Go With the Flow

Knowing the difference between tides and tidal currents can pay off when making your way through an inlet or docking.

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 that tides are caused by gravity exerted by the moon and the sun onto earth. Yet as common as this knowledge might be, it’s surprising how often we hear stories of boaters getting into tide-related trouble. It only takes a little digging into these stories to realize much of this trouble isn’t because of tides per se, but because of tidal currents—a subtle difference, but a very important one. If tides are rising and falling water, then tidal currents have to do with the horizontal flow of that water, especially 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 like 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 non-tidal. 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, North Carolina inlet could be swift at peak flow. My plan was to arrive at the inlet at the time of high tide, assuming once the tide reached its highest point it would 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. In double-checking the tables, I had successfully arrived at high tide, however 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 from high or low tide by several hours.

My friend Chris had a similar experience when timing his arrival to a marina near the inlet in Fort Pierce, Florida. 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 at the marina at high tide. His plan didn’t work out so well, because at the Fort Pierce inlet, slack water doesn’t occur for another two and a half 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 interesting phenomenon of current simultaneously flowing in opposite directions in the upper and lower sections of rivers and bays leading inland from the ocean.

It’s 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 NOAA. It is not uncommon to find free local tide table booklets at waterfront businesses serving boaters, however in my travels I have never seen a similar table of current predictions freely available. Fortunately, current tables are available in the software of most modern chartplotters. Typically, separate icons on the chart screen distinguish current from tide.

Consult current tables to time your entrance to an inlet or marina when the current is favorable. Knowing tide height is certainly important to determine safe navigational depths, but knowing tidal current flow can be even more important.

This article originally appeared in the October 2019 issue of Power & Motoryacht magazine.