Compression Failure in Boat Hulls

Discussion in 'Boat Design' started by Iridian, Oct 13, 2021.

  1. Iridian
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    Iridian Junior Member

    I've been thinking a lot about the use of Kevlar in boat building.. Generally, it seems the best use of Kevlar is as internal layers, due to its issues of:
    1. Abrasion Fuzzing
    2. Water Absorption
    3. UV Sensitivity
    4. Floating


    Kevlar's extreme abrasion resistance and graceful failure could make it of interest in toughening hulls against impacts and abrasion damage from grinding..

    https://i.imgur.com/Pi8Kp4C.png

    As far as elongation to failure matching, Kevlar matches quite closely with Basalt Fiber in elongation properties, making a potential combination of Basalt-Kevlar-Core-Kevlar-Basalt potentially useful?

    The layers of basalt fiber would protect from cosmetic abrasion and reduce the possibility of either UV damage or water intrusion to the Kevlar..

    The issue is the compression failure of kevlar.. Has compression failure been an issue in boat hulls? It seems like the majority of point loading is going to be in tension, and compression would largely be caused by either frontal ramming or wave motion. Frontal ramming should largely be managed by crumple zones, so I'm not as concerned by that. Wave motion doesn't seem like it would typically stress the hulls to the point where the kevlar compression weakness would become an issue, especially with the interior bulkhead structure made entirely of another material.

    Any thoughts would be appreciated.
     
  2. rxcomposite
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    rxcomposite Senior Member

    Not quite so. Whenever a beam is in tension, the other side is in compression. And we are talking around the center area of the laminate.

    Since the panels are fixed on both ends, the fixity rotates to counteract the movement. What is in compression in the center of panel reverses and is now inside at the base of the stiffener. The only time it is zero is at about 20% of the length.

    In plane Shear (layers moving against each other) is greatest at midpoint/neutral axis of the laminate thickness. It diminish towards the inner outer skin.
     

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    cracked_ribs and fallguy like this.
  3. Iridian
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    Iridian Junior Member

    I don't think that's quite accurate. If you consider the boat in heavy wave conditions, which I would argue are the most stressing of wave based conditions, the entire hull should be considered as a beam, not just the hull bottom.

    In this configuration, when the boat is supported at both ends, as if bridging a trough, the top skin of the top layer of the boat should be in compression, but the bottom skin of the top layer as well as both skins of the hull bottom should be in tension. When the hull is balanced or falling off a wave, the opposite should be true.

    The question more so lies on what designers are designing the tensile strength of their composites around primarily.

    What is the most stressing case?
    Point loading, wave motion, head on collisions, rig loading?

    If a tensile strength requirement is high enough that the resulting strength compressive strength of a kevlar 29/basalt fiber blend could manage the compression requirements, that could lead to some significant positive characteristics (improved toughness, stiffness, lower cost, reduced weight vs s glass).

    Kevlar and basalt fiber are the only two fibers with extremely similar elongation characteristics.

    From a less theoretical standpoint, do compression failures occur in boat Hull skins?

    Lastly, specific to point loading, would the outside fabric even be in compression given the elongation characteristics? If the core failed in sheer I expect all of the forces would be in tension.

    Thanks for entertaining my theory crafting. All I can do until I get a shop ready for my build.
     
  4. rxcomposite
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    rxcomposite Senior Member

    That is basic beam theory and I did not mention just for the hull bottom. There are many forms of Load models that you have mentioned. 3 moment equation for entire hull (hogging/sagging), impact or concentrated load (horizontal/vertical), partly distributed load, cantelever load (rig), vertical shear and horizontal shear, to name a few.

    Ask a specific question so that members of the forum can answer you correctly.
     
    Last edited: Oct 14, 2021
  5. Iridian
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    Iridian Junior Member

    Put basically, I guess the question is:

    If you were to build a hull skin made of a kevlar basalt fiber mix with sufficient tensile strength & stiffness, would it be likely to fail in compression?

    If it would, how much would you have to overbuild to make up for the compression shortcomings?
     
  6. cracked_ribs
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    cracked_ribs Senior Member

    I haven't yet seen a basalt-kevlar-core-kevlar-basalt build yet but I have seen basalt over core. I think Cerny Yacht are designing around basalt a bit now and like it. I don't follow their stuff closely or anything but I believe they're doing a bunch of basalt over strip-planked core.

    So empirically I'd say if the elongation etc are extremely similar, I don't see why basalt would work but basalt in combination with a functional equivalent would fail.

