ballast calcs

Discussion in 'Stability' started by wdemilly, May 30, 2018.

  1. wdemilly
    Joined: May 2018
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    Location: Key West

    wdemilly New Member

    I should say right away that I'm not a marine architect, and that I hardly know the vernacular of boat design. So forgive me if I'm using improper terminology here!

    I've got a partially complete boat which was designed as a ketch (see drawing) but now looks like the photo beneath it. I'm calling it a shallow-draft, swing-keel boat and with its 2.5' draft keel up, it is meant to take me into green waters to the west of the Keys and in Bahamas.

    At this point the only purpose of the keel (which has nearly 0 ballast in it) seems to be a bit of roll dampening.

    Because of the shallow draft and the fact that I don't know the weight of the boat I'm guessing that the CG is about a foot under the waterline.

    I've calculated displacement at about 23,000 lbs (assisted by 8,000 lb of ballast). This has her floating right at the design waterline.

    But as you might imagine she still feels light, and rolls considerably.

    Would adding ballast to the keel help?

    I've also considered removing the keel entirely and putting ballast at the lowest point of the 8' by 4" void created by the removal of the keel. (this would also allow me to design a more workable interior).

    Any advice?

    Thanks


    5285818_20150806104204210_1_XLARGE.jpg boatwindows.jpg
     
  2. TANSL
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    TANSL Senior Member

    The bilge keels can help a lot to reduce the rolling.
    The CG should not be below the waterline. That is an important point that you should try to calculate in one way or another. An experience of stability would help you to know it with enough accuracy and this would allow you to calculate the necessary ballast, if it were necessary to place some ballast.
     
  3. wdemilly
    Joined: May 2018
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    wdemilly New Member

    Thanks. To get an accurate CG I'd need to know the boat's weight, right? I can do that next time I haul her (formula would be gross weight-ballast?). N

    Not long ago I was speaking to a boat builder who had said something to the effect that I should add ballast beneath the "rotational axis." Does that make sense?
     
  4. gonzo
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    gonzo Senior Member

    The easiest way to add ballast is to bolt a lead shoe under the keel. The displacement of the boat is the gross weight. You can calculate the submerged volume and multiply it by the density of water; that is the displacement and the total weight. To calculate the submerged volume, divide it into sections of one or two feet (you can use chalk to mark it). Measure the angles and lengths at the marks and then calculate each individual volume. Add them up, and you are done.
     
  5. wdemilly
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    wdemilly New Member

    Thank you. I did one calculation similar to what you suggest, using Simpson's rule. I'll look into the lead shoe.
     
  6. TANSL
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    TANSL Senior Member

    You could calculate the total volume of water displaced but calculating the position of the center of gravity is another very different issue.
    There are procedures, both afloat and in the air, hanging from a crane, to calculate the position of the CoG
    The rotation of the ship takes place around axes that always, when the ship floats freely, pass through the center of gravity of the water plane. Therefore, placing ballast near the axis of rotation would be nonsense, totally useless.
    Before going into more disquisitions, you should know if you need ballast and how much. Without knowing that, it is not worth thinking about where to place it. Of course, always, as low as possible. But the longitudinal position of the ballast is also very important. Therefore, do some numbers, or ask someone to do them and, with them, but not before knowing the weight of ballast, you will decide the type of ballast and where to place it.
     
  7. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    You do not need to know the weight to figure out where the centre of gravity is.
    A simple 1' = 1" cardboard mock-up of the boat would indicate within a foot where it is.
    The fact that she floats on her lines speaks volumes too!
    Are you in touch with the designer or builder?

    It won't roll as much with masts and even less with sails in a breeze!

    Could you show the entire drawing of the boat?
    From where did it come?

    Yes, beneath the rotational axis makes sense.
    As low as possible, as Gonzo said.

    Thanks
     
    Last edited: May 31, 2018
  8. TANSL
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    TANSL Senior Member

    Beware, in many ships the CoG is above the rotation axis afloat. If you are referring to the position of the ballast, of course, it is good that it is as far below the axis of rotation as possible.
     
  9. wdemilly
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    wdemilly New Member

    Everyone, thank you for taking the time to respond. If any of you have an interest in continuing the discussion, here's what I've got at this point: I have a large print of the drawings and will try to get them scanned. (The plans state that the displacement is 16,256 lbs, LWL is 34'7", beam 11'4", 4,000 lbs ballast, D/L ratio of 1.87 and Lbs in Immersion 1,143lbs)

    Here's info from the listing page: It was built "using shoal draft design principles by Philip C. Bolger in his shallow draft designs. A flotation model was built to a scale of 1”= 1’-0” and weighted to that scale for float testing."

    For any of you curious about the design, the original builder was 6'7" tall.

    When the boat was first splashed (still incomplete interior, no rigging) the bow was about 12" above the designed waterline and the keel, about 2" above.

    I immediately added about 1,500 lbs of lead astraddle the keel trunk as well as 150 lbs of additional chain in the chain locker. This helped to get her level but I felt that she was still too light, bouncing around in 1' seas. So as an experiment I put 20 bags of concrete (each one sealed with a contractor's bag) on the floor and that helped.

    If I'm interpreting the advice here correctly the best thing to do is rig her, and/or possibly add a lead shoe to the keel.

    Thanks again for your help.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    OK..noted and understood.

    No.

    The "rotational axis" as you have called it is the CoG of the boat. The rolling, for example, will roll about the CoG.

    The "rotational axis" as you have noted, is its moment of inertia about said axis which gives us what is called the radius of gyration (r ). The summation of all the weights at a certain distance (r^2) from said axis = mass moment of inertia.
     
  11. TANSL
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    TANSL Senior Member

    Since you are going to have a large space where the keel was before, it could be that you did not need to bolt the ballast to the keel. On the other hand, depending on how you want the trim of the ship to be in balance, the longitudinal position of the ballast can be one or the other very different. I insist that you do not go with preconceived ideas of what material to use for the ballast, weight of the ballast and where to place it. If you have a body lines plan of the boat, I will gladly do some of the necessary calculations. With them, once the boat is afloat, an inclining test can be done to improve, if necessary, the estimates of the calculations. Send me a PM if you think it's convenient.
     

  12. gonzo
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    gonzo Senior Member

    The boat will have a short period of roll without the masts (she will be lively).
     
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