I believe I heard that the weak point of the ship is around the bow of the ship. Try ramming the front of the boat or only firing there to see if you can get the weak point to show. Try using chain shot before hand to slow down the ship first though.
I hit it with the flaming cannonballs and for some reason I wound up getting close enough to where you could almost board it and I hit it with the guns near the bow. It wound up setting the gun powder on fire and made it explode.
I don't think you can do that until it's been damaged.
Frylock you're like the A bomb. When you show up everybody dies - Master Shake ATHF