Hardtop: Attaching alloy composite panels to alloy frame?

Discussion in 'Boat Design' started by rustybarge, Oct 26, 2013.

  1. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Hi All,

    My 'Cat in a shipping container' project is underway. I've bought the 25' by 7'8" hull moulding from Cheetah cats in the UK, it's a commercial cat design used by fishermen, patrol boats, harbour masters etc. The bare hull weighs in at just under a ton.
    http://www.cheetahmarine.co.uk/en/

    Options are twin 40hp OB's for a 15kts cruise, 20kts max, or a single 90hp which is what i'm going with(25kts max), to save 20% on fuel burn. Hoping to buy the new Suzuki leanburn when I've saved up the £9000 price tag!

    Here's the 40mm box section frame, 3mm wall thickness:

    [​IMG]

    and bolted into the hull:
    [​IMG][/URL]

    [​IMG][/URL]

    The design brief is weight, weight, weight: max 300kg for hardtop, windscreen, side-screens, aft wheelhouse door......

    So after much consideration i've chosen these composite Aluminium plastic sandwich panels, that weigh in at 10kg for a 8' by 4' sheet:
    http://www.aalco.co.uk/datasheets/Aluminium-Alloy_Aalcopanel-Composite-Panels_258.ashx
    http://www.aalco.co.uk/_downloads/literature/aalco-panel.pdf

    So that makes the weight for the frame and roof panels : 95kg

    How do I attach the aalco alloy panels to the frame?
    All suggestions welcome......:D
     
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Glue and a few rivets would be best/easiest.

    BTW...depending upon what loads you may get, you need to be careful of racking of your frame. If you pushed the top end, what is there resisting the frame moving...just those small connections on the bulwark/side of hull. If you can put in a diagonal of sorts, would stiffen up considerably, especially in a strong breeze and occasional green seas on top...and where someone stands up falls grabs the post and hey presto...eccentric loading!
     
  3. michael pierzga
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    michael pierzga Senior Member

    Will you be able to bend the stiff composite panel to the shape of the top ?
     
  4. PAR
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    PAR Yacht Designer/Builder

    The panels will bend to that curve except at the very forward portion, where you'll likely have to slit it or maybe a few pie wedges to get it to lay down on the frame. A pretty costly way to go. An 1/8" (3 mm) plywood panel, sandwiching a 1" foam core, would weigh the same at much less cost.
     
  5. michael pierzga
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    michael pierzga Senior Member

    Will the mechanical fasteners dimple the panel ? And how will you trim the perimeter ? After 5 years of service will the panel be dented and distorted by physical abuse ?

    That alloy panel looks difficult to work with. You might investigate a different material.

    Another detail is that the panel is thin. A thick panel is advantageous because utilities such as electric cables can be routed inside the panel rather than surface mounted. Additionally local reinforcements for any hardware can be burried inside the panel
     
  6. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    I will be raising the sheer-line by about 1' on top of each gunnel, so I can bolt plywood to the uprights to give extra rigidity to stop racking.
    the aalco panels will be too heavy, adding another 40kg.

    would a 1/8" ply bolted to the uprights be strong enough?

    I chose the alloy/plastic sandwich because it has the same stiffness as 2.5mm sheet of aluminium, but is 70% lighter.
    hoping it will be strong enough for one person to walk on.
     
  7. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    i considered plywood foam core, but by the time you finish with epoxy/ matt it would nearly double the weight. the aalco panels are already painted, covered with a plastic membrane, so I felt they would be the lightest solution at 22lbs/sheet.

    weight is my obsession! (diet is next::rolleyes:)

    if you look at the spec's they advise machining a groove in the panel to bend it.

    i was thinking of using a angle grinder to cut grooves?
     
  8. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    it's only a gentle 4" bend over 4'.....hopefully.


    They sell these panels as hoarding for use on building sites, painted one side.
    only €38 for 8'by4'. much cheaper than marine ply.....
     
  9. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    The sandwich material is a polythene type plastic(3mm), so should be very strong. they mould small boats in the same plastic, used for tenders on hire boats. they get the hell nocked out of them, and survive.

    Do i bond to frames with epoxy, and pop rivets?
    would a butt joint on top of the frame 40mm, be enough?
    larger overlaps might be stronger, butt joint with a strap, or overlap?
    Will bond crack with the movement in the frame?
    flexible mastic type 'construction' adhesive better than epoxy?
     
  10. michael pierzga
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    michael pierzga Senior Member

    Go for it ! Its cheap, light and strong.

    Keep the painted side on the inside. Sanding and painting inside surfaces is very time consuming. Perhaps remove any mechanical fasteners after the adhesive has cure, fill the holes and paint. Use a flexible adhesive . Perhaps sika flex type construction adhesive

    Perhaps use a full length handrails each side. Mechanically fasten this hand rail thru the panel into the longitudinal. Leave the fasteners in. This area is were the load with be and any adhesive disbonding would be trouble
     
  11. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    the boat is going to be a mini-livaboard, so i'll need to insulate the inside surface of the roof.

    sika-flex is very exensive.
    what do you think of this stuff....http://www.tec7.ie/products/x-tack7

    I like your idea of handrails to stiffen the frame.:)
     
  12. michael pierzga
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    michael pierzga Senior Member

    That adhesive should do the trick. To make a strong bond the surface has to be correctly prepared.

    A hand rail could also be a alloy L. or T section that also acts as a rain gutter to direct water
     
  13. rustybarge
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    rustybarge Cheetah 25' Powercat.

    I'll have look for some alloy hand-railing used in office buildings.......but it might be too heavy.
     
  14. michael pierzga
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    michael pierzga Senior Member

    When you have a large surface area like a roof it gets used for storage. Boat hooks, boarding planks.....the mounting for these can also form part of the roofs structure
     

  15. michael pierzga
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    michael pierzga Senior Member

    Also ..dont join panels On a frame. The frame acts like a cantilever, stresses the joint and produces a hard spot that prints thru the profile. Keep the joint between frames
     
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