Because when the first ship unloads, all three ships have to wait for the unloading time of the first ship, and when the second ship unloads, the first ship has already left, so the waiting time of the two ships is the unloading time of the second ship, and when the third ship unloads, the first two ships have already left, so the waiting time is the unloading time of the third ship. It is not difficult to draw a conclusion that the sum of their waiting time should be: the waiting time of the first ship ×3+ the waiting time of the second ship ×2+ the waiting time of the third ship. In order to make the time shortest, we must sort small, medium and large, and the result should be 29 hours. It should be noted here that no matter which ship starts unloading, it has to wait for its own unloading time, and other ships have to wait for the corresponding time.