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My last question was about why mermaids would allow people to ride them, but someone pointed out that a conventionally sized mermaid wouldn't be able to support someone. That got me thinking: how large would a mermaid have to be to support an average human being?

By support, I mean carry a human on their back, and for long and short distances. Think a horse, or a dragon (though I know that's probably more than a little weird....). I envision most of this riding being on the surface, but underwater is possible too.

This question is about the minimum mermaid size to support the average human being's weight (136 pounds or 62 kilograms). If your answer is about or includes ways that a conventionally sized mermaid can still support a human being, magically or otherwise, I would greatly appreciate that.

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  • $\begingroup$ If I need to reformat this, please let me know. $\endgroup$
    – Alendyias
    Jan 1 '21 at 1:59
  • $\begingroup$ Support them how? Keep them afloat? Carry them for a thousand miles? What is being asked of the mermaid? Do they need to keep the human dry? $\endgroup$
    – jdunlop
    Jan 1 '21 at 3:05
  • $\begingroup$ Thank you for pointing that out! I just edited in the answers you wanted, jdunlop. $\endgroup$
    – Alendyias
    Jan 1 '21 at 3:08
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Ehm dude really no one ever went to pools or beaches? To carry a human all you need is a lot of breath not size, not magic and certainly not strength. Dogs are able to carry humans in water, humans are able to carry humans in water.

I lived near waters all my life and in more than one occasion I saved people heavier or taller than me, cause I'm a manlet.

Certainly a half human half fish can carry a human in water.

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  • $\begingroup$ this answer could use some sources or evidence $\endgroup$
    – Topcode
    Jan 5 '21 at 19:57
  • $\begingroup$ @Topcode google lifeguards $\endgroup$ Jan 5 '21 at 20:01
  • $\begingroup$ hmm, never knew google had a lifeguard service that carried people over long distances. $\endgroup$
    – Topcode
    Jan 5 '21 at 20:03
  • $\begingroup$ @Topcode you learn something everyday, don't you? $\endgroup$ Jan 5 '21 at 20:18
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Bigger than you might think.

Water is about 750 times more dense than air. This makes it a challenging medium to move through. As a result, practically all aquatic animals have streamlined bodies to make movement more efficient.

In terms of an existing biological model, the closest you will likely find is a remora hitching a ride on a manatee. This is a rare occurrence, but it has been known to happen. A typical remora is about 3 feet long and a typical manatee is about 10 feet. Scaling this up to a human rider, the mermaid would have to be approximately 20 feet long.

This is not a perfect model. A mermaid is not all that streamlined, and a person riding on a mermaid is even less so.

Additionally, having the rider above water presents another challenge. While an object is submerged in water, the water provides a buoyant force, effectively reducing the perceived weight of the object. If the rider was out of the water, the mermaid would have to constantly exert energy to keep the rider's full weight above water.

Fish control their buoyancy using swim bladders. To avoid too much math, let's assume both the human and the mermaid (with a deflated swim bladder) have a density that is equal to that of water. In this case, the mermaid would need to inflate its bladder to roughly the same size as the rider to maintain neutral buoyancy while supporting the rider above water.

Swim bladders are about one third the length of the fish they are a part of. This would require our hypothetical mermaid to be approximately 25 feet long.

Using both of these models, and factoring in some room for error, a good length to make your mermaids would be 30 feet.

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  • $\begingroup$ Good heavens, magic must be involved here! $\endgroup$
    – Alendyias
    Jan 1 '21 at 5:44
  • $\begingroup$ Not that a 30-foot-long mermaid wouldn't be amazing or anything, but it really would limit the practical uses here, and greatly increase the burden on those responsible for logistics.... $\endgroup$
    – Alendyias
    Jan 1 '21 at 5:47
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    $\begingroup$ Good answer, I appreciated reading it. I will note that other creatures, like sharks, get by without swim bladders, having oily livers. @Alendyias , since Jack's answer was helpful, you ought to vote for Jack's answer, and consider accepting it. $\endgroup$
    – Johnny
    Jan 1 '21 at 18:31
  • $\begingroup$ Thank you, I will. $\endgroup$
    – Alendyias
    Jan 1 '21 at 19:55

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