TL;DR then scroll down, genius

This question is set in a planet with a human population, about the size of Earth with earth-like formations, that has appropriate lifeforms to the conditions following. The planet has multiple continents, but one continent is about the size of Asia and Africa combined (pretty big), and on this particular continent, there is a lake about the size of Australia (pretty damn massive).

These humans are advanced to about a 19th century degree, so we aren't looking at motors, nor electricity.

The lake isn't connected to any oceans, but there are plenty of convenient fresh water rivers that run through the continent, so plant, animal and human life isn't hindered by such a lake. Because of its size, rain doesn't accumulate directly on top of it, and it is mostly in an arid area, so we don't have to worry about water accumulating on top. You know what? Don't even think about weather at all.

The lake, as per the question, is made of a metal-like substance. This is similar to mercury, in which in remains a liquid in common temperatures (it would 'freeze over' in some areas during the winter months like typical water lakes would), and it is incredibly dense, although not as much as mercury.

This metal is not dangerous chemically, but it obviously poses the threat of drowning, if you get stuck somehow (although humans can comfortably stand on it for a few seconds, so anyone who drowns... deserves to drown). It doesn't let off any dangerous fumes, and is a pretty useless substance but stands in the way indefinitely.

This is because, to my consideration, it would be considerably difficult to cross such a lake... with conventional methods. I found this, which doesn't really offer any insight, apart from a comedic view.

Keep in mind that I am looking for how one might sail, or whatever floats your boat (on a mercury-like lake, mind you), on a mercury-like lake. I don't want any flying, and simply walking across it with camels is not an option. You'd sink. It's not as dense as mercury. And even if you could keep up the speed, where would you sleep?

I'm expecting physics-based boats that are designed to go on long trips across this liquid. I would prefer for it to be powered by nature, or mechanically powered, but any vessel that matches the human's technology advancements is a step in the right direction.

And for the lazy people who didn't read all that:

How might one use a boat effectively in (on?) a mercury-like lake?

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    $\begingroup$ did you read this? :) what-if.xkcd.com/50 $\endgroup$ – Pavel Janicek Feb 3 '15 at 8:39
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    $\begingroup$ Just as an aside: it's unlikely that a lake made of anything but water would freeze over. $\endgroup$ – overactor Feb 3 '15 at 9:22
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    $\begingroup$ I don't get it. Why it would be difficult to cross this lake with conventional methods? Heavier Fluids only make the use of boats easier! $\endgroup$ – T. Sar - Reinstate Monica Feb 3 '15 at 9:58
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    $\begingroup$ It's the viscosity you should be concerned about. If it is like honey, your sailing ships can't sail and it's a big nasty metal swamp. $\endgroup$ – Serban Tanasa Feb 3 '15 at 11:57
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    $\begingroup$ How dense is it? How viscous? $\endgroup$ – frodoskywalker Feb 3 '15 at 12:50

11 Answers 11


Firstly Michael Faraday invented the electric motor in the mid 19th century so there is the potential for something interesting there. Since this mercury like metal is so buoyant then the boats design is not that integral as almost anything would float . In terms of powering the vessel I would suggest wind power. Just a big sailing boat similar to what was used in that era accept with a much broader and shallower hull as a regular ship would simply tip over due to it sitting so high in the metal.

It would need to be closer to the picture on the right: wider hull, higher in the water.


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    $\begingroup$ High-riding, unstable hulls can also use outriggers. Because they're out on poles you get more leverage for less structure compared with a very wide single hull, aka a raft ;-) $\endgroup$ – Steve Jessop Feb 3 '15 at 13:47
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    $\begingroup$ Another thing worth noting: your boats would go significantly faster since the whetted surface would be so much smaller. The increased fluid density would also make planing easier. $\endgroup$ – ckersch Feb 3 '15 at 16:47
  • $\begingroup$ What about a catamaran for minimizing contact with fluid and maximizing stability? $\endgroup$ – mouviciel Feb 5 '15 at 8:12

Physics of boats does not really change based on the fluid. So they'd use normal boats modified for new medium. I'll assume viscosity similar to water since no mention was made and just talk about density.

In fluids with higher density, your boat will float better, with less drag for the mass. This will either make boats smaller or, I think, heavier with less empty space below surface. Being as ships were limited by construction technology, I think the extra flotation would be used for extra cargo or heavier armoring. Since most cargo would be relatively light and armor would, obviously, be mostly above surface, ships would need more ballast to be stable.

Alternately, catamaran or trimaran hulls might be preferred. On Earth these were not really used much for large ships. But a trimaran might be a practical solution to staying stable without increasing waste mass and evolve naturally from outriggers. And less need for flotation and presumably lack of lifeforms attaching to hulls, would make having higher surface area more attractive.

