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

    PVC pipe as a SOF frame material?

    I've been playing with 1/2" schedule 40 PVC pipe, as a frame material for a SOF sea kayak prototype, that is planned to be a "break-apart folder", that can break down to pieces that will fit into my car. (I have roof racks, but as I get older, it becomes harder to lift boats on top. Besides, I think folders are cool.)

    I like PVC pipe because it's ridiculously cheap; is fairly light; is available at local hardware stores; is easy to cut with a hacksaw (or jig saw) and put together with friction fits and/or glue and/or readily available 2-way, 3-way and 4-way fittings; is somewhat flexible cold, and supposedly can easily be formed to shape when filled with boiling water. I don't need a band saw or steam box.

    But I'm not an engineer, and don't know the downsides of PVC pipe, as a long term structural material. I guessed that its flexibility would be a good thing for a lightweight boat, because it would bend somewhat with the waves, rather than break. But I don't know whether it can keep doing so for years on end, and whether that flexibility goes far enough.

    E.g., www.creativemechanisms.com/blog/everything-you-need-to-know-about-pvc-plastic claims that PVC is brittle. I don't know how brittle, and what they are comparing it to. Maybe that means it wouldn't be a good material. I don't know how it compares in strength and durability to materials like epoxy infused plywood or oak, aluminum, or carbon fiber, that are commonly used for the frames of SOF boats.

    Someone involved with the design of whitewater boats once told me that vertical moves (like cartwheels, and forward and backwards summersault-like moves) put a lot of torque stress on the ends, and that is a major design element. Those are less commonly used in sea kayaking, but are still sometimes used as escape moves from breaking waves. In addition, very high breaking waves can sometimes cause a sea kayak to "pitch pole" end over end even if you don't try to make that happen, though I will probably try to avoid those conditions. I wonder if PVC pipe (+ a skin) can be strong enough for vertical moves.

    I also don't know whether those friction fits will stay tight through many cycles of assembly and disassembly. If not, can I re-tighten the fit by coating them with a paint-on material? I would of course glue some of the fits, so the little pieces don't get lost, and would attach bungie cords or ropes to the skin, to maintain tension across the unglued joints, as well as to pull the skin tight on the bottom of the boat, and to attach perimeter lines and tie down points.

    Can anyone help me answer the above questions about PVC pipe strength and durability?

    Other musings, which I haven't really thought through yet:

    No one will think such a boat “authentic”. Neither the planned shape nor the materials will look like a real arctic kayak. Doesn’t trouble me in the least. As I think Dyson pointed out, arctic kayak builders used the materials they had readily available – and might have used more modern materials if they were available.

    Frame formation and shape: My tentative experiments have used just 3 long stringers (one on the bottom, two on the sides), each composed of two 10' pipes, connected by friction fits. I plan to cut this down to 16’ total length. I will use several cross pieces to form the cross-sectional shape, and to provide support for rear deck re-entries and foredeck T-rescues.

    So far I have used flexible tubing, duct taped to the stringers, for the cross pieces that form the cross section shape, but will eventually cut and bend PVC pipe, and use 4-way fittings instead of tape, to replace that, once I am happy with the shape, because that is stronger and more durable.

    I want a frame that is hip width at the widest point. Beginners often prefer wider boats for flatwater stability, but I've been kayaking a while, and prefer easy controllability and rollability, and would like something narrow enough to step across and sit down in quickly, especially when putting in into surf. (Also, thinner boats let you use longer, more vertical paddle strokes.) One critical fit issue for me is I want to be able to slip my knees snugly underneath the side stringers, to give me full lean angle control over the boat. In commercially made whitewater and sea kayaks, I have instead glued carefully shaped foam into the sides of the boat, but in my initial on-floor experiments, it looks like tucking the knees under the stringers provides better control, and also is a beautifully elegant solution.

    I want my knees splayed sideways enough, and with some height over them, to improve my forwards bend flexibility (with my legs out straight in front of me, I can’t bend forwards). There must be enough height over the feet for them to be comfortable while wearing shoes (NOT true in my current sea kayaks). The cockpit must be high enough for at-sea top re-entries that don’t flood the boat (unlike my current SOF, which is essentially roll-or-die), and small enough that I can rapidly attach the spray skirt on while underwater during a re-entry and roll. Also, if I need to piss while at sea, I want to be able to do it into a bottle in privacy, inside the boat – so that part of the boat must be high enough to do that.

    The intention is that the ends would be formed using a 3-way fitting with an acute angle (20 or 30 degrees?) on the bottom stringer (which I will have to mail order - not available in local hardware stores, they only have right angle fittings). I will then bend 2 pipe lengths to meet the other stringers. The idea is to avoid creating much if any overhang or bottom rocker, to avoid uncontrolled pitching, and to keep the ends relatively low to the water, so they don't catch waves and winds much.

