Reducing floating cockpit deflection.

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Nov 2, 2018.

  1. fallguy
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    fallguy Senior Member

    Building a floating cat cockpit. About 100" wide and 100" long.

    There will be two panels joined in the middle on the 100" seam. So, each panel will be 50"x100" for handling. The join will be done with 316 stainless angle.

    The panels will be assembled using 12mm corecell M and a laminate schedule like 800g triax.

    Tophats will stiffen the undersides.

    But I am wondering if I will get an improvement in stiffness if I add a layer of 6mm core and/or if an extra layer of laminate in between the two cores would be better still.

    Any experienced advice appreciated. Thanks.
     
  2. JamesG123
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    JamesG123 Senior Member

    Are these panels ever going to be taken apart? Why have the angle iron between them?
     
  3. fallguy
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    fallguy Senior Member

    Yes, the boat is demountable. Sorry for the omission.

    I expect to need some silencing. The panels will be screwed in from above into 316 angle with nuts welded. We will use a flat headed machine screw to hold the panels down when assembled.

    I am planning the laminate three panels 33" wide by 100" long each and glassing together using tape seams. The top seams would be rebated 2mm.

    Just want to reduce deflection. There will be a helm seat and riser sitting on it as well.

    Like I said, I have 12mm and 6mm corecell available. I would like it to be light enough to handle without too much headache, so like 100# max per panel with stiffeners and thixo edges, etc.
     
  4. groper
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    groper Senior Member

    Its far too thin...
    Stiffness is proportional to sandwich thickness cubed. I built a floor similar width in my cat- the core was 20mm thick and was just adequate to span 1200mm with triax eglass laminates.

    Putting glass in the middle of the sandwich does nothing but add weight - dont do it.

    If you glue both your 12mm foam together for a 24mm thickness core- you will be sweet. You also will not need to build top hat stiffeners either.

    All the best mate :)
     
  5. fallguy
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    fallguy Senior Member

    Thanks Groper.

    I need the tophats for conduits; so they are dual purpose.

    I just don't want the cockpit to be too flimsy. The tophats sort of go diagonally across each 50x100 panel.

    I might have enough 6mm material to double them up so the stack would be 12-6-6.

    I can't really stagger the joints as my vac table is 49"x400". I thought about modifying the table, but it seems like a lot of work to avoid a few hand laid seams. Any thoughts?

    I was just going to build three 33x100 panels and rebate the long sides and tape seam and butter them on a butt join. Then I would cut them at 50" and edgefill them all the way around :(
     
  6. fallguy
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    fallguy Senior Member

    If you know you need a solid edge for hardware would it be better to purchase or build a solid glass pultrusion or could I leave hi density edge exposed?
     
  7. JamesG123
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    JamesG123 Senior Member

    How do you have a solid edge if your foam is exposed?
     
  8. fallguy
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    fallguy Senior Member

    Well you can use a fly cutter and defoam and refill with thickened epoxy, but there must be a way to do it in the laminating process. I just don't know what it is...

    I need fasteners on the edges of the floating cockpit all around and not sure if edges of hd core can be used for the fastenings and also left edge exposed to avoid the decoring or if some other solution is better; like laminating in solid glass.

    So, you can decore and refill or fill with hd core (and leave edges exposed?) or use a 12mm solid glass insert? What would be best here?

    I'm half telling and half asking!
     
  9. fallguy
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    fallguy Senior Member

    And the reason I am asking is if I build three 33"x100" panels; any solid glass insert would be about a 50mm strip 33" long in the center of the 100" and another say 25mm strip on the sides.

    I need a screw every 16" or so around the panel edges.

    If I laminate a solid glass edge; I think I could avoid overboring each hole and filling with thickened and avoid edge filling.

    Has anyone seem it done that way?
     
  10. groper
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    groper Senior Member

    Hi Fallguy,

    First - theres a million ways of skinning a cat (pun intended).

    Your original question was how to reduce deflection - the answer is to increase core thickness. Core thickness is THE most effective way of increasing stiffness of any of your sandwich panels - id avoid the top hats entirely if i were you which will cost you more time and effort in the long run. You can place the conduits within the core before you lamainte the panels if you need them for cabling etc by laying in a PVC electrical conduit and buttering up the edges with bog and pushing it together so the conduit is surrounded by bog which then also glues the foam sheets together at the same time. Then youll end up with clean flat and smooth panels which are easy to fair and paint afterwards rather than dealing with lots of edges and coves which are time consuming to prepare for paint with the top hats - its just more work you dont need. Every cove or seam you have in boat costs you time and money to fair and paint - avoid as many of them as possible (speaking from experience here). Will you pull out all the cabling/piping etc when you dismantle/unbolt it everytime?

    When your joining foam core sheets together to make a larger panel - you dont need to go crazy with glue or seams. Just a light butt glue is all that required inside the sandwich. I glued my foam cores together with a moisture cure PU adhesive available in a calking tube designed for structural timber gluing. Apply it to the edge of the foam, butt them together and hold in place with some staples from a staple gun until cured. Then proceed with laminating. Alternatively - you can just allow some thickened resin between the sheets if your doing a wet layup buttering the edges as you lay them on the go.

