112 seamanship secrets
We plan to arrive about 1 hour before slack water. Using the 50-90-100
Rule, we estimate that the current will be at 50% velocity about 1 hour before
slack and about 1 hour after slack. So our calculation would be:
2.5 knots × 50% = 1.25 knots
Calculate slack-water duration. From the estimated current speed approxi-
mately 1 hour from slack, we can estimate slack-water duration (i.e., the length of
time during which the current will run at ½ knot or less). Simply divide 0.5 knot
by the estimated current speed 1 hour from slack, convert this result to minutes,
and subtract and add the minutes to the slack-water time.
0.5 knot (slack-water velocity) ÷ 1.25 knots (current speed at 1 hour from
slack) = 0.4 hour = 24 minutes.
Now let’s apply this to our 1306 slack-water time.
1306 hours – 24 minutes = 1242 hours
1306 hours + 24 minutes = 1330 hours
We estimate that our slack-water interval will run from 1242 to 1330
hours. Our total time of slack water will be 48 minutes. (See also the Duration
of Slack Water table in Appendix I.)
Based on the tidal current table for Charleston Harbor, the closest slack-water time to our
arrival is 1306 hours.
SEA-CRET TIP
X Arrive earlier than your estimated time of slack interval. Use binoc-
ulars to study the water inside the transit area. Do you still see
current tails streaming from buoys or pilings? If necessary, heave-to
or anchor and wait for slack water. Remember, frontal passages,
heavy rainfall, or prevailing winds can alter the time, strength, and
direction of any current.