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As mentioned by Shad in a recent video ( https://youtu.be/0t8ZrI5JqCw ), a big problem with giant fantasy swords is that they are often simply too heavy for the wielder regardless of superhuman strength. Would creating a hollow sword be an effective workaround for this? Or would it create more problems? Below I have an illustration of an example “hollow sword” design.

Hollow sword

Basically, you have a long/large metal rod that serves as the hilt and base of the sword. Then you weld three smaller rods into the big rod, the bottom rod serving as a cross guard, and the top two serving to give the blades stability. The blades themselves would basically be giant razors (four of them), thin sheets of fitted metal that could easily be replaced/repaired as needed. If looking at the sword from a top/down viewpoint, the blades would form a closed diamond shape.

Is this hollow sword design viable or problematic? What tweaks would it need to work? If it’s too problematic, what would be a better design?

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    $\begingroup$ Why do you need the sword to be giant? $\endgroup$ – Alexander Apr 10 '18 at 16:39
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    $\begingroup$ What exactly is the use case? Looking cool on a wall and sometimes cutting ribbons might work with razor blades and foil, but if you expect to kill someone who isn't cooperating robustness is pretty important. $\endgroup$ – user25818 Apr 10 '18 at 16:46
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    $\begingroup$ Yes but an actual sword of same mass and smaller volume is always going to be better, hence why we used them $\endgroup$ – Ummdustry Apr 10 '18 at 16:50
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    $\begingroup$ This proposed sword would be unsurpassed for fighting triffids. $\endgroup$ – Willk Apr 10 '18 at 18:27
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    $\begingroup$ For such questions there is usually very simple way of finding an answer: "Has anything like that ever been used as a weapon of war?" No? Then that's your answer. $\endgroup$ – M i ech Apr 10 '18 at 20:50

11 Answers 11

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To paraphrase the late Sir Terry Pratchett, A sword is for the messy business of dynastic surgery.

OK, I just really wanted to say that.

You have a number of things you need to consider when constructing a sword of any size.

Fighting style is chief among them. Your fighting style shapes the blade, so to speak. A Style in which your opponents are going to be wearing minimal armor and is geared toward stabbing, you use a rapier or an epee. You want something that can be moved very quickly and thrust into your opponents squishy bits. This style needs a lightweight, long, thin blade with a very sharp point. It's not going to have a focus on a sharp edge because you are stabbing, not slashing.

A Style that focuses on men in armor is going to need a much heavier blade in order to punch through. This blade may be single or double edged and would rely on a broad swing or a two handed thrust with the point.

Yet another style that relies on slashing cuts to cut armor away and damage mounts and stuff will look more like a samurai sword. Single edged and very sharp.

So why would you need a 7 foot long samurai sword or a Cloudbuster bastard sword from Final Fantasy 7? Let's just roll with it. Something like Cloud's sword I could imagine being a useful tool to take out mounted opponents and to smash it's way through things That means it needs Mass, and lots of it. That creates a problem for you, since you are trying to get it down to a realistic weight. Reducing the mass means less energy at the tip of the sword. There are better weapons for that, called pikes and halberds. They aren't as sexy, but they work.

The 7 foot samurai sword runs into the same problem. There are better ways to deal with your enemy if you want to kill them from a moderate distance.

This is all just to point out some basics. If you are not going to invoke Handwavium or the "Rule of Cool", over-size swords don't make a lot of sense.

Now we come to your sword design. It looks like something that would be used to smash your opponent, kind of like Cloud's blade. It's not going to lend itself to a finesse kind of style. If you want it to have a hollow core to save weight, you need to design it so that the sharpness of the edge focuses the energy of the swing into the smallest possible area. In addition, you might want your style to focus on hitting joints. It's dumb to just go after the torso of a man in plate mail with a slashing attack. Also, if it's hollow, it's going to be fragile. Once the integrity of a side is compromised, the whole darn thing is going to crumple around it. So your guy is going to have a style that focuses more on dodging attacks rather than using his sword to outright block an attack.

As Always, the fighting style must match the blade. You are just coming at the problem backwards. You can excuse some of the problems of a giant blade with Handwavium or Magic, but if you ignore the style of fighting, you end up with silliness.

Edit something I thought of that might be helpful.

