Flaws with a folding/inflatable sailing dinghy

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

  1. Sea Moss
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    Sea Moss Junior Member

    I have been planning on building a sailing dinghy with a design similar to this Russian dinghy

    I plan to scale this design down to about 12 feet and use wood for the ribs and keel, and PVC for the stringers. I would have to add lots of diagonal supports to keep it rigid.
    The side sponsons would be made using this method. The entire frame would split into pieces no longer than 40 inches. This would enable me to fit it in any car and take it anywhere.

    My question is, are there any fundamental design flaws with the Russian boat I plan to adapt my design from, that would cause it to sail or row poorly.

    I want to be able to sail and row this boat in calm to mild waves, while comfortably fitting two sailors on-board.
  2. laukejas
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    laukejas Senior Member

    That's an awesome thing you decided to build. I also considered making a folding sailboat some time ago, but it proved to be far too difficult for my first build. When scaling the design, make sure to check displacement figures - they don't scale linearly, but rather in a cubic manner. You might want to increase the rocker to compensate for it.

    The Russian design doesn't seem to have many flaws to my eye. The sheet says it can carry 3 or 4 comrades. That boat seems to sit pretty high in the water with one sailor, but in the photo with three of them, the transom does seem submerged a bit. With four, transom line is at least a thumb's length below the waterline. That is a killing blow for a sailboat. Whenever you'll be sailing alone or with some company, make sure you design it so that the transom corner never touches the water, not even when heeling. Add plenty of rocker. Judging from the pictures, I suspect Russians didn't give that boat enough of rocker: might be because the base frame is made straight for simplicity. Or it might be because of the fabric wrapping problems, which I'll address later. In any case, never make a sailboat with zero rocker. Never, ever. Might work for a powerboat, but it is a death sentence for a sailing vessel.

    I assume your boat will be folding too. If so, the structure will need strong and have simply manufactured joints, because there will be plenty of them. I have my doubts about using wood, because it takes a lot of work to make such joints, and given the expanding/contracting nature of the wood, you might find it very laborious to make sure these joints don't get stuck or loose over time. Using aluminum structure makes more sense: aluminum is pretty stable, requires no finish, comes in perfectly manufactured sizes, and joints are very easy to make. Weight will be close or even less for the same strength compared to wood structure. You can, of course, use wood for structure, metal for joints. In that case, it's all about finding compatible hardware to make these joints. When I planned my boat, one of the reasons I abandoned the whole idea was because I could not find proper hardware in stores to make these joints.

    German-made sailboat Klepper Passat is built in such a way. It uses wooden structure with hardware joints. They are, however, manufactured specifically for the needs of that particular boat - not a luxury most of us have. One of the reasons why the damned thing used to cost a fortune. For some reason, this boat has been discontinued, even though the reviews for it were very favorable. There are still some of these Passat's floating around in Europe.

    Take a peek.

    Some other thoughts: I don't know how rigid the Russian design is, but if you'll be using rig with shrouds, make sure that the structure is VERY rigid around that bulkhead, or you'll never get proper tension into these shrouds. If I were to build this boat, I'd consider the kind of rig that does not need shrouds - after all, it is a folding boat, and tuning shrouds each time takes a lot of time. If you'll choose free-standing mast, then of course, make sure that the leverage forces that come from the mast would be properly transferred to the rest of the structure to prevent twist. Traditionally built boats somewhat rely on the "skin" of the hull to prevent twist - plywood, metal, fiberglass, plastic, whatever. In your case, fabric, PVC, won't do anything to prevent twist. The structure must be designed to compensate for it.

    One more thing: if you'll be using leeboards, make sure their hinges are hella strong. After all, the inflatable sides of the boat won't help much to support the leeboard. Everything will depend on the strength of the hinge. Make it as large as you can to minimize the lever arm.

    And the last thing: if you're using PVC, find a reliable method of making watertight seams. Either cement glue, or some other stuff that is up to the task. Make tests before using it on the actual boat. Check out Yostwerks - the guy does kayaks from PVC, and gives detailed explanations on how to make those seams. You WILL need seams below waterline because if your boat is to have proper rocker (and as I suspect, it will need a lot), you will find it impossible to wrap PVC fabric around the boat without creating wrinkles. PVC has almost no stretch, so it's really unforgiving. Make a scale model to determine the best places to make those seams. If you're aiming for some performance, try to make those seams parallel with the waterflow.

    That's all I can think of for the moment. If you have any questions, go right ahead. Don't take my word for granted, because I didn't actually build such a boat, but I did quite a lot of research into the potential problems of such design, so I might be of some little help. I'll be looking forward to seeing your progress.
  3. Sea Moss
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    Sea Moss Junior Member

    Sorry for the late reply, I've been quite busy with school.

    Thank you for all your feedback! I've actually been following your progress on your folding boat. Good luck with your design.

    I should be able to add some rocker to this design. It looks to me as if the Russian's didn't give their boat any rocker. I plan to use aluminum joints, or aluminium around wood, connecting everything with wingnuts.
    As for the rig, I planned on using an unstayed mast, with just a single cable holding it in place, like the mast of the Sunfish. I plan to use Vinyl coated polyester fabric, and flexible vinyl glue. I plan on mounting a dagger board in the center, with a sleeve made through the skin.
    kind of like This Design Although A leeboard might be a simpler option.
    I plan to make the stringers out of PVC pipe, and have them fit together much like tent poles. I will mount these PVC pipes to the ribs by putting Screws with spacers on them into the wood, drilling a hole that just barely fits the screw, and fitting the PVC pipe on. See photos below for reference. In order to properly transfer forces from the mast to the main structure, there will be diagonal braces at the base.

    I have made a rough sketchup model of this design, shown below. It will have four rib sections, attached together with four keel sections. There will be two floor sections that fit in the cockpit. This assembly will be held rigid using diagonal braces probably made out of wood or aluminum. On top of this the pvc stringers will be put in place along the bottom. This model puts the boat at 11.5 feet long, 3 feet wide. Currently I plan to attach this all with wingnuts, bolts, and Aluminum L-brackets. each keel section will have L brackets on either end, to connect it to the ribs. Each rib will have three holes drilled all the way through it, allowing three bolts to attach the keel section on either side of the rib to the rib itself.

