Calculating ballast for a motorsailer

Discussion in 'Motorsailers' started by Annode, Sep 22, 2019.

  1. Annode
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    Annode Junior Member

    What are the variables, what are the margins?

    Assuming the boat is a heavy metal hull trawler or somesuch, adding ballast inside the hull might not be possible. Is it feasable to weld on a keel with ballast. Perhaps even a retracting keel?
     
  2. TANSL
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    TANSL Senior Member

    In general it can be said that, in a metallic hull, it is possible to do it, but to give a definitive answer it would be necessary to know the shapes of your hull.
     
  3. gonzo
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    gonzo Senior Member

    Welding or bolting ballast to the keel is a common approach. It lowers the CG more than inside ballast and doesn't take interior space. However, you need to look at the specific design and what your goal is.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    First question..why do you wish to add ballast?

    One only adds ballast for 1 of 2 reasons:-
    1) Because the floating/running waterline is not correct.
    2) Stability.
     
  5. Annode
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    Annode Junior Member

    A trawler is not designed to sail, sooooooo it needs ballast when you add a mast yes?
    At some angle of heel, the boat will capsize if the CogG is not lowered. The question of this thread is; how to calculate ballast, and what margin of safety is normal.
    A gust front should be able to blow the boat to an angle of heel (wihtout capsizing) that allows the majority of the wind to spill out of the top of the sail I imagine. These are the things I am asking to discuss.
     
  6. TANSL
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    TANSL Senior Member

    The amount of ballast and its position can be calculated but you should give some more information. Do you have a body lines plan of the ship? Do you know the current weight and CoG of the ship? That to start talking. And, after a detailed study, it is possible that the answer is that it is not worth trying.
     
  7. gonzo
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    gonzo Senior Member

    The outriggers on a trawler are heavier than a sailboat mast. The wind on the sails will add a considerable force though. However, you can always take the sails down. On thing to consider, is that trawlers are always on ballast. Even when there is no fish in holds, there will be shaved ice. Most conversions use the hold for accommodations, which will elevate the center of gravity. You should consult a naval architect or engineer to inspect the boat and do the calculations. Are the original plans available? That would really save a lot of time and money.
     
  8. Annode
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    Annode Junior Member

    >First question..why do you wish to add ballast?
    to prevent the wind capsizing the boat

    >Do you have a body lines plan of the ship? Do you know the current weight and CoG of the ship
    No unfortunately. This is still a theoretical discussion. I know that a Naval Architect would be needed to do this properly, I am just trying to get a sense of it.
    Trawlers do have a lot of ballast in the hull yes. Large heavy engine, combustible fuel tanks at the bottom of the hull which is replaced by fish in the hold . The hull is not a sailboat hull. It has more of a fat, flat bottom. I am just curious what the factors are in a ballast calculation.
    Assuming the boat rotates around its center of buoyancy, and the CofG is somewhere close (ie not a large net effect) , if the force on the sails generates a force of 1000lbs at 205kts at 20ft from the waterline, what is the desired counter ballance force from the ballast weight? as a fraction 1, 1.5, 0,8 0,6. What amount of force is the boat designed to resist?
    Racing sailboats that are blue water capable seem to be able to recover from the mast touching the water line. I assume this not the norm?
     
  9. TANSL
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    TANSL Senior Member

    No, that is not correct. The ship rotates around axes that pass through the geometric center of the water plane area.
    As a first approximation, you can estimate that the center of gravity of the light ship is approximately 95% of the ship´s depth.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    That is incorrect, as is the reply above too.

    A vessel rotates about its centre of gravity. As also noted here.
     
  11. TANSL
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    TANSL Senior Member

    Perhaps the image, very repeated in many calculations, explains this issue better than me.
    Snap10.jpg
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Tnasl,
    Would you please stop posting incorrect information merely to protect your ego.
    Time and time again, you are incorrect yet reply with polemics misdirection and subverting your opinion as fact. Your comprehension is so lacking it is no longer laughable. Posters come on here for correct valuable verifiable (independently) information; not chest beating of how quick a poster can reply to appear knowledgeable or important.
    Each of your replies continually and consistently highlights to anyone with a hint of NA experience your lack of understanding of the subject you claim to know.

    They always do...yet you continuously refuse to accept your lack of knowledge on the subjects you continually and incorrectly post on.

    Just read a text book..and then understand it, if that is possible:

    upload_2019-9-27_9-28-59.png
     
  13. TANSL
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    TANSL Senior Member

    You could, in a theoretical exercise, rotate the ship around that point G indicated in your image. I will not discuss that, your image is very clear (but, of course, that point G is not the CoG of the ship). A freely floating body rotates around axes that pass through the geometric center of the waper plane.
    How do you interpret the line that I have underlined at the end of the attached file?
     

    Attached Files:

  14. Ad Hoc
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    Ad Hoc Naval Architect

    So, what is G then... a bowl of G-reen soup?
    What utter nonsense. Please carry on and change the definition of anything you wish merely to satisfy your own misguided and incorrect narrative.

    If you are going to attempt to read a book, or even learn a subject, then please take the time to understand it totally and holistically, rather than a lame attempt at cut n pasting your way out of your continually and consistent incorrect understanding of the subject matter at hand:

    upload_2019-9-27_10-4-17.png

    Now please, go and try learning up on the subject before posting.
     

  15. TANSL
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    TANSL Senior Member

    It is clear that in the image of your last post the axes of rotation are being defined around a point G, CoG of the ship. Suppose I try to learn something from the wise lessons you are giving me. You will agree with me that the point G that you show in the figure of post 12, which is not the hull CoG, has nothing to do with the point G shown in post 14, which could be, in effect, the CoG of the hull. Please do not make things more difficult to understand than they are and, try to explain to me how these two figures can describe the same phenomenon: rotation of a hull floating freely, not immersed, in a fluid.
    I appreciate, in advance, your patience and interest in teaching.

    Page extracted from "Principles of Naval Architecture" published by SNAME
    CoF.jpg
     
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