I understand the concept for not allowing stringers to touch the inner hull, but...

Discussion in 'Fiberglass and Composite Boat Building' started by Skua, Jun 5, 2013.

  1. Skua
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    Skua Senior Member

    frames and bulkheads to touch the inner hull. But if these componets are then bedded in epoxy/poly/vinyl, then filetted in, does not the same problem arise? Or are the adhesives, considered flexible enough not to hardspot?? I have some hard cured epoxies, and they are considerably harder than wood, or FRP.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    And what problem is that...your point is not clear? :confused:
     
  3. Skua
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    Skua Senior Member

    Hardspotting on the hull
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Then i think you're going to have to define what your definition of "hard stopping" is and how this is effects a frame to hull joint, in any material.
     
  5. Skua
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    Skua Senior Member

    On a FRP hull allowing wood stringers to contact the hull is concidered bad form due to flexibility difference, and the resulting cracking of the hull. But after allowing a standoff of 1/4 inch or so manufactures, they then fill the gap with epoxy/poly or vinyl, and fillet. Tabbinbg of the sides of the bulkheads, is common but then they fill and fillet to the bottom of the hull again. If the wood is concidered too hard to allow it to touch the inner hull, I don't understand how the epoxy doesnt do the same thing. Why tab one are of a part then direct bond in another? Or is this simply a method of preventing a small stress riser from acting, say a burr, on the hull ?
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    So, the 1st Q to ask your self (to understand this), is the wood being used bare (glued direct to hull nowt else), or is glass laid over it?
     
  7. Skua
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    Skua Senior Member

    In all the examples I've seen it's stringers, epoxied to hull then laminated over. same with where the bulkheads/frames are laminted after bedding, but then only tabbed to the hull sides.
    My reason for understanding this, is I'm in the process of redoing some stringers, and the transom, and adding another stringer where currently, there isn't one,
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Ok...what I am trying to get you to appreciate is what does the joint do?

    To transmit out of plane loads from one surface to another. This being the hull to the frame/stiffener/WTB vertical side. The problem is, is that the joint must be flexible and is of equal importance as its strength. The dichotomy is the strength (thick over laminate) and flexibility (thin over laminate) are contrary to each other. So, you need to find a "happy medium". The trouble is, every build is different. The amount of over laminate, the fillet radius, the materials, the properties of the materials and of course the quality of the finished joint. All these affect the strength and flexibility. Therefore, you can only answer this by doing tests of your layup with variations to see how to get the strength and flexibility that is desired.

    No two boats are the same in this regard.
     
  9. Skua
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    Skua Senior Member

    I will attempt to replicate the original design. Just trying to figure out the apparent contradiction
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

  11. PAR
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    PAR Yacht Designer/Builder

    Skua, it might be easier to look at load paths for an understanding. If a "hard point" exists, the potential for a stress riser is created, whereas a load traveling along a panel quite suddenly encounters a new, much denser path. The result is a likely break or crack, just before the dramatic increase in density.

    [​IMG]

    Hijacking John's previously posted laminate schedule drawing, for a perpendicular panel joint, you can see the tabbing gradually "ramps" up the density of the laminate, as it approaches the perpendicular element and just as importantly, gradually decreases this density, as the load path travels away from the perpendicular element. This "transition" from one panel density to the perpendicular and then back, allow the loads to be absorbed and transmitted, along predictable avenues. With this, you can engineer a structure for the anticipated paths, without surprise "avenues of escape".
     
  12. michael pierzga
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    michael pierzga Senior Member

    To avoid hard spots, the component is bedded into epoxy bog. Then the bulkhead or structural component is epoxy filleted and tabbed in with 45 x 45 cloth .

    When useing several layers of tabbing reinforment, this tabbing is tapered , 2 inch offset typical or as described in your construction plan. . this spreads the load and avoids the hard spot .

    Just as the above pic shows.

    In the picture above I would be happier if the bulkhead edges had been relieved, radiused.
     
  13. tunnels

    tunnels Previous Member

    I would also like it if the bulk head was sitting 5 mm away from a 10mm wide 3 layered glass pad and then do all your glass tabbing to hold it in place a 5mm space of filled with HD foam core and a radius each side is a much better option the any thing else . if you used that method in a race boat you'd be in a little strife not only with print through but possible breaking the hull where the bulkhead is POINT LOADING !!!:eek:
     
  14. PAR
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    PAR Yacht Designer/Builder

    The drawing above clear shows an elastic structural adhesive used as bedding (Crestomer 1152PA), which is far better then a blob of epoxy or other resin. There's absolutely nothing wrong with the method shown in the drawing. Both you and Micheal should learn to read the drawings, rather than attempt to understand the physics, which you clearly don't get.
     

  15. tunnels

    tunnels Previous Member

    after being in the boating industry for a long time and been round the block a few times and attended boat shows and walk by beautiful boats in deep blue colours and seen every bulkhead location and every stringer or shelf that's been glassed into and attached in that method you describes and same as the drawing and on cored topside and solid bottoms they always showing the internals .
    In high speed water craft bulk heads are kept away from any part of the hull by a good 15 mm to 20mm sometimes even more AND a pad is usually used beneath where the bulk head could possibly strike the hull side or bottom .:p
    There's a perfect example on boat-test a while back where all the wooden structure is suspended within the whole of the boat 12 mm away from all and any panels anywhere , and nothing was used between . Don't remember which brand of boat it was , but have seen the video a few times and told others to watch and learn :D
    Its a good drawing but almost right !! :mad:
     
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