When you purchase a charter yacht they usually guarantee you the monthly mortgage payments for the 4 or 5 years they keep it in charter.
After the 4 or 5 years it's your responsibility to finish paying off the 5 or ten years you have left on the mortgage.
If you put it into a secondary charter company they charge you for everything they have to replace on your now 4 year old boat and sometimes you wind up owing charter company.
Now the most logical way to do it would be to purchase a boat that has already been through it's charter contract so as you can find one (there are plenty of after charter boats that they have to get rid of)with the least abuse and negotiate a decent price.