Watertight Bulkheads?

Discussion in 'Metal Boat Building' started by ImaginaryNumber, Jun 9, 2012.

  1. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    In a welded steel or aluminum hull, with stringers and bulkheads, how are watertight bulkheads sealed around the stringers?

    It seems to me that unless the stringers are welded full-length, even if the bulkheads are welded both to the stringers and the hull, water will leak through between the stringers and the hull. What am I missing?
     
  2. TANSL
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    TANSL Senior Member

    In general, the bulkhead perimeter is welded directly to the hull, double bottom or deck. It is not necessary a perimetral stiffener. And, of course, continuous welding, both sides.
     
  3. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    My question is how the stringers are sealed to the hull, if they are skip welded, which I assume (incorrectly?) is typical? The bulkhead can be welded to the hull, and the bulkhead can be welded to the stringers, but if the stringers penetrate the bulkhead, but aren't continuously welded to the hull, then they will be a point of leakage through the bulkhead. At least that's the way it seems to me. There must be a common solution that I'm not thinking of.
     
  4. TANSL
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    TANSL Senior Member

    I'm not sure I understood your question but I think that reinforcements should be continuously welded to the hull on both sides of the reinforcement. You could attach a sketches to accurately analyze your problem.? I am sure we will find the solution
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

  6. TANSL
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    TANSL Senior Member

    Perhaps some of these solutions is right for you
     

    Attached Files:

  7. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    TANSL and Ad Hoc,

    The collar plates you depict will solve the problem of filling in the mouse holes in the bulkhead. My question was how to seal the small gap between the stringer and the plating, if the stringer was not continuously welded on both sides? The books I have on building steel boats say that the stringers are typically not continuously welded. Perhaps the amount of leakage through these very small gaps is considered to be trivial, given that if a watertight bulkhead was actually needed, the implication would be that there was a much larger leak somewhere else than would be created by these "leaky" stringers.

    I could imagine making a notch in each stringer on either side of the bulkhead, then welding the stringer to the plating from the bulkhead (or collar plate), through the notch, and then back to the bulkhead. This would seal the leak I'm so worried about. :) But it might be tricky to weld through the notch.
     
  8. TANSL
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    TANSL Senior Member

    The longitudinal reinforcement should have a continuous weld of, let's say, 350 mm on each side of the bulkhead, and both sides of the stringer.
    The gap between the collar and the reinforcement may be about 2 mm, around the whole profile, to allow good penetration of the welding.
    Collar is placed only on one side of the bulkhead, the side opposite the one where bulkhead's vertical reinforcements are situated. From the other side you have to weld the bulkhead and the collar.
    See detail.
     

    Attached Files:

  9. Ad Hoc
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    Ad Hoc Naval Architect

    Ahh..ok, i get what you're on about now.

    Traditionally with WTBs and Tanks, either side of the watertight or Oil tight bulkhead, the stringers would have a small mouse hole in them. These are called "stoppers". The purpose is exactly that, should a leak occur in the bulkhead, it is confined to the periphery of bulkhead (and its weld) and the small amount of weld from the bulkhead to the stopper. Nominally some 50mm from the bulkhead, and a mouse hole of nominally 20mm radius.
     
  10. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    A "stopper." I like the name. Thanks for the information. :)
     
  11. waikikin
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    waikikin Senior Member

    Ad Hoc, I've seen on some barges & motorized work platforms the longti framing members being bracketed through water tight bulkheads- away/inboard of the shell to bulkhead intersection, just wondering as these vessels have been as small as 12M if this is a "quick & dirty" approach on a small vessel to ease/aid fabrication & welding out & more typical of ship/tanker construction as the written/illustrated reference Ive seen of it is in D. J. Ayres on ship construction, this is a pretty old text- if still valid. Also wondering that in Aluminium construction if there's a different or similar treatment for the consideration of tri-Axial/radial cracking stresses at theses intersections. Jeff.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Jeff,

    As always, it depends.

    Do you mean both the long.t stringer and the bracket pass through the membrane (the WTB)...and it also depends upon what other structure is around to support the penetration too. Do you have a sketch/image to clarify better what you mean?
     
  13. waikikin
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    waikikin Senior Member

    Ad Hoc,

    Just the bracket only passes through the watertight bulkhead(& is welded around) & the longitudinal(angle) finishes short of the bulkhead, the "slot" for the bracket is inboard or away from the perimeter weld of the bulkhead to skin & perpendicular to the skin also & the bracket laps partly(maybe halfway) onto the leg of angle that's welded to the skin. Regards from Jeff.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    I have seen this type used before, as you mentioned. I wouldn't recommend this type, generally. But have used in the past for some applications.

    However, if the vessel is high-speed and thus prone to slamming, it is not a recommended joint. It is a stress raiser. The toe of the bracket will crack and/or the high impact loads, or shear loads, can rupture the thin web of the WTB plate/frame. The critical area being the toe of the weld.

    But, if you want to do this type, then of course the only way is to have the toe of the bracket to finish on another bit of structure, like a rider bar (of the frame), or use a stiffener:

    WTB-bkt-1.jpg WTB-bkt-2.jpg
     

  15. TANSL
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    TANSL Senior Member

    The solution "Section XX" results in a misalignment between the horizontal and vertical reinforcement and therefore is not correct. The webs of both stiffeners should be aligned and their flanges placed on the same side. The brackets displaced in order to overlap both webs (yellow bracket) or, other solution, also aligned with reinforcements (blue brackets).
    Otherwise we create shear forces highly undesirable.
    The radial shape of the edge of the console is more expensive, from the constructive point o view, and reduces its effectiveness.
    Here I show two possible solutions that maintain the alignment of the reinforcements.
     

    Attached Files:

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