PVC pipe as a SOF kayak frame material?

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

  1. clmanges
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    clmanges Senior Member

    Mitchgrunes, if you want a boat that flexes that much, buy an inflatable boat and only blow it up to half pressure or so. Then you'll find out why no one builds 'hyper-flexible' boats.

    And the reason you can't find "clearly explained PVC strength ratings" is because PVC is not just one material; it's a whole family of formulations with different properties. I have an inflatable kayak--a Sevylor Tahiti Classic--that is made entirely of PVC. Cheap raincoats are covered with it, it's the waterproof coating on modern oilcloth, and I have no idea how many other forms it can take.

    Also, the reason that tensile strength is the wrong property to be concerned with is that in a skin-on-frame boat, the frame members (most if not all, I'm certain) never experience tensile stress; they are compression members, and it's the skin that's under tension.

    Finally, do a web search on 'build a skin-on-frame kayak' and notice what everybody specifies as frame material.
     
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  2. upchurchmr
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    upchurchmr Senior Member

    As Gonzo said, you will never get everything to match your evaluation.
    What kind of a SOF do you have?
    My boats clearly flex in waves where the plywood one we have does not.
    The sof is a softer ride and quieter.
    I don't know how you would ever evaluate a kayak to meet Dyson's reported criteria.
    I got my skins from Dyson by the way.

    While the skin in under tension, and the frame has to support that in compression, those loads are relatively small.
    The major loads in SOF are bending of the frame.
    That requires tension and compression in the same structural element.
    If you want extreme flexure of the boat, you need smaller frame elements.
     
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  3. KeithO
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    KeithO Senior Member

    You can buy fiberglass driveway markers (qty = 20 pieces) 72" long for $45 on amazon.
    https://www.amazon.com/Driveway-Mar...ywords=fiberglass rods&qid=1628564949&sr=8-54
    Using that as your base material, I'm sure you could come up with a geodesic style structure for your frame that just involves a way to bond the attachment points and then you could choose whether you bond at some of the points where the poles cross or not, depending on what you want to achieve. For locally stiffer areas, use more than one rod and bond together (wrap with glass tape and epoxy). To have unstressed curves you would have to heat above the softening temperature and then strap to a form of the appropriate shape and let cool... Experimentation required. But would not break the bank...

    We have all seen peoples hoop gardens made from PCV pipe destroyed after the first big wind or snow load. It looks like it can support a load, but in fact it is barely holding itself up.
     
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  4. Skyak
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    Skyak Senior Member

    When you changed your max beam dimension to 21 inches you opened up a significant selection of commercially available folding kayaks. Your requirement for safety offshore dictates that you would buy one of these anyway.
    If you still want to experiment with different hull dimensions you should make non-folding kayaks with wood stringers -easy, effective, proven.
     
  5. mitchgrunes
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    mitchgrunes Senior Member

    As I said, it has happened to me twice.

    The first time for me was merely inconvenient - I had to paddle extra hard to keep up with a group. The second time I more or less lost control. That is a substantial part of the reason I want a different boat.

    Other kayakers have mentioned having similar problems. Dyson has a long discussion of the issue specifically because it is one of the most difficult to deal with problems a sea kayaker encounters. (He was also trying to explain the extreme design flexibility and odd shapes of traditional baidarkas. Maintaining good speed and control were a matter of survival in the far north - they hunted for scarce food, and sometimes went to war.)

    I'm well aware that most people believe a rigid boat is optimal - and maybe it is in flatwater, though some racing boats have sliding seats.

    I've lost control before - in whitewater - but that is part of whitewater play. In whitewater, I try to go places and times where I can survive losing control. Deliberately using resonant pitching to cause a whitewater kayak to stand on end or flip end over end are common playboating maneuvers. Unfortunately, at sea, weather can be unpredictable, so it is essential to have good speed and control under all plausible conditions, not just common ones.

    To a large extent, sea kayakers can change travel direction and speed to eliminate resonance problems. But there exist specific places and conditions (e.g., paddling into oncoming surf), as well as group paddling, where that isn't always practical. E.g., the second time I had the problem, I was swept rapidly cross-shore, had to paddle forwards hard to avoid being swept under a low dock, then had to turn quickly into a wave shadow where I could regain control.

    A better kayaker, or one in better practice, might have had less trouble. In retrospect, I already knew alternate stroke techniques that would have helped, or might have allowed me to instead be alternately swept across the source wave & wind directions, like tacking a sailboat. But it still makes sense to choose a boat that minimizes problems. And it would be an interesting experiment. It's a shame that PVC isn't tough enough for the experiment to be easy.

    Wouldn't an undamped system be even harder to control? A damped flexible kayak could approximately conform to the water. An undamped kayak would keep flexing and bouncing.

