Cross-Track Error on a Narrow Strait Passage
You are the master of a 32,000 GT container vessel with a draft of 11.8 m, transiting a narrow strait at night. The passage plan requires a minimum under-keel clearance of 10% of draft and a cross-track limit of 0.3 NM. The bridge team is using a dual ECDIS/paper backup arrangement. At 0215 local time, the ECDIS GNSS position feed begins to show a slow, steady offset to starboard of about 0.15 NM over 12 minutes while the radar overlay shows the vessel tracking correctly relative to charted buoys. The second officer is conning, the helmsman is steering by autopilot, and the pilot has just stepped out to the bridge wing. Traffic density is moderate, with two inbound vessels within 3 NM. The ECDIS is not displaying a position integrity alarm.
1. Identify the most probable cause of the discrepancy and explain the immediate actions you would take as master, including the order of priority.
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The most probable cause is a degraded or spoofed GNSS position input, or a datum/offset error in the ECDIS position source. Because the radar overlay agrees with charted features and the offset accumulates steadily, the GNSS feed is suspect rather than the radar. Immediate priorities are:
- Safety of navigation first. Order the helm to hand-steer, reduce speed to a safe speed for the visibility and traffic, and tell the pilot what is happening.
- Cross-check position using radar ranges and bearings to at least two charted fixed objects, and compare with the paper chart. Compute the true cross-track error before making any large course alteration.
- Isolate the faulty source. Change the ECDIS position source to a second independent receiver if available, or revert to radar/visual navigation and paper chart plotting. Do not continue to rely on the suspect feed.
- Inform and record. Call the master's standing orders into effect, log the event with times and offsets, and prepare a report. If the vessel is in a VTS area, advise the authority.
- Review passage. Confirm the vessel remains within the 0.3 NM cross-track limit and the under-keel clearance criterion; if not, adjust course to regain the planned track.
The key judgement is not to trust a single electronic position source and to preserve the vessel's safe track before investigating the cause.
2. Explain how you would verify whether the under-keel clearance requirement is still met, and what factors could invalidate the planned figure.
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Under-keel clearance is confirmed by comparing the charted depth plus height of tide against the vessel's actual draft, allowing for squat, heel, and chart datum. The planned 10% of 11.8 m is about 1.18 m. I would:
- Read the echo sounder and compare with the predicted depth at the vessel's actual position, not the planned position.
- Apply squat: at the planned speed in confined water, squat may be 0.3–0.6 m depending on block coefficient and speed. If speed has been reduced, squat falls, which improves clearance.
- Check tide height against the predicted tide and any surge or meteorological effects.
- Confirm the vessel's actual draft and trim, including any recent ballast changes.
Factors that could invalidate the plan include: charted depths based on an old survey, undetected shoaling, the vessel being off-track into shallower water, a lower actual tide than predicted, and increased squat from speed or from interaction with passing vessels. The master should not accept the plan's figure without confirming actual position and actual depth.