    On the bottom surface, cross section shape should be almost flat in the middle, V at the ends, rounded in between. I.E., a compromise between stability and speed.

    Cockpit: Form the same way - though it doesn't need to be as strong, so maybe flexible tubing will do.

    The Skin: Sew webbing loops into the skin, and attach the cord or ropes to them, then coat the fabric to seal the sew holes and otherwise waterproof it, as has been described by others in books and videos. Sew an outer durable fabric to the very bottom first, in case of scrapes or gravel beaches – not sure what that should be. Is Kevlar fabric too stiff? Dyson says the waterproofing material must be more flexible than the fabric, for long term durability. I wonder if Dyson is selling skin materials yet again, or if I will have to make do with sail fabric, or fabric store material… Anyway, I’m not there yet.

    The skin will be white, not black like sealskin. Black is unsafe at night. And I’ve had black kayaks that smoked in the sun. Maybe add fluorescent safety tape.

    I may eventually create two skins and two sets of cross pieces, so the boat can also be used as a surf ski. But that is long term, and I'm not there yet.

    Sprayskirt: I currently buy neoprene commercial skirts, because it fits and seals better than nylon, and is fast to put on. But if I cut and sew to my cockpit shape, and insert bungie cords at the edges, maybe coated nylon will fit good enough – and is less permeable to water.

    Seat: The bottom seat would be made from a closed cell foam camping mat, that would also spread the weight across more of the bottom while getting into the boat, so I don't step through the skin. I might also later create a saddle so the boat can also be kneeled in like a canoe.

    Flotation/storage bags: Initially sewed to shape over the frame, and fill all possible available space that I’m not in. Make two bags for each end. Since all bags eventually leak, put one bag inside the other in pairs. Switch inside/outside bags every few trips and look for leaks each time. Not sure how to make the tube and sealable valve. Maybe I will cut them off cheap inflatable toys.

    I have found hand-held sea kayak water pumps awkward and extremely slow to use, and am not sure they would be practical to use under the conditions that are likely to flip me. (E.g., I would have to stop using my paddle to hold them, waves would wash into an open cockpit, and if I try to operate them while stuffed beside me in a spray skirt, that would be even more awkward.) The obvious alternatives are deck pumps - which might require a stronger deck than a SOF would provide (??), and still tie up one hand, and a foot pump. I may try a cheap plastic foot pump made for inflatable toys. (I would probably have to take it apart, and coat the spring with something so the spring wouldn't rust in salt water.)

    Paddle: I love Greenland paddles. I might make one out of PVC pipe, by filling each end with boiling water, then flattening it with a rolling pin or sitting on it through a board. That might be lighter than my wood Greenland paddle – maybe as light as my Epic wing paddle. If it isn’t strong enough, or doesn’t work well, I won’t have wasted much time or money.
    Last edited: Jul 14, 2021
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Too flexible and too weak for the weight. Maybe fibreglass tube to cover the strength, but will be quite flexible
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  3. Rumars
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    Rumars Senior Member

    I have seen a PVC pipe framed kayak on a lake. It was straight on the beach and rockered on the water. Use Al tubing or wood, it's cheap enough to not make a difference.
    If you ever find a brush on coating that can survive repeated folding and is available to the general public, let me know, I'm interested.
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  4. Rumars
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    Rumars Senior Member

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  5. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    PVC pipe would be too flexible and way too heavy.
    Al pipe or box channel would be okay but expensive.
    Wood would be my choice by far.
    Fabrigating something that breaks down will be time consuming.

    Have you looked for plans?
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  6. gonzo
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    gonzo Senior Member

    If that was the only material available you could build with PVC. However, it is heavy and the stiffness very low. PVC is not very brittle though. As a crude test, get a piece of 1/2" pipe and hit it with a hammer. The amount of material needed for the framing of a kayak is not very large, so it wouldn't cost too much to use wood.
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  7. Dave Gentry
    Joined: May 2010
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    Dave Gentry Junior Member

    - I wonder if Dyson is selling skin materials yet again, or if I will have to make do with sail fabric, or fabric store material…
    George Dyson has been selling fabric for SOF for decades, and still is. Email him at GDyson@gmail.com and check out his offerings here:
    Dropbox - MaterialNotes.pdf - Simplify your life https://www.dropbox.com/s/fbujyoc1o96je8r/MaterialNotes.pdf?dl=0

    - Seat: The bottom seat would be made from a closed cell foam camping mat, that would also spread the weight across more of the bottom while getting into the boat, so I don't step through the skin.
    FYI, you will never be able to step through 7oz or better nylon or polyester. Unless you strap razor blades to your shoes. Don't do that.

    - Flotation/storage bags: . . . Not sure how to make the tube and sealable valve.

    You can by valves at NRS, etc. Likely Amazon, as well.

