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The Rise and Fall of the Tides

There are few things more valuable to know about when selecting an anchorage in many parts of the world, especially if you are a sailor with a deep draft vessel, than the tides and currents. Fortunately, there are many useful resources and many chartplotters have tide prediction capability built right in. We touch on tides in various parts of our book, Happy Hooking – the Art of Anchoring. What a joy it is to know exactly where you are on the tide when preparing to drop anchor. It also helps to understand why the sea suddenly turned so choppy (wind opposite direction from tide) or why you're turning 2400 rpm but making no headway (heading against a strong current). In some places, tidal variation doesn't make that much difference (Chesapeake Bay) whereas in other places it makes a huge difference (Bay of Fundy).

Tides are the changing depth of water over the course of time – usually changing more than once in a day.

The physics of tides is an immensely complicated topic, which far surpasses what a boater needs to know. We will cover the only the basics that affect boating here. More detail is, of course, available in the scientific literature.

What causes the tides?

In simplified terms, tides are caused by the effect of the gravitational forces exerted by the moon and sun combined. The moon is the primary mechanism that drives tidal action and explains two equipotential tidal bulges, accounting for two daily high waters. The moon’s gravity, for example, pulls the water on earth towards it, as in the adjacent graphic. This is how it would look if the earth were a simple, stationary ball entirely covered with water.

The earth’s rotation and the land masses cause the tides to rotate in harmonic waves in circular patterns around what are called amphidromic points.

Tidal waves and variations, which can, in places, be quite significant.
Courtesy of Goddard Space Flight Center

These tidal waves sweep the shorelines and may or may not overlap. Where they do overlap they may add up or cancel each other out. They may also be slightly out of phase. Some places will thus experience a semi-diurnal tide, with two nearly equal high and low tides each day. Other locations may experience a diurnal tide with only one high and low tide each day. A "mixed tide", or two uneven tides a day is also possible.

Tides are not limited to the oceans, but can occur in other water bodies as well. Large lakes such as Superior and Erie can experience tides of 1 to 4 cm. However, that is not something anyone is likely to notice.

While tides are usually the largest source of short-term sea-level fluctuations, sea levels are also subject to forces such as barometric pressure changes and wind. The latter potentially resulting in storm surges, especially in shallow seas and near coasts.

Tides also produce reversing currents known as tidal streams. The moment that the tidal current ceases is called slack water or slack tide. The tide then reverses direction and is said to be turning. Slack water usually occurs near high water and low water.

The ocean bathymetry greatly influences the tide's exact time and height at a particular coastal point. There are some extreme cases. The Bay of Fundy, on the east coast of Canada, is often stated to have the world's highest tides because of its shape, bathymetry, and its distance from the continental shelf edge. Measurements made in November 1998 at Burntcoat Head in the Bay of Fundy, Nova Scotia recorded a maximum range of 16.3 meters (53 ft.). Likewise in the Bay of Mont-Saint-Michel in France it can also exceed 16 meters.

Tide tables list each day's high and low water heights and times. To calculate the actual water depth, add the depth on the chart (shown in MLLW) to the tide height for a given time found in the tide table. Take this tide into account when crossing shallows and when anchoring. Clearly, if there is a 10 foot tidal variation between low and high tide and you anchor close to the time of high water, you need to be certain you won’t be sitting on the bottom when the tide goes out. Similarly, you need to allow adequate scope for the increasing tide if you are anchoring closer to the time of low water. 

The Rule of Twelfths

The Rule of Twelfths is useful for determining approximately how fast the tide will be coming in or going out. It also helps in understanding how tide driven current speeds will vary over time.

Simply put, in the first hour after low tide, the tide will rise 1/12 of the way to high tide. In the second hour, it will rise 2/12. In the third and fourth hours it will rise 3/12, in the fifth 2/12, and in the sixth, 1/12. (Totaling twelve twelfths) This same sequence of twelfths is repeated in the subsequent six hours as the tide goes back out.

A logical consequence of this fluctuation in the rate the tide comes in and goes out is that the water currents being driven by these tide changes will also speed up and slow down accordingly. A complication to this arises in many instances where the current change may be offset by a matter of hours from the tide change. This can involve coastal currents around headlands or in and out of bays or inlets. These currents may be quite strong when passing through narrow passages or over shallow ground. Examples of this are the Corryvreckan in Scotland, or the East River along Manhattan. The latter is not an actual river, but rather a connection between Long Island Sound and the Hudson River. These currents can well exceed 7 knots. Information on local currents and tidal variations can be found in the almanac covering the cruising ground in question.

Tide Heights

Another complication with tides is that their amplitudes vary with the phase of the moon and the alignment of the sun, moon and earth.

Shortly after the full and new moon, one may expect spring tides, which can be considerably higher (and lower) than the neaps following the quarter moons. During the times of the equinoxes, where the sun is aligned with our planetary system, the springs can be much higher again.

Some definitions:

  • Mean Higher High Water, MHHW: The observed average of the higher high water height of each tidal day  *
  • Mean High Water, MHW: The observed average of all the high water heights *
  • Mean Low Water, MLW: The observed average of all the low water heights *
  • Mean Lower Low Water, MLLW: The observed average of the lower low water height of each tidal day *
  • Chart Datum: The datum to which soundings on a chart are referred. It is usually the MLLW.

    * The National Ocean Service adopted a 19-year period as the official time segment over which tide observations are taken and reduced to obtain mean values.

Tide Information Resources

There are many resources for determining what the tide in a given location is predicted to do. There are printed tide tables, which are handy to have for one’s local cruising ground. Tide tables are also available for most locations online. The almanacs mentioned earlier contain tide tables.

There are also many electronic options today. Most GPS chart plotters have tide tables for the regions covered in the maps. There are also numerous excellent apps available for your smartphone, as well as charting apps that also include tide tables.

Illustrations by Alex Blackwell, Copyright



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