Hard Top Build (Gluing foam to aluminum)

Discussion in 'Boat Design' started by Capt. Bill11, Jan 4, 2011.

  1. Capt. Bill11
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    Capt. Bill11 Junior Member

    Looking at building a hard top here in Costa Rica.

    One idea I had was to build the frame/skeleton of the top out of aluminum square tube welded together to form a grid. And then gluing foam to the top, bottom, sides, shape it, skin it with fiberglass, fair, prime and paint it. The wire chases would be PVC pipe.

    So my questions are,

    Is it a stupid idea?

    If not what kind of adhesive would be best to use to attach the foam to the aluminum grid?

    Is there a better material to make the grid out of?

    What kind of foam board would be best to use for something like this?

    How would one go about calculating the maximum cantilever that a structure like that could span?

    Thanks
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Sikaflex is best all round adhesive.

    You need to be very carefull glueing foam to ally. You need to make sure the foam is not hygroscopic and/or fully painted or very well protected. Since if the foam becomes wet and is touching bare ally, you'll get Poultice corrosion.

    To work out the structure is straight fwd. All you need to know is the span of the member, the load breadth and then what laod to appy, ie one perosn standing on it, 2 or more and any others, that is it.
     
  3. couch
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    couch Junior Member

    Not sure ... are you planning on leaving the aluminum in place or using it for a temporary support? A composite panel will give you more strength with less weight (subject to design & materials of course) so why waste $ & time on the al if you are going to cover it up? Plus, unless the al is actually loaded (bearing / contact surfaces) it will not serve any structural purpose and only add weight / cost / corrosion issues, etc.

    If the al is for temporary bracing (to be removed), a hot melt glue will work fine - I have used it to temporarily fasten core planks to a male mold (hint bundle the glue gun and a compressed air feed together so that you can apply the glue and then cool / set the glue rapidly with the compressed air).
     
  4. Capt. Bill11
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    Capt. Bill11 Junior Member

    Like I said, it might be a stupid idea. :)

    But the frame would be encapsulated in foam on all sides and the glassed over. So the underside will be smooth with lights.

    The reason for the frame is because the top will be 15’ 6” X 19’ 3” And I was thinking it needed a frame to add rigidity and help with support of the cantilever and also to make the top strong enough for people to walk on. I may be totally off base.

    Also the hollows between the grids would make running wire and mounting lights easy.

    The back edge of the top will come off, and be bolted and glued to, the existing arch. (I’ll post a picture tomorrow.) I was thinking that I could add tappered box beams along the sides of the top that the frame would be tied into and then they would tie into the arch and help add more rigidity to the top and help with the distance that we would like to span with a cantilever. We are trying not to have any supports to far forward along the sides or in front so it would give a nice open viewing field. Again, I don’t know if it can or should be done that way and any suggestions and questions are gladly accepted
     
  5. Typhoon
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    Typhoon Senior Member

    Make frame out of light timber, Staple/nail foam to frame. Glass over outside of foam, let cure. Turn over and glass other side. Finish and done.
     
  6. indianbayjoe
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    indianbayjoe Senior Member

    When i repaired my motorhome, i rebuilt the roof from the inside since the glues they used 30 years ago reacted to the foam and destroyed it and it fell apart. We tested many types of adhesives and 5200 seemed to produce the best bond. Sikaflex was also good but more expensive i believe. Most of the usual ones like liquid nails and some others either reacted with the foam or the bond was poor.
     
  7. Capt. Bill11
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    Capt. Bill11 Junior Member

  8. PAR
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    PAR Yacht Designer/Builder

    The problem with using aluminum is two fold. First is oxidation, which can causes issues with any adhesive, second in the inside of the tubing which will be a condensation collection system.

    Your description sounds as if the aluminum tubing is the load carrying element, with the foam offering a surface to lay 'glass on. If this is the case, why bother with foam, just use chip board or other inexpressive sheet goods to "skin" the frame work. Conversely, you could make up skins on some Formica sheets (or whatever) and then bond these to the frame work afterward, skipping the cost and bother of foam all together.

    If the aluminum frame work isn't intended to be the structure, then why use something so costly. The laminate as a single skin or a sandwich can easily do what you are asking. A simple mold made from MDF and furring strips, covered in 'mica or plastic sheeting can be built very cheaply, skinned with the laminate thickness necessary and popped off to perfect fit and shape.

    I guess my point is, don't over think this thing, make a decision about what you want and how strong it needs to be, then use an approach with the least fuss and expense. In short, aluminum tube, foam and 'glass composite structures, sounds like a lot of trouble and cost for a hardtop, when a simple single skin, 'glass hard top, laid over a cheap, throw away mold will do
     
  9. Capt. Bill11
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    Capt. Bill11 Junior Member

    The frame is to add support. It's going to be fairly large at 15 X 19 feet. And a single skin will not work because the top needs to have lights mounted into the underside of it.

    The potential condensation issue is a good point.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Now, why wouldn't a single skin work, when clearly single skin yachts hardtops, roofs, etc., of considerably large sizes and much higher loading have been built? Use an angle stock frame and single skin if you must believe that only metal can work, but honestly, it sounds like over engineering to me.
     
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  11. Capt. Bill11
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    Capt. Bill11 Junior Member

    Perhaps I don't understand the term as you, or it, is used.

    So feel free to educate me.

    When you say "single skin" I think of just layers of glass with nothing like foam in between to give it body and thickness.

    Of course I don't only believe metal would work. If I did I wouldn't have bothered asking these questions. In fact I'm leaning towards a foam cored top at this point.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Yes, single skin is a pure fabric and resin laminate. No core, no frame, just laminate, though if required structural "shapes" can be employed to make the single skin stiffer. Just like the vast majority of boat hulls until relatively recently. Cored structures means you laminate twice, once on the outside, then again on the inside. A single skin is laid up just once.
     
  13. Capt. Bill11
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    Capt. Bill11 Junior Member

    OK, that's what I thought you were talking about.

    I don't see how that would work in this case for at least a couple of reasons.

    Unless it was very thick I don't see how it could be rigid enough to with stand walking on and torsional loads without lots of bracing. So you would have to add "shapes" to the under side. Which could make the underside not as easy to clean and/or esthetically pleasing.

    Recessed lighting and perhaps speakers need to be added to the underside. And A single skin would not have the depth for that without adding more shapes. Plus where would you hide the wire runs?

    All the hard tops I've seen on large boats have some core to them to deal with just these issues.
     
  14. gonzo
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    gonzo Senior Member

    A cambered cored panel will only need a frame on the edges. You can use foam or honey comb.
     

  15. Capt. Bill11
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    Capt. Bill11 Junior Member

    Thanks. That is what I am starting to realize.
     
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