Floatation/Buoyancy perspectives

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

  1. 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.
  2. 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??????
  3. 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.
  4. 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?
  5. Exactly! The shape changed, which changed the displacement, which changed the buoyancy.
  6. 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.
  7. 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:
  8. 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.

  9. 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?

  10. 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 ...
  11. Car Talk puzzler to wrap your head around buoyancy:

    Tommy's Cubed Car

    One day, Ray secretly takes Tom's beloved black 1965 AMC Ambassador convertible to the crusher, where it's transformed into a 3-by-3-by-3-foot solid cube of rust, headrests, and mice and placed on a garbage barge. While the barge is crossing a lake, it falls overboard. Assuming water can't escape from the lake, does the water level go up, go down, or stay the same?​

    I'll post the answer tomorrow. Don't worry, the answer contains an explanation.
  12. The water level goes down.

    While on the barge the car displaces an amount of water equal to it's own weight. It does this because it is pressing down on the barge which presses down into the water enough to displace a volume of water equal to the weight of the barge/car.
    Remove the car from the barge and now it only displaces a volume of water equal to the weight of the barge.
    The barge rises slightly, the water level goes down.

    But then the car hits the water...
    Water floods all the air space inside the cube and it sinks to the bottom of the lake. Now the car displaces a volume of water equal to the volume of the parts of the car. A given volume of steel is heavier than an equal volume of water so the car sinks.

    When the car was on the barge it displaces a volume of water equal to it's weight. When the car is in the water it displaces a weight of water equal to it's volume.

    Remove car from barge: water level drops equal to weight of car (water amount A)
    Put car in lake: water level rises equal to volume of the car (water amount B)

    Because water amount A is greater than water amount B the car floats on the barge.
    Also because water amount A is greater than water amount B, the lake rises when the car falls in, but not as much as it lowered when the car fell off the barge.

    Lake level goes down.

    That's my take on the subject.
  13. Oops, double post.
  14. One would think that with all factors considered, like if it is raining during this time period, or is the lake spring fed, or just where did the water that makes/creates the lake come from? in addition to the coefficient of friction, equaling the air mass that travels over the lake surface @ a given rate of wind velocity, or that the temperature changes in a 24 hour period does, or does not have any effect upon the evaporation rate @ a given set temperature. With all items matching exact conditions of the time & day of the cube falling into the water, the water level would rise in displacement of the weight of the sinking cube, measured one hour after the cube fell into the water, but if measured @ the same time as the cube fell into the water it would not affect the water level at all, because the weight was supported by the barge which displaced the same volume of water as the total of the two pieces together or separately. There will be different displacement values of the articles placed into the water but the water level will remain the same, but only because of the total weight placed into the water.

    But following the story plot,

    In which everything is okey-dokey until Tom find's out about Ray's betrayal, & kills off all of Ray's whole family by placing their bodies in another boat & driving it into the rocks @ max speed of the boat which Tom sprayed with fuel, the resulting explosion opens another investigation & all trains of thought about floatation & buoyancy are forgotten in the ensuing questioning of the local Coast Guard/FBI/DNR officers assigned to the wreck & deaths of Ray & his family.

  15. Don't forget... they're brothers: http://www.cartalk.com/content/tom-and-rays-bios-photos-0

    Here's the answer:

    It went down. When the 3,000-pound cube is in the barge, it's displacing its weight in water. A cubic foot of water weighs about 62.4 pounds, so the barge displaced about 48 cubic feet (3,000 pounds of steel divided by 62.4 is about 48).

    When the cube sinks, it displaces an amount of water equal to its volume. Since it's a 3-foot cube, that's 27 cubic feet. So the water level goes down about 21 cubic feet (48 minus 27 equals 21). Moments later, more cubic feet are displaced, as Tommy jumped in to try to save the sleek black beauty.​
  16. Why would Tommy be trying to save a crushed 3'x 3' cube?

    LOL, Would you be trying to give him a glass of water?
  17. Yes! Now I understand the real reason Bob Triggs posted his weather warning....folks trapped indoors by the stormy weather comment on threads like this one!:rofl:

    I'm getting out of here and heading out to my shop. Got projects!
  18. I got a headache.....
  19. Lmfao! no head, just aches!
  20. :ray1: >>>>.....:hmmm:...>>>>>:ray1:.....:hmmm:....>>>:ray1:>>....:confused:....:beathead:......iagree Lmfao! ! !

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