Radar overlay offset, wrong safety contour and an unchecked ECDIS route
MV Northwind is a 190 m container ship drawing 11.8 m, inbound to Aldenham on an ECDIS route. The pilot has not boarded. On the approach the OOW notices that on the radar overlay the training wall and No.4 buoy appear about 0.15 NM to port of their ENC positions, while the radar echo of the wall agrees with the visual bearing by compass. GPS status shows 12 satellites and the ECDIS position is from GPS1. The ENC was last updated three months ago. The safety contour is set at 30 m and the safety depth at 12 m. The route was planned by the second officer with a cross-track limit of 0.15 NM and the route check was not completed. The ECDIS has produced five alarms in the last ten minutes and the OOW has silenced them. The echo sounder reads 13.6 m below the keel and the charted depth at the current position is 12.0 m.
1. Diagnose the 0.15 NM offset. List the checks in order and state which source you would trust for pilotage.
Read the worked answer
0.15 NM is about 280 m, far too much to dismiss as plotting error. Work from the simplest cause to the most serious:
- Compare the ECDIS own-ship position with a second independent GNSS receiver and with a visual fix from two bearings. If ECDIS and chart agree with the visual fix, the fault is in the radar overlay, not the chart.
- Check the gyro. An offset that is tangential to the range is the signature of a heading error: at 1.7 NM, 0.15 NM equals about 5°. Compare gyro heading with the azimuth of a charted object or with GPS course over ground on a steady leg.
- Check the radar heading marker alignment and the antenna offset entered in the radar or ECDIS.
- Check the ENC datum and the chart edition; a datum shift would displace the chart, but WGS84 ENCs should not show one.
For pilotage, trust the visual bearings, radar ranges and the echo sounder, all of which are independent of GPS and of the chart overlay. The chart remains useful for shape and for depths; the overlay must be corrected before it is used for cross-track control. Advise the master and the pilot, and plot a paper or visual back-up.
2. Critique the safety depth and safety contour settings and state what they should be, showing the calculation.
Read the worked answer
Required depth below the keel = squat in confined water at about 12 kn, say 0.8 m, plus the company UKC policy of 10% of draught, 1.2 m, = 2.0 m below the keel. With draught 11.8 m, the required depth of water is 13.8 m. With a height of tide of about 1.5 m, the required charted depth is 13.8 − 1.5 = 12.3 m, rounded up to 12.5 m.
So the safety depth should be about 12.5 m, not 12 m, and the safety contour must be set at a charted contour at or deeper than the safety depth-probably the 20 m contour in this channel. Setting the safety contour at 30 m is a serious error: every depth between 20 and 30 m is then treated as safe, the four-colour shading shows no hazard over the shoal bank, and the audible and visual warnings that would flag the bank never appear. The contour must not be shallower than the safety depth, or the display contradicts itself.
The echo sounder reading of 13.6 m below the keel against a charted 12.0 m plus about 1.5 m of tide gives about 13.5 m and is consistent, so the tide and the sounding are probably right; the error is in the ECDIS settings, not in the water.
3. What should be done about the route check, the alarms and the CATZOC and update status before the ship enters the channel?
Read the worked answer
The unchecked route is the largest single risk. The route check function verifies each leg against the safety settings-cross-track, wheel-over, isolated dangers, no-data areas. It must be run, and the route independently checked by a second person against the ENC and the paper chart: leg by leg, checking the cross-track limit against the channel width, the wheel-over positions, the turn radii, and whether the 0.15 NM cross-track limit equals the very offset now being seen, which would leave no margin at all.
The five silenced alarms are a symptom of settings that are wrong and of an OOW who has been conditioned to ignore them. Alarms must be acknowledged only after the cause is understood, and the event logged; repeated silencing of safety alarms is exactly how a grounding happens.
The ENC being three months old does not mean it is current. Check the update status and apply any outstanding notices, and check the CATZOC of the channel: a zone category C or D means the survey is old and the position and depth may be significantly in error, so the charted 12.0 m may not be reliable. Treat no-data areas as unsafe. Restore the correct settings, complete the route check, brief the bridge team, and enter the channel with a visual and radar cross-check running continuously.