tides, currents, and leeway 105
speed of a tidal current. In the United States, you can order tidal current tables in
book form. Th ese show the current at select stations (also available online, see below).
Surveyors use the same sequence for mapping tidal currents as they do for tides. Th ey
record the times (in Eastern Standard Time) of the maximum ebbs and fl oods and
the slacks at the reference station for each day of the month. Th en they determine the
maximum speeds of the ebbs and fl oods. Th e results are placed into the tidal current
book in Table 1. Next they conduct surveys at nearby substations to fi nd time and
speed diff erences for Table 2. Look over the three steps below for fi nding tidal current
information from the Tidal Current Tables. (See page 106 for samples.)
- 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 next to the geographic name. - Enter Table 2 (substations). Enter Table 2 with your substation number.
Q Look at the Time Diff erences section. Columns headed Min. before Flood
and Min. before Ebb (Min. means minimum, or slack) show slack-water
time diff erences. Write down each time diff erence. Now look under
the columns headed Flood or Ebb. Th ese show maximum current time
diff erences. Write down each time diff erence.
Q Slide over to the column headed Speed Ratios. Write down the fl ood and
ebb speed ratios.
Q Move over to the Average Speeds and Directions column. Write down the
maximum fl ood and maximum ebb current directions.
Q Look above your substation line to fi nd the Table 1 reference station; turn
to this page.- Enter Table 1 (reference stations). Find the month and date you need.
Copy the times of slack water and maximum current. Copy the maximum
speed of each fl ood or ebb. Apply the substation time diff erences to the
appropriate reference station times. Multiply the speed ratios by the
appropriate fl ood or ebb speed. Jot down the fl ood and ebb direction (set)
next to your fi gures.
SEA-CRET TIP
X Wind or sea in opposition to a tidal current creates a steep chop. In
an inlet, an ebb current creates rougher conditions than does a fl ood
current. Ocean swells fl owing into an inlet collide with an outgoing
ebb. Onshore winds add to the danger. In marginal conditions, choose
a safer route.