Motivated to gather my scant notes on converting the trade route system GURPS Traveller: Far Trader (a project on which I was the junior author) for use with a medieval setting after talking with Rob Conley of Bat in the Attic at North Texas RPG Con.
Settlement Trade Number
This is a function of population and the quality of port and market (in GT is is starport). Population is straight forward enough. For port and market, I think some generic modifications can work.
Port / Market
This list just describes things that might indicate or be typical of the various classes, it isn't a list of requirements.
- Class V: Major docks, ship and vehicle construction and repair. High quality roads. Permanent market space, probably multiple and/or specialized. Good storage, warehousing, factors, trading houses, vendors, currency exchange, labor for loading, crews for hire, provided security, defensible position.
- Class IV: Minor docks, ship and vehicle construction and repair. Medium quality roads. Permanent market space. Some storage and trading houses, and labor. Medium security and defensibility.
- Class III: Minor ship and vehicle repair. Medium to low quality roads. Regular (weekly or less) single market space. Light storage. Light security and defensibility.
- Class II: No dock or ship repair, minor vehicle repair. Low quality roads. Weekly market. Little to no storage.
- Class I: No dock, limited shore access. No improved roads. Irregular market days (monthly to seasonal)
- Class 0: No dock. No shore access. Difficult roads, if any. No market.
In GURPS Tech Levels, the medieval period is 3, which helps simplify some math so we can simply cross-index the population and port/market to looking up the Unmodified Settlement Trade Number (USTN).
Bilateral Trade Number
To calculate the trade routes, we need to find the Bilateral Trade Number between any two settlements. To do this add the two Settlement Trade Numbers and any Trade Classification Modifiers then subtract the Distance Modifier.
BTN = STN 1 + STN 2 + TCMs - Distance Modifier
Trade Classification Modifiers
These are limited in GT:FT and intended for use in a far future space travel setting.
- One settlement is Agricultural and the other is Non-agricultural or Extreme: +0.5
- One settlement is Industrial and the other is Non-industrial: +0.5
- The settlements are of different political allegiances: -0.5
With some flexibility, I think these can still be useful in a medieval and fantasy setting. I think realistically every settlement would be agricultural with a very large percentage of its population dedicated to providing food, I think it's reasonable to still have a modifier for settlements that are very much less so, that are dependent on agriculture from a greater range than average - perhaps Paris, Rome, or Constantinople would be examples of this.
"Extreme" meant an inhospitable environment, hostile to life - that could be a cursed place.
The medieval setting is all non-industrial, almost by definition, but I think there is still value in adjusting trade values to reflect that some settlements are much more focused on producing finished goods than others.
Distance Modifier
In Traveller, the main unit of distance is the parsec, which is 3.2 light years, a bit outside scope of medieval trade. However, it takes one week to make a jump so I think substituting one week for one parsec might work. (I haven't tested this and am not likely to be able to anytime soon).
An aside for short distances
(The distance modifier is subtracted from the bilateral calculation but for distances less than a week, I wonder if adding a modifier might be interesting? Thinking out loud, perhaps something like +.5 if less than 3 days).
The Silk Road was about 6,400 kilometers long but I'd wager that most of the goods transported along it would be classified as "speculative trade" in Traveller terms, i.e., not contracted, regular trade. Contrast that with trade amongst the Hanseatic League, which I believe would have more regular trade.
Example 1: Paris and Rouen circa 1300 BCE.
- Paris population: 200,000+. Assume Class V Port/Market: STN 3.5
- Rouen population: 40,000. Assume Class IV Port/Market: STN: 2.5
- Distance: 120 km, approximately 4-5 days. Modifier: 0
- Trade Classification Modifiers: none apply.
- Bilateral Trade Number: (Paris) 3.5 + (Rouen) 2.5 + 0 - 0 = 6.
In GT:FT terms this equates to 1 Million - 5 Million trade in Credits/Year (see GT:FT page 16). That's the value of trade in both directions. The volume and weight of cargo is different and is unlikely to be balanced. GT:FT assumes a value of freight between 10,000 and 50,000 credits per displacement ton (which is 14 cubic meters in Traveller) - I have my doubts that value works for a medieval setting but it would be 100-500 displacement tons/year or 5-10 displacement tons per week. (I started looking into medieval wagon and ship cargo capacity but that is a rabbit hole I'm not prepared for).
Having said all that, 5-10 wagon loads per week going each way between Paris and Rouen seems reasonable?
Example 2: Rouen to Le Havre- Rouen population: 40,000. Assume Class IV Port/Market: STN: 2.5
- Le Havre population: 2,000 (a total guess). Assume Class III Port/Market: STN 2.0
- Distance: 120 km, approximately 4-5 days. Modifier: 0
- Trade Classification Modifiers: none apply.
- Bilateral Trade Number: (Rouen) 2.5 + (Le Havre) 2.0 + 0 - 0 = 4.5
- Credits/Year: 50,000-100,000.
- Volume: 5-10 displacement tons/year.
Example 3: Paris to Marseille
- Paris population: 200,000+. Assume Class V Port/Market: STN 3.5
- Marseille population: 40,000. Assume Class V Port/Market: STN 3.0
- Distance: 750 km, approximately 4 weeks. Modifier: -1
- Trade Classification Modifiers: none apply.
- Bilateral Trade Number: (Paris) 3.5 + (Marseille) 3.0 + 0 - 1 = 5.5
- Credits/Year: 500,000-1,000,000.
- Volume: 50-100 displacement tons/year, 0-5/week.
Paris-Marseille has a step less than Paris-Rouen but the distance explains that. Does that gives results that are interesting and useful? Remains to be seen. But a couple of quick examples that might be helpful.
Example 3: Paris to Cairo
- Paris population: 200,000+. Assume Class V Port/Market: STN 3.5
- Cairo population: 400,000. Assume Class V Port/Market: STN 3.5
- Distance: 4,400 km, approximately 25 weeks. Modifier: -2.5
- Trade Classification Modifiers: different allegiances: -0.5
- Bilateral Trade Number: (Paris) 3.5 + (Cairo) 3.5 - 0.5 - 2.5 = 4.0
- Credits/Year: 10,000-50,000.
- Volume: 0-5 displacement tons/year.
Example 4: Paris to Hangzhou
- Paris population: 200,000+. Assume Class V Port/Market: STN 3.5
- Hangzhou population: 800,000. Assume Class V Port/Market: STN 3.5
- Distance: 14,500 km, approximately 2 years (100 weeks). Modifier: -4
- Trade Classification Modifiers: different allegiances: -0.5
- Bilateral Trade Number: (Paris) 3.5 + (Hangzhou ) 3.5 - 0.5 - 4 = 2.5
- Credits/Year: 500-1,000.
- Volume: --
Displacement tons might benefit from a conversion to wagons or multiple wagons, maybe small caravan? Similarly, Traveller credits might better be converted to silver pennies or something.
