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The setting; there’s a large population of merfolk living in the ocean, and a developing seafaring human nation on the neighbouring coastline.

These merfolk are entirely aquatic. They have gills and not lungs, they would suffocate if removed from water. They can survive on the surface only as long as they can hold their breath, and their bodies are totally unsuited for any movement on land. They have upper torsos, arms and heads that are broadly human-like, and fish-like tails. They live in fairly deep waters and in scattered colonies under the ocean. They farm and hunt sea creatures, they harvest the reefs, and their villages are at least a hundred foot below sea level.

These merfolk are well-used to fighting off large predators like sharks, or fighting skirmishes against rival underwater colonies.

Technology-wise, humans would class the merfolk as being primitive. They have no advanced tool-making or metalworking (because, well, no fire), but they craft coral weapons, and they can bind seaweed into rope. They primarily wield harpoons and spears, but some have crude crossbows. The merfolk are very good at farming fish, their society is very organised.

In the past, humans and merfolk have got along pretty well. They’ve interacted occasionally, but merfolk have had no interest what happens on the land, and humans don’t care for under the sea.

The problem comes when the human society begins to develop and expand quickly. Their towns become cities, they start building larger ships, and the human nation starts to trade and interact with other nations around it. Imagine these people as culturally similar to early Vikings.

To power this expansion, the humans have begun very heavily fishing in what was previously merfolk territory. Larger populations need more food, and the ocean is the only place they can get it. In particular, human whalers start to target schools of whale and dolphin pods, which seriously upsets the previously docile merfolk.

The humans aren't overfishing the environment, but the local increase in fishing is the point of contention.

In retaliation, the merfolk start sinking ships. Raiding groups of merfolk warriors use long ropes of seaweed to bind the ships’ rudders and anchor them in place. From then, the merfolk use coral saws and blades to hack through the hulls from below, and they drag the helpless ship down. Any human that jumps overboard is promptly slaughtered in the water.

This is difficult work for the merfolk (sawing through hard wood with crude materials isn’t easy), but the human sailors have no real way of defending against them. Harpoons work somewhat, but the merfolk have the huge advantage of movement under the water compared to the men trapped on boats above. The merfolk successfully sink several of the human fishing and trading vessels.

Back on shore, this is an outrage. The human king stamps his boot, and says this is enough. The humans declare total war on the merfolk.

The only problem is… how?

How would the humans fight against the merfolk? Submarines don’t exist, and diving with weapons would be foolish - so how can they launch counter-assaults against the merfolk’s underwater villages? Drop barrels on them? Poison the water? They have no idea, they’re looking for a way.

Is there any means to protect their ships from an organised attack from below? To trade, they must travel through merfolk-infested waters, but all of their ships are (literally) sitting ducks for the merfolk.

What do they do?

For answers, assume that this is a low-fantasy setting. Solutions that rely on magic are possible, but discouraged.

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    $\begingroup$ Is paying other merfolk to fight with them an option? Many wars are won with making good allies. $\endgroup$ – Raditz_35 Aug 30 '18 at 16:57
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    $\begingroup$ Making arrangements with other merfolk might be possible, yet made difficult because humans and merfolk have incompatible languages. Nobody is sure how to communicate with them, and relations between man and merfolk are pretty strained all around. $\endgroup$ – user54563 Aug 30 '18 at 17:07
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    $\begingroup$ No fire? What about vulcanic action? They could use those to heat stuff and work metals, although they likely have to be more creative than humans to get complicated or specific designs. Then again you can question the usefulness of wearing heavy metals when essentially "flying" through your local space and each swing and thrust is going to be much more labor intensive abd measured than above ground. $\endgroup$ – Demigan Aug 30 '18 at 17:52
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    $\begingroup$ @Demigan "What about vulcanic action?" No, because #1 water is too good of a heat absorbent, and #2 convection spreads the heat too much. $\endgroup$ – RonJohn Aug 30 '18 at 19:35
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    $\begingroup$ @Ronjohn the water might not be able to reach over 100 degrees, but the magma and byproducts can reach anywhere between 600 and 1600 degrees Celsius. The heat around it is also heavily influenced by the waterflow directions, down to bacteria that live above the 482 degree vulcanic vents but surrounding water at the sides is still near freezing. $\endgroup$ – Demigan Aug 30 '18 at 21:06

14 Answers 14

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A mercenary war.

