In my world, there are vehicles that can spatially "lock" themselves to a single altitude. The locking mechanism is a magical core, combined with runic metal bands extending around the ship, remaining almost completely static at a single altitude. The rest of the ship is built off of these bands, and any other material except the bands and core is subject to gravity as normal.

The Issue

So with the above established, if the inhabitants of the world don't have the ability to actively propel the airships, (that is, don't have propellers or engines) what could they use?

I was wondering if the up-down resistance from the altitude lock could be used in order to implement sails, as I have read about how most airships lack the resistance generated by the water and thus cannot them.

Edit: The ships are locked to a specific altitude that is a couple kilometers above a gas-giant(ish) planet, with floating islands resting on the same magical field. And the lack of mechanical propulsion is due to the world being post-apocalypse. The unknown event that shattered the planet, and turned it into a gas giant erased most known information, but the creation process for the spacial locks was retained.

  • $\begingroup$ How magical is this world, are there wizards that can create wind? $\endgroup$
    – Rob
    Commented May 2, 2019 at 16:33
  • 1
    $\begingroup$ So the airship remains a fixed distance from...sea-level? The center of the planet? Or does it remain xx number of meters above whatever point its currently over? Its confusing because you state they are locked at a single altitude but then later mention using up-down resistance. $\endgroup$
    – Tim
    Commented May 2, 2019 at 16:37
  • $\begingroup$ What do you mean by "don't have the ability to actively propel the airships"? Not having engines is one thing, but can they also not have a guy sitting at the back of the airship waving a fan? $\endgroup$
    – Giter
    Commented May 2, 2019 at 17:45
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    $\begingroup$ (a) Magic solves all problems. Why are you avoiding it? What is the real reason you want a non-magical solution? (b) This sounds like a "technology dichotomy" in that they have air ships (yes, via magic) but don't have props. Why don't they have props? (c) Have you done research into tacking into the wind? Because this sounds like sailing (wind on the Z axis rather than the X or Y axis thanks to the magic). $\endgroup$
    – JBH
    Commented May 2, 2019 at 19:13
  • $\begingroup$ @JBH, as noted below you can't tack into the wind in an airship. Sails won't work. $\endgroup$ Commented May 3, 2019 at 5:47

10 Answers 10


Yes, you can use sails but they would not look like normal sails and they would work differently.

Basically sails work by ships having lots drag in the width axis this means that the component of wind on that axis can be turned to go backwards. This changes the momentum of the wind on the length axis which creates an equal force to push the ship forwards.

In your case instead of high drag on the width axis, you have functionally infinite drag on the vertical axis. This means that your sails need to work on the vertical component of the wind. That means either the rising or descending air currents.

Birds and gliders make use of such currents, so this is definitely possible. The really difficult part is figuring out what the air currents in your setting would be like. I certainly have no idea. In our world these currents are largely driven by the shapes of the underlying terrain but in your case this would only apply over the islands. Presumably the currents would come from the currents in the underlying "gas giant".

Note that if you can alter the altitude, which you really should, you can easily fake vertical air current by changing altitude.

As for the appearance. The sails would probably be on the sides of the ship, so that the hull does not obstruct the air flow. Alternately or additionally, for stability it might be better to have a catamaran arrangement and you could then have sails between the hulls. Additional sails would be added in front and behind the hull. In any case the ships and sails would extend horizontally instead of upwards. So the ships would be quite flat. With maybe some "castle" structures on the deck for defense and to give officers clear vision of the crew.

The spars the sails attach to would be in the direction of the width axis. They cannot be along the axis you want to move on or the direction the air current you want to use is on, so that is actually the only usable option. Normal ships can use wind from sides or behind but your airships only have usable drag on a single axis so they are more limited. That said if the wind is going in the right direction you'll drift along so you are not actually losing anything apart from rigging complexity.

In fact, you can probably assume that shipping routes will generally be in the direction of the wind and that the actual sails are needed for control and to move sideways to the main air current.

The sails the extend from the spars on the length axis. In practice they would attach to the spar in the forward direction and the "ropes" on their backward edge would attach to the next spar. This would allow the back edge to move in the direction of the air current while the forward edge stays still. This forces the sail into a curved shape that turns the air current backwards and the ship gets forward momentum.

Control would be by trimming the sides differently. The ship will turn away from the side with more sail. In practice the ship would generally be directed 90° from the main wind, since the sails would really only be useful for movement sideways in the wind. Near islands where strong updraughts would exist they would additionally be useful to stop the ship in harbor or to move the ship away from harbor.


