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Long ago when apes and hominids split off a third group began to live in the seas and became mermaids/seafolk. Among several species of mermaids, there is a species of "giant mermaids" who have orca like skin patterns.

Pin by amyvanzi98 on Aaron Blaise favs | Animation portfolio, Mermaid art, Fantasy art

To avoid any major conflict, most nations who rely on the sea for trade or a way of life will pay a portion of their goods for safe passage through mermaid-controlled waters. But this still doesn't stop some rogue mermaids from raiding ships and taking all the loot. This presents a problem as a group of giant mermaids could capsize a cargo ship and steal its loot.

Some basic characteristics of these giant mermaids pirates include:

  • being 26 feet (8 meters) long
  • having human-level intelligence
  • have an excellent sense of vision and hearing
  • have a thick layer of blubber
  • having Stone age level technology
  • usually attack in groups of 10 to 15
  • can hold their breaths up to 13 minutes

Given this how could sailors with at most medieval level technology protect their cargo (and their lives) from giant mermaids and to the lesser extent human-sized mermaids?

Note: Magic does not exist in my story

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    $\begingroup$ If mermaids don't have any particular vulnerabilities, humans can only treat and negotiate. $\endgroup$
    – Alexander
    Commented Mar 29, 2021 at 23:11
  • $\begingroup$ I almost want to do a spinoff of this asking for strategies of how the mermaids will attack...I have some ideas that would make it much harder to defend against them! $\endgroup$
    – user3067860
    Commented Mar 30, 2021 at 20:11
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    $\begingroup$ @user3067860 i'll post it soon $\endgroup$
    – icewar1908
    Commented Mar 30, 2021 at 20:33
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    $\begingroup$ @Alexander If push comes to shove, most things have a "being shot with lots of [insert projectile weapon of choice]s" vulnerability. $\endgroup$
    – user3482749
    Commented Mar 31, 2021 at 0:25
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    $\begingroup$ @Alexander Humans are really quite good at dealing with things that don't have any particular vulnerabilities other than application of significant amounts of firepower by means other than "treat and negotiate". $\endgroup$
    – user3482749
    Commented Mar 31, 2021 at 0:34

17 Answers 17

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Employ mermaid escorts.

  1. They are smart.

  2. They like stuff we have because they accept it in payment from ships crossing their territory.

  3. Presumably they can deal with criminals of their own kind.

Mermaids associated with the area in question accompany boats. They ward off rogue mermaids whom they identify by their ruddy complexions. The mermaid escorts (and their bosses) get the payment themselves. Also, future travelers will choose this safe route and so mermaids associated with this system are ensured a future supply of land loot.

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    $\begingroup$ This is the answer. Most other answers are forgetting these mermaids have human level intelligence and are treating them as beasts. $\endgroup$
    – DKNguyen
    Commented Mar 30, 2021 at 2:04
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    $\begingroup$ "... by their ruddy complexions." Your joke is less relevant since someone's edit to OP's typo. Might be best to drop it. $\endgroup$
    – Michael come lately
    Commented Mar 30, 2021 at 14:30
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    $\begingroup$ I think you could add "mermaid hostages" to this answer. The escorts might only be escorts because the humans have their young hostage. Or, alternatively, the would-be pirates know that some of their young are held hostage and will be killed if certain ships are attacked. $\endgroup$
    – user1547672
    Commented Mar 30, 2021 at 19:01
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    $\begingroup$ @user1547672 - that idea is a whole different flavor from mine and an excellent one. If you post it you have my upvote. It smacks of a desperation move - making the humans unethical monsters (maybe they are pirates?) and offers the prospect of juvenile mermaids up on deck, where they can be characters. $\endgroup$
    – Willk
    Commented Mar 30, 2021 at 19:24
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    $\begingroup$ Perhaps the orca merfolk thought of this first, and the whole mer pirate vs mer escort business is a protection racket? And how could we landfolk tell the difference? $\endgroup$
    – Codes with Hammer
    Commented Apr 1, 2021 at 12:53
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Bomb fishing

While your post specifies medieval technology, that does not inherently exclude gunpowder, which was described as early as the 13th century, even in Europe. While sophisticated cannons and firearms are likely unreasonable stretches, simple explosive shells with timed fuses should be entirely doable.

