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Complete Guide to Understanding Tides in Southwest Florida


As saltwater boaters, one of the constantly changing conditions that we are responsible for monitoring is the rise and fall of the water level, commonly referred to as the tide cycle. Fortunately these cycles are well predicted and are generally accurate with the exception of storm surges. In this blog post we will attempt to simplify the science of how tides work, what has the greatest effect on them and how to read and interpret a tide chart. 


How Tides Work


Think of tides as really long waves that move across oceans and the Gulf of Mexico, or in more simple terms, like waves going back and forth in a swimming pool. As the crest or high point in a wave reaches the coast or one side of the swimming pool, a high tide occurs. When the trough or low point of the wave reaches the coast, a low tide occurs.


Gravitational forces of the moon, and to some extent the sun, have the greatest effect on tide cycles. When the moon and the sun are in alignment, they exert the greatest amount of gravitational pull. During these times we may experience higher high tides and lower low tides, which can also be defined as a greater tidal range, or the difference in water levels between high and low tides. 


When the moon and the sun are perpendicular to each other, there is less gravitational pull and therefore less tidal range. This means that high tides won’t be as high, and low tides will not be so low. There will be less tidal range. The phases of the moon occur on a monthly basis, and this is not to be confused with the varying heights of tides in the daily cycle, which we will discuss next. 


There are three types of tide cycles for you to be aware of. A diurnal tide cycle means that there is only one high and one low tide in a 24 hour period. A semidiurnal tide cycle occurs when there are two highs and two lows each day, and they are consistent with regard to tidal range. Here in the Naples area, we have what is called a mixed tide. We have two highs and two lows in each 24 hour period but they are unequal with regard to tidal range. One high tide may only rise 1.5 feet, while the next one rises 2.5 to 3.0 feet. The same is true for the low tide cycles within the 24 hour period. One cycle may only have a 1.5 foot decrease in water level, while the next one may see the tide go out a full 3.0 feet or so.


Now let’s discuss how to read a tide chart and how tides can affect you as a recreational boater.


How to Read a Tide Chart


The ability to read a tide chart is one of the fundamental skills that a saltwater recreational boater must have. With the help of our infographic below, it should be relatively easy to master the skill. 


Have you ever looked at a nautical chart and wondered what all of the numbers mean that are scattered all around the chart? The answer is that they are an estimate of the low tide water depth readings, also known as MLLW or Mean Lowest Low Water. These calculations are not only used to tell you the estimated water depth in a certain location, but they are also used as the basis for measuring tidal range, or how high the water level will rise or fall during a tide cycle. 


In the example below, we see several MLLW depth readings circled in yellow. On the tide chart we can see the graph showing the tide cycles including the height of the tide. If the first low tide indicates a height above MLLW of 1.07 feet, then we would simply add 1.07 feet to the numbers that are circled in yellow on the chart to know how much water we should expect in that location at that time in the cycle. The same would be true for the high tide of 2.40 feet and the next low of 0.16 feet. 




But what happens when the tidal range becomes a negative number? The answer is that we subtract from the MLLW. For example, if the phase of the moon should cause a very low tide, you may occasionally see the tide dip below the MLLW on the tide chart. If the MLLW is, say, 4 feet on the chart and the tide graph says negative 0.50 feet, then the estimated depth in that location at that point in the cycle would be 3.5 feet approximately. 


This brings us to an important point for recreational boaters. Out on the deeper waters of the Gulf, the tidal range is less important. But closer to shore or in shallow areas of the backwaters, what was safe navigable water at high tide may not be at low tide. The same is true for bridge clearance. Some boats can only make it under certain bridges at lower tides. 


There is one more aspect of tides that is important for boaters to understand. While tides are the vertical rise and fall of water levels, currents are the horizontal movement of water. Currents on the open Gulf are generally light and variable, but not so on some of the canals and backwater areas. The inlet to Collier Bay in Marco Island has very strong currents during both the incoming and outgoing tides. Currents are at their strongest at the midpoint in the tide cycle, meaning halfway between high and low tide. At the high and low tide when water has stopped flowing in or out, there will be very little velocity. This is known as Slack Water. High-powered vessels can easily fight strong currents, but slower vessels with less power, or sailing vessels, need to be very aware of the effects of currents on their safe navigation. 


If you are interested in learning more about tides and currents and how they impact you as a boater, consider taking our Coastal Boat Navigation Course. Additional tide information and resources can be found at NOAA.