What is the difference between RAM and NUC?

NUC (Rule 27a, two reds vertical) means the vessel cannot maneuver due to exceptional circumstance — engine failure, steering casualty. RAM (Rule 27b, red-white-red) means the nature of the work restricts maneuvering — cable laying, dredging, pilotage transfers.

COLREGs Lights, Shapes & Sound Signals Lookup

Enter vessel type, visibility, and the sector you're observing from. Returns required navigation lights with sectors and ranges, day shapes, sound signal in restricted visibility, and the COLREGs rule citation. Replaces a thumbed PDF on a wet chartplotter.

Masthead arc

225°

Sidelight arc each

112.5°

Sternlight arc

135°

Fog signal interval

≤ 2min

Vessel type Rules 23–31 cover the standard vessel categories

Visibility Day → shapes only. Night → lights. Fog → sound signal added

Sector you are observing from Filters lights to those visible from the observer's bearing

Rule citation

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Select inputs

Required lights (visible from sector)

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Required day shapes

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Sound signal in restricted visibility

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Visual sketch from observer sector

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Tip On US inland waters (Western Rivers, Great Lakes, NY Harbor) special inland-rules variations apply per 33 CFR §83 — most notably yellow stern and flashing yellow towing lights for tugs. Always confirm whether you are operating under International or Inland rules before relying on a profile.

Worked example

100 m container ship anchored at night, observed from her port beam. Apply the Rule 30 anchored-vessel profile, then check what is visible to port.

1. Vessel category Anchored ≥ 50 m → Rule 30(b) 2. Lights required Forward all-around white (9 nm) Aft all-around white (lower than forward) Deck lights illuminating decks (Rule 30(c) for ≥ 100 m) 3. Visible from port beam at night Two all-around whites vertically separated, deck illuminated. → Identify as anchored large vessel. 4. Day shape (had it been daylight) Single black ball forward (Rule 30(a)(i)) 5. Sound (had it been fog) Rapid bell forward 5 sec every 1 min, plus gong aft 5 sec for vessels ≥ 100 m (Rule 35(g))

Quick reference — light/shape profile by vessel category

Category	Rule	Distinctive lights	Day shape
Power-driven < 50 m	23(a)	Masthead, sidelights, sternlight	—
Power-driven ≥ 50 m	23(a)	Two masthead (aft higher), sidelights, sternlight	—
Sailing < 20 m	25(b)	Tri-color OR sidelights+stern	—
Fishing — trawling	26(b)	Green-over-white all-around + sidelights/stern	Two cones apex-to-apex
Fishing — other	26(c)	Red-over-white all-around + sidelights/stern	Two cones apex-to-apex
NUC	27(a)	Two reds vertical, sidelights/stern when making way	Two black balls vertical
RAM	27(b)	Red-white-red vertical	Ball-diamond-ball
Constrained by Draft	28	Three reds vertical (in addition to power lights)	Black cylinder
Pilot on duty	29	White-over-red all-around	—
Tug + tow ≤ 200 m	24(a)	Two masthead vertical, sidelights, stern, yellow towing	—
Tug + tow > 200 m	24(a)	Three masthead vertical, sidelights, stern, yellow towing	Black diamond
Anchored < 50 m	30(a)	One all-around white forward	Black ball forward
Anchored ≥ 50 m	30(b)	Forward all-around white + aft (lower)	Black ball forward
Aground	30(d)	Anchor lights + two reds vertical	Three balls vertical

Common mistakes

Showing tri-color and side+stern simultaneously on a sailboat at night. Annex I forbids it — pick one combination, never both.
Forgetting day shapes when visibility allows. A vessel at anchor in daylight still needs the black ball forward — the white anchor light is not a substitute.
Sound-signal confusion at anchor. Anchored vessels ≥ 100 m owe BOTH the rapid bell forward AND a gong aft sounded for 5 seconds, under Rule 35(g) — not bell alone.
Reusing International profile on US Inland waters. 33 CFR §83 modifies tug towing lights, special-flashing lights for barges, and Western Rivers conventions. Verify which jurisdiction applies.

Warn This calculator is a STUDY AID, not a substitute for the actual COLREGs text or USCG-issued chart updates. Annex I dimensional requirements (mast height, vertical separation, sector accuracy ±1°) must be verified against the boat's documentation before USCG / Coast Guard inspection. License candidates: do NOT use this in lieu of memorizing the rules.

FAQ

Why does sailing > 20 m get the tri-color exception removed?

Annex I §3 limits the tri-color masthead lantern to sailing vessels under 20 m. Above that, the vessel must show separate sidelights and sternlight at the standard heights — the combined lantern is not a legal substitute on larger sailing yachts and ships.

What's the difference between RAM and NUC?

NUC (Rule 27(a), two reds vertical) means the vessel cannot maneuver due to exceptional circumstance — engine failure, steering casualty, severe damage. RAM (Rule 27(b), red-white-red vertical) means the nature of the work restricts maneuvering: cable laying, dredging, pilotage transfer, replenishment at sea.

Are sound signals required in clear visibility?

Rule 34 maneuvering and warning signals apply when vessels are in sight of one another: 1 short blast for altering to starboard, 2 short for altering to port, 3 short for engines astern, 5 short (or more) for doubt or danger. Rule 35 fog signals apply in or near restricted visibility regardless of whether other vessels are in sight.

Range vs sector — what wins on a small boat?

Annex I §2(d): vessels under 12 m may show one combined masthead+stern lantern (all-around white) plus sidelights, in lieu of separate masthead and stern lights. The 2 nm range minimum still applies; the relaxation is only on the sector geometry.

Sources

IMO COLREGs 1972 (with 2013 amendments) Annex I Positioning & technical details of lights and shapes Annex III Technical details of sound signal appliances USCG Navigation Rules & Regulations Handbook (2021) Bowditch American Practical Navigator (2017) Vol. 1 Ch. 32 Cockcroft A Guide to the Collision Avoidance Rules (8th ed.)

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