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I have a society of merfolk in fantasy novel (humans from the waist, fish tail from waist down, biology isn't important in this story, nothing complicated). While the humans are around 1850s tech, the merfolk are probably more like a hundred to two hundred years behind (it's complicated as they have different kinds of tech underwater).

My question concerns the question of neutral buoyancy. I'm thinking about how in a well developed merfolk civilization, they'd need caravans for trade, supply lines for warfare, and just general ways to move around goods. Thus, I'm wondering, could they find a way to make wagons of sorts underwater? The material the wagon is made out of, I'd imagine, isn't important. I'm wondering though if they could add some kind of floats to the side of one to make them neutrally buoyant so they could be easily moved through the water.

They do have access to human materials that they steal from ships they attack - so wood and other materials you'd expect from the mid 19th century would be available in some supply.

If this doesn't work, I suppose they could get by using domesticated pack animals for bigger uses, but I'm sure that a floating cart of some kind would be very useful in many situations.

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  • $\begingroup$ Also look into this Q (worldbuilding.stackexchange.com/questions/241486/…) and the As of MishaP, Karl, and Nosajimiki - let's get a few underwater-sail-kites on these things! $\endgroup$
    – bukwyrm
    Commented Jul 13, 2023 at 11:47
  • $\begingroup$ there is an idea that the Egyptians may have floated massive stone blocks with skin bladders and rope. $\endgroup$
    – John
    Commented Jul 13, 2023 at 15:08
  • $\begingroup$ I don't imagine you would necessarily need complete neutral buoyancy. Pushing a near-weightless object around is still a great advantage over having to carry the full weight. $\endgroup$
    – Flywheel25a
    Commented Jul 14, 2023 at 0:59

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I think buoyancy would be a natural development

Your merefolk live in the ocean! They're surrounded by creatures of all types that use bladders to change their buoyancy underwater. I not only think they'd develop means of dealing with buoyancy early in their technological development, I think they'd start by using harvested animal air bladders and graduate to weaving their own bladders out of strong (e.g.) silk-like fabric. Oxygen would be extracted in the same way it's extracted by gills.

Honestly, I wouldn't be surprised if your merefolk's first step wouldn't be to domesticate a critter or two and use a directed breeding program to create, functionally, a biological cargo carrier. If nothing else, use nets and strap the cargo to the back of a hump-back whale. Then as technology and/or knowledge of how buoyancy works in nature develops, shift to a synthetic solution. It depends on how much more advanced in this regard you want your merefolk to be compared to the surface dwellers.

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  • $\begingroup$ I think that animal organs is indeed a good suggestion. I honestly forget how often they were used in the past, as our modern taste can find that a bit gross sometimes - but yeah, it was definitely utilized for a lot of things. $\endgroup$
    – WasatchWind
    Commented Jul 13, 2023 at 3:51
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    $\begingroup$ You don't need exactly oxygen to fill those bladders, any gas would do as all of them have less density than water. Methane, nitrogen, H2S, you name it, whatever would be available in their vicinity. Yep some gases form hydrates and drop off, especially at high pressure, but making a floating island (or harnessing an existing one based on algae) would suffice for a while. $\endgroup$
    – Vesper
    Commented Jul 13, 2023 at 6:26
  • $\begingroup$ This would not be stable. If it started rising then the air would expand, the buoyancy would increase, and it would rise faster. If falling, then the same would happen. This is not an insurmountable problem because fish can manage neutral buoyancy. But you might want to make a rigid container rather than a bladder to avoid this. $\endgroup$
    – Richard Kirk
    Commented Jul 13, 2023 at 10:11
  • $\begingroup$ @RichardKirk both are needed I think, a bladder as the main lifting device and a rigid container as a gas storage, and a compressor in order to avoid bladder explosion or collapse, to pump the gas both sides as needed. I wonder how this is implemented in fish tho, they can change depth pretty fast while maintaining buoyancy. A rigid container alone would have to sustain quite a lot of internal pressure when lifting to surface, or a lot of external pressure when diving, or both, and I wonder if a civ without ability to meld metals can build such a container. $\endgroup$
    – Vesper
    Commented Jul 13, 2023 at 13:31
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    $\begingroup$ I think the fish swim up and down. Their swim bladder adjusts so they don't have to swim to stay still. If they have a neutrally buoyant 'cart' they can add a 'tail' and 'wings' so pulling it straight and level is easy even if it is not quite balanced. $\endgroup$
    – Richard Kirk
    Commented Jul 13, 2023 at 15:15

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