You are not going to win this fight human to merfolk. But as you mentioned the merfolk are not a unified people. There are warring tribes. You also have technology that they don't have. You can make leather armor and metal weapons.

Send your diplomats. Find the most hated, misplaced tribe. Maybe there is an uncle that believes he deserves the throne. Arm them with human weapons and in exchange let them kill your enemies. Besides weapons there must be a ton of surface things murfolk find valuable. Buy them out, hire bodyguards for your ship. Crush them politically if you can't win a direct battle.

What do you do in 50 years against merfolk armed with swords? That's the next admiral's problem.

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    $\begingroup$ This makes a lot of sense. $\endgroup$ – Mike Wise Aug 31 '18 at 8:50
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    $\begingroup$ Basically, Merjahideen $\endgroup$ – problemofficer Aug 31 '18 at 13:03
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    $\begingroup$ But as OP mentioned, communication is an issue, the languages are incompatible and neither can understand the other. $\endgroup$ – tox123 Aug 31 '18 at 14:20
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    $\begingroup$ Language is an interesting point. I have based my answer on how this has always been solved historically. In history you never hear that there was a language barrier. I guess I find it hard to believe it has not been solved long ago by someone $\endgroup$ – Andrey Aug 31 '18 at 15:15
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    $\begingroup$ In 50 year's time the swords will have dulled with use and corroded in the salt water. Not only will armed merfolk not be your problem anymore, they won't be a problem at all. $\endgroup$ – Alexandre Aubrey Aug 31 '18 at 15:55
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When I was a kid I remember there were two types of amateur apnea fishermen:

  • those who knew how to catch an octopus out of his lair using an harpoon
  • those who had no clue how to use an harpoon, and relied on copper(II) sulfate. Spreading it in front of the octopus lair forced the animal to escape it and become an easy prey.

Your middle ages men are in the same conditions: they might have some skilled pearl or sponge diver, but they are surely clumsy against the meerfolks. Your only hope of having a fighting ground is to pollute the waters around the ships with some chemicals: quick lime and vinegar are those coming first in my mind. Probably copper sulfate was not available during middle ages.

Once this is done, the humans can throw nets in the water, aiming at fishing as much stunned meerfolks as they can.

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    $\begingroup$ I'm going to post an answer, but to be honest, this one (chemical warfare) is probably the best and most flexible solution. In fact, many things could work, like ground glass that would be pulled into the gills and start lacerating tissue, or something like this that would temporarily rob the surrounding water of oxygen, just long enough to suffocate the merfolk. $\endgroup$ – JBH Aug 30 '18 at 17:14
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    $\begingroup$ I don't think you are going to poison anything 100 feet under the water without poisoning the planet $\endgroup$ – Andrey Aug 30 '18 at 19:19
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    $\begingroup$ @Andrey, no, but you can poison anyone who comes close to your ships. $\endgroup$ – Mark Aug 30 '18 at 19:44
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    $\begingroup$ @Mark They would just back off and hit your ship 10 minutes later assuming you are moving. Maybe if you can use oil so the poison would cover a huge area a few molecules thick. $\endgroup$ – Andrey Aug 30 '18 at 20:05
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    $\begingroup$ I was going to post a similar answer. Copper sulfate was called 'blue vitriol' or 'roman vitriol' and there are records of a method for creating it going back to the writings of Theophrastus, and so it would likely be known to a medieval civilisation. Whether they'd know to use it in this way is another question. $\endgroup$ – Jack Aidley Aug 31 '18 at 12:26
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  • If you can live with the drag, hang long lines from the bow of the ship having sharp barbs. The merfolk would need to deal with them before cutting through the hull or be cut up.

  • Mount short blades (10" - 20") along the hull. Of course, this would lacerate anything else that comes near the ship, too.

  • Double-hull the ship and fill the space between with something nasty (see L.Dutch's answer, which I upvoted and you should too, but this would basically work only once).

  • Mount the trawling nets on a gimble so they can be swept fore or aft to capture annoying merfolk.

