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AI-assisted editorial review · version 3 ·

Restricted visibility: three different information conditions

Visual contact, radar-only detection and a heard signal do not provide the same information.

9 min read
Manual contents
In this guide
Rule 19 scope: not in sight, in or near restricted visibility. 19(d): radar-only detection. 19(e): another vessel’s fog signal heard apparently forward of the beam, or unavoidable close quarters with another vessel forward of the beam; qualification where absence of collision risk has been determined. Rules 4–10, lights and sounds remain applicable within their scope. No manoeuvring course advised.
Rule 19 · information and scope

Original conceptual schematic, not to scale; fictional cases, not a manoeuvre procedure.

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Rule 19 scope: not in sight, in or near restricted visibility. 19(d): radar-only detection. 19(e): another vessel’s fog signal heard apparently forward of the beam, or unavoidable close quarters with another vessel forward of the beam; qualification where absence of collision risk has been determined. Rules 4–10, lights and sounds remain applicable within their scope. No manoeuvring course advised.

Application depends on two conditions

Rule 19 applies to vessels not in sight of one another when navigating in or near an area of restricted visibility. The Rule 3 definition includes causes such as fog, mist, falling snow, heavy rainstorms and similar situations.

Both conditions must be retained: the relevant area and absence of visual contact. Fog on the horizon does not by itself describe the relationship with every vessel present.

In sight has a defined meaning: one vessel can be observed visually from the other. An AIS target or radar echo does not satisfy that definition merely by existing.

Transition between conditions requires updating the encounter information, not just selecting an equipment mode. Rules 4 to 10 continue to apply in any visibility; Rule 19(c) requires due regard to restricted-visibility circumstances when complying with them.

Detecting by radar alone

Rule 19(d) concerns a vessel detecting another by radar alone. She must determine whether a close-quarters situation is developing and/or risk of collision exists.

Where it does, avoiding action is required in ample time. By radar alone delimits the described condition.

A display symbol does not automatically establish vessel type, intention or the accuracy of all the information shown.

If that action consists of an alteration of course, the rule identifies alterations to avoid so far as possible. For a vessel forward of the beam, it refers to an alteration to port, except for a vessel being overtaken.

For a vessel abeam or abaft the beam, it refers to alteration towards that vessel. These are different relationships, not a general instruction always to turn to starboard.

The signal apparently forward

Rule 19(e) establishes a different condition: hearing apparently forward of the beam another vessel’s fog signal, or being unable to avoid a close-quarters situation with another vessel forward of the beam. The provision excepts the case where absence of collision risk has been determined.

That qualification must not be erased, nor absence of an identified target treated as proof that no risk exists.

Under those conditions the text requires reducing speed to the minimum at which course can be kept, taking all way off if necessary and in any event navigating with extreme caution until collision danger is over. No numerical speed is supplied.

Keeping course here does not mean that every yacht steers at the same speed or that stopping at a particular place is safe; capabilities and circumstances remain essential.

Different information, continuing duties

Safe speed adapted to restricted visibility also belongs to 19(b); power-driven vessels must have engines ready for immediate manoeuvre. Lights and sound signals are addressed elsewhere and do not disappear because radar is available.

When consulting Rule 35, the IH edition has a confirmed discrepancy in its English appendix concerning paragraphs (j)/(k); the cross-checked international reference retains that explicit documentary issue.

A report saying only “we heard a signal” has not stated whether it appeared forward of the beam. “A target was detected” has not stated whether detection was by radar alone.

Adding these words does not merely complicate prose: it retains precisely the facts the rule uses to distinguish conditions. Missing information should be identified as missing.

The figure separates information sources and conditions without simulating radar or advising courses. MCA guidance is used as declared interpretation, not an amendment to the international text.

For learning, explain why a case belongs to 19(d), 19(e), both or neither. That question is more rigorous than associating all restricted visibility with one phrase about manoeuvre direction.

