My plans for a campaign with the system include large scale piracy (ala the Barbary Coast), so I've been looking into threads on the forum re: piracy and merchant shipping. A good starting point imo for piracy rules can be found here.
Beyond that, I'm thinking that piracy should have an economical impact, because you could be prosecuring a campaign of piracy not just to get wealthy off the rewards, but to have an effect on the target realm. So, I've been thinking about ways to factor in this impact.
To begin thinking about modelling this, I'm looking at Alex' posts here and here.
So, each urban family creates 100 gp worth of trade for a market. For the minimum Class I Market of 20k families, this is a total trade volume of 2M gp. Alex comes to a round conclusion of that adding up to 30 large trade ships per month. My calculations* come up to 31.75 ships, so dunno how you want to round that out. I'm assuming Alex just rounded it down further to a nice number?
* I tried calculating this two ways: 1) 2M / 180 (average load of cargo) = 1111 (rounded down), which is then divided by 350 (the average number of loads in a large ship) to get 31.75 (rounded up); 2) 2M gp / 64k gp (average cargo value of a large ship) = 31.75
Anyway, that's the math backbone. Now the real question is how to factor in the effect of losing a ship full of cargo? We can quite easily see how much money certain people are losing, but what is the effect on the realm/domain? Also, that cargo is bound for a destination which needs that cargo to, well, function effectively. Perhaps the effects should be felt at both ends of the trade route?
Do we need something more complex here, or is a simple Morale effect enough to factor in the effects of commerce raiding? Or even simpler, is the fact that some people are losing money enough to factor it in? I'd like to hear some ideas!
If a 1gp change in tax is sufficient to cause an impact on morale, the equivalent thereof in piracy could do the same.
If urban family creates 100gp of trade goods and ~8gp in urban revenue, then for every (100/8) 12gp of lost trade goods we might see 1gp lost in urban revenue from 1 family.
Therefore multiply number of families x 12gp x 3 (months) and that's the amount of piracy required to cause a -1 morale penalty during that season's morale check.
The Judge might also impose a penalty of lost urban revenue in the season following to reflect the delay in loss of trade impacting the region.
EXAMPLE: St. Vincenzia, a port town, has 5,000 families. 5,000 x 12 x 3 = 180,000gp. Pirates capture 200,000gp worth of merchandise along St. Vincenzia's trade routes over the spring. That's enough to impose a -1 penalty on domain roll, and to reduce next season's urban revenue by 1gp per family per month (200,000 / 12 = 16,667gp ~ 5,000gp/month ~ 1gp per urban family per month).
To tag on to this, I recently calculated the average value of a stone of goods from the Merchandise Tables, using the percentages from the tables and sub-tables to weight each type of cargo. The average is 56.48 gp, but it's going to be highly variable, because 49.10 of that comes from the 15% of Merchandise that is Precious Merchandise. This suggests roughly a 2:3 ratio; that is, every 2 stone of merchandise pillaged will inflict a morale penalty on 3 families, so St. Vincenzia, with 5,000 families, would need to have at least 3,334 stone of cargo pillaged over three months to cause a morale penalty.