PVC pipe as a SOF kayak frame material?

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

  1. Rumars
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    Rumars Senior Member

    I would not use PVC pipes for a sea kayak, maybe for something that goes on a calm lake. You can use it for prototyping, wrap the frame with saran and go into the pool or calm lake to see how it feels, but it would be ill advised to take one into the sea, with waves and swell.

    Anyway, a folder needs a system to tension the skin, both longitudinally and transversly. Older kayaks did transverse by frame tensioning, wich involved bending the frame parts inwards and snapping them together, for wich you also need a big cockpit. More recent boats have mostly gone to side bladders for this. Longitudinal tensioning is usually by means of a knee lever on the keel.
    Boats with full lenght deck openings can use other systems, like an excentric lever or a screw, and transverse tensioning by rope. 5min assembly time is overly optimistic, that's for inflatables.
     
  2. BlueBell
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    BlueBell "Whatever..."

    There are some surprisingly good inflatable kayaks available.
    Way, way, way easier, faster, cheaper, nicer than designing and building your own.
    But, I don't think the OP is interested in anything other than making his idea work.
    Good luck my friend, get video.
     
  3. mitchgrunes
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    mitchgrunes Junior Member

    I'm not an engineer, but I suppose the internal aluminum tube reinforcement idea (and the similar internal bamboo stake idea that someone else mentioned) would turn them into "composite" structures, wherein one is using multiple materials to overcome the deficiencies of each. Of course skin-on-frame boats are already composite structures.

    How practical is either of those ideas - i.e., using internal aluminum tubes, or an internal bamboo stakes, to overcome the deficiencies? It seems likely that the joints would be high stress points - but the fittings are bigger diameter than the tube itself. And either would increase the weight. PVC isn't all that light - though thin PVC tubing is.

    I used an old lightweight whitewater kayak (a Phoenix Cascade) for over 10 years and possibly a few thousand hours (I don't have a precise estimate) that was made out of nothing but thin fiberglass (held together by a plastic resin). Such boats were fairly popular in the 1980's and I guess before. One had to be more careful with them than the "plastic" (polyester, and later, cross-linked polyester) boats. I.E., I tried to avoid hitting anything solid as much as I could in whitewater, or scraping on the bottom. The modern practices of deliberately "boofing" (bouncing) whitewater boats off of rocks, and of sliding them down a hill into the water, or dragging them over rocks, would not have been good ideas. (Even fairly recently whitewater prototypes have been built out of fiberglass - but they use very thick fiberglass.)

    So perhaps a PVC pipe boat, or one reinforced with aluminum or bamboo is practical, given gentle treatment?

    An interesting question is - is that kind of gentle treatment practical on a sea kayak, used in the Chesapeake Bay (where I have occasionally been in 6-6.5' waves, and someone I know passed through a 10' wave during a storm)?.

    How about the Finger Lakes in upstate New York, where I may move? They often have waves a few feet high, occasionally 5' or so high.

    I'm not really capable of learning the skill levels to use a sea kayak much on the ocean itself. I did some a little bit around Florida (The Florida Keys, Everglades, Inland Waterway and Gulf of Mexico) - but that was a plastic boat (Current Designs Caribou), mostly in good weather, and mostly carrying enough stuff to give it a longer waterline than unloaded, with a paddling partner. When I tried to take it to a surfing beach, with 6-7' breakers, I rapidly had to give up.

    Even in sturdy plastic whitewater boats, I haven't even deliberately gone into class 5 whitewater (once only, when I missed the "right" move; I escaped by an accidental move I didn't actually know) , and only a little class 4 (supervised). In whitewater, I have preferred to stay in class 2 or low class 3. I'm about 65 and am not going to try serious expedition kayaking. So a boat I build won't have to take many big waves. Usually below 3.5' high, except for occasional unexpected storms, where it might have to weather 5-6' waves for at most a few hours. Could a well-put-together PVC tube boat, or one reinforced by internal aluminum or bamboo, take that?

    P.S.
    Could I learn anything useful from an introductory mechanical engineering textbook?
    I eventually learned that almost everything in my introductory physics text had been crude approximations that only applied under carefully selected conditions, and that some of it was completely wrong or out of date.Would reading an introductory mechanical engineering text do more harm than good, if I took what it said to be exactly correct, or didn't know what parts to be skeptical about?

