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

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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