Restricted Visibility

What changes in fog

In sight of one another, COLREG splits responsibility cleanly: one vessel gives way, the other stands on. Rule 19 dissolves that split. With visibility down to where you cannot see the other vessel, neither ship knows the other's exact heading or course-change with the same confidence as visual sighting allows. The rule therefore says:

• Both vessels proceed at safe speed appropriate to the prevailing visibility (Rule 6 reinforced).
• Engines ready for immediate manoeuvre.
• If a close-quarters situation is developing or risk of collision exists, action shall be taken in ample time.
• If only altering course, avoid an alteration to port for a vessel forward of the beam (other than for one being overtaken). Reasoning: the other ship may herself be turning to her starboard to clear; turning together to port creates head-on conflict.
• Avoid an alteration of course towards a vessel abeam or abaft the beam.
• Hearing apparently the fog signal of another vessel forward of the beam — or unable to avoid close quarters — slow to bare steerageway, or stop.

The radar / ARPA workflow

Rule 19 cannot be applied without systematic observation of every detected target. Rule 7 backs this up. The simulator's restricted-visibility scenarios force you through the workflow that real OOWs use:

1. Acquire every target on radar within 6–12 NM. ARPA does the maths automatically; manual plotters draw the relative-motion vector by hand every 3 minutes.
2. Read CPA and TCPA. CPA below your safety margin (typically 1 NM open sea, 0.5 NM coastal) = developing close-quarters.
3. Decide early. A 30° starboard alteration at 8 NM beats a 90° emergency turn at 1 NM — every time.
4. Verify after the manoeuvre that the new vector clears every target, not just the one that triggered the action.
5. Listen. Sound signals (Rule 35 — see Sound Signals module) tell you what the other vessel is doing. One prolonged blast every 2 minutes = power-driven vessel making way; two prolongeds every 2 = stopped.

The simulator scenarios

• RV Ahead — visibility 0.5 NM, single target dead ahead at 6 NM, slowly converging. Free demo.
• Multi-target Fog — three vessels in different sectors, one closing fast, one overtaking on radar. ARPA tracks all three; you choose order of action.
• Fog at TSS Boundary — northbound TSS lane, visibility 1 NM, ferry crossing track. Combine Rule 10 and Rule 19.
• Sound-signal recognition — fog horn audio plays, you identify the vessel type. Solitary long blast = under way; two longs = stopped; long-short-short = NUC, RAM, fishing, sailing or constrained.
• Bare steerageway — close-quarters confirmed, you must reduce to bare steerageway and possibly take all way off.

Common mistakes

• Looking for the give-way vessel — there isn't one. Both vessels manoeuvre.
• Altering port for a vessel forward of the beam — explicitly prohibited (Rule 19(d)(i)).
• Slowing down "to be safe" without alteration — speed reduction alone does not guarantee CPA improvement; in some geometries it makes CPA worse.
• Trusting AIS and ignoring radar — small craft, fishing buoys, semi-submerged debris and unflagged vessels all show on radar but not on AIS.
• Not committing — Rule 19 demands action in ample time. Last-minute corrections are exactly what causes collisions in fog.