Transom Thickness for MerCruiser Sterndrive

Discussion in 'Metal Boat Building' started by TinMan007, Apr 5, 2021.

  1. TinMan007
    Joined: Apr 2021
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    Location: Portland, OR

    TinMan007 Junior Member

    Hi, I have a aluminum 1979, AMF Crestline '19 Norseman with MerCruiser 140 with a Mr (pre Alpha 1) drive and need to replace the plywood transom. The existing plywood is 1" spanning the entire transom with a smaller 3/4" piece (total plywood transom thickness 1 3/4"). There is no epoxy or fiberglass adding to the thickness. My marine mechanic believes the transom should be 2" thick. Given the boat is 42 years old, it's unlikely the plywood I'm replacing is original so it's anyone's guess what was originally spec'd for the boat. Prior to my mechanic's feedback, I purchased Coosa board (instead of plywood) to match the plywood thickness. My question is, is 1 3/4" thickness of Coosa sufficient for this application? Any help is deeply appreciated.
     
  2. missinginaction
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    missinginaction Senior Member

    So you're going to laminate that COOSA to get it to 1.75 inches? If the old 1" ply was sufficient that COOSA will be even more so. Based on my experience 1.75" of COOSA will be stronger than the aluminum hull itself.
     
  3. gonzo
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    gonzo Senior Member

  4. TinMan007
    Joined: Apr 2021
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    Location: Portland, OR

    TinMan007 Junior Member

    Thanks for the reply, yes, I'm laminating the 1" and 3/4" COOSA to get to the same thickness and set-up (a smaller 3/4" square laminated to a large 1" rectangle) as the existing plywood setup equaling 1 3/4". It sounds like you believe that combined thickness/setup of 1 3/4" COOSA is sufficient. All comments are appreciated.
     
  5. TinMan007
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    Location: Portland, OR

    TinMan007 Junior Member

  6. TinMan007
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    TinMan007 Junior Member

    The instructions (referencing flatness uniformity) and the diagram suggests the specification of 2" to 2.25" total thickness is for a fiberglass boat. My guess (that could be trouble) the aluminum transom plate combined with the 1" 3/4" Coosa would have a comparable strength to a 2" wood and fiberglass transom. Any thoughts?
     
  7. missinginaction
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    missinginaction Senior Member

    I don't disagree with Gonzo. Reading the manual carefully though, Merc states that the 2-2.25" thickness applies to the area 8" either side of the transom center line. They don't call for a 2-2.25" transom thickness all the way from port to starboard.

    I think you'll be fine with what you are doing. That COOSA and an aluminum plate will be here long after we're all gone as well.

    MIA
     
  8. Howlandwoodworks
    Joined: Sep 2018
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    Howlandwoodworks Member

    Just beef it up with some aluminum angle stock somewhere if you can.
    Crestliner 12' sportman
     
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  9. TinMan007
    Joined: Apr 2021
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    Location: Portland, OR

    TinMan007 Junior Member

    Thanks MIA, I appreciate your input. Once I have the outdrive removed I can measure the transom aluminum plate (the hull) and add that thickness to the Coosa (1" 3/4") to determine the entire transom thickness (inside surface to outside surface). I think it's safe to assume (that could be trouble... again) the 2-2.25" spec'd in the 2001 manual is for materials going back 20 plus years (pre composite board/Coosa) and the increased strength of Coosa (compared to marine plywood) would allow for a slightly thinner transom. Any additional thoughts are appreciate.
     
  10. TinMan007
    Joined: Apr 2021
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    Location: Portland, OR

    TinMan007 Junior Member

    Thanks Howlandwoodworks - funny you say.... there is a piece of aluminum angle stock running across the entire transom (inside, 8" above outdrive) starboard to port. I'm starting to feel good about what I've got going. Btw, nice 12' Sportman, such a cool boat. That must have some get-up-and-go. Any additional input is appreciated.
     
  11. gonzo
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    gonzo Senior Member

    The thickness requirement is for the bolts between the gimbal housing and the transom plate to work.
     

  12. Howlandwoodworks
    Joined: Sep 2018
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    Howlandwoodworks Member

    It would be best practice to find out what horsepower your boat is rated for. 140 is a large engine. Check for stress cracking.
     
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