I read a story about an interstellar flight of a couple of ships to a distant star system that takes centuries. Generations of families live on the ships. They are fleeing from an earth at war. Periodically they receive updates to their drive system, to allow them to reach the stars earlier, but they cannot trust all of the updates. Rivalry breaks out among the ships because some want to apply the updates to reach the star system earlier and therefore occupy the most promising planets. There is also a tale about a ghost ship following the cluster.