Attacking a pirate hideout or a town, you often have to fight against multiple hostile convoys. At the latest the third manual fight in a row will get difficult as your own ships are already heavily damaged.

It might be handy to use automatic sea battles: pressing the “max.”-button during a battle in the convoy details will result in a ship exchange for the next fight – only if you have additional ships in your convoy, of course.

Source: http://www.forum.kalypsomedia.com/showthread.php?tid=14540&pid=131819#pid131819