Substitute aluminum rectangular tubing for wood stringers?

Discussion in 'Materials' started by Carlazzomark, Sep 4, 2021.

  1. Carlazzomark
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    Carlazzomark Junior Member

    I am rehabbing a 6 meter fiberglass fishing boat whose floor support stringers were all rotten. Can I use aluminum stringers of approximately the same dimensions as the original wood and glass over them?

    Unless there is an issue of epoxy adhesion to the metal, it seems to me that the aluminum would be a strong and forever-lasting substitute. All placements are flat.

    Thanks in advance for your thoughts.
     
  2. hoytedow
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    hoytedow Carbon Based Life Form

    I wouldn't use aluminum. It isn't flexible enough and once bent, stays bent.
     
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  3. Blueknarr
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    Blueknarr Senior Member

    Do those rotten floor supports go all the way down to the hull?
     
  4. missinginaction
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    missinginaction Senior Member

    So you're talking about "sole stringers", right? The stringers are NOT an integral part of the hull, rather they rest on the transverse bulkheads that are perpendicular to the hull stringers and are between the hull stringers and the sole or floor, correct?

    If I have this right I suppose you could use aluminum but have you priced it lately? When I rebuilt my small cruiser years ago I used pressure treated southern yellow pine for the application you're contemplating.
     
  5. kapnD
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    kapnD Senior Member

    Rectangular tubing is a poor choice of structural shapes available.
    A T, C, or I shape has no hidden surfaces to breed corrosion in a wet environment.
     
  6. rxcomposite
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    rxcomposite Senior Member

    Why use aluminum tube when you can use mailing tube carton? Just split it in half and laminate over it.
     
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  7. bajansailor
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    bajansailor Marine Surveyor

    +1 - the cardboard simply acts as a former for the fibreglass, and all of the strength is in the fibreglass.
    It would be a real waste to use good aluminium for this purpose - and you might find that even with epoxy you might not get good adhesion if you have an ally extrusion as a core.
     
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  8. rxcomposite
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    rxcomposite Senior Member

    Add to that that fiberglass and aluminum have different Coefficient of thermal expansion. It will be pulling and pushing against each other at different temperature. To glue aluminum to fiberglass you need expensive industrial glue, adding to the cost.
     
  9. Kayakmarathon
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    Kayakmarathon Junior Member

    Aluminum is fairly porous and bonds well to epoxy resin (personal experience with footbraces in kayaks). Never bond aluminum directly to carbon fiber because it creates a corrosive galvanic reaction. Instead, place a layer or two of fiberglass between the aluminum and carbon fiber to prevent corrosion. I have to agree with everyone else's structure concerns using aluminum.
     
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  10. Barry
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    Barry Senior Member

    How is aluminum porous? I have attached a photo of a ductile break. Looks like porosity but it is not
     

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    Last edited: Sep 6, 2021
  11. rxcomposite
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    rxcomposite Senior Member

    Yup. I have seen Aluminum micro etched to increase surface area to increase adhesion. Not porous.
     
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  12. Ad Hoc
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    Ad Hoc Naval Architect

    Unfortunately the image is too small to investigate. However, brittle failure...really??? :oops:
    Aluminium is a ductile material, it would have to have been some really really extreme conditions for the item to fail in a brittle nature.

    What grade/temper of ally is that pic?
     
  13. hoytedow
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    hoytedow Carbon Based Life Form

    At what temperature did it fail? Extreme cold would make it brottle.
     
  14. gonzo
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    gonzo Senior Member

    Nothing lasts forever. If you want to use aluminum to frame the deck, why are you using epoxy? Either welding or using fasteners would make more sense. Also, replacing wood with aluminum will not require the same dimensions. You need to match the properties of the old beams.
     
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  15. Carlazzomark
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    Carlazzomark Junior Member

    Thank you all for your feedback. To answer a few of your questions, The rotten stringers rest on the transverse bulkheads, and do not seem to be integral to the frame; and I have a stock of aluminum already. I had the thought that the aluminum would provide a never-rotting strong support.

    Based on you comments, I will use wood.

    Thanks
     
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