I've seen no evidence so far that proves that grouping ships has any benefit. It is possible that there is some benefit to stacking the lower command ships under a higher one, but I haven't seen it. I tend to break my ships into individuals and attack seperately as you would with assassins. about 2 or 3 faster ships and a few more heavier ones. The Dromon is faster than the Barque or Dhow anyway so you may still not catch it, but sending more individual faster ships against it increases the chances of stopping it.