Boats on a low-gravity world

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

  1. clmanges
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    clmanges Senior Member

    Consider this amusement. I was thinking of a science-fiction scenario of an earthlike world, but with notably lower gravity, say, maybe 60-75% Earth-normal. And at first I thought that the lower gravity might reduce buoyancy, but then the boats would also weigh less proportionately.

    I'm assuming for this scenario that water is the same there as here, and that, then, made me wonder about surface tension. Surface tension doesn't change with gravitation; we see this with astronauts playing with water droplets on the ISS.

    Because of this I think waves would get steeper before breaking; rain would fall more slowly and in much bigger drops, and waterfalls would produce large drops and maybe even big blobs of water instead of fine spray.

    How do you think the performance of boats would be affected by this? Would they sit a little higher in the water? Would it be harder to move them through the water? Would prop cavitation need to be addressed differently? Would the difference even be noticeable at all?

    Hoping to get some fun out of this; thanks.
     
  2. jehardiman
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    jehardiman Senior Member

    It would affect any hydrodynamic calculation (or any dynamic calculation for that matter) that includes g (until we are down to micro-g.. then we start having to worry about pressures also) . If g is in the numerator, the outcome is diminished, if in the denominator, the outcome is enhanced. Realistically, gravity and atmospheric pressure are so very entwined (assuming a non-closed environment) that pressure will start to become the real issue.
    You are correct that buoyant volume will not change...that is a mass density issue, not a gravity one as long as the pressures and temperatures are near equal.
     
  3. BlueBell
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    BlueBell Ahhhhh...

    Wow, that's quite the concept.

    Boats would use less HP, better fuel mileage, better airtime!
    Everything is better when your boat's lighter.
     
  4. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'm guessing waves would be same shape, and displacements would stay same, but waves would be slower, and that the speed of a wave is governed by the amount of force (gravity) acting on a given mass of water to pull it back down and send the wave on its way. Planing would take less energy but displacement boat movement would stay the same.
     
  5. jehardiman
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    jehardiman Senior Member

    Except for the airtime comment we need to send you back to high school physics.
    The drag of a vessel in water is not dependent on the gravity...that is because the mass of the vessel and the mass density of the water is the same. So the only effect on powering is that the vessel may be lifted onto plane sooner as Squidly said. All else will be equal and depending on the exact hull shape, some planning vessels may not plane because of surface effects in the wave train. Lower g will cause the wavelength to get shorter, but the period to get longer until g is so small that surface tension dominates.
    While there have been investigations of waves at high and low g's (think liquid tanks in rockets and spacecraft... https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880006559.pdf). I'm not sure if anyone has looked at planetoid ocean waves yet.
     
  6. gonzo
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    gonzo Senior Member

    Force (Buoyancy) = Displaced volume x Density of displaced fluid x gravity.
    As gravity goes to zero, buoyancy also goes to zero. With less gravity, there is less weight ergo less displaced volume since the density is a function of mass.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Of course on a liquid methane ocean, you need to be well rugged up, and have a supply of oxygen to power both the boat crew and the engine, the fuel could just be drawn from the immediate environment. But would the boat be too brittle in the low temperatures ? We are very much creatures of the environment we evolved from.
     
  8. Ike
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    Ike Senior Member

    What an interesting concept. I have been a big sci-fi fan since I learned to read. I have read literally thousands of sci-fi stories and I don't recall this ever being addressed. There are lots of sci-fi stories that have water worlds, boats, both powered and sail, but I can't remember any that were a low gravity world. Why take a slow boat when you have anti-grav flying machines? LOL. Anyway, certainly a nice topic for discussion.

    Of course the classic sci-fi story, The Martian Chronicles, has water (canals that we now know don't exist) and boats on the canals. But Bradbury never addressed the low gravity issue, not with the boats or with the rest of the story.
     
  9. Ike
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    Ike Senior Member

    Given a particular boat, static stability would be affected. All things being equal except gravity, the boat would be slightly less stable on a low gravity world. This could be easily corrected by lowering the CG or by increasing the beam. There are other solution as well.

    Dynamic stability would probably not be affected much. But as has been said it would require less HP, use less fuel. In the case of sailboats there would have to be some serious adjustments to sail area, mast height, keel depth and ballast weight.
     
  10. Squidly-Diddly
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    Squidly-Diddly Senior Member

    related question: What effect does "ambient" pressure have on waves (discounting the effect of greater density of the gas over the liquid, and diffs in little quirks (viscosity of different gases, etc).

    Say you take a Test Tray full of water and use a bar to create a Wave at sea level, then take that same Test Tray in a chamber full of H2 gas and increase pressure so H2 is same density as air at sea level, then do the same with heavier gas like Argon at low pressure. Would the waves of same mechanical creation look substantially different at diff pressure? I guess "no", since pressure is pressing the wave the same in every direction.
     
  11. clmanges
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    clmanges Senior Member

    Sorry, no methane here, but you write your story and I'll write mine, lol!
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Low gravity has a bad effect on humans, as space-faring humans have shown. Colonies on Mars look more like fantasy than reality. And let's not worry about being showered with cosmic radiation on the long trip to get there.
     
  13. Dejay
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    Dejay Senior Newbie

    Haha that would be a totally new visually interesting sci-fi world for a movie!

    I figure hydrofoiling would be much easier and less power intensive and have less structural requirements. So a civilization evolving there might invent the hydrofoil before the displacement boat :) Then you can just glide over those pesky giant waves everywhere haha.

    Since the waves are slower but bigger but water has still the same mass you'd have a lot more hassle by the crushing force of waves.

    You'd also still need the same force to displace water and would create bigger bow waves right? So hull speed with displacement boats could "interesting".

    I think. NASA is planning a mission to the underwater oceans of Jupiter's moon Europa. Although probably not much changes there underwater.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    To paraphrase David Bowie, everything would be "floating in a most peculiar way" !
     
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  15. jehardiman
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    jehardiman Senior Member

    I'm sorry people, but I can't in good conscience let a statement about physics so blatantly wrong stand without correction.
    As we all know, Archimedes' principle is the statement that the buoyant force on an object is equal to the weight of the fluid displaced by the object. For a item or vessel to float, the buoyant force must be equal to its weight. Weight is the concept here and so we must define what weight is. Weight is actually the force of acceleration on the mass (m) of the item. For a boat, static in a fluid on any planet, with any gravitational acceleration (g), weight on that planet = m*g. (just for those people who like to be sticklers, the symbol of standard gravity on earth is g0 (g zero)) Now we need to determine the weight of the fluid displaced. Again, the weight of the fluid is the mass of the fluid times the gravitational acceleration of the planet. In this case mass of the fluid = rho*V where rho is the mass density of the fluid and V is the volume displaced.
    So now we have everything we need to answer clmanges question: Would they sit a little higher in the water? The answer is no.

    mass of vessel*g of planet = Volume displaced*fluid rho*g of planet
    we can now divide both sides by g of planet to remove it from the equation
    mass of vessel = Volume displaced*fluid rho

    So, regardless of what the gravity is, the vessel will always float in a specific fluid at a constant volume.
     
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