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I have a mer civilization that does have magic, with a technological advancement to about flintlock musket level, so nothing overly complicated. I need the soldiers to have effective weaponry, underwater. With the drag force water has I haven't been sure if swords or slingshots would be effective weapons. I know harpoons would work but they aren't suitable for close combat. The magic is more personal and doesn't really work to change the rules of underwater combat, though magic users ability to have area effects is why close combat is so important. This primarily for mer vs. mer combat. I'm not looking for really technical details more about which weapons would work. I am also dealing with a civilization that can trade and scavenge from and with land dwellers but are themselves still using stone and wood.

And no I'm not looking at open warfare, so please no more wmds

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  • $\begingroup$ Who are you fighting? Humans? If I were the humans, I would simply drop a ton of depth charges on a regular basis, and not give you the chance to hand to hand combat me. Sure modern depth charges are way better, but they have been around for a long time, and in a long war humans would focus their effort to make them better. $\endgroup$
    – cybernard
    Dec 4, 2016 at 19:09
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    $\begingroup$ Note that beings living in water would have a very tough time developing chemical or other processes of (a) soluble (in water) chemicals or (b) concentrated heat. It's not impossible, but it's so very much more difficult than in air -- which doesn't conduct heat well, supports fire, and doesn't dissolve away most source materials. $\endgroup$
    – Catalyst
    Dec 4, 2016 at 21:10
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    $\begingroup$ Toxins from jellyfish and such loaded on a breakaway tip of a very heavy and slim propelled dart or close combat trident good against wildlife as well. $\endgroup$
    – KalleMP
    Dec 5, 2016 at 0:33
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    $\begingroup$ A fly-swatter would be very ineffective under water. $\endgroup$
    – donquixote
    Dec 5, 2016 at 10:56
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    $\begingroup$ @donquixote Obviously. There's no flies underwater! $\endgroup$
    – Luaan
    Dec 5, 2016 at 15:23

10 Answers 10

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Basically everything that you thrust.

The issue with underwater combat is, as you mention yourself, the drag. Thus every weapon that relies on swinging or other movement in order to abuse its weight to make it hit harder will be at a disadvantage due to it being harder to move it against the water.

Everything you thrust straight forward though will be very easy to use, because it has only little water to push aside.

So (make your choice or improve upon):

  • Spears
  • Tridents
  • Harpoons (basically barbed spears)
  • Guns (Yes, projectile spewing devices of murder! Well you'll likely be using pressure to fire ammunition AND due to the already stated principles there's not much distance they can cover, but it's still a valid principle.)
  • Knives (stabby stabby)

Other than these, you could also make use of the fact that water lends itself as a transmitter for sound- and shockwaves by e.g. creating underwater mines/bombs which would have a concussive effect on everyone/-thing around it.

Addendum: Due to the many comments received on this answer I feel a need to clarify some perceptions on guns.

Gun
1 c: a device that throws a projectile

A gun is a device that shoots/throws directed projectiles. A gun does not have to propel these projectiles by using gunpowder; there's many different ways to propel a projectile.

Rifles and pistols can be easily powered by means of spring-pistons, pneumatics and/or pressurised other gasses.

Projectiles for underwater-guns, called Flechettes, will look differently from over-water-guns as they will have take a more streamlined look so they do not have to fight against drag that much.

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    $\begingroup$ Mythbusters did bullets in water (admittedly the bullet had to break the surface, so is not quite the same as firing them underwater). Effective range likely is only a few metres, because bullets are only dangerous at very high speeds and drag is proportional to square of speed. Beyond that, a harpoon does better since it has higher mass/profile ratio so it maintains speed longer. $\endgroup$
    – Steve Jessop
    Dec 4, 2016 at 16:37
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    $\begingroup$ @SteveJessop Mythbusters episode was for normal guns fired underwater, not guns designed to fire underwater, correct? en.wikipedia.org/wiki/Underwater_firearm $\endgroup$
    – Ranger
    Dec 4, 2016 at 19:47
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    $\begingroup$ Guns designed for underwater operation are a bit different from atmospheric guns. Much longer projectiles for one thing, for more mass to drag ratio and less risk of tumble. And of course the action will be designed to work under the drag. The range is still much worse than in air, but much better than trying to fire an atmo gun underwater. $\endgroup$
    – zstewart
    Dec 5, 2016 at 4:01
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    $\begingroup$ What about a crossbow? $\endgroup$
    – donquixote
    Dec 5, 2016 at 9:30
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    $\begingroup$ @donquixote Crossbows & Bows will have to fight against drag when fired. While their projeciles are a good match for underwater fighting (see spears, harpoons, flechettes).The issue stems from the increased drag both the bow-arms and bowstring have to fight against underwater, thus a more compact buil could be used but would still lose much of its power. $\endgroup$
    – dot_Sp0T
    Dec 5, 2016 at 9:41
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spears, spearguns, thrusting knives, maybe thrusting swords and that is about it. As Dot pointed out drag is the problem even firearms lose all force after only a few feet. A bow and arrow might work at a much shorter distance. Grenades work though as long as they have an internal fuse, concussion grenades actually work better underwater becasue water conducts the shockwave better.

