Flaws with a folding/inflatable sailing dinghy

Discussion in 'Boat Design' started by Sea Moss, Nov 19, 2016.

  1. laukejas
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    laukejas Senior Member

    Be sure to abuse that seam in every way possible! Be cruel and do not hold back. Also, check it for leaks afterwards.

    Exactly. Ideally, you'd want to minimize these gaps, but of course, number of stringers is a limiting factor. Obviously, you can make these gaps larger above waterline, because the water won't be exerting pressure there - unless the boat heels, so be sure to account for that too. In other words, try to maximize the number of stringers on the floor, and then gradually increase the gaps between them towards the sheer.

    Also, don't forget to make some concave curves in your bulkheads between stringers to accommodate the inward deflection of the fabric. Like here. Notice these slight concave curves between the cut-outs for the stringers. They ensure that the fabric won't press against the bulkheads when deflecting from the water pressure.

    If engineered correctly, this lever could also become your king plank once lowered into position, just like in the link Rumars provided.

    I feel you! I was student myself when I made my first boat. My budget was also $500, including the rig and the sail. In your case, are you including rig and sail into that budget? If you want a proper rig, the cost of a decent Dacron sail will cost nearly as much as the rest of the boat. If you choose "one-off" Tyvek/polytarp sail, you can make it as cheap as $20. What about the plywood you said you have access to? Did you mean you can get it for free?

    With careful planning and access to needed materials, it is possible to fit in that budget, I think. But in any case, a good friend of mine once told me an accurate way of estimating how much a boat will cost to build... Calculate the material costs, tools included. Count every nail, every brush, even the pencils. Assume that tools that will break down and will need replacement. Assume you will make mistakes and ruin materials. Timber often has a knot in the worst possible place, and thus you need a new piece. Epoxy batches get ruined. Varnish bucket tends to end up on the floor, upside down. No matter how careful you are, sh*t happens. Estimate the transportation costs - how many times you're going to drive to the hardware store. Add every little expense you can think of. When you're finally done, and you have your final sum... Multiply it by a factor of 2. AND, if you're very lucky, you will only go a little over it. :)

    Same goes for the time that it will take to build the boat. But don't let that discourage you. It's all part of the fun.

    Not really. No matter how well you seal the plywood, some moisture will get through. And then it's a mater of the type of wood veneers and the glue that was used to make that plywood. Only true marine plywood has the quality to last decades, and even it needs proper sealing to fulfill it's potential. No preparation or finishing can substitute marine plywood. However, exterior plywood can be good enough for your particular use. If sealed and treated properly, it might last longer than the rest of the boat, which is what ultimately matters. Since this boat won't be sitting in the water all the time, the plywood will have plenty of time to dry after being dissembled. So don't fret over it. Just give it 2 or 3 good coats of epoxy, plus paint or varnish, and you're good to go. It will last longer than you'll ever need. And if it doesn't, you can always cheaply replace it - that's the beauty of a modular boat.

    Both Mellonseed and Anabelle are beautiful boats. Personally, seeing Anabelle gives me the same feeling in the gut that I get when I see an amazingly beautiful girl. Love at first sight. That being said, I am not sure how well any of these two boats would handle the transformation into folding version. Here's the frame photo of Anabelle... I might be wrong, but I don't see it happening. It just wasn't designed with fold-ability in mind. Maybe it can be done, but that wouldn't be my first choice.

    I don't mean to distract you from your choices, but if it were up to me, I'd rather try to build a boat that was designed to be folded in the first place... Like Klepper Passat, or that Russian boat. Passat would be more suitable, because it is made of materials that match what you have access to. I don't think plans for it are available, but you could re-create it in Delftship from the photos and listed dimensions, with some liberty here and there. This is what I did when I wanted to make my own folding SOF. Of course, you won't get great resemblance from photos alone, but it might be close enough. I agree with Rumars that taking a proven design will save you a lot of time. But if you can't find plans for a folding SOF boat that utilizes materials you have at hand, it might be better to re-create a design of a folding SOF from general info and photos, rather than buying plans of a SOF that wasn't intended to be folded, and then trying to add joints. But I might be wrong about this, so don't take my word for granted. In any case, it will take some considerable time at the computer to plan it all in advance. The more you plan, the faster and cheaper the actual build will be. Delftship is great for hull shape creation or re-creation. For specific part design, you might want to look into some CAD software that can important Deltship files. I used Solidworks myself. But it can be done with Deltship too.


