Hull Mount Port Light upgrade

Discussion in 'Boat Design' started by zstine, Jul 19, 2014.

  1. zstine
    Joined: Sep 2013
    Posts: 15
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    Location: New Jersey

    zstine Junior Member

    Practically every production boat manufactured now has some hull mounted port light. My 1975 Tartan 41 has little light, so I would like to cut an 18" x 8" hole over the pilot berths and put in hull mounted, non opening, port lights. Right now, I'm investigating what's involved, real risks and costs to determine if its feasible for me to do.

    Is anyone familiar with the construction of these lights in production boats? What structural reinforcement is employed around the hole? Are the windows Polycarbonate (Lexan) or Acrylic (plexiglass)? Are they typically bonded in or mechanically fastened? If mechanical, are gaskets or caulks employed for sealing? If bonded, what type adhesive is used?

    My thought... The mounting location of my hull is 3/4 balsa cored & about 4 ft aft of main bulkhead. I plan to cut the exterior skin and core 3" larger than the inner skin, leaving a 1.5" wide "shelf" for the window seat. I would use glass tape to build up the shelf surface and seal the core. This would provide "L" shape structure piece and make a flat surface for the window. Then Bond in the window using 1/8 standoff for flex. Any external water pressure would push the window into the seating surface. What issue do this plan have?

    Thanks,
    Zach
    s/v Diablo
    1975 T41 #74
    New London, CT
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    Polycarbonate (Lexan) is tremendously stronger than plain plate glass (about 200 times so), more vulnerable to UV and scratches easier than acrylic. The acrylics are much less costly, tend to look better (shine, more transparent) and still is 5 - 10 times stronger then plate glass averaging about what tempered glass is, plus has better UV resistance.

    Lexan is the hands down preferred product for ports and lights. Instead of cutting the light frame opening bigger than the light frame, cut it with just enough gap to fill with bedding (polyurethane), allowing the frame's flange to cover the opening. Around the edges of the cut opening, remove the core and replace with Coosa or similar inert, strong core material, that can take compression for fasteners and bond this to the inner and out skins. I'd use epoxy, just to make it bullet proof, but polyester or vinylester can be employed too. As to the flange, well the light frame will give you hints as to how it should be shaped, but typically the bottom of the opening will have some slope to it, in case of leaks. The local reinforcement will come from the bonded core replacement product, surrounding the opening and the strength should come from the light frame itself.
     
  3. UNCIVILIZED
    Joined: Jun 2014
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    Location: Land O' the Great Lakes

    UNCIVILIZED DIY Junkyard MadScientist

    I've just got a moment, but here's a couple of thoughts. Be careful about where & how big of a hole you're cutting, as odds are there are some serious loads in the hull from the chainplates in that neck of the woods.
    Also, there's something of a similar discussion going on over here http://www.cruisersforum.com/forums/f55/which-boat-window-materials-131721.html And you might do a bit of studying there, albeit the "noise" level's a lot higher, & the general boat IQ's lower (than on here).
     
  4. zstine
    Joined: Sep 2013
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    Location: New Jersey

    zstine Junior Member

    Thanks everyone... Uncivilized, I read the other post. The window material question is much like 'what is the best anchor?"! But for the one guy who replaced his hull light, which will go underwater, Makrolon polycarbonate sounds like a good choice.

    In my case, I'm modifying the hull of a cored boat and want to install a window without a frame. The inner skin cut to a smaller size than the core and outher skin. The 'shelf' that this leaves would be layered up with glass/epoxy maybe 1/4" thick, then the window set onto the shelf with an adhesive, 4200, silkoflex, etc.

    My main concern is structural rigidity, flex and failure. I'd hate to be pounding upwind toward St. Thomas days offshore and have an underwater window fall out!
     

  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You pretty much need a frame. Mechanical seals are the only reliable way to insure these don't leak, which requires a frame, usually two pieces. You could make one portion of the frame, the part that replaces the core material around the light opening, with another ring like arrangement, fastened down over, sandwiching the glazing. Attempting to "glue" the light to the skin, will just cause it to pop out very quickly, likely when you'd most prefer it hadn't.
     
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