Because of my work background I spend a lot of time thinking about interest rates...
And this causes me to be sensitive to the fact that the interest rate paid by loan shark is way too high, from all of a gameplay, balancing, and realism perspectives.
First off, to make clear I'm not the only one who thinks this is a problem, here is Hylie Pistoff and Day Jack two years ago calling attention to this problem: Guide - How to get a very good start | Page 7 | PiratesAhoy!
Hylie Pistoff: "11% interest is waay too high! 3% tops I say."
Ok, so how does it work currently: if you give money to a loan shark, he pays you 1 + your personal commerce skill in interest per month. If you borrow, he charges you 11- your personal commerce skill per month.
What his means is, at commerce 6, the loan shark starts paying you a higher interest rate than he changes you, which is 100% wrong. Whoever originally programmed this misunderstood how interest rates are set: bargaining is a very minor element, almost everything is about risk (certainly risk sets the baseline around which things are determined). Anyone who works in the financial sector could tell you the loan shark would never offer you a higher interest rate for your money than what he is currently offering to lend to you at, unless he has gone quite insane.
The reason is, the loan shark is offering you a risk free investment (he will not default), while you have some risk of default, however small. Another way of looking at it, the loan shark is only willing to take your money if he has profitable uses for it, and if he is lending out at a lower interest rate, he would be taking a loss (i.e., even if in the specific transaction he only did one or the other).
So, to make a quick fix, let's pretend the player commerce skill is a reasonable proxy for how established he is as a safe investment (it probably gets at it pretty well, as it grows the more trading done, though maybe later a combination formula involving reputation, personal wealth, and fame could be employed instead).
Leaving interest rate for borrowing alone as above, quick fix proposal for investing rate:
Set the interest rate the loan shark pays your for money to 1% per month (12% per year) (there is little good reason to make it dependent on anything involving the player, as long as you have a basic level of competence you should get about the same amount depending on his general cost of capital)
This has advantages from all three game design perspectives:
Gameplay: the player should be encouraged to go out there and actively make money, just having capital compound on capital at a high rate is not what this game is about, players should make their money through privateering or active trading.
Balancing: the loan shark already bypasses the need for a merchant license to keep crew happy and keeps your money safe, 12% per year/1% per month on top of all that is more than enough.
Roleplaying/realism: 12% per year/1% per month risk free investment in the historical time period we are talking about is already insanely high. Going any higher than that is immersion breaking for role players with a finance background. Additionally, this ensures that the loan shark never offers a higher rate than he asks for.
If there are no objections, I can make the needed modification someday soon.

And this causes me to be sensitive to the fact that the interest rate paid by loan shark is way too high, from all of a gameplay, balancing, and realism perspectives.
First off, to make clear I'm not the only one who thinks this is a problem, here is Hylie Pistoff and Day Jack two years ago calling attention to this problem: Guide - How to get a very good start | Page 7 | PiratesAhoy!
Hylie Pistoff: "11% interest is waay too high! 3% tops I say."
Ok, so how does it work currently: if you give money to a loan shark, he pays you 1 + your personal commerce skill in interest per month. If you borrow, he charges you 11- your personal commerce skill per month.
What his means is, at commerce 6, the loan shark starts paying you a higher interest rate than he changes you, which is 100% wrong. Whoever originally programmed this misunderstood how interest rates are set: bargaining is a very minor element, almost everything is about risk (certainly risk sets the baseline around which things are determined). Anyone who works in the financial sector could tell you the loan shark would never offer you a higher interest rate for your money than what he is currently offering to lend to you at, unless he has gone quite insane.
The reason is, the loan shark is offering you a risk free investment (he will not default), while you have some risk of default, however small. Another way of looking at it, the loan shark is only willing to take your money if he has profitable uses for it, and if he is lending out at a lower interest rate, he would be taking a loss (i.e., even if in the specific transaction he only did one or the other).
So, to make a quick fix, let's pretend the player commerce skill is a reasonable proxy for how established he is as a safe investment (it probably gets at it pretty well, as it grows the more trading done, though maybe later a combination formula involving reputation, personal wealth, and fame could be employed instead).
Leaving interest rate for borrowing alone as above, quick fix proposal for investing rate:
Set the interest rate the loan shark pays your for money to 1% per month (12% per year) (there is little good reason to make it dependent on anything involving the player, as long as you have a basic level of competence you should get about the same amount depending on his general cost of capital)
This has advantages from all three game design perspectives:
Gameplay: the player should be encouraged to go out there and actively make money, just having capital compound on capital at a high rate is not what this game is about, players should make their money through privateering or active trading.
Balancing: the loan shark already bypasses the need for a merchant license to keep crew happy and keeps your money safe, 12% per year/1% per month on top of all that is more than enough.
Roleplaying/realism: 12% per year/1% per month risk free investment in the historical time period we are talking about is already insanely high. Going any higher than that is immersion breaking for role players with a finance background. Additionally, this ensures that the loan shark never offers a higher rate than he asks for.
If there are no objections, I can make the needed modification someday soon.