NUC vs RAM vs CBD

Side-by-side comparison

Not Under Command (NUC) — Rule 27(a)

• →NUC means the vessel cannot maneuver as required by the Rules due to an exceptional circumstance. Typically: engine failure, steering casualty, or grounding.
• →Two all-around RED lights only. No masthead lights. No exceptions based on vessel size.
• →When making way: add sidelights (red/green) and sternlight (white). Still no masthead.
• →NUC dayshape: two black balls in a vertical line. Shown when circumstances exist by day.
• →NUC vessels are at the top of the Rule 18 hierarchy — all other vessels keep clear of a NUC vessel.

Restricted in Ability to Maneuver (RAM) — Rule 27(b)

• →RAM means the vessel's work limits her maneuverability — she is not broken, but her operation prevents her from deviating from course. Examples: dredging, cable-laying, minesweeping, replenishment at sea, diving operations.
• →Three all-around lights: RED (top) — WHITE (middle) — RED (bottom). Memory aid: 'RAM sandwich — red bread, white filling.'
• →Unlike NUC, RAM vessels DO show masthead lights when making way. The three-light stack is added to the normal power-driven configuration.
• →RAM dayshape: ball — diamond — ball in a vertical line. The diamond is unique to RAM.
• →Dredging operations: when restricted in ability to maneuver, shows RAM lights plus two all-around red lights (or green over green) on the side with the obstruction and two all-around green lights (or red over red) on the side clear to pass.

Constrained by Draft (CBD) — Rule 28 (COLREGS only)

• →CBD applies to a power-driven vessel so restricted by her draft relative to available water depth that she cannot deviate from a course through a channel. Typically deep-draft tankers in confined straits.
• →Three all-around RED lights in a vertical line — same number as CBD, and same color as NUC, but CBD shows them in addition to normal power-driven underway lights (masthead, sidelights, sternlight).
• →CBD dayshape: a cylinder. No other status uses a cylinder — this is unambiguous on the exam.
• →Rule 28 uses 'may exhibit' — it is optional, not mandatory. A vessel constrained by draft is not required to display the signals; she may choose to. This nuance appears on exam questions.
• →CBD does NOT exist in the U.S. Inland Rules. Any question about CBD that asks about Inland waters has 'not applicable' as the correct answer.

Common exam mistakes

• ✗Adding a masthead light to NUC. Rule 27(a) is explicit — NUC shows no masthead lights, only two red all-around lights (plus sidelights/sternlight when making way).
• ✗Confusing RAM and NUC. Both show red lights. NUC = two red. RAM = red-white-red. The white middle light is the discriminator.
• ✗Saying CBD is three red all-around lights alone — CBD shows those THREE lights PLUS full power-driven underway lights. Contrast with NUC (two red, no masthead) and RAM (red-white-red plus masthead).
• ✗Applying CBD rules to Inland scenarios. CBD does not exist in the Inland Rules. A deep-draft vessel on Inland waters has no special signals — she just has to be careful.
• ✗Forgetting CBD is optional ('may exhibit'). A vessel constrained by draft that does not display the signals is not violating the Rules — she is simply not exercising the option.

USCG exam questions — Rules 27 & 28

Drawn from the same pool used in real USCG licensing exams. Answers and explanations are shown.

1. 1. A vessel not under command (NUC) shall show:

   • A.One all-round red light and a flashing white light
   • B.Two all-round white lights in a vertical line
   • C.An all-round red light over an all-round white light
   • D.Two all-round red lights in a vertical line and, when making way, sidelights and sternlight✓

   Why: Rule 27(a) prescribes two all-round red lights in a vertical line for a NUC vessel. When making way, she adds sidelights and a sternlight. Red over red — not white — is the critical identifier distinguishing NUC from fishing (red over white) or RAM (red-white-red).

2. 2. A vessel engaged in dredging or underwater operations, when underway and making way, shows in addition to the RAM lights:

   • A.Two all-round red lights on the obstruction side and two all-round green lights on the clear side✓
   • B.A flashing red light on the obstruction side
   • C.An all-round white light on the side where vessels may pass
   • D.A yellow diamond shape

   Why: Rule 27(d) requires a dredging or underwater operations vessel to show two all-round red lights vertically on the side where the obstruction exists and two all-round green lights vertically on the side where passing is safe. Red means restricted passage; green means go.

3. 3. A vessel engaged in underwater operations that is unable to deviate from its course shall show the RAM lights PLUS, on the side with the obstruction, which additional lights?

