Testing boat structure with FEA

Discussion in 'Boat Design' started by EddieGreen, Feb 13, 2021.

  1. EddieGreen
    Joined: Feb 2021
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    EddieGreen Junior Member

    Hello,
    I am trying to learn how to use FEA to test the structure of a steel boat (isotropic material simpler than wooden strucutre below) by using Rhino with the Scan & Solve plugin.

    While it is usually pretty straightforward to add material, loads ans restraints, i still don't fully understand the concept for a boat structure e.g. frames and longitudinal stringers

    i.e. loads can be water pressure on the hull, shrouds tensions to chainplates and main frames, etc ... but how about the RESTRAINTS for the whole boat structure ? I am still puzzled !
    As you can tell i am a newbie !

    For instance on this analysis :
    Testing the wooden boat structure with FEA. A modern approach to the structural design of wooden boats by finite elements analysis of the keel- frames-deck beams structures | Boat Design Net https://www.boatdesign.net/gallery/testing-the-wooden-boat-structure-with-fea-a-modern-approach-to-the-structural-design-of-wooden-boats-by-finite-elements-analysis-of-the-keel-frames-deck-beams-structures.27011/

    where are the RESTRAINTS positionned ?

    Thanks for shedding some light on this !
    Ed
     
    Last edited: Feb 13, 2021
  2. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Eddie.

    I do not know anything about FEA, but I am sure that other folk on here who do will stop by soon.

    Re your steel boat structure, can you give us a bit more info about it?
    Is it just a theoretical exercise, or is this a boat that you are planning to build, or maybe it is an existing boat?
     
  3. TANSL
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    TANSL Senior Member

    The ship as a whole is considered as a beam that is simply supported at its ends, or in the center, depending on the case under study. The elements that make up the structure are considered:
    - the plates with its four recessed edges (Although this is not always the case. That depends on what you have projected), according to plate theory
    - the reinforcements as beams with their ends fixed as you prefer.
    Having said all this, I don't know if it will help you much, but it is the basis for trying to calculate a structure, I think you should first learn how to design a structure, before calculating it. That is, establish the elements it consists of, its approximate shape and its restrictions and, when your entire structure is coherent, you can proceed to calculate the scantlings. For which you need to know the solicitations that are produced and how to calculate them. The various existing regulations will give you formulas to calculate them and will indicate the allowable stresses in each case.
    But you should start at the beginning.
     
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  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Welcome to the forum Ed.

    From your post, it seems you know very little about FEA.
    In which case your first task, is the understand what is FEA and how it works and thus, how to use it.
    It is not a case of import a model and press go... it is a very sophistical method of analysis and if you do not fully grasp this complexities of how FEA works, you'll end up with GiGo.
     
  5. EddieGreen
    Joined: Feb 2021
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    EddieGreen Junior Member

    Hi all and thanks for your messages !
    As this stage i am not building anything, just trying to learn from examples. I understand (more or less !) the theory but learning from examples seems more enjoyable.
    I have started to replicate (or at least get the overall idea) the structure of existing and older boats and looking at what it yields in terms of loads, stresses and areas of tension, etc

    I understand pretty well for instance how to calculate the scantlings of an arch to lift an heavy tender with its engine on a sailboat transom, but still puzzled /lost when it comes to a boat hull and its structure.
    Not talking of long commercial boats such as tankers ( beam theory), but rather small pleasure motor or moreover sailing boats less than 40 feet.
    Scan and Solve is pretty straightforward, but when it comes to a hull i would like to see where the restraints are positionned with several examples (that's easy for a beam or my a.m. arch).
    I have seen the iso regulations with the water pressure calculations on hull plates and stringers.

    I would like to start as an exercise to check the scantlings of some main frames (loads from the shrouds/chain plates with RM, mast compression on the deck, etc),
    but right now would be interested in understanding what has been done on this example mentioned in my first post above and also on this page : Scan-and-Solve for Rhino http://www.scan-and-solve.com/ (Andrei Rochian videos)

    For instance if someone could tell me where the restraints have been positioned (at least roughly) on e.g. this analysis :


    that would be helpful

    Thanks !
     
  6. gonzo
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    gonzo Senior Member

    The structure is the easy part of FEA. You simply enter the CADD with all the constraints, material characteristics, welds, etc. The applied forces is where the expertise really comes in. For example, the force vectors of accelerations , water pressure from slamming, cargo loads, forces on the rigging, live loads, etc. By the way, do you mean constraints when you say restraints?
     
  7. TANSL
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    TANSL Senior Member

    I will try to answer in a simple way:
    • force vectors of accelerations: I don't know what that can be
    • water pressure from slamming: always perpendicular to the element under study
    • cargo loads: directed towards the center of the Earth
    • forces on the rigging: each in the direction of its cable
    • live loads: I don't know what is that, sorry.
    And by the way, to finish clarifying those questions, from Wikipedia : both restraint and constraint refer to limitations placed upon things and people • While outside restrictions like laws and customs cause constraints, restraints are inside restrictions that an individual places upon himself.
    My opinion is that may be "restraint" is the correct word.
     
