Rule 27 A vessel engaged in dredging

As you can see there are two round balls on the starboard side of this dredger (as seen from her transom). The port side has two diamonds. the middle has two balls separated by a diamond.

Rule 27: a vessel engaged in dredging or underwater operations, when restricted in her ability to manuever, shall exhibit

i) two round balls or red lights in a verical line to indicate the side on which the obstruction exists;

ii) two all around green lights or two diamonds in a vertical line to indidcate the side on which another vessel may pass; and

iii) when at anchor , the lights or shape prescribed by this paragraph, instead of the lights or shapes prescibed in rule 30 for anchored vessels. [US Dept Transportation ; USCG Naviagtion Rules] International-Inland

In this case her obstruction was on her starboard (where the boom is extended) so we could pass on the her port side which was a little tight being as that temporary buoy showed the end of the channel. It helped that a tiny bit of pipe was hanging out of the water on starboard. good clue.