How to "scale" stability

Discussion in 'Stability' started by Erwin Ried, Aug 6, 2018.

  1. Erwin Ried
    Joined: Aug 2018
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    Erwin Ried New Member

    I have no idea about boat building, please dont destroy me :) I am just looking for tips.

    I am building a small model of a boat that I want to use to do experiments about stability. The model will be 3d printed and my intentions is to keep everything parametric so I can adjust the design iteratively:
    hull.PNG
    My question is how can I scale the accelerations a boat experiences?
    • Density of the model, vs real boat
    • Density of the liquid (mix alcohol/other in water?)
    hull2.PNG
     
  2. TANSL
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    TANSL Senior Member

  3. Mr Efficiency
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    Mr Efficiency Senior Member

    So far as I'm aware, stability scales to the 4th power, volume scales to the cube, surface area the square. None of which likely is of any use to you. I think you need to give the background to your project, and what the aim is.
     
  4. Erwin Ried
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    Erwin Ried New Member

    What? no idea. But I see you are from the land of "Torrente el protector" so it must be a joke :D

    Thanks Mr Efficiency, basically I am trying to scale the movement that a boat experiences. The final objective is to use machine learning to predict dangerous situations in fishing boats, but starting with small scale experiments in a controlled pool with wave generators.

    I know that in the old times, wooden boat builders used to make a small model of the boat to test buoyancy and so, so this is like the reverse exercise.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

  6. TANSL
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    TANSL Senior Member

    Of course, of course, it was a joke.
    There are several criteria, or calculations, that are applied to determine whether the stability of a ship is sufficient or not. In some of them, it is enough to change the flood point to "improve" that criterion. Others have to do with the moment of inertia of the water plane. Others with the position of the ship's center of gravity. You will understand, therefore, that talking about "how to scale stability" is a bit ambiguous.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    " Stability is even nastier for scaling problems, and you don't see model tests of stability very often"
    That was quoted from the link given earlier, by a seemingly respected forum member here, years ago. It suggests that a can of worms is opened, when you scale model, with stability as the parameter under test.
     
  8. rwatson
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    rwatson Senior Member

    You dont see model scaling tests often because ...

    Its so much easier to send the CAD files to an NA to calculate the Stability Curves.

    Model testing would be a crazy way to get the results.
     
  9. gonzo
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    gonzo Senior Member

    Another thing to consider is the ratio of boat size to weather conditions. For example, if you measure the wind speed or wave height in boat lengths, as the boat gets bigger, the ratio of wind/waves to boat gets smaller.
     
  10. TANSL
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    TANSL Senior Member

    That's totally accurate, Gonzo, it has nothing to do with stability but, good point !. This is so true that if, for example, there is no wind or there are no waves, the ratio of length / wind speed or length / height of the wave could reach infinity. But there is still more, if the boat is not in the water, those parameters could lead to unpredictable situations.
     
  11. Erwin Ried
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    Erwin Ried New Member

    Thanks for the comments. I haven't decided yet how to continue but I still need to read more about the topic.

    Yesterday I discovered that doing some fluid simulations is quite simple (this is the small model at 10 and 25 knots), so maybe I just dont need the background to get a solution for the density required for my model, I just need one case where the big one behaves in the same way as the small one inside that software.
     
  12. gonzo
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    gonzo Senior Member

    You shouldn't trust simulations too much.
     

  13. TANSL
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    TANSL Senior Member

    Why not?. You are pulling down one of the best current tools to get better designs.
     
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