Seamanship_Secrets_185_Tips_-_Techniques_for_Better_Navigation-_Cruise_Planning-_and_Boat_Handling_Under_Power_or_Sail_(Re)_e..

tides, currents, and leeway 111

1933

1411

0632 1306

1829

1151

1516

0919

1.9

S L A C K S L A C K

F

L

O

O

1.0 D

1.3

E

B

2.3 B 2.3 1726

1.3

0728

0824 1.8 1.8

1.0

1622

2.5

1035

Using the 50-90-100 Rule, we can compute the speed of current fl ow during fl ood and ebb cycles.

SEA-CRET TIP

X High winds blowing onshore for hours on end increase high- and

low-water heights. Flood speeds increase and ebb speeds decrease.

The beginning of the ebb may be delayed for hours.

Slack Intervals: Nature’s Gift to Mariners

Power and sailing vessels oft en need to know how much time they have to make it
through a narrow passage, bridge, or inlet. Dangerous currents make some transits
impossible. In these cases, make your run in slack-water speeds of ½ knot or less.
You can plan your passage by calculating slack velocity and duration. Below, we
use a midday arrival at Charleston Harbor as an example.
Find the closest slack time to your arrival time. First, look at the tidal current
table to fi nd the closest slack water to your planned time of arrival. For example,
we plan to arrive at Charleston Harbor around noon on September 20, 2008; the
closest slack time to our arrival, based on the table, is 1306.
Estimate the current speed you may encounter at your ETA. On the
table, look at the maximum current velocities bracketing the 1306 slack water.
The maximum flood velocity is 1.9 knots, and the maximum ebb is 2.5 knots.
For safety, use the higher of the two. Remember, this is just an estimate. Let’s
use 2.5 knots for our calculation.