Stringer Bedding, cushion or gap spacing

Discussion in 'Hydrodynamics and Aerodynamics' started by member74761, Aug 16, 2021.

  1. member74761
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    member74761 Junior Member

    10/4
    *not sure what xps is*
    I have several options for sure.

    thanks much
     
  2. ondarvr
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    ondarvr Senior Member

    What we're talking about is 99% cosmetics, structurally it has little effect. We're assuming the laminate will be strong enough to do what it's supposed to do, if it's not, then it will fail with any of the methods discussed.

    The pic fallguy provided does an excellent job of showing point loading or a hard spot being created no matter what the the end product is or where it's located.
     
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  3. ondarvr
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    ondarvr Senior Member

    Ex
    Extruded polystyrene foam, it's the pink stuff you see at the big box store. And EPS, the white stuff cheap coolers are made from. Both are dissolved by polyester resin so you need to use epoxy when using EPS or XPS.
     
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  4. TANSL
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    TANSL Senior Member

    Now everything is clear. Many thanks.;)
     
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  5. Ad Hoc
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    Ad Hoc Naval Architect

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  6. member74761
    Joined: Aug 2021
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    member74761 Junior Member

    Guys I appreciate all the helpful info.
    I really appreciate understanding or getting the right understanding of what a hardspot is. (I had that ALL wrong )
    I don't trust xps or eps as in my mind enough pounding and those could distort and not reform and xps I wouldn't put in a boat anyway.
    So I learned several things on here in the last few days.

    Thx again.
     
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  7. ondarvr
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    ondarvr Senior Member

    Don't get confused by thinking using 5200 would help make it stronger.

    5200 has so much elongation that the laminate will fail long before the 5200 is subjected to much of a load.

    Once the laminate fails the 5200 doesn't have the strength to hold the stringer in place, so it fails too.

    And again, XPS or EPS is just the spacer and shape, they offer no additional strength and aren't required to. They become irrelevant once the stringer is laminated in place, almost every other material becomes irrelevant in the same way.
     
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  8. member74761
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    member74761 Junior Member

    That was not my thoughts, I was thinking of unloading the imposed preloading of curing/cured epoxy. and maybe a cushion for temperature changes.

    since the spacers and everything else is pretty much irrelevant. AND they provide O strength for adhesion.

    I figure I will take my chances with spacing and PB, this provides plenty strength esp. in my case using all epoxy. And I use milled and chopped fiber in my pb.
    Then do 2-3 layers tabbing and a wrap over the stringers. interlocked between tabbing. tying everything together.
    With all that I think I will be better than good.
    And I'm taking everything slow and steady.
    I feel much better at this point.

    I think I have a plan now.
    Thanks again
     
  9. fallguy
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    fallguy Senior Member

    You don't need to use milled/chopped in stringer bedding pb. All it does is add weight. The only time I use it is on an edge I want to return to square like transom bottom or top, or if I have an area where I don't want shrinkage. The stuff is heavy and harder to make smooth fillets. The amount of shrink bedding stringers or bhs is irrelevant because it is all tabbed to size.

    I've not seen a good comprehensive source on types of pb. These are conclusions I arrived at on my own with destructive sampling.
     
  10. member74761
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    member74761 Junior Member

    with all respect.
    I always use "something" be it milled fibers or chopped strand in pb(always), 1 or the other or both in pb regardless what I use it for.
    I realize the chopped fibers make smoothing difficult at best(been there done that) and will pick how I apply or whether or not that I use any.
    May use it in center only and contour the corners without CSF and only M.fibers.
    I have experimented with PB and it is brittle and easily broken after cure. 3 different kinds of epoxy.
    Add some fiber of some sort and it changes to a strength that I find acceptable. (I have played with this a bunch)
    I know what a bag of this material weighs and how much of it that I use per gram of pb.
    I typically use 5-10% depending, my batches all match unless I'm building bulk.
    I'll leave something on the dock to make up for the weight added. lol.
    The weight of it I'll accept for the strength that is gained.
    I do appreciate all your help and information.
     
  11. fallguy
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    fallguy Senior Member

    Engineering properties are all that matter.

    Today, I noticed a riser I painted a few weeks ago had four screw holes that 'rose' from the surface about 0.050". Disappointing, I can tell these were screwholes filled with cabosil n epoxy. Heated up to maybe 110F in the space today...

    Maybe adding fibers would result in less change.
     

  12. Scuff
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    Scuff Senior Member

    AdHoc is the crestomer compatible with epoxy? I see you aren't using the pad to spread the load like in Fallguys diagram .. does the adhesive perform a similar function as well as filling the gap between bulkhead and hull? Thanks for sharing that.
     
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