2008 REED’s East Coast Edition - Current Differences
Current Differences Table
The current differences table that follows reference table. When determining slack be-
provides data for calculating tidal current fore flood, use the entry in the primary table
information for various secondary station under slack time that shows a current speed
locations along the North American east and (in knots) under the adjacent flood column,
gulf coasts between southwestern Nova Scotia to the right of the max. time column. When
and Aransas Pass in Texas. This information is determining slack before ebb, use the entry
keyed to the primary reference current tables under slack time that shows a current speed
found earlier in this chapter. (in knots) under the ebb column.
Mariners should bear in mind the effects of Some of the secondary stations in the cur-
local winds and weather conditions. Addition- rent differences table exhibit a double flood
ally, rivers often create local currents near their or a double ebb stage or both. In those cases,
mouths that may reinforce coastal currents or a separate time difference (and sometimes
may flow in opposition to local currents. other data) is listed for each of the flood or
For a discussion of the primary reference ebb phases. These values should be applied
current tables, please refer to page 537. only to the daily maximum flood or ebb phase
HowTo Use The Table
at the primary reference station. (A double
flood is a flood tidal current where, after the
The current differences table on the follow-
flood begins, the speed increases to a maxi-
ing pages is organized geographically begin-
mum, the first flood; decreases to a minimum
ning with stations in Nova Scotia, moving
near the middle of the flood period—and at
south along the Atlantic coast to Florida, and
some places may actually run in an ebb direc-
then following the Gulf of Mexico coast to the
tion for a short period—and again floods to
U.S. border with Mexico. There are no sec-
a maximum speed, the second flood; after
ondary stations with current data in Bermuda
which it decreases to slack water. A double
or the Bahamas.
ebb is similar.)
In the Place column, find the location of
The Speed Ratios columns give ratios for
the station in which you are interested. The
determining approximate current speeds at
name of the primary reference current table
the current difference station for a specific day
to which that station is keyed is listed above
at the time of maximum flow. The appropriate
the chosen location in bold type under the
maximum flood or maximum ebb speed from
Time Differences column, along with the page
the primary reference table is multiplied by
number where the table may be found
the relevant flood or ebb speed ratio given for
The two Position columns to the right of the
the current difference station.
station’s name give its approximate latitude
The Current Direction & Max Speed col-
and longitude. Because the positions are listed
umns give the average directions of current
according to the exactness recorded in the
flow at the times of the maximum flood and
original survey records, the preciseness of the
ebb (in degrees true), with corresponding
stations’ locations varies.
average speeds of maximum flood and ebb
The Time Differences columns express the
(which may differ from the speeds calculated
differences that must be applied to the times
for a specific day using the speed ratios). Fig-
derived from the primary reference table.
ures for these columns are read directly from
Differences are expressed in hours and min-
the table, making it easy to make quick assess-
utes. The term slack in the headings refers to
ments of average local maximum currents.
the time when the current is at a minimum
Notes: In the table, dashes (– –) are substi-
(usually, but not always, 0), before turning to
tuted for data that is unknown, indetermi-
flood or ebb. The time differences for slack
nate, unreliable, or not applicable.
before flood and slack before ebb should be
Endnotes particular to specific secondary
added or subtracted as indicated by the plus
stations are listed at the end of the current
(+) or minus (–) sign to the times from the
differences table.
column labeled slack time on the primary
690