The situation:
Version 1.3.2
Turn 312 167 Ships 1284 Maint cost
Turn 314 257 Ships 4097 Maint cost
I go looking for the cause. I find that if I go into the diplomacy area and trade with another civ that I now have 33 new ships each of three different types (99 total ships). They don't exist on the map, but I am paying for them. Using the designer to determine main cost I find that those 99 ships are costing me 2914 per turn. This has happened before, but this is the first time I have tracked down the cause.
I traded the phantom ships to another civ. I hope they don't actually get them.
Thanks!