How are you getting your ship above 12?

I’ve had a brig since season one but for the life of me I can’t get above level 12. I have Divines on my sides, long 9s on my bow and the season 1 bombards that all shoot at once that I can’t remember the name of in the stern. Highest level mortar in the middle (that I could find). Armor is rated 420. My furniture could use some work but I still can’t seem to get anything to raise my level. Ideas?