Plywood for interior of a large cruising sail boat

Discussion in 'Wooden Boat Building and Restoration' started by theoldwizard1, Sep 29, 2016.

  1. theoldwizard1
    Joined: Sep 2016
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    theoldwizard1 New Member

    A friend is planning on building a 50+' cruising sail boat. Needless to say, there will be many "compartments", a couple of state rooms, head, etc, etc. that will be built out of plywood.

    To save money, he is thinking of using interior cabinet grade plywood or exterior plywood, NOT marine grade. His logic is, "It won't be directly exposed to water" and several coats of varnish should provide adequate waterproofing.

    I know that marine plywood typically uses a rot-resistant wood. The front and back are usually grade A-A or A-B and the cores have no (or very small) voids. They are also typically assembled with WBP glue. Besides making the plywood less likely to rot and delaminate and are stronger for the same thickness.

    What is your opinion on non-marine plywood for interiors ?
  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    do not use interior grade cabinet plywood, anything in and around a boat will get damp, interior grade plywood will slowly delaminate over time just from the damp air. This is also true of particle board and similar lumber made of saw dust and glue.

    Any exterior grade plywood, that uses exterior grade glue, should work well however, fir is fairly rot resistant anyway and should hold up well. It will not be exposed to the weather directly so it should hold up okay. the surface finish is not as good, but you can either laminate a thin veneer on the exposed surface or paint the surfaces rather than finish bright. Actually I think cabinets painted white, with sawn lumber wood trim finished bright looks great, and keeps the interior much lighter and cheery than all bright varnished wood (makes the interior dark and depressing).
  3. gonzo
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    gonzo Senior Member

    I agree with that. Also, sealing the edges will go a long way to protect the plywood. Epoxy would be the best choice.
  4. JSL
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    JSL Senior Member

    All plywood I have seen here (BC) has waterproof glue and should be okay. If you want to test, cut a small billet and boil it. If not waterproof it will delaminate in about 5-10 minutes. One item is to make sure there are no voids and that any knot cutouts are patched. There used to be 'good-solid' grade but I am not sure if available anymore.
    Regular plywood often has voids and they will bloat after a while. However, sometimes you can drill and fill.

  5. PAR
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    PAR Yacht Designer/Builder

    Cabinet grade plywood typical has two major flaws in the marine environment, the first is a non-WBP adhesive and the second is a paper thin outer veneer. The outer veneer will delaminate very quickly, from the non-WBP glue, as will the other internal veneers. Buy a sample piece of cabinet grade plywood and leave it on a covered porch for a year and watch what happens.
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