Floatation/Buoyancy perspectives

Discussion in 'Watercraft' started by themaninthemoon, Nov 4, 2011.

  1. Salmo_g

    Salmo_g Active Member

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    Mr. Roamer's simple explanation is accurate. Until I read it I was going to offer this:

    Buoyancy = specific gravity of water - the specific gravity of the object being floated (or sunk, in which case the answer is - ).

    Sg

    PS: the specific gravity of water (distilled) is 1 gram per cubic centimeter (equivalent to its mass) at sea level.
     
  2. Fleshfly

    Fleshfly New Member

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    Specific gravity is the ratio of an objects mass to that of an equal volume of water. A cubic foot of solid fiberglass weighs about 115 lbs and a cubic foot of fresh water weighs about 62.40 lbs. Therefore 115 lbs รท 62.40 = 1.84 Specific Gravity. Fiberglass is 1.84 times heavier than fresh water. An object with a specific gravity over 1.0 will not float and will have negative buoyancy. An object with a specific gravity less than 1.0 will float and an object with a specific gravity of 1.0 will have neutral buoyancy.

    Also, when determing displacement and flotation is used it needs to be accounted for in your calculations like this: 2lb foam (2 pounds per cubic foot) would be deducted from the cubic foot of water being displaced. 62.40 - 2 = 60.20.

    Here are some examples of specific gravity for different materials:

    Aluminum 2.60
    Bronze 8.00
    Concrete 2.25
    Fiberglass 1.5 to 1.8
    Douglas-fir .50
    Gasoline .72
    Glass 2.50
    Cast Iron 7.00
    Lead 11.00
    Mahogany .60
    Human 1.10
    White Oak .80
    Oil .83
    Sitka Spruce .42
    Steel 7.80
    Teak .83
     
  3. Salmo_g

    Salmo_g Active Member

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    Fleshfly,

    Cool data! Of course the thing that makes most boats float the way we want them to, whether constructed of wood, fiberglass, aluminum, or steel, or pontoons, is the air surrounded by the structural pieces. So what is a the specific gravity of air?

    Sg
     
  4. Krusty

    Krusty Krusty Old Effer

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    Specific gravity only has relevancy with a fixed density for a fixed volume. A gram of lead, has a fixed density for a fixed volume. Though gases (and mixtures of gases such as air) certainly have mass, and weight, they vary tremendously in density. Water has a fixed density at a fixed volume (yeah, I know its density slightly varies with temperature....and in a somewhat inverse manner, being less dense at freezing than when liquid, and for a 'incompressible fluid' it still displays compressibility at extreme pressures).
     
  5. Salmo_g

    Salmo_g Active Member

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    Krusty,

    True, but for the sake of this discussion of buoyancy, specific gravity seems totally relevant to me. While air can vary, I don't see the variance as significant in the context of this discussion. Same with water, only much more so.

    Sg
     
  6. Bonefish Jack

    Bonefish Jack Strictly FF

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    OK, I know I promised to be done with this, but I can't be done yet ... Apologies ...

    Specific gravity has nothing to do with buoyancy. :ray1:

    For example, steel has a high SG, and therefore, should sink. However, we can make steel float, if we can form it into a shape (like, say, a steel-hulled boat) that displaces a lot of water.
     
  7. Blue

    Blue Active Member

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    Okay, let's say you have a pontoon made of balloon material, and one made of basketball material....same size. Would one float a 200lb person better than the other??????
     
  8. Bonefish Jack

    Bonefish Jack Strictly FF

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    The short answer is "No."

    Of course, the additional answer is, if both materials have the strength to remain intact under the stresses. For example, if the balloon material is too thin to withstand the stresses, then the balloon would break - this eliminates the displacement, and eliminates the buoyancy. If the balloon material is strong enough, the buoyancy provided would be the same.
     
  9. Blue

    Blue Active Member

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    I realize that the strength of the material matters. I am just thinking old pool days. It was impossible to push a basket ball under water, but I was able to submerge a balloon the same size. Granted the shape changed on the balloon which would effect buoyancy wouldn't it?
     
  10. Bonefish Jack

    Bonefish Jack Strictly FF

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    Exactly! The shape changed, which changed the displacement, which changed the buoyancy.
     
  11. Fleshfly

    Fleshfly New Member

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    The relevency to designing boats is that while similar shaped hulls of fiberglass and steel will provide the same bouyancy the weight added by the hull material itself needs to be factored to determine true displacement. In other words they would both provide the same bouyancy but one would be much heavier than the other and therefore float lower in the water. If you use the chart I provided you can see that if you designed a drift boat hull and built one of fiberglass and one of aluminum the fiberglass boat would ride higher in the water.
     
  12. Bonefish Jack

    Bonefish Jack Strictly FF

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    Agreed that two hulls of the same shape could provide the same buoyancy, and the lighter hull would ride higher in the water. And I will also say, this is not because of specific gravity. Rather, it is about the weight of each hull. In the example you describe, you could make the fiberglass hull ride much lower in the water (perhaps even lower than the aluminum hull) simply by filling the inside of the hull with lots more fiberglass. This would change the weight, without changing the specific gravity of the fiberglass.

    So, again I will say, specific gravity has nothing to do with buoyancy :ray1:
     
  13. Salmo_g

    Salmo_g Active Member

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    B Jack,

    OK, you're right. I was thinking of the combined specific gravity of a hull made of any material plus the very low specific gravity of the air contained within. If the hull were filled with another material, like your fiberglass in fiberglass example, the specific gravity of the material matters, but it matters in the same way as the displacement of the water by the hull. You said it better. And a lot more succinctly.

    Sg
     
  14. themaninthemoon

    themaninthemoon Just waiting on warmer weather, .......

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    So, basically what you're saying is that if I add 5% to say 15% helium to my air mixture on the pontoon, then that will change the specific gravity of the way that my toon floats by increasing the buoyancy, & lessening the displacement factor?

    Yes/No?
     
  15. Bonefish Jack

    Bonefish Jack Strictly FF

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    Well, Yes and No :confused:

    There are a lot of factors mixed into your question. First, if you add Helium to your toon, the mass (or weight) will be decreased, so the density will be decreased, and that is the key. Forget Specific Gravity - Specific Gravity has nothing to do with it; your toon will simply be less dense. For an object of a fixed volume (like your toon in this example), the less it weighs, the less density it has, and the higher it floats.

    As to the lessening the displacement factor ... your toon will sink into the water until it displaces an amount of water equal to it's weight. The more your toons weighs, the more water it will displace. At some point, it will either sink or float, depending on how heavy it is. Remember, we are talking about an object of a fixed volume - your toon. So, when we talk about weight, we are also talking about density. There is a far better explanation here:

    I hope this helps ...