Pre-gluing stringers in stitch and glue panels

Discussion in 'Multihulls' started by rayaldridge, Jun 13, 2011.

  1. PAR
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    PAR Yacht Designer/Builder

    It all boils down to the amount of curve you have to put into the panel with the stringer(s) on them and the thicknesses of the plywood and stringers. From what I'm hearing, you're probably be fine. Heavily curved panels wouldn't take kindly to this treatment, but you seem to have fairly soft radii to conform to, so should be okay.

    Personally, I don't do this because it takes some of the compound out of the panel as it's curved. (okay, technically it's not compound, but it looks like it) Some of you may have noticed that when you bend plywood over a decreasing radius, it tends to cup and actually seems to compound a little. This is commonly seen when bending on bottom planks on a V bottom hull. The forward sections will have considerable roundness at station 1 through 4. In fact, the forward bulkheads need to have curved not straight lines from keel to chine for this reason. If a stringer is glued on first, you'll lose some, in not all of this and the hull shape could change. How much, well who knows, but you could make a convex set of forward water lines concave if you're not careful about the stringers. This is why I don't "preload" the panels with stringers or other stuff. I want the plywood to take a "natural set" before I lock it down along the seams and bulkhead lands. I want the stringers to fit the hull, not the other way around.
     
  2. rayaldridge
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    rayaldridge Senior Member

    Par, very interesting post.

    These are dory hulls with flat bottoms, so no bilge panels. The builkheads from chine to gunwale are slightly convex, to introduce a bit of compounding into the topside panels. Anyway, I think you're probably right, and that I should, at least the first time I build these hulls, see how they look stitched up before I add stringers.
     
  3. bruceb
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    bruceb Senior Member

    pre built panels

    The Buc has compound curves built in to all of its bulkheads- the full size frames are drawn with a curve built in, I have no idea how Crowther arrived at the proper amount, but they work and really stiffen the panels as they are bent. Most panels in the boat have center and edge stringers that are pre-glued before bending to the frames. I replaced several panels on my floats with no problems. B
     
  4. Randy@bb.o
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    Randy@bb.o New Member

    Watch for obtuse angles...

    Ray, I have pre-glued stringers on relatively wide 18' kayaks with good success. For long skinny craft with light stringers, I would not expect this approach to cause problems.

    I would look over the section drawings for the boat. If the angle between the deck and the topsides is larger than 90 degrees, you will want to hold the sheer clamp above the edge of the hull panel to allow for shaping later. This can happen if there is much crown in the deck, even if the topsides are flared. For flat bottom boats, this issue will come up at the chine as well.

    Pregluing the stringers does make it a little trickier to shape the stringers where they come together at the stems in a pointy-ended boat. You can't dry fit the stringers independently of the panels. But this approach has worked for me in the past.

    Randy
    www.boatbuilder.org
     
  5. ThomD
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    ThomD Senior Member

    The gougeons on boat building show this strategy being used for making hulls. They staple the stringers to the panel, clamp the ends, and bend a curve in to add the final little bit of clamping pressure. They use this technique for flat panels and tortured ones. This is pretty efficient build-time wise and also preloads the stinger in a way that is conducive to resisting water loads between the bulkheads. If you were building a global element like a beam, or keel (some cases), your best bang would be to lay it up flat, and then place it bent, or better still pre-laminate the stringer curved, and make it flat during the build out, by starting with the center high, and pressing it out. This increases the capability of the stringer to take a global load transfer deep through the panel side, rather than just being a stiffener. Of course for any of that to happen, you have to have all the parts in the correct proportion.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Again, it all depends on the hull shape and amount of preload the stringer will have to accept in the bending process. On canoes, some multi hulls and kayaks, also some skiff like hull forms, the curves are "soft" enough this would be possible. On the other hand I can think of lots of places this wouldn't be possible.
     
  7. gagepants
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    gagepants Junior Member

    I just did exactly what Rayaldridge is asking about and thought that the final product came out with a really nice smooth curve to it, and not all that much stress built in. Mine is a 20 foot ama for a 24 foot trimaran. The ama is about 26" at the widest point, and the only tough part was torturing a little at the transom. Measure twice, cut first.
     
  8. ThomD
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    ThomD Senior Member

    If you do it the Gougeon way there isn't much pre-load at all. The result should be the same as when inserting stringers in the hull after the sides were bent on.
     
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  9. PAR
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    PAR Yacht Designer/Builder

    Typically, ama shapes aren't especially severe, so it should work well, though again it's all about shape and radius. Try this with a V bottom runabout and you'll end up with suction creating concave sections, rather then the convex shapes that were intended.
     
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