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Hard Top Build (Gluing foam to aluminum)

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

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  1. Capt Bill11

    Capt Bill11 Senior Member

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    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. dennismc

    dennismc Senior Member

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    Hard top

    Does the boat have a radar arch ?? if so, you could make the frame out of aluminum angle iron, do the top out of al. sheet tack welded to the angle iron , and rivet the frame and top to the arch , I did that on my 70 footer to replace a soft top, used wood framing slats under to mount ceiling panels which hides the wires etc and the panels are removable, is fully structural and I can walk on it, is primed and painted on the exterior and is fitted with two drain pipes (hardest part)
  3. Bill106

    Bill106 Senior Member

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    Why even bother with the square tubing aluminum? It will be a bear to clean and squeegee the underside. Most all hardtop builders do it completely out of foam or honeycomb cores now with excellent results and a laminate schedule is easy enough to figure when you add up all the details (spans, foam density/thickness, glass type/number of layers, resin type). A lot of which will depend on what you have available in your area but can be adjusted accordingly.

    There are several types of foam available on the common market but you should try and avoid the cheap blue house insulation foam which can have an adverse reaction to certain types of resins (especially ones containing styrene).

    While many of the high-end shops are using molds to vacuum bag or infuse tops, you can easily build one yourself without the mold using some cheap lumber to make a bending jig you can put together in an hour. Building the "chip" is not all that difficult or time consuming. What takes time (and $) is fairing and painting it and you can easily spend the same amount it cost to build it to make it as shiny and beautiful as I'm sure you will want it to. PM me if you'd like more details.
  4. SHAZAM

    SHAZAM Senior Member

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    Make a simple mold from plywood and formica and lay up a foam or nidacore cored top. Forget about all this skeleton business.
  5. Capt Bill11

    Capt Bill11 Senior Member

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    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


    I'll PM you tomorrow Bill, thanks.

    Dennismc, do you have a picture of your top?
  6. CaptTom

    CaptTom Senior Member

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    Bill,
    You must have some free time on your hands to take this on. Can you get the parts/supplies you need in CR? How long do you expect this to take?

    Here are some photos of the yacht in question to give folks a better idea.

    Attached Files:

  7. Capt Bill11

    Capt Bill11 Senior Member

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    Thanks for the pictures Tom.

    The owner of the boat and his partner build Maverick sportfish boats (http://www.maverickyachtscostarica.com/) here in CR. So their factory guys will be handling the build. But they have never built anything quite like this. So myself and the owner are doing the design and research on how best to build the top. We can ship in an materials we need.

    The factory will also be adding some more teak decking and railing along the aft deck of the flybridge, building a deck box and a chest fridge/freezer on the flybridge. As well as building some new cabinets in the saloon and galley.

    We'll need new pictures Tom. :)

    "The back edge of the top will come off, and be bolted and glued to, the existing arch."

    Rereading it I think that this sentence may be confusing. I should have just said the rear edge of the top will be bolted and glued to leading edge of the existing arch that you can now see in the pictures.

    The top needs to be a bit wider than the arch. So we are going to have the edge of the top come around the sides of the arch a bit and then die into it.

    That is what got me thinking about having box beams made that would bolt to the sides of the arch and run forward with the grid welded in between them. I was thinking that might eliminate the need for more than one additional vertical support on each side and give us a nice clear view forward and to the sides of the top. But of course there are side loads to consider as well as the cantilever.

    Anybody know of any free or inexpensive CAD programs that I might use to draw up design ideas for this kind of thing?
  8. CaptTom

    CaptTom Senior Member

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    Bill,
    Doesn't that arch hinge to fold down, or am I thinking of another boat? Let me know when you're done with the work and I'll come down to shoot. Passport being renewed right now.:eek:
  9. Capt Bill11

    Capt Bill11 Senior Member

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    Yes it did fold at one time. But the screw jacks are long gone that apparently let you drop it with the push of a button. And I can't ever see us needing to fold it down. So it's a non-issue.
  10. Grecko

    Grecko New Member

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    A roof that large that is stout enough to walk on is going to be a lot heavier than the present canvas top. Stiffness and light weight being key, a cored fiberglass construction is probably a good idea. Bonding to aluminum is difficult in that the aluminum forms an oxide layer and unless properly etched just before bonding good adheasion is difficult to attain. The tubing will also be a corroison issue since there will be condensation in the tubing and this will cause the tubing to rot from the inside. Better to increase the amount of cloth around the edges of the structure and add local reinforcemnts inside of the part to allow bolting of the support structure.

