Aluminium hull scantling optimisation with

Discussion in 'Materials' started by Stefano Dilena, Jul 13, 2021.

  1. Stefano Dilena
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    Stefano Dilena Junior Member

    Hi,
    Some Aluminium hulls are fitted inside with spray foam/ insulation and then glassed over the stringers in the interiors to make it more liveable. So the interior fit out starts with some sort of laminating the inside hull.
    An example is the Tim Mumby catamaran

    Are the structural strength provided by the interior glass layer and foam "core" taken into account for the scantling calculations, because in effect with such a construction a sandwich panel is created.
    It seems to me that scantling of the hull is done prior to applying the insulation and fit-out, which will go there anyway ( Who would keep a bare aluminium hull in the interiors?)

    I have read on other topics that when you build an aluminium boat you are effectively building two boats, one inside the other one.

    I am working on an aluminium design and the idea would be :
    - Weld the hull using stringers and bulkheads of minimal thickness, say 25/30mm max, but strong enough to support the structure until laminated;
    - Use minimal hull thickness above waterline (2-3mm) and adeguate thickness below waterline and on the bows for impact resistance.
    - Insert core and laminate / gelcoat making sure proper bonding is achieved between hull/core and core/glass.

    Please don't go hard on me as I am not a professional in the field..
    I am working with a naval architect on this project, but I just want to hear more opinions on this.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Trouble is that come the day you need to weld to make repairs, your boat will start to smoke and burn. with that foam
     
  3. Stefano Dilena
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    Stefano Dilena Junior Member

    This
    Surely there must be a reasonable way of doing repairs on a boat such as the one above.. I guess the damaged part should be cut out with "cold" tools and foam/ glass removed to an appropriate distance to the intended weld.
    The thing is, an aluminium hull should be much less prone to damage than other hull types, so repairs should be seldom.

    The question is wether the glass layer applied inside can be take into consideration for structural calculations or not, since it will be there at the end of the game..
    Of course this implies that the boat shouldn't be sailed until the interior laminating is completed.
     
  4. Alik
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    Alik Senior Member

    Technically, I see no problem to calculate it as sandwich.

    However, foam has to be of certian grade, to comply ISO12215-5:2019 requirements to foam core. Foam core shear and compression strength are checked versus size of craft and design load.
    Another problem could be shear between alu layer and core - how bonding is done.
     
  5. Stefano Dilena
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    Stefano Dilena Junior Member

    Another way to think this of would be as "building the aluminium hull and use it as a mould for laminating foam and glass inside"

    We could go as far as using vacuum bagging core and glass onto the aluminium hull, using appropriate foam core as @Alik suggests.
    Foam panels could be cut to size and simply inserted between the stringers, glass on top and vacuum bag.
    Cable conduits could be integrated in the sandwich.
     
  6. Stefano Dilena
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    Stefano Dilena Junior Member

    Thanks for posting the relevant standard for this.. much appreciated.

    I see already on the market some Aluminium/ foam sandwich panels. the problem is welding those panels with the foam already inside them is impossible.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    And when you say "foam", what are you referring to ? Spray foam ? That will look rough as guts
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    In any case, in the event of hull damage, weld repairs would be greatly complicated
     
  9. Alik
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    Alik Senior Member

    Modern boats are designed for replacement, not repairs ;)
     
  10. Stefano Dilena
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    Stefano Dilena Junior Member

    Spray foam was used to illustrate the concept, but as @Alik pointed out, a suitable foam would have to be selected. PVC foam cut to size between the stringers could be an option.

    Below waterline/ floor level the hull can be left uninsulated and appropriate scantling applied. Same at the bow, with a water-tight uninsulated compartment.
    These are the most damage prone areas, and not applying foam there will facilitate any repairs in the future.
     
  11. Rumars
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    Rumars Senior Member

    While technically possible it does not really make sense. When you calculate the panel you will get a very thin required Al skin, something under 1mm. That's because of the thick core required for insulation and Al mechanical properties. This presents you with the problem of welding it to itself and the frame structure, wich is the principal reason 3mm is normally the minimum thickness used in boatyards. To avoid the need for welding you can go to adhesive bonding or riveting, but if you go there the core is not required anymore, you can just decrease frame spacing and achieve the same effect for less money and trouble.
    This is all a question of the realities of building, cost, workhours, and available technology. Welding very thin Al with minimal distortion is certainly possible, but pretty uncommon, bonding foam is not difficult, you just have to wetsand the epoxy into the Al.
     
  12. Stefano Dilena
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    Stefano Dilena Junior Member

    Thanks for the reply, and certainly this is true.
    I agree with the fact 6-7cm of foam core is a lot and would give a very thin hull requirement, but it could also mean that keeping a 3mm hull, the distance between stiffeners and stringers could be tripled? ( not necessarily a true linear relationship, but you get the idea).

    Point is, insulation and interior lining will be part of the final structure, so we might as well make a good bond and take this into account during scantling.
     

  13. Rumars
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    Rumars Senior Member

    If you keep the 3mm Al outside skin with a thick core, there is no need for any stiffeners at all. Even with less Al they would not be necessary (except for bulkheads and local point reinforcement). Fiberglass boats have been buildt that way before, by injecting PU foam between two shells (Etap for example, but there were others with very thin layups).
    If you want to laminate in place over the foam you have to do a lot of fairing to achieve a good interior surface. The foam has to be structural, wich means double the weight of typical insulation foam. If you build the liner separately in a mold to cut fairing hours you get other problems, like sculpting the foam to fit the liner, and a lot of putty to glue it in place. It's all doable but in the end you will not have gained any weight advantage over the usual way, and increased the cost considerably (more expensive materials, more manhours).

    If you really want to loose weight you need to go to thin skin, glued, with lots of omega stiffeners made out of the same thin plate, also glued. Over it you put the lightest insulation you can find (for the desired insulation value, a heavier foam can have better properties and require less thickness, thereby ending lighter) then build the interior conventionally with lightweight panels.
     
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