Jon boat water ballast

Discussion in 'Stability' started by curttampa, Apr 6, 2020.

  1. rwatson
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    rwatson Senior Member

    Dont give up just yet. Wballast.png

    So, to make sure I have got this right.

    You are saying that in the diagram above, the righting moment of this vessel ( assume just a plain prism on its length) will be identical whether there is water ballast below the waterline or not ?
     
  2. DCockey
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    DCockey Senior Member

    Two different situations with water ballast:

    A: Water ballast is external to the original hull shape: In this situation the water ballast has no effect on stability until the boat is heeled/trimmed sufficently that the water ballast is above the waterline. The change to CG and displacement will be exactly offset by the buoyancy of the appended volume. (The ballast tank walls, etc will affect stability to the extent that the density of the ballast tank walls, etc differ from the desnsity of water.)

    B: Water ballast is internal to the original hull shape: In this situation the water ballast has exactly the same effect on stability as adding ballast of any density as long as the mass and center of gravity of the added ballast is identical. (The ballast tank is assumed to be completly full without a free surface). One disadvantage of water ballast can be that the center of gravity of the internal water ballast tanks usually can not be as low as the center of gravity of higher density ballast.
     
  3. Barry
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    Barry Senior Member

    Can you clarify this statement. It appears that you are saying that if you are standing in a boat holding a gallon of water and move it above or below the static waterline, that below the waterline, the water will produce buoyancy if this is the case\
    then you surely are not suggesting that the boat will float higher than if the gallon of water is above the static waterline. Perhaps something is missing in the interpretation.

    The comment that everything below the static waterline is in the fluid, but yet "everything" is not surrounded by fluid so how can it produce a buoyant force.

    Note that with a person holding the gallon of water before and after the draft of the boat has not changed. Ie not displaced any more water before or after the shift of vertical position of the gallon of water
     
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  4. kapnD
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    kapnD Senior Member

    This water ballast discussion is far off topic!
    The craft in discussion is a 12’ flat bottomed jonboat, 42” wide, no keel, no vee, obviously very light.
    TheOP wants it to be more stable when he stands on the foredeck.
    In my experience, standing on the bow of such a craft will raise the stern clear of the water, magnifying said “tippyness “.
    A partner seated in the stern who can counteract the anglers weight on the bow would likely do it.
    Or a few bricks.
     
  5. jehardiman
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    jehardiman Senior Member

    No, read carefully, I never said that. Additionally, the figure above is irrelevant to CurtTampa's question and this discussion which is about water ballast. What you are totally missing in this discussion is the function of ballast. Ballast is added to a) trim a vessel to its proper lines and b) improve stability. Just because adding water will do (a) does not mean that it will do (b) in all cases. The case I stated; "water ballast only works when it is above the waterline", specifically refers to case (b) which is CurtTampa's question. This is because the weight, volume, and CG of the added "ballast" is identical to the added buoyancy. While adding water "ballast" may change the location of both CG and CB (and sometimes Iwp), you could just have easily changed the shape of the buoyant volume and had the same effect on the relationship between CG, CB and possibly Iwp. In the case you draw above, it is impossible to have the same displacement after adding the "water ballast" unless a weight is also removed. In that case, stability and righting moment has to change...unless...the weight removed had the exact GC as the water added, in which case the stability is identical. So the figure and your understanding of what is happening in it is poorly formed.
    No, what I am saying is that if the gallon of water was below the waterline, I could effectively remove the gallon of water and that much buoyancy and the boat would have the same draft and improve stability. Ok, I'm going to bite the bullet and do the drawing to show a good way to think of buoyancy. I hope this helps.
    water ballast.png

    Edit: I was working on the figure when kapnD posted, but really other than altering the shape, adding heavy ballast low is the answer.
     
  6. rwatson
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    rwatson Senior Member

    So, what you are saying, by "water ballast only works when it is above the waterline"
    is really

    ""water ballast may improve stability in some cases"

    Can you see the reason for my query?

    These are totally different statements.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I think that is true of vee hulls, less so with a flat bottom that does not taper to a point forward. I think the water ballast idea is a non-starter, but he can certainly try fixed ballast.
     
