Understanding Tides and Currents for Boaters

Tides and currents are among the most important factors in coastal boating. The rise and fall of the tide determines when you can enter or leave a harbour, how much water is under your keel, and how fast you will travel. Tidal currents can help or hinder your passage, adding or subtracting several knots from your speed. Understanding tides is essential for safe and efficient coastal navigation — this guide covers how tides work, how to read tide tables, and how to plan passages around tidal flow.

What causes tides

Tides are caused by the gravitational pull of the moon and the sun on the Earth's oceans. The moon has the strongest effect because it is much closer to Earth. As the moon orbits the Earth, it creates a bulge of water on the side of the Earth facing the moon and a corresponding bulge on the opposite side (caused by the Earth's rotation). As the Earth rotates through these bulges, coastal areas experience a rise in water level — high tide — and between the bulges, a fall — low tide.

Most coastal areas experience two high tides and two low tides each day — a semi-diurnal tide. The time between high and low tide is approximately 6 hours and 12 minutes, meaning the tide cycle shifts by about 50 minutes each day. Some areas, such as the Gulf of Mexico, experience only one high and one low tide per day — a diurnal tide. A few areas have mixed tides with unequal high and low waters.

Spring and neap tides

The sun also exerts a gravitational pull on the oceans, about 46% as strong as the moon's. When the sun and moon are aligned — at new moon and full moon — their gravitational forces combine, producing the highest high tides and lowest low tides. These are called spring tides and occur every two weeks. The tidal range (difference between high and low water) is at its greatest during spring tides.

When the sun and moon are at right angles to each other — at the first and third quarter moons — their gravitational forces partially cancel each other, producing the lowest high tides and highest low tides. These are called neap tides and also occur every two weeks. The tidal range is at its smallest during neap tides.

For boaters, spring tides mean stronger tidal currents and less water at low tide — more careful passage planning is needed. Neap tides mean weaker currents and more water at low tide — easier conditions for navigating shallow areas.

Tidal heights and datum

Tidal height is the vertical distance between the water surface and a reference level called the datum. On most charts, the datum is Lowest Astronomical Tide (LAT) — the lowest tide that can be predicted under normal meteorological conditions. Depths shown on charts are measured from LAT, so the actual depth at any given time is the charted depth plus the current tidal height above LAT.

To calculate the actual water depth at a given time:

Actual depth = Charted depth + Tidal height

For example, if the chart shows 2.0m and the tide table gives a tidal height of 3.5m, the actual depth is 5.5m. If your boat draws 1.5m, you have 4.0m of clearance — comfortable. But if the chart shows 0.5m and the tidal height is 1.0m, the actual depth is 1.5m — and with a 1.5m draft, you would just touch the bottom.

Reading tide tables

Tide tables list the predicted times and heights of high and low water for standard ports. For secondary ports (smaller harbours and anchorages), corrections are applied to the nearest standard port — these are published in tide table supplements. Tide tables are available as printed booklets, online, and as apps. For more on tide apps, see our boating apps guide.

When reading a tide table, note the time (usually in local time, adjusted for daylight saving), the height in metres, and whether it is high water (HW) or low water (LW). The tidal curve — a graph showing the rate of rise and fall — allows you to estimate the tidal height at any time between high and low water. Most tide tables include a tidal curve for each standard port.

The rule of twelfths

For a quick estimate of tidal height between high and low water, boaters use the rule of twelfths. This assumes that the tide rises and falls in a predictable pattern over approximately 6 hours:

• 1st hour: 1/12 of the total range
• 2nd hour: 2/12 of the total range
• 3rd hour: 3/12 of the total range
• 4th hour: 3/12 of the total range
• 5th hour: 2/12 of the total range
• 6th hour: 1/12 of the total range

For example, if the tidal range is 4.8m (from 1.2m at LW to 6.0m at HW), the tide rises 0.4m in the first hour, 0.8m in the second, 1.2m in the third and fourth, 0.8m in the fifth, and 0.4m in the sixth. The rule is an approximation and works best in areas with semi-diurnal tides — always use a proper tidal curve for critical passages.

Tidal currents and streams

As the tide rises and falls, water flows in and out of coastal areas, creating tidal currents (also called tidal streams). The incoming current is called the flood tide, and the outgoing current is the ebb tide. The period of minimal current around high and low water is called slack water — the best time for manoeuvring in harbours and through narrow channels.

Tidal current direction and strength are shown on charts as tidal stream arrows, with the rate (in knots) printed above. Tidal stream atlases provide detailed information on current direction and strength for each hour of the tide cycle, covering coastal and offshore areas. In areas with strong tides — the English Channel, the Strait of Messina, the Bay of Fundy — tidal currents can exceed 5 knots, making them a major factor in passage planning.

Planning a passage with tides

Tidal planning involves choosing a departure time and route that maximises favourable current and minimises adverse current. Key principles:

• Go with the flow: Depart so that the tidal current is in your direction of travel for as much of the passage as possible.
• Plan arrival at slack water: For harbour entries through narrow channels with strong currents, time your arrival for slack water.
• Calculate course to steer: In cross-tide situations, you must steer a course that compensates for the tidal set — this is calculated using a tidal vector diagram on a chart.
• Check depth at critical points: If crossing a shallow bar or channel, calculate the tidal height at your planned crossing time and confirm sufficient depth.
• Allow for wind against tide: When wind opposes the tidal current, waves build up steeply — a common cause of rough conditions in coastal waters.

Tidal diamonds

Tidal diamonds are lettered symbols on charts that reference a table giving the direction and rate of the tidal stream at that point for each hour of the tide cycle. The table is keyed to a reference port and shows the stream direction (in degrees) and rate (in knots) for springs and neaps. Tidal diamonds are a quick way to get current information for a specific location without consulting a full tidal stream atlas.

Common tidal hazards

Overfalls and races

Where strong tidal currents interact with shallow seabed features, overfalls and races form — areas of rough, confused water that can be dangerous to small boats. Headlands and underwater ridges are common locations for races. These are marked on charts and should be avoided or passed at slack water.

Tidal bores

In some rivers with a large tidal range, the incoming tide forms a wave that travels up the river — a tidal bore. The Severn Bore in England and the Qiantang Bore in China are famous examples. Tidal bores can be dangerous and should be treated with respect.

Drying out

In areas with a large tidal range, a boat can be left high and dry at low tide — intentional drying out is common for hull maintenance, but unintentional grounding on a falling tide can leave you stranded for hours. Always check the tide times when anchoring or mooring in shallow areas.

Tides and currents are a fundamental part of coastal navigation. Every boater should know how to read tide tables, calculate tidal heights, and plan passages around tidal flow. For more on navigation, see our marine charts guide and our safety equipment checklist.