Strongback materials for cedar strip construction

Discussion in 'Wooden Boat Building and Restoration' started by cthippo, May 10, 2011.

  1. cthippo
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    cthippo Senior Member

    I'm putting together the materials list for building a cedar strip 'yak and I'm wondering about what to build the stations on the strongback out of. I've seen at least one person who used 1/2 plywood, but that seems like expensive overkill. Is there any kind of consensus on a best material for this? Would 3/8 OSB work OK?

    For the strongback itself I'm thinking two 18' 2x4s on edge with little 2x4 pieces between them in a ladder shape. I could then bolt the stations to the crossmembers.

    Also, what is the best way to attach the bow and stern shapes to the strongback?
     
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  2. TeddyDiver
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    TeddyDiver Gollywobbler

    Almost anything works. OSB quite well. For a kayak 2by4 sounds a bit overkill.. my 33'er has 3/4by4 boards as... fastenings with screws and 1,2mm nailgun.
     
  3. PAR
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    PAR Yacht Designer/Builder

    OSB works, though it doesn't accept fasteners in it's edge well, nor does most "sheet goods" like plywood or MDF. I use #2 pine, soild wood, with gussets when necessary. They can be cut up easily, plane easily and don't cost much. They can be used for other things later when you disassemble the building jig and they accept fasteners in their edges well.

    Long 2x4's are usually the ugliest things you can buy. If buying dimensional lumber (2x4', 6's, 8's, etc.) buy 16' 2x10's and 2x12's. The reason is these have to be cut from older, bigger trees, so you can get them with straight grain and often knot free. Rip them to the dimensions you need on a table saw. Trying to buy 18' lumber will prove costly. Just piece shorter stock together, with a simple lap joint. Lastly, use screws, not nails. Screws hold, nails let things move. I use bugle headed deck screws for setups. They're stronger then the black phosphorus drywall screws by a large margin and cheap. Nothing worse then being half planked up, just to realize the jig has distorted from the building process, because nails are letting the separate pieces move a bit under load.
     
  4. cthippo
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    cthippo Senior Member

    Thanks Teddy and PAR.

    I checked at the Re-Store and found out I can get OSB or MDF for next to nothing, or sometimes nothing, so I'll probably make the stations out of that. I like your idea about the stringers for the strongback, PAR.

    Couple of questions...

    Will I need to do anything special for stripping the ends? My design has upswept ends and I'm wondering what headaches I should anticipate and how to mitigate them.

    Is there any reason using gorilla glue to attach the strips together wouldn't work?

    Can someone direct me to a source for plastic brads? I like the idea of using them to stick the strips to the strongback while the glue sets.

    What about station spacing? When I laid the model out in Freeship I spaced the stations every quarter meter. My thinking is to use this spacing on the strongback at the ends, but go to only every half meter towards the middle where the curvature is less. Is this reasonable?
     
  5. BATAAN
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    BATAAN Senior Member

    I think Gorilla glue is hard to clean up, isn't it?
     
  6. txriverrat
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    txriverrat Senior Member

    Go with titebond II glue and set your stations every 12 inches. you probably already know this but cover the edges of your stations with tape so you dont glue them in.
    Ron
     
  7. cthippo
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    cthippo Senior Member

    I've used Gorilla a lot in my current project and I'm pretty comfortable with it. It has it's quirks, but once it sets I know it's completely waterproof. Cleanup can be a pain, especially in tight quarters, but a #2 x-acto knife with a chisel blade makes it go pretty fast.

    I did my table of offsets in metric so my choices are about every 9" or every 18". Do you think I can get away with the 18s in the middle or do I need to go 9s all the way down?

    I found a source for plastic brads so I'll be using those to hold the strips to the stations during construction.
     
  8. PAR
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    PAR Yacht Designer/Builder

    TiteBond II is only water resistant, TiteBond III is waterproof (just barely). PU adheasives isn't something I would consider. It foams, it's a mess to work with, tack times, clamping pressures (also with PVA's) and it costs more then epoxy. Personally, I wouldn't consider anything but epoxy for a strip build, mostly because of all the problems it can handle and solve. No clamping pressure issues, good working times, given reasonable hardener selection, joints and fits can be done with a hatchet with no ill effects and it's compatible with sheathings.

    There are many ways to attach your strips to the jig, each has it's good and bad points. A relatively new one is fishing string, which leaves no holes. Of course you'll have lots of little bits of string every where, but it's a clever system.

    Station spacing needs to be wide enough so you can reach around stuff, but tight enough so you don't have sag between them. Strip dimensions usually dictate the spacing.