Sources and references

  1. RIEAM / COLREGs — Rule 11: Application of Section II ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 11, normative Portuguese printed p.17 (PDF p.23); International English cross-check in USCG handbook.

    Rule 11; IH normative Portuguese printed p.17, PDF p.23; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  2. RIEAM / COLREGs — Rule 19: Conduct of vessels in restricted visibility ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 19, normative Portuguese printed p.26 (PDF p.32); International English cross-check in USCG handbook.

    Rule 19; IH normative Portuguese printed p.26, PDF p.32; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  3. RIEAM / COLREGs — Rule 20: Application of lights and shapes ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 20, normative Portuguese printed p.29 (PDF p.35); International English cross-check in USCG handbook.

    Rule 20; IH normative Portuguese printed p.29, PDF p.35; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  4. Navigation Rules and Regulations Handbook — corrected August 2024 ↗

    United States Coast Guard Navigation Center · International column only, Rules 1–38 and Annexes I–IV. The adjacent U.S. Inland column is excluded.

    Official PDF directly consulted. International/Inland differences are not European rules. Filename says 08-12-2024; landing page says corrected 8 August 2024: no unsupported exact correction day inferred.

    Checked on
  5. MGN 369 (M+F) Amendment 1 ↗

    Maritime and Coastguard Agency · Sections 4–8 and 11: Rules 4–10/19, lights, sound signals and terminology in restricted visibility.

    UK guidance supporting declared interpretation; no universal CPA limit, manoeuvre recipe or local pilotage.

    Checked on
  6. RIEAM / COLREGs — Rule 4: Application of Section I ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 4, normative Portuguese printed p.7 (PDF p.13); International English cross-check in USCG handbook. Rule 5, normative Portuguese printed p.7 (PDF p.13); International English cross-check in USCG handbook.

    Rule 4; IH normative Portuguese printed p.7, PDF p.13; English International column cross-check. Individual provision commencement not established: valid_from remains null. Rule 5; IH normative Portuguese printed p.7, PDF p.13; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  7. RIEAM / COLREGs — Rule 35: Sound signals in restricted visibility ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 35, normative Portuguese printed p.55 (PDF p.61); International English cross-check in USCG handbook.

    Rule 35; IH normative Portuguese printed p.55, PDF p.61; English International column cross-check. Individual provision commencement not established: valid_from remains null. Review discrepancy: IH English appendix printed pp.89–90 omits current paragraph (j), the under-12m alternative, and labels pilot clause (j). USCG International Rule35(j)/(k) supplies the current distinction; Editorial source concordance checked 2026-10-11: IH Portuguese normative Rule35(j)/(k), PDF61, and USCG INTERNATIONAL Rule35(j)/(k), PDF113, agree. The IH English appendix omission/misnumbering remains recorded; no professional approval is claimed.

    Checked on
  8. RIEAM / COLREGs — Rule 2: Responsibility ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 3, normative Portuguese printed p.3 (PDF p.9); International English cross-check in USCG handbook.

    Rule 3; IH normative Portuguese printed p.3, PDF p.9; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  9. RIEAM / COLREGs — Rule 6: Safe speed ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 6, normative Portuguese printed p.8 (PDF p.14); International English cross-check in USCG handbook. Rule 7, normative Portuguese printed p.8 (PDF p.14); International English cross-check in USCG handbook.

    Rule 6; IH normative Portuguese printed p.8, PDF p.14; English International column cross-check. Individual provision commencement not established: valid_from remains null. Rule 7; IH normative Portuguese printed p.8, PDF p.14; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on
  10. RIEAM / COLREGs — Rule 8: Action to avoid collision ↗

    Instituto Hidrográfico / International Maritime Organization · Rule 8, normative Portuguese printed p.9 (PDF p.15); International English cross-check in USCG handbook.

    Rule 8; IH normative Portuguese printed p.9, PDF p.15; English International column cross-check. Individual provision commencement not established: valid_from remains null.

    Checked on

Regulatory references

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Restricted visibility In sight of one another Risk of collision

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