  • Mount a log inside the ship to act as a battering ram against the hull. You want to hit the hull as hard as you can without disloging planks. The goal is to cause a sharp concussive wave at the point where the merfolk are sawing, hopefully to knock them unconsious (might not be able to bring enough force against the hull to make this believable).

My goal with the list was (a) to have reusable things (supplies can run out) and (b) to not necessarily be on the offensive. After all, most of the ships are just fishing trawlers that want to go about their business, not hunt merfolk. Making them go away is as valuable as killing them.

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There are a bunch of options, each with benefits and drawbacks. And don't forget that the merfolk are smart too. So here's a few things to chew on:

1: Just plate the hulls with metal. It doesn't need to be thick, just hardened. Coral and stone tools have a difficult time cutting wood. A thin plating of metal would prevent them from finding purchase at all. Just be sure to weld the seams so they can't wedge something under a corner and pry it loose.

2: Jamming the rudder won't stop the ship, only keep it from turning. Sailing ships can also be steered with the sails if the crew know what they're doing, it's just less precise and more labour intensive. Design the rudder to be retracted to prevent damage and just sail away from the enemy.

3: Biological warfare: Find a water creature with sharp teeth and claws and train it to attack merfolk relentlessly. Otters and seals are probably about the right size. The merfolk can adapt to this, but it will make their work more difficult.

4: Most water creatures have extremely sensitive ears. A sufficiently loud siren using your ship's entire hull as a transducer might well deafen them, perhaps permanently.

5: The fishing nets are the human's best weapons. Drop them off the bow, drag them to the stern, and lift them up. The quicker the better. Whack the captured mer on the heads with a club as you trundle the nets back to the front and add them to the catch, it's not cannibalism if they're not human!

6: Gills work by having the water go in through the mouth and out through the gills. For the humans to go underwater for extended periods requires pumped or compressed air. At a medieval tech level that's not impossible, but it is difficult and expensive. For the merfolk to go on land for an extended period requires... a bucket... And since the water will exchange CO2 and O2 with the air based on surface area, it might well be possible to design a garment that re-aerates and collects the water using nothing more than sea-creature leather and intestines... Humans beware! The land isn't safe either any more! (And things are going to get messy for the men as the mer trade with human groups they're not at war with and learn how to use fire on the beaches for working metal.)

7: The merfolk could just ignore the ships and attack the fishing nets. Nets have to be large and therefore difficult to defend to be effective, and making them out of materials that the merfolk couldn't cut would be prohibitively expensive. (Although why they'd cut them I have no idea. Just steal the catch and let the silly landfolk figure out what's going on as best they can.)

8: The merfolk don't need to cut holes in the ship to sink it. Drive some spikes into the side, attach a rope, and capsize it. For smaller ships, muscle and a bit of rocking would suffice. For larger ships anchor a windlass to the sea floor and just pull it over. There will be plenty of spikes and other metal pointy things salvageable from the first few ships they sink.

9: Why is this even an issue? Humans aren't nearly as well suited for fishing as the merfolk are. They'd probably trade the humans fish and whale meat by the ton in exchange for metal items, not to mention all the other neat (to humans) things that bespeckle the sea floor that humans can't get to but that the mer could just go pick up by the basketfull.

10: Since both sides are composed of essentially fractured tribes, both sides would be able to hire mercenaries of the other species from outside the conflict zone to take the fight to the enemy.

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    $\begingroup$ I like your point about capsizing. I think these merfolk would likely be too formidable to be worth being at war with for long. I can't see why they would have to engage a warship. In the early stages of war they will likely easily capture/capsize civilian ships, and will now have an arsenal of rope, sharp weapons and possibly even a resource for starting fire if they accidentally start one or get the information from torture, which would be a massive weapon if they can use it. They might even have domesticated faster sea creatures which can transport blades when organising an attack. $\endgroup$ – James Aug 31 '18 at 14:34
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    $\begingroup$ YES! Number 4! I was gonna say something like that $\endgroup$ – Aethenosity Aug 31 '18 at 15:21
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    $\begingroup$ Number 9 - There seems to be no reason they would be at war. The mutual benefits of cooperation are massive, and you didn't even mention underwater mining!! $\endgroup$ – josh Sep 1 '18 at 16:32
  • $\begingroup$ @josh That applies to lots of historical wars too though. People are stupid and often want to just grab everything for themselves up front instead of working together even though it makes them worse off in the long run. $\endgroup$ – Perkins Sep 3 '18 at 18:56
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It will hardly be a fair fight, until the right genius's ideas are put into practice. So, without further ado, we present the War in the Deeps:

