at the moment nations have a simple ship tax law; X gc per ship owned, where x is set per rank.
i suggest to change this to a system that promotes having lots of fleets.
it could be like: X*(Y/100) where x is set per rank and Y is on a downward scale from 100 to 1 and reflecting the number of sailing fleets (those on auto trade, fishing or salvaging).
0 - 10 fleets; Y=100
10-20 fleets; Y=95
20-30 fleets; Y=90
--
100-110 fleets; Y=50
--
200+ fleets; Y=1
another possibility is that Y is related to cargo space. this way it promotes using larger ships as well.