Understanding Tides for Sailors

By Peter18 mins read

In some sailing areas the tide is barely noticeable. In others, the water rises and falls several meters twice a day and tidal streams run faster than a small yacht can motor. Either way, understanding tides lets you answer the two questions that matter most on a tidal coast: will there be enough water? and which way will the stream be running?

Quick answer: tides are caused mainly by the gravitational pull of the moon and, to a lesser extent, the sun. Most coasts have two high and two low waters a day. The depth at any time is the charted depth plus the height of tide from the tide tables. The rule of twelfths (1-2-3-3-2-1) estimates how much the tide rises or falls in each hour. Tidal streams are predicted with tidal diamonds, tidal stream atlases or current tables, and the best passages are timed to carry a fair tide and avoid wind against tide.

Illustration of a sloop sailing near a coastline where the tide rises and falls

What Causes Tides?

The moon’s gravity pulls the ocean into a bulge on the side of the earth facing the moon. A second bulge forms on the opposite side, because the earth itself is pulled toward the moon more strongly than the water furthest from it. As the earth rotates through these two bulges, most places see two high waters and two low waters in a little over a day.

Because the moon moves along its orbit while the earth turns, a lunar day is about 24 hours 50 minutes. That is why high water typically comes around 50 minutes later each day, and why successive high waters on a semidiurnal coast are roughly 12 hours 25 minutes apart.

Local geography then shapes the result. Coastlines, basins and the seabed amplify or dampen the tide, which is why the range is over 15 meters in parts of the Bay of Fundy, around 4–7 meters on much of the English Channel coast, and often well under half a meter in most of the Mediterranean.

Types of tide

Semidiurnal: two high and two low waters of similar height each day (much of the Atlantic coast of Europe and North America).
Mixed: two highs and two lows of noticeably different heights (the US West Coast and much of the Pacific).
Diurnal: one high and one low each day (parts of the Gulf of Mexico and Southeast Asia).

Spring and Neap Tides

The sun also raises tides, about half as strongly as the moon. When the sun, moon and earth line up at new moon and full moon, their effects combine to produce spring tides, with higher highs, lower lows and stronger streams. The name has nothing to do with the season. When the sun and moon are at right angles, at the first and last quarter moons, they partly cancel each other out and produce neap tides, with a smaller range and weaker streams.

Springs usually arrive a day or two after the new or full moon, and the cycle repeats roughly every two weeks. Tidal streams at springs can be close to twice as strong as at neaps. The largest spring tides of the year tend to come near the spring and autumn equinoxes.

How to Read a Tide Table

Tide tables list the times and heights of high water (HW) and low water (LW) for each day at a standard port (the UK term) or reference station (NOAA’s term). Heights are given above chart datum, the same low reference level used for the depths on your chart, which is explained in how to read a nautical chart.

Things to check every time:

Time zone: many tables are in standard time; add an hour for daylight saving time if the table does not.
Units: meters or feet, matching your chart.
Secondary ports: smaller harbors have time and height differences relative to a standard port. The UKHO publishes these in Admiralty Tide Tables and almanacs; NOAA lists subordinate stations with corrections, and its online predictions apply them for you.
Range: the difference between HW and LW tells you whether it is springs, neaps or somewhere in between.

Predictions are astronomical. Weather changes them: high atmospheric pressure lowers sea level by roughly a centimeter per hectopascal above average, while low pressure and strong onshore winds raise it, sometimes dramatically in a storm surge. Allow a margin, especially when the barometer is high and you are cutting it fine.

The Rule of Twelfths With an Example

Tides do not rise or fall at a steady rate. They start slowly, run fastest in the middle and slow again near high or low water. The rule of twelfths approximates this for a semidiurnal tide lasting about six hours:

Hour after LW (or HW)	Fraction of the range	Cumulative
1st	1/12	1/12
2nd	2/12	3/12
3rd	3/12	6/12 (half)
4th	3/12	9/12
5th	2/12	11/12
6th	1/12	12/12

Worked example

Low water is at 06:00 with a height of 0.8 m, and high water is at 12:00 with a height of 4.4 m. You want to cross a bar with a charted depth of 1.2 m. Your boat draws 1.8 m and you want at least 0.5 m under the keel.

