Sea Sled madness. It’s in my brain.

Discussion in 'Fiberglass and Composite Boat Building' started by DogCavalry, Nov 11, 2019.

  1. fallguy
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    fallguy Senior Member

    I mean of the space between bh3/6.
     
  2. DogCavalry
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    DogCavalry I aim to misbehave.

    20210408_194818.jpg

    Here you go!
     
  3. fallguy
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    fallguy Senior Member

    I don't see any stringers.
     
  4. TANSL
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    TANSL Senior Member

  5. fallguy
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    fallguy Senior Member

    See DogCavalry post monday 7:26
     
  6. fallguy
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    fallguy Senior Member

    Oh. Duh. I see them now. They are the transom knees...

    yeah...I'd run them all the way to the front...and not worry about if they end on the steps

    Next time, the lovely Anne can point them out for me like Vanna White or those Price Is Right models...
     
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  7. DogCavalry
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    DogCavalry I aim to misbehave.

    The thing about strip plank construction is that the hull surface is absolutely anisotropic, unlike any metal construction, or fiberglass, or glass on foam. There is vast strength in the axis of the planking, and basically none across it. The compression yield of each plank is roughly 2 tons. Tensile yield strength is around 5 ton. Per plank. All the layers of glass I apply add strength equally across the planking and with it, because it's 45/45 biax. So there's no need for stringers unless they address a particular load. At the transom, the load from the outboards, 20200222_184458.jpg at the ram, pushing loads from a barge. Where longitudinals are added, they must not weaken transverse structure in any way. So I can add longitudinals between bulkheads, but not cut the bulkheads around them.

    See these strips? Stringers would only weaken the boat, because they would make the interior glass completely discontinuous, unless I added enough glass to make the hull surface isotropic, in which case I wouldn't need stringers or even bulkheads.
     
    Last edited: Apr 28, 2021
  8. TANSL
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    TANSL Senior Member

    With all due respect and amazement for the work you are doing, but I believe that your theories, which I don't know where you have learned it, fail in several things, but fundamentally in that glass or glass on foam are totally anisotropic materials. Therefore it is not easy for adding a biaxial glass fabric to convert the wood into an isotropic set. On the other hand, regardless of the isotropy or not of a panel, you can stop placing stringers if you make the panel strong enough (perfectly isotropic materials, such as metals, need transverse and longitudinal reinforcements). With regard to bulkheads, there is, for example, the phenomenon of global hull torsion that may make it necessary to place transverse bulkheads. I'm not saying this is a problem for your boat, because I don't know. It would be necessary to study the structure as a whole and the real stresses to which it is going to be subjected, not only the bottom and side but also the decks and soils (if any), hydrostatic, hydrodynamic and any other type of stress (of which you have not told us anything).
    In short, this is your boat and you build it however you want, but I don't see any theoretical background to back up some of the things you say.
     
  9. DogCavalry
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    DogCavalry I aim to misbehave.

    Sorry TANSL. Wood strips are almost totally anisotropic. You misread my intent. An all glass panel, or glass on foam is close enough to isotropic that stringers can contribute.
    But the completely anisotropic assembly of edge glued strips already has sufficient strength in their grain axis that more is meaningless. They are completely weak normal to the grain axis. A small square that could easily handle 20 tons compression in one axis, I could easily break across my knee in the other direction. More bulkheads or frames are needed there, not stringers. I will add longitudinals to carry external loads from the ram to the transom, so that my hull doesn't concertina when pushing a very heavy barge.
     
  10. TANSL
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    TANSL Senior Member

    Agreed, wood strips are almost totally anisotropic and there is no way to do them isotropics, imo. Unless I'm wrong again, which is not uncommon, glass or glass on foam are totally anisotropic materials.
     
  11. DogCavalry
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    DogCavalry I aim to misbehave.


    I think you get tripped up by language.

    Glass or sandwich can be isotropic if the layup is designed that way. Generally designed to match the expected loads, of course.

    I just like wood!
     
  12. TANSL
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    TANSL Senior Member

    Several layers of different fabrics and / or mats, joined by a matrix, also different, can never give an isotropic product.
     
  13. fallguy
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    fallguy Senior Member

    ...except you get a tangible benefit of reducing spans for the sole, unless furnishings or others are involved, etc.

    I have a strip canoe and the bottom ripples a wave thru it at any speed. Now, it is only 5/16" thick and only has 12 oz woven on each side, but I'd be willing to bet a doubly thick hull with double skins at 10x the speed might behave similarily...

    It'll be interesting to hear from the man in Japan. He is probably just starting his day.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Hmmm… seems you have several ‘issues at play here, so I won’t try to ‘soil’ this thinking.. :D

    Anisotropic = different properties in all 3 axis.
    Isotropic = same properties in all 3 axis.

    Composites tend to be anisotropic. I say ‘tend’ to be since they can be either: resin dominated or fibre dominated. Bog standard GRP tends to be resin dominated. And as such can ‘tend’ to be ‘isotropic’ owing to the resin being dominant.

    With unidirectional fibres, such as wood, the properties along the grain, across the grain and through the grain, are all different.

    As for the ‘no need for stringers’.

    It all depends upon how you wish to design the structure and the means to absorbed and shirk the loads from one member to another v weight.

    A panel that has a length and breadth = area. The area determines the load being applied to it and then the supports, along the l x b, to take this load. You can design a structure to be totally transverse in nature as you can all longitudinal in nature. The choice is yours, but each has their own quirks.

    All traverse framing means under a large hull girder load, the hull can concertina like an accordion.
    All longitudinal ‘framing’ means under an asymmetric load the hull girder will twist along its length.

    If the area of support = a thickness of skin, and the thickness selected satisfies the load case and does not yield under said load, then the area of support is satisfactory. One then assumes the l x b is also satisfactory and that the member in the l direction and b direction are suitably ‘stiff’ to support the applied to the area.

    However with composites the designs are deflection driven, not stress driven. That's the stiffness part..or the EI.

    So you need to establish, is the panel stiff enough? So whilst the panel may satisfy the strength requirements from the applied load, it shall deflect. Thus, is this deflection satisfactory? If the deflection is less than say 1/5th the thickness of the panel, then one could say – yes. If the deflection is say 5 times the thickness then one can say – no.

    The same logic applies to the supporting structure, over their respective given spans. Does the frame deflect – yes – but how much and is it a concern? You don’t want frames deflecting so much that the sole (don't ya just love Google Translate) is constantly flexing as waves pass it becomes difficult to stand. Doesn’t give a degree of confidence!

    Similarly, one doesn’t wish to see long.t’s flexing visibly too, between frames, as this too tends to imply a degree of uncertainty in the overall strength.

    Deflection – in itself, is not a bad thing….if it is “designed” into the arrangement. Just like an aircraft wing…it flexes…so what? It is designed that way.

    So, the bottom line is, is the 45/45biax sufficient to prevent excessive deflection of that panel under an applied extreme load? If it is, then we look to ask: is the supporting structure stiff enough – for stress check – and then if so, is the supporting structure’s deflection is also satisfactory.

    So, if the answer to all these Qs is – yes – then, it matters little how the structure is arranged. Since the structure is then doing the job it is design to do. Whether it complies with “norms” or “expectations” is not relevant.
     

  15. fallguy
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    fallguy Senior Member

    I have to ask, if you load the front of that boat with 1500 pounds of cargo, doesn't that lend itself to the very notion of the large girder load?

    I'd run the stringers.
     
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