The difference, I think, is that the trim and Lift crystals are not actual thrusters, they are anti-gravity pumps.  They can tilt a vessel by applying less lift to a port-side trim crystal than the starbard-side, but the forces are always acting only in the vertical plane in direct opposition of natural gravity, and so they cant actually apply force in a horozontal direction.  I think the horozontal rotation (ie "Yaw", for those that dont want to look it up like I had to :p) comes from teh interplay of the "Control planes" (wings) and the asymetric pulling from the various Etheric Webs.

That's if you're assuming that lift and trim crystals work in precisely the same manner, which I'm not. But you're certainly correct to point out the control planes; while they wouldn't provide the same order of magnitude of effect as a keel in a denser medium, there'd certainly be torque available.

Because to turn a ship left/right you need to give the crystals the ability to move sideways (not just up-down).  And if that's the case, then this means you can actually use the crystals themselves to move the ship forward.

Think of them as the attitude thrusters on a space vessel: they can move the ship, but they tend to provide a few orders of magnitude less thrust than the actual propulsion system. They're the reason I admit I'm making assumptions about the attitude crystals: any airborne vessel needs to have a way to change its attitude in three dimensions and not just move.

That's what I said when I suggested that the crystals act as a large gyro, but Shecky said no.  I'm trying to figure out what other alternatives there are.

Lift crystals only negate gravity's effect; it's the attitude crystals that cause the ship to pitch/roll/yaw and can therefore provide a differently vectored force. Still, considering that air currents are in three dimensions and not just two, I can see how setting the lift crystal to provide downward or upward force could play into that as well.

So we're saying that crystals alone are sufficent to keep a ship in a specific direction?  Kinda like a massive gyro?

No, the combination of webbing (to "catch" the etheric current) and the lift and attitude crystals would do that.

The point I made (perhaps not clearly enough) regarding airplanes was that they move with the air mass, in addition to their progress through it, and that this movement must be taken into account in determining the flight path over the ground.  Of course this would be inapplicable regarding the comparison you mention, and which I didn't make.

I'm willing to concede that theoretically the mass may never reach 100% of the speed of the air mass, but approach it asymptotically.  But practically speaking, I consider it close enough to make sails pointless for motive force for at least a large mass.  As long as you have a force (the motion of the air mass) acting on a mass, it will continue to accelerate.

Regarding static wind speed, yes, that was assumed to simplify the example; the effect of changes on a very large mass was considered negligible for the purposes of that example.

It's not close enough. I'll grant that moving with the air current cuts down on the drag from the main body, sure. But there will always be far more coupling between sails and air than between hull and air, and when the ship as a whole has reached its maximum speed in the air current, that gap (although certainly reduced) will never entirely zero out...and in the meantime, the ship is still moving (ergo, sails as motive force work), and there will still be at least breezes on the deck. Gale-force winds, not so much, but current shifts will increase perceived windspeed on the deck.

Just because it works differently from ships on the sea in no way means it doesn't work. To address your original post:

So how do sails propel these airships, since there's no "wind" within an air mass?

Perhaps it's revealed later?  I just started this book.

...they do propel the ships. And there is perceived air movement.

I can see density and inertia coming into play when we talk about the time it may take to accelerate to the speed of the air mass upon departure from the surface, but eventually an aircraft will reach that speed.  Every airplane that flies utilizes that speed in calculating its flight path. 

If you wish to postulate a mass so large that the airship will not reach the speed of the air mass during the time of its flight, then I'd expect that any sails would have negligible effect in imparting additional speed to that airship.

"a mass so large" = any nongaseous object. And when a heavier-than-air aircraft that relies on lifting surfaces to remain aloft matches the speed of the air mass it's in, that's called stalling and ends *very* uncomfortably for all involved. All of which means it's a completely inapplicable example, as the airships in TAW do *not* rely on lifting surfaces to remain aloft; the lift crystal is what negates gravity (to a controllably variable extent), while the sails provide the motive force. And the coupling between the air current and the airship can never achieve 100%; it's physically impossible when the differences in density are measurable. Furthermore, to address your comment about inertia applying only until windspeed is matched (which, as I've shown, never happens, but for the sake of argument), you're assuming not only a perfectly smooth laminar flow but an absolutely static vector value (i.e., the air current never changes velocity or direction or meets another air current, etc.). This is more of a practical consideration than one that addresses the fundamental principle I'm trying to convey to you, but there it is, all the same.

Try this on for size: Remember those little "paratrooper" toys from many years ago (i.e., essentially just an action figure with a toy parachute attached to it)? Go outside when the air masses are moving (i.e., the wind is blowing), unfold the parachute and place the action figure in your hand without constraining it. It *will* be pulled out of your hand, but it will never equal the speed of the wind.

This is, of course, a horribly rough illustration, but it *is* illustrative. Sails *can* propel an entirely airborne object, and that object will not quite match the speed of the air current it's in.

Coupling vs. aerodynamics. Think of the main body of the ship: wood and such. Dense things that don't tend to be moved easily by wind and whose main contribution to the body's physics is largely in their inertia; i.e., the coupling coefficient is very low. Even with a lot of sails deployed and flying with the wind, the airship will never be able to move at the precise speed of the surrounding air mass. Ergo, there will be a significantly perceptible "wind." More so when balancing crystal-fed etheric force against the sails to be able to tack; there *will* be a crosswind in that case.

And don't forget that airflow is almost never a perfectly smooth laminar force; there will always be turbulence, interacting air currents and the like.

You did very well to point out that on-water sailing relies largely on the difference between the media, but it doesn't depend entirely on that particular difference. Greatly, yes (see the multiple mentions in the text about how most airship captains deplore using the wind and instead prefer to use etheric propulsion exclusively; if you stop to wonder why, you see why "greatly" ≠ "only"), but never entirely. And it's that gap that allows these airships to sail...and produces wind that the crew can feel.

TL;DR version: Throw a grocery bag in the air when there's significant wind. It moves with the air mass. Ergo, propulsion, and not quite at the speed of that air mass.

Basically, because people didn't get the reference they didn't understand the sarcasm.

The word "terrible" has shifted similarly in just the past century.