FreeRadical said...
COLREGS Rule 25
(a) A sailing vessel underway shall exhibit:
(i) sidelights;
(i) a sternlight.
(b) In a sailing vessel of less than 20 m in length the lights prescribed in paragraph (a) of this Rule may be combined in one lantern at or near the top of the mast where it can best be seen.
(c) A sailing vessel underway may, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit at or near the top of the mast, where they can best be seen, two all-round lights in a vertical line, the upper being red and the lower green, but these lights shall not be exhibited in conjunction with the combined lantern permitted by paragraph (b) of this Rule.
(d)(i) A sailing vessel of less than 7 m in length shall, if practicable, exhibit the lights prescribed in paragraph (a) or (b) of this Rule, but if she does not, she shall have ready at hand an electric torch or lighted lantern showing a white light which shall be exhibited in sufficient time to prevent collision.
(ii) A vessel under oars may exhibit the lights prescribed in this Rule for sailing vessels, but if she does not, she shall have ready at hand an electric torch or lighted lantern showing a white light which shall be exhibited in sufficient time to prevent a collision.
(e) A vessel proceeding under sail when also being propelled by machinery shall exhibit forward where it can best be seen a conical shape, apex downwards.
And just for completeness:
Rule 22:
Visibility of lights
The lights prescribed in these Rules shall have an intensity as specified in Section 8 of Annex1 to these Regulations so as to be visible at the following minimum ranges.
B) In vessels of 12 metres or more in length but less than than 50 metres in length:
a masthead light, 5 miles; except where the length of the vessel is less than 20 metres, 3 miles.
a sidelight, 2 miles;
a sternlight, 2 miles;
a towing light, 2 miles;
a white, red, green or yellow all-round light, 2 miles.
C) In vessels of less the 12m in length:
a masthead light, 2 miles;
a sidelight, 1 miles;
a sternlight, 2 miles;
a towing light, 2 miles;
a white, red, green or yellow all-round light, 2 miles.