Answering the comment by JoeBloggs brought few rather important aspects of this to my mind, so I'll add them here as they are kind of crucial.

Since the ships only move, and only can move, between islands that also float with the same technology and the setting does not have terrain that would make the winds highly variable or unpredictable, your destination would almost always drift along the exact same wind the ship and island of origin drift along. There would be turbulence and storms to mess things up but in general you can ignore the drift along the wind as it does not move the ship significantly relative to the nearby islands.

This obviously makes this type of sailing lot more convenient and practical since the motion the sails extract from vertical air currents would not be just the controllable part of the ships motion, it would probably be the major part of the motion relative to islands and other ships. This would promote the ships from primitive sailing ships to something more similar to early steamships that do not need coal. A big upgrade.

Also, navigation needs to be considered. When your destination is constantly moving with the wind using stars for navigation is not very useful. Even if the winds are very even and predictable the errors would rapidly add up and you wouldn't be able to predict the position of your destination relative to stars for very long or very accurately.

So you'd need to supplement your navigation by observing air currents, birds, and probably spyglasses and lighthouses.

  • $\begingroup$ So if I understand correctly, sails that only use vertical air currents, alongside the resistance from the lock would work? Or are you saying the sails should disregard wind from the sides and only use wind from the back of the ship. $\endgroup$
    – Janrien
    Commented May 3, 2019 at 14:44
  • $\begingroup$ This was my initial thought, but it only works if the wind is primarily vertical or any motion gained from the sails is utterly negated by the lateral wind speed. Like you noted this is more useful for adding sideways motion to a ship that’s travelling with the wind than for performing tacking operations. $\endgroup$
    – Joe Bloggs
    Commented May 3, 2019 at 16:21
  • $\begingroup$ @Janrien The ship drifts horizontally with the wind, only the vertical component of the air flow can be used by sails. Or rather does anything that would need sails. Drifting can be done without sails... You are of course free to change the specifications of the lock for better performance. It might actually nice to have some locks that they do not know how to duplicate that give more control. Something like altitude control would be very powerful for ships like this and pretty obviously something that people originally creating these would have been able to do. $\endgroup$ Commented May 3, 2019 at 17:33
  • $\begingroup$ @JoeBloggs It doesn't negate it, the drifting with the wind and the motion sails get from vertical air flows are additive. I get what you mean but early sailing ships had fairly limited ability to maneuver as well. They still were effective methods of transport. Another aspect which is bit off-topic but probably should be considered is that the islands drift with the wind as well and that the horizontal component of the winds would probably be very even due to lack of terrain. So the extra speed from the sails might be almost all of the motion relative to destination. $\endgroup$ Commented May 3, 2019 at 17:40
  • $\begingroup$ @VilleNiemi: true enough at being effective modes of transport even without the ability to sail against the wind. Who knows: if these floating islands exist in a constant strong updraft then this could be the dominant mode of locomotion (Wingsuits, anyone?) $\endgroup$
    – Joe Bloggs
    Commented May 3, 2019 at 17:54


These could be implemented in a couple different ways; the simpler would be a pretty conventional oar (scaled similar to those on a Roman galley), with the blade replaced by a cloth sail surface. The rower would (as rowers always should) rotate the oar so the blade is edge on to the wind during the recovery stroke, and hold it flat to the air on the power stroke.

An operationally simpler (if mechanically more complex) version, suitable to be powered by animals walking on a treadmill or wheel, would be oars with sail surfaces hinged in the center, so they fold when pushed one way and open when pushed the other (drive these with a crankshaft) -- potentially simplified into cups like those on an anemometer (or elongated for more working area).

I'd expect a few generations of skyships powered this way would lead to discovery of the principle of lift, followed by the airscrew (fabric covered, low RPM, more or less like the ones on the da Vinci helicopter or the Langley Aerodrome).


NOTE: this answer was made *before* the OP stated that there was no ground on the world in question

So, I'm going to take the unconventional route here, and suggest that your airships be towed by animals on the ground.

Thank about it. You don't need well metalled roads, you don't have to worry about ground friction or axle friction, you don't need canals or rivers, you don't need to worry about water resistance. You just need strong enough animals to be able to overcome the inertial mass of the airship and its payload and oppose some amount of wind. On really windy days you'd probably have to just lay anchor. If you were particular clever, you might be able to domesticate big, long-legged, strong animals to do the hard work for you... just imagine your aerial fortress or palace being towed around by a herd of caribou or elephants! Or on a simpler scale, imagine towing your hut behind you, tying it up for the night and resting safe in your bed out of reach of predators and pests.

