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Boats and balloons both work because of buoyancy - they are pushed up by a force equal to the weight of displaced water/air respectively. I want my (otherwise earthlike) planet to have 'seas' of a liquid or gas that is

  • Denser than air, so it sits on the ground
  • Light enough that normal boats made of wood, steel, etc immediately sink (unless they were impractically large and light).
  • Ideally semi-inert (at 1atm 15 C) and nontoxic (other than being a simple asphyxiant)

I plan to have balloon-boats - which are basically lighter-than-air craft (with hydrogen or helium balloons), except they aren't lighter than air - just 'lighter' (less dense) than whatever the seas are made out of.

What substance should I use?

(Bonus points for an explanation as to how a such a sea could form.)

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    $\begingroup$ Can you use a superfluid sea? The liquid would simply flow up and into any open boat design. $\endgroup$ – Samuel Feb 9 '18 at 22:02
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    $\begingroup$ Boats function exactly as you describe them, so they are already baloons in a way. Also, replacing seas with something that's non-water would have... interesting consequences. And perhaps we're thinking backwards. Why adjust the composition of the water when you can adjust the composition of the boat? I'll have to think more about this $\endgroup$ – Andon Feb 9 '18 at 22:26
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    $\begingroup$ Immediately remembered this: what-if.xkcd.com/50 It's a pity that none of them satisfies all the requirements. $\endgroup$ – justhalf Feb 9 '18 at 23:47
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    $\begingroup$ @Renan I read your answer before I commented. It doesn't change my comment. The situation your answer describes is one where boats are designed for one density and then at some times or some places that density is very different. It's possible to design boats that could float on top of a mud volcano, we just don't do it because those are too rare to make it worthwhile. But it's conceivable that at some point on a world filled with mud volcanoes that someone would make a boat that can float on top of them. $\endgroup$ – Todd Wilcox Feb 10 '18 at 3:00
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    $\begingroup$ This will never work. If you make your sea less dense, I make my boat less dense by building it wider or with more draft. I will never need to add balloons. Container ships are made out of tons of steel would sink in an instant as individual pieces, but due to shape, it floats. Further, some college kids once made a canoe out of concrete. Again, shape was the key. $\endgroup$ – cobaltduck Feb 10 '18 at 16:39
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Start with regular seas like those of our own world. Then add lots of marine mud volcanoes. These are not true igneous volcanoes - they form when underground mud, or mud-like stuff breaches the surface of the crust and erupts.

Mud volcanoes are one of the proposed explanations for the disappearance of so many ships over the Bermuda Triangle. As per the wiki, with emphasis mine:

An explanation for some of the disappearances has focused on the presence of large fields of methane hydrates (a form of natural gas) on the continental shelves. Laboratory experiments carried out in Australia have proven that bubbles can, indeed, sink a scale model ship by decreasing the density of the water; any wreckage consequently rising to the surface would be rapidly dispersed by the Gulf Stream. It has been hypothesized that periodic methane eruptions (sometimes called "mud volcanoes") may produce regions of frothy water that are no longer capable of providing adequate buoyancy for ships. If this were the case, such an area forming around a ship could cause it to sink very rapidly and without warning.

Publications by the USGS describe large stores of undersea hydrates worldwide, including the Blake Ridge area, off the coast of the southeastern United States.

There is great skepticism about the amount of sinkings on the Bermuda Triangle, but lab tests show that mud volcanoes can give ships a hard day. On a fictional world of your writing, you just have to taylor the amount of those volcanoes and frequency of their eruptions to suit your need and taste.

This keeps rivers navigable. Also, the presence of these volcanoes and their gas output does not exclude the possibility of marine life - there is plenty of it in the Bermuda Triangle.

One interesting consequence of this is that some people might be able to build ships that would be buoyant over the shores, and then they would think they have a seaworthy vessel. They might just keep their boat afloat for some time even, hours or days depending on how cruel you wish to be with your characters. Then an eruption happens and the vessel sinks "very rapidly and without warning".