    I can see from these discussions that it is impossible to expect consensus on kayak design. Is there better consensus on the optimal design of large ships?
     
  6. gonzo
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    gonzo Senior Member

    Can you explain and describe what you mean by "resonant pitching".
     
  7. mitchgrunes
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    mitchgrunes Senior Member

    Other people on this forum understand this sort of thing better than me. But I will give the explanation a try.

    The reference I gave to Dyson explains it pretty well - if a bit technically. (Sometimes too technically for me. It doesn't help that sometimes he describes distances in meters, sometimes feet, sometimes miles, sometimes nautical miles; sometimes he describes time in seconds, sometimes hours; sometimes he describes speed in m/sec, sometimes mph, sometimes knots. He doesn't always define what the units are in his formulas. I'm guessing his formulas come from books on subjects like naval architecture, so the naval architects here know what the units of everything are, because they know the formulas - so maybe it wouldn't be a problem for them. I don't.

    I think it's somewhat similar to "porpoising" on a motor boat.

    If you pass waves that have a half-wavelength equal to the length of the boat (or ship), then you get times when the peak of the wave is under one end. and the trough is under the other end, and vice, versa. So, as you pass the wave, that causes the boat to rock (pitch) forward and back. (Of course, for a boat moving over the surface water, the wavelength is modified by doppler shifts. I suppose it's even more complicated - wind sheer causes the water to move different speeds at different depths, and waves are affected by a range of depths. So we are actually talking about "apparent wavelength".

    BTW, if the wavelength of a surface water wave is much less than the boat length, there are many little waves under the boat at once, and you don't get much pitching. Likewise, if the wavelength is much greater than the boat length, the boat approximately follows the profile of the wave - and again you don't get much pitching. It is for that special match where the wavelength is twice the length of the boat where you get much of a problem.

    There is a natural resonant period that most rigid boats have, when they rock forwards and back. When a boat pitches forward or back, buoyancy creates a restoring force to make it approximately level again; so, like a swinging pendulum, it naturally has a resonance period created by its moment of inertia and the restoring force. For the most part a boat doesn't rock all that much, unless you have very high waves. But when the resonant period of the boat matches the period of the wave, that match causes the boat to pitch forward and back much more. Again, we are talking about "apparent period" of the waves.

    When this happens, several problems result. Each time an end crashes down on the water, it generates a splash which wastes energy - so you can't go very fast. When an end pitches high into the air, winds, and waves that are at an angle to the motion, can turn the boat, and make it very hard to control.

    (It's a little more complicated for the flexible boats Dyson discusses. Instead of being resonant to the fundamental frequency ("first harmonic") of the boat, he thinks the ideal is to make the resonant motions of the ends oscillate with the "second harmonic", so the center (approximately) of the boat doesn't oscillate much, but the ends do. If you studied vibrating strings in physics that makes sense. If not, ignore it.) Of course, this is a controlled flexibility in one direction. A simple raft or "rubber ducky" doesn't have that, though there are inflatable "kayaks" that have semi-rigid rods that control the flexibility somewhat.

    In principle, I think (I'm not an expert) deep water swell (waves generated far away, that you encounter in water much deeper than the wave height) has a fairly simple relationship between wave height, wave length, and wave period. A boat can therefore be designed so that waves with a half length equal to the boat length have a period that is not resonant to the natural motions of the boat, and no problem should occur. But wind waves change that relationship in very complex manners, so sometimes you can't avoid the resonance. In addition, interaction with the bottom surface (or if there are multiple layers of water moving at different speeds) can also change the relationship, and again, sometimes you can't avoid the resonance. For example, when waves pile up on a beach, the period doesn't change, but the waves get higher and steeper, and the peaks get closer together, so you go through a large range of wavelengths. Again, breaking waves behave in extremely complex manners - so you really can't design a boat to avoid strong pitching, and the resultant loss of speed and control, under all possible conditions.

    (On top of that, in stormy weather, instead of simple surface waves, waves from different directions interact with each other - e.g., the strongest oscillations often occur when two or more wave trains sort of add to each other, to create constructive interference. But let's leave that be.)

    Is that all clear?

    If not, someone else can do a better job.
     
  8. upchurchmr
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    upchurchmr Senior Member

    Can you give us a reference to Dyson's reports?
    That might make it easier rather than taking a "summary" form someone who keeps saying he doesn't completely understand.

    BTW, have you seen the book "The Starship and the Canoe" by Kenneth Brower? Its about Dyson and his famous physicst father.
    Very interesting contrast in two different paths in life.
     
