Whats up with these enemy ships?

[bpg2012]

Okay, I've noticed that sometimes when i am sailing ill sometimes find a ship that can do a 180 degree turn in 2 seconds and have unrealistic speeds, dodge everything you throw at usually and always hit you no matter what you do!

Is this a bug? Or is it just a extra challenge?

Either way, i don't like these enemy ships, there hard to hit and usually always hit you, and I've also noticed them getting in front of you sailing away turning about 180 degrees then they launch thunder, fire what ever it is they shoot at you then turn around go farther ahead and repeat this over and over.