    P.P.S.
    I've now been told that my idea of trying to completely fill the unused space with flotation bags is flawed. A person in a kayak store told me that they have seen boats damaged by air bags, which had expanded when they got hot - they said to leave a little space all around the bags. I already knew the bags themselves could burst if filled too tight and left in the sun. But I didn't realize bags can damage a boat. I guess that makes sense - sea kayak bulkheads have small holes near the middle so the cargo compartments don't break in the sun, or collapse at night.
     
    Last edited: Jul 19, 2021
  4. upchurchmr
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    upchurchmr Senior Member

    You can't count on "gentle" conditions from the weather.
    Don't use PVC.
    I don't want to see a RIP.
     
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  5. mitchgrunes
    Joined: Jul 2020
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    mitchgrunes Junior Member

    O.K. Thanks!

    I guess I might still try one as a prototype in completely sunny weather near shore, just to see what shapes work for me. I want to see if I can use a fairly tippy boat if I can lock my knees in to the sides. If it's only a fair weather near-shore prototype, that I don't need to paddle fast, I could maybe even make do with tarp for a skin, or paint simple cloth.

    Are aluminum poles (along Yostwerks lines) actually good enough to make a folding SOF sea kayak viable? Or are they also strongly weather limited, in bays or large lakes?

    I wish there were major commercial brand folders or sturdy inflatables that already met my needs. Lots of people sell used folders at fairly reasonable prices. But all the ones I have seen were made for bigger people, or are wider than I want, or have a lower top deck than I need to get much forward bend. And so far, the frame tensioners on the wood frame folders I tried to assemble required more strength than I expect to have as I age.
     
  6. clmanges
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    clmanges Senior Member

    Not very, I'm afraid, because each piece would be mostly separate from the other, so their individual strengths would not combine well. An alternative would be to take the bamboo, for instance, and fiberglass the outside of it; this is a treatment used to strengthen wooden boat masts and spars. This person made samples and did a crude comparative test, with results that were convincing to him, at least:
    Sailboat Mast Breaking Experiment https://pdracer.com/mast/mast-breaking-experiment/
    I'm disappointed that he didn't even do the relatively easy force calculations, though they may not have been broadly applicable.

    You can also make structures that are both lighter and stronger than tubes, but they have to be aligned properly to withstand the anticipated stresses. Web girders are an excellent example of this; they're mostly air, yet tremendously strong when oriented properly. You can find them holding up the roofs of industrial buildings. Useless against side-loads, though. You might consider something of the sort for bulkhead frames or seat supports, but for the longitudinal members you should stick with tubes, because those parts will be subject to stresses from a variety of directions.

    I'll recommend two books that will help your understanding of all this:
    https://www.amazon.com/New-Science-...ords=j e gordon&qid=1626720466&s=books&sr=1-2

    https://www.amazon.com/Structures-T...ords=j e gordon&qid=1626720466&s=books&sr=1-1
     
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  7. portacruise
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    portacruise Senior Member

    "How practical is either of those ideas - i.e., using internal aluminum tubes, or an internal bamboo stakes, to overcome the deficiencies?"

    That depends on how you structure them, and what kind of adhesives/ fasteners/ squeeze fit, etc. is used. The sum strength should not be less than the strength of the strongest of the 2 parts, so long as there are good solid mechanical connections and the two tube bonding holds as well as the strongest material (Al, CF, etc.). It's hard to find adhesives that stick well to PVC with other materials, but I have found that can be improved by using mechanical bonding like pins when using adhesive. If more strength is needed, heavier internal aluminum tube struts can be inside the PVC keeping the same OD.

    Bamboo strut strength can vary more widely within a batch than aluminum, and degradation with environmental exposure, insects, etc. may require special periodic maintenance, or special coatings. Torsion, tensile, side load and compressive strength of each strut needs to be uniform and assured at some particular values.

