i hated that mission..its when you try to escape from that one island.. i needed to restart it over since i got always occupied with enemies, but i couldnt kill them fast enough
so i used my mortar and it made it lots easier. mortar and heavy shots should take quick care of enemy ships.
My mortar keep missing.... and how do you use heavy shots again?
for heavy shots you dont need to aim your broadside guns, only press teh "fire" button when you look to your left/right without aim :)
As for the mortar, if ships are moving, you have to fire it a bit in front of the ships , if they are moving since it takes a few seconds until the shots will drop from the sky. (thats why its way better against immobile enemys like forts^^)