Calculating displacement for scale model ship

Discussion in 'Boat Design' started by Peter Binns, Mar 25, 2021.

  1. gonzo
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    gonzo Senior Member

    How can he build a model without lines?
     
  2. TANSL
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    TANSL Senior Member

    Well, many rates applicable to ships are based on that "theoretical" amount. Many shipowners wish it was only useful for comparing similar ships.
     
  3. TANSL
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    TANSL Senior Member

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  4. DCockey
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    DCockey Senior Member

    Same way as many models are built. Look at photos and any other available information and then make your best guess about the shape of the hull.

    Whether it should be regarded as an accurate model is a different question which depends on what is meant by accurate.
     
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  5. rxcomposite
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    rxcomposite Senior Member

    The George Moorsom method is described in the old Principles Of Naval Architecture book, the single volume edition. Briefly, it is to estimate the earning capacity of the boat, not its size,

    But to reverse engineer the boat using Moorsom method, the internal volume of the ship has to be measured and certain deductions are made. The complete method, except for some deductions allowed are no longer in print. Add the fact that the boat has a skeg and other appendages that does not constitute the internal volume of the ship.

    However, this ships lines are common and similar lines can be found in the internet. The best way to start is tracing the old drawing with a new sheet to get a fairly good estimate.
     
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  6. TANSL
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    TANSL Senior Member

    @rxcomposite I think we are saying the same or, at least, very similar things.
    I do not remember how the gross tonnage calculations are carried out, because they are very cumbersome, but I am very clear that it includes many spaces that are not submerged and, on the contrary, it does not take into account some that are below the waterplane.
    So, in my opinion, moorsom tons do not serve to deal with the problem posed by the OP.
     
  7. rxcomposite
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    rxcomposite Senior Member

    We are.
     
  8. RAraujo
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    RAraujo Senior Member - Naval Architect

    Using the gross tonnage, calculated according to Moorsom, to estimate the displacement is a far shot.

    Technically it is supposed to give an idea of the total internal volume of the ship (not the cargo volume which is related to net tonnage) however the volume is calculated to the internal line of the stiffeners which gives a very different value. Sometimes the stiffeners were made deeper to decrease the tonnage... Other times some spaces were not included...

    Also superstructures and deckhouses were included in the calculation.

    In some way it was similar to the way Suez Canal gross tonnage is calculated.

    I think you will have a better estimation by using an estimated block coefficient.
     
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  9. Earl Boebert
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    Earl Boebert Senior Member

    Well, I've done a lot of models, and if I were to tackle that project I'd start by shaping a hull in Delftship by eye to match the photos and plans I have, checking my work using its perspective display. I'd make guesses of the section shapes from the pictures and practices of similar vessels of the period, or by the same boatyard. She seems to have a hollow entry and exit with round bilges in the middle. Once you get a shape that looks right in Delftship, the software will give you a displacement and the LCB.

    An alternative is to carve a half hull. I would laminate a block on the buttock lines rather than the water lines. Hold the laminations together with staggered screws near midships so you won't carve into them.

    Start with a squared-off shape to the plan view you have and gradually carve off corners and make the hollows until it looks right. Then take it apart and make a lines drawing from that, and calculate the displacement by drawing the sections on graph paper and counting squares to get the areas.

    Good luck, she should make a very handsome model.

    Cheers,

    Earl
     
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  10. bajansailor
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    bajansailor Marine Surveyor


  11. gonzo
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    gonzo Senior Member

    That is not a scale model. It is rather the kind of thing they sell at souvenir shops. Scale models are accurate representations.
     
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