Rule 27 — Vessels Not Under Command and Restricted in Ability to Maneuver: Lights and Shapes

TL;DR — A NUC vessel shows two all-round red lights (vertical); a RAM vessel shows red-white-red all-round lights (vertical). Neither vessel shows those special lights in place of underway lights — when making way, both must add sidelights and a sternlight. 33 CFR §83.27

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What the Rule Says

General Application

All light and shape requirements under Rules 20–31 apply in all weathers. 33 CFR §83.20 Lights must be shown from sunset to sunrise; shapes must be shown by day. 33 CFR §83.20 During restricted visibility, lights shall also be shown from sunrise to sunset if carried. 33 CFR §83.20

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Not Under Command (NUC) — §83.27(a)

A vessel not under command shall exhibit: 33 CFR §83.27

1. Two all-round red lights in a vertical line, where they can best be seen. 33 CFR §83.27 2. Two balls or similar shapes in a vertical line, where they can best be seen (day signal). 33 CFR §83.27 3. When making way through the water, sidelights and a sternlight in addition to the two red lights. 33 CFR §83.27

Key point: a NUC vessel that is stopped but not at anchor shows only the two red lights — no masthead light, no sidelights, no sternlight. The moment she gathers way, sidelights and a sternlight are added. No masthead light is required for a NUC vessel under any circumstance described in this rule. 33 CFR §83.27

An all-round light, by definition, shows an unbroken light over 360 degrees of the horizon. 33 CFR §83.21

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Restricted in Ability to Maneuver (RAM) — §83.27(b)

A vessel restricted in her ability to maneuver (except mine clearance) shall exhibit: 33 CFR §83.27

1. Three all-round lights in a vertical line — highest and lowest red, middle white. 33 CFR §83.27 2. Three shapes in a vertical line — highest and lowest balls, middle a diamond. 33 CFR §83.27 3. When making way through the water, a masthead light or lights, sidelights, and a sternlight, in addition to the red-white-red lights. 33 CFR §83.27 4. When at anchor, the anchor light(s) or shape prescribed by Rule 30 (§83.30), in addition to the red-white-red lights and shapes. 33 CFR §83.27

The RAM display is therefore: red — white — red, top to bottom, all-round. The mnemonic "Red over White over Red — Restricted" is consistent with the rule text. 33 CFR §83.27

Unlike a NUC vessel, a RAM vessel making way must also show a masthead light or lights in addition to sidelights and sternlight. 33 CFR §83.27 A masthead light is a white light on the fore-and-aft centerline showing 225 degrees, from right ahead to 22.5 degrees abaft the beam on each side. 33 CFR §83.21

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Severely Restricted Towing — §83.27(c)

A vessel engaged in a towing operation that severely restricts the towing vessel and her tow in their ability to deviate from course shall exhibit the RAM lights and shapes (red-white-red / ball-diamond-ball) plus the lights or shapes prescribed by Rule 24 (§83.24) for towing. 33 CFR §83.27

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Dredging and Underwater Operations — §83.27(d)

A RAM vessel engaged in dredging or underwater operations shows the standard RAM lights and shapes and, when an obstruction exists: 33 CFR §83.27

• Two all-round red lights or two balls in a vertical line on the side where the obstruction exists. 33 CFR §83.27
• Two all-round green lights or two diamonds in a vertical line on the side where another vessel may pass. 33 CFR §83.27

When at anchor, this vessel shows the dredging lights and shapes instead of the Rule 30 anchor signal. 33 CFR §83.27

Dredge pipelines (floating or on trestles) require, at night and in restricted visibility: one row of yellow flashing lights (50–70 flashes per minute, all-round, visible at least 2 miles, 1–3.5 meters above water, approximately equally spaced, not more than 10 meters apart in a navigable channel); and two red all-round lights at each end of the pipeline, visible at least 2 miles, one meter apart vertically with the lower light at the same height as the yellow lights. 33 CFR §83.27

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Diving Operations — §83.27(e)

When vessel size makes it impracticable to show all dredging/underwater lights and shapes, a vessel engaged in diving operations may instead exhibit: 33 CFR §83.27

• Three all-round lights in a vertical line — highest and lowest red, middle white. 33 CFR §83.27
• A rigid replica of International Code flag "A" not less than 1 meter in height, with measures taken to ensure all-round visibility. 33 CFR §83.27

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Mine Clearance — §83.27(f)

A vessel engaged in mine clearance operations shows the lights for a power-driven vessel underway (Rule 23) or anchor lights (Rule 30), as appropriate, plus three all-round green lights or three balls: one near the foremast head and one at each end of the fore yard. These signals indicate it is dangerous for another vessel to approach within 1,000 meters of the mine clearance vessel. 33 CFR §83.27

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Small Vessel Exemption — §83.27(g)

A vessel of less than 12 meters in length, except when engaged in diving operations, is not required to exhibit the lights or shapes prescribed in Rule 27. 33 CFR §83.27

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Distress Signal Clarification — §83.27(h)

The signals prescribed in Rule 27 are not distress signals. Distress signals are contained in Annex IV (33 CFR part 87). 33 CFR §83.27

