An Irate Pirate: How to Double Your XP - Naval Action Devs made XP bonus loophole
TL:DW, Since 2019 you used to get bonus experience by attacking ships in the open world that had a significantly greater battle rating (BR) than your ship. As of May 15 you get the XP equivalent to the BR you sink.
Unless you do Group Kill Missions. Soloing one of these can yield an unusually high amount of XP. Use this tip to rank up faster. Note: This is a long video, but I try to fill it with useful commentary.