  • Our medieval navy will first have to adopt a novel design for its modern day warship, namely the catamaran.
  • Next a corps of valiant Meerknechten will be equipped with an apparatus allowing them to enter the watery deeps and bring the battle to the Meerfolk;
  • For weapons, the knights will carry, apart from sharp filleting knives, one or more kinds of kinetic spear.

As I see it, modern (that is to say, medieval!) under sea warfare will look something like this:

Sailing out to the nearby Meerfolk settlements in their dugout Catamaran warships whose thick nearly solid hulls make hole boring & sawing nearly impossible, enter image description here

the king's Meerknechten are equipped with Undersea Spectacles and Breathing Apparatus, with double leather walled construction, reinforced with a fine mesh of brass chain mail (in order to confound the coral saws of the Meerfolk). enter image description here

Each contingent of Meerknechten is accompanied by leather finned yeomen armed with daggers and charged with "minding the lines". enter image description here

All the knights are armed with the DaVinci Mark VII Sperregonne enter image description here

and eight inch filleting knives for close up work, enter image description here

They will easily be able to dispatch King Neptune's finest Meerfolk Warriors: enter image description here

Meerfolk are quick swimmers, but their weapons are not ranged. The knights have the advantage with their spear throwers, allowing them to fire off one or perhaps two volleys before the enemy approach too closely.

A tactic recently devised involves a kind of windlass affixed to the deck of the warship which will, when engaged, draw the meerknechts up and away from the enemy, allowing them to fire volley after volley into their ranks from a safe distance.

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    $\begingroup$ I really liked your post. Even upvoted it. But I don't think underwater combat is the best choice. The merfolk are probably way more proeficient in 360º combat than humans. Aside from the harpoon gun, there's little other ways to damage them. And they're supposed to be fast - so good luck trying to hit one of them. I really liked your ship idea, though. $\endgroup$ – Magus Aug 30 '18 at 19:21
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    $\begingroup$ The merfolk would use a simple tactic. The front line would draw the man deep down. Then a small wave would come in from above and cut the air hoses. All your men are dead. $\endgroup$ – Andrey Aug 30 '18 at 19:24
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    $\begingroup$ @Magus - the main reason I chose undersea combat is the simple fact that in the 1200s or 1300s or so, they either probably wouldn't know much about chemical or poison warfare and even if they did, I doubt they'd be able to manufacture enough poison to counteract the effects of near shore current flow and the shear volume of sea water in the region. I suspect that even if you dump all the vinegar in the kingdom into their waters, the Meerfolk would probably react much as we do to a particularly smoggy day in a large city. Once the vinegar runs dry, dilution and currents will clear the mess. $\endgroup$ – elemtilas Aug 30 '18 at 19:36
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    $\begingroup$ @elemtilas, it doesn't take much of a cut. Once your divers get more than about 20-30 feet down, even a pinhole cut in the hose near the surface can be deadly: the air in the diving suit rushes out, and water pressure crushes the diver. $\endgroup$ – Mark Aug 30 '18 at 19:48
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    $\begingroup$ @elemtilas I see your point. I don't think that throwing stuff in the water is a viable option. If anything, it's probably even less effective than underwater combat. I'm really puzzled by this question. I can't find anything that even remotely resembles a viable solution haha. $\endgroup$ – Magus Aug 30 '18 at 21:03
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You are not winning without explosives. It is probably not that hard for a medieval society to invent depth charges. China invents gun powder by the 9th century.

These weapons would be devastating. Ships head for villages and then drop these depth charges. Protect the ships how other questions suggest. Spikes spikes on the hull, ropes with nets and razors dangling under the ship. The amazing thing about water is how well it caries explosions. A submarine can be damaged at 28 meters, but the range of hard to a body can go to a hundred meters. Eardrum damage may be even more.

A couple of such bombings targeting civilians may very quickly convince the murfolk not to engage civilian targets.

At that point if they want to fight a proper civilized war, they can figure out how to come to our battlefields.