  8. EddieGreen
    Joined: Feb 2021
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    EddieGreen Junior Member

    Hi Gonzo,
    Yes, restraint is the terminology used by Scan & Solve, see : ()
    With a small pleasure and displacement slow sailboat, i am not worried about slamming, cargo loads, hogging/sagging, etc, and believe i have a pretty good understanding where to apply the forces/loads (starting with a static analysis) static pressure, rigging (shrouds, forestay, backstay, etc), mast pressure, ... ,
    but dealing with the constraints, with say my 35ft hull floating, is more nebulous to me.

    So any example showing these constraints would be helpful
     
  9. jehardiman
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    jehardiman Senior Member

    Let me help, the first is calculated for all attached items (such as radar domes, pumps, electronic cabinets, etc.) anywhere in the vessel using DOD-STD 1399, Sect 301a or similar methods. Most non-naval architects are surprised how large the side load of a 2kg light on the masthead is. Live loads are the moving load of people and/or vehicles inside the ship, There are several DoD/CFR standards for these loads and maximum allowable, and I believe that there is an ISO one also. Sometimes, structurally, the live load requirements come at you sideways, like having to line up all the passenger weight for the stability test along the deck edge.
     
  10. gonzo
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    gonzo Senior Member

    He is talking about a small sailboat, so DOD standards would not apply. However, for a small low performance sailboat, FEA may be required only for an attachment bracket or other high stressed part. Doing a global FEA doesn't make much sense. EddieGreen, you can't ignore slamming loads until you have done an analysis of the design and its operating conditions. For small boats, there are many rules of thumb or scantling rules that generate safe designs. If it is small and slow, that is all you need.
     
  11. jehardiman
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    jehardiman Senior Member

    Without knowledge, I could see how someone could not understand the usefulness of DoD standards and requirements. Almost every modern nut, bolt, fitting and rigging component is linked directly or indirectly to one or more DoD standards. Try to buy a shackle that's not RR-C-271 compliant, and I know that the best speedboat seats are DoD compliant.
     
  12. TANSL
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    TANSL Senior Member

    Thank you @jehardiman, very kind trying to clarify my doubts. I know what is a force, what is a vector and what is an acceleration, but a "force vectors of acceleration", I don't know what it is, frankly. But, in order not to spoil the thread further, I will stop commenting. Thanks again.
     
  13. EddieGreen
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    EddieGreen Junior Member

    I don't disagree with that, just looking at a section at the mast would be a good first step... but i am not trying to build a boat here

    As mentioned above, this is an exercise. My purpose is to UNDERSTAND how a tool like the Rhino+ Scan and Solve combination can be used to optimize a metallic boat structure with frames and stringers, bulkheads, etc

    Usually applying these restraints with S&S is pretty straigthforward : beams, architectural carpentry, even some local marine analysis, but as a beginner with S&S (and FEA practical analysis) i still dont fully understand how it is used for an overall boat structure. Unlike a house structure steady and restrained for instance on the ground bedrock foundations, a boat is a floating & moving object.

    I had the idea to isolate a few frames (+ stringers and hull plates) around the mast to see stresses on chainplates and main frames from the shrouds loads, but it doesn't seem the best way to use that powerful tool

    The only explicit example i have seen so far is a Kayak wooden structure where restraints are applied to the seat. Not too sure why on the seat ...

    Anyway if someone understands how the examples shown by Andrei Rochian (links a.m.) have been set-up, i would be interested.

    Moreover if someone already has some EXPERIENCE using Rhino/SnS to optimize a metallic boat structure, giving me a few hints and starting points notably re. type of analysis/ restraints set-up would be quite helpful !

    Thanks
     
    Last edited: Feb 13, 2021
  14. Ad Hoc
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    Ad Hoc Naval Architect

    This is no different from a simple beam.

    Thus the questions remains, it appears your knowledge of FEA is limited?
    If that is the case, you need to understand what and how to use FEA.
    With this being stated:

    In Rhino, simply:

    1. Select the material
    2. Apply the restraints
    3. Apply the loads
    4. Click [Go!]
    5. View, Report, Bake or 3D print the results
    Is a gross over simplification of FEA.

    Since it does not appear you can control the mesh size nor even the type of elements sued for the analysis.
    This is basic FEA, being able to control the input!

    Thus understand how FEA works...and then how this Scan-Solve is different...and thus, how this influences the accuracy of the results.
     

  15. baeckmo
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    baeckmo Hydrodynamics

    A good way to gain insight into the working procedure of any engineering software is to start with a simple geometry, like a prism or half cylinder, that allows a straightforward manual calculation in parallell to give relevant validation results. It is tempting to go for "the full monty" directly, but that route will be far longer before you see the goal.
     
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