Building a stitch & glue boat in Fiji

Discussion in 'Boatbuilding' started by Saqa, Oct 20, 2013.

  1. PAR
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    PAR Yacht Designer/Builder

    The reason you tape the seams first is because without it, the boat isn't structurally sound at that point of the build. The seams have to be filleted and taped to make the boat a rigid structure. Once the internals and other refinements are installed, it's stiff enough to consider rolling it over and applying a sheathing (if desired). The sheathing isn't absolutely necessary (on some designs), but is usually highly recommended on plywood hulls, because of the protection it provides. Plywood end grain is very vulnerable to moisture, so special attention is necessary in these areas if you don't have a sheathing over the whole hull.

    Technically, you could apply the sheathing and seam tape in one shot, but you generally use different materials. Biax on the seams, if the plywood is anything heavier then 1/4" (6 mm). A typical hull sheath is 6 - 10 ounce (200 - 350 GSM) regular cloth, which is a whole lot easier to fair than biax.

    As far as bulking up epoxy, you'll need to learn about this, as it'll save you tons of work. Fillets and bond lines will use thickened epoxy, sometimes under tape. There are several materials used for these procedures. The last thing you want is a bunch of unthickened epoxy, that needs to be sanded down. This is just a waste of epoxy and it adds weight, without any appreciable strength.

    You'll be better off if you use the dry method of applying fabrics (tape or sheathings). This method has you apply the fabric or tape, dry over the area you want, then you wet it out with straight epoxy. This is much less messy and easier to control, then the wet method you've described. All surfaces that will get fabric, should already have had some epoxy applied previously. Plywood does soak up epoxy, though it's not as bad on some hardwoods as other woods. If the plywood will get a sheathing or tape, the area should have at least 2 previous coats, before the fabric goes on. If these previous coats are still tacky, it'll help hold the fabric in place as you apply yet another coat of straight epoxy, to wet out the fabric.
     
  2. Saqa
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    Saqa Senior Member

    Will be interesting to see how my next stages come together!

    Btw, I have never used epoxy on fg fabric. Does it wick? I use a bar heater that has elements behind my rod lathe. It warms the blank and nylon thread wraps. When I apply epoxy with a stiff brush it just wicks into the thread. I keep the rod stationary and let any excess drip and wick stubborn bits back off with the brush. In that medium I found it to be the best way to avoid bubbles for a crystal clear finish. Will warming the ply and fg fabric help turn the resin less viscous and wick it in? and drip off the exess?

    I did pick up some 200gsm plain tape too. Thought it might come in handy to do narrow edges like the transom top and gunwales and such.
     
  3. Saqa
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    Saqa Senior Member

    if 50/50 is the best then why do fishing rod blank manufacturers use extremely high pressures to squeeze out as much of the resin as possible while the fabric is on the mandrel and the resin curing?
     
  4. michael pierzga
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    michael pierzga Senior Member

    Well...you are perfectly welcome to vacuum bag your skiff and drive down resin content.

    The 50 percent rule is what is generally considered good craftsmanship when doing a hand layup.

    This ratio is for the resin content of the eglass only and does not consider waste or absorption into the plywood.

    Resin added after cure is not considered structural .
     
  5. michael pierzga
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    michael pierzga Senior Member

    And that 12oz fabric you bought will wet out beautifully....

    Thin resin is not desirable in that gravity makes it run downhill , out of your fabric, and it ends up coating your shoes.

    Epoxy saturated shoes are difficult to walk in....your friends will laugh at you.
     
  6. groper
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    groper Senior Member

    Definitely dont warm the plywood before laminating... it causes it to outgas and will get bubbles and pinholes everywhere... better to do your laminations over timber as the temperature is dropping such as after 3-4pm in the late afternoon.

    Did you get some cabosil and cotton flock to make the fillets?
     
  7. upchurchmr
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    upchurchmr Senior Member

    Saqa,

    The best you can do for strength is about 64% cloth, 36% resin.
    That is with vacumn first to set the bag down, then pressure to squeeze it to a minimum, but you also have to have some place for the squeezed resin to go, its called bleeder cloth.
    All that is more than you want to pay for in tooling for minimum benefit in a home build.

    PAR was giving you the practical limit to shoot for in the home shop. Many of us don't get as good as that and add unnecessary weight and cost.
     
  8. Saqa
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    Saqa Senior Member

    Epoxy coated crocs are the worst!

    I had thought about pre glassing the ply plates with vac bag before stitching but couldn't find decent plastic sheets. Would be too hard for me to manage with the boat shape though. I'll try to aim for 50/50 or better when pupping the glass down

    I can get the fillers here. I am now also setup to make wood dust as fine as flour.

    So? Does fg fabric wick resin?
     
  9. groper
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    groper Senior Member

    Yes, ofcourse it does... and so does the plywood - some species alot more than others.