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  • $\begingroup$ Was JUST planning on posting this same thing. Depth Charges would do a real number on merfolk trying to attack your ships too, due to the pressure waves created by the explosions. $\endgroup$ – Morris The Cat Aug 30 '18 at 19:42
  • $\begingroup$ This mixes well with my other answer. Merfolk traitors may be key for this plan. Still i think they are different enough of ideas to lump together. $\endgroup$ – Andrey Aug 30 '18 at 20:15
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Depth charge

Your people are upset, the army has tried everything but as it's imposable for any man (or woman) to stay underwater long and the merfolk have an advantage underwater a frontal assault would be foolish, luckily a young brash pirate (he prefer being called privateer) has the answer, during is travel in a far away land he came across a substance that is roughly translated to "black powder", such a substance was used by the locals for a long time (According to Wikipedia The first military applications of gunpowder were developed around 1000 AD in RL), for the right price he is confident he can steal\buy\bribe the secret of making that substance, and once that's in the local navy hands all it takes is making a lot of it and stuffing wooden barrels with it, cover in raw hide\use varnish\etc to make them watertight, add a fuse (make it internal and keep enough of the barrel empty to leave enough air for it to burn) and you got a rudimentary Depth charge, next time the merfolk attack light the fuse and throw them overboard, even if the merflok avoid a direct hit the shockwave of the explosion should cause some serious damage (water transfer shockwave much better then air).

best part of it is there is noting the merfolk can do to defend against it, noting made underwater naturally is strong enough to survive an explosion so their only choices are to stop attacking or to die trying.

Poison

You know how after a chemical spill there is a bunch of fish floating death? merfolk are basically fish with hands so why not poison them? it doesn't have to be high tech it just needs to be a lot of poison that's dumped out the side of the ship, even feces and urine will work with enough of it but even in the middle ages there where much stronger poisons known that would require fewer gallons carried onboard.

Bigger fish

You mention the merfolk fighting sharks, so why not attract them and let them kill (or distract) the merfolk while the ship sails away, blood in the water is one way to attract them, fish chum (gutted fish guts) is another proven way.

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  • $\begingroup$ Exactly. If we consider weapons as force multipliers, adding water doesn't do most much good. You can't see down there, and movement is inhibited. Large nets could be very effective, particularly in shallow waters. Paying the ships with nails or copper would at least buy time. But underwater explosives are just too effective not to be used. Compression does nasty things to soft tissues. $\endgroup$ – canhascodez Aug 30 '18 at 20:05
  • $\begingroup$ I can't name a chemical that's available in a medieval setting, toxic to animals, and toxic enough to be practical in a sea battle. $\endgroup$ – Spencer Joplin Sep 2 '18 at 8:24
  • $\begingroup$ @SpencerJoplin en.wikipedia.org/wiki/Timeline_of_chemical_warfare - 590BC (which is way before the midevil ages) used a plant named Hellebore (en.wikipedia.org/wiki/Hellebore) to poison a water supply, a planet animal avoid eating due to it's toxicity. $\endgroup$ – cypher Sep 2 '18 at 19:47
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Attack the food source

By the later middle ages, but before cannon were on the scene, castles were nearly impregnable. Knights still wanted something to do, so they rode around killing farmers and burning crops.

If you want to drive the merfolk out, get rid of their food source. If they eat fish, get rid of the fish. One way to do this is to try to get rid of all the phytoplankton. That's not easy with Medieval technology, but there are critters that can do it for you, and at the same time the merfolk won't be able to use to their advantage. Breed and release lots of salps or jellyfish. You can also try to coat the ocean floor with mussels.

If the merfolk are technologically primitive, they would have trouble competing with these 'invasive' organisms.

Alternately, protect your ships better.

Now, from a Medieval knowledge point of view, it is not obvious that the previous solution will occur to anyone, not knowing much about invertebrate life cycles. As an alternative approach, you could do a better job protecting your ships. If you have metalworking technologies, and the merfolk don't, then copper plating on hulls would be a long lasting deterrent. If the merfolk can't drill through the bottom into the ship's under-sections, then they have to come to the surface to fight and their advantage disappears.

