It's got finite amounts of oil. The cannon balls might not be able to crack the citadel, the core armor, but they could sure destroy just about every other element of the ship. I'm not sure it'd work that well with the funnels caved in from repeated bombardment, the engines would just choke. Anything weakly armored is going to get pounded to hell.

It would take a lot of ships, but it could be defeated.