guns have been developed for underwater use but they are basically needleguns, but they won't work with flintlock level tech.

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    $\begingroup$ So would a slingshot or a blowgun work? $\endgroup$
    – A Mari
    Dec 4, 2016 at 18:08
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    $\begingroup$ sling shot no, but if they can make slingshots they can make spearguns. , a blow gun with water instead of air might work provided you designed a flechette like dart, but you need dart that leaves its wadding behind. $\endgroup$
    – John
    Dec 4, 2016 at 20:01
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    $\begingroup$ There is a device called a hawaiian sling which is the precursor to a speargun and is basically what you have if you made a slingshot to fire spears. $\endgroup$
    – John
    Dec 4, 2016 at 20:14
  • $\begingroup$ How does an underwater civilization make all those metal weapons? $\endgroup$
    – Innovine
    Dec 4, 2016 at 21:56
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    $\begingroup$ Rubber? I'm talking about wood, bone, and stone. $\endgroup$
    – John
    Dec 6, 2016 at 5:13
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Here's an idea you could play with: a https://en.wikipedia.org/wiki/Cavitation - based weapon.
A sudden cavitation causes intense pressure waves. The pressure may be lethal at the end of a cavitation lance, but not harm the wielder. Cavitation bombs??

If conflict takes place at considerable depth, then the sudden implosion of evacuated spheres could produce a usefully short shock. Implode one of these (imagine a super-strength evacuated glass 'bubble') at the focus of a parabola and you basically have a pressure-wave death ray. (Just watch for the sidelobe radiation; real-world parabolas miss little bits of the wave.)

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  • $\begingroup$ My thoughts exactly. $\endgroup$
    – JDługosz
    Dec 5, 2016 at 6:06
  • $\begingroup$ Can one use whip to cause cavitation at whip's tip? $\endgroup$
    – Daerdemandt
    Dec 5, 2016 at 9:28
  • $\begingroup$ I suspect you won't be able to swing a whip fast enough under water to cause a crack. $\endgroup$
    – jorfus
    Dec 6, 2016 at 2:35
  • $\begingroup$ It's also possible to cause cavitation with your projectile tip which can surround the projectile in a pocket of air significantly reducing drag. It's pretty advanced fluid dynamics, but submarine creatures would probably be quite good at advanced fluid dynamics. $\endgroup$
    – jorfus
    Dec 6, 2016 at 2:37
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Chemical warfare...

Swim over the enemy location and drop containers on them which burst and pollute their water with poisonous chemicals.

If you could locate yourself in a current upstream of your enemy you could just release poison chemicals. Might be useful as defence.

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  • $\begingroup$ I wasn't planning on having a war, but good to know. $\endgroup$
    – A Mari
    Dec 4, 2016 at 17:57
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    $\begingroup$ If your story is set in the oceans then "upstream" has a certain difficulty in application. Oceans do flow technically but it is subject to somewhat unpredictable forces and ultimately it is possible that poisoned water will recirculate and cause problems for everyone. It's hard to un-poison water. $\endgroup$
    – Wossname
    Dec 4, 2016 at 18:03
  • $\begingroup$ @wossname poisons could also break down to harmless compounds over time. And for currents, that's also up to the author to decide. Perhaps they all live in an underwater cave system, a ravine, or on the edge of a cliff or continental shelf.. Perhaps they live in shallow water and experience tidal flows, or in the estuary of a large river (feeding on all the freshwater organic material being poured into the sea)... $\endgroup$
    – Innovine
    Dec 4, 2016 at 21:51
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    $\begingroup$ @a mari chemical warfare is still popular on a smaller, melee combat scale too, look at jellyfish stingers, fire urchins, puffer fish, the list of poisonous fish is long. It seems plausable to me that your mer civilization would carry small personal weapons based on chemicals more than projectiles and blades (and how are they supposed to forge metal without fire, anyway?) $\endgroup$
    – Innovine
    Dec 4, 2016 at 21:54
  • $\begingroup$ @Innovine now that is helpful, I hadn't thought about natural toxins like that thanks $\endgroup$
    – A Mari
    Dec 5, 2016 at 0:51
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Tsunami!

