When did Lights get engaged ?
A vessel engaged in dredging or " underwater operations " , when restricted in her ability to manoeuvre , shall exhibit the lights and shapes prescribed in sub-paragraphs ( 1 , 2 and 3 above ) of this Rule and shall in addition , " when an obstruction exists " , exhibit : two all - round red lights or two balls in a vertical line to indicate the side on which the obstruction exists ; two all - round green lights or two diamonds in a vertical line to indicate the side on which another vessel may pass ; when at anchor , the lights or shapes prescribed in this paragraph instead of the lights or shape prescribed in Rule 30 .