104 seamanship secrets
Th e Secret to Understanding Tidal Currents
All the rivers run into the sea... yet the sea is not full.
—King SolomonHow will currents infl uence us as we move from one point to another? Will they
push us along or hold us back? Will they set us to the left or to the right of our
trackline? We have looked at the vertical rise and fall of water, called tides. Now
we’ll discover how those tides directly infl uence the horizontal fl ow of water, called
tidal currents. Other factors, including geography, water depth, and weather,
infl uence tidal current strength and direction. Tidal currents have their own
unique prediction tables, called tidal current tables. Here’s a list of terms you’ll
want to know.
Flood. Th e horizontal fl ow of water from the sea toward inland waters. In most
places, but by no means in all, the fl ood lasts 6 hours.
Ebb. Th e horizontal fl ow of water from inland areas back out to the sea. In
most places, but by no means in all, the ebb lasts 6 hours.
Maximum current. Th e highest speed, or velocity, of the fl ood or ebb during
one cycle.
Set. Th e direction (in degrees true) toward which a current is fl owing. For
example, if the set is 180 degrees true, the current is fl owing toward the
south. Note that this convention is the opposite of that applied to winds,
which are named for the direction from which, not toward which, they blow.
Drift. Th e speed of a current expressed in knots.
Slack current. When speaking of tidal heights, we talked about a period of rest,
called the stand, that follows each rise or fall. Th ere is an analogous interval
of rest following each fl ood or ebb, and this is called slack water. During this
period, currents fl ow between 0 and ½ knot. It is not unusual for the time of
slack water to be off set from the corresponding stand. At river mouths, for
example, the ebb—enhanced by river fl ow—may continue for as much as
2 hours or more aft er the tide begins to rise.
A DAY IN THE LIFE OF A TIDAL CURRENT CYCLE
Let’s look at a typical day of tidal currents on the U.S. East Coast. Th e dark black
horizontal line in the accompanying illustration marks slack water. Flood cycles
rise above the slack line; ebb cycles dip below the slack line.
HOW TO USE U.S. TIDAL CURRENT TABLE
PUBLICATIONS
Before, we talked about how to fi nd the time and height of the tide from the tide
table books. You can use a similar but diff erent book to fi nd the time, direction, and