Handle design is also critical in your sword's design. A Katana has a long handle and is designed to be used two handed. From my (very brief, and a long time ago) exposure to kendo, The grip is supposed to be loose right under the hand guard with one hand. The other hand is at the bottom of the grip, with the pinkie actually under the grip. This hand grips the sword firmly. This gives you a fulcrum at the hand guard so that strikes are faster and more controlled than what you could manage with one hand or with both hands gripping the sword handle right on top of each other. combine this with the old kinetic energy = mass times velocity squared. The energy at the tip of the weapon is greater if you can get the sword moving quickly. A one handed grip on an oversized sword is not going to work especially well unless the guy wielding it has arms like a cross between Dwayne "the Rock" Johnson and Popeye.

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  • $\begingroup$ What's the functional difference between a hollow sword and a spear or other pole weapon? You're still dealing with a moment of rotational inertia that makes it difficult to quickly swing the further end around, making it much more useful for thrusting than slashing or blocking. $\endgroup$ – jamesqf Apr 10 '18 at 18:42
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    $\begingroup$ @jamesqf mass distribution. With spear you can practically deliver the same energy with lower total mass. Also, it is easier to maneuver. Have you tried to balance a hammer on your finger. Head up is (surprisingly if you didn't play like that) easiest. Head down is hardest. Equal mass rod is in the middle. $\endgroup$ – Mołot Apr 10 '18 at 20:06
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    $\begingroup$ @jamesqf I would argue that a pole weapon would be superior, but the OP wants a really big sword. I'm just looking for a way to make it happen ;) $\endgroup$ – Paul TIKI Apr 10 '18 at 21:20
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    $\begingroup$ When you think about it, many heavy non-sword weapons can be thought of as a giant sword with all the bits you don't need removed. A pike is the stabby part with the slashy part removed. An axe is the opposite. Just want something heavy for smashing? Use a hammer or a mace. A giant sword is like wielding a pike, an axe, and a hammer all at once, which is too heavy. $\endgroup$ – IndigoFenix Apr 11 '18 at 8:30
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    $\begingroup$ "So your guy is going to have a style that focuses more on dodging attacks rather than using his sword to outright block an attack." aka the Dark Souls strategy $\endgroup$ – MikeTheLiar Apr 11 '18 at 15:27
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The reason for large blades in historical weapons like an Ōdachi, Miaodao or a Claymore is to develop enough leverage in the swing to take on large, hard targets.

enter image description here

Odachi, with a typical user for scale

In armed combat, this might mean trying to strike down an armoured Knight or Samurai, or cutting down a horse carrying a mounted fighter of some sort. Long swords could also provide an advantage for people defending walls on castles, being able to reach farther and stab or strike attacking troops coming up over ladders. Swords like this were something of a compromise, they were not as handy or easy to use as a regular sword, but less bulky and unwieldy as a Pole arm like a Glaive or Halberd. Pole arms have much greater leverage and reach, which explains why they became the primary arms, displacing swords.

enter image description here

Naginata, a Japanese pole arm

So a "hollow" weapon such as you propose has neither the mass, the reach or the leverage to effectively engage the sorts of hard targets that real life "great swords" were able to, and indeed your swordsman would be rapidly killed as the sword broke against either an effective block or hard armour like plate or chain (indeed it would probably not even cut through boiled leather armour).

There are plenty of real life large weapons, use them as your guide.

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    $\begingroup$ The guy in the pic looks like an unrivaled bad-@ss. Notice the long handle. treated like a lever and fulcrum that thing would be devastating. $\endgroup$ – Paul TIKI Apr 10 '18 at 21:46
  • $\begingroup$ Devastating in the right context, but such a warrior would have to carry a second short blade for close-quarters fighting. $\endgroup$ – SPavel Apr 11 '18 at 0:48
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    $\begingroup$ the largest sword that we can confirm was used in battle ws six and a half feet long and weighed 15lbs,of course the wielder Pier Gerlofs Donia was a big fella. $\endgroup$ – John Apr 11 '18 at 4:49
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    $\begingroup$ Matt Cauthon would approve of the last image. $\endgroup$ – Caleb707 Apr 11 '18 at 19:48
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Your design would probably work as a killing weapon, to a skilled wielder. But whether it would work as a sword is a somewhat different question.

Swords are effective weapons because not only are they sharp enough to pierce human skin, but they're also sturdy enough to block other swords and penetrate certain defenses. A person wielding one of these could potentially get in close and deal some damage, but they would have to keep the weapon itself largely out of harm's way; it seems like a brittle contraption, as weapons go, at least the portion of it that would make it dangerous. The razor blades you describe, with their advantage of being easily replaced, would also have a big disadvantage in that once one is broken, an entire section of the weapon would be rendered largely useless. Someone wielding your hollow sword would probably have to rely on attacking by surprise, or using an evasive fighting style against lightly-armored opponents.

