plywood the need for stringer?

Discussion in 'Materials' started by Racman, Feb 14, 2019 at 1:36 AM.

  1. Racman
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    Racman Junior Member

    Plywood GPR yachts and the need for stringer in there construction.

    My question to the more learned here, is the need for stringer in a yacht using glass fiber reinforcement and plywood hull skins. It has been pointed out on past threads discussion regarding cores that plywood has very high strength property is very stiff and considered not a core but a structural element.

    So is the only reason for stringer to provide a means of attachment of the sheets?

    I am hoping to gain a better understanding.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Stringers spread the load around, otherwise you could have stress concentrations at bulkheads, for example.
     
  3. Racman
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    Racman Junior Member

    (Stringers spread the load around, otherwise you could have stress concentrations at bulkheads, for example.)
    Mr Efficiency
    thank you yes that a fact never thought of speading loads, I guess that FRP with cores is more flexable so spending the loads though stringer is not a common practice.
    I have heard of one yacht cat in this case say stinger can weaken the hull.
    I would have to say i don't understand why, i am not an engineer.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Perhaps you should mention what the actual boat is, that you are interested in.
     
  5. JamesG123
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    JamesG123 Senior Member

    In fiberglass on ply, or stitch and glue, etc. the "core" of plywood acts as the major structural element. The glass and resin is more of a waterproofing, sealing component, that also provides some tensional/torsional strength. So the structure is "semi-monocoque". It needs longitudinal bracing from stringers and bulkheads to hold the skin plywood rigid.

    In contrast, full FRP sandwich core structures, the inner and outer skins are the primary (only) structural part and is fully monocoque. The core is there to provide tensional strength between the inner and outer composite skins.

    I hope this simplistic explaination helps and doesn't confuse further.
     
    Last edited: Feb 14, 2019 at 7:55 PM
  6. Racman
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    Racman Junior Member

    Can i first thank you both for help here, i am think about building a 10 metre cat that is design to be built in ply but using pvc foam core and pp honeycomb FRP instead.
    using flat table infusion where possible. the whys are many the time involved in fairing with ply is great, time involved in treating ply to provent rot. over all appearance.
    that the reason for the question. Any input here would be greatful , i have limited expereance.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    You need minimal or even no stringers with sandwich construction, but bulkheads/hull connections need to be properly executed to lessen stress concentrations.
     
  8. JamesG123
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    JamesG123 Senior Member

    No. If you are not an engineer and/or have limited experience you should build exactly to the plans of someone who is and who has proven their design is sound.
     
  9. DCockey
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    DCockey Senior Member

    Longitudinal stringers and bulkheads are not needed if the skin is sufficiently thick. Many plywood boats, stitch and glue, and cold-molded boats have been built without longitudinal stringers.
     

  10. JamesG123
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    JamesG123 Senior Member

    But not particularly large ones and at a certain point you need them well before you need reinforcement in a composite monocoque. Which was my point.
     
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