A great way to plunder bigger ships early in the game is to acquire the mortars and heavy rounds. Look for Frigates at anchor (ships with no sails) usually around warehouses, which are in a bay protected by mines. You can start hammering them with mortar from outside the perimeter protected by mines, by the time that they start moving (if they actually do, i had some which were sitting ducks all the time) they have half life remaining. Then get very close using heavy rounds you can finish them before boarding them.
I boarded some frigates without taking any damage....