched, and extremely light weight. Does something like that seem like it could be possible to build ???.
     
  7. lobsterman
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    lobsterman Junior Member

    I forgot to mention the awesome, waterproof, bulletproof, adhesive glue, to use would be HH-66, that stuff is great a creates a bond that is stronger than the materials being glue together.
    HH-66 PVC Vinyl Cement Glue with Brush 8oz (1): Automotive Adhesives And Sealants: Amazon.com: Industrial & Scientific https://www.amazon.com/HH-66-PVC-Vinyl-Cement-Brush/dp/B005LL3ZXW
     
  8. mitchgrunes
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    mitchgrunes Junior Member

    I assume you are talking about a typical carbon fiber layup, using a typical carbon fiber cloth, a typical resin and hardener, in typical concentrations, with a typical mass ratio of epoxy and fiber, and perhaps with a typical decision of whether to vacuum bag. If in fact there is a "typical" set of decisions.

    Is it possible that with other mixes, e.g., where there is much more epoxy volume and mass than fiber, this might not be completely true? I.E., that the total resistance to compression might have a significant contribution from the epoxy? Or is that unrealistic?

    I admit that I based my assumptions in part on non-technical descriptions of a completely different composite, steel reinforced concrete, which is sometimes pre-stressed to place the steel under tension before the concrete is formed, and which typically contain much more concrete than steel.

    At least some epoxy resins expand as they cure (link.springer.com/article/10.1007/BF00550593). Does that mean that if epoxy is applied in a layer inside the carbon fiber cloth layer, the epoxy layer is compressed while the carbon fiber is stretched, and that the initial stiffness (at low force levels) would therefore be due in part to that fact? I notice that many kayaks with an outer carbon or carbon/kevlar cloth layer have a very smooth skin, which suggests that the cloth layer might have come under tension during formation.
     
  9. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I built a PVC SOF with plywood cross frames as a kid. IIRC 3/4" 40 stringers and 2" 200 keel. I drilled big holes in outside of stingers then ran long 1/8" through bolts with washers and nuts to secure to 3/4" plywood frames which were notched to help hold the PVC in place. Yeah, its bit heavy and weak, but its cheap and easy to work with so good for prototyping.
    "Heavy" if you were going to portage carry for a couple miles up hills but not really a factor for 1/2 of a kayak frame in and out of car.

    For a dis-mountable all PVC frame I'd again start with big pipe of 200 for keel then instead of cutting the keel into sections for X or T fittings, cut into the fittings so they fit over the solid keel and secure with glue and/or screws. Use 22.5 and 45 connectors create chines of cross frames, then notch completed cross frames to help hold stringers in place. Drill big hole on inner side of frames and drill through small screw into inside of stringers from outside of frames. Stringers don't need to match elbows of connectors; you might have 3/4" OD stringers 4" apart 2" to either side of 22.5 elbow and their 3/4" hold off would still keep fabric clear of frame (depending on depth of notch).
    To connect fore and aft frames and still keep stringers and keel outside smooth, grind down connectors on the outside with belt sander.

    As Rumars said you might have "flat on the beach but rockered on the water" so if too much rocker consider the ancient technique of rope under tension to compensate. Run cords from the top of the bow and stern into the keel just in front and behind the seat.
     
  10. clmanges
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    clmanges Senior Member

    I could be wrong in this, but I was under the impression that engineering software was good enough now to model the behavior of such structures under loads. It might just need more computational power than most of us can afford.

    Not even close. Have a look:

    Birds have hollow bones – Dickinson County Conservation Board https://dickinsoncountyconservationboard.com/2019/10/21/birds-have-hollow-bones/

    In a foam sandwich, the holes in the foam are within a narrow range of sizes and more or less uniformly spaced. The foam acts similarly to the web in a web girder, to separate the tension and compression elements of the structure, but it's homogeneous, therefore, a certain amount of it is extraneous. If it were possible to regulate the density of the foam according to load, we could build lighter structures that were equally strong. I wouldn't advocate trying that with entire hulls that are designed to need no additional framing, but for something like a SOF kayak, it makes perfect sense.
     
  11. upchurchmr
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    upchurchmr Senior Member

    Mitch,

    The only thing you do with more epoxy is to reduce the strength of the panel. Doesn't matter if it is cloth or unidirectional laminate.
    The difference in properties between epoxy and fiber is just so large that you can only have a smaller strength with more epoxy (assuming you are comparing equal weight panels).

    The only way to get more stiffness is to layup the graphite under tension, so that the fiber is straighter.

