Ballast in and out

Discussion in 'Boat Design' started by Oioi, Jan 2, 2021.

  1. Oioi
    Joined: Feb 2020
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    Oioi Junior Member

    Hi these number came from a Swaggie,is roughly that,Pilgrim as well has some keel with ballast and according to John Weslford a client have reduced the keel ballast and added inside ballast in form of water,there are other boats with the option of keel like the Biscayne Bay 14, I think we can look at some boats and get inspiration and imagine how would we build our way,thinking,dreaming I believe is still not a crime,asking simple basic question in my opinion is not offensive,abusive or stupid
  2. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    I used some very preliminary, preliminary math assumptions (which I had guessed the OP was really looking for).
    My thinking when as follows:
    1.) I thought in meters x kilograms.
    2.) Since I didn't know how the ballast depth was being measured (from the top, from the bottom, or from its CG), I picked from the top. O. 50 m is a rather shallow draft for an 18 ft keel boat, so I based my choice on that. My calculation was from top of external ballast to top of internal ballast.
    3.) I guessed that the distance from top to top would be about 0.75 m. This is because internal ballast can be spread out to maybe 1/3rd the length of the hull or less, and spread out width wise as well.
    4.) Since I'm now trading kilograms for meters, I gave the external placement a value of 1.0 for kilograms. Since I'm giving up 0.75 m in lever arm, I'm going to need that much more ballast (75% more).

    Yes. This is pretty sloppy thinking. But I wouldn't be too surprised if it's not too far from the truth (maybe 1.5 to 2.0 times the original ballast).
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Not at all, of course not, but if you were to give some more specific information about the ship of your dreams, perhaps the calculations that some members of this forum are making could serve you something useful. I hope you will apologize if my advice is too crazy and I hope I was not rude this time.
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  4. DCockey
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    DCockey Senior Member

    What did you assume as the point about which moment would be the same / lever arm pivot? Waterline?
  5. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member


    I forgot that this forum is not using your primary language. The translation may be to blame. The tone of your previous post surprised me. Your post are usually far from rude

    Yes the OP needs to provide more information before useful advise can be given.
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  6. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    As I said above, I'm not a NA, but I have had a couple of Physics Classes. In designing ballast for a sailboat, there would have to be four factors to consider. The center of buoyancy, the center of mass, the distance between those two points, and the center of effort.

    I'm sure I'm simplifying things. The center of buoyancy would have to change as the boat heels, since only the submerged profile can account for that center's location. A float at the top of a mast wouldn't contribute to buoyancy until it hit the water, for example.

    The center of effort would also shift as the boat heeled. Sails spill air as they heel, trim to create top twist, etc.

    Calculating the balance at various points of heel would be tedious, but not hard math. Knowing the exact forces involved would be the most difficult part. I think designing in tolerances would be necessary.

    -Will (Dragonfly)
  7. TANSL
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    TANSL Senior Member

    Do not forget, among many other things, the displacement of the boat for which you want to calculate the ballast, and the center of gravity of the boat. One would still have to consider the position of the metacenter, that of the sail center, and probably something else.
  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Notwithstanding other factors that may come into play.

    It is a simple moments calculation. That is to say:

    Weight x distance (keel) = weight x distance (hull ballast)

    But for this to be sensible, you need a datum...and what is that datum... one ordinarily would use the VCG.
    Thus the VCG does not move, whether adding ballast from the keel..or ballast in the hull i used. Since the resultant moment of each option, must be the same, i.e the VCG does not move.

  9. Oioi
    Joined: Feb 2020
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    Oioi Junior Member

    Thank you, I just wanted to have an idea of how much it would be, not exact numbers, now I know, well I know much more
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