An Introduction to Tides and Tidal Currents

Tides are a phenomenon every sailor encounters when sailing offshore on the sea – sometimes affecting even large inland lakes. Tidal streams can be a danger especially for smaller dinghies. Knowing about the causes of the tides can be crucial for your health and safety. In this article, I try to outline how the tides caused, how to predict them and how to take advantage of the torrents they create.

Everybody knows what tides are: the rising and falling of the sea in something like 6-hour cycles. For the layman, this definition might be enough – sailors, however, should know more about the causes and dangers of the tides. The tidal shift is not equally strong all over the World. It affects coastal areas the more intensely, the closer they are by moon. There is much less of an effect of tides in the Mediterranean and inland lakes.

The distance between the highest sea level during high tide and the lowest sea level during low tide is called tidal tidal range or tidal shift. It is often given as an average value over the course of a year as an indicator for the intensity of tides in a given area.

Some very big inland lakes do get measurable tides, but they are nothing compared to the shift of several meters you can get in the Atlantic Ocean. I was originally trained as a sailor on European lakes, sailed mostly in the Mediterranean later on – you can imagine how impressed I was when I first experienced the Atlantic Ocean at low tide. But what are the causes of tides?

Basic Astronomy: Causes of Tides

The moon is a round, yellow bowl not dissimilar to Swiss cheese that rotates around the Earth. I am sure you have noticed the moon before. Since the moon is heavy, it generates a gravitational force that acts on the Earth. Solid objects on Earth, such as mountains, continents, or buildings, can resist this force. Liquids, however, can move and thus get “sucked” towards the moon.

This is the main cause of tides, but not the only one. The sun is much further away from Earth than the moon, yet it still has a gravitational force acting on the seas. This either increases the force of the moon (in case the both celestial bodies are aligned) or weakens it (if both celestial bodies act from opposite directions). Together, these forces cause two high and two low tides a day on any place on sea in varying intensity.

The maximum gravitational force is achieved with full alignment of Earth, Moon and Sun – this happens every full and new moon. This results in the strongest tides, which are called “spring tides”. The opposite (the sun’s gravitation weakening the tide) happens during the first and last quarters of the moon and is called “neap tides”. A tide can be raising or “flooding”; or it can be falling, also called “ebbing”.

Predicting Tides and Safety Implications

Before you set sail, you should make yourself familiar with the state of tides in your area. There are tide information services, and especially if you are new to an area, tidal reports are as crucial as weather reports. Information about the tidal range can be found is so-called “tide tables”, and good chart books include tidal maps. Check your anchor line and make sure it is sufficiently long to allow you to anchor in case your boat is getting caught in an unexpected stream.

To predict where tidal streams are most dramatic, it is useful to think of a funnel: if you pour water into it, you can expect that it will spread and be less vicious in the wide part of it. Once the water runs down into the narrow part of the funnel, it accelerates and flows in a stronger torrent. The same principle applies to tides: if flowing water gets funnelled, the torrent becomes stronger.

This can be observed in rivers; they often have very strong torrents caused by raising or falling tides. Similar things are true for access streets to fjords or large, natural harbors. Much harder to anticipate are torrents caused by uneven grounds, resulting in underwater overfalls and alike – this is the most dangerous setting in which tidal torrents can hit you.

Keeping an Eye on the Tide

There are several simple tricks that you can use to keep an eye on the tides. Waves often make a good warning sign. Do they appear to be steeper than one would expect if they were caused only by the wind? In that case they are likely to be caused by a tidal stream running against the wind.

Are they smaller than one would expect under the current wind conditions? In that case the stream might flow with the wind. Estimating such things requires experience, so try to “calibrate” your skills with a tide map in your hands and practice predictions before you really need them.

Other valuable hints can come from buoys, anchored boats and other swimming or drifting objects. Boats often point against the stream, as long as they can orient themselves in free motion. This can be misleading if the boats are attached to two anchors or if they are very light and exposed to strong wind and therefore, might not point into the stream.

Buoys often lean with the stream and sometimes even show a bow wave caused by the tidal torrent. In any case, whenever you sail – especially offshore on sea or in waters new to you – you should always take care somebody knows that you are off and where you plan to go.

Further Reading

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Wikipedia on Tides

NOAA's Tides and Currents Information

Wikipedia on the Tidal Range

National Weather Service of the National Oceanic and Atmospheric Administration