  9. gonzo
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    gonzo Senior Member

    I think you are trying to describe an oscillating system. However, a boat is a damped and forced oscillating system, which is rather complicated. However, in the rare cases where the natural frequency of the boat coincides with that of the waves, a small change of direction and/or speed will solve the problem. Porpoising is a completely different issue on powerboats.


    If the period doesn't change, the wavelength won't either.
     
  10. mitchgrunes
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    mitchgrunes Senior Member

    I guess Dyson has written a lot of stuff about kayaks.

    Form and Function of the Bairdarka; The Framework of Design[/url

    BTW - I made a mistake. It isn't exactly the boat length that matters, it is the boat length projected onto the direction of the waves. I've had the most trouble when I was paddling at about a 45 degree angle into oncoming waves and wind.

    The boats he discusses there are not exactly what I want. They probably aren't stable enough for me - I've had trouble stabilizing modern racing kayaks, though maybe that's because I've only spent a few minutes in them, so my reflexes are all wrong. Also, he is talking about boats paddled by very strong, fit paddlers. E.g., there is no way I could go 10+ knots for any reasonable distance.

    No. Sounds like a fun read.

    I used to read a lot of science fiction, and his father's ideas were often mentioped.

    As waves move into shallow water, they usually slow down (see any first year physics book), all other things being equal - so the wavelength drops. Though sometimes after they fully breakup into foam, the most forward part surges rapidly forward as it collapses downwards - perhaps because water is approximately uncompressible, so it needs somewhere to go to as the height drops. You can sometimes see both effects if you look at breakers on a beach.

    E.g., see here

    At one point, where I used to work, one guy thought he could map water depth in shallow water by looking at suface wavelengths. I don't know how far he got with the idea. It's one of those ideas that sounds good at first, but you can think of a number of conditions under which it might not work quite right.
     
  11. Blueknarr
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    Blueknarr Senior Member

    I read somewhere that ancient Polynesians navigation was partly based on the interaction between the wakes of islands several hundred miles distant.
     
  12. gonzo
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    gonzo Senior Member

    You can find the mechanism that makes waves break in many websites. It is a well known phenomenon. Also, the wave height rises not drops, which is evident if you look at breakers in the beach.

    Why do waves break? https://www.surfertoday.com/surfing/why-do-waves-break#:~:text=When%20the%20Energy%20Meets%20the%20Ocean%20Floor&text=As%20waves%20reach%20the%20shore,friction%20with%20the%20shallow%20bottom.&text=The%20wave%20breaks%2C%20and%20it,1.3%20times%20the%20wave%20height.
     
  13. Skyak
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    Skyak Senior Member

    Kayaks have a pitching resonance, but it is overdamped due to the low weight/length and it's central location. The full weight of the paddlers body doesn't even contribute, just the lower half that is fixed relative to the boat. Notes -significant weight in the bow and stern would lower the frequency and increase the amplitude of pitch resonance (bad) and asymmetry of the hull would increase damping and cut amplitude (good). You can measure this yourself -sit in the kayak at the dock and have someone pull the bow down and release. If you take video with a clock and ruler in the picture you can calculate all the parameters. Look up 'first order system resonance, step function input'.
    Because kayak pitching is overdamped your issue is almost certainly about the wave driven motion (not resonant motion) and it's interference with your paddling/body motion. There are lots of reasons in waves that you need a good stroke immediately for control, and lots of reasons in waves that you must wait until a specific phase of the wave to take that stroke. If you immediately take the stroke in the wrong phase of the wave you just wasted the energy and your body is out of position when you should have taken the stroke. Then you need to wait until the phase of the wave is right again, and that is a long time from when you felt the need for the stroke. Think back to when you experienced the problem -was the frequency of the pitching, faster, slower or the same as your paddling frequency? Was your stroke rate regular or irregular, faster, slower, or the same as normal in flat water? Did your paddling strokes lose efficiency?
     
  14. mitchgrunes
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    mitchgrunes Senior Member

    gonzo: yes, waves pile up on the beach - especially before they break, and sometimes a little afterwards. But after they break, they sometimes abruptly collapse and spill a surge of foam forwards at higher speed than the main wave was moving.

    Sometimes the surge evens turns into a separate wave ahead of the main wave.

    Skyak: If it is a resonant motion, of the type that Dyson describes for Aleut baidarkas, that resonant pitching motion is still wave driven. A resonance would just drive the ends higher. As I said, I was told by an expert instructor to use that resonance to go vertical in whitewater boats - though by your criteria, whitewater boats are shorter and have more weight / length.

