1.16 NUC, RAM, CBD, Pilot, Anchored & Aground (Rules 27–30)

Special Lights Decide Hierarchy and Night Safety

After ordinary power, tow, sail, and fishing pictures, Rules 27–30 mark vessels that are impaired, working, piloting, or stopped on the bottom or at anchor. For ONC06 Q100 at 90%, these codes are pure memorization with lethal scenario value: a Master of a passenger vessel who mistakes NUC red-over-red for fishing red-over-white will misapply Rule 18 and may close a vessel that cannot get out of the way.

Rule 27 — NUC and RAM

Not under command (NUC)

A vessel not under command shall exhibit:

• Two all-round red lights in a vertical line where they can best be seen
• Two balls or similar shapes in a vertical line by day
• When making way through the water, also sidelights and a sternlight

No masthead lights as part of the NUC identity. NUC means through some exceptional circumstance (steering failure, main-engine casualty, etc.) she cannot maneuver as required. Your Master 100 passenger vessel with a lost rudder is NUC — show the lights if fitted and able; more importantly, other vessels must keep clear under Rule 18.

Restricted in ability to maneuver (RAM)

A vessel restricted in her ability to maneuver, except a vessel engaged in mine-clearance operations, shall exhibit:

• Three all-round lights in a vertical line: red, white, red
• Three shapes in a vertical line: ball, diamond, ball
• When making way: also masthead light(s), sidelights, and sternlight
• When at anchor: also the anchor lights/shapes of Rule 30 (in addition to RAM signals)

RAM is about the nature of the work (dredging, diving ops, cable work, underway replenishment, severe towing restriction, etc.), not a casualty. Exam drill: NUC = red over red; RAM = red-white-red. Day: two balls vs ball-diamond-ball.

Dredging / underwater operations — obstruction side

When a RAM vessel is engaged in dredging or underwater operations with an obstruction, she shows the RAM lights/shapes and two all-round reds (or two balls) on the obstruction side, and two all-round greens (or two diamonds) on the side on which another vessel may pass. Master takeaway: pass on the diamond / green side, not the ball/red obstruction side.

Mine-clearance vessels

A vessel engaged in mine-clearance shows three all-round green lights (one near the foremast head and one at each end of the fore yard) or three balls in the same positions. Other vessels shall not approach within 1,000 meters of a mine-clearance vessel (International text — know the standoff concept for the exam).

NUC / RAM comparison

Rule 28 — Constrained by Draught (International Only)

A vessel constrained by her draught (CBD) may, in addition to Rule 23 power-driven lights, exhibit three all-round red lights in a vertical line, or a cylinder by day. CBD exists under International Rules only — Inland Rules have no CBD category. On Master 100 Q100 this is a dual trap: know the three reds / cylinder, and know which book you are in. Inside the COLREGS Demarcation Line, do not invent CBD privilege for a deep freighter under Inland Rules (though Rule 9 shall-not-impede and good seamanship still protect deep-draft traffic in channels).

Rule 29 — Pilot Vessels

A vessel engaged on pilotage duty shall exhibit:

• At or near the masthead, two all-round lights in a vertical line: the upper white and the lower red
• When underway: in addition, sidelights and a sternlight
• When at anchor: in addition to white-over-red, the anchor light(s) or shape of Rule 30

Memory hook: "white over red, pilot ahead." Do not reverse it into fishing's red over white. A pilot vessel not engaged on pilotage duty shows ordinary lights for a vessel of her length.

Rule 30 — Anchored and Aground

Anchored

A vessel at anchor shall exhibit where it can best be seen:

• In the fore part, an all-round white light or one ball
• At or near the stern, a second all-round white light at a lower level than the forward light — required when length is 50 m or more; vessels under 50 m may exhibit a single all-round white where it can best be seen

A vessel of less than 7 meters in length, when at anchor not in or near a narrow channel, fairway, or anchorage, or where other vessels normally navigate, shall not be required to exhibit the anchor light or shape. A vessel of less than 20 meters in a special anchorage area designated by the Secretary may also be exempt from anchor light/shape under the rule's terms — know that small-craft exemptions exist so you do not assume every dark skiff is illegal.

Vessels of 100 meters or more at anchor shall also illuminate their decks with working lights (International text often tested as "use available working lights to illuminate decks").

Aground

A vessel aground shall exhibit the anchor light(s) and in addition two all-round red lights in a vertical line, and three balls in a vertical line by day. Small-vessel exemptions parallel the anchor rule: vessels under 7 m aground outside channels/anchorages/normal navigation areas need not show the aground signals; the detailed length cutoffs track Rule 30's structure — exam emphasis is anchor lights + red-over-red and three balls for a vessel aground that must signal.

Anchor vs aground vs NUC

Aground is "stuck on the bottom" with anchor-style whites plus NUC-like reds; NUC is inability to maneuver while still afloat as a vessel not under command. Do not interchange three balls (aground) with two balls (NUC).

Master Scenario: Fog Clearing in a Busy Anchorage

You command an 80-foot passenger vessel entering a bay at dawn. One freighter shows a single forward white all-round and a ball — at anchor, under 50 m or using the single-light option. A dredge shows red-white-red and ball-diamond-ball, with two reds toward a spoil bank and two greens toward the channel — RAM with obstruction; pass on the green/diamond side. A pilot launch underway shows white over red plus sidelights — on pilotage duty. Seaward of the demarcation line a tanker adds three all-round reds to her power-driven lights — CBD (International). None of these vessels is "just another white light." Your passenger schedule does not override their special status under Rules 18 and 27–30.

Master special-light cheat sheet

Exam Framing for Q100 (90%)

Highest-yield pairs: NUC red-over-red / two balls vs RAM red-white-red / ball-diamond-ball; pilot white-over-red vs fishing red-over-white; CBD three reds / cylinder / International only; anchor white + ball vs aground + two reds + three balls; 50 m second anchor light; small-vessel anchor exemptions. On a Master LT 100 exam these appear as light-picture stems and as "which vessel must you keep clear of" items. With only five misses allowed on Q100, special lights are not trivia — they are pass/fail material.