Understanding how tides work is essential if you plan on sailing in tidal areas. As the depth of water changes throughout the day, sometimes ports and harbours may not be accessible as they are too shallow, or sailing to a particular destination may be more difficult as the flow of tide is going in the wrong direction.
Below we have briefly explained how tides work and how tides in some parts of the world differ from others. We will start with a simple image; imagine a rectangular fish tank half full of water, when tilted one end will become deeper and the other end shallower. So when one area has a high tide another area will have a low tide. Tilt the tank the other way and the opposite happens, low becomes high and high becomes low.
So what causes the water to move from one area to another?
The moon orbits Earth and Earth orbits the sun, both the sun and the moon have a gravitational field that draws the water on Earth’s surface towards it. If we consider the sun as a constant force because we orbit it and the moon as a variable force because it orbits us then the moon, relative to the position of the sun, has a dramatic affect on the surface water and therefore the tides. The effects of Earth’s gravitational pull as it rotates also helps the movement of the water. Returning to the image of the fish tank, in simple terms as Earth rotates, this movement helps to ‘slosh’ the water from one place to another.
When the sun and the moon are in-line with each other we get what is known as a full moon as the sun is shining on the surface of the moon and we can see the whole face.
When the moon is between the sun and Earth we get what is known as a new moon, but we cant see anything as the sun is lighting up the surface that we cant see.
Due to the combined effect of the sun and the moon the gravitational pull is at its maximum during a full and new moon and we refer to the tides it produces as ‘spring’ tides. Spring tides have very high, high waters and very low, low waters producing a large tidal range. But due to the time taken to move such a mass of water, the spring tides do not occur exactly on a full or new moon, they are likely to be a day or two later.
Due to the combined effect of the sun and the moon the gravitational pull is at its weakest during these times and we refer to the tides it produces as ‘neap’ tides (‘neep’ being a Medieval word for ‘small’). Neap tides have the very low, high waters and very high, low waters producing a small tidal range.
As it take the moon 28 days to orbit Earth, every 7 days we get either a new moon, first quarter, full moon and last quarter or spring, neap, spring and neap tides. As the range of the tides is greater during springs this means that there is a greater volume of water flowing in the same period of time therefore it must flow faster as the tide is coming in or going out. This means that the flow or tidal streams are stronger during spring tides than they are in neaps.
Most of the Pacific Ocean has what is known as a diurnal tides, this means that there is one high water and one low water every day.
Most of the Atlantic Ocean has what is known as a semi diurnal tides, this means that there are two high waters and two low waters every day, both high water and low water are about the same height although there may be slight differences.
The Indian Ocean and Australia have semi-diurnal tides, this means that although they have 2 high and 2 low waters each day, the heights vary greatly.
Not all coastal waters are tidal. Areas like the Mediterranean and the Baltic Sea are so vast and have a narrow opening to the open water that the percentage of water that can flow is so limited that the overall effect of the tide is a few centimetres during the day, so these areas are referred to as non- tidal.
Other coastal areas like the Solent have tidal anomalies like double-high waters, young flood tides, making the tides unique. http://www.southamptonweather.co.uk/doubletides.php Portland on the South Coast of England has double low waters.
Sailors can find the time of tides in a tide table. Tide tables for a particular port or marina can be found in almanacs or they are available from most ports and harbours. The tide tables include details or the heights of tides and if they are springs or neaps. Tides are very predictable as we can predict how the sun, moon and earth will interact, so it is possible to predict tides many years in advance.
The theory of tides and how best to use them is covered in RYA Day Skipper and RYA Yachtmaster theory courses. If you would like to attend one of our classroom courses in London or perhaps complete the course on line please contact us on 020 7002 7676, or email [email protected]