One of the biggest problems about writing merfolk warfare is related to weapons. You can not slash underwater without expending a lot of energy, stabbing is much easier. Water has more drag than air and makes the sword move slower. That’s why in my merfolk society, swords are very rare. Most merpeople prefer to use tridents, spears or harpoons.

But one of my protagonists is an exception. He uses a sword, because his magical powers allow him to do so. He has the ability to increase water temperatures, thereby boiling water and turning it into steam. While using his sword, he uses magic to heat the weapon, turning water around it into steam. The steam condenses when it comes in contact with water again.

Could the protagonist use this technique to brandish the sword more quickly through the water?

Notice that the sword was not designed to slash enemies. It’s the protagonist’s way of directing his boiling powers. He cannot boil all the water around him, as that would boil everyone around, including himself. However, I suppose a blow from this super hot sword would be enough to hurt his enemies badly.

Important information:

  • The protagonist is resistant to high temperatures, so heating the sword WON’T HURT HIM. His enemies don't have that resistance
  • He can boil water at a very high speed
  • The sword is made of an imaginary material that does not melt and is not damaged by consecutive heating and cooling
  • He has considerable physical strength, so it's okay if you have to make a little effort to move the sword, as long as it doesn't become a major disadvantage

Taking all this into account:

  1. Would it be possible for this sword to work, and to make rapid movements through water?
  2. Would it be worth using that sword, or would it be easier to just use a spear?
  3. What material could this sword be made of?
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    $\begingroup$ so basically like how mantis shrimp supercavitation punch/undersea shockwave is? do you want it to cut from the distance? like wind slash but underwater? or simply shockwave that punch the opponent without cleaving or cuttting them? if it the latter i dont think it qualified as sword anymore. $\endgroup$
    – Li Jun
    Oct 7, 2020 at 16:53
  • $\begingroup$ but after thinking again some sword can do murderstroke after all.... $\endgroup$
    – Li Jun
    Oct 7, 2020 at 16:58
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    $\begingroup$ Wouldn't his arms still have to deal with the water resistance slowing down his movements, even if the blade itself doesn't? $\endgroup$
    – Pahlavan
    Oct 7, 2020 at 22:31
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    $\begingroup$ This questions seems to ignore that swords can be used for stabbing - in fact, that is the only way to score when fencing, because rapiers, foils, estocs, and épées are specifically designed as thrusting weapons (i.e. 'stabbing swords'), not slashing ones. $\endgroup$ Oct 8, 2020 at 8:57
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    $\begingroup$ It should be noted that pole-arms have a lot of irl advantages over swords: They have greater range, they're easier to make, maintain, and use, and they are more effective against armored opponents or opponents on horseback. I am no expert, but I believe that irl oftentimes soldiers would carry a pole-arm as their primary weapon with the sword being more of a side-arm or useful for one-on-one combat. $\endgroup$
    – user73215
    Oct 8, 2020 at 15:59

5 Answers 5


Don't use magic to heat it up, you will end up having more troubles than you had before you enchanted it.

Instead, use magic to give it speed. As you move something inside a fluid, you cause the fluid surrounding the thing to flow. If the thing is streamlined, that can lower the pressure of the fluid. That's important for ship building because below a certain water pressure, it cavitates:

Cavitation is a phenomenon in which rapid changes of pressure in a liquid lead to the formation of small vapor-filled cavities in places where the pressure is relatively low.

When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate a shock wave that is strong very close to the bubble, but rapidly weakens as it propagates away from the bubble.

This tends to love up propellers and hulls. But if you really go fast enough and the whole object is quite streamlined, it can supercavitate:

Supercavitation is the use of a cavitation bubble to reduce skin friction drag on a submerged object and enable high speeds. Applications include torpedoes and propellers, but in theory, the technique could be extended to an entire underwater vessel.


A supercavitating object is a high-speed submerged object that is designed to initiate a cavitation bubble at its nose. The bubble extends (either naturally or augmented with internally generated gas) past the aft end of the object and prevents contact between the sides of the object and the liquid. This separation substantially reduces the skin friction drag on the supercavitating object.

Russians have been experimenting with supercavitating torpedoes for decades now. The torpedo forms a bubble of vapor around itself, so only the tip of the torpedo actually touches water. The whole thing then literally flies inside the moving bubble at nearly 370 km/h (about 230 mph).

Anyway, back to your guy. If the sword is just fast enough (> 25 m/s, or 82 ft/s), it will form a bubble around itself which will allow you to apply a very fast cut followed by an implosion that will surely hurt a lot. If your protagonist is able to hold the hilt as the sword moves forward, they can also literally fly underwater just like Thor does on air with Mjölnir, which also adds Rule of Cool to the whole thing.

