Substitute for plywood?

Discussion in 'Materials' started by Chotu, Feb 27, 2022.

  1. Chotu
    Joined: Mar 2018
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    Chotu Junior Member

    I have a chain plate design that uses plywood in the mounting, with screws going through it. Yuck.

    What can I use as a plywood substitute?

    The plywood is placed with grain as +45/-45 to the pull of the shroud. There are 3 layers of 3/8” plywood and each layer has biaxial glass between it. All of this is tapered at 8:1 onto the hull laminate.

    the metal chain plate is bolted to this arrangement with screws that go right through it. This leaves a lot to be desired in terms of longevity.

    What can I replace this plywood with that won’t rot?

    Also... the plans show the chain plate inside the hull with the leaky, awful, old fashioned, metal hull penetration. That’s no good either. Leaky.

    I’d like to put this all outside. How can I do that? Just reverse the whole thing?

    Willing to put the plan drawings up here to illustrate, but have to take off the specifics to protect the designers IP
     
  2. missinginaction
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    missinginaction Senior Member

    It's not a very big piece, right? Sounds like about 1-1/8" thick? A couple of thoughts. Good exterior/marine ply with no voids is usable. You can make up the 3 layer laminate, coat it with at least three coats of epoxy and use the epoxy grommet technique to insure that no water gets to the plywood where the mounting bolts go through. I'll post a photo below that will show an epoxy grommet. You locate your bolt holes, then drill an oversize hole in the laminate. Fill the oversize hole with epoxy. When the resin is solid drill an appropriate size hole through it for your mounting bolts in the predetermined location. It takes a little time but it's worth the effort. You could also use something like Coosa Composite. It's a little pricy but will never rot.

    Epoxy grommet.JPG
     
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  3. Will Gilmore
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    Will Gilmore Senior Member

    How old is the chainplate backing and how long do you want it to last?
     
  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    solid fiberglass; epoxy; layers of 1708 are about 0.040" each

    I can't do the strength calc, but solid frp at 1.125" is like 25-30 layers.
     
    Last edited: Feb 27, 2022
  5. Chotu
    Joined: Mar 2018
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    Chotu Junior Member


    The chainplate is about negative 2 weeks old. Looking to build them in the next couple weeks. I’d like it to last forever. That’s why I don’t want to use plywood in its construction.

    However, the design I have relies on plywood. I was hoping to substitute in some G10 or something.
     
  6. Chotu
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    Chotu Junior Member

    I don’t know why I didn’t think of this. I do this all day long on deck. Why not through the hull? You did actually just make this metal chain plate idea viable with plywood.


     
  7. Chotu
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    Chotu Junior Member

    hmmmmm. So you’re saying to take glass and just use that in place of the plywood? 1708 has Matt which is a bad choice as this is Epoxy construction, but it might make sense to use glass, except the weight. Plywood would be a lot lighter. I use Okoume.
     
  8. Chotu
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    Chotu Junior Member

    Here is a heavily cropped part of the plans to protect the designer’s IP.

    See how it’s one of the old style internal chainplates with the awful, leaky, through deck piece you attach the shroud to?

    Can I just reverse the entire plan to have everything on the outside of the boat on the inside and Vice versa?

    15DC70CA-5767-4758-8204-14F9DFB9D7BB.jpeg
     
  9. missinginaction
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    missinginaction Senior Member

    One other thought Chotu. Just use a substantial backing plate inside the hull. You probably already thought of this. I don't know what size bolts you're using. I used that epoxy grommet technique to attach all of my cleats on deck. Inside the boat I used 4" x 6" hardwood backing plates. Then I used 1/8 inch thick stainless steel structural washers under the nylocks on the 1/4-20 machine screws that held everything together. The extra thick structural washers won't bend under high loads like typical fender washers that people try to stack for extra thickness.

    Fender Washers Extra Thick 18-8 Stainless Steel https://www.albanycountyfasteners.com/18-8-stainless-steel-extra-thick-fender-washers-p/1000-5.html
     
  10. missinginaction
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    missinginaction Senior Member

    I'm a powerboater so I'm not sure of the language here, but...... That piece that goes through the corner of the cabin top is some kind of stainless steel plate? I'm thinking that the fork with the pin is attached there and there is considerable pull on that assembly?
    If I'm on the right track it would seem to me that moving the plate to the outside of the cabin trunk would be stronger but maybe not as pretty. I would think that the steel plate passing through the cabin trunk would be putting a lot of stress right on that corner, maybe causing flexing that is causing the leak.
     
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  11. Chotu
    Joined: Mar 2018
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    Chotu Junior Member


    Are you sure you’re not a sailing person? Ha ha ha.

    you would be surprised at how accurate what you just wrote is. That’s the primary reason why this design is no good.

    The old boats used to do it this way.

    The amount of force on the piece you are talking about is the amount of force it takes to physically take the boat and lift it up. Proper chain plates should be able to be hooked to a crane, with a spreader bar, and you can lift the boat with them. That’s the amount of force.

    And you are exactly right. The metal piece sticking through the deck is extremely problematic on all boats. It leaks. There’s always force and vibration on it and whatever sealant you put around it will come loose. It’s a terrible design. Inevitably water gets down and promotes corrosion of the stainless. It also ruins things in that general area on the interior.

    Putting the plate outside instead is actually the more modern way of doing it. It’s also the ancient way. For some reason in the middle people decided to stick it through the deck. One of the best solutions is composite chain plates made entirely of fiberglass. However, those are a lot of work. I’m under some time constraints. I’m going with the metal ones just because I can bolt them on there. Also, given material shortages of these days, these are readily available.

    but these can look pretty nice on the outside. They are polished stainless in that case. 316L. They are very pretty to look at.
     
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  12. Chotu
    Joined: Mar 2018
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    Chotu Junior Member

    yes. Definitely. The epoxy grommet you are talking about is something I call “drill and fill.” I don’t make any penetration into any composite structure without doing that. Sealing off the core.

    There are 24 half-inch machine screws that go through this chain plate as designed.

    My cleats are done precisely the same way yours are. And good news. I’ve already had my boat through a hurricane on the two bow cleats. The two cleats held the entire anchor load of my high windage catamaran. It was a category 4 hurricane and the eye passed directly over the boat. They held just fine.

    A substantial backing plate is actually part of the design. A metal one.
     
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  13. fallguy
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    fallguy Senior Member

    1708 has mat, but the binders are not affected by the epoxy and the bonds are strong; there s mat glass in my all epoxy build; just for tabbing and molded items; regretting tabbing with it now some

    It is used for faster build thickness is all.

    otherwise, sure g10

    There are a couple of contributors here that even can spec this for you.
     
  14. Rumars
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    Rumars Senior Member

    Internal or external, any bolted on metal chainplate is going to leak. It's not a question of if, but when, that's differential expansion at work for you. The difference is what you do when that time comes, internal you just remove the top plate and replace the sealant, external you have to remove the whole chainplate. Internal is therefore simpler to maintain on a regular basis.
    You do what you makes you feel good. If you want to make them external you need to bend the metal to the exact hull profile, or pad the hull to be flat.
    Where's the plywood in the drawing? Is it the pad under the steel strap?
     

  15. Chotu
    Joined: Mar 2018
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    Chotu Junior Member

    The plywood is part of a buildup on the hull that receives the large plate that holds the strap in place. There are a few layers of 3/8 inch plywood and biaxial glass buildup. Then the chain plate attaches to that.

    and with the outside chain plate, it seems like a leak would be a relative term. Because it doesn’t really have an opening to weather like it does when it’s on deck. The openings are in the side of the boat. What do you think of that?
     
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