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    $\begingroup$ Bizarre! Truth is stranger than fiction... $\endgroup$ – F. Name Feb 9 '18 at 21:10
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    $\begingroup$ @F.Name stick around long enough and you will see that saying is more true than you can imagine. $\endgroup$ – Renan Feb 9 '18 at 21:12
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The lowest density practical (excluding hydrogen and helium) liquid is liquid methane, with a density of about 0.65g/cm3 compared with water's 1g/cm3. There are few, if any, solids that will float in this unless they contain trapped gas. Ethane, propane and butane are not far behind.

Steel ships float in water because they are mostly full of air. So your ship in a hydrocarbon ocean could be filled with atmosphere and float. Unless you have very high pressures.

If you take liquid methane and seal it in a partially filled container, you will have methane liquid at the bottom and methane vapour above. Increase the temperature and the density of the liquid rises. The vapour pressure also rises, hence the vapour becomes more dense. Finally at −82.6 °C temperature and and 46 bar pressure (about 46 times earth atmosphere) the density of the gas and liquid phases becomes the same, and the two phases become indistinguishable. This is called the critical point. The critical point exists for pure liquids that are stable enough not to decompose before boiling. Ethane's is at 32C and 49 bar. So if you have a methane atmosphere over a methane ocean slightly below the critical point, it will not be practical to build a boat.

The critical point temperature for ethane is more comfortable for humans, but the pressure is too much. 49 bar is equivalent to about 490m ocean depth on earth, but the record for scuba diving is 332m (short term.) Unfortunately these types of pressures are the only way to make the atmosphere dense enough that a boat filled with atmosphere would not float.

The only way to have a gas much denser than ethane while retaining a low boiling point would be to consider a noble gas atmosphere (such as Krypton) over an ocean of hydrocarbon. Pressures would still need to be quite high but not as high as 49 bar. But the heavier noble gases are quite rare so the possibility of finding an entire atmosphere of heavy noble gas is zero unless artifically created by some former superadvanced alien's terraforming project.

Making your balloon float is easy. Fill it with hydrogen or helium. A hydrogen filled balloon will not present a fire risk unless the atmosphere contains oxygen.

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    $\begingroup$ While reading your post I kept thinking of Saturn's moon Titan. $\endgroup$ – Renan Feb 10 '18 at 13:27
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I see two problems here:

  1. Most common liquids have fairly high density - oil is 0.8, compared to wood which is 0.6-0.7. wood still floats on most common liquids.

  2. Ships are balloons actually. Think about it: a ship is a thin layer of steel filled with air. Almost like balloons. If you have a lower density of the seas you will just need a bigger balloons - a bigger ship body with thin steel around it.

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    $\begingroup$ I think both of these issues are just restating the reasons for this question. $\endgroup$ – Michael Richardson Feb 9 '18 at 21:57
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    $\begingroup$ So what is it ? $\endgroup$ – Vincent Feb 9 '18 at 22:20
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    $\begingroup$ @MichaelRichardson The problem with the question is a matter of physics. If you have a less dense fluid on top of a more dense fluid, then it's trivial to make an object float on the more dense fluid - simply build something that encloses enough of the less dense fluid so that the difference in displaced mass is greater than the mass of the enclosure. That is pretty much always possible. We call such enclosures "boats" and "ships" when the more dense fluid is water, and "balloons" when the more dense fluid is regular earth air (the less dense fluid being helium). $\endgroup$ – Todd Wilcox Feb 9 '18 at 23:27
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    $\begingroup$ This has a few premises that might lead to a good answer but it needs to be led to a conclusion - wood still floats on most common liquids, and a boat uses similar principles to a balloon, therefore:? $\endgroup$ – Pingcode Feb 9 '18 at 23:55
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    $\begingroup$ The answer "there is no such thing" is a valid answer. $\endgroup$ – Censored to protect the guilty Feb 10 '18 at 13:06
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I don't believe this is possible. We already have "unsinkable" ships--flood them and they stay on the surface. (They are not used more because of cost and size limits.) Even if you replace that water by something considerably lighter the same thing could be done.

The basic approach is to use enough materials that are very light to build your ship such that it doesn't rely on air below the waterline for buoyancy. While it wouldn't be strong enough picture a ship made of styrofoam. You can't sink it.

