Sunday, March 8, 2009

Using Tidal Current Tables

There are two volumes, Tidal Current Tables for the Atlantic Coast of North America, and Tidal Current Tables for the Pacific Coast of North America and Asia. Other publications are by NOS which includeTidal Current Charts and Tidal Current Diagrams. Tidal Current Tables have a format that are similar to the TideTables and are used in the same manner. For a number of principal locations, called reference stations.

Table 1 of these tables lists the predicted times of slack water in chronological order in the left-hand column, and the predicted times and velocities of maximum flood (F) and ebb (E) currents, are also in chronological order, in the center and right-hand columns for each day of the year. Flood and ebb current directions are at the top of each page. All times in the Tidal Current Tables are standard times, each page of Table 1 indicates the central meridian of the time zone used. Adjustments must be made for the use of daylight time or any other deviation from standard time. Due to the length of the lunar day, approximately 24 hours 50 mintues, all days in Table 1 will not have a full set of four maximum currents and four slack waters.

Table 2 has a list of secondary or subordinate stations, arranged in geographic order. Given for each station is its position in terms of latitude and longitude to the nearest minute, its reference station, the difference in time of slack water and time of maximum current in hours and minutes in reference to its station. Also the maximum flood and maximum ebb velocity ratio's with respect to current at the reference station and the direction and average velocities of the maximum flood and ebb currents. The factors for any subordinate station are applied to the predictions of the reference station whose name is printed next above the subordinate station's listing in Table 2. The time differences are added to or subtracted from, according to their signs, the time of slack water and strength of current (maximum flood or ebb) at the reference station to obtain the times of in the current cycle at the subordinate station. The velocity of the maximum currents at the subordinate station is found by multiplying the velocity of either the flood or ebb current at the reference station by the velocity ratio listed for the subordinate station. Note that the time differences for slack water and maximum current are usually not the same, only one time difference is given for maximum current and it is used for both flood and ebb. Where unusual conditions exist, keyed footnotes, for specific subordinate stations, appear at the bottom of the page. The average flood velocity is the mean of all the maximum flood currents, and the average ebb velocity is the mean of all the maximum ebb currents.

Table 3 is used to determine the velocity of a current at any intermediate time between slack and maximum current. Instructions on its use are below the tabulated factors. This table is in two parts, A and B. Table A is for use at nearly all locations. Table B is used for a limited number of places where there are "hydraulic" currents, these locations for each volume of the Tidal Current Tables are also listed below Table 3.

Table 4 is used to find the duration of slack. Slack water, or the time of zero velocity, lasts only an instant, there is a period each side of slack during which the current is so weak that for practical purposes it can be considered as being negligible. From Table 4, the period (half on each side of slack) during which the current does not exceed a given velocity (0.1 to 0.5 knot) is tabulated for various maximum currents. (This table also has a "Table B" whose use is similar but limited to a few specified locations.)

Table 5 (Atlantic tables only, gives information about rotary tidal currents, or currents which change their direction continually and never come to a slack, so that in a tidal cycle of about 12 1/2 hours they set in all directions successively. These currents occur offshore and in some wide area's of the coast. The values given are average velocities due to tidal action only. When a steady wind is blowing, the effect of the current due to wind should be added to the current due to tidal action. This table is seldom used. Instructions for the use of this table as well as for Tables 1 through 4 are given in the publications themselves.

Note: When subtracting your times remember that there are 60 minutes, and not 100 in a hour, this might be obvious to you but its a fact it gets overlooked and mistakes will be made.