Understanding tides with Doug King

Tides are the periodic rise and fall of coastal waters. Doug King explains how tides work and recommends a few tools to help you understand them better.
Doug King: Tides are the periodic rise and fall of coastal waters. It's important to have a good understanding of tides; not just for fishing the tides, but also for launching and recovering your boat, anchoring, crossing ocean bars, and just knowing how much water is underneath you.

Tides are caused by the gravitational pull of the moon and the sun, and the moon is the primary gravitational force. Now, variations in the moon's orbit mean that we get semi-diurnal tides. That is two high tides and two low tides a day, although there are some areas where they have single tides or mixed tides. Tides also occur about one hour later each day because of that orbital variation. There are two types of tides; a spring tide and a neap tide. Spring tides, sometimes called king tides, occur when there is a full moon and a new moon. Neap tides occur when the moon is in its first and third quarters.

With spring tides, you get the highest tides and the lowest tides. With neap tides, the tidal range or the difference between low and high tide is not that great. Now, tides can also be affected by land masses, underwater topography. Also, strong winds and air pressure, particularly in enclosed waters. Tides can be predicted, and tide tables are published by port authorities, and also by the Bureau Of Meteorology. Tide tables allow you to look up a day and find the times of high tide and low tide, and also the height of that tide. Smartphone applications, like the Club Marine app, can also tell you heights of tide at a particular location.

With tide tables, you look up the month, then the day, and it will tell you the times of high tide and low tide, along with the height of that tide above a datum. Now, datum is an arbitrary level below which the tide rarely falls. Generally speaking, the heights of tide have got to be added to any water that you have underneath you already. If the location you're boating in doesn't have a tide table for it, generally you'll find a list of tidal differences in a tide book, and you can apply that difference to a standard port to find out the heights of tide where you're operating your boat.

There is a way of determining the height of tide at any given hour in a six hour tidal cycle. It's called the rule of twelfths, and it's based on the fact that tides don't rise and fall linearly. They go in a sinusoidal curve. A little bit of study is required to apply the rule of twelfths, but it's helpful, particularly in areas where there are large tidal variations. Sunset and sunrise times are also found in tide tables. An understanding of tides, and how to read them and apply them to the waters that you're on, will make your boating much more enjoyable, more flexible, and much safer.

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