Scantlings methodology for recreational boats

Discussion in 'Class Societies' started by TANSL, Dec 21, 2013.

  1. rxcomposite
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    rxcomposite Senior Member

    With big boats approaching 24 meters, there can be a substantial reduction on laminate thickness (and weight) due to allowable reduction of pressure. Sometimes up to two layers. At times, I play with the frame spacings. No rule says it has to be half station. But that is detailing and it goes with the amount of time spent in the analysis and iteration.

    I am also careful not to load the uppermost (cap) of the stiffeners. With LR software (or even Excel) this can be easily seen in the ply by ply analysis. There are instances where you can see in the analysis (LR) that the stiffeners are the one highly loaded (but no failure) but the panel is not, this calls for a reduction in laminate thickness or an increase in stiffener dimension.

    David,
    Thanks for the appreciation.
     
  2. TANSL
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    TANSL Senior Member

    Very interesting, I will study that in depth.
    David, thanks for what I care.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    As I noted before, it is related to ones knowledge and experience of the rules being used that must be satisfied. It seems a vast majority of posters use ISO, I do not. I very rarely use ISO rules. Some 95% of my work is Class based.

    Also the structural design is not and should not be a set of absolutes and nor should it be done in isolation of what the yard can produce and is this certified by Class, within the QA system.

    There are 2 basic approaches one can initially take. One can obtain the strength of a layup via theory or one can obtain the strength via coupon tests. All Class vessels require coupon testing. I never rely on theory, ever. I base all my calculations on actual real built laminates that have been previously tested and approve by Class. This is part of the QA system. If a wholly new laminate is required, then base estimates are used (from previous layups) and then confirmed, or otherwise, again, via coupon testing. The coupon testing is done as soon as practically possible, since the whole vessels structural design is reliant upon correct and accurate values of the laminates mechanical properties.

    Once knowing what the mechanical properties are, it is simple, takes no time at all to do a midship section to Class. Using LR rules takes longer, since to do by hand, the endless rules and formulae take some time to double check. One can use the SSC software, but the general data input takes forever. However, since I always prefer to do a majority of my preliminary calculations by hand, DNV are far easier to use.

    The question of whether FoS between one set of Class rules or ISO is important or not, does not come into play for me. The vessel must satisfy Class, period. Secondly designing the structure one must add margins and allow for the many what if scenarios that is ever present in design of any vessel; vessels always grow in weight, never get lighter! So, to a purists or theoreticians eye, the structure does not appear “optimal” (I detest that term). I design a structure that is fit for purpose, since these vessels must last decades without warranty issues. Class rules are just bare minimum guidelines and must be satisfied.

    Once that cognitive “leap” is made, a midship section is simply length of span, for any longt.s and the frame spacing. To establish the panel aspect ratio and the supporting spans of the structural members. Anything else, is just “noise” and a distraction to the job at hand. Thus takes me around 30mins or maybe up to an hour to do a midship section.
     
  4. daiquiri
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    daiquiri Engineering and Design

    I agree, this is imo one of the most interesting topics around.
    Rxcomposite, I very much like the way you present the layup schedule with Excel spreadsheet. It is simple, easy to read and almost idiot-proof. It was an eye-opening stuff for me, thank you very much. :)
    Cheers
     
  5. TANSL
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    TANSL Senior Member

    Ad Hoc, I will not say anything because in the previous post there is enough information for readers to draw their own conclusions.
    I just wish to ask you a question: since you usually work with vessels must comply with any CS, those 30 minutes (which now can already reach 60), how many do you use to calculate the modulus of the midship section and check if it meets the regulatory requirements for longitudinal strength?. Thanks
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    None.

    Longitudinal strength is only a concern for vessels over 50m. I have never designed a composite vessel greater than 50m...always less.

    For catamarans, transverse strength is the dominant global axis.
     
  7. TANSL
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    TANSL Senior Member

    It's funny. ISO 12215, which applies to recreational craft up to 24 m in length, in some cases requires the study of the longitudinal strength of ships, whether or not of plastic.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    As I have noted previously, I very rarely use ISO rules. Last time was about 8 years ago.

    Attached is typical Class rules on guidance as such, this is DNV's:

    DNV global rules.jpg

    Thus only over 50m is global strength a consideration.

