Scantlings methodology for recreational boats

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

  1. TANSL
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    TANSL Senior Member

    In one of the threads recently opened, has been written a response that I would bring here:
    Response that has left me stunned, speechless and very worried because scantlings of midship sections cost me much longer
    So I thought to write a flowchart of the procedure followed by me to ask for your comments and to see how to simplify the process, refine and, in short, make it more efficient. This is what I do to define a midship section scantlings.
    1. - Draw 3 "main" sections, aft , center and forward , with plates and transverse/longitudinal reinforcements.
    For the center section:
    2. - Calculate design pressures .
    3.- Calculate regulatory thicknesses.
    4. - Define a layer combination for bulwark panels .
    5.- If the chosen combination complies with regulatory , go to
    step 6 if studying the bulwark .
    step 7 if studying the side.
    step 8 , if studying the bottom.
    step 9 , if studying the keel.
    Otherwise return to Step 5 , with a new combination of layers. Optionally check if changing the dimensions of the panels I get to met the regulations.
    6.- Adding layers to the bulwark , create a combination for the side
    7.- Adding layers to the side , to create a combination for the bottom
    8.- Adding layers to the bottom to create a combination for keel
    9.- Check the above combinations are applicable to stern and bow sections .
    If they are correct , go to step 10.
    If not correct, make the necessary changes and return to step 5.
    10.- Similar process for transversals , frames , webs and bulwark´s stiffeners .
    11.- Similar process for longitudinal stiffeners , keel , bottom and side .
    12 -.Consult shipyard´s Workshop before considering definitive calculated panels.
    13.- Definitive drawings for the 3 main sections.
    Assuming an average of 5 minutes per step, take a total of 65 minutes, but the reality is that the whole process costs me a few hours.
    Thanks for your attention and for your comments I´ll certainly take into account. Perhaps this will serve also as a guide to those who pose “How should I start calculating .....?
     
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  2. gonzo
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    gonzo Senior Member

    There are many approaches. I think that starting with a profile is one of the most common. I am not sure how you can draw three sections with plating if you don't yet have the other views.
     
  3. TANSL
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    TANSL Senior Member

    Yes, of course, I need the body lines and, normally I´ve got too a 3D model of the hull. I need to know first what i am going to calculate. It must be assumed that before you start scantlings, you already have some information. For example, besides the boat, it's good to know the ways and means available for the construction shipyard.
    You can not start with a profile because it must carry an "attached plate", belonging to the accompanying panel, which has not yet been defined. Do you agree?
     
  4. Alik
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    Alik Senior Member

    Actually, we start from bottom as the heaviest part of the boat highly dependent on design speed. For composite craft in sandwich:

    1. Decide on the Rules and define design accelerations. Check versus speed readily achieved on this boat, i.e. the structure should not be overbuilt for speed/impacts the boat cant realistically experience on given seastate. Check with crew shock criteria as well.

    2. Define design loads for bottom, side, deck, cabin, bhds and tanks. We used to take panels at bow, middle and stern, plus any location of interest - for initial design.

    3. Decide the material supplier for both fabrics and core and find their properties/msmv values. Compose first approximation bottom laminate panel (from experience and technological concerns) and check the maximum shortest dimension 'b' to which the panel will comply. For this, one needs to run laminate stack analysis, but we do it in few seconds in spreadsheet or software such as Hull Scant or Lloyds SSC.

    4. Check where are natural stiffeners (bhds, furniture, chines, etc.). Trace longitudinal stiffeners on section to check distance between them and bottom panel size. Check with critical dimension 'b' of panel from #3 and make sure this is not exceeded. Change number of stiffeners and their width (if they are top-hat) if required.

    5. Add/remove layers to laminate as appropriate, repeat #3-4, play with thickness of core to reach the compliance and minimize the mass.

    6. Repeat the steps #3-5 for sides, deck, cabin, etc.

    7. Check laminate of stiffeners same way, adding layers or changing height in stack analysis spreadsheet until compliance is reached.

    8. Make lamination schedule in Excel, taking areas from 3D model, with summary of weights, to make sure weight budget is not exceeded. Keep some margins for further adjustments.

    9. For steps 1-8, we use rulebook approach with stack laminate analysis that proved to be fasted method and actually what rulebook loads are intended for. For places of interest such as lifting points or engine base, or global strength of catamaran we create the model and run FEA calculations.

    In our practice, steps 1-8 done before drawings of structure are made. We just hand sketch the structure on general arrangement, and advise our designers to move some bulkheads/furniture/tanks if it improves/simplifies the structure.

    In process of making the structural drawings, we check structural compliance for other panels (not exceeding 'b') and stiffeners (not exceeding max span), and fill calculations spreadsheets for every panel on bottom, side, and some representative panels on deck, cabin, tanks, bulkheads.

    Usually it takes one working day to get preliminary structural report complete, for the boat below 24m.
     
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  5. TANSL
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    TANSL Senior Member

    Alik, I think your method and mine have a certain logic very similar. I have described a much simpler construction without sandwich panels. I have not mentioned studies for points with special charges or FEA studies, if needed. Of course my tests include plies analysis. Without them it is impossible to scantlings.
    Generally I use Lloyd's SSC software and my own software for ISO 12215, depending on the type of vessel. The SSC for commercial ships and ISO for pleasure boats. I never use the SC for pleasure boats unless there is a very special occasion.
    The figure below tries to explain why I begin by defining the bulwarks. It tries to make the hull as homogeneous as possible so that the lamination is easy to perform, when the laminate is made by hand.
    The estimated time for you are more from my experience, which reassures me a lot.
    Thanks indeed.
     

