Anyone boating in coastal waters is familiar with ocean tides, the daily cycle of rising and falling water levels. You also likely remember from grade school science class that tides are caused by the moon and the sun’s gravity. In most coastal locations, you will see, twice daily, the tide come in and the tide go out.
But what if I told you that the water doesn’t actually come in and the water doesn’t actually go out?
Your science teacher was right. There are two bulges of water, one on each side of the Earth caused by the moon and sun, respectively. Contrary to your perception, though, the rising and falling tides are not the result of those bulges rotating around the Earth.
Rather, it is the Earth rotating inside those bulges. The water is actually staying stationary. It is the Earth at your coastal location that is passing into and then out of the deeper water.
The bulge of deeper water does rotate around the Earth as well, but that happens quite slowly and only monthly, as the moon circles the Earth. Related to this is another common misconception about tides: that we have higher tides, known as “spring tides,” on a full moon. In reality, the bulge of water caused by the moon stays the same regardless of the moon’s phase. It is actually the sun—more specifically, the alignment of the sun and the moon—that is increasing the depth or bulge of water we’re passing through.
Conversely, when the sun and moon are at right angles to the Earth, their bulges effectively cancel each other out, giving us the lowest tides in the cycle, called neap tides.
I’ll admit these are all just fun facts that may not affect how you handle your boat. Aside from the risk of running aground at low tide, most of the tide-related trouble boaters get into is more specifically due to tidal current—a subtle difference, but an important one.
If tides are the rising and falling of water, then tidal currents have to do with the horizontal flow of that water. This is especially true when the water intersects land along a shoreline.
The rising water level approaching the shoreline creates currents as the water flows in and out of restricted areas such as rivers, bays and harbors. Adding to the confusion, the terminology associated with these conditions is frequently misused. Tides rise and fall; tidal currents flood and ebb.
I grew up boating on the Great Lakes, where tides do technically occur—but they are in inches, not feet, so the Great Lakes are effectively nontidal. Taking a boat into the Atlantic Ocean for the first time, I learned the difference between tides and tidal currents.
Local boaters had informed me that the tidal current running through the Beaufort, N.C., inlet could be swift at peak flow. My plan was to arrive at the inlet at the time of high tide, assuming it would then reverse and start to recede. I thought the current should be slack during the period between high and low water.
Imagine my surprise when I encountered 3-foot waves created by the tidal current still coming in against a moderate offshore breeze. I double-checked the tables. I had arrived at high tide, but tide charts only indicate the times for the minimum and maximum water heights, not the flow. Depending on geographical features, slack current can be offset by several hours from high or low tide.
My friend Chris had a similar experience when timing his arrival to a marina near the inlet in Fort Pierce, Fla. He planned to arrive at slack current, so he wouldn’t be fighting a fast-moving current while trying to dock. Chris consulted the tide tables and timed his arrival for high tide. His plan didn’t work out so well because at the Fort Pierce inlet, slack water doesn’t occur until 2½ hours past the time of high tide.
The water may reach its highest or lowest point vertically, but the horizontal flow—the tidal current—can continue well past this point. This condition can also create the phenomenon of current simultaneously flowing in opposite directions in the upper and lower sections of rivers and bays leading inland from the ocean.
It is not surprising that this is difficult to understand, given that tide tables showing the times of high and low water are much more common than current tables showing the times of flood and ebb, even from sources like the National Oceanic and Atmospheric Administration. In my travels, I’ve found plenty of free local tide table booklets at waterfront businesses, but I have never seen a similar table of current predictions freely available.
Fortunately, current tables are available in the software of most modern chartplotters. Separate icons distinguish current from tide.
Knowing tide height is certainly important to determine safe navigational depths, but knowing tidal current flow can be even more important.
This article was originally published in the November/December 2024 issue.
