The adventurers are trekking cross-country when they see an odd shape in the sky some distance off. As it draws closer, they see it is some kind of airship, a massive balloon with a much smaller basket below. Several lengths of rope suddenly drop down with a note attached to the end of one: “Pull me in, please.”
They grab hold of the ropes and start tugging the floating device down, eventually getting the basket to ground level and tying it to some stout trees. Once secured a female satyr with a crown of yellow dandelions hops out of the basket. “Thanks, friends!” she says. “My name’s Tara, and this is my ship powered by just by the very thought of it!”
The description of the Enchantment cantrip Encode Thought is:
Putting a finger to your head, you pull a memory, an idea, or a message from your mind and transform it into a tangible string of glowing energy called a thought strand, which persists for the duration or until you cast this spell again. The thought strand appears in an unoccupied space within 5 feet of you as a Tiny, weightless, semisolid object that can be held and carried like a ribbon. It is otherwise stationary.
If you cast this spell while concentrating on a spell or an ability that allows you to read or manipulate the thoughts of others (such as detect thoughts or modify memory), you can transform the thoughts or memories you read, rather than your own, into a thought strand.
Casting this spell while holding a thought strand allows you to instantly receive whatever memory, idea, or message the thought strand contains. (Casting detect thoughts on the strand has the same effect.)
The key word here is “weightless.” It is not the same as neutrally buoyant, meaning a weightless object will not simply float stationary. It will immediately start flying off. Do not cast this spell outdoors or in a strong wind, or else you risk losing the thought forever!
Naturally, if an object can float, it has the potential to lift other objects. An encoded thought is a more efficient lift mechanism than helium or hydrogen since it has zero weight to it. Just for a quick bit of comparison, dry air at sea level and 60F has a density of 1.225kg/m3. Helium is 0.1785kg/m3, and hydrogen is 0.082kg/m3. An encoded thought is 0.000kg/m3. It’s more efficient than hydrogen and helium, but its main advantage is that it’s relatively easy to produce — just takes 6 seconds with a cantrip.
The rest of this isn’t going to get too technical because stuff is going to vary a lot based on building materials, and frankly, I’ve got 2 glasses of sparkling rosé and 0 physics background.
But, the one big (tiny) factor we do need to know is the size of the encoded thought. It’s described as ribbon-like, so I think 1 cubic inch is about right (if you imagine it folded over a bunch of times to estimate the total volume). There’s 1728 cubic inches in a cubic foot. It takes a character, casting the spell once every 6 seconds, just under 3 hours to produce a cubic foot of encoded thoughts. Also, the thoughts last 8 hours, so 5 hours later and they’re going to start disappearing. With that in mind, let’s get to work.
A Hindenberg-class airship held just over 7 million cubic feet of gas. That means it’d take 7 million casters working consistently for 3 hours to get this thing in the air for a 5 hour flight.
The Zeppelin NT airship (same as the current Goodyear Blimp) is much more sensible, with only 300,000 cubic foot balloon, so we’re talking just 300,000 casters at work. That’s likely more than the total number of people who know the spell in an entire kingdom or continent, and that’s in a magic-rich setting.
A typical hot air balloon is 77,000 cubic feet in volume. It’s also way less efficient than even hydrogen, so a thought-filled balloon could be much smaller, and this is why I’m not getting into the exact math because it’s still going to require some number in the tens of thousands.
What if we ditch the airship and just go for a balloon with a harness? That’ll be in the realm of 1500-2000 cubic feet. So, even the smallest thought-powered balloon is impracticably big.
Just going to need to dream a little bigger, darling.
Literally.
Objects in the same size category can have very different actual sizes. A tiny object can be a fraction of our 1 cubic inch thought ribbon, or it can be a 50 cubic inch bottle. But even then, take each of the numbers and divide it by 50, and you still end up needing some absurdly huge number of casters.
For Tara to have a reasonably practical Goodyear blimp-scale airship powered by just her own Encoded Thoughts, she’d need to be able to conjure weightless ribbons the volume of a 5ft cube. At that rate, she could be airborne after about 4 hours, and enjoy 4 hours of flight before needing to start replacing the thoughts.
For the record, the thought is “This is totally going to work.”
Also, “weightless” should probably be interpreted as “neutrally buoyant regardless of medium.”