Tides and currents
The Master works the height of tide and the tidal stream from the tables, the tidal diamonds and the stream atlas, at standard and secondary ports, to be sure of the water for entry and departure and to allow for the set on passage. On ocean and coastal passages the ocean and tidal currents are allowed for in the course to steer and the EP. Underkeel clearance is calculated allowing for the height of tide, squat and any swell, and a margin is kept — the depth that looked adequate on the chart is not the depth under the keel at speed in a seaway.
Electronic navigation with judgement
- GNSS gives a continuous position but can fail, be jammed or read wrong; cross-check it and know its datum.
- ECDIS is the primary means, set up correctly for the ship, with the position cross-checked and a backup kept.
- Radar and AIS support fixing and collision avoidance, each with its limitations.
- Keep an independent means of navigating if the electronics or power fail.
WARNING — Underkeel clearance is not the charted depth At speed in a seaway the depth under the keel is reduced by squat and swell, and the tide changes it further. Calculate the underkeel clearance for the actual conditions and keep a margin; the charted depth is not the water under the keel.
NOTE — GNSS is one input, not the truth GNSS is accurate until it is not, and it can be jammed or spoofed. The command navigator cross-checks it against radar, visual and depth and keeps the running EP, never navigating on one box alone.