    So you need to wetout the plywood first with epoxy, most recommend 2 coats to ensure its properly waterproofed... then whilst its still tacky, apply more epoxy and directly rollout your dry fibreglass fabric over this, which will immediately suck up some of the epoxy you just put down. Then brush or roll on some more epoxy over the top which completes the wetout. Apply your peel ply at this point if choosing to use it.

    At this point you can let it cure and then lightly key it or just wait a little until it sets up, then apply you first fill coat via a trowel or squeegee or fairing batten etc - epoxy thickened with micro balloons or q-cells etc to fill the weave left by the fabric and feather out the thickness difference left by the tape. If you do all this concurrently, you havnt needed to sand anything yet.
     
  10. upchurchmr
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    upchurchmr Senior Member

    You don't need to prewet or precoat the plywood, except where there is exposed endgrain. The flat surface of a panel doesn't suck up hardly any resin.

    Pygmy kayaks and others suggest you spread the cloth over the boat then pour epoxy onto the glass, using a squeege to move the epoxy around and completely wet out the cloth. If you squeege the cloth (not really hard) you will get the epoxy saturated, the cloth / epoxy looking like the cloth looked (not slick with too much epoxy) and the minimum resin used. The when it hardens use the fairing compound the same day to fill the "weave" of the cloth, before you start sanding. You don't want to cut into the glass when you are sanding or you just wasted some of the strength you paid for.

    I have built 4 kayaks with this method and never had a problem. Some of them were strip planked with Cedar, which sucks up more epoxy than any plywood I have used.

    The epoxy goes right thru to the wood and sticks better than the strength of the wood.

    If you apply epoxy to bare wood then place the cloth, you will certainly have to add more epoxy, as Groper said. Doing this makes it hard to drape the cloth unless you have many hands. Easier to not epoxy the wood first.

    Take your pick.
     
  11. Saqa
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    Saqa Senior Member

    k, thanks for the tips fellas
     
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    It's very unlikely a hand laid 'glass job will get anything close to a 50/50 resin/fiber ratio. You'll be lucky to get 60/40 and you'll have areas that are 70/30, with most of the laminate between these ranges.

    You're correct Saga, you want a low resin/finer ratio for strength, stiffness and weight, which is why fishing rod makes squeeze the crap out of their rods. Boat builders do too, if they have the ability. For example, if you use vacuum bagging techniques and pull a 15 pound PSI (1.05 KG/Cent 2) vacuum, this is 2,160 pounds (10546 Kg/M 2) of pressure per square foot of hull surface! That's a lot of squeezing. With autoclave infusion, you can get 70 - 75% fiber ratios, but this is well past what you'll be doing so a moot point.

    I wouldn't bother with vacuum bagging anything on your little boat. Vacuum bagging is a great way to find out how easy it is to crush things you didn't anticipate. I watched a guy destroy a well built mold this way on his first vacuum bagging experience. Nothing on your boat is so critical that you need to go to these extremes with laminates.

    Dig out your West System "User's Guide" and your System Three "Epoxy Book" and go over procedures again. Read and re-read the hand layup sections until you're pretty sure you have a grip on it.
     
  13. michael pierzga
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    michael pierzga Senior Member

    Precoating end grain is always best. Components like the transom perimeter, bulkheads and frames.


    Precoating the ply is handy for some situations when you have to laminate in awkward locations.

    Precoating ply before assembly is recommended for panels that will not be laminated...interior components like seats, boxes. Saves time when they are on the workbench and keeps you from working on your hands and knees in the bildge.

    A typical flat panel precoat goes best when you first apply the epoxy randomly in a pool. Then with your squeegee spread the epoxy over the surface in a thin coat. Let the epoxy soak in for 5min then apply a normal roller coat. This helps prevent outgassing bubbles and runs in your finish
     
  14. Saqa
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    Saqa Senior Member

    One more thing regarding prep. I use an acrylic/acetone based sanding sealer on timber when doing furniture work. This helps seal the surface and prevents bubbles. Any advantages in using this stuff? Its usually sprayed on and works as a sanding primer
     

  15. groper
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    groper Senior Member

    Sounds great as a cosmetic tool.... but think about what you are doing... the best adhesion you will get, structurally, is whatever bond strength is achieved between that sealer and the epoxy... so no, its not a good idea.

    When you use tape over plywood, the epoxy gets right into the timber fibres, and joins them to the glass fibres layed over the top via the resin matrix. The fibres are welded together. You want everything consolidated in 1 waterproof and long lasting matrix with not chance of adhesion issues which could lead to glass delamination and water intrusion at a later date.

    All the soft and easy sanding type "high build primers" are appled after youve completed the structural stuff. Its all just cosmetic stuff and makes light work of getting a nice finish at the end.

    You will need to paint this boat afterwards, because epoxy is generally not UV stable and degrades over time, weakens and turns yellow etc.
     
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