    I have also build the seat section of the boat, using plywood and aluminum L brackets. The seats are each attached to the ribs using bolts and wingnuts. I still need to add a diagonal brace or two for structure, but this design proves to be quite strong. I also need to add the small keel section to the bottom of this. Once I finish this section I can test it for strength and base the rest of the design off this.

    I still don't have a good way to tension the skin of this design lengthwise. I'm still not sure if this is completely necessary. I might be able to just use bungee cords at the rear to hold it up.

    Attached Files:

  4. laukejas
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    laukejas Senior Member

    You've been busy! Nice work. PVC pipes with holes that mount onto these screws in the frames is a good idea, provided you can drill them to match these pins/screws. If I were to do it, I would first fit the stringers without screws in the wood, mark pencil lines on the stringer on each side of the frame, then remove the stringer, and drill between holes. Then drill and insert the screw into the frame. If done carefully, screw in the frame and hole in the pipe should align. Of course, you can drill straight through the pipe into the wood (much easier, no worries about misalignment), but it would unnecessarily lessen the strength of that pipe because of the extra hole on the far side of the joint. Maybe you devised yet another method? I'm interested in how you will be solving this.

    The latest Russian design you showed in your link has some very beautiful feats of engineering. Very well thought-through. That kind of boat should be able to take a lot of abuse, which is a common thing for boats of any kind in Russia. I noticed how they cut strips of fabric of a predefined shape, and then joined it with seams that run along the hull. That's similar to plywood panel boat construction. Somehow this didn't cross my mind earlier, but it is an obvious solution to the bi-axial fabric fold problem. It is great that you have access to vinyl impregnated polyester cloth and suitable glue. When I wanted to make my boat, such materials were a luxury I only heard legends of. Somewhere in far away countries. I envy you.

    One thing that I wouldn't copy from this Russian design, is the "fractured" rocker. Meaning that the curve of the rocker is divided into several flat areas. That is a very bad idea. The rocker curve should be smooth, because water doesn't like sudden changes in particle acceleration that happen at the edges. Obviously, Russians made it so o ease the construction of the aluminum keel. But since you're using timber, it won't be difficult to make this curve smooth. From your screenshots, I see something resembling a plank for the keel. So, an easy method to make this plank curved... Make a mold on a strongback to bend that plank, bend it, then bend another plank on top, and glue them together. Clamp well. Then trim to length. I have made various bends with wood previously this way on my scale models. Very accurate, and very strong. If the bend is extreme, you can steam your planks... Or simply use thinner ones. Steaming is a lot of trouble. You can glue more than two, if you want. The more layers, the stronger it will be. You might want to use 4 or more layers for the bow, since the bend is really tight there. Or, you can cut the bow out of plywood, but it won't be nearly as strong (half of the grains run the wrong way).

    Now, regarding the rest of your design. I'm not sure what is the purpose of the plywood panel behind the seats. Is it to transfer the loads from the rudder? If so, I would trade that panel for two cross braces that run from the centerline of the transom top towards the outer upper edges of the next section. The bottom of the transom will be held by the keel. The middle plywood section, I assume, is the trunk for the daggerboard? It also does not need to be that massive. Use smaller box and braces instead. Save on weight. Same goes for the forward plywood panel in the bow. You don't need that. Curve up the keel, and add a king plank for the deck. Add a brace or two if you're expecting any loads in the bow, but keep in mind that every brace adds to the assembly time.

    In any case, if you opt to use plywood panels... Cut big holes in them. Like here, just a bit bigger. Stay away from the edges, but cut out as much meat off the middle as you can. It will save a lot of weight, yet will give only a slight reduction in strength. Folding boat must be light and assembled without too much sweat, or else it the whole idea becomes impractical.

    Could you please make some screenshots with section and side views? It would make it easier to estimate the shape of your hull. Anyway, judging from the top view, your boat now resembles a tanker, or a cargo ship. The entry and exit angles are too steep. Water particles will be forced to change direction very fast there, creating lots of drag, while middle part of the hull is wasting the length that could otherwise be used to push out water gradually. Similarly to the rocker, it is best to make your sailboat sheer lines smooth and curved all the way: there should be no flat spots. That way, water particle acceleration will be better distributed along the hull, and there will be no sudden changes in direction that create drag. Your length/beam ratio is good, but I would suggest making the maximum beam around 5% aft of the midships, while pushing the rocker 5% forward of midships. The exact number may vary, play around until it looks good. And then make sure that the sheer line is curved like a spline with it's furthest point on that maximum beam. Something like this (don't mind the proportions, just the general shape).

    Daggerboard vs leeboards - well, it all boils down to whenever you'll be racing with that boat. Leeboards are much easier to make, especially in a folding boat, take up far less space in the cockpit, and their performance isn't that much worse compared to daggerboard or centerboard. In the race, of course, every second counts, so that's different. When I considered making my folding boat, I had great doubts whenever I would be able to make the daggerboard sleeve out of fabric. Problem lies in the corners where the sleeve meets the bottom of the hull. It isn't easy to make watertight corners with fabric, because again, fabric doesn't bend in two directions well, especially one that has been resinated. It is easy to make seams watertight, but difficult to overlap fabric over the corners. Let me know if you have any ideas on this regard.

    Stretching the fabric is important. Less important than if you were to use nylon fabric - polyester is more stable - yet important nevertheless. Stretching lengthwise can be done with inflatable sections. It is possible to locate some of them on the transom, and then stretch lengthwise, but it is awkward and not really a good idea. It will also give the transom the look of a fat ***. Instead, do it like they did it on Klepper Passat: make some inflatable sections on the sides of the boat, for example, below the seats. Make sure that once inflated, they will push against something, and will try to move outwards from the boat (making it wider). Since the sheer is curved (if you will make it so), fabric will stretch lengthwise too. Maybe it can be done with bungee chords too, I don't know. But I suspect it could prove difficult, like trying to stretch pouches under the eyes with sticky tape.