While black powder is not a high explosive, and tends to deflagrate (burn) rather than detonate (explode), but notably, black powder under confinement absolutely can and does explode, and violently. That means that it can be used for bomb fishing. Bomb fishing is an incredibly effective way to kill large groups of fish, and also effectively kills mammals.

Black powder is less effective than dynamite, but if properly contained to generate maximum overpressure, the effects can be quite serious. As the linked study notes, the black powder charge in exp 44, which was a sheet metal container with two detonators, killed considerably more fish than a thin-walled can container. A military report indicated that under proper confinement, a TNT equivalence of up to 25% for pressure is possible.

In addition, since your goal is to kill the mermaids, not eat them, you’re free to add frangible metal pieces to the outside/inside of the explosive device, which can greatly increase the kill radius. The Soviet fragmentation grenade ФГ-45 has a kill radius of at least 14 meters underwater, and is a relatively small (though high explosive) charge.

Given the size of your targets, and a ship’s ability to carry large black powder charges (10lbs+) without much issue, they should be brutally effective countermeasures.

Additionally, don’t underestimate the non-fatal deterrent effects: underwater blasts can rupture animal eardrums, and are provablt effective in terrifying even sub-human-intelligence animals. Their effect on primitive stone-age cultures capable of rational thought will likely be profound.

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  • $\begingroup$ What kind of fuse did you have in mind that prevents the mermaids from learning they can intercept them and position them beneath the ship before they detonate? Both timed and pressure fuses suffer from this weakness unless you set it to detonate as soon as they hit the water which might render them ineffective. $\endgroup$
    – DKNguyen
    Commented Mar 31, 2021 at 22:27
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    $\begingroup$ I find it highly unlikely that the mermaids will develop a policy of swimming towards the lethally explosive, shrapnel-filled spheres, based on human reactions to such things. In case they do, though, the study in my third link found that explosions at the surface of the water were effective for killing fish if the explosion was of sufficient intensity. $\endgroup$
    – Daniel B
    Commented Mar 31, 2021 at 23:22
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    $\begingroup$ Civilians maybe, but not soldiers who have repeatedly seen the damage it does to their team if they know the damage can be minimized or turned on the enemy. $\endgroup$
    – DKNguyen
    Commented Mar 31, 2021 at 23:34
  • $\begingroup$ No--there's no reason for shrapnel. The water will very quickly bring it to a stop. $\endgroup$
    – Loren Pechtel
    Commented Apr 1, 2021 at 2:24
  • $\begingroup$ As for the problem of fuses: Make the bombs long and thin, much heavier than water so they go down quickly. That means the burn time is short (you light the fuse by a float coming off when it starts to sink) and the mermaids don't have enough time to move the bomb under the ship before it blows, let alone get away before it detonates. You don't need much confinement, either--the water itself will offer a lot of resistance. $\endgroup$
    – Loren Pechtel
    Commented Apr 1, 2021 at 2:28
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I approach this by thinking about contemporary whaling ships, and then thinking about how one could add to that technology. The first thing that comes to my mind is giant crossbow harpoons with drogues attached; the drogues would keep the mermaids from diving and tire them out (as they did for whalers). You could assume that the nations of the time would have dedicated some serious engineering time and energy to the mermaid problem, so the technology would likely be more developed than that of real whaling technology. Of course what is and isn't reasonable in this regard is up to your good judgement.

