Tides are the slow, regular rise and fall of the sea, driven mainly by the Moon's gravity, supported by the Sun's influence, and reshaped by coastlines and ocean basins. For the navigator, tides affect depth, under-keel clearance, tidal streams, berth access, and the timing of every coastal passage.

What you will learn

  • Why most coasts experience two high waters during a lunar day.
  • Why the tide arrives roughly 50 minutes later from one day to the next.
  • How spring and neap tides differ.
  • How to distinguish semidiurnal, mixed, and diurnal tidal patterns.
  • How chart datum, height of tide, charted depth, and under-keel clearance fit together.
  • How tidal streams alter a vessel's course and position.
  • How to use the Rule of Twelfths as a first estimate—and when not to use it.

The tide-raising force

The Moon is the main tide-maker. Although the Sun is much more massive, it is far farther away. Tides depend on the difference in gravitational pull across Earth's diameter, and that tide-raising effect falls approximately with the cube of distance. The Sun's tidal effect is therefore only about half the Moon's.

The ocean forms two idealised bulges: one on the side facing the Moon, where water is pulled more strongly than Earth's centre, and one on the far side, where water is pulled less strongly and is left behind. As Earth turns beneath these bulges, many coasts pass through two high and two low waters.

A lunar day is about 24 hours 50 minutes. This is why corresponding high waters commonly occur about 50 minutes later on the following day.

Springs, neaps, and tidal patterns

At new and full moon, the Sun, Earth, and Moon are aligned. Their tide-raising effects reinforce one another, producing spring tides with a larger range. At first and last quarter, the Sun and Moon act at right angles, partly cancelling one another and producing neap tides with a smaller range.

Real oceans do not behave like a uniform water-covered globe. Basin size and depth, resonance, friction, coastline shape, and Earth's rotation create three broad patterns:

  • Semidiurnal: two similar high and two similar low waters each lunar day.
  • Mixed: two highs and lows of noticeably unequal height.
  • Diurnal: one high and one low water each lunar day.

In many basins, the tidal wave rotates around an amphidromic point, where the tidal range is nearly zero. Co-tidal lines connect places where high water occurs at the same time; co-range lines connect places with the same tidal range.

Datums, depth, and clearance

Chart datum is the reference level from which charted depths are measured. On Admiralty charts it is normally close to Lowest Astronomical Tide. The practical relationships are:

  • Depth of water = charted depth + height of tide.
  • Over a drying bank: depth of water = height of tide − drying height.
  • Under-keel clearance = depth of water − vessel draught, after applying all required safety allowances.

Mean High Water Springs, Mean High Water Neaps, Mean Low Water Neaps, and Mean Low Water Springs describe long-term average levels. They are not guaranteed extremes.

Rule of Twelfths

For a smooth, symmetrical tide rising or falling over about six hours, the range is divided into twelve parts. The change in successive hours is estimated as 1–2–3–3–2–1 twelfths. The first and last hours are slowest; the middle two are fastest.

Example: low water is 1.0 m and high water is 6.1 m, so the range is 5.1 m. A required height of 2.7 m is 1.7 m above low water, or four twelfths of the range. Three twelfths rise in the first two hours and the next hour contributes three twelfths, so the remaining one twelfth takes about 20 minutes. Starting from low water at 07:35 gives approximately 09:55.

The rule is only an approximation. Do not use it blindly at ports with a stand, double high water, strongly distorted curves, or an unusual rise or fall duration. Use the official tidal curve.

Tidal streams and navigation

The vertical rise and fall creates horizontal movement. Set is the direction toward which the tidal stream flows; rate is its speed. A tidal stream atlas gives regional hourly patterns, while charted tidal diamonds provide set and spring/neap rates at selected positions.

A dead-reckoning position uses course and distance through the water. Applying the tidal vector and leeway produces an estimated position over the ground. For a course to steer, work the vector triangle in reverse so the combined vessel-through-water and tidal vectors produce the required ground track.

Weather and observed levels

Tide tables predict the astronomical tide under average weather conditions. Wind and atmospheric pressure can change the observed level substantially. Low pressure generally raises sea level, high pressure lowers it, and sustained winds can pile water against a coast or drive it away. Storm surges, seiches, river discharge, and wave conditions can add further departures.

Always compare predictions with current observations and forecasts. For safe navigation, use current official tide tables, charts, sailing directions, Notices to Mariners, and the vessel's approved under-keel-clearance procedures.