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    $\begingroup$ That’s an awesome idea! Thank you very much! $\endgroup$
    – ssfg
    Oct 7, 2020 at 17:51
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    $\begingroup$ What propels the sword? Particularly re the "flying" concept? (For the combat part, I assumed it was the hero who propelled it.) You said not to use magic to heat it -- do you mean use magic to propel it instead? $\endgroup$ Oct 8, 2020 at 10:54
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    $\begingroup$ @T.J.Crowder yes. $\endgroup$ Oct 8, 2020 at 12:03
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    $\begingroup$ "they can also literally fly underwater" You mean, like... swimming? $\endgroup$
    – walen
    Oct 8, 2020 at 15:36
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    $\begingroup$ @walen no. Like flying. They will not be touching the water, they will be inside a bubble of water vapor. If they don't generate enough lift, they will touch the base of the bubble and possibly collapse it. $\endgroup$ Oct 8, 2020 at 17:05

My solution is simple.

Stabbing swords and small slashes.

Stabbing swords

Rapiers come to mind when we start talking about stabbing swords. Basically they come in many forms but all of them are thrust centeric, some to the point of having no edge and others to a lesser degree.

Anyway rapiers offer excellent hand protection, they also have reach over even cavalry sabers, and are some of the best unarmored fighting weapons as they are fast and agile.

Now don't mistake the weapon for less manly sword as rapier have the same mass just distributed in a different manner. They are longer and thinner and made to thrust quickly while keeping the enemy at bay and offering excellent hand protection.

Also remember that many rapiers were war swords and those tended to be a bit shorter with a more cutting blade because duh.

Small slashes

In reality big telegraphed moves are not really that much of a thing. In reality a sword fight can be as quick as two heart beats.

I highly recommend seeing HEMA (Historical European martial arts) video on YouTube as it would give you an actual idea of how actual fights went.

Small controlled moves, a cut and a retreat. Nobody just raises the swords as high as they can like they are using a hammer. That leaves you open to probably more than enough attacks to kill you.

So even in reality you just jab the opponent or do a small cut and that's it.

So both can be done underwater with little effort, your protagonist is super powerful anyway, and can be very realistic.

On the realism aspect let me tell you a well known technique. Halfswording. Basically if you are fighting an armored opponent your chances of doing damage with the conventional sword fighting is minimal. So you grip the sword from the middle and thus turning into a dagger and try to get that little sharp edge into a gap. Sword were also used to be held as a club with the pommel at the top and using the pommel as an improvised warhamer.

Both, and many others, are 100% realistic techniques used in history.

So just say that your MUMA (Merfolk Underwater martial arts) focuses on small cuts and slashes.

Here are general sword advantages that you might like and make you incorporate the sword

  • Every part of a sword is a weapon. The pommel the crossguard and grip are all incorporated into actual fighting manuals of history. A spear is great but swords can be more versatile.
  • Swords were EDC (every day carry) side arms of the times before pistols. Obvious truth is obvious but even underwater I think trying to carry a spear or pollaxe on your back is troublesome. So while even above water the overall advantage of spears existed. People found it easier to carry a sword.
  • Paired with how every part of sword is a weapon a sword is a much more dangerous weapon in CQC. You can manipulate the blade better to defend yourself in tight spaces or from an enemy that just closed the distance. In my opinion when you introduce the underwater fighting element this might make swords more useful as the enemy can just come at you from an unexpected angle and that leaves you less time to react and fight back.
  • Armor void swords. So sorry. If you have armor your swords will suffer trying to get through it. That is if it compared to full plate armor or something similar.

Basically I think you can keep the sword even without the magic. But as "Renan" so logically suggested just have the magic be like he said if you magic.

Other than that just add the bits about ease of carry and overall advantages of swords and that the martial art itself helps and voila. You have swords underwater without magic or with little magic as possible.

Personally if I can use magic in the situation even for small stuff I would just bring a box of dagger points, or just nails for maximum cheapness effect, and accelerate my weapon of choice to hit the enemy with enough force to penetrate armor. Rinse and repeat. That would be even cheaper than bullets.

Or I'd make little spiked balls that I will shot and have a couple of them run around in the opponents body for several seconds until the desired effect is achieved.

Or for maximum serious and grimdark stuff just cut their veins from the inside or explode their heart or brain or rupture their lungs directly. I actually use this magic in a world of mine. Wizards there take like 3 second to kill a dozen .

But I digress.