    Structural continuity is always an issue though and must be understood from the outset and the structure designed accordingly.

    However, having said that, it only takes about 10mins to do a hull modulus check.
     
  9. Alik
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    Alik Senior Member

    FEA software such as Strand7 has a composite module and allows to build laminates of layers, including core. The properties of layers/materials for input we obtain either by formulas from rulebooks, specifications from suppliers (for cores), panel testing or CLT methods (such as used by GL).

    I agree that for small pleasure craft use of FEA is an overkill and the average Customer is never willing to pay for that. Meanwhile, we have been asked by IMCI to present analysis of global strength for 60' pleasure cat (though officially this is not yet covered by ISO), and we did it with FEA.

    Some FEA analysis is useful for details such as hardtop roofs where officially ISO does not apply (they are not walking areas and are not a part of watertight envelope of the craft). But those roofs can carry, say, solar panels or can badly vibrate/flex, so...

    Other useful checks we do with FEA for arches carrying main sheet loads, skegs attachment for grounding, etc. But this is done case-by-case only if the builder/customer is willing to pay for it.

    For most of rulebook methods, presence of main stiffeners and secondary stiffeners is assumed, say high frames/floors and lower bottom stringers between them. But in reality there are sometimes grillage structures where no main stiffener direction is obvious. This is to some extent covered in ISO12215-6 by introduction of coefficients, but more detailed analysis an be made with FEA.

    For small commercial craft in composites, analysis of global strength and strength of details such as engine base, lifting points, masts, hydrofoils etc. are made in FEA on regular basis, in our practice.
     
  10. rxcomposite
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    rxcomposite Senior Member

    TANSL- the global strength calculation for LR is generally not required for vessel under 24 meter (monohull) as its scantling rules is sufficient. To be more specific, it is based on S/L ratio and displacement. For S/L ratio of equal to or greater than 3.0 but light displacement compliant, the limiting stress fraction is 0.33, which is generally used throughout the Rules.

    If S/L is less than 3.0 but heavy displacement, the limiting stress fraction is 0.25 so watchout for cargo vessels and workboats. Should not present a problem since workboats and cargo vessels minimum plating thickness is increased by a factor from the very start (Service type and Service notation).

    Hull girder strength calculation is not required if vessel is less than 40 m. but hull shear strength calculation is required. Tortional strength calculations starts at 65 m. length.
     
  11. rxcomposite
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    rxcomposite Senior Member

    That is when I throw away the rulebook when designing roofs. It is not designed for walking but there is lots of clutter in there like antennae, radar mount, masthead lights, solar panels, railings, ect. It is so littered and has lots of hard points that in ships, they call it the "monkey island".

    I agree FEA is good for special applications. I know that modern FEA program (and those that knows how to use it) can be a useful tool.
     
  12. rxcomposite
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    rxcomposite Senior Member

    Thank you Daiquiri. I have made it idiot proof when the pattern is shaped to the ships outline and sometimes combined with the top view position to show the extent of the laminate coverage for local reinforcement layer.

    It takes a lot of imagination to transition from thick laminates to thin one (while complying with the fiber mix ratio rules) especially on chined craft with local thickness increase in knuckles. It saves on labor time knowing when to terminate, cut and overlap instead of laying up an extra strip.

    I thought I was good at it but the first time our draftsman has to do it, he beat me to it :p while I was sketching and he was drawing in ACAD. He is our 3D modeler, and left handed. Lots of imagination.
     

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  13. Ad Hoc
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    Ad Hoc Naval Architect

    Sorry to correct you RX..but as I noted above the cross over is 50m. I posted the DNV above, here is the LR rule:

    LR global rules.jpg

    May be you're using different LR rules?? :eek::p
     
  14. rxcomposite
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    rxcomposite Senior Member

    AH, I was referring to this LR rule, part 8, Hull Construction in Composite, monohull.
     

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  15. TANSL
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    TANSL Senior Member

    Thanks Alik, Rxcomposite and Ad Hoc. You have done very interesting contributions, I have learned with you, and I have corrected some of my mistakes. Now the thread is going in other directions and I withdraw. Thank you, again.
     
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