An iron ship will float, as we know these days, so the kingdom could spend some money to make the first 'ironclad' some 500 years early. The merfolk would not have any success cutting into or otherwise harming an iron hull. Combine that with crossbowman on castles and merfolk won't have much success with attacking the ship; or at the very least they will take heavy casualties. With your new iron ship, you can proceed to do the most destructive fishing or whaling that you can accomplish. Alternately, you could drag stone anchors (on a metal chain) over the ocean floor in an attempt to damage merfolk structures.

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    $\begingroup$ As an alternative to full iron cladding, a rough mesh of chain should be enough to keep a thick hull from being breached by coral tools. $\endgroup$ – Mathaddict Aug 30 '18 at 18:42
  • $\begingroup$ Even copper cladding should be sufficient to protect the ships -- we're talking stone saws here, not artillery shells. Still incredibly expensive, though. $\endgroup$ – Mark Aug 30 '18 at 19:51
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On deck hearths, small but fiercely hot vented furnaces, heating lead, oil, water water and small lead shot, iron BBs, medieval caltrops, small rocks, pebbles, cast off broken iron armour etc etc... slung at the gunnels in rolling racks, iron mesh netting to a half-fathom's length with heavy lead weights at the leading edge which have sharpened spikes blades etc protruding... all crusted in lime or other strong alkaline and abrasive. Heck, use Greek Fire or Automatic Fire with quicklime which only combusts in contact with water...

As soon as a merfolk attack is detected, the iron netting is percussively deployed, fore, aft and port and starboard - as soon as it's 75% down (and it will cause significant merfolk injuries on the way down) the heated slops and shot etc is deployed through runnels reaching around the boat's profile dangling to halfway to the waterline (therefore INSIDE the perimeter of the iron netting) with the admixture of a lot of small waterproof fireworks laden with iron filings.

Why?

Your merfolk will have, by our landlubber standards, very delicate skins (something like a cross between octopus and dolphins) so the abrasives will be sheer agony, the alkali will provide time-delayed chemical burn impacts and unreasoning terror, the iron filings and explosives will disrupt skin integrity - and remember that continually-wetted skin / hide in an oceanic setting is an infection barrier under constant attack from biofilms, molluscs, flukes, barnacles, and other far less cuddly things... Those merfolks skins will be incredibly sensitive, and thus a positive sensing resource for hunting at sea (not having had serious evolutionary competition at the top of the food chain) helping merfolks detect prey (think sharks and their electroreceptive sense) alerting based on the tiniest changes in water chemistry and temperature - and you have just trapped them in a space and forced them to encounter chemical attacks, massive thermal gradients and hotspots, high-speed (for a short distance) particulate metal (at temperature) AND the horrifying compressive impact of undersea explosions...

A few of these floating round as decoys amongst your fishing and shipping fleets will wreak unholy havoc on the merfolks... and as indicated in another answer - if you can, using this escort method, fish the hell out of a given area - this creates a localised food desert for the merfolks, and they must, perforce, relocate.

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While it will also affect the fishing, you could drop steel rods over the side and let them hang in the water. When being attacked, strike the rods with a hammer. Some experimentation would be required to find the best tone or combination of tones for driving the merfolk away.

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If your medieval people understand why the attacks are coming, the best approach is to find a way to solve the original problem.

Find some other way to get enough food. Farm new land, find new foods that can be farmed on land that has not been worth farming before. Trade for food with somebody else who has a reliable surplus.

If you can't do that, then you are eating up the same food the mermen need, and it will only get worse as your population goes up. They cannot surrender, all they can do is fight until their population has been reduced to the point they can survive on the reduced food, and then face further reductions later, until they are mostly gone. Or possibly they might find somewhere else they can migrate to until your fishing fleets arrive there too.

They will salvage metal tools from sunk ships and use them to attack you. Chisels. Maybe saws. Whatever works.

While you invent new military technology, they will too. They might find out how to use fire above the surface. Every bottle of distilled alcohol on a sunk ship could turn into a firebomb that could be tossed into a new ship. Bottles of oil similarly. And grease.

You might win. It will cost, and it will take a long time. If there's any way to get your food elsewhere, that's likely to be a better solution. Then start learning languages, and see if there's anything to trade with them. It might turn out it's more efficient to trade things you can make easily for fish etc, than to catch the fish yourself.

