Boats on a low-gravity world

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

  1. gonzo
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    gonzo Senior Member

    How could it have more atmosphere? That is also a function of gravity.
     
  2. JSL
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    JSL Senior Member

    One serious matter is that safety would be compromised as people could be light headed & make poor decisions.
     
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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Venus provides an example for us, it has roughly 90% Earth gravity at the surface, but the atmosphere is a whopping 90 times Earth's surface pressure. Not to mention hot enough to melt lead. How those Soviet spacecraft managed to send back pictures of Venusian rocks to Earth under those conditions, is remarkable.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Just about everything about our physiology, is tailored to the environment we evolved in, and that includes 1 g. I expect that can't be so easily changed to suit distinctly lower gravity environments, like Mars.
     
  5. TeddyDiver
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    TeddyDiver Gollywobbler

    Not, planets magnetic field, the amount of solar wind, the age of the planet, planet density and most of all what's the original composition and thickness of the atmosphere. The gravity comes in eventually but a lot of more significant variables..
     
  6. clmanges
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    clmanges Senior Member

    Different gases. Venus' atmosphere is almost entirely carbon dioxide. My proposed fictional planet has the same mixture of gases we have here. And, yeah, maybe I should give it more oxygen, but that might alter the conditions.
     
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  7. brendan gardam
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    brendan gardam Junior Member

    ground effect craft like the ekranoplane should perform well if they achieve the same ground effect with less gravity. is that right?.
     
  8. Dejay
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    Dejay Senior Newbie

    How do ground effect planes deal with large waves? I think you'd get disproportionately large and slow waves because gravity is the restoring force. Surface tension also plays a role for waves. There is airy wave theory for gravity waves, but it looks complex and it's too early for me to understand that.

    At least the pressure on the planet is the same, else my head would explode ;)
     
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  9. tlouth7
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    tlouth7 Senior Member

    Gonzo and Jehardiman are arguing the same thing from opposite sides:

    for a vessel floating to the same lines the buoyancy force is reduced (as Gonzo argues) because the weight of displaced water is reduced. However the mass of cargo+vessel that can be supported is constant (as Jehardiman argues) because weight of cargo+ vessel is similarly reduced.

    If the force to push a given vessel through the water stays the same (I am not confident of this at all) then there would be a problem for sailing vessels: the righting moment at a given angle of heel would be reduced because buoyancy force is lower. Coupled with lower atmospheric pressure (again I am not sure this follows from reduced gravity) it might be very hard to make a sailing boat work.
     
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  10. gonzo
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    gonzo Senior Member

    Well explained, thanks.
     
  11. fredrosse
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    fredrosse USACE Steam

    Back to the front of this thread, the movie Avatar, a very high end and well received movie, explained very early that gravity on "Pandora" was much less than on Earth That way they could justify much of the planet's character.
     
  12. fredrosse
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    fredrosse USACE Steam

    Atmospheric pressure – Higher pressure = increased density = greater force on a sail, with the sail area and wind velocity held constant


    Atmospheric pressure – Changing Wave character? “Free Surface Conditions” . in old school fluid dynamics, are generally based on the assumption that the density of the topside fluid (air) is very small compared to the bottom-side fluid (water). So small is the ratio that it can be taken as zero for the sake of reasonable analysis, which provides valid approximations. So, this means that, even with double atmospheric pressure, the density of air is still a very low fraction of the water density, and wave relations are not impacted by atmospheric pressure variations.


    Of course, one could argue that the atmospheric pressure on this imagined planet could be so high that its density (of air) became a sizable fraction of water density, but that is an extreme and unreasonable supposition, at least if you believe Sir Isaac Newton’s Universal Law of Gravitation. I think we all have to be “on board” with that within this discussion?
     
  13. fredrosse
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    fredrosse USACE Steam

    “……cavitation with props or foils…….” Cavitation occurs when the local pressure falls to the steam saturation pressure. The local pressure at any point on a propeller blade (for example here) is a function of three variables, 1: the atmospheric pressure, 2: the depth of submergence in the liquid (multiplied by the liquid density, and multiplied by local gravity), 3: the dynamic pressure disturbance, caused by the propeller spin redirecting the liquid flow & velocity. So, these variables, each one, has influence on the propensity to cavitate,
     

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    It is uncanny the way on Mars, wind has sculpted landscapes to appearances very similar to a desert on Earth, despite the atmosphere being very thin indeed, surface pressure about 1% of ours. The very high wind speeds, however, might compensate for that lack of density.
     
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