Hull Scantling

Discussion in 'Boat Design' started by SeaBoat, Feb 10, 2021.

  1. SeaBoat
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    SeaBoat Junior Member

    Hi altruist,

    I am a student of Naval Architecture.
    I am week in hull scantling. I can calculate resistance, desired prop, stability and everything, but when I come in hull scantling stage, I messed up hull scantling. I want to study this from beginning. I need your help. Could you please tell me elaborately how can I start hull scantling and what is the process?

    Thanks in advance.
     
  2. jehardiman
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    jehardiman Senior Member

    Generally boats and ships are built to rules. These rules are are generally required by the flag state, the financier, or the insurer to ensure the safety and economic viability of the vessel. There are several to choose from, typically for small craft see ISO - 12215-6, larger vessel will have state input, such as the Indian Register of Shipping (IRClass), an international ship classification society providing ship classification and certification as well as technical inspection services.

    Simply, there are three sets of major loads that need to be resisted by a vessels hull that the rules address. What the rules do is provide a minimum requirement of structural load analysis for the following items.

    1) Primary Loads. This the total longitudinal bending moment forces from hydrodynamic and hydrostatic forces. The loads are primarily carried through the shell. Think of standing on an upright empty soda can...as long as the shell is straight and intact it will support your weight. Twist or bend the shell...the can collapses due to loss of symmetric section shape which overloads a portion of the shell and begins a cascade failure. Now again try stepping on an upright soda can...reach down and with a sharp nail keep putting holes in the shell...the can eventually collapses by because the loss of area causes the primary loading to exceeded the yield strength of the shell.

    2) Secondary loading. This is the transverse loads generated by the primary loads. They are normally carried by the transverse frames and bulkheads. What this does is to break the shell panels up into to short lengths that will resist buckling. Go back to our soda can...cut the ends (bulkheads) off. Now stand on the can shell...it collapses immediately! Why?...the ends provided fixity for the shell. Without bulkheads or frames, the shell cannot support the longitudinal load without buckling.

    3) Tertiary loading. This the the actual hydrodynamic and hydrostatic load normal to the skin. The skin has to suport this load without deflecting or sheering. Lets lay our soda can down on its side...now step on it in the middle....it collapses because the bulkheads are too far apart...it losses section geometry. If you step on it near one end it will support weight until the bulkhead collapses. Now again try stepping on an upright soda can...reach down and dent the side of the shell...the can collapses by buckling because the tertiary load caused deflection exceeding the Euler buckling quotient of the shell between frames/bulkheads. Going back to our first example where we put holes in the can skin, this is another tertiary load that the skin is expected to resist to prevent damage from flooding the vessel.

    Additionally, there is attachment loading which functions similar to tertiary loading. These are the loads placed in the hull by things attached to it such as masts, stays, engine power transmission loads, and the dynamic loads of heavy masses attached to the hull. Going back to our soda can...weld a rod to the skin...then step on the can...now bend/push/pull the rod....see when and how the can collapses.
     
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  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    @jehardiman that is a wonderful way of answering SeaBoat's questions - I love how you describe the primary, secondary and tertiary loadings, and I shall bookmark this post for future reference whenever anybody asks me a similar question, and also keep an electronic copy.
     
  4. Ike
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    Ike Senior Member

    simplest explanation I have seen.
     
  5. SeaBoat
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    SeaBoat Junior Member

    Thanks a lot sir.

    I wanna share one thing. At present when I do scantling, I open a rule book. i.e. it is GL. Then use empirical formula. There are some parameters like L, B, D, T, CB etc. After using those formula I get the thickness and section modulus of desired members like center girder, side girder etc. Is it the method to do scantling? If the answer is yes, then I should say that there is no parameters like beam bending, buckling, yielding etc. in those empirical formula, parameters like L, B, D, T, CB etc. can be same for many models, so the thickness and section modulus would be same for them, in the consequence, different models would be same scantling, is it possible?
     
  6. TANSL
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    TANSL Senior Member

    If all the parameters are the same, why should the scantlings be different? But it is very difficult for all the parameters to be the same, keep in mind that there are also variables, such as the separation between reinforcements, for example, that make the results vary. There are other "variables" that can vary between ships even though they have the same main dimensions. Not to mention the types and qualities of the materials used, or the type of structure (longitudinal or transverse) that is needed for the ship or the class that you want to assign to the ship, or the area and navigation conditions , .....
    Even the same designer, with the same boat, can come up with solutions and consequently different scantlings depending on variables, such as, for example, looking for the minimum weight structure or looking for the most comfortable structure for the construction yard, or the cheapest, or the one that adapts to the material available in the material warehouse. In short, the possibilities are many and different.
     
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  7. SeaBoat
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    SeaBoat Junior Member

    Thank you sir.

    Sir, Could you please send me an example? I mean, I need a complete calculation at least one member like center girder as example. It will be great help for me.
     
  8. SeaBoat
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    SeaBoat Junior Member

    Sir, what about cargo loads and other small loads in superstructure?
     
  9. TANSL
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    TANSL Senior Member

    I cannot send you an example of the calculation of an element. The structure must be conceived as a whole, defining how many parts there are, where each one is, what they are like and how they are connected to each other. Once this structure is conceived, it is possible to calculate the thickness or the necessary modulus of each element.
    The part of how a certain element is calculated is clearly explained in the CS rules. Nobody can explain it better than those texts. The part of conceiving the structure is not in the rules. If you present me with a structure, explaining what it is intended to solve, I will gladly discuss it with you.
     
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  10. Ad Hoc
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    Ad Hoc Naval Architect

    Cargo loads, are "local" loads.
    Thus you address them by their individual weight and their location and of course the type of vessel.
    From this you ascertain whether it is a simple 1g loading or greater (if HSC type of craft) and whether dynamic issues are to be added (sea loads from roll/pitch/heave).
    Then you simple apply the "load", onto the structure you have arranged and check for stress, deflection, shear and any possible fatigue issues.

    Not sure what you mean by "small loads in superstructure"...as this sounds like cargo or point loads. In which, see above.
     
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  11. SeaBoat
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    SeaBoat Junior Member

    Thank you sir, I will present a midship.
    Sir, When midship should be drawn? Before Scantling? or After? If it is done after scantling, how can I get hoy many members are there?
     
  12. SeaBoat
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    SeaBoat Junior Member

    Thank you sir for your reply.
    By saying "small loads in superstructure" I wanted to say about furniture, locker etc. which is located in superstructure.
     
  13. TANSL
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    TANSL Senior Member

    I am going to propose two exercises and, once finished, you can answer your question yourself. First exercise, try to scantill the elements of a main frame without having drawn it. Second exercise, draw a main frame without having the scantlings. What is possible and what is not possible?
    And... a little help: no one in the world, except the designer, can know how many elements there are. Through calculation, study and common sense, the designer will arrive at definitively to specify how many elements there are and what they are like.
     

  14. Ad Hoc
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    Ad Hoc Naval Architect

    Then, if it is heavy, you treat it like any normal "point" load, like an engine or a crane etc.

    The scantling is the size of the structural member.

    For example, if the frame has the dimensions of 250x6mm web with a 100x10mm rider, the scantlings for the frames are: Tee = (250x6)+(100x10)

    But, to know whether this Tee is suitable, you need to provide several inputs The first being the span.
    An example is shown below:

    upload_2021-2-12_10-59-42.png

    The red distance, is the span of the frame.

    This is based upon the frame spacing - which you choose - and upon the particulars of the vessel, such as speed and displacement etc, that are inputs, into Class rules, that provide the minimum modulus (scantling) required.

    This is the basic procedure for all the structural members.
     
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