100 seamanship secretsHOW TO USE U.S. TIDE
TABLE PUBLICATIONS
In the United States, you can order tide
tables in book form (they are also avail-
able online, see below). Th ese publica-
tions show the time and height of the
tide at select stations. Surveyors study
the tides at large ports, called reference
stations. Th ey record the time (using
Eastern Standard Time) and height of
tide for each day of the month. Th ese
daily tides are grouped by month and
placed into the book in Table 1. Next,
surveys are conducted at minor harbors,
rivers, and bays called substations. Times
and heights of tide at substations are
compared with those recorded at nearby
reference stations, and the diff erences are
placed into the book in Table 2. Follow
these three simple Steps to fi nd time
and height from the tide tables.- Enter the back index. Turn to the
back of the book and look for the
substation of interest. If you don’t
fi nd the exact location, fi nd one
nearby. Write down the substation
number you see next to the name.
In a semidiurnal tide, there are two high tides and two low tides in each 24-hour period.
(Actually, the tidal day is a bit longer than 24 hours—more like 24 hours 50 minutes—
because the moon advances about 50 minutes in its orbit with each 24-hour rotation of the
earth.) Since the range of the tide oft en varies from one cycle to the next, you should not
assume that the range this evening will be the same as it was this morning.Slack Tide, Ebb Tide, and
Flood Tide Don’t Exist
The term slack refers to a tidal current,
not to the tide itself. A slack current is
any current with a velocity less than
half a knot. The tide stands, but it
does not go slack. Similarly, there
is no such thing as an “ebb tide” or
“fl ood tide.” The ebb and the fl ood
are tidal currents, but they are not the
tide itself. The poets and songwriters
have it wrong. Though we all use
these terms, they are incorrect. The
distinction may seem trivial—after
all, tidal currents are the direct result
of tides even if they’re not the same
thing—but maintaining the distinction
will better prepare you for those times
when, for example, slack current and
the nearest low- or high-water stand
are hours apart.