Mercantile Income and the 3%

Reading about income and how it is calculated this game, I always thought it was pretty well done and liked the focus on the economy and how adventurers make money outside the dungeon. That said I've come across what I think is a small snag in how something works. I might be wrong, but this is how I see it. 

Purchasing ships is a better form of investment than any other that I can find. Purchasing a large sailing ship for 20,000 gold. Using the 3% rule I see often see quoted should give you a return of 600 gold per month. But this is not how it works. That ship takes on thousands of stone of cargo in shipping contracts and ends up being worth 3% of the total average load which in the book is calculated at around 2560 gold per month on a return.

So simply by choosing to invest in a ship instead of local business or something to that effect you get to get a return as if you invested 3-4x as much into the venture by using your ship on shipping contracts. You do have to hire a broker or two, but the cost of the broker is not really all that much compared to the gold being raked in.

Am I missing something?

it makes a certain degree of sense to me: 
A) ships are a bigger upfront investment. most people will never be able to afford 20,000gp

B) if ships weren't a better investment than local business, nobody would ever leave and try to do trade abroad

C) even when shipping IS better, the people who don't want the risk and vaugary of travel can still make do with local investment.


Here's how the large sailing ship's ROI is calculated. It's assumed to be a 500 mile trip, which takes 1 week by sea.

  Purchase Cost Cargo Cost Cargo (stones) Wages Rations Moorage Total Cost Monthly Revenue Monthly Profit Monthly ROI Travel Assumption  
Sailing Ship, large $20,000 $64,800 30,000 $252 $100 $180 $532 $3,123 $2,591 3.06% Type I Market Round Trip

On average, only 9 shipping contracts of 27 loads of 70 stone will be available. That's 17,010 stone. The shipping contract will be 1gp per 10 stone per 500 miles, or 1,701gp. From that you have to pay $252 wages, $100 rations, and $180 moorage, leaving you with 1,169gp. 

That is a 6% rate of profit. That is definitely better than 3% rate of profit you get from arbitraging your own inventory (as above), but it's also only 45% of the gross profit of arbitrage shipping - you make 1169gp per month instead of 2591gp per month. 

Put another way, if you're willing to travel with half your cargo empty and cut profits by 55%, it's possible to get a higher relative rate of profit. But I think that's fine. In the real world, for instance, the larger an investment the more it approaches the standard rate of profit. 

If it irks you, you could change the fee for shipping contracts to be 0.5gp per 10 stone.