Re-do transom with plywood/fiberglass or Seacast?

Discussion in 'Fiberglass and Composite Boat Building' started by jjhamm22, Feb 11, 2019.

  1. jjhamm22
    Joined: Jan 2019
    Posts: 21
    Likes: 1, Points: 3
    Location: PA

    jjhamm22 Junior Member

    Hey all,
    I am in need of some advice before I begin to re-do the floor and transom on my 79 Mark Twain 24' cuddy with a Mercruiser 260. The boat is in nice shape for a 40 year old boat but the floor is quite soft and I am prepared to tear the whole thing out and glass in new stringers and thwarts. I am hoping the floor beneath the cuddy is ok, but we will see (I found a leak in the windshield that was trickling water into the cuddy). There are no stress cracks around the out drive and I couldn't see any deflection when I pulled up on the outdrive so I was hopeful the transom was ok. This weekend, I got to thumping around the transom with a rubber mallet and found some areas that definitely sounded hollow to me. I will get the drill out and do some test holes before I tear into it but I am preparing for the worst. I have attached a sketch and a couple pictures to show what I am up against. My question is would it be better to tear out the existing fiberglass inside the boat and redo with plywood or would it be better to use Seacast? I would have to cut holes in some upper surface to pour the Seacast I guess. I am hoping one of you old hand fiberglassers will have some helpful tips for me. Thanks in advance.


    Jeff
     

    Attached Files:

  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Verify whether these had balsa core or not before beginning any work. The 70s were known for

    Look for a telltale patter of 2" squares in unpainted hull areas; especially aft. Or areas of the bilge where it feels like there is separation or a loose 'liner'.

    If it has core and the core it rotted; the boat is not economically viable for a core replacement.
     
  3. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Do NOT use seacast for any structural core such as in a transom!!!

    Balsa is not an appropriate transom core either. If transom is rotten:
    • Remove interior glass
    • Remove rotten plywood
    • Replace marine grade plywood
    • Glass over interior
    • Replace stringers . One at a time
    Keep hull well supported so it doesn't worp while stringers are out.

    It's the labor involved which will drive this type of repair through the roof.

    Good luck
     
  4. jjhamm22
    Joined: Jan 2019
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    Location: PA

    jjhamm22 Junior Member

    Thanks for the advice all. The transom is not a core type. If you are not familiar with Mark Twain boats (I sure had never heard of them) they were built in the mid-west from the 60's to mid 90's as a heavy duty, high quality boat. They were chased out of business by the cheaper upstarts like Glasstron as they just couldn't match the low prices. The only reason I am willing to entertain all this work is that I am starting with a high quality hull in nice shape.

    So the SeaCast ads are a bunch of crap? They certainly do advertise like it the best stuff on the planet.
    Many thanks
    Jeff
     
  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I would remove the inside laminate and remove all rot and fix with marine ply.

    Not familiar with Seacast.
     
  6. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Jeff.

    The tramsom is almost certainly cored with plywood. Outdrives are designed for a two inch thick transom. No manufacturer lays solid glass transoms.

    Large outboards also require thicker transoms than are practical without plywood coring.
     
  7. jjhamm22
    Joined: Jan 2019
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    Location: PA

    jjhamm22 Junior Member

    Sorry, I forgot the word "balsa" in my reply to another post. I'm sure it is plywood as there is a hole drilled for a navigation light and I can see it. The funny thing is some areas sound really good when hit with a rubber mallet and then sounds quite different a few inches higher. I would have thought they would rot from the bottom up. I also thought I would see some stress cracks around the drive. Mine is a pre Alpha Merc.
     
  8. ondarvr
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    Location: Monroe WA

    ondarvr Senior Member

    The pound on it with a hammer test is very inconclusive at best. People do the hammer test and are frequency shocked that what they thought they heard was no indication of what they found when tearing it apart
     
    fallguy likes this.
  9. tpenfield
    Joined: Dec 2016
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    Location: Cape Cod, MA

    tpenfield Senior Member

    Seacast is more typically used when repairing an outboard transom that has a rotted core, because they can be accessed from the top (rub rail). For an I/O transom (Mercruiser . . . right ?) you are probably better off with plywood. If you are concerned about using wood, maybe you can research some synthetic core material that would be good for a transom. Most folks end up using wood.

    You could always try to get a few sheets of 'Perma Panel' (Inland Plywood Co.) or 'Greenwood XL' (Greenwood Products, Inc.), which is rot resistant plywood, since it sounds like you will be doing more than just the transom.
     
  10. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    Coosa Composite would be the ideal replacement but it is quite expensive (a little under $300 for a 4x8 sheet 1/2" thick). Don't use Seacast in a structural location subject to stress. Make sure that you want to make the investment in an older stern drive boat before you start buying supplies. Good marine plywood, thoroughly entombed in epoxy resin and glassed in properly is your most economical choice. If your transom is rotted out, how are the engine stringers? You're likely going to find more issues as you dig into this. I'm not trying to discourage you, just making the point that you need to know exactly what you are up against and whether the project is worth doing. Only you can decide that.
     
  11. jjhamm22
    Joined: Jan 2019
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    Location: PA

    jjhamm22 Junior Member

    Thanks all. I have a soft floor and I expect to have to redo stringers and thwarts. From poking and tapping, the motor rails seem to be in good shape. I know it will be a ton of work but should all pay off in the end
     
  12. tpenfield
    Joined: Dec 2016
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    Location: Cape Cod, MA

    tpenfield Senior Member

    Rotten transoms rarely travel alone. Stringers usually come along for the ride.
     
  13. jjhamm22
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    Location: PA

    jjhamm22 Junior Member

    Since it sounds like the tap test is not conclusive, should I drill a couple test holes and see if I get dark wood?
     
  14. ondarvr
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    Location: Monroe WA

    ondarvr Senior Member


  15. jjhamm22
    Joined: Jan 2019
    Posts: 21
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    Location: PA

    jjhamm22 Junior Member

    The Mark Twain brand boats were known for their thick sturdy hulls. Will I have any problems doing floor, stringers, and transom work while sitting on a trailer with rollers? Would a 2x4 brace that keeps the gunwales in place be enough?
     
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