Boats on a low-gravity world

Discussion in 'Boat Design' started by clmanges, May 8, 2020.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    I guess the physics of alien worlds will catch out the "common sense" approach, I might back jehardiman to design that helicopter they want to send to Mars, the rest of us can sit that one out !
     
  2. gonzo
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    gonzo Senior Member

    What is the buoyancy at zero gravity?
     
  3. TANSL
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    TANSL Senior Member

    I think you are confusing kg-mass with kg-force.
    The vessel's mass is measured in kg-mass while the rho of the fluid is measured in kg-force, therefore, that equation is not correct.

    Gonzo, the buoyancy experienced by the boat immersed in a fluid, in equilibrium, is always equal to the weight of the boat. Archimedes was right even in outer space.
     
  4. jehardiman
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    jehardiman Senior Member

    No, I am correct. rho is mass density (i.e. kg/m^3 or slug/ft^3 see Density - Wikipedia https://en.wikipedia.org/wiki/Density). Specific weight or weight density is usually gamma (i.e. N/m^3 or lbs/ft^3 see Specific weight - Wikipedia https://en.wikipedia.org/wiki/Specific_weight)
    Also see the ITTC recommended symbol list. https://ittc.info/media/8214/alphabetic-ittc-symbols-list-2017.pdf which uses rho for mass density, w for weight density (which is defined as rho*g), and gamma for specific gravity (i.e. the dimensionless mass density or weight density relative to water at 4C).
     
  5. jehardiman
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    jehardiman Senior Member

    First of all, that is not the original question asked, and secondly everyone who has passed high school physics will know what happens in body relative zero-G.....unless you really want to get into wetting factors....which is way beyond this discussion.
     
  6. TANSL
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    TANSL Senior Member

    Then you are correct, of course. The normal thing is to consider rho as the specific weight of a material.
     
  7. gonzo
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    gonzo Senior Member

    No, I just wanted an answer. Could you, please?
     
  8. TeddyDiver
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    TeddyDiver Gollywobbler

    My first thought was to watch slow motion video. As gravity g = 9,81 m/s² just calculate how fast or slow run the video to simulate any different gravity..
     
  9. gonzo
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    gonzo Senior Member

    jehardiman:
    I think you are confusing buoyancy with displacement. The displacement volume (edited) will not change, since the weight of the boat will change at the same ratio as the weight of the water. However, the buoyancy will be less, since the force needed to lift a mass is a function of the acceleration of gravity (weight). If buoyancy (vertical force was constant) the boat would float higher which is the opposite of your statement.
     
    Last edited: May 10, 2020
  10. TANSL
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    TANSL Senior Member

  11. philSweet
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    philSweet Senior Member

    The dynamics are maybe a bit easier to grok. If the dynamic response is governed by inertia, and the restoring force is governed by gravity, such as in vessel roll, then the dynamics depend on the ratio of inertia to gravity. This can be related to scaling laws at constant G. At low G values, the boat will behave like a larger boat at the same G values. Roll rates will be slower, etc.

    For props and foils, ventilation will be an even bigger problem.

    As mentioned by others, anything dependent on Froude number will be affected. The reason powering will be less is that Fr will increase, thus displacement speed limit will be lower. The displacement speed-limit/length formula will no longer be knots = 1.34 * sqrt(length in feet).

    A good look at wiki's table of nondimensional numbers, looking for any that have a /g/ in them, will let you see where to expect changes to happen.
     
  12. Dejay
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    Dejay Senior Newbie

    I guess it's undefined, you get division by zero. Of course you always have some gravity.
    But weird to think of a balloon floating inside a large blob of water.

    Star Trek Voyager had an episode featuring a planet entirely made up of water.
     
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  13. clmanges
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    clmanges Senior Member

    Maybe a little to the side of the topic, but that's fine with me, and I was thinking of this myself. However, instead of a balloon, just a free air bubble.
    Heinlein described such a thing in one of his novels: a swimming pool on an orbital station. The water was contained in some kind of clear envelope, or maybe just a big spherical chamber (I can't recall how people got in or out of it) and it had a big air bubble in its center. I think I recall something about people smoking in there, but they weren't supposed to because it overtaxed the little air recycler inside the bubble.

    I'm going to say I don't believe such a thing would be stable. I think it could be done, but the bubble would not stay put in the center. The surface tension of the water would try to force it out and the air bubble would be pushed to the outside of the envelope as surface tension acted to contain the water in a shape with the smallest surface area.

    And now I'm wondering if the ISS astronauts have ever done such an experiment. Seems like something they'd have tried by now.

    Also, a factor I hadn't mentioned before is that (I'm pretty certain) this planet will have lower atmospheric pressure as a consequence of its lower gravity. I don't know much about how that would affect the boats aside from needing bigger sails. I know that water would boil at a lower temperature there, and perhaps that would affect atmospheric humidity.

    I'm pretty sure that's been accounted for, although the pics and video of it do make me wonder if the rotor blades wouldn't need to be bigger. We'll see, I guess, and I hope it doesn't become an embarrassment.
     
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  14. gonzo
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    gonzo Senior Member

    At very low gravity, there will be other weird effects. I think that depending on what material the boat is made of, the capillary action of the water, which has a positive meniscus, could make the water creep over the boat and cover it.
     

  15. Dejay
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    Dejay Senior Newbie

    You can have a lower gravity planet with the same pressure if it has more atmosphere.
    So I think cavitation with props or foils would be the same.
     
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