Need help with transom thickness/layup

Discussion in 'Boat Design' started by modunlavy, Nov 29, 2017.

  1. modunlavy
    Joined: Jan 2003
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    modunlavy Junior Member

    On my inboard to outboard conversion, I need help determining the thickness and layup schedule for the transom modifications to be able to handle the outboards.

    I currently have a 1/4" thick solid glass transom (that is fortunately flat). I am going to add an aluminum flotation style bracket with 30" setback for a pair of twin 300 hp outboards.

    After some research, I have decided on using the Coosa Bluewater 26 as the core material, and it is available in up to 2" thick.

    My current thought process is to grind the existing glass back enough to build it up to 1/2" thick, use the coosa board for the coring at a total of 3" to 4" thick, then cover with another layup of about 1/4" of glass.

    Is this enough or too much? I am also thinking I don't need to use the coosa above where the bracket bolts to the transom, or maybe just not as thick? Any help or guidance would be greatly appreciated.

    Thanks,

    MD
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Isn't the compressive strength of the material sufficient to not have worries about crushing when the bolts are tightened ? Some extra large washers or plates under the nuts should be used as insurance, anyway. The tricky part might be getting your coosaboard to sit hard up against the existing transom while it sets up, it will need to be clamped or otherwise keep in intimate contact. Make sure you use a laminate designed for strength, with high glass content and the better non-woven fabrics. I'd go a minimum of 1/2 inch both sides of the core, with all that HP and weight back there.
     
  3. modunlavy
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    modunlavy Junior Member

    Thanks. I have a plan for clamping down the coosa board that should work. Is there a way to calculate the thickness of the coosa required or is it done based of experience? I have some people telling me to go 4" thick and others telling me 2.5" should be plenty.... pretty big difference in weight back there that I would like to eliminate if I could.
     
  4. fallguy
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    fallguy Senior Member

    I would love to know the calculations used to spec a transom. I am guessing on mine using corelite 3/4" laminated two pieces to 1.5" then 3 layers of 18oz biax each side tabbed into the hull; so just under 2". But my transom is very small; 3' wide only and 2' at the bottom, and my horses are 70 total. We are also putting a couple knees on, but the hull is stringerless. For something like 300hp twins; I'd want to make sure I had enough flexural strength. Isn't this a question for a paid naval architect at some point? Maybe you ought to consider paying someone. I would. I had a designer help with my specs, but he was paid. The guessing I refer to is I don't have hard numbers that say the motor will impart x force and the transom can withstand y force.
     
  5. modunlavy
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    modunlavy Junior Member

    just spoke to a naval architect, he told me I would be fine with a single 2" thick of bluewater 26 coosa with 1/2" of glass on both sides, with good overlap to hull sides and hull bottom on outer skin. he said this is typical of a lot of outboard boats being made today....
     
  6. gonzo
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    gonzo Senior Member

    If you are planning on clamping the outboards to the transom, 1 3/4" is about the thickest the motor brackets will be able to work with.
     
  7. TANSL
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    TANSL Senior Member

    There are not many boats that carry 600 hp attached to the transom so I think that, although that configuration may be close to what is necessary, it would be advisable to make a more detailed study, to check if it is better to use marine plywood, with layers of glass for both sides and, perhaps, connect by means of brackets the transom to the bottom and side longitudinals.
     
  8. fallguy
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    fallguy Senior Member

    The 70hp Yammie allows 2.25" max for the tabs. I'm not 100%, but I saw some tabs that are movable for bigger engines. They have a cam to move the hook off the vertical. I'd be surprised if the 300hp rigging sheet shows a tab limit, but smaller motors do.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    He is using an aluminium bracket, it is the structure of the boat transom that the pod(s) will be bolted to, he is worried about.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I neglected to mention the matter of the overlapping, to the existing hill, all the way round, which should be gradually tapered to blend in to the existing hull bottom/sides.
     
  11. TANSL
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    TANSL Senior Member

    You are right but I would like to know how he is going to join that alu bracket to the existing transom because, in the end, the loads and vibrations generated by the engines will be transmitted to the boat's transom.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Through bolting, presumably. It sounds like it would be a nice wide twin -engine bracket, which in itself acts as a brace to the transom, as well. His 3" total thickness transom should be quite adequate, provided the glasswork is done using the right methods and materials.
     
  13. modunlavy
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    modunlavy Junior Member

    This is what I am going to do:

    [​IMG]

    I have been to the West Systems training class and plan on using their epoxy and following their guidelines for overlapping the transom and the stringers.
     

  14. modunlavy
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    modunlavy Junior Member

    Just looked at a new 41' boat with triple (3) outboard 350 hp engines. Transom total thickness is 3" including glass and coring. I think I will be fine with 2" of the Coosa bluewater 26 and 1/2" glass on each side for a total of 3" as well. Now to start laying out the stringers - will start a thread for that I guess when I get to that point.
     
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