A classic of sci-fi, metal band album covers and posters of high-school age nerds of decades past. Big ol’ sailboat floats in air (or space but let’s forget those for now). Assuming you somehow have buoyancy solved, what are the challenges of steering such boats with conventional sails and tackle? I am thinking that you lose the keel, for one, so you cannot tack the wind and you would need a much bigger rudder, but I really don’t know what I am talking about.
Isn’t steering really just a matter of directing propulsive force? Kinda-sorta? I’d think you’d have to figure out what is making your floating sailboat move, and what means you have of controlling that force.
You may want to look at the AD&D Spelljammer rules–they may have some semi-scientific theory on point…
If your sailing ship worked, then steering (i.e. controlling the direction you’re pointed) would be more or less unchanged. You might need a bigger rudder, but it would work the same.
But …
Everything else is a bust. What you have is ship-shaped balloon. It would simply drift with the air. A boat in water relies on the fact the air is moving relative to the water, and the ship can extract force from that difference. With a little cleverness, you can build a ship to do more than just drift directly downwind. Hence sails, rigging, and 5,000 years of nautical progress.
In the air, you have nothing for the ship to work against. So it’ll drift directly downwind at wind speed, period.
And that leads to the conclusion that the rudder won’t really work. IF the ship had velocity relative to the air, then the rudder would work as well as it does in water. But the ship won’t have any relative veloicty, and the rudder will be perpetually becalmed. So you’re not only drifting downwind, but you can’t even control whether you’re traveling bow first, stern first, or sideways.
Now a floating motor boat is a different matter. That’s exactly what a blimp or dirigible is.
The best way to see why you need two different media (for a conventional sailing ship, the water and the air) is the principle of relativity. No, not Einstein’s theory, just Galileo’s version. There’s some frame of reference in which the air is stationary, and in that frame of reference, there’s obviously not anything that’ll move the airship. So the airship will always be stationary in that frame of reference, too.
You could, in principle, use air for both of your two media, if you have a significant windshear. But windshear isn’t really something you can count on having.
Well, it turns out that I knew even less than I knew I knew. Why do you need a speed difference between air and water?
I get the fact that you need the surfaces to move relative to their medium to be of use. That the rudder needs to move through the water to steer the boat. I think I am lost after that.
I understood that the boat somehow “leaned” on the water so that the sails would work as a wing (as opposed to as a balloon going directly downwind), but I now realize I have no idea what that is supposed to mean.
Imagine a boat with only underwater sails. The underwater sails just move the boat in the direction of the water currents. In fact, relative to the water, the boat is motionless. So you can’t steer the boat, you can’t turn the boat, you can do nothing except drift. You might bet a tiny bit of steering by taking advantage in small differences in current on different sides of the boat, but your boat isn’t a boat, it’s a raft.
Now consider an all-air sailboat. You are in the same situation. You are carried in the direction of the air current. You can’t steer, you can’t do anything except drift on the wind.
With a conventional sailboat you take advantage of the fact that the air currents and water currents are moving in different directions. So when the wind blows in the direction you want, you can go that direction, and when it blows in a different direction you can use the friction of the water to change the direction of the force from the wind. But with an all-air boat you don’t get that.
A boat can move forward if the wind in blowing directly behind it (square sails) or if the wind is blowing at a right angle across the sails (creating a winglike effect). For any other combination of wind direction and heading, a ship is dependent upon the fact that it moves forward easier than it moves sideways. The keel of a ship is basically a device for providing resistance against lateral movement. By combining a keel with a transverse sail it is even possible to “tack”- move at an angle partially toward the wind, and then by repeatedly switching which way the ship is tacking make overall progress directly against the wind. The resistance of the keel against sideslipping is critical to making this work.
I’m reminded of a Horatio Hornblower novel in which his sloop Hotspur is tacking upwind trying to escape a larger and more powerful French frigate. The frigate slowly overtakes them because it’s hull and keel are deeper and provide more resistance against sideslipping, allowing it to sail closer into the wind than Hotspur can.
The only aerial analogy to this was a proposal by balloonist S. A. Andrée to use drag ropes to allow a balloon to sail something less than directly downwind, and even that was a bust.
