Foam core sandwich question

Discussion in 'Fiberglass and Composite Boat Building' started by jorgepease, Aug 26, 2016.

  1. jorgepease
    Joined: Feb 2012
    Posts: 1,614
    Likes: 47, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    Hi, can someone answer this

    In the top illustration is a foam core panel with laminate skins each side.

    In the bottom, is a foam core panel with skins interrupted by another foam core running perpendicular.

    Is the rigidity of the panel running left to right affected by the interruption of the inside skin?

    Thanks

    [​IMG]
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    If the transition from the one plane to the other is well filleted and tabbed, things will be fine. Typically this would be a two step laminate, with the hull shell having its internal partitions and bulkheads, installed after it's skinned. This means you wouldn't have a foam on foam abutment. If for some reason you did have this abutment, the inside corners would need fillets or foam ramps, so the fabric can lay down neatly.
     
  3. jorgepease
    Joined: Feb 2012
    Posts: 1,614
    Likes: 47, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    Thanks for the answer!
     
  4. abourgault
    Joined: Jul 2014
    Posts: 22
    Likes: 1, Points: 3, Legacy Rep: 10
    Location: canada

    abourgault Junior Member

    I would say it depends on how it is loaded.

    To illustrate that, imagine the vertical portion is fixed and the load induce a upward bending on the top horizontal laminate. The skin under that top laminate will get in tension and could delaminate from the vertical foam. If that happen, the capacity of the laminate will be essentially the capacity of the foam.
     
  5. jorgepease
    Joined: Feb 2012
    Posts: 1,614
    Likes: 47, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    It's a hull with a bulkhead so all types of forces but not as extreme as your example, the bulkhead would move with the hull.

    I would probably throw a 12" wide layer of glass between bulkhead and hull so it ties in, no big deal on that but I would def be using a foam chamfer on the bulkhead to get a nice strong radius.
     
  6. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    If the transition zone is appropriately reinforced (tabbing), the loading wouldn't be a concern, until the laminate sheared/peeled from the core or started to break down.
     
  7. redreuben
    Joined: Jan 2009
    Posts: 1,693
    Likes: 85, Points: 48, Legacy Rep: 349
    Location: Beaconsfield Western Australia

    redreuben redreuben

    I've built a lot of hulls for a variety of boats and never installed a bulkhead like that. I'm not saying it can't be done successfully but I've never seen it.
    Perhaps now with resin infusion it is an acceptable practice ?
     
  8. jorgepease
    Joined: Feb 2012
    Posts: 1,614
    Likes: 47, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    I don't think it's done on infusions of size because when a vacuum is pulled it would compact the outer skins and pull the hull cores in each direction, outwards, towards the walls of the mold and away from the bulkheads.

    In my application, a small dinghy, I won't be using a mold.
     
  9. upchurchmr
    Joined: Feb 2011
    Posts: 2,944
    Likes: 113, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Not having a complete inner skin will definitely be weaker than having one, but a small dingy might not matter.

    IMHO
     
  10. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    i asked the same question a while back, forget where of of whom i asked it, but i received a completely different response to PAR`s. Word was it wasnt good enough - hardly a concern for a dingy, but what if this were a 50 ft boat?

    To my dead reckoning, if the inner skin were put in tension, the strength of the entire panel would be limited to the shear strength of the foam no?
     
  11. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Typically, with perpendicular abutments, like bulkheads and partitions, the inner skin remains intact, as the abutment is installed after the inner skin is applied. If the core layout has these abutments in place, before the inner skin is applied, you can run into issues, unless you bulk up the transition area at the abutment (tabbing), so the core doesn't get compressed. In high load applications, the core would be removed in this contact area and the outer laminate heavily bulked up, then the abutment would be installed with tabbing, further reinforcing the area.
     
  12. upchurchmr
    Joined: Feb 2011
    Posts: 2,944
    Likes: 113, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Groper,

    I believe you are correct.

    PAR,

    I don't understand what you are talking about - abutments.
    It would help to have a sketch.

    Seems that you are saying the inner skin should be complete first, then the bulkhead bonded in.
    That is not what the OP asked. He had a break in the inner skin (fiberglass).
     
  13. jorgepease
    Joined: Feb 2012
    Posts: 1,614
    Likes: 47, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    In my application I am sliding a piece of glass between the bulkhead and wall because it's easy to do.

    I was curious though because I understand that a panel get's it stiffness from being skinned both sides, the perpendicular wall with tabbing and reinforcement both sides of the abutment would seem to keep that stiffness intact regardless of the change in direction, especially since at the other end you got the same treatment.

    On the other hand, it would still seem that an non interupted skin, followed by the same treatment would still be stronger but I can also see where it wouldn't make much difference because to flex in that area would mean to flex the opposing fillets glass of the abutment and that doesn't seem like it's going to be any easier to do than to flex an uninterupted skin
     

  14. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    It can be done a few ways, which I describe. If the inner skin is intact, the abutment is tabbed in place. If the inner skin isn't in place, the core can land on the perpendicular core, but tabbing needs to be heavier. Conversely, the area where the inner skin is missing can be filled with laminate (common in high load situations) and the abutment can be tabbed in afterward, normally.

    Yes, without the inner skin the core will be vulnerable to crushing, which is why the tabbing needs to be heavier than normal. In high load areas, it's wise to eliminate the core and reinforce it, which can be more laminate, G-10, plywood, etc.
     
Loading...
Similar Threads
  1. fallguy
    Replies:
    4
    Views:
    801
  2. sailingdaniel
    Replies:
    17
    Views:
    3,875
  3. teneicm
    Replies:
    7
    Views:
    334
  4. SwedenYachtsDaniel
    Replies:
    7
    Views:
    521
  5. AJP
    Replies:
    11
    Views:
    497
  6. Heinrich Poigner
    Replies:
    22
    Views:
    2,544
  7. Smj1
    Replies:
    12
    Views:
    1,047
  8. guzzis3
    Replies:
    31
    Views:
    2,664
  9. Steve W
    Replies:
    2
    Views:
    1,136
  10. rivergybsy
    Replies:
    15
    Views:
    2,791
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.