How to seal/gasket this hatch cover?

Discussion in 'Boat Design' started by MJT, May 17, 2020.

  1. MJT
    Joined: Jul 2012
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    MJT Junior Member

    Hello: These teak hatches I've built have a flat top, on which will sit 1/2-inch thick gray plexiglass. The plexi will extend all the way to the perimeter. The plexi will be screwed into the tops with SS screws, countersunk so the heads are flush with the top of the plexi. I'm still unsure about how I'll seal the plexi to the flat, to-be-varnished tops. I figure I have two options:

    (1) A clear adhesive like silicone.
    (2) A gasket material, like neoprene, with a strip across the center beam where there would be no screws.

    The advantage to (2) is that I could easily remove the tops and gasket for yearly varnishing.

    All feedback welcome, and thank you in advance.
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Half-inch thick ? That is pretty solid. Why would the centre piece need sealant ? If the perimeter and screw holes are sealed, that is enough. Silicone is a menace where re-varnishing is concerned, nothing much will take over it, or any residue of it, and even slight residue will show itself, as the finish will be repelled by it. There are clear waterproof sealants that are not silicone based.
     
  3. MJT
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    MJT Junior Member

    Half inch is pretty solid, but the "center loaded flat plate" calculator HERE suggests that without the beam a 400-lb load would flex 3/8 by almost an inch based on the dimensions of my hatches. So I added a beam and upped to 1/2 inch.

    The center beam will not need sealant, but if I go with a gasket then I'd lay the same gasket material over it to make the height uniform.
     
  4. fallguy
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    fallguy Senior Member

    I wish I knew what hinges those are. I need some like them for a shoebox lid.

    I would make the plexiglass more like a lid that goes over the edges of the shoebox. Glues for plexiglass would be used for the edges which look like about 3/4"-1" wide pieces.

    Then screw it on the edges and put a bead of butyl under it.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Just put a bead around the edge, not under it. It would help if the timber frame is a little wider than the plexiglass. A neat, clear bead should be easy.
     
  6. fallguy
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    fallguy Senior Member

    I agree, but I would build the lid to cover and bead the bottom edge. This way is also okay; just harder to make look nice.

    Otherwise the bead on the edges is better than pressing it down.
     
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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    What I do know, if you try and paint anything that has had silicone on it, no matter how meticulous you are in removing it, residue always seems to remain. Forget silicone !
     
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  8. fallguy
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    fallguy Senior Member

    110% agreement

    the only time I use it is as a light locktite
     
  9. MJT
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    MJT Junior Member

    Those are Whitecap Hatch Hinges. They're dinkier than I'd like, but I'm having a heck of a time finding hefty hinges for these beautiful, heavy hatches. Still looking.
     
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  10. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Every screw that is securing the plexiglass will be a potential source of leaking.
    Rather than extending the plexiglass to the timber perimeter, would it be feasible to stop it short by say 3/4" or 1", and build a 1/2" deep teak 'frame' around it?
    You could then glue the plexiglass down with 3M 5200 and you could then forget about it - no need to worry about varnishing under it every year.
    And definitely stay well clear of silicon!
    Re hinges, could these from Jamestown Distributors be a possibility, or are they just as dinky?
    Hatch Hinges | Stainless Steel | Chrome Plated Brass https://www.jamestowndistributors.com/userportal/show_product.do?pid=2784

    I was going to suggest Defender, but I see that they have the Whitecap hinges, and they seem to be the chunkiest in the list.
    Whitecap Standard Hatch Hinge https://www.defender.com/product.jsp?id=1320112
     
  11. fallguy
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    fallguy Senior Member

    Can I be a real pest?

    I rather like the idea of perma glazing the plexi into the hatch. It would be done with a rabbetted trim piece of teak sealed out of sight. Rabbet and trim high to avoid ingress. Then seal it on forever.

    Now for the pain part. How are you latching this beast?

    And what are those callouts on the Jamestown site? Att, wht? Makes no sense.

    And is there some built in clearance requirements? Like if I have a 3" shoebox lid, will the lid bottom hit on the hinge side?

    I said I would be a pain...
     
  12. Barry
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    Barry Senior Member

    If the goal is to be able to remove the plexi annually or bi-annually, you do not want to put any type of adhesive on the wood side.
    A neoprene gasket will be to0 firm for an adequate seal as it falls in the firm or extra firm category for gasket material.

    A 2 inch wide by a 3/16 inch medium density gasket material, PSA-1, silicone rubber, closed cell (not light density foam often called sponge gasket material) will work just fine. The PSA side, ie pressure sensitive adhesive
    side against the plexi will permit the plexi to release off the teak at refinishing time. The 2 inch strip would then allow the screw line to be midpoint of the 2 inch strip. As others have mentioned, you probably do not
    want to silicone the plexi or gasket material to the wood though no one will see the finish under the plexi if it is opaque gray. The issue becomes the leakage at screws. Counter sinking with a small drop of sealant under
    the head may work but I would be inclined to use a dome stainless screw with a thin rubber washer underneath. Similar to a roofing screw that holds sheet metal roofing to wood. Certainly, they will be raised a bit but you
    are probably not going to be walking on the hatch all the time anyway.

    When mounting the screws, use a pilot screw bit to drill the shoulder and thread pilot. Rather than drive them like a self tapping. Finnish the last few turns by hand.
     
  13. MJT
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    MJT Junior Member

    Thanks everyone for the discussion!

    Steering clear of silicone now and forevermore.

    I'm not actually worried about varnishing under the plexiglass. Rather, removing the plexiglass would facilitate varnishing the exposed sides, and I can't stop thinking that ANY sealant would make a mess where I want a clean boundary between varnished teak and the plexi top.

    The JT hinges are pretty much the same dimensions.


    Latching from below, not sure exactly how yet but will use a soft foam gasket that the hatch will compress to about 1/2 its height when it bottoms out on a lip.

    Thank you for addressing the gasket idea. Great stuff!
    I was concerned about the firmness of neoprene. I will look into the other options you listed. I will keep updating this thread as the project progresses.
     
  14. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    I'm a little late

    Gasket for sure.

    But

    DON'T countersink the screws. It can cause a crack to form. The plexi and wood need to flex independently from each other. Oversized holes in the plexi and large flat washers will be far less likely to crack.
     
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  15. gonzo
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    gonzo Senior Member

    Countersunk screws will crack the plexiglass. The rate of thermal expansion is very different for plexiglass and wood. Countersunk screws won't allow enough movement. There are several ways of attaching that will allow for contraction and expansion, depending on the aesthetic requirements. On commercial boats, it is common to use flat washers under a round head screw/bolt. If you want a really pretty look, a stainless frame is the way to go.
     
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