As for a better design, if you really want a large sword you'd probably just have to use a lighter metal. In a fantasy or sci-fi setting, this could easily be explained as a magic-infused (fantasy) or newly-discovered (sci-fi) compound. In a more realistic setting, or in the real world, I'm not sure there's a viable way to make this work. Think of it this way: Humans have had thousands of years of experience in killing each other. If a weapon like this were effective at killing humans, someone would have invented it already. So if you want to use one in fiction, the world and/or the weapon itself will need to be meaningfully "unreal."

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Hollow is potentially a problem because it could be designed to strike and stab well enough, but when you tried to block with it it would crumple (think chicken eggs, ridiculously strong against pressure in one direction, but flimsy against it in another).

My recommendation would be bird bone structure. To maintain weight and strength bird bones are "hollow" but with internal struts that protect them against being flimsy...

Not an "easy" solution in terms of manufacturing, but it would be worth exploring.

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    $\begingroup$ +1, How about an interior of foamed light metal with a skin hard enough that can hold the edge and thick and toughened enough to take the beating. - en.wikipedia.org/wiki/Metal_foam $\endgroup$ – KalleMP Apr 10 '18 at 20:55
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    $\begingroup$ A foam weapon is called a wiffle bat........ $\endgroup$ – Thorne Apr 11 '18 at 1:37
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    $\begingroup$ Foamed metal. That sounds fun and dangerous. $\endgroup$ – OhkaBaka Apr 11 '18 at 20:49
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I would have to disagree with the other answers. You can make a sword that's significantly lighter without compromising its strength if you 3D print it. 3D printing allows you to build the object layer by layer, building an internal honeycomb support structure while leaving most of volume empty.

Here is an example showing a cross-section of a 3D printed object with various levels of infill:

enter image description here

It seems reasonable that you could lower the weight of the sword by roughly 70% without too much trouble.

However, I very much doubt it will be necessary. Swords are already very light - even large 2 handed great swords are under 15 lbs (~7kg). According to this essay, a nearly 2 meter (78 inches) long sword is only 5.4kg (11.9 lbs)

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    $\begingroup$ I think it would be easier to 3D print a gun if the technology was available than waste the effort on a sword. Right idea though with making it partially hollow. $\endgroup$ – KalleMP Apr 10 '18 at 20:59
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    $\begingroup$ My son bought a replica claymore that only weighs about 3 kg. we messed around, trying to mimic some style appropriate moves we found online. You would be amazed at how heavy that thing feels after 10 minutes. I can hardly imagine how a 7kg sword would feel after similar use $\endgroup$ – Paul TIKI Apr 10 '18 at 21:53
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    $\begingroup$ What this misses is that weight is as much a feature of a melee weapon as it is a problem. You need a certain weight be able to apply enough force effectively with only muscle power, especially if you're facing armored targets. Which brings up the other problem - the material (and underlying structure) you use needs to be at least as hard as the armor it will be used against, and any 3D printing techniques you use make weapons harder and lighter and generally better... will be applied to armors, too, which puts you back in the same spot we started from with solid swords against solid armors. $\endgroup$ – HopelessN00b Apr 10 '18 at 22:54
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    $\begingroup$ "According to this essay, a nearly 2 meter (78 inches) long sword is only 5.4kg (11.9 lbs)" Yeah, sure, except same essay states that swords like those were ceremonial. Heaviest fighting swords were about ~3.5 kg and had polerarm-like style as one of the primary techniques. $\endgroup$ – M i ech Apr 11 '18 at 7:12
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    $\begingroup$ Using 3D printing (additive construction) for a sword is a disastrous idea. The joins between each layer are weak, the sword would shatter the moment it hit anything. $\endgroup$ – Grimm The Opiner Apr 11 '18 at 8:05
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The problem with Really Big Swords is, that they cannot be effectively used against armed humans (and especially against armer AND armored humans) or similar creatures.

It CAN be used as intimidating decoration or cool mark of leader as long, as it is not used in fight.

But there CAN be solution in "other" worlds - it can be very effective against big soft creatures with duplicite internal organs with some kind of fast closing wounds. (Jelly clouds with many "hearts" and "brain cells" everywhere in their body for instance). Hammer make a dent, but do not destroy enought internals. pike can pierce it, but again no suitable damage done. Short weapons just hit the skin, which closes nearly immediately. But Big Bad Hollow Sword would slice it in two, which make it close the wounds and form two smaler enemies. Slice it again and again and at some point there is too much internals destroed, all others are in too small bodies to be effective and large part of body is without internals, so effectively disabled.

