Shell Arrangement of Welded Ships

Discussion in 'Metal Boat Building' started by ktimg, Nov 4, 2019.

  1. ktimg
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    ktimg Junior Member

    As far as I know, common practice is that the transverse weld joints (or butt joints) are arranged staggered, and longitudinal seam lines run continuously along the ship length. (as shown in the picture below)
    upload_2019-11-4_16-38-7.png
    I am wondering what is the reason behind this.
    Thank you in advance for your discussion. :)
     
  2. bajansailor
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    bajansailor Marine Surveyor

    It seems logical in terms of longitudinal strength purposes.
    Maybe one reason could be that it is a development from traditional wooden boat building - here you want to have longitudinal planks as long as possible, with as few as possible butt joints.
    And you want to have these joints staggered along the depth of the vessel.
    If (for example) all of the butt joints were in the middle of the vessel it would be very weak in longitudinal bending.
     
  3. TANSL
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    TANSL Senior Member

    The attached files show some examples of current ships. The yellow lines indicate the weld seams of the plates. You can see that there is no problem in placing the butt plates with continuous welding.
     

    Attached Files:

  4. ktimg
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    ktimg Junior Member

    @bajansailor, thanks for your input, it’s an interesting reasoning that it comes from wooden boat construction.

    @TANSL, Thank you for your reply. that’s the common practice I hear a lot in my environment to have staggered butt joints. I am also getting dubious about the idea now because I see photos of ships with block construction, and according to the nature of the block construction method, I think it is difficult to have staggered butt joints.
    However, is there really no complication with having weld lines as cross intersections?
     
  5. TANSL
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    TANSL Senior Member

    Absolutely no complications, if the welding is done correctly, if the roots, the beginnings and end of the welds, etc. are sanitized (I don't know if this is the correct word).
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    To minimise distortion and shrinkage.
     
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  7. ktimg
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    ktimg Junior Member

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  8. RAraujo
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    RAraujo Senior Member - Naval Architect

    I think it comes back from the days welding was not so reliable - this arrangement would prevent propagation of cracks. Nowadays this is not used anymore since it would also complicate the arragment of building blocks.
     
  9. JosephT
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    JosephT Senior Member

    I believe this is less of a concern today due to improved manufacturing quality by most ship yards. However, let us say a seam were to fail for whatever reason (e.g. bad batch of steel, collision in a harbor with another vessel). Staggering the plates would, in theory, prevent a longer split along a continuous seam. As such, a staggered seam would prevent extra water breaching the hull. If there is any doubt whatsoever about the quality of the weld I would opt for staggered seams.

    Boat yards that repair hulls may not use the identical steel plates, which introduces yet another vulnerability. Dissimilar metals welded to each other bring along yet another set of potential problems.

    Agree with others that there is history for this that goes back to wooden ships & boats. Unless we are talking about the keel of a large wooden boat, continuous wooden seams may not be as strong as staggered seams. For example, while a keel typically has plenty of reinforcement, seams along the hull may not be fully supported. When a ship or boat is bumped hard by an obstacle, a continuous seam can be more vulnerable.
     
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  10. Ad Hoc
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    Ad Hoc Naval Architect

    Well, that is a different question to your original one in post#1.

    Yes, this is problematic and not recommended. You always need to avoid welds which cross, even worse when like this " + "
    So four corners meeting at once. This is a combination of a biaxial and triaxial stress raiser, and must be avoided at all costs.
    So, welds are staggered to avoid this meeting of the edges in one location.

    When building in blocks, to some extent we cannot avoid this. So the internal structures are staggered slightly to offset this. So whilst the plate butts/seams are all in line, the long.ts etc...generally are not, they tend to be staggered.

    Also, when doing this in steel...it is not so much of an issue, as steel is a "forgiving" materiel. Whereas doing this in aluminium must be avoided. Aluminium is less forgiving in that sense. This is not to say you cannot do it, you can, you just must provide mitigation to the effects of shrinkage/distortion and biaxial and triaxial stress raisers.
     
    Last edited: Nov 6, 2019
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  11. ktimg
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    ktimg Junior Member

    Thank you @RAraujo and @JosephT for your inputs. I can imagine that large ship builders usually don't bother about staggering butts anymore due to their improved welding capability.

    This is the answer I needed. Thank you, @Ad Hoc . I think it largely impacts the fatigue life of boats.

    This is also important information, as I am currently dealing with aluminum boats mainly.

    Regarding the mitigation of effects, I am sure we can look up in the books for good welding practice and such. Nevertheless, I think it would be great if a discussion is made here of your common welding practices.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Then you should read an article in Professional Boatbuilder Magazine issue No. 151, Oct-Nov 2014, called - Order of Assembly.
    This should answer all your questions.

    For example, the sequencing of the butts and seams is shown:

    upload_2019-11-5_17-23-1.png

    It is worth your while reading it.
     
    Last edited: Nov 5, 2019
  13. Ad Hoc
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    Ad Hoc Naval Architect

    YES!!!...that is why we always avoid doing this and only if there is no other option. It is generally bad practice to weld over welds like this.
     
  14. ktimg
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    ktimg Junior Member

    Thank you @Ad Hoc for your detail explanation.
    I will also check out the article for more knowledge.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    It wasn't very detailed, just general in nature, as your question is also rather general in nature.
    But if you have any further questions, please ask...
     
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