Fog, a Shoal and a South Cardinal Mark
At 0620 you are navigating an 18 m planning motor yacht from Po Toi to Hong Kong Island via Tathong Channel. Visibility has fallen to about 200 m in fog and drizzle. Your radar is working but is cluttered by rain; AIS is intermittent. Speed is 7 knots. You have a paper chart, GPS, echo sounder, and a hand-bearing compass. The tide is flooding; predicted height at 0700 is +0.6 m above chart datum. Your draft is 1.8 m. Ahead, the chart shows a shoal patch with a least depth of 3.2 m at chart datum, marked by a south cardinal buoy. The echo sounder reads 11.8 m below the keel. You hear one prolonged blast followed by two short blasts from a vessel forward of your beam, but you cannot see it. Your next waypoint is a narrow pass between two headlands.
1. What does the sound signal tell you, and what action do you take under Rule 19?
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One prolonged blast followed by two short blasts is the Rule 35(c) signal that may be made by a vessel not under command, restricted in ability to manoeuvre, constrained by draft, sailing, fishing, or towing. You cannot tell which from the sound alone, so treat it as a hampered vessel and keep clear. In restricted visibility Rule 19 applies: there is no stand-on vessel. Proceed at a safe speed for the visibility, have engines ready for immediate manoeuvre, use radar and sound signals, and post a lookout. Because the signal is forward of your beam, reduce speed to the minimum steerage way and, if necessary, stop and navigate with extreme caution until the risk passes. Do not hold course merely because you think you have right of way.
2. Calculate the under-keel clearance over the shoal and decide whether you can pass directly over it. What other information do you need?
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Over the shoal, charted depth 3.2 m + tide 0.6 m = 3.8 m of water. With draft 1.8 m, the static UKC is 2.0 m. That is a bare minimum in calm water and is reduced by squat at 7 knots, swell, tide prediction error and chart accuracy. The echo sounder reading of 11.8 m below the keel shows you are not over the shoal at that moment, so the calculation is a warning for the route, not a present grounding. Do not pass directly over the 3.2 m patch; pass south of the south cardinal mark, whose two cones point down, and keep to the deeper water. If the route cannot maintain at least 2–3 m UKC in the prevailing conditions, turn back or wait.
3. What fix would you use in fog, and how do you verify the GPS position?
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Fix by radar ranges and bearings to identified headlands or buoys, checked with the hand-bearing compass and the echo sounder contour. Plot the fix on the paper chart and compare it with GPS; check that the GPS datum matches the chart and that AIS is not lagging. Use parallel indexing on the radar and the echo sounder as a safety contour. In fog, do not rely on a single GPS position; a GPS fix can be offset or delayed. Keep the sound-signal and radar watch, and if the fix is uncertain, reduce speed or anchor in a safe depth clear of the channel.