Tides are the vertical rise and fall of the surface of a body of water, caused primarily by the gravitational attraction of the moon and to a lesser extent, the sun. The rotation of the earth is the primary cause of two high tides and two low...
READ MORESpring tides are those with higher high tides and lower low tides than normal, or a greater range of tide. They occur when the sun and the new moon, or the sun and the full moon are in straight line with the earth exerting a larger than normal...
READ MORENow let’s see how we calculate the times of high and low water in a secondary port. Let’s take Barfleur. First we have to look up which standard port is assigned to Barfleur, and we find it is Cherbourg. For Barfleur we want to obtain...
READ MORENow we take the table for the secondary port, Barfleur. At the top we have the times of high and low water, and the time differences. To the right we have the heights of high and low water, and the height differences. Let’s first calculate...
READ MOREWe calculate the height of the tide in the same manner. 5.6 meters lies at about half the interval between plus 0.3 and plus 0.9 meters. So the height difference for high water should read plus 0.6 meters. We apply this figure to the high water...
READ MOREThere 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...
READ MORETo 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...
READ MORETo 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...
READ MOREThe 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...
READ MOREIf 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...
READ MOREThe 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...
READ MOREInformation 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...
READ MOREIn 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...
READ MORELet’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...
READ MOREThe 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.
READ MORELet’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...
READ MOREHere 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...
READ MORELet’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...
READ MOREOne of the best sailing seminars is simply to head out with an old-timer or two who generously share warning after warning, adage after adage, whopper after whopper. I always carry a notebook for such cases. Here is a sampling of the best tips...
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