   • A.Two all-round red lights vertically and on the clear side two all-round green lights vertically✓
   • B.A flashing red light on the obstruction side only
   • C.An all-round white light on each side
   • D.A yellow all-round light above the RAM lights

   Why: Rule 27(d) prescribes that a vessel conducting underwater operations that is at anchor or underway but impeded shall show two all-round red lights (vertically) on the obstruction side and two all-round green lights (vertically) on the safe passing side, in addition to the RAM lights.

4. 4. Which situation would most likely qualify a vessel as 'not under command' under Rule 3(f)?

   • A.Complete loss of steering due to a rudder casualty in heavy weather✓
   • B.Reduced speed due to fouled propeller requiring diver inspection
   • C.Maneuvering restrictions caused by towing a barge astern
   • D.Inability to alter course due to a narrow channel

   Why: NUC status requires an 'exceptional circumstance' rendering the vessel unable to maneuver as required — a complete loss of steering from a rudder casualty is the classic example of such an exceptional circumstance, distinct from operational or navigational restrictions.

5. 5. A vessel restricted in her ability to maneuver (RAM) underway and making way shall exhibit which lights in addition to the RAM identity lights?

   • A.Masthead lights, sidelights, and a stern light✓
   • B.Sidelights and a stern light only
   • C.Masthead lights and sidelights only
   • D.All-round white lights and sidelights

   Why: Under COLREGS Rule 27(b)(iii), a vessel restricted in her ability to maneuver when making way through the water shall exhibit masthead light(s), sidelights, and a sternlight in addition to the RAM identity lights (the red-white-red all-round lights of Rule 27(b)(i), with the ball-diamond-ball dayshapes by day under 27(b)(ii)).

6. 6. A vessel 'constrained by her draft' as defined in Rule 3(h) applies to:

   • A.A power-driven vessel which, because of her draft in relation to the available depth and width of navigable water, is severely restricted in her ability to deviate from the course she is following✓
   • B.Any vessel whose draft exceeds 10 meters
   • C.Any vessel that is heavily laden to the point where her freeboard is less than 0.5 meters
   • D.Both power-driven and sailing vessels navigating in charted shallow water areas

   Why: Rule 3(h) defines CBD as applying only to power-driven vessels whose draft in relation to available depth severely restricts their ability to deviate from their course — this is a COLREGS-only category not found in the Inland Rules.

7. 7. A vessel constrained by her draft may exhibit which optional lights?

   • A.Three all-round red lights in a vertical line✓
   • B.Three all-round blue lights in a vertical line
   • C.Two all-round red lights in a vertical line
   • D.A single all-round red light

   Why: Rule 28 allows a power-driven vessel constrained by her draft to optionally exhibit three all-round red lights in a vertical line in addition to prescribed lights, indicating to other vessels that maneuvering ability is severely limited by available water depth.

Frequently asked questions

What is the difference between NUC and RAM?

Not Under Command (NUC) means the vessel cannot maneuver as required by the Rules due to an exceptional circumstance — engine failure, steering casualty, or similar mechanical inability. Restricted in Ability to Maneuver (RAM) means the vessel can maneuver but her work restricts that ability — a dredge, a vessel laying cable, a vessel conducting replenishment. NUC is circumstantial; RAM is operational.

Does NUC show masthead lights?

No. A NUC vessel does NOT show masthead lights. Rule 27(a) specifies only two all-around red lights in a vertical line when not making way, plus sidelights and sternlight when making way. There is no masthead light in the NUC configuration — this is the most commonly tested NUC mistake on the exam.

What are the lights for RAM vs NUC?

NUC shows two all-around RED lights in a vertical line. RAM shows three all-around lights in a vertical line: RED on top, WHITE in the middle, RED on the bottom. A useful memory aid: RAM has the 'sandwich' — red-white-red. NUC is just two reds. RAM also shows masthead light(s) and sidelights/sternlight when making way; NUC does not show a masthead light.

What is Constrained by Draft and why doesn't it apply to Inland waters?

Constrained by Draft (CBD) is a status under International COLREGS (Rule 28) for a power-driven vessel that is severely restricted in her ability to deviate from a course because of her draft relative to available depth. CBD shows three all-around red lights in a vertical line plus a cylinder dayshape — in addition to normal underway lights. The Inland Rules have no equivalent provision. CBD is COLREGS-only, and exam questions comparing CBD to Inland rules always have CBD as the answer that applies only internationally.

What are the dayshapes for NUC, RAM, and CBD?

NUC dayshape: two black balls in a vertical line. RAM dayshape: ball, diamond, ball in a vertical line (top to bottom). CBD dayshape: a cylinder. Aground (for context) uses three balls in a vertical line. The ball-diamond-ball for RAM is the most distinctive — no other vessel uses a diamond.

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