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Using Tidal Current Tables

This is the third article in a three-part series on current and tide. (Review Part Two.)

The Tidal Current Tables are somewhat similar to tide tables, but the coverage is less extensive. The National Ocean Service (NOS) provides the information from which tidal current volumes are published on an annual basis: These are the Atlantic Coast of North America, and the Pacific Coast of North America and Asia.
Voyaging on the West Coast or Hawaii? This is what you need to understand the tidal currents there.
 Table 1 contains a complete list of predicted times of maximum currents and slack water, with the velocity of the maximum currents and the direction of the ebb and flood for each reference station. There are 22 stations listed in the Atlantic volume and 36 in the Pacific volume.

 Table 2 lists time differences and speed ratios for hundreds of subordinate stations listed by station number. Also included is a description of the location along with the latitude and longitude for each station number. The complete contents of Table 2 are listed below.
 Table 3 provides information to determine the current's velocity at any time between entries in Tables 1 and 2.
 Table 4 gives duration of slack, or the number of minutes the current does not exceed stated amounts, for various maximum velocities.
 Table 5 (Atlantic Coast of North America only) gives information on rotary tidal currents.

Scientists now have much-improved equipment, but the first tide-predicting machine, designed in 1873, was useful in quantifying tidal phenomena.
Each volume also contains current diagrams and instructions for their use. Explanations and examples are given in each table. The volumes also contain general descriptive information on wind driven currents, combination currents, and information such as Gulf Stream currents for the East Coast and coastal currents on the West Coast.

In the Tidal Current Prediction volumes, each reference station is covered for the given year in four pages. Each page covers three months and for each day in the month, the date and day of week are given; along with the current information. If the cycle is repeated twice each tidal day, the currents are semidiurnal. On most days there are four slack waters and four maximum currents, two floods (F) and two ebbs (E). However, since the tidal day is longer than the civil day, the corresponding condition occurs later each day, and on certain days there are only three slack waters or three maximum currents. At some places, the current on some days runs maximum flood twice, but ebbs only once, a minimum flood occurring in place of the second ebb.

For each subordinate station listed in Table 2 of the Tidal Current Tables, the following information is given:

 Number   The stations are listed in geographical order and assigned consecutive numbers, as in the tide tables. Each volume also contains an alphabetical station listing correlating the station with its consecutive number to assist in locating the entry in Table 2.
 Place  The list of places includes both subordinate and reference stations, with the latter given in bold type.
 Position  The approximate latitude and longitude are given to assist in locating the station on your chart. The latitude is north or south and the longitude east or west as indicated by the letters (N, S, E, W) adjacent the entry. The current given is for the center of the channel unless another location is indicated by the station name.
 Time Differences  Two time differences are tabulated. One is the number of hours and minutes to be applied to the tabulated times of slack water at the reference station to find the times of slack waters at the subordinate station. The other time difference is applied to the times of maximum current at the reference station to find the times of the corresponding maximum current at the subordinate station. The intervals, which are added or subtracted in accordance with their signs, include any difference in time between the two stations, so that the answer is correct for the standard time of the subordinate station. Remember to adjust for Daylight Saving Time when applicable. Limited application and special conditions are indicated by footnotes.
 Speed Ratios  The speed of the current at the subordinate station is the product of the velocity at the reference station and the tabulated ratio. Separate ratios may be given for flood and ebb currents. Special conditions are indicated by footnotes.
 Average Speeds and Directions  Minimum and maximum velocities before flood and ebb are listed for each station, along with the true directions of the flow. Minimum velocity is not always 0.0 knots.

Tidal current charts like the one shown here (available on line or in print) offer hour-by-hour information for specific locations.
Duration of Slack Water  The predicted times of slack water listed in the tidal current tables indicate the instant of zero velocity. There is a period of time either side of slack water, however, during which the current is so weak that for practical purposes it may be considered negligible. Table 4 of the Tidal Current Tables gives, for various maximum currents, the approximate period of time during which currents not exceeding 0.1 to 0.5 knots will be encountered. This period includes the last of the flood or ebb and the beginning of the following flood or ebb; that is, half of the duration will be before and half after the time of slack water. When there is a difference between the velocities of the maximum flood and ebb preceding and following the slack for which the duration is desired, it will be sufficiently accurate to find a separate duration for each maximum velocity and average the two to determine the duration of the weak current. Of the two sub-tables of Table 4, Table A is used for all places except those listed for Table B; Table B is used for just the places listed and the stations in Table 2 which are referred to them.

Additional Tide Prediction Publications  NOS also publishes a special Regional Tide and Tidal Current Table for New York Harbor to Chesapeake Bay, and a Tidal Circulation and Water Level Forecast Atlas for Delaware River and Bay.

Tidal extremes, like those in Anchorage, AK (shown here), make understanding and quantifying tidal current a must for mariners.
Tidal Current charts present a comprehensive view of the hourly velocity of current in different bodies of water. They also provide a means for determining the current's velocity at various locations in these waters. The arrows show the direction of the current; and the figures give the speed in knots at the time of spring tides. A weak current is defined as less than 0.1 knot. These charts depict the flow of the tidal current under normal weather conditions. Strong winds and freshets, however, may cause nontidal currents, considerably modifying the velocity indicated on the charts. Tidal Current charts are provided for Boston Harbor, Charleston Harbor, SC, Long Island Sound and Block Island Sound, Narragansett Bay, Narragansett Bay to Nantucket Sound, Puget Sound (Northern Part), Puget Sound (Southern Part), Upper Chesapeake Bay, and Tampa Bay. The tidal current's velocity varies from day to day as a function of the phase, distance, and declination of the moon. Therefore, to obtain the velocity for any particular day and hour, the spring velocities shown on the charts must be modified by correction factors. A correction table given in the charts can be used for this purpose. All of the charts except Narragansett Bay require the use of the annual Tidal Current Tables. Narragansett Bay requires use of the annual Tide Tables.

Predictions  Until recently, tidal predictions were compiled only on mainframe or mini computers and then put into hardcopy table form for the mariner. There are several types of commercial software available now for personal computers that provide digital versions of the NOS tide tables and also provide the capability to graph the tidal heights. For voyage planning, the tabular data and graphs can be printed for the desired locations. There are also several types of specialized, handheld calculators and tide clocks that can be used to predict tides for local areas. All versions of navigation software use the actual harmonic constants available for locations, the prediction equation, and digital versions of Table 2 in the Tide Tables to produce tide and current data for every reference and subordinate station. For an extra fee you can even get worldwide tidal and current data.

Emerging applications include integration of tidal prediction with positioning systems and vessel-traffic systems, which are now moving toward full use of GPS. In addition, most electronic chart systems are already able to integrate tide-prediction information. Many of these new systems will also use real-time water level and current information. Active research also includes providing predictions of total water level that will include not only the tidal-prediction component, but also the weather-related component.

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