You observe a buoy: black body with one or more red horizontal bands; topmarks showing two black spheres in a vertical line; light characteristic Fl(2) white. What is this mark, what is beneath it, and how do you pass?

A. ISOLATED DANGER MARK. Topmark: two BLACK SPHERES in a vertical line (two balls stacked). Colour: black with one or more red horizontal bands. Light: Fl(2) white (group of 2 flashes). Meaning: this mark is placed DIRECTLY OVER OR ADJACENT TO a small, isolated danger (rock, wreck, shoal) with navigable water all around it. The danger has limited extent — it is not a wide shoal area. Action: PASS ON EITHER SIDE — navigable water exists all around the mark. However, give ADEQUATE CLEARANCE — the mark indicates a specific hazard point and is moored nearby, not exactly on the danger. Do NOT pass closely — the danger extends to an uncertain radius around the buoy. Check the chart for the nature and extent of the charted hazard. The isolated danger mark is NOT a cardinal mark and does not indicate a safe side — it warns of a specific point hazard.

B. Two spheres topmark with Fl(2) white indicates a north cardinal mark. Pass to the north of this buoy.

C. Black with red bands and two sphere topmarks indicates a preferred channel mark showing the primary channel is to starboard. In IALA A, pass with the mark to starboard to take the main channel.

D. An isolated danger mark with Fl(2) means the danger flashes twice — there are two separate rocks beneath the buoy. Pass between them where the chart shows clear water.

A. IALA System A — Red to PORT when entering harbour (proceeding from seaward). Red can buoy on your port side means you are on the starboard side of the channel, heading seaward (leaving harbour). To re-enter, you would keep the red buoy to PORT. Rule: In IALA A: RED = PORT = entering harbour (coming from sea). Green = Starboard = entering harbour. Memory aid for IALA A: "Red to Port Returning." The question states the red buoy is to port — therefore you are ENTERING HARBOUR correctly, keeping the channel to starboard. Pass the red buoy keeping it to PORT (leave it on your port side). In IALA B (Americas, Japan, Philippines, Korea): colours are reversed — red to starboard entering.

B. In IALA A, red buoys are always on the starboard side of the channel. Pass the red buoy keeping it to starboard — it marks the starboard side of safe water.

C. IALA A and B are interchangeable in UK waters. The MCA has adopted a combined system since 2015 where red marks starboard in all UK harbour approaches.

D. Red can buoys are cardinal marks indicating danger to the north. Pass to the north of the buoy regardless of the direction of travel.

A. NORTH CARDINAL MARK. Topmark: two cones BOTH pointing UP (like an arrow pointing north). Colour: black over yellow (black upper, yellow lower). Light: white VQ (very quick) or Q (quick) — uninterrupted. Meaning: safe water lies to the NORTH of this mark. The mark indicates a hazard to its south — the safe side to pass is to the NORTH. Action: pass the mark to the NORTH — i.e., keep the mark to the SOUTH of you as you pass. In this scenario the buoy is bearing 358° (nearly north), so you are approaching from the south — the mark is warning you of danger to your south; you are already in the safe water north of the mark. Monitor: do not pass to the south of this buoy.

B. North cardinal marks are passed to the east. The topmarks and colour confirm this is a north cardinal but the passing convention is to leave it to port when heading north.

C. Two cones pointing up indicate an isolated danger mark. Pass on either side with caution as the danger is directly beneath the buoy.

D. VQ white light with black/yellow topmark indicates a special mark. These do not indicate hazards and may be passed on any side.