Don't see anything about centering:
Rule 30 - Anchored Vessels and Vessels Aground
(a) A vessel at anchor shall exhibit where it can best be seen:
(i) in the fore part, an all-round
white light or one ball;
(ii) at or near the stern and at a lower level than the light prescribed in subparagraph (i), an all-round white light.
(b) A vessel of less than 50 meters in length may exhibit an all-round white light where it can best be seen instead of the lights prescribed in paragraph (a) of this Rule.
(c) A vessel at anchor may , and a vessel of 100 meters and more in length
shall, also use the available working or equivalent lights to illuminate her decks.
(d) A vessel aground shall exhibit the lights prescribed in paragraph (a) or (b) of this Rule and in addition, if practicable,
where they can best be seen;
(i) two all-round red lights in a vertical line;
(ii) three balls in a vertical line.
(e) A vessel of less than 7 meters in length, when at anchor not in or near a narrow channel, fairway or where other vessels normally navigate, shall not be required to exhibit the shape prescribed in paragraphs (a) and (b) of this Rule.
(f) A vessel of less than 12 meters in length, when aground, shall not be required to exhibit the lights or shapes prescribed in subparagraphs (d)(i) and (ii) of this Rule.
(g) A vessel of less than 20 meters in length, when at anchor in a special anchorage area designated by the Secretary, shall not be required to exhibit the anchor lights and shapes required by this Rule.
Being under 12 meters in length has its advantages.