    Not to push my ideas into your design... But if you were to choose leeboards, you would not only save a lot of trouble of making the daggerboard sleeve, but you could also gain an opportunity to extend the seats up to the next frame... Twice more room for the sailors. More freedom to shift weight. Easier access to the mast. The advantages are numerous. All that for the price of a fraction of a knot of boat speed you probably won't even notice... Just saying. Your decision. :)

    I have some other things I'd like to say, but first, let us see the side view and the sail plan of the boat. It will determine a lot of design solutions. Unless you have a really bad urge to go out and sail as fast as possible, I would suggest not to rush it - there are plenty more things that can be improved with this design. Other forum members, which are far more experienced that I, might also offer some insight later on.

    Keep up the good work, you're doing great. A very interesting project.
  5. tspeer
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    tspeer Senior Member

    I believe tensioning the skin lengthwise is absolutely necessary, and with a pretty heavy tension at that. It would help to make the skin of a fairly inelastic material so it will maintain its shape under load.

    One way might be to make the angle at the forward end of the aft shear web a little less than 90 deg and to hinge the aft shear web to the cockpit at the bottom. Two cleats on the outside of the aft face of the cockpit would form a slot for the shear web to restrain it when the frame was installed. When inserting the frame into the skin, the ends of the frame would be pointed downward, with the cockpit raised. As the cockpit was pushed down, the ends would move out, tensioning the skin. Then, as the cockpit and aft shear web passed over center, the tension in the skin would snap the aft shear web up against the cockpit, raising the stern slightly and holding the shear web in place. The shear web could be latched into place at the top, but the normal sailing loads would also contribute to holding it in position.

    This would require precise construction of the skin so as to be just the right length. Too short and you won't be able to insert the frame all the way. Too long and it will be loose. But you might be able to test fit the skin with the cockpit raised slightly and stitch the skin together at the bow to properly fit the length. Then it would tension when you pushed the cockpit down.
  6. laukejas
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    laukejas Senior Member

    Old Russian baidarkas are tensioned in a similar way. My parents had one of these. Two parts of the aluminum frame are assembled, each part is inserted into the skin, the centers of the "backbone" lock together while lifted in an inverted V shape, and then the joint is pushed down until the skin is stretched and the joint locks in place. Then, the stringers are inserted into each other (slider mechanism), and finally, gunwales are attached.

    I can tell that while this method is certainly possible, it is very, very inconvenient in practice. It takes a tremendous amount of strength to assemble a baidarka that way, and it always ends up with bruises and some blood. Maybe modern designs are better in that regard, but it is certainly not something you can easily make at home. Maybe, with proper planning... But I would still choose stretching the skin with the help of inflatables.
  7. Rumars
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    Rumars Senior Member

    Yes there are some flaws in the russian design but it can be argued that they are a design compromise.
    First and foremost is insufficient longitudinal skin tension. Even the professional build examples suffers from it (you can see it in the diagonal creases). This is due to the construction principle relying on ballonets alone simplifying construction by not providing a mechanical frame tensioning device.
    Second is the fact that the boat will not row well at all. That is a design choice since the boat is a sailing dinghy and there are no provisions for oarlocks. That is why there is not much rocker. You could use a paddle to push out, but serious rowing would be only a little bit better than in a normal inflatable.

    The big problem is not the russian design but your adaptation, since you do not seem to understand the underlying principles. First I will try to explain the russian design, then your adaptations problems, and lastly propose some solutions.

    The russian design was driven by lightness, economy and safety. I have to say the solution is brilliant. The big tubes make over half the boat and provide flotation and form. PVC is also cheaper then Al even in Russia. This means that the frame can be smaller and thus being able to resist the forces by using less material. For rigidity the designer used a box keel section and for skin tensioning separate smaller air tubes.

    Now your design: first the keel. Your big plywood central web is overweight and does nothing to prevent twisting. Second the stringers. PVC pipe is overweight and not stiff enough. The stringer attachment method is a unsuitable. Third, the first bracing is wrong.
    Solutions: use a keel ladder or a box beam depending on design. For a U bottom like this design a keel ladder is lighter then a plywood box beam. Stringers: use Al tube or wood stringers. Attach them either by HDPE clips or hold them in notches with straps. Braces: in order to brace correctly the braces need to go from the keel to the upper corners of the bulkheads, like in your cockpit bracing. Your first set of bracings is wrong.
    Improvements: Use ladder gunwales. This stiffen the boat and provide the air tubes with the necessary place to brace against. Use a lengthwise skin tensioning system. This can be a levering system in the keel (articulated keel ladder) or a screw type double transom (outer transom is pushed pushed away by a screw). If you prefer a split deck you can also use an excenter lever in the bow. Redesign the cockpit area. Cut the bulkhead ears away, they will rub into the PVC skin. Attach the seats either directly to the skin with straps or floating on supports from the bulkheads. The seats either move with the skin, or do not touch it at all. You can see this clearly in the russian photos. Use a V coaming to attach the skin and stiffen the gunwale.

    I know this will mean a major redesign of your boat but in the present form it is structurally unsound. I also suspect that in the US Aluminium construction using 6061T6 tubes and a box extrusion for the keel will be cheaper, easier and faster.
  8. The Q
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    The Q Senior Member

  9. Sea Moss
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    Sea Moss Junior Member

    laukejas, my plan for drilling the PVC holes is exactly what you described, I would just split the PVC into sections and install the inner wooden dowels that hold the sections together first.
    For the skin, I used this fabric: http://www.sailrite.com/Shelter-Rite-18oz-Grey-61 It's a shame they don't ship the glue anywhere besides the US.

    Considering your advice, I will redesign this model to include a proper rocker. This model was just a rough estimate of the lines of the boat. I made that model just to show the rough shape of the boat. I should be able to add a more evenly smoothed rocker quite easily.
    I was planning on simply using plywood for the bow, but gluing planks like you suggested would probably make it substantially stronger.
    Correct me if i'm wrong, but as long as I give the boat a smooth rocker, I should be able to keep the center seating section's flat bottom, similar to the Tiwal boat.

    Yes, the transom plywood serves to hold the rudder, and give the fabric a shape to fit around. I plan on simply cutting large lightening holes in all the panels, which should cut the weight at least in half. I didn't plan on adding a king plank. Is there any reason to have a deck that far forward?
    I didn't make this clear earlier, but I plan on making two large inflatable tubes out of the PVC fabric, about 8in in diameter, the same way Tom Yost made his kayak sponsons. These tubes will run the entire length of the boat and be tapered on either end. It seems his website is down right now, but here is the web archive link: Yostwerks sponsons
    I didn't worry too much about sheer lines because the inflatable tubes on each side will give the boat most of it's curve. I'll attach more views of that model below. I'm going to figure out how to use Delft ship, and make better lines for this boat. I will definitely add a 5% forward maximum rocker, and see what I can do to get the inflatable tubes to create a 5% aft maximum beam.