    The one thing I would be concerned about would just be the hygroscopic qualities of kevlar - but buried behind the basalt it might not matter. People do use it successfully, so it's hardly unworkable. I'm not sure if it would be possible to exploit that and get it to soak up a sufficiently thinned resin, say thinned by a combination of appriopriate thinner per the resin and heat? When I work with epoxy I use heat a lot to control the viscosity, but epoxy isn't the only game in town, either. You can get some resins pretty thin, but I don't know enough about what makes kevlar hygroscopic to know if that's a trait you could work with.

    Anyway basalt is in use as a skin, and kevlar is in use in spot applications at least and someone is probably using it as a layer in the complete sandwich. I think if they were experiencing compression failures we'd hear about it, but maybe not. I haven't heard about it if they have, but I'm not involved with it professionally or anything.
     
  7. Kayakmarathon
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    Kayakmarathon Junior Member

    Is "hull compression" being defined as forces applied to the hull such that the ends of the boat move toward the sky relative to the midship keel, and the midship beam feels like its being stretched outboard? If so, that's what I understand to be a precursor to classic longitudinal failure. Some torpedoes sank ships not by puncturing the hull, but by blowing away the water under midship. With the ends supporting the entire displacement of the ship, the hull buckled into the void of water.
     
  8. DCockey
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    DCockey Senior Member

    For small boats potential point loading, due to grounding on a rock, submerged pier, etc, trailering on a poorly fitted trailer, or improper props for storage, may be of significant concern.
     
  9. rxcomposite
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    rxcomposite Senior Member

    That is what I have said.
    It is the same thing. Just a hogging/sagging condition.


    You are talking about hull modulus, not plate modulus. Hull modulus is not used for small boats (<24 m). Its design is considered strong enough to survive this condition.
     
  10. rxcomposite
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    rxcomposite Senior Member

    Post the mechanical properties of the basalt fiber/kevlar fiber you are comparing so that responses will be accurate. It must be of same material type test, ie, Woven vs woven not woven vs unidirectional.
     
  11. rxcomposite
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    rxcomposite Senior Member

    My data shows Basalt fabric compression strength is higher than Eglass. What are you compensating for?
     

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  12. Iridian
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    Iridian Junior Member

    Sorry for the late response. Been at the Annapolis yacht show all day.

    I'm trying to figure out if compression strength will be an issue in a basalt fiber kevlar layup compared to an s glass or e glass layup, or even a pure basalt layup.

    In theory the kevlar should add lightness, stiffness and abrasion resistance compared to pure basalt or s glass at the cost of compression strength.

    Will this loss in compression strength cause issues on a hull skin?

    I haven't found any retail basalt fiber for sale in the US.

    Was thinking about testing something like this on a canoe.

    https://m.aliexpress.com/item/40008...6f4e90f1152116c157f58047226f25ae63e0a9&gclid=

    But the stats aren't available.

    All the stats I've found online show basalt fiber with an elongation to break at 3.15-3.5%, not the 5.5% you have.. Where is that from? An elongation to break at 3.5% near perfectly matches kevlar 29's 3.6% elongation to break.



    Here are the kevlar stats:
    Kevlar® Properties | Kevlar® Techinical Guide https://www.dupont.com/news/kevlar-properties.html

    Here are the stats for mafic, the only American manufacturer I can find. https://www.mafic.com/product-lines

    Apologies for not posting direct screenshots but I am on mobile.

    @Kayakmarathon
    I'm specifically interested in any likely compression that might cause a hull made with a bkkb or bkckb layup to fail whereas a pure basalt or s glass hull of the same weight would not.
     
  13. rxcomposite
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    rxcomposite Senior Member

    To combine basalt and kevlar, usually it is bkbk. To find the homogenized properties, use Rule of Mixture (ROM). That is homogenised property = sum of property (individual layup) x thickness (individual) /Sum of thickness (of individual layup).
     
  14. Iridian
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    Iridian Junior Member

    If you don't mind me asking, why would you typically do bkbk instead of bkkb? I was trying to avoid kevlar fuzzing, uv, and water absorption with bkkb.

    Are we only able to use the ROM like this because of the similar elongation to break? I.e it would not work on say a e glass carbon fiber layup?
     

  15. rxcomposite
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    rxcomposite Senior Member

    Sorry typo error when I made a graph. It is 4%. I had this data long time ago when basalt was discussed in this forum.
     

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