For a more eccentric option, you could consider grappling hooks. Something like a big anchor, flung out by catapult or ballista, tethered on a long rope. Haul the rope back in with a capstan to move your airship towards the anchor, pull it up and repeat. This might be a good way to make headway against the wind regardless of what other kind of propulsion you're using.

If you're near the coast you could, in fact, sail. Put down a long keel, be happy about the lack of wave motion and the absense of sea sickness and the sheer difficulty of being sunk. But that's less awesome than moose-laputa, as I'm sure everyone will agree.

  • $\begingroup$ Ah, probably should have stated above that there isn't any ground, a good example would be that there are a bunch of floating islands above a gas giant, can't really stand on anything $\endgroup$
    – Janrien
    Commented May 2, 2019 at 21:15
  • $\begingroup$ @Janrien Yeah, thats, uh, a pretty major issue right there. Oh well. $\endgroup$ Commented May 3, 2019 at 10:07
  • $\begingroup$ Floating islands could still be connected by strings of waystations though, or magically-fixed chains that the airships can drag themselves along... $\endgroup$
    – Joe Bloggs
    Commented May 3, 2019 at 11:22
  • 2
    $\begingroup$ Disclaimer: One of the earliest worlds I built was a floating continent over a gas giant. The trailing edge had broken into an archipelago of floating islands held in place by giant chains. Due to the winds and ‘magical effects’ holding the world up the only way airships could traverse the archipelago was anchoring themselves to the chains and laboriously dragging themselves along link by link. Naturally it became the nation of wizards that could control the winds, since they had a massive tactical advantage over the other nations. $\endgroup$
    – Joe Bloggs
    Commented May 3, 2019 at 11:27

Interesting question. At first I thought the answer was yes, then no, and now I’m unsure.

Even if you can use the bands as a ‘keel’ of sorts, your airship has an extra degree of freedom that will prevent you from tacking into the wind. Basically a ship can’t move up or down because of the ocean or side to side because of the keel, so when the wind hits it has to go forward. Your ship can’t move up or down, but can still move both side to side and back and forth.

But fear not!! You have options!!

If your magical mcguffins can be turned on and off relatively quickly then you can use their spatial locking to drag your ship along mechanically (albeit jerkily). Imagine you have one of these doohickeys strapped to an oar. Push the oar down, lift the ship up and toward. The vertical resistance it offers will give you something to ‘push’ off to move the ship.

‘Aha!’ You May cry ‘but Newton says you will just move your ship back and forth that way!!’

Well, no. You can turn off the vertical resistance when moving the doohickey forwards, giving yourself less work to do to move the it back to its original position. That means you can incrementally push the ship forward by stroking the oars down and back (with added resistance) and up and forward (with less).

This effect is further enhanced if you have a large, fixed hull that will further work to oppose the downward motion of the oars: basically you push two of your altitude locking doohickeys against each other to generate some small thrust, then turn one of them off, reset, and go again. It breaks physics a little, but hey: Magic.

Disclaimer: I’m very tired and haven’t had chance to draw the force diagrams for this. It’s entirely possible it doesn’t work...

  • $\begingroup$ Now that is certainly an interesting solution! It may not work economically in the fiction unfortunately. I was thinking that the spacial lock would be one of the more expensive components of the ship, so using a lock on the end of an oar would be a bit of an odd choice $\endgroup$
    – Janrien
    Commented May 2, 2019 at 21:41
  • $\begingroup$ @Janrien: I thought it might be. You could use a similar solution with only two ‘hulls’ attached to opposing points on a large wheel, but that’s even weirder. $\endgroup$
    – Joe Bloggs
    Commented May 3, 2019 at 9:55
  • $\begingroup$ An altitude-locking "paddleboat" might be an interesting concept... $\endgroup$
    – Ruadhan
    Commented May 3, 2019 at 13:44
  • $\begingroup$ IIUC, you would need friction against the "surface" for the oars (punt poles, actually) to work. As the "surface" is a force field, I'm afraid that it is quite frictionless. $\endgroup$
    – Ralf B
    Commented Jul 26, 2019 at 21:16
  • $\begingroup$ @RalfB: the trick doesn’t require friction in the horizontal plane: it works be exploiting the infinite vertical friction you have available and locking some component of the ship off on a vector such that you can convert vertical motion into lateral motion. Sure, the locked bit is going to move back towards you, but as long as you extract some resistance from the vertical component it really doesn’t matter. $\endgroup$
    – Joe Bloggs
    Commented Jul 27, 2019 at 12:04

Problem : as written it doesn't work at all as an airship.

