Cracks ion aluminum hull, reinforce or not?

Discussion in 'Boat Design' started by Magnus W, Nov 17, 2018.

  1. Magnus W
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    Magnus W Junior Member

    I have recently welded a second crack in the hull of one of my aluminum workboats. The first time I thought "oh well, **** happens" but now I'm not so sure.

    As you can see in the pic the hull is not of an unusual design but rather pretty much straightforward. It has a constant deadrise of circa 18 degrees and measures 8 x 2,6 meters at WL. the horisontal parts of the bottom are about 13-15 cm on each side. The cracks have appeared in the angle inside the red circle (where the sheet metal is welded). Hull material is 5 mm marine grade aluminum and the boat appears to be well designed compared to other similar boats. Perhaps even on the sturdy side.

    Since I don't think I'm running it too hard there's either a problem with the welds or the design of the hull and the supports in the affected area. The "pocket" that the negative angle creates can catch sea from the sides on unless I trim the boat lead away from the sea (as in keel towards the oncoming waves) it makes for a rougher ride and the occasional hard impact. Since I'm aware of the behaviour I do my best to avoid this but I suspect that this is what's causing the cracks (i.e. they wouldn't have appeared if I only drove the boat in calm seas).

    In order to ease the stress on the welded angle I'm considering welding a say 5 cm strip along the fuselage.
    I'm curious whether you think it will have a noticeable effect on the performance of the boat or not?
    Would you recommend using 5 mm as in rest of the hull of something thinner (more flexible) so as to not move the problem somewhere else but rather just bring some relief to the current welded angle.
    Would you weld it all the way or spot weld it to allow for more movement. Maybe like this –_–_–?

    Pic is not to scale.

    Skärmavbild 2018-11-17 kl. 09.54.42.png
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    It might be a case of more internal support being needed. Can you access that part from inside ?
     
  3. Magnus W
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    Magnus W Junior Member

    In some places yes, in others no. I would agree that that would be the best solution but the effort required would render it cheaper to build a new hull (factoring in the 0 vs 3000 hr and 20 year difference).
    So I’m looking for a compromise.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't think it would make much difference to the performance, but whether it helps or hinders the structural aspect, vis a vis cracking, is another matter. Are there any strakes inboard of that wide flat at the chine ?
     
  5. Magnus W
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    Magnus W Junior Member

    If I understand correctly the question regarding strakes is wether there are any other plating joints between the cracking one and the keel.
    And as far as I can tell no. Save for the cracking joint the bottom plating is single sheet from chine to keel.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, over what length is the cracking, is it both sides of the hull, and where is it, nearer the bow than the stern for example ?
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Magnus W

    This is a classic location of failure. The reasons for the failure is two fold

    1) The weld prep detail is poor
    2) There is not support for the weld at the joint.

    LR has its own method of detail improvement:

    upload_2018-11-17_21-51-48.png

    Basically you need to support the joint and have a better weld prep.
     
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  8. gonzo
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    gonzo Senior Member

    Can you find out what kind of alloy was used in the hull?
     
  9. kapnD
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    kapnD Senior Member

    How about some photos how about some photos of the cracks and internal structure in the area?
    Without more info, any answer is a shot in the dark.
    It could be the material, the weld, how the weld was done, supporting structure/framing, hull design, or even how the boat is loaded and what it is asked to do.
    My first guess would be as Ad Hoc has suggested, chine flat material inadequate.
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Magnus, I've seen (and repaired) some of these hulls with crossing welds in the zones close to the frames, like in AH's left picture (frame welded continously to the skin and crossing the longitudinal welds). The cut-outs shown in the right pic are essential to reduce cracking in alu structures. The trick in repairing is to remove crossing welds and open up the crossing members so there is some flexibility behind the weld.

    If you go for the strip addition, you should check where there will be crossing welds.
     
  11. Magnus W
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    Magnus W Junior Member

    Both cracks are on the port side, the first one circa 1/3 from the bow and the other one 2/3 from the bow. Both cracks are about 10 cm (and to be clear it's the welds that have cracked, not the material besides them).


    I don't have the proper vocabulary but the stringers run from bow to stern, not like in the pic. And there are cutouts and the stringers aren't fully welded to the plating but from memory about 1/3. (OT: At least over here this has, over 50 years, proved to be a good general design. The first aluminum designs from my own home island dates back to the 60's but since they initially built them like they did the old wooden boats – with transverse ribs rather than longitudinal stringers – they cracked. These are the same guys that help me with my boats and they haven't said anything about this one being obviously of a poor design.)


    I doubt it. The boat is a one off copy (licensed) of another design and I have already tried to track down the builder for other reasons. Judging by the color and shine of the aluminum, as well as how it behaves as far as corrosion goes, it's definitely some kind of marine grade aluminum able to withstand salt water.


    I don't think I'm asking more of the general design than it should be able to handle. I'm more inclined to suspect a problem with the welds or a design flaw in this area (or poor drawing interpretation of the builder). I'll get some pics come Monday or Tuesday since I had to leave the boat at the yard as the replacement of the gimbal bearing in the transom turned out to be a slightly bigger problem.


    If I add a strip it will cross some transverse welds in the plating. Is this a problem too?
     
  12. JSL
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    JSL Senior Member

    questions.
    how old are the cracks.
    are they stable or 'growing'
    what alloy is it.
    are welds of chine flat/bottom plate on both sides of plate.
    if welds cross, you may? have a tri-axial stress problem
     
  13. Magnus W
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    Magnus W Junior Member

    I don’t know how old the cracks are but I pay close attention to my boats and if the water in bilge taste of salt I find the problem and fix it asap. But I suppose a fine crack can sit for quite a long time without leaking anything more than shy over nothing so... I can’t say if they are stable or not because I fixed them instantly.

    Welds on both sides (inside and out).

    No idea on the alloy, other than some kind of marine type being able to withstand salt water.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Yes please...piccies always help :)
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    If it is a boat that has already done a lot of work and isn't leaking to any noticeable degree, and the cracks are only in the welds, the obvious solution seems to be grind out the cracked welds and re-weld those areas.
     
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