Lessons

Rule of twelfths 1

There are two methods of calculating the tidal height. The first uses the tide tables, which is a very accurate method, and the second uses the “Rule of Twelfths”, which is not so accurate and should be used with caution! To use the...

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Rule of twelfths 2

To find the times, heights and range of the tide it is necessary to consult the tide tables. Let’s use the “Rule of Twelfths” for the port of Portsmouth on the 1st of November. The first high water is at 00:45 UTC and the height...

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Rule of twelfths 3

To calculate the fall of the tide for 02:45 UTC we now use the “Rule of Twelfths”. First we have to calculate the time. 02:45 UTC minus 00:45 UTC gives us 2 hours. As we already know, in the first hour the tide will fall 1/12 of its...

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Tidal curve 1

The Tidal Curve is another method of determining times of high and low water. Before you use the curve, note the times and heights of tide for the day from the tide table. Let’s again use Portsmouth on September 1st. First you have to enter...

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Tidal curve 2

If you need the height of the tide for a particular time, for example for 13:00, just enter the required time at the timescale grid. 13:00 is roughly 3 hours after high tide. Then check whether the tidal range is close to a spring range or neap...

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Introduction

The tidal stream is the horizontal flow of water which is caused by the rise and fall of the tide. The direction of the tidal stream is called “set” and its speed is called “drift”. In a harbor we call the tidal stream the...

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Tidal diamonds

Information about the set and drift of the tidal stream can be obtained from Charts or Tidal Stream Atlases. On charts the tidal streams are marked with a letter within a diamond, and a table provides information about the set and drift for...

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Atlas and infos

In a tidal stream atlas you can find information about a specific area, such as the English Channel. On separate pages you can find information about the tidal stream for each hour before high water, and each hour after high water. Each page...

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Tide heights chart datum 1

Let’s have a closer look at “Tidal Heights” and “Chart Datum”. On nautical charts, soundings and drying heights are measured from Chart Datum. Chart Datum is the Lowest Astronomical Tide or L A T , meaning the lowest...

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Tide heights chart datum 2

The difference of height between Mean High Water Spring (MHWS) and Mean Low Water Spring (MLWS) is referred to as spring range. Also, the difference in height between Mean High Water Neap and Mean Low Water Neap is referred to as neap range.

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Tides for standart port 1

Let’s now use the tide table to calculate the tide for a standard port. But first, what is a standard port? In the tide table we distinguish between standard ports and secondary ports. A standard port is a port for which the times and...

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Tides for standart port 2

Here is an excerpt of a Tide Table. In the first line you see the name of the standard port and where it is located. In the second line information is provided about the time zone in which the times of high and low water are calculated and the...

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Tides for standart port 3

Let’s now find out the times and heights of high and low water on September 1st. Note that all times are in UTC (Coordinated Universal Time), formerly known as GMT (Greenwich Mean Time). Watch out – if you are on British Summer Time...

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