Tide is the name given to the periodic rising and falling of the level of the sea.
The tide-generating forces acting on the oceans of the world are:
The gravitational attraction between the earth and the moon.
The gravitational attraction between the earth and the sun.
The gravitational pull of the moon produces a heaping up of the water on the side of the earth facing it and a similar heaping up on the opposite side. As the earth rotates it exposes a different side to the moon so that every point has two high and two low tides in each 24 Hr and 50 min. This process was first analysed by Issac Newton 300 years ago.
It is important that we are able to calculate the tide times as divers, as there are times we will not be able to dive depending on the current state of the tide.
We may not want to dive due to fast currents which occur between high and low tides.
At low tide there may be a drop in sea level which would prevent us from being able to launch or recover the boat in certain areas.
When the sun, earth and moon are in the same straight line (at new moon and full moon) the sun and moon "pull together" and then the range of tide is at its greatest. These are called "Spring tides". (i.e.) High High Waters and Low Low Waters.
When the sun and moon are pulling at right angles to each other then the range of tide is at its least. This happens at half moon. These are called "Neap Tides". (i.e.) when high waters are low and low waters are high
The United Kingdom Hydrographic Office also produces tidal predictions for Irish coastal locations, as part of a global prediction system catering for over 4000 ports world wide. 73 ports, harbours, towns, cities, bays, quays and headland sites around Ireland are included. This gives 7 day predictions so is very useful for those weekends we are away diving!
Information about the tides can be found in the Tide Tables. They predict the time of High and Low tide for each day of the year. They cover standard ports and local differences. Tide tables can be obtained from Harbour Masters Offices, Admiralty Chart Agents or Newspapers. They can also be found in the January edition of SubSea.
This is a fixed level used in the measurement of depths, drying heights and the heights of tides below which the tide very rarely falls. Currently a new more accurate level is being adopted as the standard for chart Datums around the British Isles. This level is known as "Lowest Astronomical Tide" (L.A.T.). It is the lowest predictable tide under average meteorological conditions.
Tide Tables only give the time of high and low water, and the chart reads from chart datum, so to find the depth of water at any given time we refer to the "Rule of Twelfths". This is a rough guide to the rise and fall of a 6 hourly tide and from which it will be seen that the maximum rate of rise or fall occurs at half tide.
1/12th of the tidal range in the first hour
You intend to launch your diving boat at 09.30 and land again at 10.30.
What height of tide will be available?
5m above L.A.T.
1m above L.A.T.
Tidal range is therefore 4m
To calculate height of tide:
08.30hr - 1st hr. of fall - 1/12 of range
09.30hr - 2nd hr. of fall - 2/12 of range
So at 09.30 we have a total fall of 3/12ths or 1/4 of range of 4m, which is:
4m ÷ 4 = 1m
Height of tide at 09.30 = 5m - 1m = 4m above L.A.T.
10.30 - 3rd hour of fall - 3/12ths of range.
Total fall of 1/12 + 2/12 + 3/12 = 6/12 or 1/2 of range, which is:
4m ÷ 2 = 2m
Height of tide at 10.30 = 5m - 2m = 3m above L.A.T.
From this example you can now determine what the height of the tide will be when you want to launch and recover your boat.