    We're not exactly making this easy for you, I know. :) Don't worry. You're doing great. All these decisions are important, take the time to make the right ones.
     
  2. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    I do not happen to agree with Laukejas on the topic of transforming said boats to folders. It's not rocket science, and you do not need any computers. You assemble the boat with some cheap pine (or your PVC pipe) just like in the instructions, as if it were non folding (do not epoxy the stringers, just lash them on with zip ties). Then you have to build the real stringers in sections on the mock-up. If something is to curved to be bent laminate it in place or cut it out of plywood.

    The real problem is actually your need for speed. Both are non planing dinghys, so their speed will be limited by the waterline length. If you want speed you may need to go to a outrigger canoe. Gentry has Splinter http://gentrycustomboats.com/splinter.html but it can only carry 250lb, so for all purposes a one man canoe. I suppose 2 athletic teenagers could have a ton of fun on it provided they fit the weight limit and are not afraid of body contact (it's a short boat). It would also be a speedy and easy build with that straight sections. You could also do an Ulua http://homepages.paradise.net.nz/garyd/plans.html this one is longer and would carry a load with confidence but a little more complicated to build.
    I suppose the Splinter could be made light enough to carry by bicycle.

    Something about your choice of fabric. The material you choose is ordinary PVC tarp with a nice description and inflated price. That's marketing for you. If you want to use a no name / no technical description product you can have it a lot cheaper from a tarp supplier. If you want to use a good brand product go for Valmex PVC. Just google "Mehler Texnologies USA" and give them a call to ask how much the 7318 Mainstream costs per yard. That is what half of the worlds brand inflatables are made of.

    With your tools you can build in wood, no problem. Exterior plywood can be fine if it has no voids in the layers.
    With a 500$ budget try to find out what Al pipein 6061 T6 costs in your region. Then compare to wood+varnish (or paint). What wood can you get?
     
  3. laukejas
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    laukejas Senior Member

    I figured I might be wrong about that. But is it really safe to assume that if permanent (glued, screwed) joints will be substituted for a little more flimsy, dissemble-able joints, the boat will still be strong and sturdy enough for the job? I kind of like to believe that when a folding SOF boat is designed, this is compensated for with stronger structure and more (or different type) joints, so that any slack is accounted for. Not to mention that these joints are made in a way that makes it easy to assemble and disassemble them. Is it not true?
     
  4. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    It is true in a way but not as you believe. Yes there is some flex to the structure. The way to compensate for it is not to do a bigger and more heavy structure, but to distribute the stresses and go around the flex points. So not a bigger gunwale but a ladder gunwale. The "steps" (technically the I beams flange) align the gunwale and stringer correctly and keep them locked. Normally this is the frames job in a screw&glue SoF. The connectors just keep the keel and gunwale in contact with the frames. When paddling or sailing the whole assembly twists lightly and the connectors become wedged locking everything together (assuming correctly designed interfaces) transmitting the forces as intended. The problem is limiting this twist so that the boat does not behave like a wet noodle. This can be managed with diagonal bracing, geodesic frames, etc. For example in Anabelle this is managed with the big plywood seats and the triangular plate in the bow. In a folder this plates would need to be removable so they need some type of connector. This could be screws, clamps, pins, etc. really anything that keeps them in position and allows minimum movement. When you are choosing connectors you compromise between speed of assembly and loss of rigidity.
    For example if the boat needs to be assembled rarely you could choose all bolted connections. This would require tools and a some hours and keeping spare bolts but would result in a frame as rigid as can be. If you assemble daily you need something with no tools involved so you do it rapidly and accept some loss of performance.
    As for Anabelle the only difficulty I can see are the really twisted stringers in the bow. I would either laminate them or use round stringers (with epoxy its possible to glue the web frame to round stringers without being a master carpenter). I would do a fold around skin and screwed over the top gunwale, and split plywood screwed seats.