    My main reason for choosing a daggerboard was so it takes up less space when folded. However, considering I have no way of easily making the daggerboard sleeve, I'll see what I can do to attach leeboards instead. The only way I can think of to make a daggerboard sleeve is to get some sort of PVC L bracket, glue it together to make a square frame, and glue the fabric to the bracket. I want to build this boat more for cruising than for racing, and don't mind a 1% decrease in performance that makes construction and assembly easier.

    Tspeer, Seems like a great way to tension the skin very tightly. However, I don't think I will be capable of building the skin accurately enough to get this method just right. With that method I would also be worried about any sharp corners or points puncturing the skin.
    The inflatable sections should be able to stretch the boat both lengthwise, and width wise, since they run along the entire length of the boat. The portion at the front and back, would tension the skin at an angle, resulting in slight tension lengthwise.

    Rumars, Thanks a ton for your feedback.
    Does rocker help with rowing? Kyaks and conoes have nearly no rocker, and still paddle very well. Is this just because of their long thin profile?

    My design is built around the cockpit section. The two diagonal supports keep this section from twisting. The daggerboard and mast box are attached to the cockpit section vertically, then diagonal supports keep it from pivoting. The bow and stern are not braced as well, but I don't believe they need it. I realize Moving the forward supports down so they attach the keel and bulkhead corner would help stop twisting, but this was designed as a V coaming to attach the skin to.

    The stringer attachment that I described holds the pipe quite firmly. I cold also angle the screws 10-20 degrees forward, so removing the stringers would require them to be pulled out and forward, forces they are unlikely to experience while sailing. In addition to this, I can simply add string to hold the pipe in place. HDPE clips would allow the pipe to slide in it's joint, and while that wouldn't be a huge problem, it wouldn't help keep the ribs evenly spaced the way my mounting method does.

    The 3/4 in SCH 40 PVC pipe I am using flexes slightly, but I decided to use it because there will be frequent supports and it will be held in an outward curve that will add stiffness to it.

    I am considering using a keel shaped like an upside-down T, which would resist twisting in the same way a box keel would. The problem is the keel will be split into 3 foot sections, so it cannot be completely relied on to brace twisting motion. The Russian design doesn't split into sections this short.

    I will look into ladder gunwhales. This design does not have any large surface for air tubes to brace themselves against. The only problem with this is it adds more joints that need to be attached during construction. My only plan to add support for the inflatables was to use a stringer that runs down the middle of each side. The bulkhead Ears are there to take some of the pressure off the inflatable tubes. I can use foam padding to stop the skin from rubbing.

    I chose PVC because it is easy to work with, however I will see if I can find someplace local to source Aluminum tubes. If so, it may prove better to have stiff stringers. Wood Stringers would be the hardest to join together, so i'd rather not use them.
    Lastly, Rumars, what do you mean by an excenter lever in the bow?

    The tinker traveler looks promising, I'll see if there is anything I can incorporate into my design.

    Attached Files:

  10. laukejas
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    laukejas Senior Member

    Come to think of it, I am not sure if sliding of the sections along the stringers would be a problem. The skin will definitely hold the stringers tight into their cut-outs on the sections - maybe the friction would also be enough to prevent them from sliding lengthwise. I really don't know if there will be any forces that will make the sections twist and slide on the stringers. The Russian design doesn't seem to have any fixtures for the stringers to prevent from sliding. Maybe it's simply not needed. Or maybe it's vital. Can't really say, maybe someone else here can.

    Lashing can be used to hold the stringers in place until you put the skin on, but it's tedious to work and un-work each time. If your plugs would be angled as you suggested, then maybe they would hold stringers just enough until the skin is on. I really don't know about this. There is another option, though. Take an aluminum pipe that can fit over the stringers (it's inside diameter should be just a little larger than the outside diameter of the stringers). Cut an inch of that pipe, and glue it in the cut-out on the bulkhead at the correct angle (epoxy would be perfect for this). Insert and bend a stringer into that little sleeve to make sure it sits in the cut-out at the correct angle while epoxy cures. Then take out the stringer, and trim the ends of the sleeve flush with each side of the bulkhead. This will create a similar sleeve to the Russian design. It will also create a little ridge on your fabric, of course, but I don't think it will make much of a difference in performance. Just make sure these little sleeves are glued in really strong, so that they don't break off when the stringers are bent.

    HDPE clips... Maybe they can be used instead of these little aluminum sleeves I described. I'm not sure which clips you have in mind, but some I found on google look like they could do the job. It would be far easier to make.

    I'm not sure what you meant in that last part. Do you want the seats to be flat? They can be, irrelevant of the rocker. Or do you want the center section to have a 90° angle to the bottom of the boat? It can be done, but designing around such a limitation will do no good for your boat.
    Or did you mean that you want the bottom to be straight in the length of the hull that is below the seats? Why would you want that? Tiwal seems pretty much like a flat plank with a slight vee to me. It isn't a good shape for a sailboat, unless you plan to be hydroplanning all the time - and chances are, you won't. I would recommend skipping flat sections at all. Make a curve, no matter how slight. A beautiful rocker curve can be drawn with a spline, some weights and a pencil. I'm not sure if your software has a spline tool, but if so, make the rocker a single spline from the bow to the stern, and adjust it until it looks good (and until the deepest point is around 5% forward of the midsection). Delftship makes it very easy to draw such curves.

    A good friend of mine, an architect, once explained that usually straight-line structures are not a good idea in engineering. Straight things are easier to design and manufacture, sure. A straight structure can take higher compressive loads. The problem is, it is unpredictable. When the load is too much, said structure can bend or wobble in any direction - and to stabilize it throughout it's length, you need a lot of supportive structures. When there's a slight bend, you can be sure that the structure will continue bending to that side, and not the other. For example, if the rocker is in a constant curve outwards from the boat, lengthwise compression will only try to increase the amount of that curve. That can be easily controlled with the structure above the boat's bottom. However, if there's a flat section along that rocker, it might suddenly decide to bend inwards. Which would be very bad.