In general, fantasy and science fiction tends to somehow overlook that a floating object has no resistance to sideways movement. Even for powered craft it’s a significant problem. For anything dependent on the wind, forget it.
Friction, essentially. When sailing away from the wind (running), things act essentially the way you’d expect. The wind pushes you. When sailing 30-45 degrees away from the wind (close hauled), the sail acts more like a wing. It provides “lift” sideways and forward. The shape of the boat prevents a lot of the sideways motion, because there is friction with the water. What you are left with is forward.
I suppose a big enough keel would work the same way in an air boat.
The ship is also dependent on that fact for the right-angle movement. If it weren’t for that effect, the ship would also start taking on a significant downwind component of velocity, and as it got closer and closer to matching speed with the wind, the “lift” force would become less and less.
I don’t see a problem here except for the fact that you’ll never sail into the wind. As long as your boat is double masted for and aft , you should be able to steer for at least a 180 degree sweep with the wind behind you. With the aft sail swung out in the opposite direction of the for sail , they can act as rudders and thus maintain vertical stability.
Because hang gliders use gravity as a form of propulsion. They are not just floating in the air, they move through it and can then use the airflow over the wing for control. The trade off is that the must constantly lose altitude unless they are in an area of rising air (windward side of a hill is a popular place to be.)
The problem is that there is no “wind”. Wind is just movement of air over the ground, if you are not attached to the ground then there is effectively no wind. For something that is airborne, wind is only relevant for navigating with reference to the ground.
Right. Because on a boat, your keel need only dip into a fluid medium that has a velocity distinct from the air - and, what’s more, is rather conveniently far, far denser than it. What you’re doing is juggling the force differential between the air and the water, for which you need a proportionately large sail and small keel.
Now replace your boat with one that is just barely afloat, having no freeboard (nothing above the waterline) and no sail. Your keel is now virtually useless as all it does is enable the water to push the hull around, which the water could do perfectly well anyway. That’s the position your airship is in. The only possible way to make progress is if you can somehow reach an air-mass which is moving relative to the one you’re floating in. Exactly how you then take advantage of the situation is left as an exercise for the student, but until you have some velocity difference somewhere in the picture, there is not much you can do except drift with the “tide”.
If you’re able to take advantage of a thermal or a ground wave, then you can do what hang gliders (and all gliders) do - trade potential energy for kinetic, since you now have an independent potential energy source. But without it, you’re scrod.
A conventionally rigged sail would pull you directly downwind. A sail rigged as an airfoil could pull you either plus or minus 90 degrees to the wind, depending on how it was oriented.
So if you combine the two techniques by using a pair of sails would you steer off the wind to the vector sum of the two effects, or would your ship just twist around and continue heading downwind?
Any kind of sail at all, no matter how you rig it, will just take you directly downwind. You need to have another control surface, in a medium that’s moving differently, for an airfoil sail to do anything.
Not only that but the sail itself will be doing exactly nothing. Having a sail is as useful as having nothing at all. Any object floating in the air is at best as maneuverable as a hot air balloon. You drift with the wind and the only way to have any control over where you go over the ground is by rising or sinking into areas where the wind is moving in a different direction.
Well, that’s true in an ideal steady state. But who wants to write their sci-fi airship story about a ship that never stops, never steers, etc? In particular, we know that sooner or later someone is going to be *chasing *somebody else.
Let’s say we have our air boat anchored in a stiff breeze. Clearly the sails fill and pull at the anchor. We lift anchor and the boat accelerates. The more sail we spread, the faster our initial acceleration and the quicker we get up to speed. In a chase situation, that’s a non-negligible advantage.
But, admittedly, we had been focusing on steering and on steady state speed. And, on reflection, I understand that the airfoil/sail combo is ineffective at stearing because the airfoil would also be a conventional sail so the whole ship would simply rotate to maximize the conventional sail effect. (Yeah, I know, once we’re matching the wind speed even that won’t be significant, but certainly when we are under acceleration he ship will have a preferred orientation.)
So, pitch the rudder. We’ll have to steer by seeking air currents at different altitudes. But when the pirates/sky cops/evil empire fleet appear behind us, I still want to spread some sail.