    Unfortunately any folder will weigh more than a screw&glue SoF. First you have more wood (webs, overlaps, additional or bigger braces), then some metal connectors, and lastly the big item, the skin. A typical SoF goes by with a 300-500 gr/sqm nylon or polyester skin, coated with 3-5 coats of polyurethane so maybe a total of 500-900 gr/sqm. A typical PVC or CSM skin is 1000 gr/sqm with additional rubbing strips on keel and chines, and other reinforcements, and an expedition style skin is 1450 gr/sqm. TPU skins are lighter then PVC or CSM but I don't know exactly by how much. Therefore if salt water resistance is not a requirement I would advise building with aluminium. If salt water resistance is required then wood and stainless are necessary.

    Have you seen the hydraulic jacks used by Trak Kayaks to tension their boats? They can actually put enough tension on the frame to alter the boats rocker. https://www.youtube.com/watch?v=MP-0UrUSSz0
    Unfortunately they want 499 for a set of 3 jacks.:mad:
     
  5. Sea Moss
    Joined: Nov 2016
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    Location: Houston Texas

    Sea Moss Junior Member

    Both the Annabelle and Melonseed have almost a foot of space between stringers on the floor, but this seems to be because the hull is completely flat on this section, and there is adequate skin tension to keep the skin from bowing in too much.

    Lots of differing opinions on marine grade plywood vs exterior grade. It seems that because marine grade is made from thinner layers it is stronger. But because I already have several sheets of exterior grade plywood just sitting in the workshop, I will use these. It should be able to dry out just fine in between uses.


    I thought about designing something similar to the Klepper Passat, but (according to this) the Passat weighs 200 pounds, which I find ridiculously overweight. I understand that making a boat into a folder will increase it's weight, but if I modify an 80lb hull, it shouldn't weigh more than 120lb.

    I would much rather take the time and effort to make these crucial decisions.

    I do not plan on racing with this boat, and am not worried too much about the speed of it. The Anabelle or Mellonseed seem like adequate hulls for what I want this boat to do.

    Thanks for the information. I am waiting for a reply from Mehler for a quote on the fabric. A while ago, I purchased several yards of the sailrite fabric, but not enough for the entire boat. Good thing too.

    I have not looked into what wood is available locally much, but there is lots of cheap pine and cedar.

    Salt water resistance is a major concern. I will be primarily using this boat in the ocean.

    cool jacks. As long as I can get the cost of the skin, frame, and sail down to $1, they'll be perfect! :D
     
  6. Rumars
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    Rumars Senior Member

    It will be heavy don't you worry. And 223.7 pounds is not a ridiculous weight for a fully rigged Passat. The naked boat weighs 143 pounds and is longer, wider, and with a lot more structure than the boats we are talking about. They are also made from heavier materials all around.

    I'm afraid "cheap pine and cedar" is out of the question. You need a tough wood that bends well and is air dried. Suitable woods would be hickory, ash, black locust, and maybe Alaska yellow cedar. Hickory and ash are non-durable and will need lots of varnish and regular care.

    What type and thickness of exterior plywood do you have?
     
  7. Sea Moss
    Joined: Nov 2016
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    Sea Moss Junior Member

    The sheets I have are scrap 2' by 4' pieces of exterior plywood, 5/8 in thick.

    I need to shop around and see what else is available at lumber stores.

    I have tested several samples of fabric seams. The glue is seemingly invincible. The seam always breaks by ripping off a layer of the PVC skin. The glue joint itself will not break. A 1 inch thick seam, three inches wide, is more than capable of supporting over 100 pounds. A half inch seam showed signs of ripping where the fabric wrinkled. So 1 inch seams or greater should be perfectly capable of holding the skin together. I just need to make sure the skin is evenly tensioned.
     

  8. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    Fundamentally it is a poor sailboat/rowboat due to high drag, low stiffness, and high dependence on air pressure for primary structure. Russians have lots of aluminum welding talent to make a decent boat from a bad design.

    Making a shorter version will still not be easy, and wood will be difficult at every joint. If you like the difficult build and would be happy even if it just floats then have-at-it. The old german folding boats are great examples. There is another guy doing a respectable job on a wood folder here;

    http://www.imagebam.com/gallery/njq1lf6ul5lj3t34ktd24928cegwm239

    My strong recommendation would be to give up on the skin hull and build a nesting plywood dingy like Richard Woods duo. You can make it longer, triple nest, add inflatable wing seats or even go with a mild trimaran.
     
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