Tier system is a system that judges the certain ship to have certain parts or traits to do certain tasks required for that tier. Normally it is impossible to upgrade a tier unless captain has upgraded its ship through the country power for example ( research breakthrough and etc), modified under the shipyard, risky situation like forcefully install a huge cannon on the ship

Through other hidden methods and etc, it is possible to rank up the tier, After Tier 5, it may choose to change into other class

Destroyer class

Tier 0 Raft

Tier 1 Sampan and Boats

Tier 2 carvels, Carrack, Barge

Tier 3 Junks, Galleon, Frigates, Triremes, Yachts, Man of wars

Tier 4 Steamboat

Tier 5 Steamboat with metal cover and lower hull base, ironclad