COLREGs · Rule 27 · 33 CFR §83.27
Vessels Not Under Command or Restricted in Their Ability to Maneuver
Rule 27 gives four different vertical light signatures to four different kinds of vessel that cannot get out of your way, and the exam tests which is which. Two all-round reds mean she cannot manoeuvre at all; red-white-red means she can steer but must not be asked to; three all-round greens mean mine clearance and a 1,000-metre exclusion.
The signals at a glance#
| Light / shape | Means — Inland |
|---|---|
| 2 all-round reds · 2 balls | Not under command — she cannot manoeuvre as the Rules require |
| Red · white · red · ball-diamond-ball | Restricted in her ability to manoeuvre — she can steer, but the work she is doing stops her keeping out of the way |
| Red-white-red + 2 reds one side, 2 greens the other | Dredging or underwater work — pass on the green side, the reds mark the obstruction |
| Red-white-red + rigid flag “A” replica | Diving operations from a vessel too small for the full signal |
| 3 all-round greens · 3 balls | Mine clearance — keep 1,000 metres clear. This vessel does NOT show red-white-red; paragraph (b) excepts her from it |
| Row of flashing yellow lights, 50–70 per minute | A floating or trestle-supported dredge pipeline, with two red lights at each end. Inland only, §83.27(d)(iv)differs |
What does Rule 27 actually say?#
Source
Practice this rule#
One real exam question on this rule, cited to source. No sign-up.
Under both the International and Inland Rules, another vessel must not approach within ______ of a mine clearance vessel.
How does Inland Rule 27 differ from COLREGs?#
Dredge pipelines
International
Rule 27(d) ends at (iii). The International Rules prescribe no lights for the pipeline itself — only for the dredge.
Inland
§83.27(d)(iv) adds them: one row of yellow lights flashing 50 to 70 times per minute, all-round, visible 2 miles, 1 to 3.5 metres above the water and no more than 10 metres apart where the pipeline crosses a navigable channel — plus two red lights 1 metre apart at each end, including each end where the pipeline is parted to let vessels through.
21 of 38 rules differ somewhere; the five the exam leans on are Rule 9, Rule 10, Rule 24, Rule 28 and Rule 34, side by side on Inland vs International Rules.
Why it shows up on the exam#
Learn the pair as opposites: two reds mean the vessel cannot manoeuvre at all, red-white-red means she can but must not be asked to. The asymmetric dredge signal is the other reliable question — two reds mark the side to stay away from, two greens the side you may pass, and candidates routinely reverse them. Mine clearance is the trap inside the trap: paragraph (b) excepts it from the red-white-red signal altogether, and three all-round greens carry a 1,000-metre keep-off that no other signal in Part C carries. Note also that under 12 metres is exempt from Rule 27 entirely, except when engaged in diving operations.
What does the exam actually test here?#
Not under command
Adds sidelights + sternlight when making way
The two reds (two balls by day) are shown wherever they can best be seen. When she is making way through the water she adds sidelights and a sternlight under §83.27(a)(iii); when she is not making way she shows the reds or balls alone — which is how you tell an NUC vessel with steerage from one dead in the water.
Restricted in ability
Masthead lights underway, Rule 30 at anchor
When making way she also shows a masthead light or lights, sidelights and a sternlight. At anchor she keeps the red-white-red and the ball-diamond-ball and ADDS the Rule 30 anchor lights — the one place in Part C where a working signal and the anchor signal are shown together.
Mine clearance
3 greens · 1,000 metres
A vessel engaged in mine clearance is expressly excepted from the restricted-in-ability signal by §83.27(b) and shows something else entirely: three all-round green lights or three balls, one near the foremast head and one at each end of the fore yard, in addition to her Rule 23 power-driven lights or her Rule 30 anchor lights. They mean it is dangerous to approach within 1,000 metres. No other Rule 27 signal carries a distance.
