I need to stiffen the flybridge deck/cabin ceiling

Discussion in 'Boat Design' started by Skua, Jul 22, 2015.

  1. Skua
    Joined: Apr 2013
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    Skua Senior Member

    It's a tad trampoline like. It's 94 inches across and 64 inches long in the span that needs attention. In the back there is a vertical frame that forms some seating on the fbridge, and in front of the curve, it is formed with several joints and curves. It's that middle span that is the bugaboo. The seat mount is more or less centered in the floppy spot. (circle of bolts in first pic).

    I laid a 1x3 made up straight edge across so you can see the curve in the panel. There is a little core damage from leakage at the seat mount, this will be cut out and replaced. The coring is made of a green semi coarse, foam circa 1986. I drilled several holes to check for water, and found none, but included a pic of the coring style as best as I could get.

    One idea I came up with was a series of 3 beams made up from 2x4's cut to follow the contour and laminated in place, then supported by beams that transfer some of the load to the frame/rib you see in the 4th pic. The 3 frames/ribs per side are intended to support the gunnels/ toerail, and provide attachment method for the interior walls. They where not originally intended to have anything to do with the ceiling.However the overall headroom in the cabin is a lackluster 6'3" to begin with and the beam method would eat 2-3 inches more. As well it's rather inelegant and the beams across the windows downright inconvenient. Would a layer, or 2 of 1808 0-90 add any appreciable stiffness?? I see no need to recore the deck, and that's not really a consideration at this time.
     

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  2. messabout
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    messabout Senior Member

    Without further information, I would be suspicious of the sides where the top is attached. The top has some curvature. When the middle of that curve is depressed by the weight of the chair and its occupant, the curve tends to straighten and the chord length becomes longer. That means that the sides will be deflected outward unless you added some bow strings across the chord.

    Ninety four inches across constitutes a beam that would need to be fairly robust in order to firmly support the concentrated load of the chair. A few layers of glass might help but I would not expect it to help much. The generality for beam strength is that it varies as a function of the third power of depth. Adding a quarter inch of glass thickness would matter but that is a lot of glass to apply upside down.
     
  3. Skua
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    Skua Senior Member

    I had considered a post, but in the middle of the cabin, that is less than ideal.

    The bridge deck/ cap is one piece, that is bolted to the window frames using #12 machine screws , which in turn are bolted to the gunnels using #12 screws as well. There is a 1 1/2 x2 board between the cap and the window frames. The entry bulkhead, also, is screwed to the cap and to the window frames I think some of the issue is that the laminate appears to be mostly a very resin rich CSM on the bottom.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Cutting the beams from 2x4's wouldn't be advisable across that distance. A laminate would be stiffer. You can rip the edge off the 2x4, say 3/16" or 1/4" thick, stacking them up in place with epoxy, to match your roof crown. I agree that the edges of the roof are where your real problems lie. If the crown is flattening out, it has to be moving the edges.
     
  5. alan white
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    alan white Senior Member

    Drill more holes. Be absolutely sure you have no more saturation. Remove any hardware and see if holes are jacketed with resin. They should all be sealed with a ring of epoxy to prevent future problems.
    Then, drill a few holes in a circle around any hardware a couple of inches out. Make sure all is dry.
    I suspect the oil-canning is an indication of much more damage than you are aware of. If constant flexing can occur, even sound, dry areas can internally shear and weaken.
     
  6. Skua
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    Location: Hunt's Pier WW NJ

    Skua Senior Member

    No doubt. There are stress cracks in the gunnel/window area, at the entry bulkhead, gunnel interface. Sometime in the past, the lower frames under the walking deck had ruptured from top to bottom, likely due to stress from the deck,and pounding through the surf with all the flexing. Lasted 28 years without a real issue, but looking for some type of remediation for the bridge flex. Cables with turnbuckes would work to hold the crown, but thats a little too much like a government bridge repair for me. It's not like there is a huge deflection, only evidence is gelcoat cracking, laminate is intact. I'm fixing all of the issues I can, and this is one of the last on the list.
     