    A practical example from my field... (I'm a musician, a guitar player.) A classical guitar holds a string tension load of around 130 lbs while only weighting 3-4 lbs itself. The thin veneer that is called the top (or the deck) and it's internal braces are expected to hold these 130 lbs without deforming for decades. Every decent luthier curves that top outwards before gluing the braces, so that the shape is stable. The top might bend a fraction of an inch more under load, but it will always bend in a nice and predictable curve. Now, some luthiers (or manufacturers) make this top flat, for simplicity of construction. This is what usually happens after a year or two.
    To prevent that, they try to compensate by adding oversized internal braces or additional veneer layers... But in most cases, even that doesn't help. It's like trying to keep a piece of paper straight while pushing it from both sides. The principle is the same on the boats, or any other structure. Straight lines are a bad idea. Avoid them if you can. It applies not only to the rocker, but to the sheer lines as well, which is one of the reasons why I suggested a sweet sweep. As I have mentioned before, water also likes curves more than straight lines and abrupt transitions.

    You can cut big holes in the transom, or you can offset the shape of the transom inwards, and then cut out the shape, so that the transom "edge" is of a constant width throughout. This applies to other panels as well. A little more tedious, but you won't be adding dead weight to your boat. Of course, try to predict the points where larger loads are expected, and leave more "meat" there.

    King plank will help the mast-holding bulkhead from collapsing towards the bow of the boat. The bulkhead itself does a fine job of taking the bending forces from the sail, but only the king plank can take the forward (driving) vector of the sail directly. In other words, it won't let the mast to break the bulkhead in two due to the driving force of the sail. I wouldn't skip this part.

    Your inflatables are a good idea, but don't rely on them to give your boat the sheer line it deserves. Sweet curve on the sheer won't do your boat any good unless a similar curve is down there on the waterline. Sure, it can be done with inflatables too, but then you'd need to cover half of your hull with these inflatables, which would render stringers nearly useless... Also, it is very difficult to predict on what shape the inflatables will take once inflated. It might mess up your hydrodynamics big time. I would suggest designing the structure to give the boat a proper shape by itself, and the adding inflatables just for emergency flotation and skin tension. Inflatables must have something to push against if you want to them to stretch the fabric. Rumars is correct that you need a ladder gunwale. It is very easy to make from some timber and thin plywood. Look how it's done in Klepper boats for reference.

    You can combine two methods. First, you stretch the skin on with the straps on the transom as much as you can, cleat off those straps on the inside of the transom, and then inflate the sides to finish up the stretching.
    Don't take this particular piece of advice for granted - I never tried this, and might be delusional about it. Rumars and Tspeer are likely giving a much better advice on this.

    Rocker helps with just about everything. Rocker curve complements the sheer curves. Primitively speaking, sides of the boat split the water stream sideways, while the rocker does it vertically. With no rocker, the sides of the boat must make up for the loss of displacement, and that makes the boat too wide. Kayaks and canoes can get away with it because they are very long and slender, so their entrance angle is very low anyway. But if the boat has a transom, then without the rocker the transom will drag water. That is just about as bad as having a blunt bow that "pushes" the water with it's flat surface... Or maybe even worse - pressure creates less drag than suction, which is in the case of a dragging transom. Seriously, I once sailed a boat with a transom that dragged. It felt like trying to tow a barge.

    What are your actual limitations on this design? Are you trying to fit this boat in a car once folded? Or into some small room for storage? If you were to tell more about what limitations forced you to consider a folding design, maybe some of us would have some ideas on how to find the best compromise. A wise person once told me that when you're asking for advice on building the boat, you should always start with:
    1. What exactly do you want the boat to do;
    2. What are your limitations.
    The more detailed that part is, the better we can help you. :)

    Hey, if you transfer your design to Delftship, maybe you'd like to share the design files? I also have Delftship, and could take a look at your design, run some tests on it. Many people on this forum have this software as well, maybe they would be willing to take a look too.
  11. Sea Moss
    Joined: Nov 2016
    Posts: 7
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Houston Texas

    Sea Moss Junior Member

    The HDPE clips I have been talking about are the clips often used on folding kayaks. These are simply sections of HDPE sheet with holes cut into them near the edge. Since the HDPE flexes slightly, the stringer snaps firmly in place.
    The only problem I see with your stringer mounting method is it would require a generous amount of epoxy on each joint. For this to work I would also have to split the stringers close to their mount to the ribs, but that isn't a major problem.

    I originally wanted the bottom of the hull below the seating section to be completely flat, however I agree, this is a mistake. I have been focusing more on the folding portion of the boat, and less on the 'boat' portion of it. :p

    I will definitely do this. I'll come up with a good hull design first, then split it into sections that can be folded. Since the fabric I am using does not stretch, it won't act like the skin of an ordinary SOF boat. The hull of this boat needs to be designed like a plywood hull boat, but the frame needs to be a sort of beefed up SOF boat frame.

    So a king plank serves to brace the mast against the side rails and bow skin? Since the skin of this boat doesn't provide any structural support, and the side rails provide very little support, shouldn't I have a support that holds the mast back instead?

    My plan was for the inflatables to provide most of the shape, the same way the Russian boat in the OP does, or the Tiwal, or Tom Yost's Sonnet series boats. This would allow the boat to fold much smaller and lighter, and be simpler to inflate. However, having 8" tubes on each side inflated to high enough PSI to support the boats weight and keep its shape puts enormous stress on the seams. This design would also require more fabric and precise shaping of the inflatables to fit the hull profile. I am unsure of the strength of the glue I am using under these conditions. Tom Yost's sponsons are only about 4" in diameter. Additionally, the inflatables would likely need support stringers on the inside. This means the design has just as many stringers as a non inflatable design.
    Because of these drawbacks, and because I do not have the resources to make professionally sealed inflatable bladders, I am going to take your advice and downsize the inflatables. I will come up with a design similar to the Klepper boats, with ladder gunwales, and a rigid wood frame.

    Thanks for the explanation. This makes a lot of sense now. Boats like the Laser or Tiwal are purely planing hulls, and would not row very well.

