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In this world, a nation's army uses sailing ships to travel across a desert. They only travel at one time of year, during which a very powerful windstorm blows from the East. They leverage the wind to sail west with the wind, and do not need to navigate or change course extensively. Their primary focus is on speed of travel.

The army uses two types of ships:

  • Trimarans, which are large enough to carry ~20 men, and are very fast and maneuverable
  • Large, multi-masted windjammers that carry the bulk of the army (perhaps ~300 per ship). These are flat-hulled and have giant wheels on the sides, similar to those of a watermill, that help with load bearing and forward propulsion.

The desert is largely made up of soft rolling sand dunes. Steam or internal combustion engines do not exist in this world. What other features might such ships have to make them a viable transport option in this environment?

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    $\begingroup$ First, you would need to explain (or handweave) the solution for friction. For landships to be any practical, they need a very good bearings. Ball bearings are too advanced for a preindustrial society. So, you would have some kind of "magic grease"? $\endgroup$ – Alexander Jul 6 '17 at 0:33
  • $\begingroup$ uhm, wheels i guess? $\endgroup$ – Fl.pf. Jul 6 '17 at 8:24
  • $\begingroup$ Sand would grind a wooden axle to nothing very quickly, and friction will make a metal one seize from heating. hence Alexanders statement. $\endgroup$ – John Jul 7 '17 at 1:26
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Land Sailing is a thing in the real world. You might want to take a look at how some of those vehicles are built.

It's not a perfect analog for your situation, though, since actual sand yachts tend to a) be small, one or two-person vehicles, and b) be designed for running on smooth, flat surfaces- not soft desert dunes.

If they are looking for speed, note that sailing vessels can actually move faster at a steep angle to the wind than they can running directly with the wind. This may or may not be relevant depending on the advancement of these people's sailing technology, however.

These vessels are going to have to overcome two major challenges around which they will be designed: minimizing friction, and avoiding sinking into soft sand.

To minimize friction, none of them will want to let their hulls touch the sand at all. Pushing through sand just isn't practical, so all of these, not just the giant windjammers, will be running on wheels. Even advanced real-world racing sailboats try not merely to streamline their hulls, but to actually avoid contact with the water at all where possible, using hydrofoils to lift the hull completely out of the water at high speeds. The trimaran design thus actually makes a lot of sense, since you can angle the struts to lift the main hull relatively far above the surface, with wheels mounted on the secondary hulls.

You'll also want to minimize friction of the wheel bearings. I don't know if it would be absolutely necessary, but contrary to a comment on the question, ball bearings are not entirely out of reach. Modern, high-efficiency metal ball bearings would be impractical, but a pre-industrial civilization that can build land ships in the first place would easily be able to produce wooden ball bearings, potentially even with metal races. Larger wheels, which do not have to turn as quickly, may be desirable to reduce the friction load on bearings.

To avoid sinking into the sand, you'll want to maximize the load-bearing surface. Note that this is in conflict with the desire to minimize friction. Once again, however, the multi-hull design shows promise, since with wheels on two auxiliary hulls you can spread the weight out over a wide area. The major design impact here, however, will be on the design of the wheels themselves; specifically, you'll want wide wheels. If they can produce flexible rims somehow, that would certainly help, although I'm not sure how you'd manage that without modern air-filled tires. So, large and broad is good, but "like a watermill" is not so good, if that includes the gaps and paddles that are meant to bite into water/sand.

Rather than having a rudder, like any other wheeled vehicles these are going to need to be able to rotate (some of) the wheels for steering. For the big windjammers, I'd expect some kind of massive bar-and-pinion arrangement linking either the two front or two back wheels, assuming there are four wheels total- you won't want to steer all four wheels, because that introduces unnecessary complication. Six or eight wheels might be better, in which case you would want to steer both the front and back sets; although this requires more steerable wheelmounts, each one can be weaker, since it's carrying less of the total load. Steering control would be accomplished with a capstan that has a rope, or several ropes, wound around several times to ensure proper friction engagement with the wheel, and with the free ends lashed to the steering bar.

Arranging to mechanically coordinate the wheels on different hulls of a trimaran would be overly complicated, since you'd want the tie-bar to be angled up and built in sections, rather that passing under the main hull and potentially hitting sand. Since these are intended for maneuverability, you can simplify the design by simply requiring a separate "driver" for each steerable wheel (or pair on a single hull), and relying on extensive training to ensure they all remain synchronized. If you have, e.g., 8 steerable wheels, two on the front and back of each secondary hull, with four "drivers", then you can pull off very sharp turns, drifts, and strafes fairly easily. Steering in this case could be simplified, since each "driver" does not need to shift nearly as much mass, and could involve a simple handle attached to the tie rod, but a capstan or vertical steering wheel arrangement with rope attachments would provide significant advantages in terms of mechanical advantage and stability.