Oars would work better. The mass of fluid the oars could practically move would increase with density. This would mean that lower oar velocity and energy would be needed for the same thrust. Same would apply to paddle wheels and propellers. So steam ships might be fairly attractive.

Possibly the biggest difference would be that waves would probably be shallower, so ships might get by lower freeboard. So they'd evolve from boats for rivers or lakes?

  • $\begingroup$ Wouldn't the decrease in drag due to better floatation be offset by the increase in density of the medium you are traversing? $\endgroup$ – Taemyr Feb 3 '15 at 13:29
  • $\begingroup$ @Taemyr: to first approximation, I suppose any floating object must displace its own mass of fluid in order to move one object-length, regardless of the density of whatever it's floating in. This results in a density-independent contribution to drag. More complex (i.e. actually useful) drag models don't necessarily result in a straight cancellation, though :-) $\endgroup$ – Steve Jessop Feb 3 '15 at 13:52
  • $\begingroup$ @Taemyr I am making some assumptions that I didn't really fully elaborate. Mainly that the size of ships you can build is limited by construction technology and if a ship of same size needs less displacement to float, it will have better geometry and less drag with other things being equal. Also that less wood exposed the better... I think these are reasonable assumptions, but the magnitude of reduction is something I don't want to even guess. I doubt it would be large enough to be worth investigating. $\endgroup$ – Ville Niemi Feb 3 '15 at 18:16
  • $\begingroup$ The only problem with oars is that, being far less dense than the lake, it would take a lot of effort to push them under the surface. They would need to be either very thin (and strong, to prevent from snapping under the increased mass of the fluid they move) or very heavy (and difficult to move out of the lake). A gondola-type oar might work though, if you don't have to lift it out and it's balanced correctly) $\endgroup$ – 2012rcampion Feb 3 '15 at 19:25
  • $\begingroup$ @2012rcampion Oars were limited by the strength of the rowers you could practically have per oar and changing the fluid would be unlikely to change that. That said, nice catch with push down (and pull up). I think galley oars are massive and long enough, but... Well, they need counterweights anyway, so it should be possible to compensate, certainly with 19th century mechanics, if not with classical solutions. $\endgroup$ – Ville Niemi Feb 3 '15 at 21:23

Even more interesting would be Magnetohydrodynamic Propulsion.

Because the metallic nature of the "seas" would be a greater conductor than saltwater, the effect of passing a current through a section in a magnetic field should be even greater--and more obvious to someone on that planet who is looking for it.

19th century technology could certainly create crude DC generators (steam plants + strong magnets) that would work, and one could imagine a simple series of tech advances where this could be the standard approach for propulsion of vessels within a particular range (say, medium-to-large because of the fuel requirements)


(although humans can comfortably stand on it for a few seconds,...

Through time humans have looked at the nature for inspiration for developing tools and transportation. And i think a possible solution here would be like that.

  1. If the fauna would allow it, dire waterstriders as pack animals would be a possible solution.

  2. A mechanical machine that mimmic the waterstrider, maybe pedal, clockwork or steampowered.

a third option is hydrofoils (or mercurial foils?), something that lifts the main body and leaves a "wing" under the surface, it requires minimal power to propell because the low resistance, but is dificult to get up to speed.

  • $\begingroup$ You can't scale a water strider. They're based on surface tension. $\endgroup$ – Loren Pechtel Feb 4 '15 at 2:17
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    $\begingroup$ @LorenPechtel so just use a lot of them. $\endgroup$ – JDługosz Feb 5 '15 at 11:34
  • $\begingroup$ @jdlugosz The square-cube law will get you. Water striders work because they have a very low weight per foot. If you scale this up you'll end up with more mass in the legs than a foot can support so it doesn't matter how many feet you have. $\endgroup$ – Loren Pechtel Feb 6 '15 at 15:09
  • $\begingroup$ Mercury used to be called "hydrargyrum," so "hydrafoil" would work here. $\endgroup$ – Darth Wedgius Jun 23 '15 at 17:39

Steam-powered paddle-boat should work within your limits. Catamaran or trimaran for stability.

If you prefer no paddles (if liquid is dense enough that putting something in is hard), try aero boat - powered by a propeller in air.


If it's dense enough then something like skies would actually work quite well. You have "ski poles" with large surfaces on the end, you stand in the skis and you push back behind you with the poles to keep sliding forwards. You might even be able to move like ice skating, without the poles, although it would be interesting doing that without tangling the skis.

When you get tired a few floats would allow you to lie down on the lake.


Let's have a look at what methods of propulsion could be used.

  • Sailing (wind)
  • Paddle power
  • Steam engines

Sailing requires a large sheet of sailcloth attached to a mast. Paddles require big paddle wheels attached to the side of the boat. Steam engines are big and heavy and need to be attached to something.

