Under-Keel Clearance and a Crossing Tow in a Dredged Channel
Your 225 m LOA bulk carrier, draft 13.80 m even keel, is instructed to join the northbound convoy at 0400 UTC. The approach channel has a charted depth of 15.20 m below chart datum; predicted tide at the entrance is +0.70 m at 0400 and +0.30 m at 0800. The canal authority requires a minimum under-keel clearance of 10% of maximum draft at all times. Your bridge team calculates squat of 0.90 m at 10 knots and 0.55 m at 8 knots in the confined channel; the convoy speed is set at 10 knots. At 0350 a radar contact shows a tug-and-tow crossing the channel ahead from starboard at 4 knots, range 2.1 NM, bearing steady. Visibility is 2 NM in haze, wind NE 18 knots, and the current sets across the channel at 0.8 knots. The pilot is on board but has not yet given a speed reduction instruction.
1. Calculate the available under-keel clearance at 0400 and 0800 for 10 knots and 8 knots, and decide whether to enter the convoy on schedule. Show each step.
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Required UKC = 10% × 13.80 = 1.38 m.
At 0400: depth = 15.20 + 0.70 = 15.90 m; static UKC = 15.90 − 13.80 = 2.10 m.
- At 10 kn: 2.10 − 0.90 = 1.20 m, which is 0.18 m below the required 1.38 m. Not compliant.
- At 8 kn: 2.10 − 0.55 = 1.55 m, which is compliant by 0.17 m.
At 0800: depth = 15.20 + 0.30 = 15.50 m; static UKC = 1.70 m.
- At 8 kn: 1.70 − 0.55 = 1.15 m, below 1.38 m.
- At 10 kn: 1.70 − 0.90 = 0.80 m, clearly non-compliant.
Conclusion: the convoy's 10-knot speed cannot be used at 0400. The only compliant option in the stated data is to enter at 0400 at about 8 knots or less, if the convoy and pilot accept a reduced speed; waiting until 0800 makes UKC worse because the tide falls. The 1.55 m at 8 knots leaves only 0.17 m above the minimum, so I would treat it as a thin margin, consider 7 knots, confirm actual trim and water density, and keep the echo sounder and UKC display monitored. If the convoy cannot accept 8 knots, the ship should not join and should wait for the next suitable high water.
2. Assess the crossing tug-and-tow and state the COLREG action you would take.
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Risk of collision exists: the tug-and-tow is on a steady bearing at 2.1 NM and the range is closing; even at 4 knots, a steady bearing means the CPA is near zero unless something changes.
Rules: The ship is in a narrow channel, so Rule 9 requires her to keep as near to the starboard outer limit as is safe and practicable. The tug-and-tow is crossing the channel; under Rule 9(d) a crossing vessel should not cross if she would impede a vessel that can safely navigate only within the channel. But a tug with a tow may be hampered and unable to manoeuvre as an ordinary power-driven vessel; I would not assume she will give way. Rule 15 may also make own ship give-way because the tug is on the starboard side, and Rule 2 requires the ordinary practice of seamen.
Action: immediately reduce speed and be ready to stop; sound at least five short and rapid blasts if in doubt or if the tow does not appear to be taking action; call the pilot and master; try VHF but do not rely on it. I would not alter to port because that is toward the channel edge and the crossing vessel. If there is room, a small starboard alteration to pass under the tug's stern, combined with a large speed reduction, is preferable; if not, hold the ship up with engines and let the tow clear. The controlling principle is to keep the ship inside the dredged channel and avoid a close-quarters situation, not to insist on the right of way.
3. List the bridge-team and passage-plan actions to monitor UKC and current during the transit.
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Bridge team: put an officer on UKC/echo sounder monitoring with alarms set; plot positions at short intervals by visual/radar and GNSS; compare charted depths with the echo sounder; record under-keel clearance and speed at each critical point. Passage plan: mark abort/anchorage positions, no-go areas and the point of no return; calculate squat for the actual speed and trim; allow for the 0.8-knot cross-current by heading upstream of the track; maintain engine readiness; brief the helmsman on the narrow-channel track; and review the plan if the pilot gives a different speed or if the tow causes a hold-up. Also check tide height and density, and keep a written log for the canal authority.