tides, currents, and leeway 109
across to the column that shows the letter diamond of interest. Read the direction
and speed of the neap or spring current at that location.
Tidal stream atlases (also called tidal current atlases).Tidal stream atlases
are published annually from the United Kingdom Hydrographic Offi ce and can
be ordered from its website at: http://www.ukho.gov.uk/amd/paperPublications
.asp. Th e atlases contain diagrams of current fl ow at selected areas in northwestern
Europe, including Great Britain, Ireland, France, Scotland, and the North Sea. Each
area contains a series of diagrams referenced to the time of high water. You enter
the diagram based on the number of hours before or aft er the time of high water.
For example, if you were sailing in the English Channel 3 hours aft er high water,
you would turn to the diagram labeled as such. Current arrows show the direction
of the current and its speed in knots. Because the current speeds are for spring and
neap tides, you would correct the speed using a convenient table. Finally, you could
lightly pencil your track onto the page to see how the current would aff ect you dur-
ing your transit. Later in this chapter you will learn how to use this information to
correct your course.
Tap the Magic of the 50-90-100 Rule
You might expect current to move in a rather lazy motion—taking its time to
get from slack water to maximum speed during the fi rst half of the cycle, and
then slowly losing strength through the latter half. In fact, however, the current
reaches 50% of its maximum velocity by the end of the fi rst hour. Th e 50-90-100
Rule makes it easy to fi nd current speed during any hour of a cycle.
Th e numbers 50, 90, and 100 represent percentages of the maximum current
during the fi rst half of the cycle. We then reverse the rule—from 90 to 50 to 0—during
the last half-cycle. A fl ood or an ebb might not be exactly 6 hours in duration, so
for more accuracy, divide the actual interval between the slack and the maximum
current that bracket your time of interest into three equal portions. If the interval
between the bracketing slack and maximum current is 2 hours 30 minutes, each
portion thereof is 50 minutes. Follow these steps; then go to the example:
- Find the interval of time between the slack and the maximum current
that bracket your time of interest. - Divide this time interval by 3. Th is gives three equal portions of time
between slack and maximum current. - For the fi rst half of an ebb or a fl ood, begin with the initial slack and add
each interval twice. For instance, if slack is at 0900 and maximum current
is at 1212, each interval would be 1 hour 04 minutes. For the fi rst interval,
0900 + 1 h 04 m = 1004. Th en once more: 1004 + 1 h 04 m = 1108. - For the second half of the ebb or fl ood, subtract the time of maximum
current from the time of the next slack, then proceed as above.