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All questions
Tides

A shallow-water calculation only just exceeds the chosen requirement. Why must the safety margin remain in the decision?

Short answer

Predictions, weather, waves, pressure, readings, and chart detail all have uncertainty

A tidal calculation combines predictions and measured or charted inputs that each have limits, while the operating vessel and sea state add further variation. A margin does not make them exact or repair unchecked inputs; it protects the decision from plausible uncertainty. Apply it before entering shallow water and revise Depth, Height, and Margin.

Sources and scope

Reviewed by Richard Moore on . This answer is scoped to united kingdom and general recreational seamanship; check the current chart, publication, local rules and official operational guidance before acting afloat.

Quick check

A shallow-water calculation only just exceeds the chosen requirement. Why must the safety margin remain in the decision?