I'm making a world with magic and most if not all tech will run on mana. I have airships in this world but I'm having a hard time deciding how large across the world should be. I want - even with airships - for long distance travel to take months (cross a continent or from one continent to another). Should I set a slower airspeed than airplanes now, and have the world the same size as Earth, or increase the size of the world, or both?
Using LZ 127 Graf Zepplin as an example of a transatlantic airship flight from 1928, it took 111 hours to travel from Friedrichshafen, Tübingen, Germany to Lakehurst, New Jersey, United States of America, about 9,926km in length.
In a similar airship that is capable of achieving the same speed of 89km/hr, on a similar journey across a Continental divide, the distance would need to be 150,000km to achieve a two month journey taking 10 weeks.
This means your theoretical planet would need to be on a scale of 15x the size of the Earth to achieve a similar journey over a longer time.
This massive world would probably increase the likely-hood of transition to air-travel as the speed to cross long distances would be a must for world spanning empires.
If you wish for your airship to just travel slower to make the journey longer, to achieve a 2 month long journey taking 10 weeks,you would need to slow your airship to travel 6km/h to travel roughly 10,000km across an earth sized ocean.
In terms of world building, the slower option makes less sense as most people can walk faster than 6km/hr, and any naval vessel could easily outspeed this airship. Unless of course the ocean in your world is so acidic that the only way to traverse it is to fly above it!
Below is a quick table of some compromises between the above suggestions:
| Distance | Scale | Speed | Likeness to other modes of travel| | 150,000km | 15x | 90km/h | Speed of Car | | 100,000km | 10x | 60km/h | Speed of Early Model Steam Train | | 50,000km | 5x | 30km/h | Speed of Horse Gallop | | 20,000km | 2x | 12km/h | Speed of Runner | | 10,000km | 1x | 6km/h | Speed of Fast Walker |
Basing this off of JustAnotherDotNetDev's calculations:
The big problem with making the planet much larger is of course gravity. As Michael pointed out in the comments, Jupiter is 11x the radius of earth, is mostly hydrogen and we wouldn't survive for very long under the huge crushing weight of our own bodies. So find reasons to slow things down.
The one thing I can see that might help slow things down is that the people only navigate visually. so when the sun goes down they try to find a place to 'anchor' for the night. This would mostly double the time it takes to fly anywhere, and it would still be significantly faster than walking.
One more thing might be that large merchant ships and large troop transports are large enough to just wallow along at 100-200 feet above the ground, and they might need to partially empty to navigate mountain passes, slowing things down more.
Really high passes might need to actually completely empty the ship and 'carry' it over the pass to the other side, this could take weeks all by itself, and make the seasons have a huge impact on long distance travel.
Since all the tech (including the airship engines I assume) runs on mana, then in addition to bowlturners ideas, you could limit speed by the need to "refuel".
I'll have to make a few assumptions since you didn't specify how mana works, but if mana isn't a constant, unlimited resource, then it'll have to be replentished.
So the airship could be able to travel at 89km/hr, but only be able to travel for 6 hours before having to stop and recharge for a few hours. This could limit the ship to 1000km a day. Less if the engines aren't as efficient.
Add to that high mountain ranges that need to be navigated around so straight line travel is impossible. So a straight "as the crow flies" distance of 10,000km could end up being 30,000km of distance needed to travel.
Another thing that could slow it down is scheduled stops.
Say the world is heavily forested, and mountainous, so ground travel is difficult. You still have settlements and towns that need supplies and people that want to move around. The airships are the obvious choice for this, meaning that airships would have routes and have to make stops at each location along the way. Docking, unloading, and loading could take a while depending on what's being moved around. This would slow down the travel time too. You could still have express airships that only make a few stops.
One other thing would be weather. Airships have trouble in high winds, so if your world has frequent storms, or even something like the Santa Ana Winds or Chinook winds where the wind will blow hard for days/weeks at a time. The airships would have to anchor and wait for things to die down. Either that or travel really slowly, fighting the wind the whole way, and making frequent stops.
Another weather issue could be fog or low clouds full of things like tall trees and rocks. It could even be kind of like river navigation, where you had to have a navigator that knew where all the sand banks were in order not to run-aground. If this happened frequently, they would be forced to travel very slowly or risk crashing until the clouds lifted.
You can increase the radius/mass of the planet, as well as the density of the atmosphere.
The bigger the radius, the bigger the circumference, and thus the further apart places will be.
The bigger the mass, the higher the gravity thus you need more lift per pound to fly.
The denser the atmosphere the more drag you'll get. Denser atmosphere provides more lift, but the higher gravity easily makes up for that.
At the end you end up with slower airships (due to the higher gravity you'll have weight restriction, won't be able to carry enough fuel etc), more drag and greater distances to travel.
On top of that add finicky airships that require ground service every few days and inability to navigate at night most nights and it could easily take months to go transcontinental.