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  • $\begingroup$ Not a bad idea, care to elaborate on further strategies? (You can edit to add or change stuff), We invite you to enjoy our splendiferous tour and make use of our help center for reference as to our ways, welcome to worldbuilding. $\endgroup$
    – Escaped dental patient.
    Commented Mar 30, 2021 at 1:49
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    $\begingroup$ I doubt drogues will be at all effective against intelligent mermaids with opposable thumbs who are working together. Harpoons still sound painful and effective, but I think the drogues would be more bother than they're worth for the defenders. $\endgroup$
    – Gregor Thomas
    Commented Mar 30, 2021 at 15:15
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Mermaid hostages

Maybe mermaids don't attack their own kind, or if their own kind are under threat. The ships may keep young or smaller mermaids aboard, under threat of killing them if the ship is attacked. Or perhaps some mermaids serve as members of the crew (or entertainment) beyond just being a hostage/prisoner.

Of course it seems like the true answer to this question would be a combination of all the answers. Depending on the culture of the people involved, and their history of the mermaids, they may try weapons, negotiation, or intimidation to various levels of success.

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  • $\begingroup$ If the story allows it, another option is to use them effectively as engines. Hiding the exact location of the mermaids will make the attackers hesitate for fear of hurting the hostages. Depending on how evil you want the humans to be, you could use language barriers and different customs to make the humans unaware of mermaid intelligence. Or have a split in what people do, Some run with hostages, some don't. But if the hostages are hidden. The mermaids can never really be sure whether you have them or not. $\endgroup$
    – skippy
    Commented Apr 1, 2021 at 12:57
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Are these mermaids susceptible to poison? Toxins? Ships could carry a cargo of substances dangerous to mermaids and dump these into the surrounding water when under attack or even just if being scouted out. Or they might coat darts or other weapons with the poison/toxin and have at it (presumably there would be people especially trained to use these implements while fighting underwater, for example; though then the substance has to be water-resistant).

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    $\begingroup$ Thiy would be my approach as well, use some nice biodegradable poison and base the ships' defense on M.A.D. "You wanna sink my ship? Fine, feel free to die in this puddle of poison then. The next time your buddies might decide otherwise." $\endgroup$
    – zovits
    Commented Mar 30, 2021 at 14:13
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    $\begingroup$ Ships constantly move, unless they are becalmed, which is a bigger problem than mermaids. So the mermaids would just dive to avoid the poison, and resurface, they are not fish, they breathe air, and anyway, this poison, available to medieval sailors, will be diluted by the oceans upon use. How toxic is this stuff? Is it liquid? Liquids become gases sort of, via evaporation. Solid? Won't diffuse fast enough, nor will it stay near the hull. Storage? Is it on deck? During a rainstorm? Heavy waves? Kills giant mermaids at a touch, are your medieval sailors barefoot, during said rainstorms? $\endgroup$
    – chiggsy
    Commented Apr 1, 2021 at 17:39
  • $\begingroup$ Capsaicin would be a good choice here. Non-Lethal, (potentially) highly irritating and very available for the ships. We've tried it on sharks, of course, who lack proper receptors and simply ignore the stuff, but everyone knows Mermaids can't handle spicy foods, right? Catapult some skinfulls of Carolina Reaper and your problems are solved. $\endgroup$
    – JohnHunt
    Commented Apr 2, 2021 at 3:48
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Based on what we know about other marine mammals: Sound travels more efficiently (faster and farther) in water than air, so it seems that medieval tech could produce a loud enough noise to disrupt them, particularly if they have sensitive hearing. https://www.sciencealert.com/this-is-the-horrifying-reason-why-sonar-makes-beaked-whales-beach-themselves

Similarly, if they have excellent underwater vision, with similar eyes to terrestrial humans, I would imagine them to have very light-sensitive pupils. They might be a bit blind when surfacing as they adjust to the light levels, and this could be exploited by creating a bright flash. There are some interesting ideas for hazing marine mammals from oil spills.

You don't mention if they dive, but if they live in water they likely have some adaptations to pressure differences. Anything that causes them to surface rapidly could cause the bends, maybe some kind of giant baited fishing pole that yanks them up so they explode? If they're intelligent, though, that may only work a few times.