    A smooth skin is probably from a large layer of epoxy. I once made a strip planked skin coated with glass epoxy, with additional epoxy to fill the "weave" which was mistaken for aluminum.
    Smooth was from lots of OCD attention to sanding with smaller grits until it was painted, then polishing the paint.
    Somebody is overthinking micro/micro effects for expansion of epoxy. Graphite is effectively so close to ZERO expansion for any thermal changes that it is a non issue.
    You could also say that epoxy absorbs water over time and therefore expands.
     
  12. upchurchmr
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    upchurchmr Senior Member

    Clmanges,

    Foam is not just a separator.
    It has to support the shear load internal to the panel, and it has to be stiff enough to cause the panel to act as a sandwich.
    Just because it is homogenous does not mean it is extraneous.
    In a well designed/ light weight sandwich, the shear is effectively the same thru the core.

    What do you mean "regulate the foam according to load"?
    The foam density is one variable in the design of the sandwich structure, and is chosen according to the load and stiffness desired ( and impact criteria and other things).
    Foam density is available in a wide range according to the needs of the design
     
  13. clmanges
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    clmanges Senior Member

    Granted.
    Reread my previous post.
    Reread my previous post.
     
  14. mitchgrunes
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    mitchgrunes Junior Member

    I thought about it and realized what I proposed made no sense at all. If an outer layer of fiber cloth were under tension, it would compress the inner layers - but in order to do so, there would be a counter-acting compressive force on the cloth fiber - and it would be across the fibers, not along them. So to do that, the cloth would need to have compressive strength too.

    Incidentally, I guess there are reasons why a foam core or support might optimally vary pressure level to be greatest in the places it is most needed. E.g., I suppose if you used a foam core inside a paddle shaft, you could create the most outwards pressure (by using denser foam, packed tighter) where the bending stress was highest. I have no idea if anyone does that. Likewise, when a foam pillar is used to help support the top deck, I suppose you could vary the pressure or width of the pillar to be greatest where you needed the most support. E.g., people commonly re-enter kayaks at sea by initially climbing onto the back deck, which places downward and some sideways stress there, and even more downward (and sideways) stress is created when you do a "T-rescue" - wherein you pull an inverted kayak that has up to a few hundred pounds of water inside across your foredeck, and rock it back and forth to empty it out. I personally don't much like having a foam pillar under the foredeck of a sea kayak, because it makes it harder to re-enter the boat while at sea. But it is a very lightweight way to create vertical support - often much lighter than having strong arch-like structures near the top of the boat to support the load, which is more or less the way most commercial sea kayaks are made. (Some people also hug a central foredeck pillar inwards with their knees, instead of pressing outwards against the sides, like I prefer, to help roll kayaks back upright, and to allow better control of the tilt of the kayak.)

    In a similar way, it is very common among serious whitewater boaters and a few sea kayakers to glue foam onto the sides of a kayak or canoe to fit one's knees, to make a snug fit, and on the bottom seat or saddle, to fit one's body there too. You might prefer to vary the density of the foam, if for some reason you want your vary the pressure against different parts of your body - but I don't know if many people do that. In principle, it would be very much like varying the fit of stuffed upholstery, or inside a pillow or back support. But that all comes under the domain of outfitting, which isn't what this thread is about.

    Off-topic, many shoes and boots have insoles whose foam density varies, in an attempt to properly support the "arch" (i.e., medial longitudinal arch) of an "average" foot. Unfortunately, "average" feet aren't universal, so a lot of people end up unhappy with their store-bought shoes and boots. Regulating foam density according to desired pressure is quite common in shoe and boot insoles (and "orthotics" made by medically qualified personnel). I tried to play with it a little in ice skates - but it is people who pronate or supinate who seem to most need different amounts of pressure on different parts of the foot. In contrast, the bottoms of my feet are just a little tilted, because one leg is a little longer than the other (not all that unusual, BTW), and I also need to reshape the insides to fit the shape of my feet, which have significantly wider toes than heels - so it makes more sense to make insole whose thickness varies as needed, and to wrap an extra wide back of the insole around the sides of my heels to make them snug. Before I did that, I had a lot of ankle sprains. (My skate boots were custom fit, at extra cost, so shouldn't have needed any of that - but the fitter did it wrong.) I wasted a lot of years figuring out how to compensate for badly fit boots and shoes, and made a web page about it. But that is way outside the domain of boatdesign.net.
     

  15. upchurchmr
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    upchurchmr Senior Member

    Do you really think I didn't read your post?
    What you wrote did not make sense in some manner.
    How about an answer if you can.
     
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