Zipper Seam Construction

Discussion in 'Wooden Boat Building and Restoration' started by ancient kayaker, Jul 10, 2007.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    - “machined step scarfs” is B&B’s own wording at Graham’s page http://www.bandbyachtdesigns.com/cs17kit.htm - I agree it isn’t accurate but it lets me continue using the term “zipper seam” for my own idea without infringement.



    - the notched planks should work on any ply that can be used for stitch and glue, the notches are more robust than a hole and a wire stitch. Certainly 4 mm and 3 mm, probably 2 mm too if you can get it.

    My idea using stick-on tabs should also be fine with thin ply but won’t handle thick ply; I'd guess 6 mm is the limit. The concept is aimed at lightweight narrow hulls like canoes, kayaks and amas that have many narrow planks, where S&G becomes a hassle.
     
  2. JEM
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    JEM Senior Member


    Oh ok... I misundestood what the stepped scars were.
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    Hoping to start the 1:6 model soon. I got the plank developments printed to the correct scale and have bought some 0.8 mm Baltic Birch plywood. I think I will give the ply a coat of varnish on one side so the temporary spray adhesive does not leave a residue on the surface. I did some test cuts on the ply, I found that a sharp pair of scissors makes a nice clean, square cut.

    I will have to cut some 1/8 x 1/16 "lumber" for the gunnels and keel. Not sure how to do scale inner stems at this point, probably use 1/8 ply if it's not too flimsy. Cutting 1/64" laminations for the outer stems is going to be a challenge though . . .

    I'm trying out West System epoxy for the first time. A 7g coffee spoon measure of #403 microfibers added to 1 pump measure each of epoxy & hardener provided a non-sag mix, and a veterinarian's hypodermic without a needle does a great job of filling the seams; I will try different sizes and nozzle shapes to see how best to form the bead.

    I've only done one test piece so far but it looks like the mix may be strong enough for the seams of the Wee Lassie without glass tape. I beveled the edges to increase the glueing area. I will try precoating with non-thickened epoxy, as I don't think the thickened mix penetrated into the seam full depth. I plan to do several test pieces and break them to see if the joint is as strong as the wood.

    Previously using 5 minute epoxy with plain butt joints I was getting joints with about 58% of the strength of 3 mm ply. The microfibers seem to increase the flexibility of the epoxy - the 5 min stuff was brittle.
     
  4. ancient kayaker
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    ancient kayaker aka Terry Haines

    This is taking forever! I got waylaid by a medication problem which is clearing up but now I have even more medications to take; sigh! hopefully when the underlying condition clears up I can discontinue the treatment AND the three (so far) medications I have to take to deal with its side-effects. It will be nice to have all of my immune system back to working order too. *****, ***** . . .

    Anyway, I did manage to put together a 1:12 model in card. It assembled easily enough and produced the expected hull shape, which validates the plank developments from FreeShip, but I didn't use the Zipper Seam technique.
     
  5. rwatson
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    rwatson Senior Member

    While zoning out pulling wire stitches out of hulls ... I pondered this silly stitch process that makes lots of holes in the nice wood, as you have been doing.

    My thoughts are that your zipper idea may not need continuous teeth for much of the length of the hull. Apart from sections with a lot of bend near the ends, maybe the majority of the hull can get a way with one 'tooth' every say ... 200 mm ?

    So, dreaming on, every few inches in the sections where the panels are at larger angles, one of the panels can have a tooth sticking out that matches a notch in the other panel ?

    My ultimate dream would be to have a mould made up, with the exact shape and inner dimensions of the panels, and with the aid of small drill holes, use vacuum to hold the ply planks in place while I glass the outside. I could make that out of fibreglass I suppose.

    Does the world need that many new ply Kayaks I wonder ?
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    I think you’re right about the spacing of the teeth, or tabs as I call them, I was planning on 4" but that’s overkill for the straighter sections. Speaking of teeth fitting into notches, did you see PAR’s post #5?

    I like the concept of a vacuum mold for ply boats so they can be planked and glassed in one step, but it would have to be for mass-production, I’m not sure if the market can absorb the output! Might be an idea for anyone still using one of the old-style molds made for manufacturing rib and strip boats with copper tacks; 90% of the work is already done.


    - Gasp! Splutter! I’ll just pretend I didn’t see that . . .
     
  7. Chuck Losness
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    Chuck Losness Senior Member

    Having only made one 10' stitch and glue dinghy my experience with this is limited to say the least. If the panels are fully developed shouldn't they just lay against the molds? Why can't you just temporarily screw the panel to mold with a couple of screws. Tape and epoxy all the seams and remove the screws. Just a thought.
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    Chuck: there's no reason you can't screw the panels to the molds and then tape as you suggest, but as with RW's vacuum suggestion you will need a full mold to define the hull shape - although it needn't be airtight.

    if you're going to drill holes why not go the S&G route which eliminates the need for much more than a midships station mold. My own desire (obsession?) is to build using the plank developments as is done for S&G but without the holes, stitches and tape, but I still want to avoid needing a full mold the way S&G does. That's what the thread's about.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Most developed shapes have some forced edges in the design. Typically these will be in the fore foot, the forward and aft portions of the chine, maybe on the centerline aft too. As a result you have a fair bit of tension along these edges, which can cause the panels to cub and distort a good deal. Most designers take advantage of these distortions, which can force convex sections in the fore foot, a wee bit of tumblehome aft, etc.