Given that these species seem to lack blubber and fur for thermoregulation (based on the sketch), I would guess that the oceans are either a lot warmer or they are restricted to warmer waters. There could be some efforts to minimize travel around tropics, but this isn't really very helpful unless they could figure out a way to cold shock them...travel with ice in the ballast water and dump?

Again, based on the sketch, those long, skinny arms and head/neck shape are problematic. They would create a lot of drag without adding much to propulsion. What do they use them for? Grabbing ships? Marine mammals (marine anything that swims at a decent speed) are all similarly shaped to reduce drag because moving in water is so much harder than on land. So, these mermaids must be a lot slower than other marine mammals. So a fast ship should be able to outrun them given a distraction (like one of the above methods). If you want them to be realistically fast, their shape needs to be adjusted to their environment.

Otherwise, I would go with the mermaid guards or perhaps some kind of mermaid hostage situation.

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  • $\begingroup$ my actual mermaids have blubber and other such adaptations for sea life i just used the pic as rough example of what they look like $\endgroup$
    – icewar1908
    Commented Mar 30, 2021 at 19:05
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I think it mostly depends on how fast the mermaids can swim, and for how long. Technically, these mermaids wouldn't be that much more difficult to defend against than normal humans - they might be able to capsize a smaller ship, but they wouldn't necessarily be able to tear through the a wooden hull any easier than a person. Their main advantage is that it's easier for them to reach the ship, potentially without being detected, and easier to keep up with it while moving.

Given that they swim somewhat slower than a ship, I'd say the best defenses would be a sail, at least as backup, since oars would be too easy to grab and cripple the ship, and investing in an extra-thick coat of pitch so it would be harder to grab the sides. Maybe carry a couple of sealed barrels of water-soluble poison that can be smashed if mermaids attack, and which will kill them if the ship sinks? Could be too risky, since the crew would probably die too.

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    $\begingroup$ "but they wouldn't necessarily be able to tear through the a wooden hull any easier than a person." I thing being giants ("being 26 feet (8 meters) long") might help a bit with that part, along with the use of tools ("having Stone age level technology"). So technically yes, they wouldn't be able to tear through the hull any easier than a 8 meter tall person with stone tools, which I wouldn't bet my life on. $\endgroup$
    – zovits
    Commented Mar 30, 2021 at 14:09
  • $\begingroup$ By that I mainly meant that, using their bare hands, they would have a hard time smashing wood in open water without a good foundation to push against. Assuming the wood used for the hull is a few inches thick, it would be the relative equivalent of swimming up to, say, a wall of 0.5" to 1" thick wood planks and punching through it: not impossible, but very difficult. As for tools, a human can also break wood using stone weapons, so being a giant or a mermaid doesn't make it any more or less possible. $\endgroup$
    – Duncan Urquhart
    Commented Apr 2, 2021 at 21:43
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  • Think about defense in layers, not just the far fight...

  • International law, order, and punishment installed

  • Weighted or spiked nets

  • Hire mercenary mermaids to combat threats (might add an interesting dynamic to the story and make available some good complexity and character depth)

  • Poisons, venoms, toxins

  • Harpoons & Crossbows

  • Spears & Pikes

  • Release predators into the water (i.e. shark, etc) maintained on-board

  • Spiked oars to prevent mermaids from grabbing them

  • High frequency sound for disorientation

  • Tethered cages free-divers or other mermaids use to get into the water and combat the threat with spears, pikes, etc.

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    $\begingroup$ I don't think predators are practical - these mermaids are the size of orcas, which are apex predators. A shark big enough to be a minor threat to the mermaids would be very difficult to "maintain" on a boat using medieval technology. $\endgroup$
    – Gregor Thomas
    Commented Mar 30, 2021 at 15:20
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Giant mermaids is a nice idea, it seems very plausible that a sea-version of humans would become much bigger with time.

I think the best bet is some poisoned harpoons. Just bows probably don't cut it, they don't look like they have as thick skin as whales, but probably still much thicker than land animals, for isolation in water.

If the medieval people are really smart and lucky, maybe they'll be able to spread some mermaid-diseases, perhaps by spreading body parts of a diseased mermaid they caught (or found after death). Biological warfare was a thing in medieval times, though limited.

