Cabin skin thickness and window mounting for cabin of an aluminum boat

Discussion in 'Materials' started by cthippo, Dec 17, 2020.

  1. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    Than's to a seasonal infusion of cash, i am working on the Vonda Lynn again and am once again hitting a roadblock. The cabin frame will be made from 1 1/4" 6061-T6 square tubing, but what is a good thickness to use for the skin? I have relatively little experience with sheet metal and trying to translate gauge to something I can wrap my brain around is a challenge. Also, what is the best way to mount lexan window into such a skin? The largest area to be covered, which will also have the largest window, is about 3x4 feet.
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I had to remind myself as to who Vonda Lynn is - to save others from looking up your past posts, here is a link :
    Aluminum boat rebuilding project - The Vonda Lynn https://www.boatdesign.net/threads/aluminum-boat-rebuilding-project-the-vonda-lynn.64071/

    Re a shell thickness for your ally cabin - aluminium is strong stuff, and you could rivet on 18 SWG (approx 1.5 mm) thick plate. We made a roof for the powercat in my avatar using 1.5 MM thick plate, and I can walk on it quite happily (although it does flex a bit).
    It will depend on the panel size (distances between the 1 1/4" square hollow sections) as to how 'floppy' it will be.
    It might be better to increase the thickness a bit.
    Or, just build a square tube framework, and then skin it with plywood or fibreglass sheets?
    Re attaching a lexan window to the aluminium (or plywood) - you could just use a good adhesive like 5200 - if you prepare it all properly, it will be plenty strong enough.
    Re the 3' x 4' unsupported area, how large will the window be in this area?
     
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  3. fallguy
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    fallguy Senior Member

    I have the same question, but not a big fan of the 5200 idea. If one of the lights break, oi vey, what a pain to remove.
     
  4. kapnD
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    kapnD Senior Member

    3M VHB tape works well for fastening plastics to aluminum or fiberglass, no mechanical fasteners required.
    It has enough “give” to handle thermal expansion, and the edges can be sealed with caulking.
    I pre paint the overlap area with black Krylon So the tape doesn’t show.
    3342DFD0-03DC-435F-98A5-4AE868CE4A81.jpeg
     
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  5. fallguy
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    fallguy Senior Member

    damn that looks good, can you explain it in detail!
     
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  6. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    The Lexan windows. Just stationary, right? I looked at the thread that Bajansailor posted above. Pretty nice job on the framing lumber mock-up. It occurs to me that you might be able to secure the Lexan with something as simple as clear silicone caulk. I've used GE Silicone in a few places around my boat that weren't structural. Silicone is often used in metal building construction. You would want to clean the fresh aluminum with acetone or another solvent to get any oils off and then wash the aluminum with metal etch and apply the Lexan as soon as the metal is dry. You keep the Lexan neat by dry fitting and masking off the Lexan so that the squeeze out doesn't make a mess of the window. If you ever wanted to replace the windows they would be much easier to remove than ones secured with 5200.

    Just a thought.

    MIA
     
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  7. kapnD
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    kapnD Senior Member

    This one is over 1/2” plywood, glassed both sides. I didn’t take photos of the process, sorry!
    I cut the polycarbonate windows 1 1/2“oversized, and painted the inside surface of the overlap with Krylon.
    Did a test panel because I thought the paint would be the weak link, but it is stronger than the laminate, which only failed with extreme point pressure from a very big pry bar, and was largely the plywood splitting.
    I have very high confidence in the bond, no leaks or evidence of failure yet.
    All surfaces to be painted/taped/caulked were thoroughly cleaned with acetone first.
    Used 4000 series black caulking, and after 4 years, the thin edges of it are Starting to show some drying and flaking.
    I set up a ledger to support the windows when setting them against the tape, which is VERY sticky.
    It’s a lot like working with contact cement in that it is a done deal once contact is made, so alignment is critical, especially where I had those 4 panels with only 3/8” between them.
    I also used the VHB tape to install a window on the outside surface of a bait well, that Never leaked either.
    My current project utilizes VHB to join the floor panels to the tops of the stringers and bulkheads.
     
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  8. fallguy
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    fallguy Senior Member

    So, you didn't rabbet the panels? I guess a rabbet would help line them up easier.
     
  9. cthippo
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    cthippo Senior Member


    AHA! This is useful information! Thank you so much!

    It looks like what I can get here is mostly fractional, So maybe 1/32 (.032) for the roof panels, which are about 28x68" and then 1/16 (.064) for the sides and rear. I shouldn't ever need to walk on the roof, and can reinforce antenna mountings if need be.

    The biggest panels are the sides of the cabin which are 40 tall by 54 wide (fore and aft) and the windows which will go into those will be 36 tall by 48 long.I plan to use 3/8 Lexan for the windshields, and the sides, with probably 1/4" for the smaller rear and roof windows..

    Thanks for the help!
     
  10. cthippo
    Joined: Sep 2010
    Posts: 790
    Likes: 41, Points: 28, Legacy Rep: 465
    Location: Bellingham WA

    cthippo Senior Member


    AHA! This is useful information! Thank you so much!

    It looks like what I can get here is mostly fractional, So maybe 1/32 (.032) for the roof panels, which are about 28x68" and then 1/16 (.064) for the sides and rear. I shouldn't ever need to walk on the roof, and can reinforce antenna mountings if need be.

    The biggest panels are the sides of the cabin which are 40 tall by 54 wide (fore and aft) and the windows which will go into those will be 36 tall by 48 long.I plan to use 3/8 Lexan for the windshields, and the sides, with probably 1/4" for the smaller rear and roof windows..

    Thanks for the help!
     
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  11. rangebowdrie
    Joined: Nov 2009
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    rangebowdrie Junior Member

    Their are some specific requirements for polycarbonate windows.
    Sikaflex has a product just for that application, #295UV.
    Here is a PDF that includes instructions for installation of said windows,, scroll down for your application;

    Structural Bonding Marine Applications (sika.com)
     
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  12. kapnD
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    kapnD Senior Member

    The panels were only 1/2” plywood, so I didn’t want to sacrifice any strength by reducing thickness and interrupting the laminate with a recess.
    A major goal was to keep it lightweight and uncomplicated.
    So the windows are surface mounted on VHB tape with a bead of 4000 series caulking sealing both sides.
    The fastening is totally reliant on the tape.
     
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