Power-driven Vessel Underway

Under Rule 23: a masthead light forward, a second masthead light abaft of and higher than the forward one, sidelights, and a sternlight. Below 50 metres the second masthead light is optional, and below 12 metres she may substitute an all-round white light plus sidelights.

At 50 metres and over. A vessel of less than 50 metres may exhibit the second masthead light but is not obliged to, so seeing a single white masthead light does not by itself tell you the vessel is small — it tells you she is either under 50 metres or has chosen not to fit the second light.

Because the two lights together form a range. When you see them vertically separated you are looking at the vessel roughly abeam; as she turns towards or away from you the lights close together, and when they line up you are looking near her bow or stern. It is the cheapest aspect indicator on a ship.

An air-cushion vessel — a hovercraft — operating in the non-displacement mode. She still shows the ordinary Rule 23 lights; the flashing yellow is added, warning that she may be moving fast and cannot be expected to manoeuvre like a displacement hull.

It depends on her length and on which rulebook the question is asking about. At 12 metres and over she carries the full Rule 23(a) set — masthead light forward, sidelights, sternlight, the second masthead light optional below 50 metres. Under 12 metres, in both rulebooks, she may substitute an all-round white light and sidelights. Under 7 metres with a maximum speed not exceeding 7 knots, the International Rules alone (Rule 23(d)(ii)) let her show the all-round white light by itself, with sidelights if practicable; 33 CFR 83.23 has no such paragraph, so on inland waters the sidelights are not excused.

Under the International Rules, if her maximum speed does not exceed 7 knots, an all-round white light is enough on its own — she must also exhibit sidelights if that is practicable. Both conditions have to hold: a fast 6-metre boat is not covered by Rule 23(d)(ii) and falls back to the under-12-metre option of an all-round white light plus sidelights. Inland has no 7-metre paragraph at all, so on inland waters the under-12-metre option is the smallest reduction available to a power-driven vessel.

Yes, in three places. Inland 33 CFR 83.23 lets a power-driven vessel on the Great Lakes carry an all-round white light in place of the second masthead light and the sternlight, positioned where the second masthead light would be, and COLREGs has no equivalent. In the other direction, COLREGs Rule 23(d) runs to three sub-paragraphs and Inland 83.23(d) to one: the under-7-metre, 7-knot option and the provision allowing the masthead or all-round white light to sit off the fore-and-aft centreline are both International only.

Under the International Rules, yes, where fitting it on the centreline is not practicable — Rule 23(d)(iii) applies to the masthead light or all-round white light of a power-driven vessel under 12 metres, and it is conditional: the sidelights must then be combined in one lantern carried on the fore-and-aft centreline, or as nearly as practicable in the same fore-and-aft line as that white light. Inland 33 CFR 83.23 contains no such provision.