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

Rules 27-30: NUC, RAM, Constrained by Draught, Pilot, Anchored, and Aground

This section carries the densest block of pure memorization on the entire Rules exam: the special light and day-shape signatures for the vessels at the top of Rule 18's pecking order, plus pilot boats and vessels at anchor or aground. Learn the master table, then the traps.

The master table

Committing it to memory

• NUC = "red over red, the captain is dead." Two all-round reds, and — critically — no masthead light and no way-related lights unless she is actually making way through the water. A drifting, powerless vessel shows only the two reds. If she is being carried along and making way, she adds sidelights and a sternlight, but never a masthead light.
• RAM = "red-white-red, restricted I said." The white light in the middle separates her from NUC. A tug restricted while handling a heavy tow, a cable-layer, a dredge, or a survey vessel shows this. When making way she does show masthead lights and sidelights (unlike NUC), and when at anchor she shows the red-white-red plus the anchor light.
• CBD = three reds / a cylinder — and International only. There is no constrained-by-draught signal in the Inland Rules. On Inland waters a deep-draft ship is just a power-driven vessel (protected instead by the narrow-channel rule).
• Pilot = "white over red, pilot ahead." Do not confuse it with NUC's red-over-red or a fishing vessel's red-over-white.
• Anchored = one ball / one or two whites; aground = three balls / anchor lights + two reds. Aground simply adds two red lights (or a third ball) to the anchor signal.

Special RAM configurations

Two RAM sub-cases show up on the exam:

• Dredging or underwater operations with an obstruction: the RAM shows the red-white-red plus two all-round red lights / two balls on the obstructed side and two all-round green lights / two diamonds on the side a vessel may pass. Read it as "reds = danger, keep off; greens = go, pass here."
• Diving operations on a small boat that cannot show the full RAM signal exhibits three all-round lights vertically — red, white, red — and, by day, a rigid replica of the International Code flag "A" (blue-and-white) at least 1 metre high. On U.S. waters divers are additionally marked by the red-and-white "diver down" flag, but flag A is the one the Rules require.

Length exemptions

Vessels less than 12 metres are generally not required to show the Rule 27 NUC/RAM signals — with the notable exception of a small vessel engaged in diving operations, which must still display the red-white-red diving signal or flag A. For anchoring, a vessel less than 7 metres at anchor not in or near a narrow channel, fairway, anchorage, or where other vessels normally navigate need not show the anchor light or ball. A vessel less than 50 metres may show a single all-round white anchor light rather than the two-light (forward and aft) arrangement.

Worked example

At night, a large vessel ahead shows three all-round red lights in a vertical line above her ordinary running lights, and you are on International (offshore) waters. Diagnose it: three vertical reds is a vessel constrained by her draught. Under Rule 18(d) you must, if circumstances admit, avoid impeding her — stay out of the dredged channel she is confined to and do not force her to alter. Had you seen only two reds with no masthead light, it would instead be a not-under-command vessel drifting.

Traps

• Count the reds: two all-round reds = NUC; three = CBD (International only); red-white-red (a white between) = RAM.
• NUC shows no masthead light; RAM does when making way. That single difference separates two look-alike signals.
• CBD does not exist under Inland Rules. A "three vertical reds" question set on inland waters is testing whether you know that.
• Pilot = white over red, not red over white (fishing) and not red over red (NUC).
• Aground = anchor light(s) + two vertical reds (three balls by day); it builds on the anchor signal.