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How Can the Moon Affect Tides?

© Capt. Matt

Don't believe everything you hear, especially around marinas and bait stores where the "old salts" spin their yarns. When you hear boaters referring to ebb tide, or incoming tide or outgoing tide, it may be familiar language, but it is not accurate. Tide does not ebb, nor does it come in or go out.

Tide is defined as the vertical movement of water and only goes up and down.

Current is the horizontal or sideways flow of water. It floods in which makes the tide rise and ebbs out which makes the tide fall.

It is this up and down tidal movement that you should be concerned with and be able to estimate with some accuracy, especially when entering a potentially shallow port or harbor. Let's explore some definitions concerning tide.

First of all, tide is the rise and fall of water caused by gravitational forces of the moon and sun on the oceans of the earth. Generally speaking, tidal cycles contain two high tides and two low tides each day. During the time between high and low tide there will obviously be current flow. The time between high and low tides is a little over 6 hours and the entire tidal cycle repeats itself approximately fifty minutes later each day. So if you know that low tide is at 0800 today you can estimate that it will be at 0850 tomorrow.

The difference between the high tide and low tide is called the range of tide. For instance, if the water depth at high tide is 20 feet and at low tide is 18 feet, the range of tide is two feet.

There are two types of currents that you can expect in regard to tides.

Flooding current is experienced when the tide is rising.

Ebbing current is experienced when the tide is falling. When the tide has reached it highest and lowest points there is a brief period where there is no current ebbing or flooding, this is referred to as slack water.

Although there are irregularities in some parts of the world, as a general rule the following can be predicted using publications such as the Tide and Current Tables, using software specifically designed to estimate tides or by accessing the information about tides online at http://boatsafe.com Boating Safety Links.

Times of high and low tide and depth of water at each of these times

Set and drift of strong tidal currents, including times of maximum current