    If you don't control the edges, these distortions and edges stresses will never permit you to lie one edge causally against another as the boat is "folded" into shape. This is the logic behind stitches, to control the edge tension associated with the build method particulars.

    I too don't like, nor do I often use stitches, preferring other methods, but I have no issue with molds and do prefer them for accuracy sake, especially if building more than one. I've found it's possible to use maybe two molds and a transom on boats under 18', assuming the shape isn't too wild. Boats under 15', I can use a single mold and a transom. This isn't asking much, though the hull is a floppy thing, until remaining structure is in place. An eye on keeping things square isn't hard with a well setup building area.
     
  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    I learned about the stresses in plank edges the hard way. On the Wee Lassie stations the first two planks are parallel at every station so I merged them into a single, wider plank. FreeShip's development for that wide plank was kinky along the bottom edge. "No way that is going to work" I decided so, unable to recognize the hint I faired it. When I tried to form it into shape the bottom developed more than 4" of unwanted rocker.


    I am considering doing just that as an experiment, using a midplank stringer, outwale, and temporary external chine log to induce a convex shape into a sheer plank. I have done some calculations* that suggest the curvature is predictable and I want to check it out. Could be an interesting design and construction tool, perhaps for a canoe. It’ll need careful gluing though, to prevent “exploding canoe syndrome” . . . I also found the lines of Herreshoff pram that could use the idea instead of requiring lapstrake.


    * retired engineer: you can take the rat out of the race but you can't take the race out of the rat.
     
  11. LP
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    LP Flying Boatman

    Terry,

    I think that the amount of curvature that is induced is simply the aligning of the centroids of the given sections. Simplistically, 1/2" stringers are going to give you 1/2" of camber across the panel. 1" stringers will give you a 1" camber, give or take the thickness of the panel. The only caveat that I see here is, can the panel that the stresses induced by the stringers?

    I was able to get 3/8" to 1/2" of camber on a 3mm x 9" wide panel, but the 3/4" attempt was disasterous.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Panel curvature is governed by the panel stiffness. The amount you get is dependent on thickness, number of veneers, thickness of the veneers, species employed, number of internal repairs, the amount of "convincing" you're asking of the panel, etc. From a mathematical perspective it might be predictable, but I've found only to a degree. After trimming countless station molds and frames, to accommodate the anticipated curvature in comparison to what you really get, when bending a panel into position, I can tell you, it's mostly a crap shoot. For a taped seam build, not really much of a concern, as you can get it close enough, that a fillet will fix everything, but a plank on frame build is more challenging, usually requiring a low angle plane or belt sander, to fix the best of calculations.
     
  13. Steve W
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    Steve W Senior Member

    30 years ago i designed an 8ft junior trainer with which we founded a sailing program which continues to this day, we built 10 boats stitch and glue style at community centers all over the city with kids doing the building and then learning to sail after launching,good times. Small boat journal did a story on the program.The boats had a lot of shape in the forefoot which i didnt want to change and it was very difficult to stress the bottom panels into place so we just sanded away the inner veneer, laid a folded up towel over it and poured boiling water over it and it became easy,after the boat was all together and dried, out glass cloth and epoxy made up for the inner veneer, it worked out great, never an issue and allows for shapes that may not otherwise be attainable. Other little tricks i use are using 1/4" threaded rod and plywood washers to spread the load to pull panels together and take the load off the ties in the always difficult fwd end of the bottom panels, i never bevel the panels and just use dabs of hotmelt to keep things in line if needed, i also never pull ties anymore, just snip them off and bury them. I havnt done a S&G boat in years but if i anticipated doing more than one the same i would build a simple mould,either male or female and skip most of the ties.

    Steve.
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Wow! A lot of interest in this idea, perhaps I should have made it a new thread *. Between the time I went to bed and the time I turned on my computer you night owls asked and probably answered most of the questions!

    I agree with LP that simplistically the stringer centroids will want to line up if we ignore the stiffness of the ply. Steve’s experiences highlights the problems of excessive shaping forces; with my personal preference for using chine logs instead of tape which is probably stronger, I worry whether the boat can stay together - although such stresses do relieve over time. So that leads to the question, how far can this technique go in terms of creating curvature *. That will be limited by the ply, but I wonder how close we can get to the usual bend radius limit for a single axis bend which most sources put around 100 x thickness for regular ply.

    PAR raises the issue of predictability, a good point if a design using this concept is to have boat-to-boat consistency. In theory predictability can be assured by taking LP’s point in my 2nd para to the limit by ensuring stringer stiffness is sufficient to overwhelm that of the panel, but using paper-thin ply or massive unbendable stringers won’t lead to practical designs. However, if the midline stringer is sized to produce excess curvature then after a dry run the stringer can be planed down to produce the design curvature.


    * This idea came up before and LP did some experiments in his thread, the topic starts from post #9 - http://www.boatdesign.net/forums/wo...tion/15-lapstrake-kayak-40486.html#post517361 . . . so LP should have a better feel than most for how far this idea can be taken. I should have posted my comments there . . .
     

  15. LP
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    LP Flying Boatman

    I think your OK to carry it here if the thread is flowing with it.

    One additional bit. I glued my stiffener panel up with a reverse curve so when the panel was laying flat, the camber (crosspanel curvature) was fully developed. With continued bending, the camber remained intact. I may be mistaken, but I feel this method of pre-induction of the camber in the panel made the compound curvature easier to attain than trying to induce the lateral and longintudinal curvature concurrently from a flat panel.
     
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