First, how can you have a 25.5 debt? That's more than the original loan. Probably just a wording issue. I think I get what you are saying.
Due to interests and bank financing. The MCr 36 (before you redeem MCr 6) or 25.5 (after redeeming) come from what you will pay in all your remaining montly payments, not from what principal you still owe.
Typically the monthly amount due is the monthly amount due. Payments are not recalculated just because you are paying extra against the principle. It usually just means that the ship loan would be payed off sooner.
For what I've seen from mortgages, you can refinance them each year (or twice a year). Of course other mortgages may work differently...
I've never seen a ship last long enough that you'd need to worry about it. Just jot it down in case they trade the ship in or sell it or claim insurance and you can go online and figure it out if and when you need to.
But you surely have seen ships with some years paid (e.g. a Merchant rolling 3 times a ship in mustering out, so having 20 years paid)
Optionally, they could refinance and you just need to calculate the loan payoff and then determine the new loan parameters. Why not take it out to 40 years again instead of 30 and really lower the payments down or, if you can afford it, pay it off in 20 years or whatever. Just go online to figure it out.
In this case, you're not refinancing your mortgage, you're just redeeming part of your principle. Those are two different things.
Using a monthly recapitalization and monthly payment reducing principle prior to recapitalization, and no points... 12 payments per year, 13 months per year (to match imperial calendar) I get 5.15%.; using 12 months, it's 5.56%, and you can use real world calculators to do it easily.
Traveller talks about paying for 480 months (see quote below), not 40 years. So, as many reference are made to those 40 years, I assume there are 12 payments a year, either by paying each day divisible by 30 (as hans suggests, and so having 5 "free" days a year) or by skipping one month a year (probably the one for maintenance).
If you make 13 payments a year (to match Imperial calendar) you'll pay your ship in a little less of 37 years, not matching with the many references to 40 years.
I'd look at it like this:
Initial calc:
Ship cost = 30M
Down Payment = 20% = 6M
Loan Amount = 24M
Repayment = 1/240*24M = 0.1M/mo
Initial debt = 0.1M*480mo = 48M
10 yrs later:
120*0.1M = 12M paid, outstanding debt = 48-12 = 36M
I'm afraid you calculated it wrong. The 20% down payment isn't subtracted from the price of the ship when calculating financing, so that you end paying 220% of the ship's cost
From Bk2, page 7 (or TTB page 52):
after a down payment of 20% of the cash price of the starship <snip> Standard termsi nvolve the payment of 1/240th of the cash price each month for 480 months. In effect, interest and bank financing cost a simple 120% of the final cost of the ship, and the total financed price equals 220% ofthe cash purchase price
BTW, I guess Will's tables have the same error, as they begin (as I understand them) with a principle of MCr 0.8 for a MCr 1 loan.
If you pay off 6M, there are two low maths methods depending on the nature of the Mortgage contract (ie whether your payments are made from the sum owing at the time or from the sum loaned. Method 1 should be illegal because you are paying out money for a loan that you no longer have):
Method 1.
Current debt is 36M to be paid over 360mo = 0.1M/mo
New debt is 30M to be paid over 360mo = 0.08333/mo
Method 2.
New Nominal Debt = 24-6 = 18M
Repayment = 1/240*18 = 0.075M/mo
New debt = 0.075*360mo = 27M
This ignores that for the initial payments you're paying mostly interests, and few principle (see Will's tables, above), so, the same 1/6 of the remaining debt at the begining of your mortgage will be worth more than at the end of it.
In 10 years, you have paid MCr 12 and have 36 to pay, as we both calculated, but that does not mean you've paid 1/4 of your principle. In fact, according to Will's tables you whould have only paid about 10% of it.