It seem that with 3 raised mast and rolling 2 gold, 1 silver ad 2 black dice for steering. (even more gold dice from overlord upgrade)
It is quite easy to move off the edge of the map every turn. (unless a very bad roll) And the overlord ship disappear during the hero turn. There is no way the hero can attack the overlord ship. unless all of them place a guard order adjacent to their cannon.
On the next turn, overlord reappear again and fire its cannon and move off the egde of the map again.
Another question is what happen if the hero ship block the space of the overlord ship entering from the east edge of the map?