    Your stated attraction to PVC appears to be ease of assembly/ disassembly and low cost, and easy repairs. I think it is a good material in principle, for prototyping, and testing in a safe place like a swimming pool. But I think that strength relative to weight, volume, and stiffness is just not there, for your particular application. You will probably have to go to a larger diameter than 1/2 inch or go to a stronger, higher schedule PVC if it is available in smaller diameters. That might balloon or distort the prototype considerably compared to keeping 1/2 in struts, unless you use reinforcement inside the PVC. The approximate size of aluminum struts needed is already known from other existing SOF constructions, presumably the aluminum part could at least partially be reused if the PVC does not work out.

    PS: if you can find a used Al strut SOF cheap enough, do you have the skill to rebuild it to your particular specifications using the PVC as a model? Maybe it might save a lot of time and money chasing down parts....

    Hope this helps.
     
    Last edited: Jul 19, 2021
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  8. Skyak
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    Skyak Senior Member

    There are certainly production folding kayaks. Here is a list
    Folding Kayaks | Paddling.com
    It is safe to say that none of them are as narrow as you plan but I don't think 16" is practical for SOF. I think I told you to go as narrow as you were comfortable with on your rudder thread, but that seems too narrow for a skin on frame. My narrowest boat is a CLC Pax 20 is 17" wide (20'L zero rocker) and if my butt is separated from the plywood by anything more than a couple layers of cloth it is too unstable to ever have the paddle out of the water. With skin on frame you must be suspended above the skin at least an inch so the skin can deflect and still be fair. SOF is less stable than a solid kayak of the same dimension because of this skin deflection and seat position so they tend to be more than 20" wide. It makes sense if you think about a recovery in the very rough conditions you are asking about -empty the kayak the best you can, invert it, enter and attach the skirt, roll upright, gasp for breath and you now have at least 2 gallons of water compromising your stability until you pump out.

    Are they good enough for the sea? I would say they the best are not much worse than plastic. I think there have been Atlantic crossings in SOF kayaks but if I recall, a guy crossed from Europe to the Caribbean, then disappeared on his way to Florida.
     
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  9. gonzo
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    gonzo Senior Member

  10. mitchgrunes
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    mitchgrunes Junior Member

    I'm sure that depends a lot on other factors. My current SOF is 19", and I briefly tried it out on 6-6.5' waves, which it handled much better than my fiberglass boat; I have tried someone else's 16" SOF (in calm water). Both were quite manageable, but neither meets all my desires. I've known one avid "storm paddler" who prefers SOFs - claims the flexibility makes them safer - but perhaps he builds or has them built very tough and heavy.
     
  11. mitchgrunes
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    mitchgrunes Junior Member

    I glanced at a later (legit?) free edition at
    www.gutenberg.org/files/50828/50828-h/50828-h.htm

    While the required tools are pretty crude, it would still require much better woodworking skills than I have, and a lot of time (for me).

    Based on page 155 of that edition, there was quite a lot of variation in shape - many did not have flat bottoms.

    Anyway, I see no reason to limit myself to traditional materials, constructions, or shapes. Regardless, materials derived from sea mammal bodies wouldn't be politically correct now. And course my recreational application might be best served by boats that differ significantly from those of traditional fishing, hunting, fur trade and war paddlecraft.

    I wonder if a reasonably easy approach would be to use a PVC-pipe based prototype as a mold for a fiberglass+epoxy boat. The only thing I've ever done with fiberglass & epoxy was to do an emergency repair of a broken paddle blade, and it didn't last more than a few weeks (I think). I do have a copy of Charlie Walbridge's Boat Builder's Manual. Wouldn't be a folder, but if weren't much heavier than my current 30 pound SOF, it might be reasonably easy lift to the top of my car. My old 4 meter (13'2") fiberglass+epoxy (I think) Cascade was also about 30 pounds, but it was wider, and taller at the ends than my current concept, so maybe a 16"x16' boat of the same materials wouldn't be much heavier.

    But - if I don't ake the PVC pipe prototype into rough water, it will be hard to guess how it will handle under conditions. I could take it into shore break, with waves a few feet high, so I could swim back to shore if it breaks up and hope I don't get wrapped by the skin and drown. I could of course wear a PFD and sheath knife to try to get free, but I don't know if that would be enough. And it wouldn't be environmentally correct to leave the broken pieces in the water. Also, shore break alone couldn't tell me how it would handle under the potentially wide range of wave and wind conditions. It took me over 40 years of paddling to happen to run into severe resonant pitching problem. How can I possibly test for that kind of phenomena?