The harpoons do assume the ability to chase mermaids down until they come up to breath. If that's not an option (i.e. they are too fast), it's going to be much harder. I don't think they would need to come up to sink the ships - they could probably poke holes in ships using metal/rock/sharp wood, or capsize them using rope stolen from other ships.

Another thing that would help is making bigger boats. If boats are big enough, they can't be capsized by a pod of mermaids, unless the mermaids find some leverage point, which seems unlikely in the ocean. Width is especially helpful, although of course makes them slower. Bigger boats would also naturally have stronger hulls, hopefully preventing the mermaids poking holes. There were no metal ships in medieval times, but maybe the technology for them existed?

If making individual ships bigger doesn't work, then travel is a fleet, and tie the ships together when mermaids are spotted. Maybe you can get advance warning by training birds to scout for you.

The mermaids are big mammals, so a lot of resources and time probably goes into growing each one. Plus they're smart, so I think you don't need to inflict too high a casualty on them to make them give up their piracy. They're probably already apex predators, hunting human ships is not their only means of survival, they'll switch to something easier if ships get too hard.

Some things I don't expect to work:

  • I don't think adding spikes to the bottom/side of ship would work well. The mermaids have hands and look pretty nimble, so a spike is probably just something for them to hold on to while capsizing the ship.
  • They're probably apex predators, so training other animals to fight probably doesn't do any good. They're most similar to orcas, who are apex predators. These things are smarter, similar sized, and live in similar or bigger pods. Plus mermaids are going to be much better at domesticating sea animals than humans are.
  • I think dumping poison in the water would very quickly dilute too much, or be left behind. Similarly with oil. Also it's probably not cheap.
  • Solutions with electricity or sound or explosives or such are probably too modern.
  • Trying to scare them somehow, like scaring land predators with fire, probably doesn't work since they are very smart.
  • I can't easily find anything about medieval methods to detect whale sounds, so I'm assuming it's not possible, but not sure.
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Here be mermaids.

In the Iron Age, the sea was a very unsafe place: storms, real sea monsters (whales, giant squid, sharks), mythical sea monsters and pirates. What could an Iron Age ship captain do about these things? Not much. The sea was just dangerous place and people accept and avoid particular dangerous routes.

Maps of seas often warned of dragons; maps of your world would warn of place with rogue mermaids and more ships would avoid these parts of the sea. That forces mermaids to police themselves or lose business.

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  • $\begingroup$ Basically, avoid them? With respect, not insiteful enough for me. The question states you can't avoid them, even in mermaid-controlled waters (Which we assume are policed to some degree). $\endgroup$
    – n00dles
    Commented Mar 30, 2021 at 11:37
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What if you were to chum the water and attract sharks to come in and create a feeding frenzy, could be too dangerous for the mermaids to stay around?

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  • $\begingroup$ You'd need giant sharks to be a threat to orca-sized mermaids. $\endgroup$
    – Gregor Thomas
    Commented Mar 31, 2021 at 20:28
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The Obvious Answer: Diplomacy Or Control

Look, you specified that they were giant mermaids. That means they are a female-only species. That means they rely upon human men (most likely sailors) to continue the species. This gives the humans huge diplomatic power over them unless, of course, the mermaids are smart enough to kidnap people for this purpose, but doing so would likely result in retaliation, better ship defenses (or a complete stop to all naval trade since ships aren't coming back) and since they're smart enough to raid ships, they are smart enough to negotiate instead.

This means humans and mermaids will have good relations, which means giant mermaids will likely attack and defend against rogues. For example, there may be soldier mermaids stationed along popular shipways, as well as mermaid escorts (which are covered in other answers). Or maybe giant mermaids only monitor sailing ships so if one is sunk or attacked by a rogue, they recognize it and hunt down the offender (this would mean either lax commitment or really peaceful relations overall with the merpeople).

