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Thread: Plywood vs. Marine Plywood

  1. #1
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    Default Plywood vs. Marine Plywood

    I was talking to my grandfather and I was telling him how i didnt think that it would cost very much to build the egret sharpie. That is when he said that it would be expensive with plywood being at 100$ (give or take) a sheet for marine plywood. I was thinking of using just regular plywood. This is a big number that I would not be able to afford and was wondering what the integrity of the hull would be if you used regular Home Depot plywood and then epoxy coated it rather than using marine grade plywood. Would this work? Would one outlast the other to make an etreme difference? Would it be cheaper to glass the boat rather than buy marine plywood?


    Thanks,
    Tommy

  2. #2
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    Default Re: Plywood vs. Marine Plywood

    I used doug fir exterior plywood to build a six-hour canoe once. I coated the hull exterior with epoxy, then painted with system three epoxy primer and paint.

    I kept the boat under a cover, but outdoors. In two years I had lots of checking in the plywood.

    I think glass would have been a good idea. I bet you would get the same checking with marine plywood--maybe wouldn't happen as soon, though. I think you will get some better informed opinions soon.

    From what I've heard, glass is pretty standard over a plywood hull, marine or otherwise, in which case you might save a few bucks with the exterior grade. The exterior grade had a lot of footballs and was pretty difficult to get a smooth surface finish, too. I think the marine stuff is a little smoother.

    good luck

    Vince

  3. #3
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    Default Re: Plywood vs. Marine Plywood

    The total cost of materials to build a boat is relatively insignificant in comparison to your time etc. Home Depot plywood will likely not be made of waterproof glue and will have many voids including some that could be quite large. Marine fir is much less than Okoume and far better than standard ply. Exterior ply should be somewhat waterproof but will have the voids. You'll have a much nicer boat if you go with some type of marine ply and its not a bad idea to have some thin glass cloth/epoxy coating on the outside but not necessarily required. The finish if using marine fir will be nicer with the cloth. Another way to go is to use MDO which is basically made for exterior sign work. Some folks have had good luck using it though I don't have any experience using it on boats. It would be cheaper than marine ply.
    Personally I'd try to go with the best material you can get if at all possible. You can glue up a boat with plain old exterior ply and it will float, may not be pretty and may not last a long time but heh, it will be a boat and will float.
    Good Luck.
    GAry

  4. #4
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    Default Re: Plywood vs. Marine Plywood

    Most of the folks on this forum will tell you that using fir marine ply will save you money over using cheap construction ply and having the boat break or rot in a few years. But you'll have to glass it all to avoid checking.

    They will also tell you that using fir marine ply and having to glass it all will cost more than using Okeme or Meranti marine ply and not having to glass it, or only glass the underwater parts of the hull. Epoxy and glass are not cheap, and involve a lot of labor with generally toxic materials.

    Kinda like building a nice house or a cool car out of cardboard -- it can be done if you ignore code, but your work and carpet and paint and fittings will be wasted when the cardboard fails....

    Bottom line = if you can't afford decent materials to build a 20' boat, get the plans for a 10' boat you can afford to build properly.

    I ignored the warnings about cheap ply and used it to build the centerboard for my dory skiff, just coating it with a thin coat of epoxy, no glass.

    Surprise, surprise, surprise...it failed, broke in half, and could have caused the loss of the boat under more extreme conditions. I rebuilt it from Meranti marine ply and fiberglassed it, and expect it to outlast the hull planks.

    Non-marine ply bottom from my dory skiff - previous owner's big mistake, note the rot -


    Cheap non-marine ply centerboard before installation -


    After two hard seasons -


    Replacement Meranti marine ply with glass -
    Last edited by Thorne; 03-05-2008 at 06:58 PM.
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

  5. #5
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    Default Re: Plywood vs. Marine Plywood

    Jim - Why polyester and nail the polyester fiberglass to the boat instead of just using epoxy? And what is Cuprinol?

  6. #6
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    Default Re: Plywood vs. Marine Plywood

    Wow, Thorne you make a good point.

  7. #7
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    Default Re: Plywood vs. Marine Plywood

    He makes a great point.

  8. #8
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    Default Re: Plywood vs. Marine Plywood

    Hey, that's my speciality -- WB Forum Poster Boy for bad dory bottom planks! Two boats and counting...

    ;0 )

    "If you can't be a good example, then you'll just have to serve as a horrible warning."
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

  9. #9
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    Default Re: Plywood vs. Marine Plywood

    There have been many forum posts addressing this issue over the years. Non-marine won't last as long, and won't look as good, and may have other problems, but if the boat will not be stored in the water and you really want to build and can't afford marine plywood, why not? Your first boat is seldom your last anyway, so you could consider this a 'starter' boat.

  10. #10
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    Default Re: Plywood vs. Marine Plywood

    My two cents: You are building a boat, presumably in part because you expect to enjoy the building process. Fir ply is miserable stuff, splintery and impossible to sand smooth. Meranti is much nicer to work with and you can get a decent long lasting finish without fiberglass sheathing.

    Noah's Marine sells 1/2 BS6566 meranti to $53/ 4x8 sheet. Big box AC fir must cost almost that.
    Denny Wolfe
    www.wolfEboats.com

  11. #11
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    Default Re: Plywood vs. Marine Plywood

    Ok I hear you !!!
    My plans call for 10’ sheet stock. However my supplier only stocks 8’ Okoume and I do not want to scarf this boat… I hate the lines.

    My supplier dose have 5’x 10” exterior luon. Will this be ok to use? The boat is a lap strake, 4 planks per side.

    If I cut though a void could I just fill the void with epoxy? I plan on costing the planks before in I install them.

  12. #12
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    Default Re: Plywood vs. Marine Plywood

    If you are going to use "cheap" ply, at least make sure it is an exterior grade, so you know the glue holding the plys together is waterproof - it will probably be a dark reddish-brown colour.

    Pete
    Don't underestimate the power of stupid people in large numbers!

  13. #13
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    Default Re: Plywood vs. Marine Plywood

    Quote Originally Posted by Jim Ledger View Post
    Use the regular fir plywood. Go for a workboat finish. Glass the outside with POLYESTER resin and use the mat-roving-mat material that comes in one sheet.. Fasten it up with hot galvanized common nails and bolts. Slather the inside regularly with cuprinol (green). Bulletproof hull.

    Twenty years from now build a better one.
    I know of many 'chop' boats built that way and the thought was, there was enough strength in a CSM layup that if the wood were to rot, the channels formed by the glass wrapped around structural members would still kind of hold the thing together collectively for the most part. Anyone that has replaced decks or transoms in fiberglass hulls will know what I mean. Plus, with this kind of construction, the irregular grain or poor grain of the cheap ply is soon lost in the rough cloth layup that you wont want to try to tame with a long board anyway. Supposedly, 6566 is relatively inexpensive and is supposed to be better than modern marine 'exterior' fir ply, which I was told, was really designated to be used on 'buildings' on the waterfront, not boats but still better than typical lumberyard materials. As of late, anything with marine near the label is considered boat wood.
    Last edited by pipefitter; 03-06-2008 at 12:56 AM.

  14. #14
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    Default Re: Plywood vs. Marine Plywood

    Quote Originally Posted by tchiffriller View Post
    ... egret sharpie.... plywood being at 100$ (give or take) a sheet for marine plywood...
    Virginia? Find a local sawyer. You can build it better for less money.

    Woodweb, Woodfinder, Forestry Forum, sawmill manufacturers, etc.

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