    Folding Limitations:
    I want to be able to fit this boat into the trunk of an average sedan. therefore, any rectangualr section cannot be longer than 40" or wider than 16"
    Anything long and thin such as the stringers or mast can fold up to around 48", but I would prefer they also fit within the 40" limit.
    I don't mind setting up the boat taking 45 minutes to an hour, but setup time cannot be ridiculous. I want to be able to set it up in the morning, sail for a day, and take it down at night.

    I want to be able to sail this boat, with a crew of 1 to 2, along beaches in moderate to heavy winds, and moderate waves.
    I am not too worried about performance, as long as it's reasonably easy to get moving.
    I also want to also be able to fit a single set of oars on the boat and be able to row it well.
    For the sail I'm planning on using a balanced lug initially, for its adjust-ability.

    I originally designed the split seating setup based on my experience sailing a Sunfish, but I realize this kind of sailing puts a lot of stress on the hull.

    I'm going to work in DELFTship from now on, google sketchup is terrible at modeling boat hull shapes.

    Also, how do I properly quote specific people?
  12. laukejas
    Joined: Feb 2012
    Posts: 766
    Likes: 19, Points: 18, Legacy Rep: 128
    Location: Lithuania

    laukejas Senior Member

    I see. I think Yost used that kind of HDPE clips. Then again, his bulkheads were plain HDPE, not plywood. Perfect for SOF, but outrageously expensive, unless you can find some used and thrown-away large cutting boards from industrial kitchens. My method with epoxy would use too much epoxy if you cut an approximate angle cut-out in the bulkheads before installing these aluminum sleeves... But then again, this might be too tedious a solution. I'm just brainstorming here.

    Another brainstorm idea: you know these straps for webbing? If you found a mechanism which would allow quick insertion and removal of the webbing, you could use them to secure stringers in place during assembly. These things can be tightened real tight, real fast, and would hold much stronger than HDPE clips. Whenever it is practical or not would totally depend on the kind of mechanism that is on the end of that strap. There are dozens of them. If it takes forever to push the end of the webbing through the throat of it, then it's not a good idea.

    All fabrics stretch, but not to the same extent. If you stretch it in any single direction, wrinkles will appear along the stretch vector. To smooth them out, you need to stretch the fabric in the other direction as well. If you ever dealt with 4-sided sails, you know what I mean. If not... Take a fabric napkin. Stretch it along one axis. Notice the wrinkles that will appear. Now try to stretch it also on another axis perpendicular to the first one. With enough force, wrinkles will smooth out... Unless you overstretch it and create new wrinkles along that axis. Now, of course, the fabric you have will be much, much more stable. Still, don't assume it will be as stiff as plywood. If it will be - great, but be prepared for the worst.

    While we're on the subject - try to make some sample seams with the glue you can get, and then stretch the hell out of it in your workshop. Try to determine how much force can a seam hold. Leave it stretched overnight. Submerge it. Heat it. Cool it. Fold it and stretch again. Beat with a hammer while stretched. Use your imagination to find new ways of abusing this experimental piece to make sure it is strong enough for your boat.

    Also, try to minimize unsupported sections of the fabric. By unsupported I mean anywhere where the fabric does not lie against the stringers. The larger gaps are, the more fabric will want to deflect inwards from the static water pressure. It will it anyway, but it's best to minimize it. When calculating displacement, add some extra, because once the fabric deflects inward, the actual carrying capacity of the boat will decrease somewhat.

    I'm not sure if we're on the same page. The king plank is the beam that goes along the centerline of the boat, starting at the mast partner bulkhead, ending at the bow. It's purpose is to hold against the forces that try to pivot the mast forward. These forces aren't very large when compared to the sideway forces, but still enough to drive the boat at speed, so design accordingly. Here's a picture of my own boat in construction - you can see this king plank (the narrow beam) that goes from the bulkhead to the bow. This is the most primitive type of king plank, but it works just fine.

    As far as I know, there is no need to hold the mast from falling backwards - usually the sail forces mast forward, not back (and mostly sideways, of course, but that is up to the bulkhead to hold up against).

    Let me know if that answered your question, I might have simply misunderstood you there.

    Well, Tiwal type boats are certainly possible to make, but as you correctly stated, it is very difficult to predict the hull shape that these inflatables will create. If you had time and resources, you could make numerous inflatable scale models, and tweak them until you get proportions right... Very tedious. Maybe there is some computer software that can spare you such experimentation... But I'm not aware of it. The Tiwal is very primitive in it's shape (V-shaped plank). Good for planning, but then again, look at it's rig. Unless it had such a powertrain, it would be overtaken by most traditional sailboats with proper rockers. Since we're talking about a displacement boat here, that means rocker... Which complicates hull shape even more.
    Even if you made such a inflatable boat, you'd still need a very strong structure - look at these oversized tubes in Tiwal... In short, it's doable, but I wouldn't recommend it for a first homemade project of this sort.

    Very true. Now, someone might come and correct me, but I think rowing boats are not that much different in hull shape from sailboats. They are almost always displacement hulls, unless it is rowing/motored hybrid. They are usually a bit narrower than sailboats. They also usually have skegs that help with tracking. But generally, a hull that sails well should also row reasonably well. If you expect to row a lot, make sure the thwart is comfortable, and oarlocks are in the right positions. There are some guidelines on oarlock placement on the internet. It is very important. Oar length is also critical. You might also want some sort of foot brace. All these are tiny details now, but will save you a lot of trouble later on.

    Okay, this part is troubling. If you want no more than 2 people onboard, then you can make your boat from 3 sections, which should come to a LOA of around 9-11 feet, depending on whenever your limit is 40" or 48". Such a short boat won't be fast, but it makes sense with your limitations. I wouldn't want to have more than 3 parts. The more joints there are, the weaker the structure.

    Does your car have a roof rack? If so, it would make your life much, much easier. You could extend your limits a lot. 40", even 48" is really not much. Especially for spars. Lug is a good choice, but to have any decent sail area, you'd need to make your spars at least from 2 parts, mast would need 3. If you make your spars from aluminum, it's doable, provided you can get correct diameters. But so many joints might not be a good idea from a structural standpoint. Joints also add weight aloft. Best to minimize them.

