"Warhorse of Seas" Flagship which can be obtained by reaching Fortress Lv.30, 

"Flagship Shipyard" which is unlocked by completing the Fortress Lv.30 Quest, can increase the level.

The performance of the Flagship will increase as you level up Flagship, and you can acquire additional buffs exclusive to the Expedition Fleet

* Warhorse of Seas Blueprints are consumed to increase the level of Flagship.

* Warhorse of Seas Blueprints can be obtained via Exclusive Mission.