Why the sea breathes

Twice a day around the UK the sea rises and falls, and the difference matters more to a small boat than almost anything else on the chart. Get the tide right and a drying harbour is a car park you can drive into; get it wrong and your keel meets it at the wrong time.

The pull comes from the moon and, to a lesser degree, the sun. When the two line up — at new moon and full moon — their pulls stack and the tide swings hard: high highs, low lows. These are springs. When the moon is at quarter phase it pulls at right angles to the sun, the effects partly cancel, and the swing is gentle: neapsneapsThe fortnightly small tides after the half moons — the gentle half of the cycle.full glossary →. The cycle from springs to neaps and back takes about a fortnight.

Chart datum — the chart’s idea of empty

Every depth printed on a chart is measured from chart datum (CD) — a level chosen so low that the real tide almost never falls below it. So a charted depth of 2 m means: even on the worst low water you can reasonably expect, there is about 2 m there. Whatever the tide is doing sits on top of that.

Heights of tide from the tide tables are also measured above chart datum. That is the whole trick of tidal calculations: depth available = charted depth + height of tide. Two numbers, one sum.

Some features dry. A green-tinted bank with an underlined figure — say 2̲·1̲ — stands 2.1 m above chart datum and is exposed whenever the tide falls below that. Water over a drying bank = height of tide − drying height.

The levels you will meet

Almanacs quote four mean levels for each port: MHWS and MHWN (mean high water at springs and neaps) and MLWN and MLWS (the matching low waters). They tell you instantly whether today’s tide is spring-ish or neap-ish: compare today’s range — HW height minus LW height — with the mean spring and neap ranges.

One more level matters: HAT, the highest astronomical tide. Overhead clearances — bridges, power cables — are charted from HAT, the meanest possible case. If the chart says 16 m under a cable, you have at least 16 m even at the biggest tide of the century, and more whenever the tide is below HAT.