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Why It Matters on the Exam

Exam questions on Rule 27 test three things repeatedly:

1. Color and arrangement of the special lights. The exam will present a light configuration and ask you to identify the vessel type, or describe a vessel type and ask what lights it shows. Burn in the vertical sequences:

• NUC: red / red (two all-round reds) 33 CFR §83.27
• RAM: red / white / red (three all-round lights) 33 CFR §83.27
• Mine clearance: green / green / green (three all-round greens, specific placement) 33 CFR §83.27

2. What additional lights are required when making way. This is the most frequently missed concept. The special lights are not a substitute for normal navigation lights when the vessel is moving through the water. A RAM vessel making way must show masthead light(s), sidelights, and sternlight in addition to the red-white-red. 33 CFR §83.27 A NUC vessel making way adds only sidelights and a sternlight — no masthead light. 33 CFR §83.27

3. Day shapes. The exam tests shapes as frequently as lights. NUC = two balls vertical; RAM = ball-diamond-ball vertical. 33 CFR §83.27 Shapes are required by day. 33 CFR §83.20

4. The 12-meter exemption. Vessels under 12 meters are generally exempt from Rule 27 requirements, with the specific exception of diving operations. 33 CFR §83.27

5. Dredging obstruction side signals. Red lights/balls = obstruction side; green lights/diamonds = clear (passing) side. 33 CFR §83.27 The exam will ask which color indicates which side.

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Common Pitfalls

Pitfall 1: Confusing NUC and RAM light sequences. NUC is two reds — no white light at all. RAM is three lights with white in the middle. Candidates who memorize only "red lights" without the count and color of the middle light will miss RAM questions. 33 CFR §83.27

Pitfall 2: Assuming a NUC vessel making way shows a masthead light. Rule 27(a)(iii) adds only sidelights and a sternlight for a NUC vessel making way. There is no provision requiring a masthead light for a NUC vessel. Rule 27(b)(iii) explicitly requires a masthead light for a RAM vessel making way. These are different requirements. 33 CFR §83.27

Pitfall 3: Forgetting anchor lights for a RAM vessel at anchor. A RAM vessel at anchor must show the Rule 30 anchor signal in addition to the red-white-red lights and shapes. 33 CFR §83.27 The special RAM lights do not replace anchor lights.

Pitfall 4: Applying the 12-meter exemption to diving vessels. The exemption in §83.27(g) explicitly carves out diving operations. A vessel under 12 meters engaged in diving must still comply with Rule 27. 33 CFR §83.27

Pitfall 5: Treating Rule 27 signals as distress signals. The rule itself states these are not distress signals. 33 CFR §83.27 Distress signals are governed by Annex IV (33 CFR part 87). 33 CFR §83.27

Pitfall 6: Confusing dredge obstruction colors. Red = obstruction side (danger, stay away). Green = clear side (pass here). 33 CFR §83.27 Candidates sometimes reverse these under exam pressure.

Pitfall 7: Forgetting that lights apply at night and shapes apply by day. Both regimes are tested. Lights are required sunset to sunrise; shapes are required by day. 33 CFR §83.20 In restricted visibility during daylight, lights shall also be shown if carried. 33 CFR §83.20

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Quick Check

Q1: A vessel not under command is stopped in the water (not making way). What lights does she show at night?

Two all-round red lights in a vertical line, where they can best be seen. No sidelights, no sternlight, no masthead light are required while not making way. 33 CFR §83.27

Q2: A vessel restricted in her ability to maneuver is making way. List all lights she must show.

Three all-round lights in a vertical line (red-white-red), plus a masthead light or lights, sidelights, and a sternlight. 33 CFR §83.27

Q3: What day shapes does a RAM vessel display?

Three shapes in a vertical line: the highest and lowest are balls, and the middle shape is a diamond. 33 CFR §83.27

Q4: A dredge has an obstruction on her port side. What lights indicate the obstruction side, and what lights indicate the safe passing side?

Two all-round red lights in a vertical line on the port (obstruction) side; two all-round green lights in a vertical line on the starboard (clear/passing) side. 33 CFR §83.27

Q5: A 10-meter vessel is engaged in diving operations. Is she required to show Rule 27 lights and shapes?

Yes. The exemption for vessels under 12 meters in §83.27(g) does not apply to vessels engaged in diving operations. 33 CFR §83.27

Q6: You observe three all-round green lights arranged with one near the foremast head and one at each end of the fore yard. What does this indicate, and what is the safe approach distance?

The vessel is engaged in mine clearance operations. It is dangerous for another vessel to approach within 1,000 meters of her. 33 CFR §83.27

Q7: A RAM vessel is at anchor. What lights must she show?

The three all-round lights in a vertical line (red-white-red) plus the anchor light(s) prescribed by Rule 30 (§83.30). 33 CFR §83.27

Q8: Are the signals prescribed in Rule 27 considered distress signals?

No. Rule 27 signals are not distress signals. Distress signals are contained in Annex IV (33 CFR part 87). 33 CFR §83.27