Fog Approach to Ploče with a Crossing Ferry and a Falling Tide
At 04:20 on a dark, foggy morning, your 180 m bulk carrier, draft 12.4 m, is inbound to Ploče pilot station at 10 kn. Visibility is 0.3 NM. The dredged channel is 0.35 NM wide with a charted depth of 13.8 m; the tide is falling, height 0.4 m, and the minimum required UKC is 1.5 m. A 2.0 kn tidal stream sets across the channel. Radar shows a ferry on a constant bearing, range 1.2 NM, bearing 020° relative, speed 14 kn; CPA is 0.25 NM and TCPA 5 minutes. The ECDIS route has an XTD of 0.05 NM and two position sources disagree by 40 m. The pilot boarding ground is 1.5 NM ahead. The master must decide whether to continue at 10 kn, reduce speed, or abort and anchor outside the channel.
1. Assess the collision risk with the ferry and set out the immediate actions you would take as OOW, including bridge team, sound signals and VHF use.
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Risk assessment. A constant bearing with decreasing range is a risk of collision under Rule 7. The ferry is on the starboard bow at 1.2 NM; CPA 0.25 NM is less than visibility 0.3 NM, so it is not a safe passing distance. In fog there is no visual aspect, so the radar plot and relative-motion display are the primary evidence. Do not assume the ferry will give way merely because a crossing rule may apply: in a narrow channel, Rule 9 and local traffic may constrain both vessels.
Immediate actions. Reduce speed and take way off as necessary; a bold alteration to pass astern of the ferry is preferable to a small alteration across its bow, but only if the alteration can be seen on radar and does not put you outside the channel. Sound one prolonged blast at intervals not exceeding 2 minutes while making way; if stopped and making no way, two prolonged blasts with about 2 seconds between. Switch to hand steering, call the master, station a bow lookout, ring engine room standby, and use all available radar ranges and parallel-index marks.
VHF. Use VHF only to exchange information, not to negotiate collision avoidance. A passing arrangement agreed by VHF does not replace the Rules. If in doubt, stop and hold clear rather than commit to a narrow channel at speed.
2. Evaluate the ECDIS position disagreement and the XTD. What checks and contingency actions are required before continuing?
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A 40 m disagreement between two GNSS sources is significant in a channel only 0.35 NM, about 648 m, wide. An XTD of 0.05 NM leaves roughly 46 m either side if the vessel is exactly on centreline, so the plan has almost no margin for position error, cross-current or helm error.
Checks. Compare the ECDIS position with radar ranges, visual transits and soundings; check gyro error, speed input, datum, ENC updates, safety contour and UKC settings. Verify the GNSS receivers' integrity and RAIM status, and identify which source is suspect. Do not simply select the source that agrees with the chart. Confirm the echo sounder is working and compare indicated depth with charted depth plus tide.
Cross-current. At 10 kn with a 2 kn beam current, the drift angle is about arctan(2/10) = 11°, and the vessel will be crabbing. Over 1 NM that gives a lateral offset of about 0.2 NM, far more than the XTD. The plan must use a heading offset, parallel-index marks and a wider safe corridor.
Contingency. Reduce speed, increase XTD to the safe navigable width, post a second radar observer, prepare the anchor, and be ready to hold outside the channel. If position integrity cannot be confirmed, do not continue on ECDIS alone.
3. Calculate the UKC available at 10 kn and at 6 kn and make a recommendation on proceeding or anchoring.
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Static UKC. Actual depth = charted depth + tide = 13.8 + 0.4 = 14.2 m. Static UKC = 14.2 - 12.4 = 1.8 m.
Squat estimate. Using the approximate open-water formula squat = Cb × V² / 100 with Cb ≈ 0.82:
- At 10 kn: 0.82 × 100 / 100 = 0.82 m; UKC ≈ 1.8 - 0.82 = 0.98 m.
- At 6 kn: 0.82 × 36 / 100 ≈ 0.30 m; UKC ≈ 1.8 - 0.30 = 1.50 m.
In a confined channel, squat is greater than the open-water estimate because of blockage, so these figures are optimistic. At 10 kn the UKC is clearly below the required 1.5 m. At 6 kn it only just meets the minimum, with no allowance for heel, silting, swell, tide prediction error or the cross-current crabbing angle. The tide is falling, so the margin will reduce further.
Recommendation. Do not proceed at 10 kn. Reduce to 6 kn or less, verify actual depth by echo sounder and confirm the controlling depth with the port or pilot. If there is any doubt, anchor outside the channel and wait for the flood tide and better visibility. If continuing, keep the anchor ready, use hand steering and parallel index, and have tugs or a pilot available.