  7. Skua
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    Skua Senior Member

    PAR would you advise making a buck to develop the laminate curve, or apply the layers
    individually to the ceiling and build it up. You know while typing this I think I answered my own question. There would be no real way to get a satisfactory joint applying the layers to the ceiling.

    However, once in place, I think a layer or two of the 0-90 to reinforce the bonding of the beams may be sufficient.
     
  8. Skua
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    Skua Senior Member

    This is not a Bert or Viking. The hardware was drilled through the laminate and simply laid in place with caulk. All of the repairs and mods I'm doing are overdrilled and filled with epoxy, then remounted. I think basic engineering is the problem, with not enough support for the load. It had a sticker stating that the max load was 700 lb on the flybridge. LoL. Aside from the entry bulkhead the was no support for anything above the rubrail from the factory. Even the foredeck has some bounce to it. I have added a forward walk through bulkhead at the vberth entrance.
     
  9. PAR
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    PAR Yacht Designer/Builder

    You could build a "buck" for the roof crown, but I'd laminate in place on the underside of the roof. I'd do this in two stages, the first over some packaging tape, so the laminate stack can't stick. This will let you over bend it a bit with some shims and you can dress up the sides and faces on a workbench, before bonding them in place on the roof.
     
  10. Skua
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    Skua Senior Member

    Sounds like a plan. Thanks.
     
  11. rambat
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    rambat Member at large

    top deck

    I recently had this issue on a large MY, the deck needed to be strengthened for a CL mast. The deck core was compromised, even replaced, but such secondary bonding never provides the original stiffness. As Messabout mentioned its a problem in modulus, beam depth is the best way to increase stiffness. Like you we had a height issue so instead of a 6" deep beam I chose a number of shorter vertices spot welded together, it worked great. I used steel box tubes rolled to the deck camber that landed on a angle placed along the top of the P&S headers. Use well coated steel, or galvanised even SS. Steel gives you confidence when you are being whipped around in the flybridge chair that it will hold.

    A attempt to fix this by adding any fiberglass onto old fiberglass is called a cold bond. The new laminate will always have the potential of splitting away from the old surface. You end up "gluing" your laminated beams to the deck with resin if you tab it to the deck. Poly resin is not the best adhesive so if you make laminated beams glue them to the underside with 5200. Otherwise do steel and sleep well.
     

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  12. Skua
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    Skua Senior Member

    Interesting idea. Might be cost overkill for a 28ft'er
     
  13. rasorinc
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    rasorinc Senior Member

    Could you epoxy a layer of marine ply over the top say 3/8" or 1/2", even 5/8" then fiberglass over. Then inside install a layer of 1/4" ply, stain and finish. This would give you the stiffness needed and equal weight distribution but does not address the side issures. Just
    another idea for you that may be easer to do. Does not lower headroom much.
     
  14. Skua
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    Skua Senior Member

    I had thought of that, but I didn't think that a layer of plywood would really stiffen up that much. With the internal beams, I can attach them to the perimeter header board. I am thinking of an alternating layer, laminate beam of selected grain DF and epoyglass in between. I am thinking with the grain structure across oriented up and down , and the glass thread 0-90 across, this should result in a beam of minimal weight with great stiffness, Also allows me to attach my ceiling panels and insulation, easier.
     

  15. PAR
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    PAR Yacht Designer/Builder

    You have a moderately stiff 'glass roof. You could core it, simply applying some foam and skinning it with more fabric. Athwart beams would be nice, but I don't think all that necessary. I think perimeter beams would be more helpful for the same work and weight. You just need to make it stiffer, so the edges don't splay out, when crew are on it and the crown is depressed. This is the real trick to panel construction, controlling the edges. This is why taped seam construction works so well. The middle of the panel has to remain in place, if the edges are restrained.

    Cored construction is a different philosophy and basically makes the whole panel, uniformly rigid. Both engineering methods work, so all you need to decide is which you want to play with. Overhead goo work isn't pleasant, even on a good day, so consider it carefully.
     
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