Inbound to Aktau: Shallow-Water UKC Versus a Position Disagreement
At 0210 in clear weather a 5,800 GT general cargo ship with drafts 5.9 m forward and 6.5 m aft is inbound to Aktau on 078° at 12 kn. The passage plan takes her over a shoal patch charted at 9.2 m, where the chart carries a 2009 survey note and the port has advised up to 0.4 m of recent siltation. Company policy is an under-keel clearance of 10% of mean draft plus allowances for squat and heel, and never less than 0.8 m. The second officer's ECDIS puts the ship 0.25 nm to port of track, inside the 5 m contour, while the radar shows the fairway buoy bearing 315° at 1.1 nm and the port leading lights, which should be in line on 078°, are clearly open. Wind is 240° at 14 kn, there is no appreciable tide, and a 0.4 m seiche was reported yesterday. The master is resting; the chief officer is down in the hold.
1. Compute the available under-keel clearance on the shoal patch and decide whether the vessel may pass at 12 kn. State the action you would take.
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Start from the most pessimistic depth the ship may actually meet, not the charted figure.
- Depth to rely on: 9.2 m charted − 0.4 m siltation = 8.8 m. The seiche can take another 0.4 m off in a worst case, so treat 8.4–8.8 m as the band.
- Mean draft: (5.9 + 6.5)/2 = 6.2 m.
- Geometric UKC in 8.8 m: 8.8 − 6.2 = 2.6 m.
- Squat at 12 kn with Cb ≈ 0.75, open water: 0.75 × 12²/100 ≈ 1.08 m; in a dredged/confined cut it roughly doubles to ≈ 2.16 m.
- Heel/rolling allowance: 0.15 m.
- Required by policy: 0.62 m (10%) + 1.08 + 0.15 = 1.85 m (the 0.8 m floor is not the governing figure).
Remaining UKC after squat is 2.6 − 1.08 − 0.15 = 1.37 m against 1.85 m required, a shortfall of about 0.5 m; with seiche or channel effect it is worse. The vessel must not pass at 12 kn. There is no tide to wait for in the Caspian, so the levers are speed and timing: reduce to about 6 kn (squat ≈ 0.27 m), which restores roughly 2.2 m of UKC, confirm the actual depth with the echo sounder ahead of the patch, and only then proceed, ideally after the port confirms a current survey.
2. Three position sources disagree. Explain how you resolve the conflict and what you do before reaching the shoal.
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The three sources are not equal in value. The leading lights in line and the radar range/bearing to a fixed, charted object are ground-truth observations; the ECDIS position is a computed output that inherits every error upstream of it.
Likely causes, in order of probability: (1) GNSS error or an offset datum inside the receiver (local chart datum versus WGS-84, or a stale correction feed); (2) an ENC georeferencing or chart-datum shift on this particular cell; (3) the buoy has moved or been re-laid and the chart is uncorrected, in which case the radar-derived position is right and the chart is wrong; (4) a genuine cross-track error by the watch.
Routine to apply: check which GNSS is feeding the ECDIS and its HDOP/status; compare the ECDIS cursor position against a radar range and bearing taken to a second fixed object (two ranges give a cut with no compass error); confirm the leading lights are the correct pair for this reach and are in transit on the planned heading; if the lights are open and the radar agrees with them, navigate on visual/radar fixes and treat the ECDIS position as suspect.
Before the shoal: call the master, ring engines to standby, change to hand steering, reduce speed, post a forward lookout with a portable VHF, plot paper fixes at short intervals, apply a parallel index on radar, and inform port control of the discrepancy and of the reduced speed.
3. Set out the orders and records you would make from now until the vessel is safely past the patch.
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Orders: inform the master immediately, since the decision to proceed over a shoal with a marginal UKC is his; engines to standby and manoeuvring speed; hand steering with the helmsman repeating every order; echo sounder watched continuously and its readings logged; forward lookout and a lookout aft if the channel is narrow; VHF call to port control giving name, position by radar fix, draft, speed and the intention to pass the patch at reduced speed; request the latest survey and any notice of a shifted buoy. Nothing is done on VHF that cannot be done by the Rules - the call is information, not a right-of-way agreement.
Records: deck log and bell book entries for time, position (state the fixing method), speed, engine movements, soundings, and the moment the discrepancy was detected; a note of the master's decision and its basis; a defect/discrepancy report so the ENC or buoy position is corrected for the next ship; a near-miss entry in the SMS. If the vessel later grounds or touches, these records plus an immediate report to the company and flag state are what establish that the bridge team detected the error and acted on it rather than following the display.