(Note that once we have enough space-based industry expect to see this in widespread use on Earth. Metal foam is considerably stronger than the same weight of pure metal and it can be made lighter than water. It's just that it can't meaningfully be made other than in zero-g.)

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    $\begingroup$ "Unsinkable" ships still are widely used, but more for small/private craft. My father had a 22 foot sailboat with enough foam inside to keep it afloat pretty much regardless, and pretty much any kayak will have them as well. Sure, they're not ships in terms of oil tankers or battleships, but they're still boats that are "unsinkable" $\endgroup$ – Andon Feb 10 '18 at 6:02
  • $\begingroup$ @Andon Exactly--plenty in the small craft world where it doesn't cost that much and it's easy to compensate for the lack of strength. The bigger the ship the harder it is to do. $\endgroup$ – Loren Pechtel Feb 10 '18 at 19:45
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From a physics perspective, you are pretty much out of luck: Air has a density of only $1.2 \frac{kg}{m^3}$, hydrogen, the lightest gas there is, has a density of $0.1 \frac{kg}{m^3}$. This means that you need to displace at least $0.9m^3$ of air to make a single kg float.

Now, if you want it to be more practical to displace air than "water", you need your water to weight less than $2.4 \frac{kg}{m^3}$. Radon, which is the heaviest noble gas, weights about $9.7 \frac{kg}{m^3}$, and still it mixes with air. That is, you cannot have a clear cut "sea" surface with it at room temperatures. And I didn't even take wind into account yet. Point is, at these densities, you won't have anything that we would recognize as a "sea".

To put this into perspective: If we assumed the "sea" to be $10 \frac{kg}{m^3}$, you'd be able to generate a lift of $8.8\frac{kg}{m^3}$ by displacing the "water" with air, and only a lift of $1.1\frac{kg}{m^3}$ by displacing air with hydrogen (which is not a very good idea, btw.). Displacing "water" gives you roughly eight times the bang for the buck, even though you do not even have a well-defined sea surface yet. And the heavier you make your "sea", the more clearly physics favor normal boat designs over your proposed balloons.

On the practicability of boats on very light weight materials: You just need to scale up your boats. The mass a boat can carry is directly proportional to the amount of water it displaces, and that is base area times depth. If your boat is ten meters below the "water" line, every square meter will carry $88kg$ of load with the numbers above. That's an average human plus 18 kg for one square meter of floor. Not much, but definitely something to work with. So you build your boats by simply laying down a large "water"-tight floor, attach a high wall around it, and you are ready to float. Much easier to manage than producing balloons of sufficient size and strength to carry you through the air.

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Heavier air?

Have you thought of making a heavier gas make up some of your atmosphere? Sulfur hexafluoride is a heavy gas, and would cause ships to need much more of it to float. Its still less dense than water, though. It may be poisonous to humans, but your planets inhabitants will have adapted to it. Here is a link to a website that explains some more: https://science.howstuffworks.com/environmental/earth/oceanography/invisible-water1.htm

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  • $\begingroup$ You could just make the atmosphere more massive with the same components of our own and you would get the same result. $\endgroup$ – Renan Feb 9 '18 at 22:50
  • $\begingroup$ There's no such thing -- AFAICT -- as NaF6. And SF6 is poisonous because fluorine is so stunningly reactive. No adapting to that... $\endgroup$ – RonJohn Feb 10 '18 at 1:53
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    $\begingroup$ @RonJohn, sulfur hexafluoride is non-toxic: the fluorine atoms are so busy holding on to the sulfer that they can't be bothered to react with anything else. $\endgroup$ – Mark Feb 10 '18 at 3:40
  • $\begingroup$ @Mark thanks for the correction. Are there any molecules that it reacts with slowly enough for life (as opposed to combustion)? $\endgroup$ – RonJohn Feb 10 '18 at 3:50
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    $\begingroup$ @Syro33, it's an asphyxiant, much like helium: it kills you by displacing the oxygen in your lungs. Unlike helium, it's (much) heavier than air, and will take a while to leave your lungs once you stop inhaling it. $\endgroup$ – Mark Feb 10 '18 at 4:32

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