Floatation/Buoyancy perspectives

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

  1. themaninthemoon

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

    Buoyancy, is regulated by
    A). The amount of air pressure you can maintain for a constant/consistent period in a given area, more air pressure equals more buoyancy, until the weight of the contents/air "trapped" overcomes the space provided. ie; a propane tank, empty, will float, on top of the water, full, it will sink, until the pressure is equal to the pressure of the depth of water, & if it weighs more than the structure of the tank allows for by the capacity of it's internal strength, then it will implode as the pressure increases with the depth.

    B). The displacement of mass, in a given area, vs the weight of the water
    ie; a 100 lb block of steel, sinks, vs a 100lb., also made of steel 12' boat floats
    It is the increased displacement of the mass over a larger surface area that allows one form of steel to float, & the other to sink. By reducing the impact of weight per square inch, allows for the 100 lbs. to float. Increasing the area of coverage by utilizing the same weight will increase the buoyancy, & reduce the amount of draft of the contact surface of the craft.

    Something to consider thinking about? No?
    This also applies to the inflatable pontoons. No?
  2. Krusty

    Krusty Active Member

    Yes, it's a convoluted way of saying that bouyancy is directly related to the weight of the water displaced by the craft's weight (including cargo)....though increasing air (via higher pressure)in a rigid vessel does not increase bouyancy. Higher pressure, up to the point of failure, slightly increases the displacement volume of an expandable bouyant tank, i.e., bladder....and increases bouyancy directly by altering the vessel's displacement. Conversely, if a rigid hollow kayak (a sit-on-top kayak) were able to perfectly sustain it's volume under internal vacuum it would be slightly more bouyant than one filled with air at ambient atmospheric pressure. Open vessels are, of course, filled with air at ambient air pressure.
  3. jeff bandy

    jeff bandy Make my day

    B) is right on, but theory A) is full of holes. The reason a propane tank sinks full and floats empty is directly related to it's weight. Full, it probably weights 20lbs. more. Yet the size remains the same. The tank doesn't change shape.
  4. CC898

    CC898 Member

    In theory "A" what he is describing is the displacement of what creates the bouancy. In this case AIR, when the container is filled with a compressed gas, be it any kind, your are displacing the internal air volume. If you were to use compressed air, which we all know is made up of many components, the result would be the same, just at a much higher pressure. Which does not, as you stated, does not have anything to do with changing the shape of the structure.

    Density of the given object. In order for anything to float, it must displace a greater volume of water by mass than it weighs.
  5. Methow Roamer

    Methow Roamer Seeker of the Exotic and Aquatic

    Buoyancy is equal to the weight of the water displaced by the vessel. Period.

    Spreading the pontoons farther apart, for example, will not increase the buoyancy. It will only change the shape/location of the buoyant surfaces.

    This can provide more stability, but the vessel will still displace the same amount of water. (excepting the added weight of modifications)
  6. Plecoptera

    Plecoptera Active Member

    Good simple explanation. This is why longer or larger diameter pontoons are more buoyant and can hold more weight with less draft than smaller ones. Changing the frame dimensions does not effect this unless you are adding or reducing weight.
  7. Bonefish Jack

    Bonefish Jack Strictly FF

    Thank you :thumb:

    Pressure has nothing to do with buoyancy ...
  8. Ian Stubbs

    Ian Stubbs New Member

    If a pontoon is not inflated to a pressure great enough to keep the pontoons shape the vessel will displace less water therefore become less bouyant therefore pressure does have something to do with bouyancy even if the pressure is only to maintain shape.

  9. Krusty

    Krusty Active Member

    Yup, because without the contribution of adequate positive pressure to maintain bladder rigidity, the vessel is unable to displace as much water....hence, less bouyancy.
  10. themaninthemoon

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

    Wow! So many explanations as to perceive the separate experiences of defining a term that we just take for granted. This is great! I look forward to anyone else that would like to put their 2 cents in. Some of these comments are really, really good.

    But, just one question, ....if pressure, has no bearing on the buoyancy of a rigid structure, then how does a submarine surface from a deeper depth, to a shallower one? Seems rigid enough to me, ....just asking, .....
  11. Ed Call

    Ed Call Mumbling Moderator Staff Member

    A low pressure pontoon (weakly inflated) displaces less, carries less, drafts deeper when loaded. A fully and properly inflated boat will carry more, draft less, track better and respond better.

    The walls of those intentionally sinking cans flex a whole lot under pressure.
  12. cabezon

    cabezon Sculpin Enterprises

    See http://science.howstuffworks.com/transport/engines-equipment/submarine1.htm for a description on how a submarine changes its depth. More interestingly, from a fishing perspective is how fishes with swimbladders can add or subtract gases from their swimbladders to maintain neutral bouyancy after changing their depth. This has been worked out for many years and is contained in any good fish biology text book, but you can read a summary here: http://thegab.org/Illness-and-Treatment/swim-bladder-anatomy-and-physiology.html.

  13. Bonefish Jack

    Bonefish Jack Strictly FF

    Sorry to argue, but the issue you describe has to do with displacement, not pressure. If you were to fill the pontoons with styrafoam, and leave the valves open, there would be no pressure (differential), and the pontoon would be very buoyant. Now, no question that sometimes the pressure inside the pontoon can impact the displacement, as you describe - however, just because you have pressure does not necessarily mean that you will have the displacement you need to have buoyancy.

    cabezon posted a great link expaining the displacement issue, and how it applies to submarines. In fact, it also applies here. If you were to put a lot of water in your pontoon (just as is done in the sub when it dives), you could put a lot of "pressure" in the remaining air space, and the pontoon would still not be buoyant. And if you were able to push the water out of the pontoon (just as is done in the sub when it surfaces), the pontoon would become buoyant.

    OK, I promise to be done with this topic ...
  14. themaninthemoon

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

    Great answers guys, & I can see by the various answers that everyone who responded, have differing, (yet similar in nature), views on this topic. But this is how we interact to learn from each other, & I think that the response has been great. I know that I've expanded my view, & have picked up on a number of tips regarding this issue.

    Is there another topic that can generate as much conversation ? ? ?
  15. Methow Roamer

    Methow Roamer Seeker of the Exotic and Aquatic

    So, what do you guys think about nymphing under a bobber?
  16. Salmo_g

    Salmo_g Active Member

    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 - ).


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

    Fleshfly New Member

    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
  18. Salmo_g

    Salmo_g Active Member


    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?

  19. Krusty

    Krusty Active Member

    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).
  20. Salmo_g

    Salmo_g Active Member


    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.