Power-driven Vessels Underway.
(a) A power-driven vessel underway shall exhibit:
(i) a masthead light forward;
(ii) a second masthead light abaft of and higher than the forward one; except that a vessel of less than 50 metres in length shall not be obliged to exhibit such a light but may do so;
(iii) sidelights;
(iv) a sternlight.
(b) An air-cushion vessel when operating in non-displacement mode shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit an all-round flashing yellow light.
(c) A WIG craft only when taking off, landing and in flight near the surface shall, in addition to the lights prescribed in paragraph (a) of this Rule, exhibit a high intensity all-round flashing red light.
(d) (i) A power-driven vessel of less than 12 metres in length may in lieu of the lights prescribed in paragraph (a) of this Rule exhibit an all-round white light and sidelights.
(ii) a power-driven vessel of less than 7 metres in length whose maximum speed does not exceed 7 knots may in lieu of the lights prescribed in paragraph (a) of this Rule exhibit an all-round white light and shall, if practicable, also exhibit sidelights.
(iii) the masthead light or all-round white light on a power driven vessel of less than 12 metres in length may be displaced from the fore and aft centerline of the vessel if centerline fitting is not practicable, provided the sidelights are combined in one lantern which shall be carried on the fore and aft centerline of the vessel or located as nearly as practicable in the same fore and aft line as the masthead light or all-round white light.
On all power-driven vessels, the masthead light is in fact the light that is on the foremast. If the vessel is over 50m in length (or under in some circumstances) then the second masthead light is the one, on top of the accommodation (if all aft superstructure). At the risk of pedantically pushing this point, the history of why this is so only goes back a couple of decades, when most vessels had a centre-castle midships where the bridge was located, on top of which was the “masthead” light. The second masthead light would have been on top of the accommodation that was at the aft end of the vessel. The important part however, is that you cannot say with certainty whether a vessel is greater than 50m in length, as vessel’s less than 50m in length may show the second masthead light, if they so wish. Therefore, when you see two mastheads lights and a sidelight, you can only determine that the vessel is PROBABLY greater than 50m.
The remainder of the Rule is self explanatory, detailing the lights that vessels of shorter lengths than 50m should display in lieu of, or in addition to the standard navigation lights, that are detailed in paragraph (a).
Also note that these lights are what a power-driven vessel UNDERWAY shall exhibit. The vessel could very well be stopped and not making way through the water, but she is still underway. Therefore, assumptions cannot be made when you see masthead lights and sidelights of another vessel that she is underway and making way. She could be, for example, be stopped and picking up a pilot. The only way that you can accurately determine whether the other vessel is moving through the water is to take a series of compass bearings and/or radar plot her.
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