Structural Interior Furniture Options

Discussion in 'Materials' started by KnockedDown, Dec 5, 2020.

  1. KnockedDown
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    KnockedDown Junior Member

    Hull and deck are balsa cored.

    Maybe I'm terribly confused (or using imprecise wording). My understanding... The bulkheads run latitudinally partway across the boat. They serve as attachment points for seats and storage lockers. The chainplate bulkhead also provides attachment points for the shrouds. Structurally, the bulkheads keep the hull shape true and the sides from coming inward and the deck rising as shroud tension is increased which would result in lower headstay tension and decreased racing performance. I believe they provide very little longitudinal support directly but if incorrect, would appreciate a clarification.

    These boats are a bit flexible. I've heard accounts of rudders binding against the hull with enough backstay tension. So to help prevent this, one could run a structural member from the headstay stem fitting to the backstay attachment. But this is impractical. So I'd have thought moving that structure lower in the boat and adding a pair since they have to be off-center, while not as effective, would still help with stiffening the boat fore and aft. I believe this is the purpose of the unbroken line of plywood and tube that runs from just aft of the bow to just forward of the rudder post. Another way to say this is the lower 4" of the panels could be tabbed to the hull and the rest of the panel cut away except the top 4" creating a "stinger" along the hull and a beam 14" above it. Alternatively, I could make panels 1, 2 and 3 out of one piece of material (ply or composite) and lay it along the inside edges of bulkheads 2, 3 and 4 though this introduces some aesthetic challenges.

    My goal is to replace the plywood sections of that unbroken line with something that is at least as strong but lighter enough to make the effort worthwhile.
     
  2. fallguy
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    fallguy Senior Member

    I'd say if the boat is known to move that much to replace the furniture similarily to what was original with lighter materials.

    A seat or bench or locker is similar to an I-beam, or two i-beams. It creates a path for loads to be transferred; not just on one area. While it is likely a stringer or stiffener could accomplish the same; the i-beam might work better unless the top is simply a lid. If you remove the areas with the red strikethrough; it is a bit weaker and you lose the locker. If you do some calculations; you'll likely discover the ply was heavier than foam. And if the seats are rarely used; you could easily switch it all to 12mm foam.

    Do the weight analysis. It it pretty easy. Use all the glass weight times two for resin hand laid. Use something like 80% for vaccuum as the tapes would still be 100%.

    Build a spreadsheet and compare using different options and you'll probably drop the weight by half and create a more permanent solution.

    B78777CE-3EF4-4201-B70B-915987198196.png
     
  3. gonzo
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    gonzo Senior Member

    Ultralight boats are very carefully engineered. There was a lot of time and expense that went into the design. The fact that it lasted 39 years is good proof of good design. Rot is caused by neglect, so there was no structural failure. Why do you want to change the design? The new structural reinforcements should be engineered to match or better the originals.
     
  4. KnockedDown
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    KnockedDown Junior Member

    Terrific. Yes, the weight calcs are relatively straight-forward. The engineered characteristics not so much (at least for me). Will make up a few test panels using versions of the layup proposed and test the results.
     
  5. KnockedDown
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    KnockedDown Junior Member

    This is the key question. I'm actually not looking to change the design. As suggested, I'm looking to find better (meaning stiffer and/or lighter) materials to replace the 35lbs/ft^3 ply that should come out because it was neglected, looks ugly and has likely lost some its strength.
     
  6. fallguy
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    fallguy Senior Member

    Be aware when you test the panel you are not testing for flexural rigidity in the plane, but against the plane. Most of the strength will be developed. That is, when you add things. The raw panel will not be acted upon in its thickness.

    If you have a vertical 14" high panel by 18" long; flexing the panel is invalid as a test. The forces acting on the panel in the hull will be against the 14" dimension which will be supported by taping to the hull and the twist in the panel will be offset by the horizontal section and the tapes bonding that together.

    You are creating a system; not a singular panel working alone.

    For semantics, I offer you a corecell I-beam. This I beam is an ultralight, ultra thin method of making a span of 30" not flex under walking loads. Each piece alone is useless, but combined and a piece of glass and it becomes incredibly strong.

    Your challenge will be keeping the weight down. Calculate using 20 ounce 1208 tapes versus 25 ounce 1708 tapes; it is a savings of 10 ounces per yard..
    E47E7C3A-08B9-4238-B50F-804D827FC878.jpeg
     
  7. fallguy
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    fallguy Senior Member

    I agree with the heart of the post, but using 9mm ply is not an ultralight solution, generally. A 4x8 9mm ply panel with 6oz glass is 60 pounds. A 4x8 corecell panel with 24oz glass each side is 28 pounds, vac bagged at 30% resin reduction is 24.8 pounds. 6 oz glass is not really considered waterproof and would typically require more resin at about 14 ounces per panel for weave filling; call it another pound or two..(this points to the failure, btw).

    It might require a bit more tape for the core panels, but not 30 pounds

    And honestly, a corecell panel with that much glass is used for decks. Or walking loads. So, using a 24 ounce glass on each side would be major overkill.
     
  8. rxcomposite
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    rxcomposite Senior Member

    Lightweight high performance boat uses the interior furnitures/cabinets as part of the structure. From the sketch, the furniture forms part or is the longitudinal stiffeners of the boat.

    Since the original is 9 mm ply, you can substitute it with a 16 or 20mm core with 2 layers of glass, minimum 1 mm thick. Connection to the balsa cored hull must be solid, all edges filled with bog and reinforced with 2 layers of biax on the hull and bulkhead sides.

    I will use PVC core like DIAB. Corecell is tough but a little flexible. Not a good match for a balsa core substrate.
     
    Last edited: Dec 6, 2020
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  9. fallguy
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    fallguy Senior Member

    Rx has a lot more knowledge and experience than me. Pay close attention to his words.
     
  10. gonzo
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    gonzo Senior Member

    I think that it is impossible to decide without an engineering analysis. The bulkheads may have large point loads which a for which a foam core laminate would be inadequate. Also, the OP indicated that the plywood was not laminated over.
     
  11. gonzo
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    gonzo Senior Member

    I think that it is impossible to decide without an engineering analysis. The bulkheads may have large point loads which a fo
    Stiffer is not necessarily better. Rotted plywood is de
    The OP indicated the bulkheads were plain plywood without laminate. A foam core panel may not be adequate if they are point loads or large shear stresses. At the end, he may spend thousands of dollars for a questionable structure that will save less than 200 lb.
     
  12. gonzo
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    gonzo Senior Member

    More stiffness is not necessary better. If the original design was successful, a stiffer panel may cause stress riser and a structural failure. The difference in weight is minimal, particularly compared to the cost and complication.
     
  13. Rumars
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    Rumars Senior Member

    Just use Okoume. Today most ply manufacturers will say the density to be 500kg/cum+-40, wich translates to a range from 28.7 - 33.7lbs/cuft. Seal the edges with epoxy, paint or varnish the rest. There is no reason to glass the ply, if you want an increase in stiffness just use thicker ply, 10mm or 12mm.
     
  14. rxcomposite
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    rxcomposite Senior Member

    Be forewarned. Replacing the original furniture with cored panels will be challenging. It will be slightly lighter but there will be more work and planning.

    That is probably the reason they used marine plywood for the interior. It is cheaper and easy to fabricate.
     

  15. Rumars
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    Rumars Senior Member

    I don't think he can save any weight with composite panels, not without a full optimisation. A panel like you described, 1mm glass each side of 16mm 80kg foam is ~5.6kg/sqm. 9mm Okoume ply at 540kg/cum is 4.86kg/sqm.
     
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