    Outside the sandwich on the skins - yes you can rebate the core to allow for tape seams which will speed up your fairing as the finished seam is close to flush rather than raised up and needs lots of feathering to make it disappear.

    Edge treatment.
    Yes you can run a router around the edge with a special rebating bit to decore the edge and backfill with thickened resin or bog. It doesnt take all that much time to do this once you have the correct size router bits. Alternatively you can use a high density material around the edges of the foam core when your laminating your panels. You can use all sorts of materials here such as rigid PVC or ABS sheet plastic which matches your core thickness. Cut up a few strips to run around the edges. The the resin will bond to it, just give it rough up and hit it with a gas flame prior to laminnting. You cannot use Polyetheylene type plastics tho or the resin will not bond to it. You could also use G10 or other premade glass products/extrusions or make your own etc. You could even use aluminium extrusions or flat bar in the edges if you wanted to all incorporated into the sandwich as your laying it up. At the end of the day you can use many things that provide a hard edge which resists compression for the fasteners you plan on holding it together with. Choice is yours and the biggest variable is cost.
     
  11. fallguy
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    fallguy Senior Member

    I can't insert the conduits in the laminate. I am using hydraulic steering and those cables need to go through my tophats cuz they are a bit big for a 24mm laminate. And while the idea of laminating a chunk of pvc sounds easy; it won't be as stiff as a tophat.

    However; it is an intriquing idea for my roof. I am using 12mm scrim and unsure about running wires. The obvious issue is a 12mm space is a pretty small tube/chase.

    RW leaves some of this artsy stuff out of the plans.

    How did you run wires on the rooftop? I want some interior cabin lighting and want solar panels up top and want a spotlight on the front top of the cabin. I suppose I could find a 12mm od tube or just run a wire chase on the exterior.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    This is the simplest way:

    But the question remains, how much?

    Would be a good idea to put a value on how much you would like or expect as a maximum deflection you wish to experience on the centre of the panel. The design to suit.
    Or just over engineer it - simple :cool:
     
  13. fallguy
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    fallguy Senior Member

    331D0448-7562-4F61-A10D-6D5761D17422.jpeg
    Here is a sketch.

    It'd be good to limit deflection to 1/4". A bit nervous the helm riser will squeak with even 1/8". Might need a rubber base no matter what.
     
  14. groper
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    groper Senior Member

    You CAN and the hydraulic lines will fit thru :) i used 3/8in dia lines for my twin 115hp outboard engines and it was fine...
    You can also consider running them elsewhere - bacause if you disassemble this and have to disconnect the hydraulic lines and bleed the system everytime, isnt it becoming kind of non demountable in the process???
    You can hide hydraulic lines and cables behind cowlings or other non structural items if need be, those things are easily solved. Focus on the main structural problem first.

    Not necessarily - it depends on thickness of the core of your floor panel, skin modulus, the section modulus of your top hats, and the spans between stiffeners(tophats or other) and edge supports. These can all be calculated btw.

    without calculating it - which we can do later - i can say via dead reckoning that the 24mm floor will be much stiffer than your original design using 12mm + the top hats.
    The reason is partly due to the geometry of the layout- the worst deflection will be in the area farthest from any supported edge and tophat -ie towards the rear in the center of your drawing. The panel span there is quite large and you dont have any stiffeners there - the top hats are way out near the edges back there. Your angle irons will also not add much stiffness unless they are very deep and large in section (+4in)- which im assuming they are not due to weight issues? You have not included any transverse supports in your drawing which means you have an unsupported span of 100inches athwartship. This is very large and difficult to design for without exceeedingly thick core material. I think you need to put some more thought into this design or consider revising :)

    With a 24mm core throughout - you will have less deflection everywhere-not just near the stiffeners or edges and springy panel in between like when you use too thin a cored panel on a widely spaced support grid.

    I used 20mm core with eglass lamaintes for my cockpit floor and the max span i designed for was about 1200mm (between supports) on that or the floor would start to feel springy. I ran a beam down the center which cut the floor span in half to acheive the required maximum span support of 1200mm. The transverse bulkheads above the floor and structural furniture panels completed a support grid for the entire floor so that no unsupported floor span exceeded 1200mm in any direction. I only made this center beam to add stiffness to my floor or i would not have included it. You need to design your floor so that you dont have any unsupported span greater than your panel stiffness provides, so that you can acheive your deflection goals. Thicker core = more distance between supports :)

    upload_2018-11-5_18-59-15.png
     

  15. groper
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    groper Senior Member

    Long time no see Adhoc - hows life mate? Ive been on a sabatical from BD.net - busy sailing yachts from the mediterean to the USA for the last year, 14,000NM later, a little wiser, and now back home and back to the grind :( take me back!
    upload_2018-11-5_19-28-3.png
     
    Last edited: Nov 5, 2018
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