    - If you ever find a brush on coating that can survive repeated folding and is available to the general public, let me know, I'm interested.

    Loctite's PL Premium polyurethane construction adhesive. My old video on the process:

    I encourage you to experiment, Mitch - you'll only be out a few of hundred bucks and your time. I suggest you plan for the first one to be a proof of concept, to be revised and improved with a subsequent build.

    BTW - with some exceptions - assembling and taking down a take-apart kayak is a huge time suck. After the first few times, impromptu paddles after work and such tend to no longer be worth the effort, and they get used only on weekends when you have lots of time to waste. So, if you just want to get on the water with a folding boat, Rumars' Yostwerks suggestion is spot on, though I recommend Tom's inflatable designs, which will go together far, far quicker than his folders do. The same is true with what you are planning.

    Have fun!
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  8. Rumars
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    Rumars Senior Member

    Thanks for the tip Dave, I will have to experiment with it. Is the PU glue UV stable, or does it have to be coated?

    Folder assembly time should be around 20min for a simple design with full lenght deck opening.
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  9. clmanges
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    clmanges Senior Member

    Please find another material. PVC pipe was never intended or designed to be used as a structural material. Period. Study professionally built SOF boats and find out what they used, then get the closest equivalent you can afford. That would be: carefully selected wood, 6000 series aluminum tube, and perhaps fiberglass or carbon-fiber tube. Nobody wants you to have to swim, or, worse, wash up on shore.
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  10. mitchgrunes
    Joined: Jul 2020
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    mitchgrunes Senior Member

    Wow! So many replies. Thanks, guys.

    It looks to me that both wood and aluminum require fairly complicated, bulky and expensive equipment to fabricate, especially wood. At the moment I only have basic hand tools, and have no room for tools like a band saw, drill press (I do have a basic "drill guide", because that doesn't take much space), steam box, or metal bending tools. I also don't have much in the way of carpentry skills. I'm not all that clear how much the metal working tools would cost or how much space they take to set up.

    Perhaps PVC is good enough for initial prototypes, to learn about how boat shapes interact with the water - since the boat I propose to build will be a lot different in shape from any of the boats I've had or tried. The only boat I've tried that is the 16" x 16' size I've been aiming at was a Greenland shape kayak (another home-made SOF). I liked the way it handled, but I only tried it in calm water. The only other narrow waterline boats I've tried were race boats. They were very tippy, and they weren't fit for me, so there was no easy way to control the tippiness with my knees.

    What I am mostly concerned about is the resonant pitching that can occur in wind and steep waves of a specific height range, which I mentioned in another thread. When that recently happened to me in my Current Designs Caribou (possibly a Caribou S? - it's an older model with dimensions that are different from the modern models), a Greenland-shape boat, with lots of rocker and overhang (at my 145-150 lb weight - admittedly less than the Caribou was designed for), I felt very out of control. Not only did it cut my speed, but it was extremely hard to steer. I was out of practice and made mistakes - it didn't even occur to me that in such shallow water I could have regained control by poling my paddle against the bottom - but it was still quite scary. I need to feel in control. I also worry a little about sea-sickness, which I once experienced in beam chop in another boat (a Phoenix Cascade - a 1970's era 4 meter slalom kayak with a very rounded hull - not an ideal boat for open water). Part of the problem is that the specific conditions which created those two problems aren't all that common. I've been paddling since about 1980, but have only experienced resonant pitching twice (the first time only cut my speed, but didn't create control problems), and sea sickness only once. I'm not sure I can re-create the worst conditions for a given boat for test purposes. I also want a boat shape I can get in and put a spray skirt on very quickly, for launching between waves into surf, and putting the spray skirt on while underwater during re-enter and rolls.

    I think the only way I can test for what I want is to build a prototype. I can't even pay someone to build what I want for me if I don't know exactly what shape I need.

    If PVC is brittle, there are limits on what type of testing in wind and waves I can safely do with such a prototype. I could do near-shore testing in waves that are washing into shore, so I could safely swim back to the beach if the boat broke up - but that is pretty limiting, and a lot of such places cut the wind, one way or another. I'm not sure resonance pitching would show up without wind. I could do pool and drop whitewater, so I could swim safely out of the pools - but in my part of the country, whitewater has a lot of rocks, which might easily damage a PVC skin on frame boat. You have to be careful even on fiberglass and carbon/kevlar boats. Also, pool and drop whitewater might not develop the resonant pitching problem, because the waves aren't repeated - and local whitewater is in twisty stream and gorge valleys that cut the wind too.

    The specific plans I have so far seen are much to wide for my tastes, and create the problematical Greenland hulls. I want something that will fit my body, that will be fairly efficient, and that won't have the resonant pitching problem.

    I might be a bit wrong about that - obviously I need a little overhang and rocker at the ends, so the boat can put in and land on uneven beaches.

