You have to defeat an enemy convoy possessing more than 60 guns with a pinnace.
On the paper, it's impossible : Not for a trophy hunter !
It's "simple": click on the dock of a city, go to the tab equipment and buy as much as possible of explosive barrels for your pinnace. Don't forget sailors, and guns.
My pinnace had 20 explosive barrels, and i used 6,8 barrels to win.
So if you have 2,4 barrels, try to increase your captain capacity, i don't know if it's necessary; but my captain was good in attack; maybe not.
Make a save when you're ready, ok, so now attack the first convoy of a nation which appears and having more than 60 guns. I suggest you to wait near a big city, Port Royale is a good place.Move with your pinnace to the three enemy ships then make circles : release your barrels and if you do it well, the enemy ships will explode !
If it's not the case, reload your save and begin again.
It took me a single try, it is much easier than what we could believe.Edit : To play with the dlc increase the maximum barrels of your boat, and iv'e got the three dlc, so if anybody can confirm that the original game has fewer barrels.