In dense fog you hear the following signals: one prolonged blast every 2 minutes. Then separately you hear: two prolonged blasts every 2 minutes. What does each signal indicate?

A. One prolonged blast every 2 minutes: a POWER-DRIVEN VESSEL UNDERWAY AND MAKING WAY through the water — Rule 35(a). Two prolonged blasts every 2 minutes: a POWER-DRIVEN VESSEL UNDERWAY BUT STOPPED AND MAKING NO WAY through the water — Rule 35(b). The distinction between making way (moving through water) and stopped-no-way is critical: stopped-no-way means the engines may be running but the vessel has no steerage and is not moving through the water. This has implications for collision avoidance — a stopped vessel may be drifting with the current. Both signals require action under Rule 19 in restricted visibility: treat any sound signal from forward of the beam as indicating a potential close-quarters situation; reduce to safe speed if not already done; do not alter to port for a vessel forward of the beam.

B. One prolonged blast means full speed ahead; two prolonged blasts means the vessel is reducing speed. These are manoeuvring signals under Rule 34, not fog signals.

C. One prolonged blast every 2 minutes means a vessel at anchor. Two prolonged blasts every 2 minutes means a vessel aground. These are the standard anchor and aground fog signals.

D. Two prolonged blasts means a vessel engaged in trawling. Fishing vessels use two prolonged blasts to warn approaching vessels of gear in the water.

A. Two all-round red lights in a vertical line: this is a VESSEL NOT UNDER COMMAND (NUC) — Rule 27(a). The vessel is unable to manoeuvre as required by COLREGs due to some exceptional circumstance. Obligations: Rule 18 — all power-driven vessels must keep out of the way of a NUC vessel. You are the give-way vessel regardless of aspect or crossing/overtaking geometry. Actions: alter course and/or reduce speed early and substantially to keep clear; do NOT cross ahead of a NUC vessel. Monitor: watch for any green (starboard) or white (sternlight) becoming visible — this confirms aspect and direction of drift; watch for any additional lights (towing, dredging); check for any DSC/MAYDAY on DSC Ch70 or Ch16 — a NUC vessel may be in distress. Rule 35 fog signals for NUC: two prolonged + one short blast every 2 minutes if making way; two prolonged if stopped.

B. Two red vertical lights indicate a vessel constrained by draft. You must keep well clear but can pass ahead if the range is sufficient.

C. Two red lights in a vertical line mean the vessel is at anchor. Anchor lights are all-round white — two reds in a line indicate an anchored vessel with emergency lights on.

D. Two red lights indicates a fishing vessel with gear deployed. Maintain your course and speed as stand-on vessel; the fishing vessel must keep clear.

A. All-round red over white in a vertical line WITH sidelights: this is a PILOT VESSEL underway — Rule 29(a)(i). The red over white is the internationally recognised pilot vessel identification light. The sidelights confirm she is underway and making way. Obligations: no specific Rule 18 obligation to keep clear of pilot vessels — they are treated as power-driven vessels underway for collision avoidance purposes (Rule 18 list does not include pilot vessels). Apply normal Rules 12-17 based on aspect and situation. By day: pilot vessels display the international flag "H" (Hotel — vertical halves, white and red).

B. Red over white all-round lights indicate a trawler — the white is the towing light showing the direction of the gear, and the red indicates a vessel with nets deployed on the port side.

C. Red over white vertical lights indicate a vessel constrained by draught displaying the optional CBD lights under Rule 28.

D. Red over white in a vertical line means a vessel restricted in ability to manoeuvre (RAM). Stand-on rules apply and the pilot vessel should give way to you.