Tides and currents

Tides The gravitational pull of the moon and to a lesser extent, the sun, on the seas and oceans of the earth, causes tides. This pull causes the water of the sea at any particular place to get deeper or shallower in a definite rhythm. The rising tide is called the “flood” tide and the falling tide is called the “ebb” tide.

The time taken by the tide to flood and to ebb varies in different places but around the Irish Coast the average is about 6.25 hours each way. This means about 12.5 hours between two high waters (HW) and 12.5 hours between two low waters (LW).

Spring and Neap TIDES At the times of full moon and new moon the high tides are higher and the low tides are lower than average. These are called “Spring tides”. When the moon is at first or third quarter the high tides are not so high and the low tides are not so low. These are called “Neap tides”. Spring tides occur every fortnight. Neap tides also occur at fortnightly intervals between Spring tides. Tide times Times of HW and LW are found in “tide tables”. You should know how to read tide tables and find the times of high and low water for your normal boating area.

Tidal currents As well as the rise and fall of the level of the sea due to the tides, there are also horizontal movements called “Tidal streams”. In some places these streams or currents can be quite strong. Strong currents can cause overfalls in some places. You should learn about Tidal currents in your own area and know which way they run at different stages of the tide. The flood stream, when the tide is rising, is usually in the opposite direction to the ebb stream, when the tide is falling.

Effect of wind Wind blowing over the water causes waves; the stronger the wind, the bigger the waves. Wind blowing against the tide can cause bigger and breaking waves, sometimes dangerous for small boats. Find out about local currents in your Troop’s boating area, and what problems may be caused by a change of direction of tide or wind.

Tides and Currents

Depths The charted depth of water is usually the least depth, that can be predicted by calculating the gravitational pull of the moon, and to a lesser extent the Sun. The lowest tide that can be calculated is called the “Lowest Astronomical Tide” (LAT). This level may be altered by unpredictable events such as wind strength and direction. Most of the time the actual depth of the water will be greater than that marked on the chart, and will only fall to the charted depth a couple of times in the year.

The difference between the charted depth and the actual depth is the “Height of the Tide” at that time. For example, if the chart shows a depth of 4 metres at a certain place and the Tide Table shows the height of High Water is 3 metres, then the depth of water at that place at the time of high water will be 4 + 3 = 7 metres.

Areas of sand, mud or rocks which are covered by high tide, but exposed at low tide are coloured green in metric charts, and the height above the Lowest Astronomical Tide is indicated by a figure with a line underneath. This is known as the “Drying Height”. All heights or depths are related to the Lowest Astronomical Tide except the heights of lighthouses and clearances under bridges which are related to the height of High Water Springs. The “Range of the Tide” is the difference between the height of High Water and the height of the preceding or following Low Water.