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Tea Break Talk – Tides
Unless you are going to do all your sailing in either the Mediterranean or the Caribbean, it is extremely important that you get to grips with and really understand tides. You always want to know that you have enough water beneath your keel!
What causes tides?
Tides exist solely because of the moon and the sun. Tides are the result of the gravitational effect (pull) of the Moon and, to a lesser extent, the Sun on the Earth’s bodies of water.
The Earth rotates every 24 hours so you would expect to have just one tide a day when the moon has the greatest pull on a particular area closest to it at any given time, but this is not the case. The Earth’s water doesn’t just bulge towards the moon but actually bulges on both sides. So following this, we have a high tide on the same side of the Earth as the moon and simultaneously on the opposite side. As sailors, we don’t really need to know why – just that it happens! This, therefore, gives us two tides a day.
Each tidal cycle is approx. 12 hours – 6 hours of Flood (in coming) and 6 hours of Ebb (out going).
Chart Datum and Tidal Heights
Chart datum is the level of water that charted depths displayed on a nautical chart are measured from. Chart datum is generally the same as the lowest astronomical tide (LAT) or the lowest recorded tide. The tidal height, at any given time, is the vertical distance between chart datum and the sea level.
The tidal range is the vertical difference between the high tide height and the following low tide height.
Springs and Neaps
A Spring tide is when the difference between high and low tide (tidal range) is the greatest. So, you get the highest high tides and the lowest low tides. Spring tides occur when the moon is either new or full. When the Sun, Moon and Earth are aligned their collective gravitational pull on the Earth’s water is strengthened.
A Neap tide is when the difference between high and low tide is the least. As with Springs, Neap tides occur twice a month when the Sun and the Moon are at right angles to the Earth. When arranged in this way their total gravitational pull on the Earth’s water is weakened because it comes from two different directions.
As you can see tides cause the depth of water to change at any given time. You mustn’t expect the water depth to be the same in the morning as when you anchored overnight. If you do, it could mean that you’re sitting high and dry as your climb sideways out of the cabin in the morning! 2m below the keel might seem safe but if you have a 2.3m tide then you will have a problem. The depth can change significantly in a few hours and thus sailing in a shallow bay can mean what was once deep water will leave you touching the bottom. It is embarrassing, to say the least, and you may be very late for dinner, but in some cases it may cause significant damage to your hull as you touch the sea bed in shallow (previously deep) water.
If you don’t want to get caught out, why not sign up to an online sailing theory course. The start of your sailing journey is just a few short clicks away!