108 seamanship secrets
Make a Tidal Current Graph from Online Data
- Slack-water line. Draw a thick, horizontal slack-water line fi rst. Remember,
each current cycle begins and ends at this line. - Max current. In the column for Maximum Current in the online tidal
current tables, a plus (+) sign indicates fl ood current; a negative (–) sign
represents ebb current. Use that information to draw each cycle, beginning
and ending with a slack-water time marked on the slack-water line. Show
fl ood above and ebb below the slack-water line. - Current direction (set). Look above the columns on the tidal current table;
note the true direction of the fl ood (313°T) and ebb (127°T). Write these
directions onto your graph next to the appropriate fl ood or ebb. Compare
this direction to your tracklines. Current from ahead will slow you down
(boatspeed minus current speed = actual speed). Current from behind will
add to your boatspeed (boatspeed plus current speed = actual speed). Any
current not directly in line with your trackline will set you off course. Check
the chart and mark any dangers toward which a tidal current will set you.
BRITISH ADMIRALTY TIDAL CURRENT RESOURCES
Tidal station diamonds. Tidal station diamonds are plotted at various locations
on British Admiralty (and some U.S.) charts. Look for a letter enclosed by a
diamond border. You will fi nd a tidal stream table printed on the chart nearby.
Enter the column to the left with the number of hours before or aft er high water.
For example, for 4 hours aft er high water, fi nd “4” in the column. Follow this line
A graph made from the time table and maximum current for the fl ood and ebb at
Charleston Harbor on September 20, 2008.