When I modeled the revenues of large sailing ships, I assumed they traveled from one Class I route to another, a distance of 500 miles which took 4 weeks. At each port, they stayed for three weeks, where they picked up 6 passengers and 9 shipping contracts, then filled up the rest of their cargo bay with arbitrage cargo. I assumed margins on arbitrage work at 5%. As a result, most ships make most of their money from shipping fees, not from arbitrage.
One of the interesting questions is how much trade should be going on in any given city. In a civilization of 6M families there is approximately $4.3B gold pieces of net worth, or 716 gp per family.
From there, the bottom 77% own 15% of the wealth.
The top 23% own 85% of the wealth.
The top 5% own 75% of the wealth.
The top 2% own 55% of the wealth.
Let’s assume city dwellers are drawn from the top 23%, who therefore own $3.65B gold pieces across 1.4M families. That means the average city family’s net worth is 2,607gp. That, in turn, translates into a per-capita monthly income of about 80gp per city dweller (remember ACKS’s secret 1/33 ratio) per month.
That gives us a sense of the demand for goods of a city - 80gp per family. A Class I city with 20,000 families therefore has a demand of 1,600,000gp per month for goods. An average load of merchandise is worth 180gp, so approximately 8,888 loads of merchandise can circulate through a Class I city each month.
Per ACKS, in a Class I city, average number of merchants willing to do business with a PC each month is 9, while average loads per merchant is 27, or 243 loads per PC. 243/8888 = .027, or 2.7%. So at any given time, there are actually probably (9/.027) 333 merchants available, enough for (1/.027) 36 PCs to do business.
A typical large merchant ship carries about 350 loads, so that would be 25 large sailing ships per month in port, or 300 per year. Is that plausible?
Rome, when it peaked at a population of 1,000,000 was served by 1,200 large vessels carrying food each year. At 1,000,000, people, that is 200,000 families, or 10x the size of our basic class I city. So we project 3,000 vessels docking at Rome per year. Looking at the Merch tables, about 30% are foodstuffs of various sorts, so that would suggest ~ 1,000 food vessels, not too dissimilar from our totals.*
*To be fair, ACKS skews grain to a smaller proportion and increases salt, meat, beer, etc. to a higher proportion than history would suggest. This was purposeful as otherwise the merchandise tables are rather dull… Oh, more grain!