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

    Hello I would like to ask the very knowledgeable forum people if there is a calculator to give the weight of ballast going inside hull to have the same effect of the one down in the keel? For example let's say there is a keel 50 cm dow with 500 Kg,how much ballast would I have to put inside to have the same effect?
    Thank you
     
  2. TANSL
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    TANSL Senior Member

    It is necessary to know the height of the center of gravity of the ballast in each case and it would also be convenient to know the shapes of the hull.
     
  3. DCockey
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    DCockey Senior Member

    Same waterline, same initial stiffness, or same righting moment at some specificed heel angle?

    Same waterline requires the same.

    For the same intial stability the ballast will need to be increased which means the vessel will sit lower in the water. The vessel sitting lower in the water will also affect stability. Similar situation for same righting moment at a specified heel angle. The answer to how much ballast in either case depends on the shape and size of the vessel.

    How large is the vessel? Exterior ballast is 50 cm down from what? How high would internal ballast be?
     
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  4. Oioi
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    Oioi Junior Member

    18 feet sailboat 500 kg 50 cm from bottom panel ,how many kg would have to go inside right on top the same bottom panel?
     
  5. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Based on the following assumptions:

    1.) the 18 ft is the actual length of the boat and not its Water Line (WL) length (if the 18 ft is just the WL length, the actual length of the boat's hull could be anywhere from 18 ft to 24 ft, or even longer),
    2.) the half meter ballast depth is measured from the bottom of the ballast slab or bulb (this is likely to be the case if the boat's hull length is 18 ft, but far less likely to be the case if the hull is considerably longer),
    3.) that the fully loaded displacement of the boat is around 1,500 kg (it could be considerably more, or significantly less. The empty weight of the boat could be as little as around 800 kg),
    4.) that the boat has relatively round or moderate to deep "V" sections (most likely to be the case if the hull is 18 ft long, and least likely to be the case it is considerably longer. We could be looking at a 24 ft IOR influenced design, or a more traditional 18 ft one)

    The Center of Gravity (CG) of the ballast will likely be moved up about 0.75 m. If we let the original CG placement be multiplied by 1.0, the new placement will have to be multiplied by 1.75.

    So this means you will need around 875 kg of ballast to keep the boat's total CG near where it originally was.

    Now this is just a ballpark figure, and it assumes that the weight of this extra ballast, 350 kg, is going to be subtracted from the carrying capacity of the boat. If not, the boat will end up displacing that much more, which would be about a 25% increase in total displacement.
    Most likely the boat's WL would then have to be raised by about 1.5 inches. This will mean a likely increase of the WL beam and maybe even some transom immersion.

    We must keep in mind that the two main purposes of external ballast are (in descending order of importance) :
    1.) to improve the range of stability of the boat, per given amount of ballast, and
    2.) to improve the initial stability (sail carrying ability) of the boat, per given amount of ballast.

    Typically, it does the former much better than it does the latter. This is why canting ballast keels were invented.
     
  6. TANSL
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    TANSL Senior Member

    I am afraid that, if we replace a ballast located in a very low point `with another ballast located higher, the weight of the new ballast must be much greater than the weight of the old one (to be equivalent to it) so that the displacement of the ship will be greater. On the other hand, the longitudinal position of the new ballast, given that it is greater than the previous one, must also be studied so that unwanted trim does not occur.
    I honestly do not believe that a definitive answer can be given without knowing the ship, its shapes and its general arrangement. One can only venture opinions that I do not think are useful in practice.
     
  7. sharpii2
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    sharpii2 Senior Member

    I agree.

    Posted this just to give the guy an idea of what he may be dealing with--likely needing nearly twice as much ballast.
     
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  8. DCockey
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    DCockey Senior Member

    How was the factor of 1.75 determined?
     
  9. Oioi
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    Oioi Junior Member

    Ok ,the 18 feet is total length,not waterline,the 50 cm are to the bottom of ballast,I wanted to have an idea of how much it would increase the amount of ballast being inside,doesn't have to be a very precise number,thank you
     
  10. TANSL
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    TANSL Senior Member

    Why don't you give us some useful information about your boat? For example, the overall length is useless. It would be important to know the general arrangement. Do you have any reason why I can't show it?
     
  11. Oioi
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    Oioi Junior Member

    I don't have a boat yet,I am thinking to build one,so just imagination,try to know what the difference will be,to have an idea,to look at the pros of dipper ballast and having it inside and a shallow one
     
  12. TANSL
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    TANSL Senior Member

    Yeah, now I see it clear. I was wrong thinking that you had a real problem to solve and I was willing to help. Your approach was strange but one should never be surprised by the problems that arise on ships. But I see that you are only playing "little boats" and, in that game, I am not going to participate. With all due respect, what you are raising is stupid. A ship, if there is no other solution, should carry as little ballast as possible and as low as possible. Why don't you place the helmsman under the water? With a periscope he could see the horizon perfectly and would be safe from enemy fire.
    Good luck with your naval architecture "experiments" but do yourself a favor, with your current knowledge, don't design / build any ships yourself.
     
  13. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member


    Tansel

    Be nicer!!
    The OP was very clear with the small dimensions.
    Your missunderstanding is no excuse for rudeness.
     
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  14. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    Many small sailing skiffs from 18' down, use no ballast at all. The crew take on the task of countering heel from the sails.

    May I ask, where did these numbers come from?

    I am not a builder nor a NA, I am a cabinet maker and an artist who loves boats and dreams, like you. Were I to design my own boat to really build, I would take a look at some designs that were similar to what I had in mind and compare.

    As a sailor, I can also tell you there are advantages to external ballast beyond righting moment. This usually comes in the form of a keel that tracks better than a daggerboard or centerboard/swing keel. Everything is a trade off.

    -Will (Dragonfly)
     
    Last edited: Jan 4, 2021

  15. TANSL
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    TANSL Senior Member

    @Blueknarr, perhaps the way of expressing my feelings has been rude, I don't know, it is difficult for me to calibrate those nuances in a language that I do not master. But what I can assure you is that I was frustrated to find that the OP's question was very unserious. There are people who want to help, who invest part of their time in the interest of others, and who like to feel respected. Some have even made their own numbers, calculations that we now know are totally useless (doesn't that deserve more respect?). If the OP is musing with wild ideas, I, modestly, would prefer that he think about it a little more, that he provide some data, before committing the time of others. I know that no one has asked me personally for this help, but it is precisely that altruistic intention to help that, I think, deserves the most respect.
    It is well known, as it should be, that the same problem, on different boats, has a different solution. That is why it is essential to define which boat we are talking about in order to request and obtain valid help. If not, it would be desirable for the OP to elaborate a little more on the problem he wants someone to solve for him, to work a little more on his own before asking others to work.
    If something I said in my previous post or in this one is not correct, you can tell me. If the way I express myself is not correct, you can also tell me and I will improve my use of English. I'm sorry to have seemed so rude but I humbly believe that, forgetting my rudeness, my final advice is still valid.
     
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