    When I wanted to make a folding SOF, it was for similar reasons... Transporting with car, and no place to keep the boat (I live in an apartment in a middle of the city, and at the time, I didn't even have a garage). When I realized SOF is not feasible with the availability of materials in my country, I opted to make a super lightweight 2 person stitch&glue boat which could be car-topped single handed. Took me a year of planning, but I did it. Roof rack is a life-saver. If it exists or can be added to your car, it would be great.

    You are quoting correctly, I think... The way I do it: find the post of the person you want to quote, press "quote" button beneath it, and then copy the text that appears in the next window. If you want to quote in parts, just copy the opening and closing tags - [ QUOTE= some name; post number] and [ /QUOTE] along with the text in between.
  13. Rumars
    Joined: Mar 2013
    Posts: 1,699
    Likes: 1,001, Points: 113, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    Sea Moss I think you must decide what you want to do. Do you want to learn how to design a good boat, or do you want to build a folding dinghy? I can not help you with the former, but I can help you with the later. So here is my proposal to you:
    You stop fooling around with boat designing, trying to learn delftship, etc. You go to http://gentrycustomboats.com/Planspage.html and choose between the Melonseed http://gentrycustomboats.com/Melonseed.html and Anabelle http://gentrycustomboats.com/Annabelle.html Then you buy plans for the one you like best and fits your need most. This will free you from designing the boat and give you full size frame patterns. The money (98 or 79$) is well invested, you bypass countless hours of study and trial and error and go directly to building. From now on you only need to adapt the construction from fixed SOF to folding SOF. There are many ways to skin a cat (or folder in this case) and you do not need to reinvent the wheel.
    All you have to do is say what your capabilities, woodworking tools and budget are, and we can select appropriate solutions for you. If you do this I promise to help you step by step with what I know.

    In principle there are two ways to make a folding boat:
    1. You assemble the frame outside the boat and the frame is rigid enough to hold shape without any skin tension.
    2. You assemble the frame in sections and it holds shape by skin tension.
    I would choose option 1 for any boat and invest in a good modern waterproof zipper for central deck closure, or use separate deck pieces on bolt ropes on the gunwales.

    Some technical solutions to motivate you (anything I present here can be made at home with basic tools):

    Longitudinal tensioning solutions:

    1. Skin tensioning by way of excenter:
    The stem (or stern for a double ended boat) is split and inside pivots a plate that extends out when locked in position. This gives you some inches to get the frame into the skin then tension it.
    Photos of said mechanism (photos are not mine so I will only provide links, not include them into this post) in the stern of a folding kayak (Pouch E68):
    Lever pivoted up, excenter in, ready to be introduced into the skin: http://www.pouch-inoffiziell.de/pics/boote/s2000aufbau/heck_entspannt.jpg
    Lever pivoted down, excenter out, normal in skin position: http://www.pouch-inoffiziell.de/pics/boote/s2000aufbau/heck_gespannt.jpg
    Tensioning the skin, midway action shot (please observe split deck to be closed later): http://www.pouch-inoffiziell.de/pics/boote/s2000aufbau/spannen1.jpg
    Tensioning the skin, end of action, lever will be locked into the frame and double as deck stringer: http://www.pouch-inoffiziell.de/pics/boote/s2000aufbau/spannen2.jpg

    2. Tensioning the skin by way of screws:
    One, two or more screws push the transom out against the skin. Photos of the system used by MTW, where a horseshoe piece pushes out and the screws are anchored in the last frame: http://faltbootbasteln.de/d110ab3.jpg
    http://faltbootbasteln.de/d140ab7.jpg http://faltbootbasteln.de/d140ab11.jpg

    3. Knee lever solutions:
    a) The keel is articulated and by pushing down you push the frame ends into the skin followed by locking the two halves of the keel together. This is the "normal" method for kayaks using a keel ladder. To be observed on countless youtube videos of assembling a folding kayak.
    b) The keel is split but not articulated and the knee lever is separate and used only to push the two halves apart until they can be locked. Used on kayaks without keel ladder (mainly some deep V eskimo type).

    Transversal skin tensioning systems:

    Today most folders rely on air tubes to provide transversal skin tension. For reasons of user convenience and manufacture economics today they are inserted into skin pockets and made out of clear PE. However they can also be out of heavy rubberized fabric (or reinforced PVC or heat weldable TPU coated nylon) and attached directly to the gunwale ladders by straps. Here a photo of such an arrangement on a MTW dinghy. http://faltbootbasteln.de/d150au03.jpg

    Stringer to frame fastening solutions:

    1. Ladder gunwale to frame:
    Frame extends into the ladder and is fixed there by some means. There exists a lot of variation on these theme, from free metal hooks trough a slot, to rotating tongues, L brackets, keyhole inserts, up to a simple strip of PVC or a bungee cord. Here is a photo shoving the free metal hook system http://www.pouch-inoffiziell.de/pics/nutzung/schienen_am_boot_3.jpg In this system lateral movement is restricted by the ladders "steps", up and down movement by the stringers themselves, and compression as usual by the frame. The whole thing is locked together by the bent metal strip passing trough the frame. The strip locks itself by friction against the ladders "steps" but could be positive locked by a screw on the inside or other means. The same system can be used for a keel ladder.

    2. Stringers to frame
    In SOF (rigid or folding) boats stringers are there to define the boats shape and not to ad to rigidity. All forces are carried by keel and gunwale forming a truss with the frames (that is why you need beefy keel and gunwales, and we skip true beefy by using I beams in the form of ladders). Thus stringers need only to be located in place by the frame cutout and hold against the frame by the skin. Since PVC is not as stretchy as the old time rubber skins the usual method now is to provide a separate tensioning system with a belt. Here a photo of an inside strapping system with polyester webbing and tensioning buckle (there are also outside systems with PVC strips, individual or all stringers at once). The belt is screwed to the keel (only one side of the keel ladder visible), holds the chine stringer in place with a bracket and is tensioned with a buckle at the gunwale. Same thing on the other side of course. http://www.pouch-inoffiziell.de/pics/boote/s2000aufbau/spanten_spanngurte.jpg
    Alternative with only one buckle: http://www.pouch-inoffiziell.de/pics/nutzung/spanngurt.gif
    Outside PVC strap on hooks (actually screws): http://www.pouch-inoffiziell.de/pics/nutzung/spanngurt3.jpg

    Longitudinal locking systems for stringers, keels and gunwhales:

    Here we have a multitude of solutions with a lot of variations. Popular are interlocking ladders or fingers and metal or composite ferrules, either one with or without locking or arresting features. To select one first you have to make some other decisions like assembling the frame outside or inside the boat, building with round or square stringers, desired length of segments, choosing metal fasteners or going composite, etc.

