Transverse framing / Centerline girder

Discussion in 'Class Societies' started by ldigas, Jan 22, 2012.

  1. ldigas
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    ldigas Senior Member

    I can't find anything about this in either DNV nor GL rules (nothing explicit),
    but is it mandatory for a bottom CLG to be continuous, or can it be "in between" bottom frames?

    Is it allowable to have a centerline girder in between frames, that is - the frames be unbroken?

    I'm interested in the case of a steel yacht.
     
  2. The Loftsman
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    The Loftsman The Loftsman

    CentreLine Girder

    In my experience the CLG has always been Continuous

    cheers
     
  3. ldigas
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    ldigas Senior Member

    Do you know perhaps whether the classification societies sometimes give any "leeway" in this regard, if the vessel is close to the length which does not require the CLG, and this is a reconstruction (lengthening of the hull by a small amount)?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    As a simple rule:-
    For monohulls, yes, it is generally continuous.
    For catamarans, it is not necessary.

    Principal reason is longitudinal strength and secondary issue is fabrication.

    Monohulls principal axis of loading is longitudinal, thus all long.t members to be continuous, especially where providing strength to the hull girder, in this case the keelson. Whereas for catamarans, the principal axis of loading is transverse, so the keelson is generally intercostal between transverse frames.

    This is "in general", as some other factors can dictate. Such as, up fwd as the hull becomes the bow regions with a high deadrise, the keelson, becomes the stem bar and is then continuous, on a catamaran.
     
  5. ldigas
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    ldigas Senior Member


    What about the case where a boat has a high (steep) bow, a centerline girder and a skeg (which on its lower part has its own "centerline longitudinal keel").

    Would it be possible to make the "main" centerline girder in that case intercostal between frames, while the keel's one is continuous?

    I know that in the end it will be up to the register, but I find this way much faster to get an opinion or two, than to wait 5 months for their response (yup, 5 months).
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    You need to state what type of hull it is, as noted above.

    I think you're confusing keelson and centre line girder. A centre line girder is the keelson. That is the correct terminology.
     
  7. ldigas
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    ldigas Senior Member

    Uhmm, steel built, was a motor sailer, now a motor yacht, displacement speed regime.
    I don't know what you mean under "what type of hull" exactly?

    Center line girder (CLG) = keelson, then? Are they synonims?

    How is the element which holds the skeg plating together then called?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Is the vessel a monohull, or catamaran or something else??...ie, what hull type..1, 2 or 3 hulls?

    No. Keelson has a very specific meaning in ship terminology.
    Centre line girder can be a keelson, but it can also be "just" a girder on the centre line.

    A keelson is a girder that runs longitudinally and continuously on the bottom shell plate on the hull centre line throughout the length of the vessel

    A centre line girder can be a girder that runs in the double bottom region only, for example. When it does the same job throughout the hull, it is called a keelson as it also is sited on the keel plate. Which is often a thicker plate which is extended some distance either side of the keelson.

    Which element and where?
     
  9. ldigas
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    ldigas Senior Member

    1 hull.

    [​IMG]

    Okey, just to check if I got it right. A keelson is the one that runs along the bottom of the keel/skeg/..., and in the upper picture that would be the element which runs down the bottom of the skeg.

    A centerline girder is a generic term for an element which can be, but is not necessary a keelson. In the upper picture, it is not ... since there is supposed to be an another centerline girder which runs "in the hull".

    Does a boat, like the above, which is "just above" 15m length (societies require a centerline girder for boats above 15m) *need* a centerline girder, or could it pass with only a keelson? Acting as a centerline girder in a way?

    Also, a keelson in the above example is obviously continuous (for if not construction, then technological reasons). Could a centerline girder be made intercostal?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    No.

    A skeg is a skeg and a keelson is a keelson.

    They are both different structural members and perform different tasks.

    A keelson contributes to the global hull girder strength, when continuous. A skeg is a localised "bit" of structure that is often an "add on" under the hull plating, and often there is no load path between the keelson and skeg plating.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    As a rule, all monohulls when Classed require a keelson.

    However, you can "play" with this by making the webs intercoastal and the rider bar continuous. This is in effect a centre line girder, rather than a keelson, to confuse matters further.
     
  12. ldigas
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    ldigas Senior Member

    [​IMG]

    I'll have to re-read all of this a few times, tomorrow morning, to see if it'll make more sense then :)
     
  13. viking north
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    viking north VINLAND

    Indigas--I am only a builder and dare I venture into this above my knowledge world of vessel engineering but many of these terms relate back to old wooden boatbuilding. From there they at times are manupilated to cover slightly other engineered structural members as they apply to building materials other than wood. However i've always found it important to keep the basic use and names(wood construction) in mind when one works in other materials. Wikipedia while not always detailed and accurate do give a fairley good definition of the Kellson and one that I visualize when the term comes up. A quick and dirty definition I use is the keelson is as a rule, a seperate but attached inside continiouse longitudional reinforcing keel structure that runs parallel to and above the keel from stem to stern. It's prime but not only function is to add longitudional strength. I hope this very basic info serves as a key to unlock the door to your higher engineering enquirey---Cheers Geo.
     

  14. TANSL
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    TANSL Senior Member

    girders versus frames

    It seems more correct, in a transverse structure, to have frames continuous and girders interrupted between adjacent frames. But, if they are welded and continuity of elements is ensured, you can do as you like, the most suitable for the workshop.
     
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