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    $\begingroup$ I think you meant "et voilà".. "viola" has a totally different meaning in french (that you probably want to avoid) $\endgroup$
    – Kaddath
    Oct 8, 2020 at 8:29
  • $\begingroup$ @Kaddath, I fixed it. Thanks. $\endgroup$
    – Seallussus
    Oct 8, 2020 at 15:22

It will look awesome! Until it sticks in your opponent and you cant get it loose.

The bubbles of steam will go straight up, as will heated water. None of that stuff will stay in the vicinity of sword or merperson for very long.

The sword will appear as a swirling mass of bubbles.

If the sword is hot enough to immediately boil all the water it touches, it will be swinging thru steam. Given the rate of water change in the vinicinty of the sword this sword will be consuming tons of energy.

If you hit someone with this sort it will burn the bejeezus out of them. The sword does not cool down when it heats other things up so unlike a burning hot thing that cools down with bodily fluid, the sword will stay hot.

A bad thing about this is that the hot sword will stick. Imagine cooking a steak on an unseasoned iron pan. The steak is your opponent and the pan is your sword. Your sword will stick fast in or on your opponents flesh. Burning will hurt your opponent but this might be less likely to kill than the same force hit from a regular sword. Burn damage is less rapidly lethal than a deep slice or a stab. Also your opponent will not bleed out - the sword cauterizes the wounds it makes.

Your sword getting stuck may be worse for you - if your opponent is physically large or very tough he may be able to deal with the burning sword and come at you regardless. You might have to abandon your stuck sword. Hopefully you got some of those ninja throwing stars in your shirt pocket.


It is not helpful for moving the sword, But they can use the sword anyway, and it makes a wicked defensive tool.

first water is a very good thermal conductor so your merfolk is going to be scalded constantly. If they still have a sword hand after trying this they are lucky. Watch a reenacted sword fight in slow motion and track the movement of blade and hand and see how often they cross each other, with your effect every time this happens the merfolk is putting their hand and arm in scalding water. You say heating the sword won't hurt them how about boiling water. Of course they can use this against the enemy as well, the enemy is not going to approach through a cloud of scalding water.

Second, what you are describing is the leidenfrost effect. Which works if the METAL is hot enough to flash boil the water. Boiling the water directly will not have the same effect and will not reduce drag much unless they can boil the water in the entire arc of the swing, Which is a lot of water and something you said they cannot do.

but why bother if they are using a sword designed for thrusting (which were common) it will work just fine underwater, just as well as a spear.

Honestly if they just need metal for a focus rig up hollow metal tube spear in a spear gun and use is like super heated bullets. make one opeing smaller than the other in the tube and boil the water inside the tube, you can get it to accelerate in the water like a rocket, very fast acceleration to boot. Better yet get a gun, you can boil the water in the tube and get more power than gunpowder, use it to fire spears. Heck get a hollow copper ball with a few holes drilled in it and you have grenades.

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    $\begingroup$ Water is not a very good thermal conductor, it is quite the opposite instead. That's why we use it to quench fire. $\endgroup$ Oct 7, 2020 at 17:39
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    $\begingroup$ @Renan and why water takes quite a long time to freeze in winter. $\endgroup$
    – IT Alex
    Oct 7, 2020 at 18:13
  • $\begingroup$ What water does have that makes this a problem is much higher thermal conductivity than air -- and a genuinely immense specific heat and latent heat of vaporization. It takes more energy to boil off 100C water at standard pressure than it does to heat the same mass of water from 0C to 100C (which in itself is one of the highest figures of all known materials). Also, you might have missed that the character is immune to heat, as is the material of the sword. $\endgroup$
    – Zeiss Ikon
    Oct 7, 2020 at 18:39
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    $\begingroup$ @ITAlex that is specific heat which is a very different thing than thermal conductivity. $\endgroup$
    – John
    Oct 7, 2020 at 21:22
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    $\begingroup$ In hydrostatic / zero G situations water is a bad thermal conductor. But convection + high thermal capacity makes it very good at moving heat, which you can say is why we use it for cooking. Water+steam combo can be even better at heat transfer by taking heat for vaporization in one place and releasing heat from condensation in another (which is used in "heat pipes" to move heat in laptops). What all this means for the sword, I'm not sure :-). But lots of steam forming underwater will likely to be pressurized, thus it can be super-hot (significantly over 100ºC), which makes for bad burns. $\endgroup$ Oct 8, 2020 at 19:41

Theres videos of people lifting Olympic barbells underwater.

I doubt a sword will be a problem, the slash won't be as fast as if done in air, but fast enough. Just like how an underwater punch still hurts, even if not as fast as out of water.


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