High Water Full and Change

High Water Full and Change (HWF&C), also called the establishment of the port, is the average interval between the moon’s meridian passage at full and new moon and the next high water at a given place. It is the local lunitidal interval at syzygy, an old shorthand for predicting spring high-water timing before harmonic methods. Expressed in hours and minutes, HWF&C let a mariner estimate high water from the moon’s phase and transit. Harmonic prediction and tide tables have replaced it, but the term survives on older charts and in tidal glossaries.