Range: 4.4 − 0.8 = 3.6 m, so one twelfth is 0.3 m.
Water needed: draft 1.8 m + clearance 0.5 m = 2.3 m. The bar has 1.2 m at chart datum, so you need a height of tide of at least 2.3 − 1.2 = 1.1 m.
When: after the first hour the tide has risen 1/12 (0.3 m) to 1.1 m. So from about 07:00 the depth over the bar is 1.2 + 1.1 = 2.3 m.
At 09:00 (three hours after LW) the tide has risen 6/12 (1.8 m) to a height of 2.6 m, giving 3.8 m over the bar.

On the way back, work it the other way from high water. In practice, give yourself more margin than this calculation, because the rule is approximate and swell makes boats pitch deeper in the troughs.

The rule of twelfths works best on regular semidiurnal tides. It is unreliable where tide curves are irregular, such as the Solent with its double high waters, and on mixed or diurnal coasts. In those places, use the tidal curves in an almanac or an online prediction for the exact time you need. For how much water your type of boat really needs, see how shallow a sailboat can go.

Tidal Streams and Currents

The rise and fall of the tide is accompanied by horizontal movement of water: tidal streams, also called tidal currents. Streams usually flood in one direction and ebb in the other, with a period of slack water in between. Slack water does not always coincide with high or low water, especially in estuaries and around headlands.

Where to find stream information

Tidal diamonds: on Admiralty and many European charts, a lettered diamond symbol refers to a table on the chart giving the stream’s direction and rate (spring and neap) for each hour before and after HW at a standard port.
Tidal stream atlases: a page for each hour relative to HW at a standard port, with arrows showing direction and figures showing rates at neaps and springs.
Current predictions: in the US, NOAA publishes tidal current predictions for many stations, giving times of slack water and maximum flood and ebb with speed and direction.
Apps and chartplotters: many display predicted streams, but check which data source they use.

To estimate a rate between springs and neaps, interpolate using the day’s range. If the range is halfway between the neap and spring ranges, use a rate roughly halfway between the neap and spring figures.

Wind against tide

When the wind blows against the direction of the tidal stream, waves become shorter, steeper and more dangerous. A moderate breeze against a strong spring ebb off a headland can produce breaking seas that would be harmless at slack water. Tidal races and overfalls, marked on charts with wavy symbols, are worst in these conditions. The stream also changes the wind you feel on board, as explained in true wind vs apparent wind.

Planning Passages Around the Tide

A fair tide can add two or three knots to a small boat’s speed over the ground; a foul one can stop it completely. Good tidal planning means:

Timing departures so the stream is with you for the longest part of the trip, and especially through headlands and narrows where it runs fastest.
Avoiding wind against tide in exposed areas, even if it means a slower passage.
Checking gates: some harbors are only accessible near high water; some bridges can only be passed near low water. Work out the window before you leave.
Allowing for set: on a cross-tide passage, steer a course that compensates for the stream rather than trying to point straight at the destination. Plot the course to steer with the stream vectors on the chart.
Keeping a margin: if a delay would make you miss the window, have a plan B anchorage.

On a busy route such as the English Channel, tidal timing and shipping lanes go together; see the best time to cross the English Channel by sailboat and the sailboat right of way rules for crossing traffic separation schemes. At night, recognizing buoys and ships by their lights becomes part of the job, covered in sailboat navigation lights explained. Add tide times and the predicted stream direction to your pre-departure checklist.

Anchoring in Tidal Waters

Tides change both the depth under you and the direction your boat lies at anchor.

Check the depth at low water. Before dropping the anchor, work out how much the tide will fall and make sure there will still be enough water under the keel throughout your swinging circle.
Calculate scope for high water. Pay out scope based on the depth at high water plus the height of the bow roller above the water, not the depth when you arrive. Five to seven times that figure in chain is a common guide for overnight stays; more in strong winds.
Expect to swing. The boat will turn through up to 180° when the stream reverses. Make sure the anchor will reset and that you will not swing onto hazards or other boats.
Watch for streams. In strong currents, choose an anchorage out of the main flow, and consider an anchor alarm.

For technique and seabed choice, see how to anchor a sailboat. And if your electronics fail in tidal waters, a few traditional methods from navigating without a chart or compass can get you home.