Sailing in tidal waters: harnessing nature's power

Sailing in tidal waters: harnessing nature's power

Find out everything you need to know about the tides and how you can use them to your advantage when sailing.

Tides, or sea swells, have long been recognised by coastal peoples. Although more of a mystery in times gone by, it recurred with such iron-clad regularity that ancient and medieval fishermen and mariners learned to use it to their advantage. Nowadays, we now know that these movements of seawater result from the gravitational field of the sun and moon, and the centrifugal force of the earth's rotation and we can refer to accurate tables that show us exactly when the water will advance or recede from the shore. If you understand tidal forces, the tides can be used for efficient sailing propulsion.

Tides are like the inhale and exhale of the sea

The tides have played and continue to play an important role in the lives of coastal nations and sailors who take to the seas, whether for a relaxing seaside holiday or for competitive sailing. Similar to sea currents, comprehending and utilizing tides provide an edge over those relying solely on charts and forecasts. If you want to enjoy recreational and competitive sailing to the fullest, the tides will allow you to achieve optimum results. Plus, riding a properly trimmed boat, with its speed supported by the power of the sea, is one of the most beautiful sailing experiences you can have.

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Tides follow a recurring pattern, with two tides alternating within a 24-hour and 50-minute cycle.  The length of this tidal cycle is also known as a lunar day — the time it takes for one point on the Earth to make one complete rotation and end up at the same point in relation to the moon. A lunar day is longer than a normal 24-hour day because the moon orbits the Earth in the same direction that the Earth is spinning.

The coastlines that surround the seas and oceans around the world therefore experience varying tides. as the height of the tide varies according to the relative positions of the Earth, the Sun and the Moon. Generally speaking, the largest tides occur during the full moon because the Sun and Moon are aligned with the Earth, resulting in increased gravitational forces.

The strength of tidal forces is also influenced by the Moon's position relative to the Earth. When the Moon is nearest to Earth in its elliptical orbit (perigee), tidal forces are stronger, while they are weakest when the Moon is at its farthest point from Earth or "apogee." Fortunately, sailors today no longer need to keep track of star constants, celestial body movements, or rely on a chart, as they have clear forecast tables at their disposal.

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Tides in the seas and oceans

Ancient seafarers had already observed the variation in tide intensity across different bodies of water. They then summarised their findings in a simple guide for captains and navigators that in enclosed waters (such as the Mediterranean or Baltic Sea), the tides have smaller fluctuations compared to regions where the open ocean meets the land (for example, the Atlantic, Pacific or North Sea coasts).

Tidewater World Map

© Štěpán Bartošek

Apart from tidal phenomena related to the Earth's rotation, orbit around the Sun, and the Moon's gravitational field, the shape of the coastline and seabed topography also have an impact on the intensity and quantity of tides. When a sea mass from the open ocean flows into a bay through a narrow passage, the water is forced to compress through a smaller area, resulting in an increase in the water flow rate and intensity of the tide. Conversely, in partially enclosed bodies of water, the incoming water dynamics are weaker. Deltas of large rivers present a specific scenario where the mixing of salt and fresh water also influences tidal behaviour.

Comprehending the physical principles of tides and the capacity to interpret forecast tables of sea level changes are essential aspects of navigation for sailors, skippers, and navigators who wish to experience sailing to the fullest. In truth, the fluctuation of water levels can serve as a beneficial means of propulsion and a straightforward way to reach port, but can also present as an irritating hindrance during a voyage.

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If utilizing natural forces besides wind to navigate your yacht appeals to you, then understanding the tides at a specific location and the underlying principles of sea swell is an ideal way to master the elements of the sea. The first thing you should factor in is the difference between the length of the lunar day and the human perception of time on Earth, which also changes every year.

Sailors have up-to-date tables available for each year, which enable them to determine the timing and location of peak tides with great precision. These tables are referred to as "tide tables," and navigators are authorized to use only those tables that are relevant to their sailing dates. You will usually see the terms high water (HW) and low water (LW). Coastal states usually publish these charts annually, and the British Admiralty forecast charts are among the most reliable for European boaters.

Low tide

Low tide

Navigation in the digital age

The navigator or skipper is responsible for the smooth running of the voyage, especially with regard to anticipating the risks associated with the tides. Many a sailor has learned the hard way (by having their keel scrape against the seabed) that proper planning and a well-structured itinerary, which considers all variables, are crucial for smooth sailing. If you know you will be sailing in areas with significant sea or ocean level differences, relying solely on evaluating tidal changes is insufficient — it is necessary to also monitor tidal currents.

Tidal currents are created when water in tidal areas flows in a way that balances the water level between high and low tides. In terms of navigation, determining tidal currents and the distance they will propel a vessel is more challenging than working with tides. Navigators have tidal current atlases that record how the currents change hourly.

Compared to the past, navigators are also greatly facilitated by modern technologies such as navigation computers, electronic charts, GPS and other instruments that are capable of handling multiple variables such as wind drift, tidal current drift, boat speed, etc. Even with all the technological assistance available, it's still important to rely on human judgment and evaluate the current situation.

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Tides in Europe's seas and the world's oceans

The height and strength of the tide depend on many variables, but ultimately they act as a given constant at a particular location that can be relied upon. Tides are influenced by:

  • latitude and longitude
  • the relative positions (alignment, distances of planets and celestial bodies) of the Sun, Moon and Earth
  • the nature of the relief of the seabed and the surrounding land (straits, channels, funnel-shaped bays which accelerate the flow of a given volume of water)
  • the amount of water mass that moves periodically
  • the length of the lunar day and the number of tides

It's logical to assume that tidal heights vary in different seas and oceans. This was already known to coastal communities in ancient times, and in Europe, the writings of several Roman seafarers who observed the movement of water and used that knowledge to determine the best time for arrival or departure, have survived.

One of the most famous examples of tidal action is the Bay of Saint-Malo, between Brittany and Normandy in the English Channel off the French coast. The fortress and monastery of Mont-Saint-Michel is an island at high tide and becomes part of the mainland connected by a sandbank at low tide. The difference between high tide and low tide is one of the largest in the world at around 10 to 15 metres.



In Europe, when sailing on a yacht, you are more likely to encounter enclosed seas such as the Mediterranean or the Baltic Sea. Here, the difference between high and low tides is relatively small, although of course, it depends on the specific location. In the Mediterranean, for example, the waters of the Atlantic flow only through the narrow Strait of Gibraltar, so while you may see more marked differences in sea swell on the coasts of Spain and parts of France, eastwards the water pressure diminishes, and on the popular Adriatic, for example, the tidal range is relatively insignificant, although it still plays a role.

More significant changes in the rise and fall of the water line are encountered especially along the coasts that bathe in the Atlantic and Pacific Oceans. The huge mass of water, combined with the relief of the seabed and the long coastlines, allows the tidal action to get into full effect.

Another interesting phenomenon related to tides is when rivers appear to flow against their current. In the deltas of large rivers, such as the Amazon, during high tide, the saltwater from the ocean can overpower the fresh river water flowing from the land and push it back inland, sometimes for hundreds of kilometres. This is also observed in the Elbe River, which can experience tides up to 150 kilometres from its mouth toward the continent, making it of interest to European sailors.

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By gaining an understanding of the unique patterns of the tides in different bodies of water, you can harness this natural force to your advantage and provide your boat with extra propulsion for free, or reach places that are inaccessible to boats at low tide.

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FAQ Tidal sailing basics