A power boat and a raft both left dock $A$ on a river and headed downstream. The raft drifted at the speed of the river current. The power boat maintained a constant speed with respect to the river. The power boat reached dock $B$ downriver, then immediately turned and traveled back upriver. It met the raft on the river 4.5 hours after turning around at dock B. How many hours did it take the power boat to go from $A$ to $B$?