Let's do some math - if a human can stand for a few seconds on this metal, something a bit lighter could float. Average human mass is 75kg, so let's say 50kg could float standing on a 500cm2 foot area. That means this metal will let anything float, given that it exerts a force less than or equal to $ \frac{50}{500} = 0.1\text{ kgcm}^{-2} $, or $ \frac{50}{0.05} = 1000\text{ kgm}^{-2} $

So let's look at sailing. If you want a reasonable size boat, you also need a reasonable size sail. A fairly big wooden boat would weigh a few tons, say 4 including sails. For that to work, what surface area does this need to be spread out over? 4 square metres. So boats on this lake are not going to need to be big; they can in fact be completely differently designed than conventional boats we're used to.

Using that formula, you can determine how big your boats need to be, but in general most things will float, even with their methods of propulsion.

$$ \frac{\text{mass}}{\text{max. force (1000)}} = \text{necessary area} $$

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    $\begingroup$ Not sure. Stand for a few seconds might imply high viscosity, meaning I need to rewrite my answer... $\endgroup$ – Ville Niemi Feb 3 '15 at 9:19

A vehicle with wooden rolling bars/wheels, wind-powered. The wind should power a big propeller that is connected to the bars via toothed wheels, so the wind wouldn't push the boat-vehicle, but power its drive wheels/bars. Even if the wind stops, it wouldn't sink. It could even have a way of manual powering it, you know, put some slaves under the deck.


Well as some have already said, the physics would be the same as for water except the liquid is just denser. Therefore why not just use a propeller like they do in normal boats. The only problem might be the liquid is denser, so much more power is needed to rotate the propeller to get it at a sufficient speed. But a consequence of a denser fluid, is that more weight can be put in the boat because the liquid it displaces is as you say much heavier than water, therefore the requirement that you need more power isn't a problem because you could just get a larger,heavier and more powerful engine, without sinking the boat and to drive the propeller.

A more interesting way could be to use a superconductor. Since weather plays no part meaning the fluid will be mostly flat with no waves, and the lake is metallic, you could attach a superconductor in a vehicle and glide across the lake like a hovercraft. You might not even need a superconductor, perhaps an electromagnet would produce a strong enough field. You could accelerate, decelerate and steer the craft by applying a magnetic field unevenly from the craft onto the fluid, and if the vehicle is shaped aerodynamicly, you could achieve pretty high speeds. This in a way has already been done as a kickstarter, where some people tried to recreate the hoverboard from back to the future. It only works on metal surfaces though , but the principle is the same.


If the fluid is viscous enough, I'd consider a tread based propulsion. (Think tank.) This would be different from a paddle boat inasmuch as it's working on the friction with the environment. The surface/fluid sounds like it's non-Newtonian, so impact friction (humans can comfortably stand on it for a few seconds) would seem to be more appropriate than "dig and push" like a paddle boat.


I am making a second answer for a thick liquid with viscosity much higher than water. (See comments for my previous "same viscosity as water" answer.)

Best solution for such a "lake" would IMHO be a sled. This is because such a substance would be in behauviour most similar to snow or very thick mud. This means a wide flat bottom with overall shape from above being a elongated rectangle, because while you want a wide bottom you also would want to minimize the front aspect. From sideways aspect the hull would have very low freeboard, ie low height, and the bottom would be a smooth flat curve at both ends to make rising up from the sludge easier.

The sides of the bottom would be noticeably deeper than the center to trap the sludge underneath and to allow lifting much of of the bottom out from the sludge, if the speed is sufficient.

For propulsion sails would be preferred in civilian use. A wide flat hull should allow a large amount of canvas. And once the sled got moving the drag should be acceptable.

For getting the hull moving solutions would range from having camels pull or crew members with snow shoes push to some sort of mechanical solution. For example, the sides might have a relatively wide parts that can be pushed down when starting to move. Done fast enough this would allow the bottom of the hull to be lifted out of the sludge. With the specified technology level, a relatively efficient mechanism using pneumatics or springs should be possible.

The needed solution depends on how heavy they'd want "ships" to be. I think the default would be very light with just a push from some strong men needed, but a mechanical solution would allow much larger ships and larger cargoes, which should give more profit. It basically depends on just how well established trade over the lake is.

Military vessels would use steam engines and paddle wheels to supplement wind. They'd also have "the lift mechanism" powered by steam. Large side paddle wheels would probably be sufficient by themselves to lift ships high enough for move. In this case you'd also see lots of commercial ships having made the transition to steam and paddle wheels. If no steam engines are available, similar solutions I suggested for large civilian vessels would be used. (Sails and mechanical lift to get started.)

Something like a camel or a human with snow shoes should be able to walk on the lake. So caravans might be practical. Stops would be either on islands of solid ground or spreading sludge proof canvas with some supports to create temporary "floats". It is fairly simple to create something that floats on a dense material with high viscosity.


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