fiberglass hull thickness

Discussion in 'Boat Design' started by Gianf1041, Dec 18, 2019.

  1. Cacciatore
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    Cacciatore Junior Member

    Ciao Gianf1041 , have you designed the location of longitudinal stiffeners and transversal bulkheads (related to cabin arrangement and other fittings) . This is the first step. In bocca al lupo!
     
  2. rxcomposite
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    rxcomposite Senior Member

    If core is still a preference, end grain balsa is best for a workboat as it has greater compression properties. It is best at 4X the (total) thickness of the original single skin laminate thickness t.
     
  3. Gianf1041
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    Gianf1041 Junior Member

    I hope to correctly translate my ideas; what I'm trying to understand is if I am moving correctly in developing the project that I have in mind, that is, ........ I seem to understand that for the bottom it would be more correct to abandon the sandwich and think of one monolithic thickness ...... well ..... for the calculation of the thickness I apply the formula I posted ....... is it correct? ditto for the lower topside part; for the upper topside part I can think of thickness with sandwich combination ....... this may be acceptable according to your greater experiences than mine.
     
  4. rxcomposite
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    rxcomposite Senior Member

    A boat has 3 zones of thickness. The keel, the bottom(underwater), and the sides (above water) with the keel being the thickest part. Workboats generally do not use core as the arrangement is such that the skin is thin compared to the overall thickness to get some form of rigidity. A single skin works best as it will be subject to a lot of abuse and that the thickness is usually 1.3 to 1.4X that you would regularly use for a leisure craft.

    The choice of material plays also a major part. Veil cloth after gelcoat followed by lots of CSM on the outermost layer as a water barrier, preventing osmosis and WR/Biax on the succeeding layer. I usually keep my Biax on the inner layer as the tensile/compressive strength is not as good as WR. Lastly, the inside is finished again with CSM to promote adhesion as you will be doing a lot of secondary bonding, not to mention that the bilge is always wet. The resin rich CSM layer is a good water barrier.
     
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  5. fallguy
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    fallguy Senior Member

    did I miss workboat?

    I am glad to see rx back. I really am not helping here much.

    I ordered Gerr's book this week out of sheer curiousity.

    I don't believe OP said workboat. But I was interested to see an upside to a denser core like egb.
     
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  6. rxcomposite
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    rxcomposite Senior Member

    Well it seems the thread is already 5 pages long and the OP is still confused. And I am. ;)
     
  7. Gianf1041
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    Gianf1041 Junior Member

    Yes, in fact for the thicknesses I am thinking as you correctly indicated: 3 regions of the hull ....... keel, underwater, abovewater .......... three different thicknesses that I need to evaluate the approximate weight of the hull; at this moment of my idea I don't want to go into too much detail because I am still in a preliminary phase that can change, this also applies to the choice of materials, at this moment I prefer to concentrate on the graphic part of the idea ...... .... study of the hull lines etc.
    In my previous posts I indicated a formula for the calculation of thicknesses, those rules that I read in Dave Geer's text are a simplification - "simple scantling rule" - .... and they must be taken and verified ?? !!
    I don't want to buy with your design experiences because I am very competent, I am only at the beginning and I need to get into the design perspective, my experience is only of navigation.
    Thank you all.
     
  8. TANSL
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    TANSL Senior Member

    Before arriving at the final body lines plan you should make an estimate of the weights and centers of gravity of the boat, including the structure, as approximate as possible. For this, as in any other calculation, it is necessary to use the most exact procedures, possible at each stage, and not mere orientations. The more accurate you are in each phase of the design, the less unpleasant surprises it will take later.
     
  9. Gianf1041
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    Gianf1041 Junior Member

    1) Italian formula for calculating the average thickness of a fiberglass hull, unfortunately it must be translated.
    2) Dave Gerr formula for finding the scantling number which will be used for the formula for calculating the base thickness of the hull; ........ in my opinion it is necessary to verify that the results of the two formulas (Italian formula and scantling formula) are not too discordant.
     

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  10. Gianf1041
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    Gianf1041 Junior Member

    Thanks, I am aware of it, I am moving in that direction.
     
  11. Cacciatore
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    Cacciatore Junior Member

    I don't understand how give suggestions about the thickness without have informations about stiffeners span and building method.
    Divide the boat in (Keel Bottom) - Side and Deck and after how many transversal bulkhead and long. Stiffeners do you want and only at the end perform the scantling.
     
  12. Gianf1041
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    Gianf1041 Junior Member

    They are only rough indications read on the texts that I have made available to everyone, then, each one manages them or not as he prefers ...... I am the least indicated to give suggestions.
    However I am also considering all the irrigations as I indicated on a post.
    Thank you.
    Sorry, but it's not a "note".
    Thank you.
     
  13. Gianf1041
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    Gianf1041 Junior Member

    The idea starts from here,
    it's a bit "retro" ..... let's see how it ends.
     

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  14. TANSL
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    TANSL Senior Member

    In addition to dividing the hull into three zones, within each zone the design pressures are different depending, among other things, on the distance to the stern. Therefore, it is necessary to consider whether, for example, the bow and stern bottom can be the same or not.
    And, of course, before calculating the scantlings of each element, it is necessary to define how many transversal or longitudinal elements there are and the separation between them. It is a pity that there are no shortcuts to do things well but this is the design of ships currently, serious, rigorous, technical.
     
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  15. rxcomposite
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    rxcomposite Senior Member

    You worry about the thickness but you haven't done your homework yet. As Tansl suggested, you draw the lines first, then calculate the weights/displacement based on what you have drawn. And as Cacciatore has suggested you divide the lines into sections and define the longitudinal spacings. That will give you the grid/panel size.

    The thickness of the panels is dependent on the panel dimension, be it a square or rectangular. Then the position of each. Is it at the bottom, sides, or deck? Because each position will have different pressure. There is also the front to back relationship as Tansl has mentioned. Generally, panels forward are thicker as it plows thru the water and there is a reduction of pressure as you go towards the transom. As we progresses, the members here can determine if your boat (depending on speed) will need reduction of panel pressure towards the transom.
     
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