    You'll probably need "stealers" in the ends of the boat for your sheer sweep. This isn't hard to do and you can get creative with contrasting wood species if desired. It also depends on how you'll plank. If the planks just run out at the sheer, then your stealers will be few. If you plank to the sheer then you'll get to a point where you'll need to straight out the strips, because twist and sweep is becoming unmanageable. Some stop at the LWL and start again with realigned strips, fitted to a set of stealers, again it all depends on how you'll plank her.
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    Like PAR I like pine as a strongback, usually select grade. It’s straighter and more stable than the typical spruce available from hardware stores, easy to work with, and when I’m done with it as a strongback it gets used on a boat . . . I find it’s nice for seats, contrast stripes etc.

    When I use molds I use thin ply framed with pine strips to stiffen it; it is probably the cheapest and the pine is easier to fair and takes screws better than sheet materials.

    My stripper is a nod to old times and has ribs and no glass, so no molds were used. The ribs are at 8" spacing for structural reasons, but for clamping purposes that seemed closer than necessary even for the 3/16 thick beveled strips that I used, which can slide apart more easily than 1/4 thick bead and cove strips. I was amazed how little clamping forces was needed to keep the strips in place on the ribs; I never have enough clamps so I used cheap plastic clothes pegs on alternate ribs.

    FreeShip allows you to change station/mold spacing as easy as underwear; I imagine 12" would be about right, maybe a little more, but ½ m (20") may be too much.I don’t think station mold spacing is related to curvature unless the curvature is changing rapidly, more to keep the strips lined up; I found it best to increase spacing toward the stems to minimize the force required to get the strips to sit on the stems, but it was an unusual hull design and that might not apply to your boat.

    Stripper stems are usually attached to a stem mold, which can be used for steaming or laminating the stem, and to support it when cutting the rolling bevels for the planking; the bevel angles can be obtained from FreeShip printouts if you have the control lines lined up with the planking pattern. The bevel angles will depend on the planking method - top down, bottom up, waterline and out, with or without stealers etc. The mold has a series of holes near the edge to allow clamping and the clamps are removed progressively during planking. Planning for removing the mold from the completed hull is best addressed before construction starts!

    Gorilla glue expands, it will fill a small gap but loses most of its strength in a large gap and tends to force thin sheet materials apart unless a perfect storm of clamps is used. It’s also expensive. For a glassed hull any wood glue is fine, it’s only to hold the strips together until you encapsulate everything. I used Titebond III but wished I hadn’t, it sets dark and rock hard. I suggest choosing a glue that is fast setting, easy to sand, and color compatible with the wood if you intend a bright finish.
     
  10. txriverrat
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    txriverrat Senior Member

    I dont worry about glue being just water resistant , I am going to cover it with cloth and epoxy anyway ,so all it has to do is hold it in shape till I get the epoxy on it.
    Gorilla glue is great stuff ,but a damp rag will clean up the titebond when it is applied.
    The spacing of your stations 9 inches or 18 will be dictated by the shape of your hull.
    This is the way most of the yak builder I know do it ,but it will all work.
    Ron
     
  11. cthippo
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    cthippo Senior Member

    Just to be sure I'm understanding you guys, Stealers are the wedge shaped pieces that go between strips to fill gaps?

    What are the relative advantages and disadvantages of the different planking methods? I'm planning on using 1/4" x 3/4" cove and bead strips. I like the idea of having contrasting color wedges at the end, especially since I have a local source for exotic woods.

    What do I need to consider about removing the hull from strongback?
     
  12. cthippo
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    cthippo Senior Member

    I got the stations and bow and stern pieces lofted last night. I think I want to start planking from the sheer as I set up my stations with a shoulder for this purpose. That will also allow me to deal with the upswept ends right from the beginning instead of trying to figure it out as I go.

    What should I be considering at this point?
     
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    Just for the record.. :) and a disclaimer "don't do this at home"
    The planks lying flat are loose and only for easier walking..
     

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  14. cthippo
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    cthippo Senior Member

    Damn, Teddy, that's a BIG kayak :p

    How many layers of planking are you putting on?
     

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    Yes, those are stealers.

    Starting at the sheer leaves a football-shaped opening to fill in at the bottom, requiring you to fit strips precisely inside a hole.

    Starting at the keel requires stealers at the sheer, which is easy, but the first strips are severely twisted. The strips reach the stem tops before the midships section is fully planked so partial-length strips are needed, These can follow the curve of earlier strips but the curvature can get extreme so gored planks are often used; these are curved on the lower edge. This construction is most common in wood-and-canvas canoes where the strips are hidden.

    For both the above you may need to use stealers anyway to straighten the strips so they bend easier. Stealers are also used to stop planks curving downwards which is considered unattractive.

    You can also build the football-shaped bottom panel flat and strip from that up to the sheer. This may be the easiest method of all but limits the hull shape as the panel is like ply, it will only bend one way. I did a stripper in which I used a plywood panel: dedicated strippers have put out a contract on me . . .

    Contrasting wedge-shaped strips should perhaps be tried out by painting a carved half-model to see what it looks like first. Full-length strips are less artistically demanding.
     
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