In my world, there are vehicles that can spatially "lock" themselves to a single altitude. The locking mechanism is a magical core, combined with runic metal bands extending around the ship, remaining almost completely static at a single altitude.

So far so good ...

The rest of the ship is built off of these bands,

So the core and the "rest of the ship" can stay at that altitude. What this translates to is that the core and "rest of the ship" have an effective density equal to the surrounding air at all times.

This is what real airships aim for : an average density equal to local air density at their flight altitude.

and any other material except the bands and core is subject to gravity as normal.

Which is why it won't work.

Only the "core materials" are going to produce an average density equivalent to local air density. When you add the payload (cargo etc.) their average density will raise the average density of the ship above that of the local air. It will no longer be as light as air on average, but heavier than air on average and heavier than air means it's falls.

Put another way, the magic supplies exactly enough upward force to balance the weight of the core and "rest of ship". When you add payload that has no upward force to lift it, it just drags the whole thing down with the payload.

So it won't work.

To work the magic materials will have to extend their magical properties to anything enclosed by them (or something similar to that).

The question of propulsion is mute until you address that in some way. Of course if you do fix it we reach another problem :

If the inhabitants of the world don't have the ability to actively propel the airships, (that is, don't have propellers or engines) what could they use?

Firstly it's inconceivable that they could construct a vehicle like this and not be able to propel it. As someone said, a guy with a fan can propel it (well, enough guys with fans could). Steam could, and all you need to make steam is water, a hot fire and a pot for the water (steampunk magic beckons :-( ). Sails could work and it's not remotely realistic to say they would not have sails - heck even the propeller is inevitable to find as it's essentially just a rotor from a windmill - they'll figure these things out very quickly once they have a need to.

I was wondering if the up-down resistance from the altitude lock could be used in order to implement sails, as I have read about how most airships lack the resistance generated by the water and thus cannot them.

The altitude lock provides no up-down resistance. There's nothing to "translate" into an impulse.

Sails are just sheets of something as light as possible but strong enough to not tear stretched out to capture the wind.

Lastly it's hard to imagine they have magic to float something but no magic to propel something. This is extremely selectively magic.

So I think the whole concept is full of flaws as is.

  • 1
    $\begingroup$ I think you misunderstood the above. Density in this case doesn't factor into the equation. The airship locks itself magically to a plane of force that encircles the gas giant of a planet. The ships are not able to travel up or down, and can be as dense as the metal bands can support them. Think of the ships floating on a magnetic field, or on the surface of the sea, without any water. $\endgroup$
    – Janrien
    Commented May 2, 2019 at 21:19
  • $\begingroup$ As you describe that it's not like floating at all, it's more like a railway - they are "Locked" to the "magical tracks". It has no relation of air travel as we know it or floating - it "depends" on a magic surface they travel on - no up, no down. You need to add this to your question as it's not at all clear that's what you mean. It still raises the question of what prevents your culture from using any of a dozen normal propulsion systems. $\endgroup$ Commented May 2, 2019 at 21:27
  • $\begingroup$ Sails are useless in an airship except to go downwind. $\endgroup$ Commented May 2, 2019 at 22:03
  • $\begingroup$ @KeithMorrison Sail boats have been traveling into the wind for a long time. Watch this video. $\endgroup$ Commented May 2, 2019 at 22:24
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    $\begingroup$ It might be locked in a vertical position, but it's not restricted in a horizontal position because the air surrounding it is, roughly speaking, uniform, and all moving in the same direction at the same speed. Boats can tack using a sail because the air and water are different, and the boat exploits those differences riding the interface between the two. Same thing for an iceboat, or someone kite-skiing. In all those cases you have the movement caused by the forces generated due to the water or ground not moving in the same direction at the same velocity as the air. $\endgroup$ Commented May 2, 2019 at 22:57

Okay your conditions are a little strange but I think they can extrapolated as basically you can produce lift but you have no way of produce thrust. This is a solved problem today because that's how gliders work.

Gliders basically trade altitude for speed by changing their attitude to a certain angle. Glider pilots have to rely on types of rising air to regain altitude and maintain forward momentum but your conditions basically mean you have as much lift as you want so you can continuously have altitude which you trade for speed.

"If the glide ratio were the only factor involved, gliders would not be able to stay in the air nearly as long as they do. So how do they do it?

The key to staying in the air for longer periods of time is to get some help from Mother Nature whenever possible. While a glider will slowly descend with respect to the air around it, what if the air around it was moving upward faster than the glider was descending? It's kind of like trying to paddle a kayak upstream; even though you may be cutting through the water at a respectable pace, you're not really making any progress with respect to the riverbank. The same thing works with gliders. If you are descending at one meter per second, but the air around the plane is rising at two meters per second, you're actually gaining altitude."


