Expanded PVC vs plywood

Discussion in 'Fiberglass and Composite Boat Building' started by jjhamm22, Mar 29, 2019.

  1. jjhamm22
    Joined: Jan 2019
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    Location: PA

    jjhamm22 Junior Member

    I am restoring a 79 Mark Twain 24 footer. It has the usual rotten floor and transom. I would love to use composites so I never have to worry about rot. Of course Coosa is out there but I am not wealthy enough to afford that material. A friend told me he used expanded PVC sheet in his cabin cruiser. I called Curbell and they said their Celtec is a direct replacement for plywood and about the same price as marine plywood. I am tracking down physical properties of both to compare but wondered if anyone had used it in the past? The only fault I see so far is isn't UV stable without paint. Thanks for reading.
     
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    How thick is the current transom??

    The main thing is the thickness of previous materials needs to be maintained in strength.

    So a 2" thick plywood transom needs to be made equivalently strong.

    You cannot just swap materials simply because they do not rot.

    That said, you can use corelite or coosa, but you generally need to use 26 pound or higher density cores or sufficiently heavier fiberglass when you walk away from wood.

    Wood is surprisingly stiff stuff. In my build with foam, I decided to use ply where I wanted stiffness.
     
  3. jjhamm22
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    jjhamm22 Junior Member

    Transom is 1.5" thick. Matching the thickness isn't my concern as much as the long term suitability of the material
     
  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I do apologize here.

    Can you provide the technical datasheet for celtec?

    Basically, you would compare the properties of a known suitable plywood replacement like coosa to the material you are considering and look for outliers.
     
  5. fallguy
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    fallguy Senior Member

    You need to match the strength of the original design. The long term suitability is simply a bonus.

    I tried to find the tds for celtec quickly and was unable.
     
  6. missinginaction
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    missinginaction Senior Member

    There is nothing wrong with using decent marine ply or MDO (Medium Density Overlay) provided you are careful. A few years ago I added a larger swim platform to my inboard boat. Part of the project required reinforcing the transom. I used Coosa below the waterline and MDO above. Yes Coosa is expensive but I didn't need a ton of it. I used 17 oz. double biased cloth (no mat) against the inside of the transom and added a layer of 1/2" reinforcement then another layer of cloth another layer of 1/2" reinforcement and a final cloth layer. Seal the ply with three coats of resin before you do the installation and be meticulous about sealing any holes you might have to make and you'll be fine.
    Do a search for MDO and you'll find it's a durable product made even more so if sealed up properly. Three seal coats and pay attention to the edges.

    MIA
     
  7. jjhamm22
    Joined: Jan 2019
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    jjhamm22 Junior Member

    No worries, Fallguy. Thank you all for the consideration and replies. I dug around and found enough material properties to put together this comparison. I had to do some units conversion so they are comparable and summarized on the Celtec chart. It seems it is in the ball park as Coosa and Starboard. There wasn't much past the density of Coosa available but I found the Celtec is more dense than Coosa but not as dense as the Starboard. I did the pricing using Coosa and it was over $1000. To much for this boater. I am not opposed to going the traditional way of glass over marine plywood. But I am drawn to the idea of using material that just won't rot ever. The Celtec is about the same price as marine plywood. I look forward to hearing your thoughts
    Jeff
     

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  8. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Plywood rots because noone takes the time to overbore and redrill holes.

    If it is your boat and you have the patience to overbore and plan to use epoxy; then go plywood.

    Plywood has its place. People shirk.

    If you install a transom mount ducer and just hope the caulk seam is good, then a no rot option is wise.

    Otherwise, you screw in the ducer and remove it. Drill out the holes 1/4" to a half inch oversized and deeper than the ducer screws. Tape a straw to and plastic bag and fill with thickened epoxy and fill back to front and tape them off level. Next day, you come back and predrill and use the same caulk and install the ducer. This method won't allow water ingress to the ply. And you do it for all the holes in the transom AND you make sure the transom ply edges are not exposed to any weather. A no rot transom.

    And, this practice is also the best method for foam builds anyway.
     

  9. jjhamm22
    Joined: Jan 2019
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    jjhamm22 Junior Member

    Thanks for the detailed "how to" on transom replacement. I have done a ton of reading and research and your description is spot on to the best practices I have seen. I will likely go with plywood and fiberglass as the substitutes are just too expensive for me (4-5 times plywood). My boat must have been built well as it lasted 40 years. I think the rot started around the added on speedometer bracket and water got into the screw holes. Thanks again
     
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