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  6. Alik
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    Alik Senior Member

    Note, my time estimates includes making nice formal report that is delivered to Customer :)
     
  7. TANSL
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    TANSL Senior Member

    Well, I guess the software used makes in a few seconds an accurate report. Now we need to create the "Scantlings drawings" and the information in a format that can be used by the workshop. That can take a long time or a little, depending on the size of the boat, workshop skills, etc.. But I think we both are talking about the same thing. In 30 minutes you can not do anything serious, imo.
     
  8. rxcomposite
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    rxcomposite Senior Member

    It depends on how you allocate the time. I tend to agree with AH that midship section analysis (keel,bottom, sides, deck) can be made in 30-45 minutes. That is assuming;

    1. I am doing a monohull design. Cats has more details.
    2. I am working on single skin laminate with standard CSM, WR, UDR materials and polyester. If I shift to exotics like Kevlar, Carbon fiber, or even biax and epoxy, I will have to change the default material properties.
    3. That all preliminary load had been calculated in the condition the design will be operating, the frame spacing defined (this with a preliminary sketch of the profile)
    4. The midship section has been defined/sketched and the spacings and location from baseline of the panels, longitudinals/stiffeners defined.

    However, my time is consistent with Alik that it takes me a day to complete structural analysis, and about 4 to 6 days to do a really complete analysis and iteration for presentation to a customer. For a 20-24 meter boat, I generate up to 70 pages of calculations using LR SSC.

    To speed up things, I have in Excel various tophat stiffener proportions, and try to keep my panel in the 1:2 ratio during preliminaries.

    One thing I would disagree is the use of FEA for structural analysis. Boat composites, by layers are made thick in order to speed up production and is made up of differing material characteristics. FEA is accurate for homogenous materials or materials of the same mechanical properties built up with very thin layers. Such is the practice in aerospace. For me, the more tedious classic lamination theory is still more accurate.

    And I start first with the bottom structure, go to keel laminate (which is roughly 1.5X the thickness of the bottom and approximately 10% of boat breadth in width), then go back to side laminate (which is always less that the bottom), then to the deck (which is equal to or less than the side laminate). So add, reduce, reduce.
     
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  9. rxcomposite
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    rxcomposite Senior Member

    This is the most time consuming in terms of communicating with the shop floor. It start with the sequence, where it starts and stop, where it overlaps. It also shows the type of fiber, orientation of the laminate, and the target glass content.

    I usually make colored coded sequence of the ply schedule, layer by layer per page. So a 20 ply (layer) will need 20 pages of illustrations. Makes it easy for the leadperson and the QA to communicate. So when the leadperson shouts ply 15, the QA looks up page 15 and checks. To check resin consumption, each resin batch and fiber is totalled/weighed per layer. This way, even productivity (total weight deposited/hour/number of laminators) can be measured.
     
  10. TANSL
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    TANSL Senior Member

    rxcomposite, in my "procedure" I guess I just have a general arrangement and a body lines drawing with preliminary forms. When I start scantlings I have nothing more. I have not yet made any calculation of design pressures. I have only one idea, based on experience, on the separation of frames, and longitudinals, and the initial laminates that I will propose. Therefore, the first thing I need is a schematic of the cross sections from which I can get all the necessary dimensions. I think, more or less, this is the start situation for everyone.
    In most of the projects I use the EL-c method so FEA, on those cases, makes little sense.
    Do not you think that starting from bulwark facilitates work?.
     
  11. TANSL
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    TANSL Senior Member

    I very much like this idea.
     
  12. rxcomposite
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    rxcomposite Senior Member

    Like this? And I still have to break it down layer by layer with the illustration following the cross sectional shape of the boat.
     

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  13. rxcomposite
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    rxcomposite Senior Member

    I do not start until I have the principal dimensions of the boat,displacement, speed of operation. Then I sketch the profile, divide evenly the frame spacings, and establish the bulkhead positions.

    I draw the midship second because by then, I have a fairly good idea of what the boat is. It defines the boat as it can be longitudinaly framed or transversly framed. This gives me the chance also to allign the top and bottom longi, pillars, or vertical stiffeners with each other.

    I need only the forward cross section when I do the forward scantlings as this where the bottom frame and side frame is treated as one when the angle of bend is equal or greater than 150 degrees. In any case the forward end has closer frame/stiffener spacing because of greater pressure.
     
  14. TANSL
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    TANSL Senior Member

    rxcomposite, of course , I need a LOR , among other things, to know the type of ship , service, navigation area, etc. . I also do an estimate of weights. For the engine , equipment, etc. , have weight data , but the structure at this time is unknown. I guess a weight for the structure , based on previous boats, calculate the total weight and add a 10% margin. That is the displacement I use for scantlings and the values ​​of the various coefficients that are derived from it . Of course I need a table of hydrostatic values.
    I try that separation of the frames is the same over the entire length . I think that makes it easier for production and prevents errors . I also try , for the same reason, that laminates are as uniform as possible in all areas , stern, center and forward . That is why I need to study the three main sections simultaneously.
    It is very important , as you pointed out, ensure continuity of the cross members , which should form closed rings , if possible . Also the longitudinal members should form rings.
     

  15. DCockey
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    DCockey Senior Member

    Very interesting and informative discussion. Thanks!
     
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