    While it will cost a lot more the increased stiffness and strength of carbon fiber might be worth it. Just a thought, but this is a big piece and to make it strong enough to walk on will require a pretty strong structure. Also, you aren't going to get by with the present support structure, it's going to have to be more robust to support a roof that you can walk on.

    You also need to think carefully about the shape since that is what is going to make this a success or failure. While you will have curvature in the front view along the length, you need to make sure that you have some curvature in three dimensions as you approach the front. This will substantially increase the stiffness of the top and help prevent it from twisting.

    As far as inexpensive CAD programs, DesignCAD 3D Max from IMSI Design is only $100. It is surprisingly capable and would let you do this type of thing pretty easily. In fact, you could model the top of your yacht and then see what your new top is going to look like when installed. Before I bought a top line CAD program I used it and it was pretty darn good for doing 3D surfacing. It works like most other CAD programs, and if you don't have CAD experience you will need to read the instructions as to how to navigate through it, but once you get the hang of it it is pretty intuitive.
  11. Capt Bill11

    Capt Bill11 Senior Member

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    Thanks for all that. I'll look into that program.
  12. hamall9

    hamall9 New Member

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    This is going to cost you money and it will change the appearance of your boat.

    So the question is .... do you really need all the expense, grief and possible disappointment if you do not like the finished product.

    As has been mentioned, an e glass/ carbon fiber lay up over a multi density core is the way to go... this will give you the slimmest lightest profile.

    I would suggest that you run this by an Engineer to get an idea of cost, weight and appearance before committing yourself.

    Good luck
  13. Capt Bill11

    Capt Bill11 Senior Member

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    Thanks. We are running it by an NA and an engineer.
  14. Chevelle

    Chevelle New Member

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    A lot of very good info above. Just a thought, you can always use wood as a frame or maybe just as a mock up for shape, supports etc. it can be fully encapsulated and add strength and solid attachment for antennas or whatever.

    A very good example of a "conestoga wagon" top to a hardtop, look no further than the Delta 124' "CD III". Tom? any recent pics?
  15. Capt Bill11

    Capt Bill11 Senior Member

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    Thanks. The NA is suggestion a light weight wood frame. I'll try and find pictures of CD III online.

    I'll also post pictures of this project as we get into it.
  16. SandEngXp

    SandEngXp New Member

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    Bonding to Aluminum - Important points

    This is a very technical task. Done incorrectly will result in de-lamination.

    1) Surface prep on the aluminum is extremely important. Follow the US Paint or Akzo prep guide for painting.
    2) priming is mandatory.
    3) choose a 2 part urethane adhesive or toughened epoxy with appropriate service temperature for the exposed color of the component.

    We have done Aluminum sandwich panels for many Yacht decks and Offshore Oil Platform Structures. It works great for flat or nearly flat surfaces.

    Good Luck
  17. hamall9

    hamall9 New Member

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    hi Capt Bill11,

    Call me old fashioned, but I am not a fan of mixing mediums such as aluminum, wood, foam etc ... personally I would just stay with variable density foam core and concentrate working in some sexy compound curves to mimic the existing canvas cover.

    On the other hand ....
    Have you considered converting the existing cover into a permanent fixture.
    This would require slightly heavier s/s tubing but less of it of course giving the canvas cover a slightly "floating" appearance without the fussiness of the current folding frame. ..... clear side screens can easily be added and your electrics can be buried within the tubing.

    regards
  18. bly

    bly New Member

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    Aluminum is not a very good candidate to accept glue.

    Infact if I were looking for a good mold to not stick a part to. It welds good and can be bolted together. Aluminum will corrode all the time. When this happens any glue reacts and releases. It makes a great sacrificial metal when combined with other metals. Aluminum would not be my choice. I can give you much better options then aluminum frame with a glued foam??? I have been considering a few options my self. Tubing made from fiberglass or carbon fiber. For the top material, multiple layers of coosa board with overlapping joints to make it any size you want.
  19. garyohv

    garyohv New Member

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    stronger foam sandwich roof

    Foam is easier than metal to make compound curves. To make a foam sandwich roof stronger, lay the foam planks with a 3/8" gap. Lay "rovings" of glass in the gap to be the vertical flange of an I beam attached to the top and bottom glass. Lay extra glass cloth in the middle of the span.