Bridgedeck house top- unmatched laminate

Discussion in 'Multihulls' started by AndrewK, Oct 13, 2011.

  1. AndrewK
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    AndrewK Senior Member

    I understand that some Crowther designs used only 145/ 200gsm kevlar as the inner laminate and 600gsm glass top laminate over a 20mm core for a 40' bridgedeck catamaran.
    So when you are on the roof top the sandwich is using the high strength and modulus of kevlar that will be in tension and the top glass laminate in compression.

    How well would this sandwich work as a roof top of a modern 12m catamaran which is say 5x4m and relatively flat? too springy??
     
  2. waikikin
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    waikikin Senior Member

    Is there much framing involved for the likes of where headsail tracks land & in line of window mullions etc?, I think thats a pretty big panel & not just foot traffic to be considered, maybe a thicker core like 40mm & inner & outer laminates of greater than 850gsm would be more consistant with vessels I've been involved with. I think you should get your designer to give the advice. When you look at the connective stucture & underwing/ deck over same it looks a lot like a barge with hulls added on most bridgedeck cats. Regards from Jeff.
     
  3. AndrewK
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    AndrewK Senior Member

    My design specifies 600g inner and 800g outer glass laminate.
    Having a self tacking headsail only so no tracks on roof top, just foot traffic.
    Just wondering if inner laminate of 200g kevlar not just saved weight but also produced a stiffer panel due to its high tensile modulus.
     
  4. gonzo
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    gonzo Senior Member

    The stiffness will be determined more by the thickness of the laminate than by whether is kevlar or glass fibers. I mean for foot traffic, where the strength of the laminate is sufficient with either material.
     
  5. AndrewK
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    AndrewK Senior Member

    This is a sandwich with 20mm core, the overall thickness will be almost the same.
     
  6. waikikin
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    waikikin Senior Member

    May well have, Lock was a pretty cluey bloke, seemed to like 20mm foam too, but would agree to thicker cores to simplify structure & stiffeners, I dont think you should just reapply & exchange engineering from one specific vessel to another, the weight loss would be under 15kg I think(considering resin v fabric & specific gravity of Kevlar/glass), on a 40' cat that aint much & your current designer must have been happy with whats on his plans.
    Regards from Jeff
     
  7. catsketcher
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    catsketcher Senior Member

    I think this laminate will be too soft for a flat roof. However if you are clever you ca slways make some stiffeners that do other things as well. Some suggestions

    Don't use typical genoa cars - they are expensive and need holes in the laminate - which in turn create stress concentrations. In my experience genoa cars never get moved on cats after you get the position correct - you lead the sheet outboard for reaching instead of forward as on a mono.

    I put PVC pipe on the deck, glassed it with two layers of 600 db, and then stitched uni tow around the pipe and then around an internal stiffener inside the cabin. Then a layer of 600 DB and sand it up. No squeaks, no stress concentrations, increased stiffness of cabin and hardly any money compared to tracks.

    Put as much camber as you can in the deck anyway - compound curvature is stiff.

    Of you have solar panel mounts etc - embed them and make them as long as possible to help stiffen the roof.

    Don't bolt anything on the roof. Make composite turning sheave boxes, put a large shackle with tow running through a hole and then spread out on the inside to cope with tension fittings. There is no need to go bolts any more.

    My friend who had such a laminate had to install extra stiffeners everywhere to stiffen his Windspeed 35 deck - although such a laminate may be fine at first it will soften more quickly than a rugged one as it is much nearer its microcracking limit. I would go 750 triax on top and 600 Biax on the bottom for a good and long term stiffness top.

    cheers

    Phil
     
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  8. AndrewK
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    AndrewK Senior Member

    Jeff, my friends boat has the same style roof only a bit wider and used the same above glass laminates and 20mm core. This is nice and stiff under foot traffic.
    I am willing to deviate from the design if it makes it both lighter and stiffer without costing too much.
     
  9. AndrewK
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    AndrewK Senior Member

    Phil, thanks for the advice and feedback regarding the Windspeed 35.
    My default laminate is 750g triax on top and 600g DB inner as thats what I have plenty off.
    Andrew
     
  10. Samnz
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    Samnz Senior Member

    My tris cabintop has unsupported area 1.8m x 2m. 15mm h80 foam, 300gm carbon on inside and 400gm carbon on outside. Its very bendy. I wont let more than one person on it at a time... im putting, "no step" stickers on it too....
     
  11. marshmat
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    marshmat Senior Member

    Andrew: I don't have time to run the calcs right now (have to catch a bus in 10 min). Rough guess, the laminate you describe in post #1 is very, very light for a 4 x 5 metre panel. It'll keep you dry but would flex dramatically if you have people walking on it (would probably be OK with just one person on top, or with small people). Pick a core with a high shear modulus. If it's going to be relatively flat, some stiffener beams on the underside would be called for.
     

  12. AndrewK
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    AndrewK Senior Member

    I havent had internet connection for a while, thanks for all replies.
    I will be using the all glass laminate but when I can find the bit of kevlar that I have I will make up a small test piece as I am curious to see how the two laminates compare.
     
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