Final answer:
When backing out of a slip in a harbor with restricted visibility, you are required to sound a two prolonged blast on your horn. This signal alerts other boats in the area of your presence and situation.
Step-by-step explanation:
When backing out of a slip in a harbor, if visibility is restricted, you are required to sound a two prolonged blast on your horn. This signal alerts other boats in the area that you are in reverse and may have limited visibility. The two prolonged blasts should be given every 2 minutes, and they should not be sounded if the boat is in immediate danger or if it would be confused with another signal.