Make those enemies also poitinous or acidic or flesh melting or something like that (so basically untouchable by normal means) and now the best equipement is big hermetic armor (maybe even with evil red glases for better seeing half transparent blue enemies) and really long blade razor sharp over its full lenght and no problems with otherwise clumpsy and volatile hollows. Maybe even some big teeths on the razor would make even more damage to the enemy and split it faster, so there is shorter fight and less risc of being contamined by some crack in your big armor.

And if possible, make them also half-inteligent, so they can avoid traps and nets, so Hunter must track them, find and engage them fast before they can find a cover in some deep hole or choose another way to avoid your hero :)

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The purpose of these weapons is in the visuals, not in the (imaginary) battlefield effectiveness.

With size comes weight, but if you are going for weight, there are better weapons (e.g. hammers) to bring that to bear on your opponent. For a sword, the purpose of the weight is to give the weapon enough kinetic energy to overcome the resistance of what it encounters mid-swing, e.g. armour, bones, that kind of annoyances. But there is a reasonable optimum to that. Once you have enough energy to cut through the typical stuff, more energy does nothing for you. And beyond a certain strength of armour, cutting is not the most effective attack anymore anyways (I'm ignoring piercing, as these weapons are obviously not made for piercing attacks). Basically, if you can dent the armour into the enemies vital organs, you don't even need to break it anymore.

Your hollow sword would retain the size of the blade, without the weight. But the weight is half the point. Size by itself does nothing, except increase your reach and as other answers pointed out, if you are going for reach, there are better solutions.

So in short, a giant hollow sword is a solution looking for a problem.

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    $\begingroup$ Most war hammers are not much heavier than swords - the difference is in moment of inertia. Most armor will resist swords being swung at them, otherwise no-one would ever have bothered with it. $\endgroup$ – Pete Kirkham Apr 11 '18 at 10:25
  • $\begingroup$ A rapier and a claymore are fundamentally different in weight because their design and fighting style. Neither is necessarily more or less deadly. $\endgroup$ – OhkaBaka Apr 11 '18 at 20:55
  • $\begingroup$ Pete, that was the point. If you add weight because it does something for your weapon, there are better ways to do it than just making the sword larger. $\endgroup$ – Tom Apr 12 '18 at 4:44
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From the perspective of torque, there are two problems with a really big sword.

  1. The sword is heavy.
  2. The sword is long,

Both make the sword difficult to hold parallel to the ground or to bring back up to take the next swing.

Assuming you are able to come up with a strong hollow structure, you still have the problem of length. So a hollow sword is better than a solid sword, but if the sword is twice as long, it will still be twice as hard to use even if it is the same weight.

There is some tradeoff potential there. Perhaps you make a sword that is two thirds the weight and half again as long as a regular sword. That would produce the same torque as the regular sword (because $\frac{2}{3}\cdot 1.5 = 1$). The formula for torque is

$$\mathcal{T} = mgr \sin\theta$$

where

$m$ is the mass of the sword.
$g$ is the acceleration due to gravity.
$r$ is the distance from the pivot (the wielder's hands) to the center of mass of the sword.
$\theta$ is the angle of the sword relative to a line perpendicular with the ground that runs through the pivot.
$\mathcal{T}$ is the torque.

Higher torque helps make the swing stronger as it goes towards the ground but makes it harder to lift the sword back up to take the next swing. It also makes it harder to hold the sword unless it is perpendicular to the ground.

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Nested and squished tetrahedrons would allow for a low density, high strength sword that comes to three points, like a triple-edged sword. A central spire could be put in so as to prevent crumpling and allow stabbing.

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Sharpenability will be a problem, since you will need to make sure there is enough material in the edge area to grind a bit off repeatedly. Even worse if you want a sharpening regime like that which is proper to real "samurai" swords - material is taken off the entire blade, not just the primary bevel.

Getting the heat treatment correct while making a sword like this would also be a challenge.

Also, you would have to be extra careful not to overheat it while doing any maintenance that involves abrasives: A hollow design would have very little heat sinking capability, and once you get most hardened steels to a few hundred centigrades, you permanently alter the temper (softening the steel) - even a pedal-powered, dry grinding machine could cause problems.

Real world, practical knife blades approaching much more than a meter in length exist, eg some versions of the japanese magurokiri (an oversized sushi knife used in butchering massive fish).

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Since you are in a fantasy setting anyway, why not make the material strong but light like elven chainmail? You can make a sword like Clouds (FF7) and it would still be light and strong.

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