"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.

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.

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.

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.

That's what I was suggesting above (notice my edits).  That the Predator create a slight negative/positive boyancy (with the crystals), allowing the sails to act as airfoils.

As I pointed out, there's no "wind" for the sails.  Any research you do will usually refer to balloons as "moving with the wind", because it takes another force to move them.

Perhaps the FAA's Balloon Flying Handbook: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/FAA-H-8083-11.pdf

To expand on Quantus' response, this only happens when you are going at the exact velocity of the wind.

As do all aircraft, unless another force is applied; surface area, etc., don't matter.  Once you leave the surface, and are in the air, you're moving with that air.  And when you add power (or force), the only air motion relative to the aircraft is due to the aircraft motion because of that power (or force).  A sail wouldn't help an airship under power any more that it would help an airplane.  You'll note that sails have never been used on dirigibles or blimps.  They won't work.

For example:
When you're standing on the ground, and the air moves by at 10mph, that's a wind of 10mph.  When you launch in a balloon, that "wind" will take your balloon along at 10mph.  So the air's moving at 10mph across the ground, and so are you and your balloon.  There's no difference in the speed of your balloon and the air mass surrounding it.  You can stick out your hand and not feel a breeze.  You can hang out a sail, and it will hang limp.

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.