  8. jehardiman
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    jehardiman Senior Member

    Then to be precise the full technical statement should be:
    Water ballast may improve stability in some cases but only in the cases where the ballast is (or is lifted) above the waterline or in cases where the water ballast is needed to replace an absent cargo load to bring the vessel displacement up to its minimum stability condition. Otherwise, the need for water ballast is a indicator that the hull form and weight distribution is non-optimal (i.e. the water ballast below the waterline is needed to provide minimum stability without any weights missing) and the hull should be redesigned.
     
  9. DCockey
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    DCockey Senior Member

    Two reasons water ballast is used in small sailboats:
    1) Ability to dump the ballast and reduce weight for trailering.
    2) Ability to reduce displacement for light air performance.
     
  10. jehardiman
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    jehardiman Senior Member

    Correct, both of those are legitimate cases for water ballast as both generally fit with its usefulness. However, adding water ballast to a trailer sailer that is not and will never be lifted above the waterline is just improper design. Fine line I admit, but just removing volume in the hull to add water ballast is counterproductive on a volume limited sailboat.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    This, of course, isn't a sailboat, and whilst using water ballast might have its uses in some cases, with small non-sail propelled craft, it is far from commonly used, which speaks to its limited value. I have seen smallish, but deeply veed planing hulls incorporating it, but for every one of those boats, there would be a hundred that don't. At least.
     
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  12. jehardiman
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    jehardiman Senior Member

    Yeah, 'tippy' is one of those subjective things. If the vessel rolls too fast people think it is "unstable" when the real issue is low initial stability (or waterplane inertia dominates making the response too quick), but it may have adequate ultimate stability. Adding mass inertia (i.e. water ballast below the waterline) to the hull makes the vessel roll slower but does not change the stability of the hull. This is the function of open anti-roll blisters and the self-bailing compartments found in some small deep-V craft. On plane the vessel if fully supported and stabilized by the planning pressure, but at rest initial static stability is low while waterplane inertia is high, so mass inertia is added to slow the response to waves.
     
  13. DCockey
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    DCockey Senior Member

    One of the functions of water ballast in trailerable sailboats is usually to provide righting moment in the event of a knockdown. In that situation the CG of the water ballast would rise above the waterline.
    If positive buoyancy when swamped is a requirement then the interior volume lost due to the addition of ballast will be the same whether water, concrete, iron or lead ballast is used (assuming the boat without provisions for ballast does not have excess buoyancy when swamped). Concrete, iron, lead, etc ballast would allow for flexibility in the location of the space used for floatation to offset the swamped weight of the ballast.

    Two reasons I'm aware of for water ballast in small powerboats. One is in boats which are intended to create large wakes for wakeboarding. Water ballast is used to increase operating displacement when the large wakes are desired with the ability to empty the ballast tanks for smaller displacement at other times.

    The other use of water ballast in small powerboats I'm aware of is in deep V boats to provide increased stability when at rest or moving slowly. The tanks are typically arranged to drain when the boat startes to plane. This sue takes advantage of the concept that the water ballast is equivalent to changing the shape of the hull. Edit: I see that jehardiman discussed this use of water ballast above.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm aware of that, it is to improve initial stability at rest, but seems not to have become popular. There are a few, and sometimes the boat isn't that heavily veed. This one is around 20 degrees. Probably a structural complication builders prefer not to bother with.

    BC.jpg
     

  15. Barry
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    Barry Senior Member

    Your comment that everything below the water line is in the fluid is incorrect. This is to suggest that adding an object into the space increases the buoyancy. You need an object, statically to displace a fluid to create a bouyant force


    re your drawing, left hand bottom with the verbiage "hydrostatic pressure is reduced by the gravity head of the water ballast" The hydrostatic pressure is not conditional upon what is on the other side of the hull structure. The hydrostatic
    pressure is due to the depth of the water.

    In you drawing on the right, lower, it appears that you are suggesting that the water ballast above the interface, hull membrane, reduces the hydrostatic pressure below the hull. There is not an opening from the inside ballast to the outside
    water as you have indicated.

     
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