# What would the aerodynamics of ships sailing on clouds look like (and thus their design)?

I'm currently working on a TTRPG setting where there is an ocean of caustic clouds surrounding the continents instead of a regular ocean. Tech level is roughly Age of Sail with some deviations due to differences in resource availability. In this world, there is a type of rock that exhibits incredible buoyancy when coming into contact with said ocean and the people living there have been building ships with these rocks as the centrepiece that keeps them floating on top of it. For reference, a rock with a diameter of ~5 ft. would support a fully loaded caravel.

From researching a bit, I realised that it's the resistance/difference of movement between water and air that lets conventional ships move in any direction that is not the same as the wind. This begs the question of how this could be achieved in this setting? And what the possible implications for ship design would be?

Now, I'm not above handwaving this pretty heavily 'cause this is more about aesthetic to me than about how realistic something like this is but I think following the thought could still end up making the vessels look more unique and lead to other cool ideas.

I did look at some other questions about airships but all of them seemed to concern ships that can move in three dimensions so I hope this makes sense as its own question.

(If anyone needs/wants more information about the world, I'm happy to provide but I wanted to keep this short for now.)

• Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
– Community Bot
Nov 11, 2022 at 12:48

## The Toxic Gas Needs to be Mostly Opaque

The reason the air 100ft above us moves in the same direction (usually) as the air next to us is because they are heated equally by the sun, but in a more opaque system, the surface gas will be notably hotter/cooler from day/night, and what's below it will be more stable. This means that the surface toxic gas will move against the sub surface toxic gas creating slower and often different motion at only slightly lower depths.

## Your "Buoyancy" Field Will Interact more with Deep Gases

Buoyancy is where the weight of gravity on a fluid pushing down around you puts a displacement force on it in excess of the weight of the floating object. Since the rock does not have a negative mass (which comes with all sorts of extra caveats), and the ship as a whole has a higher density than gas, it could not possibly work by means of buoyancy unless it is displacing a very large area of gas around the ship.

To achieve this effect, the rock needs to have a repulsive property pushing away a greater mass of gas than the mass of your ship. This means its field will push far into the cooler subsurface currents, and these currents will give an opposite and equal reaction providing you with the resistance you need.

• That makes a lot of sense, I suppose if there was a centreboard that goes fairly low, this could create enough resistance for steering? In the end it doesn't have to make perfect sense, but this is good to know, thank you! I used buoyancy to just mean that it floats on the cloud layer but I can see that its actual meaning doesn't work in this context. Your way of explaining it does provide me with a very interesting consequence, which is that the quarries where these rocks are mined would be largely cloud free. Thank you for the in-depth explanation and the graphic, that helped a lot. Nov 13, 2022 at 12:47
• @ThaddeusCahill A center board would have to be huge, but you may not need a centerboard at all. If you place stones in a line across the length of the hull, then the bubble will be longer than it is wide which should provide a similar outcome. Nov 14, 2022 at 15:06
• I figured as much and was already considering configurations involving multiple stones for added stability so I‘ll explore this some more, good point. Nov 15, 2022 at 14:31

If the caustic clouds do not move with the wind, this should be possible.

The reason why good sail ships can move in almost any direction is that they can move much easier forward than sideways (for details see https://en.wikipedia.org/wiki/Forces_on_sails). Especially small sailboats have structures specifically designed to make sideway-movement essentially impossible by using water-resistance (e.g. centreboard, https://en.wikipedia.org/wiki/Centreboard). If your clouds don't move with the wind (or much slower), they can provide the same resistance as water assuming the centreboard and rudder are much bigger.

This would mean that all changes in design are subsurface, the part of the ship above the cloud would just like any convential ship.

If your clouds move with the wind I don't see any possibility of going in another direction than the wind (except maybe by a few degrees).

• The concept of the centreboard really helps with making this make sense, thank you. I do like the image of ships in the wharf or in harbour looking strange when seeing the hull but looking mostly normal while sailing. Nov 13, 2022 at 12:49