Replacement for Plywood

Discussion in 'Boat Design' started by Barry, Mar 24, 2022.

  1. Barry
    Joined: Mar 2002
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    Barry Senior Member

    This has been a topic before but the threads have not provided some answers that I am looking for. I recently purchased a small aluminum boat to keep at the lake for fishing, 17 feet. It was an open style boat and I am in the process of enclosing the deck and adding a windshield to provide some protection against the elements. It had a 60 prop and I will be installing an 115/80 outboard jet. The boat
    has a 5/32 bottom and 1/8 inch sides.
    While removing the center console and existing seating, I decided to pull the floor which was 3/4 inch plywood, ( much overkill) covered with Duradeck. After considerable effort due to the weight, I decided to put a sheet on a scale and found that the weight was about 2.5 pounds per square foot. It had absorbed quite a bit of water. (as well as the spray in foam beneath)
    So searching for a lightweight alternative, I have found that there are some options of which Coosa board is one. But availability is almost non existent where I live.
    So I am considering building a foam filled sandwich with fibreglass above and below.
    I have no real experience with epoxy, fibreglass or foam but reading some of the threads, a design limiting parameter is the inability of some foam to take shear flow and perhaps the adhesion between a thin outside layer and the foam.
    So my idea is to take say a 4 foot x 8 foot section of foam, 1/2 inch, and cut some slots into the foam so that the upper and lower thin skins are attached which should deal with the shear as well as provide some internal stiffness.
    The center to center support is no more than 12 inches between hull mounted stringers.

    Some parameters
    1) the upper skin will have to support people walking on it
    2) the lower skin is required to keep moisture out of the foam as well as provide a bearing skin to rest on the stringers
    3) I would run the slots which will fill with epoxy/resin probably beam wise to span the supports
    4) as I have the previous floor as templates, I can core out the areas so that they will fill with epoxy that will match the existing screw holes

    With this in mind,
    Can someone provide
    1) the type of foam and thickness, I can easily go up to 3/4 inch
    2) the laminate layup for the upper skin and suggestions regarding epoxy or resin to suit the foam bearing in mind people will be walking on this. (suggestions to make this anti skid?)
    3) the laminate layup for the lower skin, keeping in mind, that the foam will have adequate support with c to c spacing less than 12 inches
    4) an approximate weight per square foot (square meter) that this board will weigh

    I am hoping that vacuum infusion is not the only way to get this made. I have to create about the equivalent of 4 sheets of plywood, 4 x 8 feet

    Thanks in advance
     
  2. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    You could use okume marine plywood it's much lighter than the fir equivalent. Done correctly you shouldn't have any problems with water intrusion or rot.
     
  3. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    The foam is an extensive pita for your project.

    First, getting it laminated well and sized correctly is a chore, but doable.

    The problem is attaching it to the aluminum floors/frames. Most of the time; things are riveted. Unfortunately, low density foams will not tolerate the fastenings which pushes you to coosa which is about double the cost of good quality marine plywood unless you can source some 20# density coosa at decent rates. And coosa still requires laminating; it really isn't made for jist laying down like plywood. I've not tried to rivet 20# coosa, but I'd be a bit nervous about it delaminating the glass with fastenings...a new potential problem..

    If you had a bondable substrate, coosa would be fine, but because you are going to use rivets; stay with ply. Okume is very high quality. A lower quality, but acceptable board is roseburg marine ply in the usa. You can drop down to 1/2".
     
  4. SolGato
    Joined: May 2019
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    Location: Kauai

    SolGato Senior Member

    If you want to replace plywood with something lighter that won’t rot and can be cut, screwed and glued like plywood, check out ThermoLite by Space Age Materials.

    Yes it is expensive, but it might save you time and money over more complicated materials.

    It’s been used to replace floors/decking in boats like yours, to replace transoms, etc..

    Check out YouTube for some videos on it being used on various projects.
     
  5. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Generally in the thicker aluminum boats, there is an adequate thickness in the stringers, normally at least 1/8 inch thick. The attachment process is just 1/4 inch stainless pan head sheet metal screws with a washer underneath. For thicker stringers, drill a 1/4 inch hole, tape to 5/16 and run countersunk SS machine screws into the threaded hole. The stringer thickness on this boat is 5/32 and the original was screwed.
     
  6. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    all fine, but the typical foam won't tolerate it
     
  7. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    Not sure on the thermolite but there is a shortage on coosa. I had to wait several months to get two sheets of 3/4 26# density and it's pricey. You may want to check availability if going that route. Will the foredeck have much camber?
     
  8. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    The most important question is: What core is available to you without horrendous shipping charges? Possible cores: foam (PVC, SAN, PET, PU), honeycombs, endgrain balsa, cedar (WRC, AWC, etc.), plywood.
    Other flooring options are: hardwood, plastic lumber, premade cored panels, aluminium.

    Foams do not require epoxy, polyester is fine. Hand lamination is absolutely fine, no need for infusion regardless of resin.
     
  9. Barry
    Joined: Mar 2002
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    Barry Senior Member

    It is actually the floor that I am wanting to replace. So flat, but the stringers are very close together, 10 inch prox
     
  10. Barry
    Joined: Mar 2002
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    Barry Senior Member

    XPS and polyurethane seems to be the most available. So with this as a core, I can just apply some FG without the foam "melting" away underneath. Can you suggest what I need for the side that would be walked on and the side that will be against the stringer support?
     
  11. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    Insulation foam is not really suitable. Not that it can't be done, but it's complicated even with infusion, without it it's a mess. If you want to try say so, I'll explain what's needed.

    Just buy some 1/2 cedar and use that as a core. As long as all holes have epoxy plugs it will last as long as any foam. 12oz glass on the top, 6oz on the bottom, in epoxy. The result will be in the area of 1.2lbs/sqft, similar to high strength coosa.
     
    Barry likes this.
  12. Bobuk25
    Joined: Jan 2019
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    Location: Uk

    Bobuk25 Junior Member

    Quite heavy and not sure it is available where you are, but Vetus do something called Polywood which is a white plastic sheet that is pretty indestructible. I used some as Dodgers and after years in the sun it still looked the same. I used 12mm, but they do 6mm and 18mm too.
     
  13. Jho0101
    Joined: Mar 2022
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    Location: Philadelphia

    Jho0101 New Member

    Im building now. Im experimenting with 3/4” HPDE (starboard). Its heavy…. Starlite might work to. Its 35% lighter. Ill post when im done.
     
  14. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    91C1AB0A-8B3A-443F-BFDA-05E0F9824C7E.jpeg
    i really dislike starboard...so many things about it are bad...
     

  15. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry Soy Soylent Green: I can't believe it's not people

    Your spans are so small, I'd just use ¼ common sheathing plywood, sandwiched in glass. I have ⅜ ply in 14oz biax and epoxy on ~48" spans.
     
    Last edited: Mar 27, 2022
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