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Thread: Adhesives for scarf joints?

  1. #1
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    Default Adhesives for scarf joints?

    Since I haven't built a boat in some years, the adhesive front has changed- a lot. It seems like the only answer is 'epoxy'. That's fine, but I'm going to run into an issue, I think. I'm building a glued lapstrake pulling boat. At 18' long, each plank will have two scarf joints. In order to conserve very expensive marine plywood, I'm going rough-cut strips of plywood to match my spiling pattern, scarf them together, then do the final shaping after the glue sets. I keep reading that epoxy takes days to reach full strength. That's going to slow me down a lot, waiting for the scarfed pieces to cure before shaping and fitting them to the hull. I can do two planks (port and starboard) at a time. Then I'm going to be twiddling my thumbs waiting on the epoxy.

    In the past, I've used Weldwood Plastic Resin glue for scarf joints, and it worked well. It seems to be somewhat out of production now, though. Most vendors just say 'out of stock'. The only similar product I've found in reasonable quantities is called Cascamite, available on Amazon. Is this stuff equivalent? Like wise, resourcinol glues seem to be rare now days.

    Most people just scarf the sheets of plywood together, then get out the planks. According to my plans, I need five sheets of plywood, which I have. BUT- due to the sometimes crazy shapes planks wind up being, I'm worried about not having enough. That, plus just one screw-up would put me in jeopardy. I probably should have just ponied up for a spare sheet of plywood, but that would have meant eating cat food for a few more months.

  2. #2
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    Default Re: Adhesives for scarf joints?

    When building my HV13 in glued lap I wanted to conserve materials as much as possible. This was mostly for the challenge of it. Anyway, I would typically spile the plank using my jig then lay out where on a sheet of ply, or partial ply, to make my cuts. Then I'd scarf the lengths together. Most of the time the scarfs were cut on an angle to facilitate the overall shape of the plank. I used epoxy and left the glued up mess to cure overnight. In the morning, I'd hang the plank. The epoxy easily achieves enough strength to handle. One doesn't want to sand the curing epoxy or even scrape it much. But there isn't much of that necessary anyway.

    Jeff

  3. #3
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    Default Re: Adhesives for scarf joints?

    Your idea of gluing the individual planks, as opposed to the full sheet, is a better method. Not only it saves stock but you have the grain running closer to the run of the plank. There's a method to laying out scarfs on an angled plank which I would be happy to explain if you need.

    If you're gluing epoxy at a temperature of sixty degrees or above, you can probably work on the plank later in the day if you glue up first thing in the morning.

    Jim

  4. #4
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    Default Re: Adhesives for scarf joints?

    The only adhesive has been the only adhesive for over 50 years. It has not changed.
    I can go 3 times in one day with epoxy in the Caribbean , where it's warm. Sometimes in the PNW it takes 3 days to go one mix.
    Yachts are 'spensive.
    twiddling thumbs is free

  5. #5
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    Default Re: Adhesives for scarf joints?

    Quote Originally Posted by MushCreek View Post
    In order to conserve very expensive marine plywood, I'm going rough-cut strips of plywood to match my spiling pattern, scarf them together, then do the final shaping after the glue sets.
    I built my Eun Mara this way. The last thing I'd do during a session in the shop would be to glue up the scarfs, let them set overnight and come back the next day. System 3 makes fast, medium and slow hardeners. I use the slow when it is 70 or above or for something that needs a long open time like gluing up a birdsmouth mast. They say the fast has a 15 minute gel time, if you have everything ready to go getting two scarfs aligned and clamped in the window might just work.
    Steve

    If you would have a good boat, be a good guy when you build her - honest, careful, patient, strong.
    H.A. Calahan

  6. #6
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    Default Re: Adhesives for scarf joints?

    I never use system three for repairs... far too slow. Even their fast is like rubber on the second day . System Three should refer to three days .
    i built my boat of eye three, but if you wanna move tings along... use WEST .

  7. #7
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    Default Re: Adhesives for scarf joints?

    All epoxies I’ve used can reach close to full strength using an elevated temperature. A couple of throw sized electric blankets should work for your purpose. I typically use an electric blanket on a timer that gets the part temperature to around 100F and hold it for 3 hours and than let it cool to room temperature overnight.

  8. #8
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    Default Re: Adhesives for scarf joints?

    Another reason for doing individual planks is to prevent having to deal with a 20' sheet of plywood if I scarfed them first. The epoxy I ordered is on its way, so I'll soon find out how it is to work with. I learned what I know of boat building in the '60's and '70's, when epoxy was either unheard of, or rarely used. We should soon be in the 70's and 80's here, so even the slow epoxy I ordered should set up fast enough. Time will tell.

  9. #9
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    Default Re: Adhesives for scarf joints?

    Glue the scarfs on the boat.
    Use fast hardener if you dare.

  10. #10
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    Default Re: Adhesives for scarf joints?

    I did exactly as you described - scarfing and gluing individual strakes, not whole sheets. It was time consuming, but it came out well.
    In my experience, the epoxy doesn't neem to get anywhere near the several-day fully cured stage before you can continue working your long planks; hard-to-a-fingernail would be plenty cured to move forward with. And that should be easy to achieve in well under 12 hours with a hardener well chosen for the ambient temps.
    When I was actually doing it, I would do a glue up just before closing down for the night; it was always fine to work with by morning.
    -Leif

  11. #11
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    Default Re: Adhesives for scarf joints?

    If these are just patterns, why not just bolt the scarfs together and use them in 10 minutes?

  12. #12
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    Default Re: Adhesives for scarf joints?

    Resorcinol is still very much out there and is my glue of choice.

  13. #13
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    Default Re: Adhesives for scarf joints?

    I favor gluing the sheets with epoxy and then cutting the planks. You are then more likely to get accurate faying surfaces. Make cardboard models of the planks - 1-1/2" to 1' is a convenient scale if you're metric - and move them around. You might find that gluing two sheet length wise the ends of which are glued to th side of a third which has the same thing with two on the other side.

    Assuming 3/8" ply and 8:1 scarf, the whole thing is 7'-5/8" wide and 19'-1/4" long. Two car garage?

    When I had a problem like this I pounded some stakes with a framing atop to give a base for each seam.

    First plane and glue the two sets of two sheets glued the long way, then plane faying surfaces of the sides of the remaining sheet and the noses of the two sets of two sheets, and put it all together. It helps to have alignment blocks all over - figure them out with a dry fit - so all you have to do is drop the three units in place.

    I've used quart size plastic bags with sand inside as clamping weights. Space them close.

    G'luck

  14. #14
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    Default Re: Adhesives for scarf joints?

    My boat also took 2 scarfs per plank. Working alone it seemed much easier to scarf on the boat. Clamped with blocks on each side and a couple screws, which could stay on until cured. I could do a plank each side per day. This was System III slow hardener, but in CA summer heat it was set plenty in a day.

  15. #15
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    Default Re: Adhesives for scarf joints?

    At 70 degrees, West 105 will cure way more than enough overnight to work the next day. Even using 206 in the warm South Carolina spring and hot summer. Go for it, it is not a particularly steep learning curve.
    Steve Martinsen

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