Don't get into a genocidal war if you can help it.

And for gods sake don't get into a war with somebody you don't know how to talk to. That's a crazy thing to do.

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Realistically humans can never completely beat a merfolk civilization with Medieval technology. They can only suppress interruption of their fishing activities.

Here is why a merfolk enemy can't be fully eradicated: they can hide at the bottom of the ocean. No matter what the humans try to do to attack merfolk, if the merfolk hide deep in the ocean for example the bottom of the Mariana Trench as an extreme example, nothing will be able to reach them and do any meaningful damage. Certainly nothing that wouldn't also completely destroy the Ocean and make fishing pointless. Dropping rocks, bombs, nets, or whatever you think of will be prohibitively expensive for humans who have to ship it out to sea, but be easily avoidable by merfolk who only have to dodge these devices, or be somewhere else.

The very best a human civilization could do, is to reduce the merfolk to guerilla tactics and bands of rebels by defeating any standing armies outright. But that is all you could ever hope for.

So if fishing is what the humans are after, it would make more sense to find a way to ward off merfolk. I've seen metal plating mentioned, sonic warfare, dynamite in the water, etc.

Outright defeat is however impossible. It is like asking how a Medieval army is going to defeat an enemy that lives on the moon. It is not going to. It can only stop interference into its own interests.

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  • $\begingroup$ Ain't never gonna happen, but sure as well, this is going to stop, +1. Because every apex human sailor that returns with mermaid fins gets paid $20 per. It's not war, it's genocide, and everyone's invited. See also, The Great Westward Expansion. $\endgroup$ – Mazura Jan 5 at 7:44
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I agree with @elemenas that catamarans are a valid option. However, solid wood hull and chain mail would probably be too resource-intensive, and the chain mail would cause too much drag. I think that there is a simpler, but still related solution. First, make a few catamarans, or just tie two small fishing boats together so there is a gap in the middle. Then, station men armed with pikes and harpoons on both hulls and set sail out to merfolk infested water, and wait for their attack party to approach.

The catamaran design should prevent it from capsizing, and armed men stationed on both hulls will give you a position from which to spear the aquatic saboteurs. Basically, when a one of them tries to saw through one of the hulls, a pikeman on the opposite hull will be able to spear the merperson before he saws through the hull. This process would continue, with pikemen on each hull spearing any merperson swiming below the opposite hull. Eventually, the merfolk would learn not to attack the catamarans any more, and you would be rid of your problem.

You could even use the same concept on a full sized ship as well. Instead of using a catamaran, you could just hand a couple of platforms six or so meters off the sides of your ship and station pikemen on these. That way they could defeat any merfolk under the hull of the ship that would otherwise not be reachable by those on deck.

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Eutrophicate

This involves dumping all wastes into the sea:

  • Livestock remains.

  • Sewage.

  • Trash.

Aerobic bacteria would thrive on the food supply and consume all the dissolved oxygen. All creatures with gills would die of asphyxiation and become more food for bacteria.

Nutrients in the waste would fertilize algae, which would block sunlight from aquatic plants and create a "fog of war".

Blood would attract sharks. Reinforcing the shark defenses would presumably require resources diverted from the human offensive.

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  • 1
    $\begingroup$ Unfortunately this kind of large-scale area denial would also affect the fish that you came to the merfolk's territory to fish in the first place... $\endgroup$ – DevSolar Aug 31 '18 at 8:22
  • $\begingroup$ Depends which one would suffer more. The humans still have farms on the land, the merfolk are entirely dependent on what's in the sea. $\endgroup$ – user54563 Aug 31 '18 at 9:10
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    $\begingroup$ The sea is a pretty big place. I don't think any medieval society generates enough of the above to make any significant dent in it's ecosystem. If it were lakes it might be an issue if you throw in stuff for long enough. $\endgroup$ – Jacco van Dorp Aug 31 '18 at 14:42
  • $\begingroup$ @Wyvern: If you have the farmland to feed your population, you don't need to wage war with the merfolk in the first place, especially not on this genocide / extinction level. You posited that farm produce was insufficient and that the humans needed to fish. $\endgroup$ – DevSolar Sep 4 '18 at 7:44

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