  • $\begingroup$ It won't work in the OP's case because the magical lift apparatus can't change altitude. $\endgroup$ Commented May 3, 2019 at 5:49

Dragon farts

Every action has a reaction. All you need is something that pushes backwards to push you forwards. Now of course dragons aren't known for the power of their wind but that's all you need to think about.

You still seem to have some sort of industrial base for building these airships, so we don't need to depend much on any other sort of magic, though magical weather control may be the most effective. Perhaps what you want is a pedal powered fan on each side. You're not going to be going fast against the wind, but it's a simple construction.


Why rely on fickle wind filling your sails, the muscle of your men for oars, when you can just create wind yourself?

Use magic (since it's in your world) to create/help create a propulsion system based off blowing air behind the ship as a thruster.

You could still have technology in significant use to direct or steer the wind created by your device.


TL;DR: Drop a nozzle under a ship and use water as reaction mass to push the ship. If it runs out too soon, only huge hand-cranked propellers will save you.

As borne out in the discussion, there is no way to obtain "free" propulsion in a controllable direction through the interaction of wind and forcefield resistance, if the forcefield is orthogonal to gravity (i.e. horizontal) and frictionless.

So in general, the answer is no. You cannot propel these ships under such conditions. It is necessary to relax the stricture that the locals "don't have propellers or engines". Engines are the more challenging thing to build, but propellers are easy---windmills have propellers. They are big propellers, but that is what you want: big propellers turning slowly, rather than small propellers spinning rapidly. You could drive these large propellers by muscle power, human or dray animal.

Another way of getting propulsive power is by using reaction mass. Note that these airships don't have a major limitation of earthly aircraft: They do not need to be lightweight. Thus you can take along a big tank of reaction mass---water would be best, as it can be easily loaded from the floating islands, and will be replenished at the source by rainfall.

Next you will have to eject the reaction mass in such a way as to give it the maximum practical horizontal speed. You can do it by gravity. Mount a curved tube underneath the ship. the entrance to the tube is vertical---a simple drain opening in the bottom of the water tank. The exit is horizontal, narrowed to a nozzle, and the optimal shape of the bend---the trajectory---is a brachistochrone, but a plain quarter-circle elbow is good enough. Now to maximize the propulsive power from a gallon of water, you need to make the height between the inlet and the outlet as large as possible. The energy per unit reaction mass is directly proportional to the height the water descends inside the tube.

So the airship would have no masts above, but a comparably huge dong beneath, facing backward but twistable somewhat to the sides for steering. You could pour the water out at a constant stream, just enough to balance air resistance and give a constant speed. But it would give you much better travel time if you pour it out all at once, attaining a high speed right away; then coast to the destination along the forcefield. In practice, too high a speed will give excessive air resistance, so the best way is a compromise; pour out until you reach an optimal speed, then periodically squirt some more to maintain this speed.

All this is made more interesting by the need to keep some spare reaction mass to overcome adverse winds, or to change vector to run from pirates or a storm. How much do you keep spare? It is more economical to use it all up on a trip. And if you run out of water in the middle of nowhere, you will need to erect these windmill sails, and put your backs into the crank in order to limp ignominiously to a port. :-) Or spread out a big funnel, and hope for rain.

And finally, a compromise: a boat driven by windmill propellers, with a long rope looped around the propeller shaft, a bucket on each end. You pour water into one bucket, and as it descends the length of the rope, it drives the prop. At the end of the rope, it stops with a jerk, tripping a release to drain water from the bucket; you twist the propeller sails around (it will be rotating the other way now) and pour water into the other bucket. Keep repeating. The longer the rope, the more miles you will get out of a gallon. But there is an upper limit to rope length, because the long rope will be dragged by the wind.


The only argument against sails for airships is that they would flip them whereas ships has a natural tendency to stay upright. If your ship has 2 cores locked at different altitudes, it's vertical alignment would become rigid so it could not be flipped. In this case, sails would work very well.

  • 1
    $\begingroup$ Sails require a partially opposing force to react against. With a ship, it's the side force on the keel or hull. With an airship, there is no such side force available, unless the vertical span of sails and keel are immense, allowing you to take advantage of wind speed differentials. Even then, without the ability to control altitude, you can't go up and down to find the wind shear, and without it, you just "blow in the wind." $\endgroup$
    – Zeiss Ikon
    Commented May 2, 2019 at 17:35

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