If you have flintlocks then you have gunpowder. If you have gunpowder you have explosives. If you can blow up the side of a coastal mountain (either above water or below, either works) then you can in principle cause a rockfall sufficient to displace enough water to give rise to a tsunami.

These have precedent on a mythical and distant world known as "Earth" (what a dull name)...

https://en.wikipedia.org/wiki/Tsunami_bomb

Downsides include:

  1. Limited locations for deployment.
  2. Difficult to steer.
  3. Makes you VERY unpopular.

EDIT:

Serving suggestion...

Aim two carefully timed tsunamis towards eachother to coincide exactly above an enemy merfolk city (which is presumably 100% underwater). The combined waves will cause maybe a 2 kilometer high mass of water to come to a halt over the city. Gravity does the rest... Trillions of tons of (cavitating) water pressure bearing down on the enemy base.

Maniacal cackling is encouraged, but optional.

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  • $\begingroup$ This might be a lot more effective above water than under the wave. I can't find anything concrete, except it's apparently safe enough to scuba dive under a Tsunami (youtube.com/watch?v=L5IbDi09Yb4) and one would expect merfolk to swim at least as well as scuba divers. $\endgroup$
    – Ranger
    Dec 4, 2016 at 15:53
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    $\begingroup$ @NexTerren: a "regular" tsunami wave is only a few metres high (or often even less) in open water and moves quickly across it, whereas the record actually measured for a megatsunami caused by a landslip is 520m, in Lituya Bay, Alaska, in 1958. There's probably a difference between an unexpected but brief increase of a few metres of pressure, and an unexpected increase of 520m of pressure :-) Mind you, if the mer-city is close enough to the affected land that the wave is that big, the landslip itself may already be catastrophic. $\endgroup$
    – Steve Jessop
    Dec 4, 2016 at 16:46
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    $\begingroup$ ... basically a regular tsunami is terrible for humans because neither we nor our buildings enjoy suddenly being at the interface of land and water. A regular tsunami doesn't scour the seabed except where the wave breaks. Wossname is talking about a very different (and much larger) effect. $\endgroup$
    – Steve Jessop
    Dec 4, 2016 at 16:53
  • $\begingroup$ I think this could still work as a shock effect, but yeah bigger area effect than I was looking for. Thanks anway $\endgroup$
    – A Mari
    Dec 4, 2016 at 18:01
  • $\begingroup$ "with a technological advancement to about flintlock musket level" $\endgroup$
    – Antzi
    Dec 5, 2016 at 2:14
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Use ink to blind everyone, or ropes, like in fishing nets, to pull your foes up, or down to the bottom. Or have trained electric eels.

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  • $\begingroup$ Welcome to Worldbuilding, Paul, answer should have substance like information, facts, and reasons why whatever is proposed as the case. Your answer is very brief, please improve it with added how your suggested weapons can be wielded effectively. Have fun here! $\endgroup$
    – a4android
    Dec 5, 2016 at 6:00
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Blasting weapons are harmful, apparently: dynamite, torpedoes, mines.

See for example, Is it worse to be near an explosion on land or in water?

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  • $\begingroup$ Torpedoes are way above flintlock level of technology. How would you manufacture dynamite underwater? $\endgroup$
    – Tomáš Zato
    Dec 4, 2016 at 19:41
  • $\begingroup$ That's a question you should ask your alchemist. $\endgroup$
    – ChrisW
    Dec 4, 2016 at 19:46
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I'm not sure if you would count it as weaponry, but you could probably dress Dolphins or Killer Wale to fight and kill for you.

As a bonus, they might help you moving around !

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  • $\begingroup$ Any animal that can swim and that you can train; they don't have to be large. Built-in propulsion, built-in fangs, built-in poisons. What more do you need? $\endgroup$
    – user3106
    Dec 5, 2016 at 10:16
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Slashing weapons would work though they would have to be used differently.

You can't swing them around but you once you made contact you can drag the sharp bits along your opponent's skin.

Both sharp and jagged edges should work.

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Somebody mentioned Cavitation. There's already supercavitating rifle ammo https://www.youtube.com/watch?v=jnR1rcvku4c.

Cavitation - many bubbles

Supercavitation - one bubble

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