Dredging / underwater work
Two reds = obstruction
In addition to the restricted signal: two all-round red lights or two balls in a vertical line on the side where the obstruction exists, and two all-round green lights or two diamonds on the side on which another vessel may pass. Red is the side to avoid.
Diving operations
Rigid flag A replica
Where the size of a vessel engaged in diving operations makes it impracticable to exhibit all the lights and shapes of paragraph (d), she shows three all-round lights in a vertical line — red, white, red — and a rigid replica of the International Code flag A not less than 1 metre high, kept visible all round. Diving is the one activity that keeps Rule 27 duties below 12 metres.
Towing that severely restricts
Restricted signal + Rule 24
A vessel engaged in a towing operation such as severely restricts the towing vessel and her tow in their ability to deviate from their course shows the restricted-in-ability signal in addition to the towing lights required by Rule 24.
Rule 27 — common exam questions#
What is the difference between not under command and restricted in ability to manoeuvre?
Not under command means the vessel, through some exceptional circumstance, is unable to manoeuvre as the Rules require — a steering or engine failure, for instance — and she shows two all-round red lights in a vertical line. Restricted in her ability to manoeuvre means the nature of her work restricts her, so she can still steer but cannot readily keep out of the way; she shows red over white over red. Two reds: cannot. Red-white-red: can, but do not ask her to.
What lights does a dredge show?
The restricted-in-ability signal — red, white, red in a vertical line with ball, diamond, ball by day — plus an asymmetric pair marking the working side: two all-round red lights or two balls on the side where the obstruction exists, and two all-round green lights or two diamonds on the side another vessel may pass. Pass on the green side.
What shapes mean restricted in ability to manoeuvre?
Ball, diamond, ball in a vertical line, matching the red-white-red light signature at night. The diamond in the middle is the identifying element — a ball-diamond-ball hoist is nothing else in the rulebook.
Does Rule 27 apply to small vessels?
Mostly not. A vessel of less than 12 metres in length is not required to exhibit the lights or shapes prescribed in Rule 27, with one exception: when she is engaged in diving operations. That is why a small dive boat still displays the rigid flag A replica.
Is a vessel not under command the same as a vessel aground?
No. Aground has its own signal under Rule 30 — three balls in a vertical line by day, and anchor lights plus two all-round reds at night. Rule 27's two reds without anchor lights indicate a vessel underway but unable to manoeuvre, which is a different problem for anyone approaching her.
What do three all-round green lights mean?
Mine clearance operations, and they are a keep-away signal with a number attached. Under Rule 27(f) a vessel engaged in mine clearance shows three all-round green lights or three balls — one near the foremast head and one at each end of the fore yard — in addition to the Rule 23 lights of a power-driven vessel, or the Rule 30 lights or shape if she is at anchor. They mean it is dangerous for another vessel to approach within 1,000 metres of her. Note what she does NOT show: paragraph (b) excepts a mine clearance vessel from the red-white-red restricted-in-ability signal, so an answer that gives her red over white over red is wrong.
Do the Inland Rules and COLREGs prescribe the same Rule 27 lights?
For every condition the international rule addresses, yes — not under command, restricted in ability, dredging, diving, towing that severely restricts, mine clearance and the under-12-metre exemption all read the same in both rulebooks. The Inland Rules add one thing COLREGs does not have: 33 CFR 83.27(d)(iv) lights the dredge pipeline itself. A floating or trestle-supported pipeline carries a row of all-round yellow lights flashing 50 to 70 times a minute, visible 2 miles, 1 to 3.5 metres above the water and no more than 10 metres apart where it crosses a navigable channel, plus two red lights a metre apart at each end — including each end where the pipeline is parted to let vessels through. There is no International counterpart, so this is an Inland-only exam item.
Authoritative source#
- 33 CFR §83.27 — this section with its neighbours. 33 CFR Part 83 is public domain; checked 2026-08-09.
- COLREGs Rule 27 is published by the IMO and is authoritative internationally.
Practise this rule against the real exam.
Real exam questions. No signup, no card.
Was this page helpful?