    Yostwerks advocates PVC skins for SOF boats. But they use a different variety of PVC than is used in schedule 40 PVC pipes... And that is for the tensile skin, not the compressive frame.
     
  12. upchurchmr
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    upchurchmr Senior Member

    Bout time to just build what you want.
    Everyone's opinion is set, won't change with more talking.
    Please provide a report.
    Your experience has become more obvious as the thread went along.
    I'm interested to see if PVC works, in spite of my opinion. :D:D:D
     
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  13. portacruise
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    portacruise Senior Member

    @ 25 writes "I've known one avid "storm paddler" who prefers SOFs - claims the flexibility makes them safer - but perhaps he builds or has them built very tough and heavy."

    I used a Porta-bote for a time many years ago that has flexible panels; it did absorb a lot of punishment from waves and even solid objects without any jolts, so I think it was safer and had a smoother ride in rough conditions compared to similar boats. The downside was that rowing it deflected some energy with each stroke of the oars to the flexing of panels, so loss of efficiency. Maybe that has a bearing on your resonance issues... The flexing seemed to be less of an issue when under constant power.

    @26- using a chase boat or jet-ski might be the way to test your prototype under challenging conditions.
     
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  14. mitchgrunes
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    mitchgrunes Junior Member

    In retrospect, if there were a cheap common readily easily assembled material that had all the properties I wanted, everybody would use it. :)

    PVC alone also fails to match what I have noticed about reasonably strong lightly built boats and other stiff structures: They oppose thin layers of one or more flexible stretchable materials with a lot of tensile strength (e.g., fiberglass, Kevlar, S-glass, carbon fiber) against one or more bulk materials (e.g., epoxy, polyester resin, vinyl ester resin; closed cell foam pillars and cores) that resists compression. (Some bulk materials also act as an adhesive, so must have some tensile strength.)

    And of course when I say light, I partly mean that you don't need much of it - if I recall correctly, a fiberglass boat with no air or foam inside sinks when flooded. Fiberglass boats are lightly partly because they can be thin and still provide the desirable properties.

    I don't know why one lightweight material can't do all these things reasonably well (heavy materials like steel and some woods can); I assume it has something to do with the chemical bonding.

    BTW, a rough kayak equivalent to Porta-bote is Oru. If Oru made a boat of the right size and shape for me, I might go with it. Some people, including people in these forums, have made similar boats to Oru kayaks, and I thought about it. However, it looks to me in the videos that the material (I forget what it is called) forms into folds fairly easily. Perhaps that severely limits the impacts and scrapes they can take a lot? I do love how compactly they fold down. The concept is beautifully elegant. And you can only form the folds once - not a great prototype material.

    Perhaps in the future we will be able to design a boat on a semi-automated CAD program, push a button, and a force field with all the right properties will form in the desired shape. It will disappear with another button push. :) For the moment, my force fields have to be the chemical bonds that hold together existing materials. I have to accept that each of these materials has both advantages and limitations, and that construction can't be nearly as easy as I hoped. I may also have to give up on fitting it inside my car.

    I suppose real engineers are trained from the start to accept such compromises. I need to learn to think of the learning process for more complex construction techniques as a fun challenge, rather than a nuisance.
     

  15. upchurchmr
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    upchurchmr Senior Member

    Mitch, perhaps you should take some engineering courses.
    You have so many mis-statements above that it is not really possible to correct them all.
    Just one.
    In a carbon fiber layup, the tensile strength is in the fiber, and the compressive strength (along the fiber direction) is in the fiber.
    Epoxy or other laminating materials has no where near the strength of the fiber.
    But, the laminating material has to keep the fiber straight and connected to other fibers in order to allow the fiber to take the loads.

    Great fantasy, semi-automated CAD programs. If that were available, design and build of structures would never change, since you can only program what you know.
    New things by definition are unknown.
    For 30 years (at my work) engineers had to teach management that the computer did nothing but what some knowledgeable person made it do. Nothing is automated, except trivial tasks.
    And a computer program (CAD) does not prevent mistakes.
    Nothing replaces a knowledgeable, trained, and competent engineer or scientist.

    Are you going to build the PVC framed boat?
     
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