    There are other chapters like skin construction and attachment, fixing of appendages (rudders, skegs), rig, floors.

    Now to some of your questions:
    Apart from what Laukejas said you need rocker to be able to tack. No rocker and the boat turns like a pig. Olympic rowing shells do not need it, they are made for going fast in a straight line. For them waterline length is priority nr. 1. Kayaks and canoes have more or less rocker depending to application. For racing almost none, for wildwater curved like a banana. Sail and oar boats are always a compromise and designers have to carefully balance a lot of factors beside rocker to get the right blend. That is why I say go for a proven design, this things are not easy even for experienced designers.
    As I said PVC is not suitable. When cut and rejoined it will not bend smooth because it is not stiff enough and you stiffen it locally at the joints. If it is stiff enough to overcome this it will be to heavy to use. Use Al or fiberglass or wood. As I said stringers are there only for shape not for structure, they do not have to be rigidly attached to the frames. Round wood stringers can be connected with tubular ferrules, rectangular ones by a variety of means.
    Twisting is resisted by the right sort of connection, depending on the profile used. Even a T section can be stiffen successfully but it is better to use an I or box beam. Be advised that a 9 foot keel needs four 3 foot segments. If you use 3 segments you either end up with a 7 foot boat or the segments are longer then 3 feet. You need to factor in the length of the connectors. Gunwales and stringers are longer then the boat because they are curved. There is also the question of frame or bulkhead placement they can not sit directly on a joint.
    Using inflatable tubes to define the shape of the boat means some complicated geometry to design the tubes and good knowledge of the materials properties and a pump. It is not trivial and you are moving into inflatable boat territory. The russian knew what he was doing and you can still see the difference between his homemade boat and the ones made by the professionals (FMK builds folding kayaks and inflatable catamarans http://www.fmkboat.ru).
    A frame defined boat needs only cylindrical air tubes mouth inflated to be able to take the slack out of the skin.
    I know you refer to Tom Yost's hybrids but those are another matter. For example they were not shape locked, they where build straight, without rocker, and got rocker by bending under the persons weight. They also flexed in the waves. This were desirable things for him as a paddler but can not be applied to a sailboat without it behaving like a wet noodle.
  14. alan craig
    Joined: Jul 2012
    Posts: 367
    Likes: 121, Points: 53, Legacy Rep: 14
    Location: s.e. england

    alan craig Senior Member

    I built a folding back packable canoe for the same reason as Laukejas, I lived in a flat and had nowhere to store a boat, many years ago. I'm posting a few pictures to show how I tensioned the fabric. Tension across the frame was easy using elastic cord; tension from end to end was more difficult. First I tried eccentric cams to tension the bow and stern frames but finally used threaded rods as shown in the picture.

    I would have to say that, though the boat folded to a very small package, it was not a great boat to paddle. When you try to design and build something unique from scratch you realise that the ready-made item is not so expensive after all - it cost me more to develop this boat than it would to buy a quality inflatable or folding canoe!

    Attached Files:

  15. Sea Moss
    Joined: Nov 2016
    Posts: 7
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Houston Texas

    Sea Moss Junior Member

    laukejas, once again thanks for your feedback
    I really like this idea, makes tensioning so simple. I could also use these to tension the skin or add diagonal bracing to the frame if needed.

    I have a sample of this PVC fabric. If I stretch it diagonally, with a ton of force, slight wrinkles appear, but large wrinkles do not appear like in most fabrics. I think the plastic coating locks the fabric threads into a grid, so it does not stretch like normal fabrics.

    Tomorrow, I'll test several seams and see how well they preform.

    Thank you, I hadn't thought of that. depending on the distance between stringers and how well the skin is tensioned, deflection could have a substantial impact on displacement.

    I understood what you meant by a king plank, but wasn't clear enough in my explanation. Definitely need a king plank in this design. In my original design a king plank would cause the front of the boat to bend the bow of the boat downward. There was nothing to transfer downward forces on the bow to the rest of the boat.

    Hmmm. Prehaps I could change the specifications a bit, these aren't set in stone.

    I am a student in high school right now, and do not own my own car. I drive my parent's car, but I want to build this boat so I can take it absolutely anywhere, and don't have to worry about space limitations.

    I was using the [bold] tag and writing out 'originally posted by . . ' :p Thank's for the help.

    I've made up my mind. Thank you Rumars for your feedback.

    From the start, I wanted to adapt a dinghy to make it foldable, however, since I do not have the resources to create a boat simular to the russian dinghy. I totally agree, it's worth the cost of plans for the time saved. I'm going to take your advice on this.
    I am slightly torn between the mellonseed and Anabelle. While the mellonseed seems to handle better and will have a faster hull speed, It is larger and will need to split into more sections.

    I am fairly handy with woodworking tools, and am willing to learn anything I don't know. I have a table saw, a scroll saw, a small miter saw, and a router, as well as basic hand tools. I am willing to spend around $500 for this project, and have no particular time limit. The scroll saw is rather hard to control, so I may purchase a jigsaw to cut plywood sections. I do have an abundant supply of outdoor (not marine grade) plywood. if I seal this with epoxy, would it be just as waterproof as marine grade plywood?

    This seems like the best option to me too. It means that the frame will be stronger and more robust.
    I will look into waterproof zippers.

    The excenter method of tensioning seems promising. I'ts just a small addition to the bow, and the lever allows easy tensioning. These other methods are intriguing too. A lot of the methods to tensioning depend on the layout of the frame. For longitudinal tensioning, I will definitely use small inflatable sponsons. Strapping the tubes to the gunwales is a great way to save fabric.

    I like this method of using a metal strip as a locking pin. Perhaps the end not passing through the frame could be lengthened and bent inwards 90 degrees so it snaps in place at the joint between the gunwale and frame, locking it in.

    Ok, this makes sense, there are few forces that the stringers can help support.

    I like the idea of that belt, makes mounting stringers much simpler. maybe I could use those adjustable straps like laukejas suggested.

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