On the trimaran, it may make sense to have barrel-roller wheels centered underneath the secondary hulls, so they don't get stuck if they high-center on a dune or something. These would be intended to remain off the ground most of the time, to minimize friction.

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  • $\begingroup$ Well done on the link to land sailing. I think this answer can be improved by expanding a bit more about wind angle and tacking - especially the fact that a vessel may use side-wind to sail faster than the wind (almost impossible to do dead-downwind, but see the Blackbird land yacht which use wind energy to power its wheels). $\endgroup$ – G0BLiN Jul 6 '17 at 13:38
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    $\begingroup$ The heavier you make the "ships" the ,more likely they are to simply bog down in the sand. dune sand is extremely loose unlike the beach sand aéroplage normally operate on. $\endgroup$ – John Jul 7 '17 at 1:34
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Are you fully committed to wheels?

I suppose they're only working because of extremely light weight construction but I thought aboute Theo Jansens strandbeests. https://www.youtube.com/watch?v=MYGJ9jrbpvg

They can be fairly fast when the wind is blowing. A bit of magical handwaving and suspension of disbelief (adapting the insect like leg system for a trimaran structure) should allow them to be practical for troop transport. They would also be (at least in theory) less sensitive to wear and tear than a setup with very few large wheels as a couple of legs can break down without the crawler losing balance or speed.

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I suggest:

  • Deal with gravity. Maybe borrow from kite surfing and/or dirigibles to lift the ship up. Perhaps they have a gas that is better than hydrogen.
  • Deal with friction: Continuing with kite surfing, instead of the whole hull in the water - they use a fin - in this case a sand-skid or dual skid (catamaran-like)
  • Use natural phenomenon: perhaps there are strong winds at certain altitudes, or sand channels that are very low friction.
  • Save on weight: high-tech materials and hex- or triad- construction of panels for strength.
  • A way to adjust balance left/right fore/aft to keep weight on the proper parts of the skids for speed, steering.
  • Sand shields: scratch resistant, transparent panels that channel the sand around anyone on deck
  • Air filters to keep dust out of the interior. Probably two door layers to keep sand from blowing in when people come inside.
  • Embark/disembark doors on side and bottom or rear (like some aircraft)
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    $\begingroup$ No gas is better than hydrogen. Only pure vacuum is, but requires heavy enclosure. Low friction sand is handwavium, too. $\endgroup$ – Mołot Jul 6 '17 at 6:40
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Your first problem is weight

The bigger the "ship" the heavier is has to be to maintain its own structure. Anything big on soft sand is a nightmare. If you want them to move fast then give them kiteboards.

Your second problem is wheels.

Ideally you don't want wheels, moving parts and sand don't mix. They also focus all your weight down to a very small area. At low speeds on hard sand perhaps, at high speeds you want to be thinking skis. It's possible to ski/sandboard on softer sand than you can drive on, like the downwind side of dunes.

In summary

Lose the big multimast, make the 20man ships the big slow cargo carriers taking them down to a crew of 3. The bulk of the group should be kiteboarding for speed and maneuverability with maybe a few 2man land yacht types with skis alongside their wheels so they don't bog down too much.

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  • $\begingroup$ ski will be ground down to nothing on sand, so they had better be replacing them a few times a day. $\endgroup$ – John Jul 7 '17 at 1:35
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/ They only travel at one time of year, during which a very powerful windstorm blows from the East. They leverage the wind to sail west with the wind, and do not need to navigate or change course extensively. Their primary focus is on speed of travel./

Since you do not need to steer and you will be blown by the wind, you can get up off the sand and out of the dunes. You can use hot air balloons. At elevation there will be steady windspeeds and less sand. You will have less friction. You will move at the speed of the wind. Snakes will not bite you. Since you do not need to steer you do not even need dirigibles. Just get up there and go.

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If you're talking about wind strong enough to move a land ship, moving through a desert covered in dunes, there's going to be a MASSIVE amount of sand in the air. If your ships aren't very fast moving, they'll quickly be buried.

Also, as someone who loves to sail, I'm trying to imagine what it would be like to sail through dunes. The wind would be so impossibly unpredictable I'm not sure it would be navigable unless your sails were extremely high up. I would suggest that your ships include a spinnaker and have it fly quite high above the dunes. This could have the added benefit of adding lift to your ship, making it less heavy and less likely to sink into the sand.

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Assuming you are not looking for hard science answers you could have the winds cause the sand particles to move over the dunes and create a negative charge, then the ship's hulls could be negatively charged and 'float' over the sand like magnets with the same pole facing each other.

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