Hull metal or wood, fiber lined

Discussion in 'Boat Design' started by TANSL, Jul 8, 2013.

  1. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    I have seen in this forum, there are people who build a wooden hull and then line it, outside and inside, with fiber. Others want to make an aluminum hull and then also, wrap it with fiber.
    I thought another option : to start at the outer layers with fiber hand made, place a layer of a product that is obtained, for example, by infusion into another shop, and end with other fiber layers manually.
    A part of the utility of the system or not, would appreciate any comment on that from building expert, I would like to know how to calculate the physical and mechanical properties of the whole. This is a "sandwich" construction whose "core" also contributes to the strength of the panel, but how and how much does it contribute?
    Thanks for your help
     
  2. Mike Graham
    Joined: Feb 2013
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    Location: Maryland

    Mike Graham Junior Member

    Computing the nominal properties of a built-up section or panel uses fundamental engineering concepts from strength of materials. Most of the time for construction like you describe it's sufficient to use the properties of a rectangle for the constituent components and apply the the parallel axis theorem. A more-detailed, higher-fidelity analysis can be conducted using the finite element method. Assumptions and methodology for both traditional analysis and FEA (especially the former) are available in many vessel design rules.

    It rarely makes sense to have strong core, especially strong core that is susceptible to rotting. Because of physical laws, strength at the extreme fibers is much more effective than strength near the neutral axis of bending. Because of practicality, it's important to be able to inspect and repair components, which may be hard with this type of construction.
     
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Thanks for your answer Mike,
    The truth is I do not want to get into finite element analysis although I know that would be the correct method, especially, being a very heterogeneous group of materials. I was looking for something simpler, more affordable to a not very tecnical structural designer.
    I was thinking about how to apply the calculation to a boat whose scantlings are governed by ISOI12215-5.
    Actually, if I calculate the "core" to hold the design pressures, I have it solved. I do not need the "core" to increase the cross section module of the plate. HOWEVER, since I will have other materials, besides the "core", I wanted to use them to contribute to the strength of the panel. Something like layer by layer analysis which is performed with non homogeneous laminates.
     
  4. Mike Graham
    Joined: Feb 2013
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    Location: Maryland

    Mike Graham Junior Member

    It's a misunderstanding that FEA is especially applicable because of the heterogeneity of the materials. The part in FEA where you dealt with the material heterogeneity would normally be before the finite element method was actually employed.

    What I was trying to say is that you would handle this exactly like a normal composite layup. Methods for doing this are already pervasive in the literature. I hesitate to hand out a few equations for you to try to do this, as a forum post is no way to design a vessel.

    I also encourage you to consider whether this is a good way to design a vessels. Consider

    • Why do you think this will create an efficient (high strength, low weight/cost) section?
    • How will you inspect it?
     

  5. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    I agree in what you say.
    I am not a boat builder. I am currently trying to market a program for pleasure craft scantlings and one of my potential customers "complained" that my program was not able to do these calculations. So I tried to see if there was a simple way to calculate this particular sandwich.
    The first question I have posed was whether the system seemed useful to shipbuilders of the forum, because my arguments were not able to convince the customer.
    I'm going to forget "the idea".
    As for the finite element calculation, it has been my mistake. The method has nothing to do with the internal structure of the material.
    Thanks a lot.
     
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