In fog near the Isle of Wight, your ARPA shows a contact steady on bearing 045°, range 1.2nm, CPA 0.15nm. You have a 2-knot tidal stream setting 090°. How does this affect your Rule 19 avoiding action?

A. TIDAL STREAM IN FOG AVOIDING ACTION: tidal stream is a critical factor in collision avoidance — it affects your actual ground track even when you alter your water track (compass course). IMPACT IN THIS SCENARIO: 2-knot set to 090° means your vessel is being moved east by the tide relative to the seabed. AFFECT ON CPA CALCULATION: ARPA calculates CPA based on relative motion — this should correctly incorporate your vessel's resultant movement (water track + tidal set). However: ARPA input is GNSS-derived ground track OR water track — verify which. If ARPA is set to "ground stabilised" mode (using GNSS), it already accounts for tide — the CPA shown IS the CPA over ground. If ARPA is "sea stabilised" (using heading and log speed), it does NOT automatically account for tidal set — the actual ground-track CPA may differ from displayed CPA. PRACTICAL IMPLICATION: in tidal waters, verify your ARPA mode. Sea-stabilised ARPA in strong tidal streams can give misleading CPA readings. AVOIDING ACTION: if you alter course to starboard (recommended under Rule 19(d)) — calculate how the tidal set will affect your actual track: a course alteration to starboard in an east-setting tide will bring you further east — potentially toward shoals. Chart consultation essential. ECDIS integrated with ground-stabilised ARPA gives most accurate picture in tidal waters.

B. ARPA always uses GPS-derived ground track data, so tidal stream is automatically incorporated and there is no need to consider tidal effects separately when using ARPA in fog.

C. Tidal stream does not affect collision avoidance decisions in fog — COLREG Rule 19 is based on radar relative motion, which inherently accounts for all vessel movements including tidal drift.

D. In tidal waters, fog navigation decisions should be based solely on visual bearings rather than ARPA, as radar is unreliable in areas of strong tidal flow.

A. SAFE SPEED IN RESTRICTED VISIBILITY — Rule 6 assessment (COLREG Rule 6): safe speed requires consideration of: (a) state of visibility (300m — severely restricted); (b) traffic density (Dover Strait is one of the world's busiest waterways — separation scheme traffic plus local traffic); (c) manoeuvring ability — your vessel's stopping distance at 12 knots (a 140m bulk carrier may take 15+ ship lengths to stop); (d) background lighting at night; (e) draught — your squat in shallow Dover Strait approaches. AT 12 KNOTS WITH 300M VISIBILITY: stopping distance will greatly exceed 300m — you cannot stop within the distance you can see. Rule 8(e) requires reduction to minimum steerage way if necessary to avoid collision. IMMEDIATE ACTIONS: (1) Sound fog signal — one prolonged blast every 2 minutes (Rule 35a — power-driven vessel underway making way); (2) Reduce speed to safe speed — likely 5-6 knots or less to ensure vessel CAN STOP within visibility range; (3) Post extra lookout — Rule 5 — additional lookouts on bow in fog; (4) ARPA on — operating on appropriate scale, all targets tracked; (5) VHF Ch 16 / Ch 11 (Dover VTS DOVER COASTGUARD) monitored; (6) Navigation lights on; (7) Master already called — correct. Safe speed is your judgement call — document your reasoning in the log.

B. Safe speed in fog is defined as half the vessel's normal service speed. At 12 knots service speed, 6 knots is automatically the correct safe speed under COLREG Rule 6.

C. Rule 6 safe speed assessment applies only when vessels have radar — a vessel without radar is exempt from safe speed requirements in restricted visibility.

D. With visibility of 300m, the vessel should anchor immediately in the Dover Strait to await improvement, regardless of traffic density or depth.

A. RULE 19(d) — Action when radar contact detected (no visual contact): COLREG Rule 19(d): a vessel which detects by radar only another vessel shall determine if a close-quarters situation is developing and/or risk of collision exists. If so, take avoiding action in ample time. The action must be: (a) A LARGE ALTERATION OF COURSE — Rule 19(d)(i): if possible, avoid an alteration of course to port for a vessel forward of the beam other than for a vessel being overtaken; (b) AVOID ALTERING COURSE TOWARDS A VESSEL ABEAM OR ABAFT THE BEAM — Rule 19(d)(ii); (c) REDUCE SPEED — reducing speed, stopping engines, or reversing may be needed. IN YOUR SCENARIO: CPA 0.1nm is a close-quarters situation. TCPA 8 minutes gives time for action. RECOMMENDED ACTION: (1) Reduce speed immediately (further — to minimum steerage); (2) Alter course to starboard — large alteration (minimum 30-40°) to open the CPA; (3) Sound appropriate fog signals; (4) Monitor ARPA to confirm action is working — CPA must open; (5) DO NOT alter to port (risk of swinging your bow into the other vessel's path); (6) VHF — if other vessel is a known contact, alert them, but do not rely on VHF agreement. Rule 19 is clear — radar action must be early, substantial, and effective.

B. With CPA 0.1nm and TCPA 8 minutes, the situation is not yet a close-quarters situation under COLREG Rules. Maintain course and speed while monitoring the target.

C. Call the other vessel on VHF Ch 16 and agree a port-to-port passing. Once verbal agreement is reached, no course alteration is necessary.

D. Stop engines and anchor immediately — Rule 19 requires vessels to anchor when a close-quarters situation develops in restricted visibility.