Help with thicknesses please!

Discussion in 'Fiberglass and Composite Boat Building' started by mr mills, Dec 6, 2008.

  1. mr mills
    Joined: Dec 2008
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    Location: u.k

    mr mills New Member

    Hi Guys

    Im hoping that some of you maybe able to help me with something. I am a student in the UK studying transport design and for my final project i am designing the nect generation caravan. Now you may wonder why i am here for (i've not got lost...honest!) im planning on constructing the body of the caravan like a boat hull, so effectively it will be an upside down boat!

    There is an australian caravan that has been made this way using GRP skins and a foam core. Unfortunately i cant find any technical info on this caravan and i need to know what sort of thickness of foam and GRP skins im going to have to use.

    I've done a some reading into boat hull construction and i think i know how to calculate roughtly what sort of weight the lay up will weigh if i know teh ticknesses of the core and skins. But the thing is im really not sure what sort of thickness of foam and fibreglass skins i need? Considering that currently caravans have approx 30-40mm walls made up of aluminium skin with rockwool insulation.

    Am i right in thinking that i should use WR matting rather than CSM due to less resin needed? Therefore less weight? And i would use Divinycell H from DIAB as the foam core ( not sure what sort of density i would need though?)

    I know that there is probably a massive difference in what is required for a caravan compared to a boat but if somebody could please give me a rough idea of what sort of thicknesses it would be greatly appreciated!

    Thanks for your time, Mr Mills
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    I believe with a caravan you need a somewhat different way of thinking than with boats allthough the math is same. It's more convenient to size core thickness according the thermal isolation requirements. The strength requirements in specific areas then demands the density of the core material and the skins.
    Anyways thats how I'd start...

    my 5 c
  3. wet feet
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    wet feet Senior Member

    I suspect that if you use WR on either side of a 30-40 mm structural foam core you will have a very strong and well insulated caravan.It is also likely to be quite a costly product.It might be a good idea to investigate window fitting conventions before going too far with the concept unless the project is intended to remain within the realms of design with no construction taking place.My guess,and it is purely a guess,would be that 12mm foam might be a good starting point with not much more than 600gsm on either side.Good luck with the project.
  4. Eddy Johansen
    Joined: Dec 2008
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    Eddy Johansen Hydrosport Sportboats

    I tend to agree with wet feet when it comes to structural stiffness, but the problem with such light skins is you can almost press your finger through it.

    If you want a structure with an impact resistance and stiffness close to the deck of a, say 5m closed bow sportboat, I would go for something like this:

    300gsm mat for less printing
    2x 300/500gsm combimat mat/rowing
    Diab H80 - 10mm
    2x 300/500gsm combimat mat/rowing
    Perhaps 300gsm for smoother surface
    Topcoat or inside paint or inside finishing panels

    Total thickness of this lay-up should be in the neighborhood of 18mm.

    This gives you some pretty strong panels that will hold many square feets without flexing too much. Depending on the size of panels, if pretty big like 40-50 sq ft or more without structural backing support, perhaps the Diab H80 20, 30 or even 40mm would be recomended.

    I am sure your local Diab supplier (or supplier of any other brand core foam) should be able to help you further.

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    FAST FRED Senior Member

    The biggest hassle you would have to actually construct it is the size and heft of the mold.

    AS the human eye is fantastic at seeing differences , getting a straight enough sides to look good would require a monster mold.

    Boats curve in both planes so a slight error wont be noticed as easily as something close to flat , that may look like wiggle walls.

    Modular construction would allow the same mold to create 20ft to 45 ft Motorhomes or trailers , and repair after a bit of damage.


  6. CML UK Ltd
    Joined: Dec 2008
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    CML UK Ltd GRP & FRP Plugs Mouldings

    If it's a concept with no restrictions on cost then 12mm H80 infusions core with West's pro-set infusion resin would give you a light, thin and extremely strong strong shell. However researching the caravan market they are built cheap and are crap quality. Looking at one for research i was shocked at how poorly it was built.

    Check out

    This is a futuristic caravan commissioned by the Museum of Contemporary Art in Sydney.

    I think i might have got a bit carried away researching this....
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