    Rumar said the boat he looked at on the lake was rockered in the water, but straight on the beach. That makes sense - one's body weight is in the center. But it maybe also had to do with the way the frame fits together. If the bottom stringer was cut a little shorter, it would come under tension and be forced fairly straight. That might mean it would need to be put together with a combination of glued joints and screw-together couplers, rather than friction joints.

    Another PVC frame boat, www.instructables.com/PVC-Duct-Tape-Kayak, doesn't appear to have any overhang. But it doesn't have the shape or characteristics I want.

    I still like the idea of a break-apart folder. One thing I liked about PVC pipe so far is that it goes together very quickly. I can put together my initial prototype frame in a few minutes, though it doesn't yet have the 4 way joints or a skin. If I can manage 5 minute assembly/disassembly, that would be roughly comparable to load/unloading a boat from car racks. I'm not sure how realistic that is with a skin. (Yostwork's aluminum frame construction doesn't look too elaborate to assemble once built either - and I guess aluminum tube would let me tuck my knees under the side stringers the same way as PVC.)

    You folks say that PVC is heavy - but the six 10' lengths of 1/2" pipe I have played with so far only weigh about 9.6 lbs, much much lighter than any kayak I have had. (Based on www.midcoonline.com/web/Pages/Products/Pipe/Info/PVC_CPVC) (My Caribou is about 42 lbs; my SOF is 30; even my whitewater boats have substantially weighed more.) Then again, maybe 1/2" pipe is much too fragile even for initial trials - I'm not sure. And I haven't skin weight and outfitting yet.

    When I have tried to assemble other people's wood commercial folders (which they were trying to sell), they required more strength to tension the frame than I possess. They also told me the wood folder frames require a lot of repairs. Even my non-folding SOF (made by and for someone else, and not a folder) has developed cracks in the frame - though that may be partly because the wood was uncoated, it was left outside, and I share a home with a family whose kids play soccer. Also, by now it might be 5-10 years old, which may be too long for a wood frame boat left outside. It has a very elaborate frame, resembling those in Christopher Cunningham's Building The Greenland Kayak (a beautiful book, to be sure), though the cross pieces (ribs) are lighter than Cunningham uses. In any event, I don't have the elaborate carpentry skill Cunningham assumes.

    I don't have any feeling for aluminum tubing properties - except that I know that backpackers are told to carry pole repair kits for their tents. Then again, the poles used in backpacking tents are pretty small diameter.
  11. portacruise
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    portacruise Senior Member

    Is it possible to use high-strength struts of another material like aluminum at the high stress points mixed with PVC at the low stress points? Or you can strengthen the PVC tube with an internal aluminum tube where needed?

    You might be able to tell the high stress points once you have your prototype and examine the individual struts for stress during assembly/ disassembly and when in use.
  12. Saqa
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    Saqa Senior Member

    I can offer some first-hand experience. I was one, a temporary resident in Fiji. I had a good fishing spot just out from a groyne that was difficult to fish from the shore. I built a PVC raft with using elbows and "T" joints. Basically looked like a mini above ground swimming pool. Skin was nylon/polyester fabric doped with epoxy

    I would tie this off the end of the groyne and let it out on rope. My wife and I had a lot of fun fishing on this. Funnily enough, it didn't flex noticeably. Cant say the weight was an issue either. Easily carried by both of us and loaded onto the roof racks. We gave that away to a friend as I made another using a different concept, and it was an absolute winner. To the point where someone snapped up the prototype soon after water trials. If you want a skin over frame type design that you might be interested in what I did
  13. upchurchmr
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    upchurchmr Senior Member

    pvc pipe sucks.
    too flexible and too heavy.
    I watched a video of a guy building one, it folded up when he got in it from a dock.
    Looked like he might of been caught in it, not easy to escape.

    How many times was this said above?
    It's your money and life.

    Please report on your success or failure.
    Other people need to know exactly what happened.
    Picture would be good also.

    Yost has great light weight designs that work well.
    You could pay someone to cut the longerons, everything else can be done with a jigsaw and hand tools.
    If you were in Texas I would give one to you.
  14. gonzo
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    gonzo Senior Member

  15. alan craig
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    alan craig Senior Member

    You can build a boat which is stiff enough and strong enough from PVC tube, but I have no idea what schedule 40 is; I built the open canoe in this link from PE/PP copolymer 1.125" tube which is even floppier than PVC.
    back packable Canoe https://www.boatdesign.net/threads/back-packable-canoe.60237/#post-829145
    I think there is a thread here on Jeremy Harris's "skin on tin" actually skin on aluminium tube super light rowing boat. Not folding, but he simply pop-riveted the bent tube frames to the straight tube stringers, an "easy" method which could be adapted to a folder.
    I think you will have to make several prototypes before you get something that works well. There may be more work in making the skin than the frame.
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