    Unfortunately, I can't easily recreate the situations. I've been sea kayaking for years, and it's only occurred to me twice. And at the time of the second problem, I was out of practice, so I may have made errors - e.g., I don't recall if I was on the crest of the wave while I sweeped. Is that optimal? I think most of the involuntary turning action created by the breaking waves was while my paddle was out of the water, so if I turned on the crest, my kayak would have been in the trough while the paddle was in the air, where the wave turning action would have been greatest. In fact I know I made errors- I tried to turn into the wave using only sweep strokes, and also sweeping on one side while back sweeping on the other. Neither was sufficient. For some reason, I forgot to follow the sweep stroke with a draw stroke, let alone the compound circular stroke which is the most effective turning stroke I know, because the paddle never leaves the water and remains in control throughout. Since I was in shallow water, I could simply have planted a paddle in the bottom, ahead of my body, and pushed off of it, to turn into the waves.

    I think a large part of the problem is that I am too light for the Caribou, and it has ends that are too high volume, and it has too much overhang. The ends simply have too much volume for my 150 pound weight, so they (especially the bow) sometimes pitch up high into the air, a lot, catch the waves and possibly wind while high, then splash back down, including these two occasions. I maybe lose 2-2.5 feet of length at each end to overhang in flatwater - but of course waves of the wrong frequency can float the ends up high. Other kayakers have suggested I add a heavy ballast, but that eliminates a lot of the fun for me of kayaking - I love the playful feeling when a boat moves almost effortlessly at the touch of a paddle. There are very few sea kayaks designed for light people, and they all have a lot of rocker and overhang, except for certain racing sea kayaks, which as I said I found too tippy in the several minutes I tried them. I don't know anywhere I can rent a racing kayak for a day - plus all the ones I have tried (a downriver boat, though that was decades ago, and I didn't understand hip stabilization yet, an Epic 18, an Epic 18 sport, an Epic surfski, and a Huki [?]) were probably meant for somewhat heavier people too.

    Even though the flatwater waterline length of my SOF is slightly less - I have about 3.5 - 4' of overhang at each end - and it was designed to roll rather than go fast, it is a very low volume design, so it ends up faster for me, especially in steep waves, and is much easier to control. I'm not sure how much that is effected by other shape parameters - the SOF is 19" x 19' and 30 pounds, but the Caribou, if I remember right, is 21.75" x 18.5', and 42 pounds.

    If the SOF wasn't so low volume in the center as well as the ends that it is hard to do a re-entry, I would only paddle it. With my weight, it is a much more seaworthy craft. But as I said before, my only successful re-entries in the SOF have been re-enter and rolls, because the low cockpit rim is a small circle that floods if I climb onto the back deck, and it takes a while for me to work my legs under the low front deck. (I once managed to climb on the deck and re-enter without flooding it in calm water, by using a paddle float to support some of my weight. I doubt I could do it in waves.) (I suppose an X-rescue would work, if someone was available to help - but sometimes I paddle solo, and unlike the whitewater world, many sea kayakers don't practice rescues, so can't be relied on to help effectively.) I also haven't found air bags that completely fill the ends. I've discussed the problem with other SOF kayakers in a local club. They mostly view their SOFs as roll-or-die boats, partly because they don't think the boats are strong enough to give or receive a T-rescue. But I'm in my mid-60's and no longer feel that is good enough, plus I think any responsible sea kayaker should be able to give T-rescues.

    So my original idea was to create a shorter SOF which had low volume ends like my current on, but a high volume center, less rocker, an easier to enter cockpit, and a stronger arch structure where it would need to bear weight for T rescues, and try to make my own air bags to completely fill the unused space - though to some extent that last idea was nixed by a sales person at a local store, who told me that air bags filled to the size of the boat, after the air expands due to heat, sometimes burst kayaks, especially SOFs - and the store has seen a few examples of that.

    The low volume ends and shorter length would reduce the pitching force. Less rocker is to reduce overhang so the shorter length wouldn't cut speed. The high volume center and easier to enter cockpit would make for an easier and safer re-entry. Possibly a foot pump, or deck pump, would get rid of any water that did get in better than the hand pumps I've used. I'm not sure I can successfully use a hand pump in rough water, and they are very slow.
     

  15. gonzo
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    gonzo Senior Member

    I think "resonance" is really confusing you. There is no such thing as "resonant motion". Boats are damped oscillating systems. A forced oscillating system is one where an exterior force is increasing the oscillation. On a kayak, the exterior forces would be waves, paddling and the movement of the paddler's body. The damping is very large and it takes conscious effort to make a kayak to pitch continuously. Simply changing the frequency of the paddling makes a large difference. What you describe are the consequence of a boat that has too large of a volume for your weight and also a function of its form stability. I find that on traditional shape kayaks, dragging my body over the deck from the stern, then straddling the hull and entering the cockpit is an effective way of re-entering.
     
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