Yachtmaster Offshore Standard: Tides

All worked values in these advanced sections are fictional, non-navigational teaching data. Use the current publication, applicable observations and vessel-specific margins for a real decision.

Start by controlling the source. Record the named standard or secondary port, date, edition or service, time basis, vertical datum and the specific curve, correction or stream method in use. A direct digital prediction for a secondary port is not corrected again with a paper secondary-port table. Two apps repeating the same dataset are one prediction path, not independent confirmation.

Separate three evidence classes. A fact is directly established for this calculation, such as the printed HW time, the charted sounding or a depth trend observed on a checked instrument. An estimate is a modelled result, such as predicted tidal height or an interpolated stream rate. An assumption is a condition the plan depends on, such as boat speed, gauge comparability or an unverified weather allowance. Label uncertainty instead of hiding it inside a precise-looking answer.

Turn the result into a gate with an arrival window, minimum depth or clearance, an operational allowance, a last safe decision point and a fallback. Recalculate when actual progress, depth, stream, pressure, wind or sea state differs materially from the plan. A favourable permission, desired ETA or best-case calculation does not reopen a gate whose evidence or margin has failed.

Observed data also needs source control. Before comparing a tide gauge or stream station with the plan, check that it is the intended station, that the timestamp and variable are current, and that its datum, quality and location are applicable. An ordinary navigation buoy or a display labelled 'tide' is not automatically an authoritative live observation.