30 Coins and a Mule
With inventory economy being what it is in Knave and given that Knave uses the earn-money-to-gain-experience dichotomy for gaining levels, of course one of the first things a player will want to do is gain access to more slots. 300c for a hireling is pretty steep at the start of the game, but for a mere 30c a player could purchase a mule.
Intuitively, an animal bred specifically for its hardiness and ability to carry large loads should be able to carry more than the average person, but the only statistic we have is how many coins to pay for one. Similarly, a cart, the natural first upgrade for your new mule, costs 50c, but we don't know anything else about them.
So, how much can a mule carry? What about a cart? How much can a mule pull in a cart?
TL;DR
If you don't particularly care how this was all figured out or for the answers to any follow up questions: a Knavian mule can carry 67 slots or pull a fully loaded cart, which can hold 185 slots.
Using real world data
You could simply import statistics from your game system of choice, but would probably still be faced with the task of converting units of weight to Knave's slots. However, you'll notice that Knave leaves a lot of this sort of minutiae up to the GM's discretion, so this may end up being a common activity. And while I cannot cite a specific example, you can easily imagine a scenario where you can't find source material to import from the system you chose initially, and now you must worry whether the new system you are importing from is compatible with the other you've been using - perhaps they make fundamentally different assumptions about how much weight can be carried? Do you then make adjustments to convert from system B to system A and then to Knave? What if we find a need for something that we must draw from another system C? What if what we need can't be found in any system at all?
To avoid all of that, I decided that I would start wherever possible with real world and historical statistics as the ground truth and extrapolate and convert those as needed.
How big is a slot?
Since we're starting from real world observations, the first thing we're going to need to know is about how much weight fits into a slot. Again, the Knave rules have nothing to say on the matter, only that a human can carry 10 slots of items unless they increase their CON. So the first real world question we need to answer is: How much can a human carry?
Even this question needs to be more nuanced, because how much a person can carry will depend on the circumstances of where you are carrying that stuff. We are primarily worried about carrying capacity when traveling or delving. We will probably be hard pressed to find a real world example how much a person can comfortably carry while dungeon crawling (full disclosure: I didn’t look), but Knave makes no distinction between the two with respect to inventory management, and so we can look at backpacking and hiking guidelines: generally, a backpacker should carry at most 20% of their own bodyweight.
But now we also need to know how much the character weighs. This isn't a concept in Knave and even if it was, we don't want to get into a place where we start incorporating ethnic or gender based differences, so, we'll simply use the weight of a generic human given the whole world as the sample space of 135 pounds.
So, one slot is about 2.7 pounds of stuff (135 pounds * 20% / 10 slots).
How much can a mule carry?
The guideline is similar for mules, to only carry 20% of their own bodyweight. So, once again, we will need to know just how much the mule weighs, and, again, we really don't want to care about its breed or gender, so we'll use the average weight of 910 pounds.
And now, we know how much a mule can carry: 67 slots of gear.
How much does a cart weigh and how much can it hold?
Finding figures for medieval carts proved to be a challenge, but I did find a post in much the same vein as this one that based a handcart for GURPS on the Bringham Young design, and came up with a cart that weighs 60 pounds with a capacity of 500. This being a handcart means that not only should our mule be capable of hauling it, but so should an adventurer, if needed.
For Knave, that comes out to a cart that weighs 22 slots and can hold 185.
How much can a mule pull on a cart?
As long as the load is on wheels, mules can pull up to their own body weight, so our average mule can pull 337 slots total. Of course, some of that weight is going to be the cart itself, which leaves 315 slots, more than our cart’s maximum load.
Feeding your mule
Your mule isn’t going to carry much for you without being fed, so you had better plan on some of what it’s carrying to be its feed. Mules will eat between 1.5% and 2% of their body weight per day, but since ours is a working mule, we’re going to assume the high end at 2%, we’ll need to reserve 7 slots of space per day for feed.
Water is generally assumed to be available, but if it isn’t, a mule needs 10-13 gallons of water per day, and since it’s a working animal, we’re going to use the higher value again, which will end up being a much heavier 40 slots per day.
Riding a mule
Want to ride your mule? There’s a distinction between dead and live weight. As we’ve already said, mules can carry 20% of their own weight in dead weight, but can carry 30% of their weight in live weight, much more than our idealized average human, good enough for two. But suppose we want to know how much loot a rider can carry with them, we’ll simplify by assuming that a mule can carry a mixture of live and dead weight equal to 25% of its own weight, so a mule can carry a rider with another 34 slots of gear.