However, humans have also been domesticating animals and enslaving their own kind for years. You may not think they can do that to giant mermaids, but A) we took down whales, B) a baby elephant securely tied to a rope will grow up never trying to escape because it thinks it can't, and C) a young mermaid (which would be the most likely target) will be relatively easy to train into subservience.

Eventually, I can see entire nations of humans using enslaved mermaids to hunt down and capture the free mermaids, eventually putting every last tribe under their thumb. Of course, you may not go that direction, and I don't blame you. Diplomacy is so much better. Either way, I hope this helps.

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  • $\begingroup$ If they rely on human men to continue the species, are they not human? Their biology must be wild, if they require fertilization from a distantly related hominid, who only became accessible to them at the dawn of the Age of Sail. Since falling into the sea is a death event, they must have some crazy pheromones to make a guy think "hang on a sec, this monster who is about to kill me is HAWT! Lemme try something real quick.. aw yeah.." Which to be fair, is a staple of mermaid stories. So how do you keep guys out of the water your prisoners/farm maids are in? $\endgroup$
    – chiggsy
    Commented Apr 1, 2021 at 18:17
  • $\begingroup$ @chiggsy: the Murphy's Law of Law-if people want to do something, the only way to prevent them from doing it is to make it completely, totally, eternally impossible. In other words, you can't. $\endgroup$
    – Alendyias
    Commented Apr 1, 2021 at 18:27
  • $\begingroup$ why yes mermaids tend to be an all female species, the ones in my story have male members. plus the idea of a species needing to breed with a separate, smaller species that lives in entirely different regions and would required the invention of the sail for this process to be even mildly sustainable. sounds quite absurd. $\endgroup$
    – icewar1908
    Commented Apr 3, 2021 at 21:39
  • $\begingroup$ @icewar1908: I apologize, I was just going off what was in the OP. $\endgroup$
    – Alendyias
    Commented Apr 4, 2021 at 2:13
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How fast can the mermaids swim? You could to design your ships to be un-catchable. So make them fast and light and captains only set sail when the wind is strong. Maybe also make them have a shallow draft to make them harder to detect in the water (though that will affect the ships stability in bad weather), maybe even a catamaran shaped hull to keep most of the boat out of the water and hopefully less noticeable by the mermaids.

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I disagree on harpoons--they're slow.

If you're going to attack the weapon is the ballista. Travel in convoys so hiding under one ship doesn't prevent another ship from shooting. Long and thin enough rounds will get through the water.

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The first thing that comes to my mind is harpoons, maybe with some poison on them as well.

A few other options would be to employ/hire mermaid escorts to travel with ships to help deal with rogue mermaids and in return they get more of the goods the ships have on board or another form of payment.

Or create a large net that's made to catch these mermaids, perhaps with spikes and poison on them as well.

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  • $\begingroup$ Hello, and welcome. Harpoons and hiring mermaid escorts are already covered in earlier answers. If those answers are helpful, please upvote them. Also try to distinguish what new suggestions your answer brings. $\endgroup$
    – Michael come lately
    Commented Mar 30, 2021 at 15:10
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At the risk of getting laughed at, couldn't your ships be armored with huge metal spikes all over them? The mermaids attack and they get impaled. That, and a combination of the boats being very fast, and the sailors being very good with harpoons, might be enough to keep them away.

Also, I'm thinking that while the mermaids are clearly in their element, their bodies must still obey similar laws of physics that whales on Earth do (you said no magic). So I don't think there would a rash of giant fish, mermaid or not, jumping onto the boats, or ramming the boats.

Would they be attacking with weapons(spears)? If so, I think the sailors could defend themselves much the same way they would from other boats attacking.

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"have an excellent sense of vision and hearing"

Play music.
REALLY bad, REALLY loud music. I'm thinking piano wire harp strung from the mainmast, scraped by blunt files level of bad music. Tubular bells mounted in the ship's keel. Tin drum percussion segment, with sledgehammers.

It will drive all creatures with sensitive hearing far, far away. Will also change the expression "deaf as a doorpost" to "deaf as a sailor"

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