Hull Strengthening after Rebuilding

Discussion in 'Boat Design' started by Katoh, Dec 1, 2012.

  1. Katoh
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    Katoh Senior Member

    Colleagues
    I have being rebuilding a certain hull for quite a while now, not to get too much into that, but a brief description a 20' planing aluminum hull, with corrosion only in the bottom plates. The corroded plate sections were removed an new plates welded in to suite. Access was not an option through hull so all work had to done externally.
    The Photo shows the removed panels, exposing the internal stringers.
    The second photo shows the new plates welded in with backing plates, the only thing is I had to cut slots were the stringers were, so I could try and tie the plate to the stringer through one weld only from the outside. How successful this was, can only be guessed, and I very much doubt that it would have the strength the same as an internal welded stringer to plate.
    My proposal is as follows. the yellow lines on the drawings indicate where I would place a 6 x 12 6XXX series extrusion on the outer of the hull. This solid extrusion would would be fully seemed welded to the outer hull plate and follow the stringer line. Basically you could consider it as an outside stringer, or strengthening bar.
    Please refer to the PDF for details.

    My question, is this a total waste of time and effort? Will it have some sort negative effect on the hull? Or is this a good idea and should be followed up?
    Thanks For Reading!
     

    Attached Files:

  2. Ad Hoc
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    Ad Hoc Naval Architect

    I strongly recommend NOT to use any 6000 series below the waterline.

    The purpose, structurally, is also unclear, for these 6x12 blocks.
     
  3. TANSL
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    TANSL Senior Member

    I think the plating must be welded to the interior floors and girders. In my opinion you should look for a system to get it. External reinforcements do not solve some of the problems posed by the repair.
     
  4. Katoh
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    Katoh Senior Member

    Gentlemen
    Thank you for the replies, the 6x12 strips I was figuring that they would act like a brace or a stiffing seam along the hull where the internal stringer runs.
    Definitely point taken on the 6series aluminum , just that size came in an extrusion and handy without cutting down full sheets.
    On each of those plates I cut slots 100mm in length and 3 of them per panel so about 100mm apart. each of these slots were around 4-5mm in width and situated where the stringer is. Them from the outside welded them in hoping to bind the stringer and plate together.
    As stated I have no internal access to the hull and do not wish to go that way.
    Sounds like my idea is not going to achieve what I want, may have to look for something else.
    Thanks Again
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    It appears you have a solution and are trying to make it work. However, is the solution ‘sound’…is more the question to ask!

    It appears you primary purpose is the replace the hull plating in certain areas. Yet you are limited in your repair sequence owing to lack of access.

    I suggest the best way to do with is the “usual” manner when limited by space. Firstly cut back the 100x6mm FB stringer by 6mm, so the height is 94mm. Then weld a 50x6mm FB or 30x6mm FB or similar (of 5000 series, such as 5083 not 6000) to the now reduced height stringer, this to be fully welded, not staggered or intermittent. The 6mm FB thickness bringing the total depth back to 100mm.

    This internal strip, between frames, now forms your permanent backing bar. Make sure the butts and seams are aligned with one of this new FB’s, as a backing bar.

    Cut your plates to suit, and the slot weld the plate onto the internal backing bars....and fully weld the seam/butt onto the backing bar as a continuous run as normal.
     
  6. TANSL
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    TANSL Senior Member

    This method is used to weld the plates of the rudder from the outside: placing a support plate, welded to the reinforcement.
     

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  7. Katoh
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    Katoh Senior Member

    Gentlemen
    Once again thank you for the information.
    The drawing that TANSL has posted is basically what I have done, except for the new backing plate on top of the existing stringer. I went straight onto the stringer.
    Like most things in life, if only I knew before I started, I would put those few extra couple of pieces in for peace of mind. A little late now as the plates are in as you can see by the second photo, leading to my thought of adding the outer stiffeners as extra security.
    Again many thanks for your input.
     
  8. TANSL
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    TANSL Senior Member

    Welding directly on the edge of the reinforcement is strongly discouraged. I can not say it is forbidden, because I do not know, but I can assure you it is totally contrary to "good practice" of shipbuilding.
    The backing plates to be placed not attempt to make the construction more expensive, but are intended to avoid delamination problems that may occur if not placed.
     

  9. Katoh
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    Katoh Senior Member

    Gentlemen
    What I have done is bent with a radius 5mm 5083 to 90deg and cut them length wise to form a slight taper. these will be fully seem welded along those lines in the previous picture's and will act as strakes but will also tie those new plates into the existing, even though they are tied into the internal stringers already. The back of these strakes are open to let them drain.
    Now if anyone has a good idea how to paint inside them I would like to know.
    Thanks.
     
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